Performance Improvement of Power Conversion by Utilizing Coupled Inductors

Transcription

1 Performance Improvemen of Power Conversion by Uilizing Coupled Inducors Qun Zhao Disseraion submied o he Faculy of he Virginia Polyechnic Insiue and ae Universiy in parial fulfillmen of he requiremens for he degree of Docor of Philosophy in Elecrical Engineering Fred C. Lee, Chairman Dushan Boroyevich Dan Y. Chen Alex Q. Huang Douglas Nelson February 03, 2003 Blacksburg, Virginia Keywords: coupled inducor, high sep up, reverse recovery, power facor correcion, single sage Copyrigh 2003, Qun Zhao

2 PERFROMANCE IMPROVEMENT OF POWER CONVERION BY UTILIZING COUPLED INDUCTOR By Qun Zhao Fred C. Lee, Chairman Elecrical Engineering (ABTRACT) This disseraion presens he derivaion, analysis and applicaion issues of advanced opologies wih coupled inducors. The proposed innovaive soluions can achieve significan performance improvemen compared o he saeofhear echnology. New applicaions call for highefficiency high sepup DCDC converers. The basic opologies suffer from exreme duy raios and severe recifier reverse recovery. Uilizing coupled inducor is a simple soluion o avoid exreme duy raios, bu he leakage inducance associaed wih he coupled inducor induces severe volage sress and loss. An innovaive soluion is proposed feauring wih efficien leakage energy recovery and alleviaed recifier reverse recovery. Impressive efficiency improvemen is achieved wih a simple opology srucure. The coupled inducor swiching cell is idenified. Topology variaions and evaluaions are also addressed. The concep ha uilizes coupled inducors o alleviae recifier reverse recovery is hen exended, and new opologies suiable for oher applicaions are generaed. The proposed concep is demonsraed o solve he severe recifier reverse recovery ha occurs in coninuous curren mode (CCM) boos converers. ignifican profile reducion and power densiy improvemen can be achieved in fronend CCM power facor correcion (PFC) boos converers, which are he overwhelmingly choice for use in elecommunicaions and server applicaions. ii

3 This disseraion also proposes opologies o realize he singlesage parallel PFC by uilizing coupled inducors. Compared o he saeofhear singlesage PFC converers, he proposed opologies inroduce a new power flow paern ha minimizes he bulkcapacior volage sress and he swich curren sress. iii

4 TO MY WIFE MIAO, ON ETHAN XINGYI AND MY PARENT YUNHENG ZHAO YING WANG iv

5 ACKNOWLEDGMENT I would firs like o express my appreciaion o my advisor, Dr. Fred C. Lee. The rigorous research aiude I learned from him will surely benefi my fuure career. His exensive knowledge, broad vision and creaive hinking have been a source of inspiraion for me. I am graeful o Dr. Dushan Boroyevich, Dr. Dan Y. Chen, Dr. Alex Q. Huang and Dr. Douglas Nelson for heir valuable discussions abou and commens on my research. My life has changed so much during he pas six years. There have been some days full of pain and darkness when I could no walk ou myself. I am very lucky o have had help and encouragemen from Mr. Zhenxue Xu. My special hanks go o Mr. Wei Dong and Dr. Ming Xu, who have helped me ou of some hard imes. I has been a grea pleasure o work wih so many alened colleagues in he Cener for Power Elecronics ysems (CPE). I would like o hank Dr. Pileong Wong, Dr. Xunwei Zhou, Dr. Qiong Li, Dr. Kun Xing, Dr. Peng Xu, Dr. Yong Li, Mr. Changrong Liu, Mr. Rengang Chen, Mr. Xigen Zhou, Dr. Xiaogang Feng, Dr. ChihYi Lin, Dr. Yunfeng Liu, Dr. Zhenxian Liang, Dr. Xiaochuan Jia, Mr. DengMing Peng, Mr. Jia Wei, Mr. Yu Meng, Mr. HoPu Wu, Mr. Jiabin Chen, Mr. Yuhui Chen, Dr. Michael Zhang, Dr. JaeYoung Choi, Dr. Glenn ku for heir grea help. I am especially indebed o my colleagues in he Ballas and DP (disribued power sysem) groups. Thanks o Mr. Wei Dong, Mr. Bing Lu, Dr. Jingrong Qian, Dr. Fengfeng Tao, Mr. Bo Yang, Dr. Jindong Zhang, Dr. Zhiguo Lu, Dr. Jinjun Liu, Mr. Francisco Canales, Dr. Peer Barbosa, Mr. Alexander UanZoli, Dr. RayLee Lin, Mr. Yuanxuan Hu, Mr. Jinghai Zhou, Mr. Yang Qiu, Ms. Manjing Xie, Ms. Juanjuan un, Miss. Tinging ang, Mr. Liyu Yang and Mr. hou Wang for he delighful discussions. I was a pleasure o work wih such alened and creaive fellows. The fun ha I have had wih my volleyball eammaes has been an inseparable par of my life. I would like o hank Miss. Jinghong Guo, Mr. Kaiwei Yao, Dr. Xuening Li, Mr. Ruancheng v

6 Acknowledgemens Ren, Mr. Lingying Zhao, Mr. Dianbo Fu, Mr. Chong Han, Mr. Chuanyun Wang, Mr. Yuming Bai. Mr. Yuibin Zhu, Mr. ihua Wen for sharing such grea ime. I would also like o acknowledge he CPE adminisraive and lab managemen saff, Ms. Linda Gallagher, Ms. Trish Rose, Ms. Marianne Hawhorne, Mr. Rober Marin, Mr. eve Chen, Ms. Teresa haw, Ms. Elizabeh Traner, Ms. Ann Craig, Mr. Gary Kerr, Mr. Jeffery Bason, Ms. Lida Fizgerald, for heir counless help in my CPE work. Thanks o Ms. Amy hea, who polishes all of my publicaions, including his disseraion. My hanks also exend o Dr. Fushen Tsai, Dr. Ching Chang, Mr. Ger Burning, Ms. Faye Li, Dr. Juan A. abae, Mr. Mahew Concannon and Mr. Arne einbakk for he valuable suggesions and commens, and for supporing his research. I highly appreciae my wife, Miao Xu, for he love, undersanding, encouragemen and sacrifice. If i were no for her, my dreams would never come rue. Thanks o my lovely son, Ehan, who brings me he pride and happiness of being a faher. My deepes graiude is sen o my parens, Mr. Yunsheng Zhao and Ms. Ying Wang, for heir love and suppor. vi

7 Acknowledgemens This work was suppored by Philips Research, Aresyn Technology, Elek Energy and he ERC program of he Naional cience Foundaion under Award Number EEC vii

8 TABLE OF CONTENT Chaper 1. Inroducion Research Background Disseraion Ouline and Major Resuls Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers Inroducion Topology Evaluaions for HID Ballas Converer Advanages and Disadvanages of he AciveClamp Flyback Converer ClampMode CoupledInducor High epup DCDC Converers Design Guidelines Experimenal Verificaion Generalized Concep Applicaion Example of he New Family of TopologiesDCBackup Power upply ummary Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery Inroducion A Brief Review of aeofthe Ar oluions Alleviaed ReverseRecovery Problem wih Coupled Inducors Improved Topologies for he FronEnd PFC Applicaions Developmen of he 1U High PowerDensiy FronEnd Converer Concep Exension and Unified Curreneering Condiion ummary Chaper 4. Uilizing Coupled Inducors o Realize Parallel Power Facor Correcion Inroducion CoupledInducor Converers wih Two Inpu Volage ources viii

9 Table of Conens 4.3. Parallel Power Facor Correcion wih PF= Improved Parallel Power Facor Correcion wih PF< Comparison of he Proposed Approach wih Oher ingleage PFC ACDC Converers Experimenal Verificaion Generalized rucure ummary Chaper 5. Conclusion and Fuure Work Conclusion Fuure Work Appendix A. Topology Evaluaions for The FronEnd Converer of a HID Ballas Appendix B. Capacior hif Rule References Via ix

10 LIT OF ILLUTRATION Fig Comparison of road condiions wih differen headlamps: (a) 35W HID lamp and (b) 55W convenional lamp... 2 Fig High performance of HID lamps: (a) good focusing capabiliy and (b) longer lamp life Fig Block diagram of HID ballas for auomobiles... 4 Fig aeofhear HID ballas: (a) ballas circui diagram and (b) efficiency comparison of he fronend sepup converer Fig Differen backup schemes: (a) ACUP and (b) dualinpu fronend converers [B9]... 8 Fig The cascade boos is used for he DCinpu converer: (a) circui diagram and (b) loss breakdown... 9 Fig CCM boos converer as he fronend boos converer: (a) a fronend boos converer and (b) loss breakdown Fig Clampmode coupledinducor buckboos converer Fig Clampmode coupledinducor boos converer Fig Coupledinducor boos converer wih alleviaed recifier reverserecovery problem Fig Derivaion of PPFC opologies: (a) an equivalen circui of he converer shown in Fig and (b) opology wih wo separaed volage sources Fig Buckboos converer under exreme duy raio: (a) buckboos converer; (b) calculaed efficiency; and (c) loss breakdown Fig Analysis he feasibiliy of he cascade srucure: (a) opology srucure; (b) calculaed efficiency; and (c) loss comparision Fig Equivalen circui of he Flyback converer and he curren waveforms: (a) he equivalen circui and (b) waveforms for loss calculaion Fig Loss relaed o he leakage inducance Fig Flyback converer wih RCD snubber: (a) circui diagram; (b) prediced efficiency; and (c) loss breakdown Fig Aciveclamp flyback converer: (a) circui diagram; (b) prediced efficiency; and (c) loss breakdown Fig Aciveclamp Flyback converer: (a) opology and (b) key waveforms Fig Equivalence of isolaion ransformer and auo ransformer: (a) isolaion ransformer and (b) auo ransformer Fig Transformaion of Flyback converer o he coupledinducor converer wihou isolaion : (a) Flyback converer and (b) nonisolaion coupledinducor converer Fig Coupledinducor converer wih aciveclamp scheme: (a) opology and (b) key waveforms Fig Derivaion of he clampmode coupledinducor converers wih N s =N p Fig Proposed clampmode coupledinducor buckboos converer Fig Operaion modes of he clampmode coupledinducor buckboos converer: (a) [ 0, 1 ], (b) [ 1, 2 ], (c) [ 2, 3 ], (d) [ 3, 4 ], (e) [ 4, 5 ] and (f) [ 5, 0 ] x

11 Lis of Illusraions Fig Key waveforms of he clampmode coupledinducor buckboos converer. 40 Fig Curren relaionship Fig Aciveclamp flyback converer: (a) circui diagram and (b) key waveforms.44 Fig Comparison of circulaing energy: (a) aciveclamp counerpar and (b) clampmode buckboos converer Fig Loss breakdown Fig Normalized clamp capacior volage Fig Design curves Fig Opimizaion procedure Fig Prooype of he HID ballas Fig Experimenal waveforms of he DCDC converer for use in HID ballass Fig Experimenal waveforms when generaing 400V opencircui oupu volage Fig Prediced and measured efficiency Fig Threeerminal cell and coupledinducor swiching cell Fig Converer cell Fig ix opologies wih coupledinducor swiching cell Fig Leakage handling for he sepup converers: (a) nonideal coupledinducor converer; (b) clampmode coupledinducor converer; (c) swiching cell of he converer in (a) and (d) swiching cell of he converer in (b) Fig The cell can be applied o he high sepup boos converer Fig Topology variaions by applying capacior shif rule: (a) buckboos version and (b) boos version Fig Topology variaions and comparison of he buckboos version Fig Topology variaions and comparison of he boos version Fig Leakage handling for he sepup converers Fig Differen backup schemes: (a) ACUP and (b) dualinpu fronend converers [B9] Fig Curren seering of he proposed soluion: (a) circui diagram and (b) key waveforms Fig Design graph for DCbackup power supply Fig Experimenal es se up Fig Experimenal waveforms of inpu inducor curren and swich volage Fig Experimenal waveforms of inpu inducor curren, oupu diode curren, and swich volage Fig Loss breakdown Fig ignifican reverserecovery loss in he cascade converer Fig Derimenal effecs of recifier reverserecovery on CCM boos converers: (a) a CCM boos converer and (b) waveforms showing reverserecovery effec Fig General srucure for alleviaing recifier reverserecovery problem Fig Derivaion of equivalen swiching cell: (a) he cell discussed in he previous chaper and (b) a new equivalen cell Fig Curren seering of a coupled inducor xi

12 Lis of Illusraions Fig. 3.5 Derivaion of boos converer wih alleviaed recifier reverse recovery: (a) previous leakage energyrecovery scheme; (b) new leakagerecovery scheme; and (c) coupledinducor boos converer Fig The concep as applied o oher coupledinducor converers: (a) buck converer and (b) buckboos converer Fig Analysis model Fig Operaion modes of he proposed converer: Fig Key waveforms corresponding o he operaion modes Fig Tes bed for measuring reverserecovery charge: (a) diagram of es circui and (b) waveforms of he wopulse es mehod Fig Measured waveforms showing he reverserecovery of PHRP860 (a 25 o C).. 95 Fig Measured waveforms showing he reverserecovery of PHRP860 (a 55 o C).. 95 Fig Measured reverserecovery charge: (a) a room emperaure (25 o C) and (b) under an ambien emperaure of 55 o C Fig Experimenal seup for DCDC boos converer Fig Compleed curren shif before swich urns on when V in = 125 V dc Fig Effec comparisons: (a) severe reverserecovery problem for hyperfas recifier RHRP860 (rr< 30 ns) wihou conrolled di/d; and (b) no only is he reverserecovery curren reduced, bu he curren appearance is also delayed Fig The volage sress of recifier D o Fig Efficiency measuremens of he DCDC converer Fig The proposed converer wih a lossless snubber for he sress reducion of D o Fig Eigh operaion modes of he proposed converer wih a lossless snubber. (a) [T 0, T 1 ]; (b) [T 1, T 2 ]; (c) [T 2, T 3 ]; (d) [T 3, T 4 ]; (e) [T 4, T 5 ]; (f) [T 5, T 6 ]; and (g) [T 6, T 0 ] Fig Key waveforms of of he proposed converer wih a lossless snubber Fig Turns raio for complee curren seering Fig Facors affecing he leakage inducance of he coupled inducor Fig Three coupled inducors Fig ensiiviy o he leakage inducance Fig Inpu volage and curren waveforms a low line and high line inpu. (a) V in =120V and (b) V in =230V Fig Curren seering of he coupled inducor: (a) waveforms wih he ime scale 2µs/div and (b) enlarged waveforms wih ime scale 50ns/div Fig Reduced volage sress of he oupu recifier Fig Measured efficiency Fig Improved version wih he reduced inducor winding loss: (a) circui diagram and (b) key waveforms Fig CCM curren in winding N Fig Measure efficiency Fig Operaion modes of he converer wih minimum volage sress: (a) [T 0, T 1 ]; (b) [T 1, T 2 ]; (c) [T 2, T 3 ]; (d) [T 3, T 4 ]; (e) [T 4, T 5 ]; (f) [T 5, T 6 ]; (g 1 ) mode I during [T 6, T 0 ] and (g 2 ) mode II during [T 6, T 0 ] Fig Key waveforms xii

13 Lis of Illusraions Fig Experimenal resuls. (a) snubber urnon and (b) snubber urn off Fig Deailed waveforms of he recifier volage sress: (a) D o blocks he volage V o V in and (b) D c blocks he volage V o Fig Measured efficiency Fig Targeed dimensions of he fronend converer Fig Design graph o deermine he urns raio N Fig Comparisons of he wo designs: (a) fooprin by use of 77192; (b) fooprin by use of and (c) comparison of wo inducors Fig High powerdensiy 1U fronend converer Fig Compariosn of he proposed soluion and he hardswiching converer Fig The coupled inducor for he improved opology Fig Circui realizaion of he improved version Fig Inpu volage and curren waveforms a low and high line inpu: (a) V in = 90V and (b) V in = 150V Fig Experimenal waveforms of inpu curren, swich curren and volage a 90Vac inpu Fig Experimenal waveforms of curren seering Fig The wors case of he recifier reverse recovery has been alleviaed: (a) complee curren shif and (b) deails of he circled waveforms in (a) Fig Volage waveform of he capacior in he lossless snubber Fig Efficiency comparison for he PFC converer Fig Comparisons beween he baseline and he developed converer: (a) exising fronend converer; (b) lowprofile, high powerdensiy fronend converer and (c) picure of he lowprofile, high powerdensiy fronend converer Fig Comparisons beween he baseline and he developed converer: (a) profile; (b) volume; (c) fooprin and (d) power densiy Fig Exension of he proposed concep o oher boos PFC circuis for universallineinpu applicaions Fig wiching cell for alleviaing he recifier reverse recovery Fig Coupled inducor converer derived in Chaper 3: (a) he swiching cell and (b) he coupledinducor boos converer conaining he swiching cell Fig Derivaion of PPFC opologies: (a) he boos version derived in Chaper 2; (b) a converer wih same operaion modes as he converer shown in (a) and (c) separae he inpu volage source ino wo sources Fig Oupu power is processed wice in he wosage approach Fig Parallel PFC scheme wih uniy power facor: (a) he inpu and oupu power for PF=1; (b) he concep of PPFC and (c) circui realizaion Fig Concep of parallel PFC for PF<1. (a) boos version; (b) concep circui and (c) simplified PPFC waveforms Fig PPFC opology and he key waveforms: (a) he opology and (b) key waveforms Fig Experimenal resuls wihou: (a) lowline volage and curren; (b) harmonic curren disribuion a low line; (c) highline volage and curren; and (d) harmonic curren disribuion a high line xiii

14 Lis of Illusraions Fig Improving he inpu curren waveform: (a) waveform in a line cycle; (b) waveform in a swiching cycle corresponding he doed area of (a); (c) exension of he inpu curren conducion angle; and (d) when a small inducor is added Fig Four operaion modes of he converer wih a boos inducor Fig Key waveforms in one swiching cycle for four differen operaion modes: (a) mode I; (b) mode II; (c) mode III; and (d) mode IV Fig Block diagram of singlesage PFC AC/DC converer Fig An example of DCM 2 PFC ACDC converers: (a) wihou feedback winding and (b) wih feedback winding Fig Reducing he volage and curren sresses using a coupled winding: (a) wihou feedback winding and (b) wih feedback winding Fig CCM V 2 PFC converer: (a) he opology; (b) he comparison of he efficiency and (c) he comparison of volage sress Fig CCM C 2 PFC converer: (a) he opology; (b) he comparison of he efficiency and (c) he comparison of volage sress Fig Differen power flow paern of he proposed converer: (a) curren flow paern of exising approach and (b) new proposed new power flow paern for he proposed approach Fig Experimenal resuls wih L i (20 µh): (a) lowline volage and curren; (b) harmonic curren disribuion a low line; (c) highline volage and curren; and (d) harmonic curren disribuion a high line Fig Clamped bulk capacior a differen load condiions (Vin = 230 Vac): (a) Po = 60 W; (b) Po = 30 W; and (c) Po = 0 W Fig Comparison of bulk capacior wih volage source singlesage PFC converers: (a) efficiency comparison and (b) bulk capacior volage comparison Fig The proposed converer using a Forward converer Fig Experimenal resuls wih P o =100W and V o =5V: (a) V in =120V and (b) V in =230V Fig Comparison wih he singlesage curren source PFC converer: (a) efficiency comparison and (b) bulk capacior volage comparison Fig Topology variaions: (a) general srucure; (b) boos version; and (c) volagedoubler version xiv

15 LIT OF TABLE Table 1.1. Problems o be solved Table 2.1. Loss comparison Table 2.2. Connecion of he converer cell o he inpu and oupu Table 2.3. Measured efficiency of he prooype converer Table 2.4. Componen lis Table 3.1. Comparison of differen fasrecovery diodes Table 3.2. aeofhear fronend converers xv

16 Chaper 1. Inroducion 1.1. Research Background Insead of convenional lamps, high inensiy discharge (HID) lamps are preferable for use as he headlamps for highend auomobiles because of heir significan advanages, such as longer life, beer colorrendering properies, and higher efficacy in convering he elecrical power o visible ligh. Fig. 1.1 compares road condiions wih a 36W HID lamp and a 55W convenional lamp [B2]. As can be seen, alhough he HID lamp consumes onehird less power, i provides a much beer road condiion han ha achieved wih a convenional lamp. Anoher advanage of he HID lamp is he good focusing capabiliy of he ligh beam, as shown in Fig. 1.2(a). The focused farreaching ligh beam provides drivers wih more safey and comfor. The HID lamp also has a much longer lifeime han ha of he radiional lamp, as shown in Fig. 1.2(b). A HID lamp can las abou en housand hours, while he convenional reaches only abou hree housand. Alhough he HID lamp has so many advanages, i is he same as all oher discharge lamps ha require a ballas o conrol he lamp power during seady sae. The volage drop of he 36W HID lamp is from 60 o 100V. During he igniion period of he lamp, a 400V open circui volage is also necessary. ince he power source of he HID headlamps are he auomobile 12V baery, which provides a volage much lower han he lamp operaion volage, a 1

17 Chaper 1. Inroducion high sepup DCDC converer is necessary in he ballas o supply he low baery volage wih a volage gain of abou enfold for seadysae operaion. (a) (b) Fig Comparison of road condiions wih differen headlamps [B2]: (a) 35W HID lamp and (b) 55W convenional lamp. HID Lamp Convenional Lamp 3,000 6,000 9,000 Lamp Life (Hours) (a) Fig High performance of HID lamps: (a) good focusing capabiliy [B2] and (b) longer lamp life. (b) The requiremens for he high sepup in he HID ballas converer are as follows: High sepup volage gain. The volage of he auomobile baery is 9 o 16V, and he seadysae volage of he lamp is 60 o 100V. Abou en imes he sepup gain is 2

18 Chaper 1. Inroducion required o boos he baery o he required operaion volage of he lamp. During he sarup period, a 400V open circui volage mus be provided. High efficiency. The encapsulaed environmen of he car provides a bad hermal condiion for he converer. Highefficiency DCDC converers can dramaically reduce he hermal managemen cos. The arge efficiency of he high sepup DC DC converer is 90% [B1]. No isolaion is required. Fig. 1.3 is a block diagram of a HID ballas for auomobiles. The ballas calls for high efficiency high sepup DCDC converers ha do no require isolaion. The highfrequency DC DC converer is used o boos he nominal 916V baery volage o 60 V o 100V wih a consan 36W of oupu power. To exend he life of he lamp, one erminal of he oupu volage mus be negaive referring o he inpu ground in order o avoid he migraion of he maerial in he lamp. During he sarup process, he DCDC converer should be capable of generaing a 400V opencircui volage o ignie he lamp. An unregulaed lowfrequency DCAC fullbridge converer ha conains no magneic componen follows he DCDC converer. This DCAC converer is used o chop he DC oupu volage o a 50% duy cycle a 400Hz of low frequency o avoid he acousic resonance of he lamp. The performance of he fronend DCDC converer dominaes he performance of he whole ballas sysem because he second DCAC sage is an unregulaed lowfrequency fullbridge converer wihou magneic componens. A highperformance, high sepup DCDC converer is a crucial par for he ballas. 3

19 Chaper 1. Inroducion DCDC FronEnd 60 o 100V eady ae DCAC Full bridge Bridge 9 o 400V DCAC 16 V DC Open Circui HID Lamp Digial Conroller Fig Block diagram of HID ballas for auomobiles. The buckboos converer is he simples nonisolaion opology for his applicaion. However, a buckboos converer encouners exreme duy raios o provide he enfold sepup volage gain during seadysae operaion. In paricular, he requiremen of 400 sarup volage severely penalizes he efficiency since he device raing is abou 500V. This high volage raing device is no a good choice for he seadysae operaion because of he high inpu curren. The converer has high conducion and swiching losses under exreme duy raio operaion. The challenge for his highefficiency, high sepup DCDC converer is o avoid exreme duy raios so ha conducion loss and swiching loss can be dramaically reduced. The cascade srucure can be used o avoid he exreme duy raios; however, he 400V sarup volage remains problemaic in erms of efficiency. Uilizing a coupled inducor is a soluion. Fig. 1.4(a) shows he circui diagram of he saeofhear HID ballas for auomobile headlamps. The fronend sepup DCDC converer is a Flyback converer wih a resisorcapaciordiode (RCD) snubber. The downsream fullbridge converer does no have much impac on he performance. Fig. 1.4(b) shows he efficiency 4

20 Chaper 1. Inroducion comparison beween he buckboos converer, he cascade srucure, and he Flyback converer for his HID fronend DCDC applicaion [Appendix A]. The efficiency of he Flyback converer for his applicaion is abou 85% o 86%, which is much lower han he argeed 90%. Alhough he Flyback converer can provide high sepup volage gain, he leakage inducance of he coupled inducor no only increases he volage sress of he swich bu also induces significan loss. The RCD snubber can alleviae he volage sress of he swich, bu all of he leakage energy is dissipaed. The inpu ground and he posiive erminal of he oupu are conneced in he Flyback converer so ha one of he oupu erminals is negaive referring o he ground. This soluion does no ake advanage of he fac ha isolaion is no required in his applicaion. An advanced soluion o handle he leakage energy mus be idenified, because he loss analysis shows ha he leakage inducance induces significan loss. If he loss relaed o he leakage inducance can be recovered, he efficiency can be higher han 90%. Finding a way o improve he performance of he high sepup converers is one of he moivaions of his disseraion. 5

21 Chaper 1. Inroducion L m Igniion circui V in N 1 N 2 V o C L o k HID Transformer D o (a) Efficiency Targeed Efficiency Flyback w/rcd Cascade rucure BuckBoos V in =12 V, P o =36 W, F s =100 KHz, V _sar =400V V o (V) (b) Fig aeofhear HID ballas: (a) ballas circui diagram and (b) efficiency comparison of he fronend sepup converer. In he applicaion of HID lamps for highend auomobiles, he seadysae power is 36W and he oupu volage is 60 o 100V. A his power and oupu volage, he oupu recifier reverserecovery problem is no a major concern. For some oher high sepup applicaions wih higher levels of oupu power and oupu volage, he recifier reverserecovery problem is anoher possible concern. Fig. 1.5(a) shows a disribued power sysem (DP) wih an uninerrupible power supply (UP). The ACDC fronend converer provides power facor correcion (PFC) 6

22 Chaper 1. Inroducion and a roughly regulaed 380 o 400V DC bus. The downsream DCDC converer provides ighly regulaed oupu volage and isolaion. During a blackou period, he UP generaes AC volage from he baery. However, he UP ypically provides 30 minues of reserve ime. The convergence of compuer and elecommunicaions indusries makes he welldefined 48V DC baery plan he naural choice for providing hours of reserve ime during ouages in he AC mains [B7][B9]. The emerging DCbackup converer is a simple and efficien soluion compared wih he UP soluion, since i conains only a DCDC converer insead of an inverer. Fig. 1.5(b) shows a dualinpu fronend boos converer. A nonisolaion DCDC converer insead of a UP inverer (AC UP), boh of which would be powered by he 48V DC power plan, is more efficien and much less complex [B8][B11]. A highefficiency DCDC converer ha can generae 380V DC bus from a 48V DC plan is he mos imporan par of he DCbackup sysem. I should be poined ou ha isolaion is no necessary for he ACDC or DC DC fronend converer, since he isolaion is provided by he consequen DCDC sages. The DCinpu fronend converer mus provide approximaely en imes he volage gain. 7

23 Chaper 1. Inroducion 90 o 264V AC ACDC FronEnd 380 o 400 V DC Load # 1 DCDC Converer Load # 2 Load # 3 DCAC FronEnd V DC_baery 90 o 264V AC ACDC FronEnd P o = 1KW (a) 380 o 400 V DC Load # 1 DCDC Converer Load # 2 Load # 3 DCDC FronEnd 48 o 72 V DC Po = 1KW (b) Fig Differen backup schemes: (a) ACUP and (b) dualinpu fronend converers [B9]. In a HID ballas, he 400V sarup volage severely penalizes he efficiency of boh he buckboos and he cascade converers. For he DCbackup applicaion, differen specificaions make he cascade boos converer he saeofhear soluion, as shown in Fig. 1.6(a). However, he cascade srucure is only used o avoid he exreme duy raios for high sepup volage gain. CCM operaion of a power converer is preferred because of he low curren sress o he swich. The oupu recifier reverserecovery problem of recifier D o2 can resul in high levels of urnon loss for he swich. Fig. 1.6(a) shows he loss breakdown of he cascade converer wih 1kW of 8

24 Chaper 1. Inroducion oupu power. The loss due o he recifier reverse recovery is he mos salien porion of oal losses. This high swiching loss dramaically deerioraes he hermal condiion of he swich. L 1 D o1 L 2 D o2 P o =1KW V o = V V in = 48 75V 1 C o1 2 C o2 (a) Loss (W) V in = 60V, Po=1KW, Fs=100 KHz _cond on _off Do Cu Core oher Toal (b) Fig The cascade boos is used for he DCinpu converer: (a) circui diagram and (b) loss breakdown. The second challenge for he highefficiency, high sepup DCDC converers is o alleviae he recifier reverse recovery so ha he swiching loss can be dramaically reduced. Eliminaion of exreme duy raios and alleviaion of he recifier reverserecovery problem can significanly improve he efficiency of he power conversion. 9

25 Chaper 1. Inroducion I is also well known ha he fronend PFC ACDC converers shown in Fig. 1.7(a) have he same recifier reverserecovery problem when hey are operaed in CCM because of he 380 o 400V DC oupu volage. The difference is ha he duy raio mus change wihin a very large range in order o achieve uniy power facor. In oher words, he exreme duy raio is no a concern bu he recifier reverserecovery problem is. Fig. 1.7(b) shows he loss breakdown of a 1KW boos converer wih 90V AC inpu volage. The swich urnon loss could be dramaically reduced if an approach o alleviae he recifier reverse recovery would be adoped. The recifier reverserecovery problem deerioraes he hermal condiion of he boos swich. The exreme hermal condiion limis he power of a fronend boos converer. The performance of boh ACinpu and DCinpu fronend converers is dramaically degraded. The soluion of alleviaing recifier reverserecovery problem benefis boh he ACinpu and he DCinpu fronend converers. Finding a simple and effecive soluion o alleviae he recifier reverse recovery in he DCbackup power supply and he ACDC fronend converer is anoher moivaion of his disseraion. 10

26 Chaper 1. Inroducion L D o P o =1KW V o = V V in = Vac C o (a) Loss (W) P_bridge _cond _on _off Do P_inducor oher Toal (b) V in = 90V ac, Po=1KW, Fs=100 KHz Fig CCM boos converer as he fronend boos converer: (a) a fronend boos converer and (b) loss breakdown. As can be seen from he proceeding analysis, he exreme duy raio and severe recifier reverserecovery problem are he wo major challenges for he relevan applicaions. Table 1.1 summarizes he major problems in hese applicaions. Advanced opologies wih coupled inducors developed in his disseraion demonsrae ha he proposed soluions can achieve a significan performance improvemen. 11

27 Chaper 1. Inroducion Table 1.1. Problems o be solved. Applicaions High epup for HID Ballas DCInpu Backup Converer ACInpu Fron End Converer pecificaions V in =9 o16v, V o =60 o 100 (Igniion 400V) P o =36W V in =48 o 72V dc, V o =380 o 400V dc P o =1KW V in =90 o 264V ac, V o =380 o 400V P o =1KW Duy Raios Exreme Exreme Variable (No a Problem) Idenified Problems ReverseRecovery Problem No evere evere evere 1.2. Disseraion Ouline and Major Resuls This disseraion addresses soluions o improve he performance of power conversion by uilizing coupled inducors for differen applicaions. When a converer is operaed under exreme duy raios o provide large conversion raios, i suffers from high volage and curren sresses. When a buckboos converer is used for his HID applicaion, he efficiency is around 70% because of he penaly ha resuls from he exreme duy raios and he 400 sarup volage. For he high sepup DCDC converer in a HID ballas, he oupu power and oupu volage are relaive low. The major concern is o avoid he exreme duy raio so ha he conducion and swiching losses can be reduced. To avoid exreme duy raio and complexiy of he opology, a coupledinducor converer, such as a Flyback converer or a coupledinducor epic converer, is a good opion for lowpower applicaions. By uilizing a coupled inducor, various volage gains can be achieved by properly designing he urns raio. The problem is ha 12

28 Chaper 1. Inroducion he leakage inducance of a coupled inducor has derimenal effecs on he performance of he converers. The saeofhear soluion is a Flyback converer wih RCD snubber. The efficiency is abou 85% o 86%, which is far below he argeed 90% [B1]. The aciveclamp scheme minimizes he swich volage sress and offers an efficiency improvemen of abou 3%.. The drawbacks of he aciveclamp scheme are is complex opology and conrol circui. For applicaions ha do no require isolaion, mehods available o handle leakage energy do no ake advanage of he lack of an isolaion requiremen. In Chaper 2, a nonisolaion coupledinducor buckboos converer is firs derived from he Flyback converer. The aciveclamp scheme is hen applied o his converer. The acive swich in he aciveclamp scheme provides a discharge pah for he clamp capacior; However, analysis shows ha his discharge pah exiss hrough he secondary coupled winding and he oupu recifier. Therefore, he acive swich in he aciveclamp circui can be eliminaed. A novel leakage recycling scheme wih a simple opology is proposed, as shown in Fig L m N p C c C o V o V in L k D c N s D o I Dc I Do Fig Clampmode coupledinducor buckboos converer. The proposed clamp scheme only needs an exra recifier D c and a clamp capacior C c, and i can realize a funcion similar o ha of he aciveclamp scheme. Furhermore, he proposed 13

29 Chaper 1. Inroducion scheme has much less curren circulaing in he clamp circui as compared o he aciveclamp scheme. ignifican efficiency improvemen can be achieved for he proposed converers. The proposed opology can achieve more han 90% efficiency wih he inpu volage variaion range from 9 o 16V for he fronend converer of he HID ballas. Topology derivaion, operaion analysis, design issues and experimenal verificaion are addressed in he firs par of Chaper 1. The coupledinducor swiching cell is hen idenified. In pracical applicaions, leakage energy mus be handled properly for he family of converers ha use he coupledinducor swiching cell. The proposed concep for recycling he leakage energy is hen generalized and applied o oher nonisolaion coupledinducor sepup converers. New clampmode coupledinducor opologies are generaed. The clampmode coupledinducor boos converer as shown in Fig. 1.9 is a member of he new circuis. This converer can be used as he DCbackup power supply. For he DCbackup power supply, he oupu volage is 380 o 400V, and he oupu power is 1KW. I has been shown ha he recifier reverse recovery becomes anoher concern in his high sepup applicaion. The curren soluion for alleviaing he recifier reverserecovery problem is o conrol he di/d of he recifier during is urnoff ime period. To accomplish his purpose, an auxiliary circui is necessary. Bu for he proposed soluion, alleviaing he recifier reverserecovery problem can be achieved wihou any exra circui because he clamp scheme can inherenly realize he curren seering. During he swich urnoff ime period, he magneizing curren will shif from loop 1 (dashedline loop) o loop 2 (solidline loop). Before swich urns on again, he complee curren shifing is guaraneed so ha recifier D c is naurally recovered. When urns on again, recifier D o urns off. ince recifier D o is in series wih a coupled 14

30 Chaper 1. Inroducion winding, he leakage inducance of he coupled winding conrols he di/d of he oupu recifier Do. The recifier reverserecovery problem is hus alleviaed. The operaion of he curren seering process, design deails ha guaranee he curren shifing, and experimenal verificaion are also provided in Chaper 2. V Cc C c Loop 1 D c D o N p I Dc N s I Do V in Loop 2 Vo Fig Clampmode coupledinducor boos converer. The recifier reverserecovery problem is also a concern for he PFC fronend converer, and can dramaically degrade is performance. A mehod for alleviaing he recifier reverserecovery problem by uilizing he curren seering of he coupled inducor is discussed in Chaper 3. aring from he coupledinducor swiching cell covered in Chaper 2, an equivalen coupled inducor swiching cell is derived. A simple discharge pah for he leakage energy is hrough he oupu volage source. A family of new opologies wih alleviaed recifier reverserecovery problem is proposed. Fig shows he boos version. The curren seering process is similar o ha of he high sepup DCDC converer. Afer swich urns off, he magneizing curren will shif form he loop wih a dashed line (loop 1) o he curren loop wih a solid line (loop 2). Under proper design, he complee curren shif from loop 1 o loop 2 can be achieved during he 15

31 Chaper 1. Inroducion swich urnoff ime period. Therefore, recifier D c can be naurally recovered. When swich urns on, recifier D o urns off. ince recifier D o is in series wih a coupled winding, he leakage inducance of he coupled winding conrols he di/d of he oupu recifier. The recifier reverserecovery problem is alleviaed. In Chaper 3, opology developmen, operaion principle design issues, and parasiic resonance handling for pracical applicaions are addressed. Experimenal resuls show ha an efficiency improvemen of more han 2% can be achieved for a 1KW fronend CCM PFC boos converer wih universal line inpu. By adoping he proposed soluion, he hermal condiion of he MOFET is dramaically improved. Improved power densiy and reduced profile are achieved. In Chaper 3, he proposed soluion is also demonsraed wih a lowprofile (1U), high powerdensiy (11W/inch 3 ) fronend converer. N s D o I Do N p D c I Dc V in Loop 1 Loop 2 Co Vo Fig Coupledinducor boos converer wih alleviaed recifier reverserecovery problem. In Chaper 4, a new coupledinducor swiching cell is derived from he ideal coupledinducor swiching cell. The equivalen circui is shown in Fig. 1.11(a). When he inpu volage source is separaed ino wo volage sources, a new feaure emerges: The curren pah o charge 16

32 Chaper 1. Inroducion he magneizing inducor can be changed as he volage sources are varied. The converer hen has wo possible charge pahs, as shown in Fig. 1.11(b). This converer can be uilized o realize singlesage parallel PFC (PPFC). The goal of PPFC is o no process all he power wice in an ACDC converer wih PFC; only 32% of he oupu power mus be processed wice. This scheme reveals he benefi of efficiency improvemen. In order o realize uniy power facor wih PPFC, he converer consiss of wo sages. Because sage one and sage wo are wo differen converers, he complexiy of he circui makes he soluion less coseffecive in lowpower applicaions. The proposed converer can realize PPFC in lowpower applicaions in which he inpu curren harmonic requiremen is no sringen. Compared o he convenional singlesage approach, in which he bulk capacior volage and he swich curren sress are he major concerns, he proposed converers have much lower volage sress and he swich curren sress is minimized. The proposed singlesage coupledinducor PPFC converers realize a new power flow paern, which is differen from he curren singlesage approach. This new paern allows for a direc load curren feedback, which auomaically reduces he inpu power when he oupu power decreases. The concerns of bulk capacior volage sress and swich curren sress in he curren singlesage approach are eliminaed. Theoreical analysis and he experimenal resuls verify he proposed soluion. 17

33 Chaper 1. Inroducion N s >N p D s I Ds N p D p I Dp V in C o Vo (a) N s >N p D s I Ds N p D p I Dp V ins V inp C o Vo (b) Fig Derivaion of PPFC opologies: (a) an equivalen circui of he converer shown in Fig and (b) opology wih wo separaed volage sources. In Chaper 5, he funcions of he coupled inducor sudied in his disseraion are summarized, and he possible fuure work is discussed. 18

34 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers 2.1. Inroducion The major concern relaed o he efficiency of he high sepup converers for he HID lamp ballas is he exreme duy raio ha exiss if basic opologies, such as buckboos and boos, are used. Because of he high inpu curren ha resuls from he low inpu volage, lowvolageraed devices wih low R Don are necessary o reduce he conducion loss. The buckboos converer is he simples nonisolaion opology ha can achieve he sepup funcion and negaive oupu for he HID ballas. Unforunaely, 500Vraed devices are necessary o generae he 400V DC volage for he igniion. Furhermore, he high oupu volage and he shor pulse curren wih high ampliude hrough he oupu recifier (due o he exreme duy raio) induce high swiching loss. Insead of nonisolaion converers, he Flyback converer could be used [B4][B5][B16]. The coupled inducors are used no for isolaion bu for high sepup volage gain in his applicaion, because here is no isolaion requiremen. Converers wih coupled inducors can easily achieve high sepup volage gain, uilizing low R Don swiches for his lowpower applicaion. Unforunaely, he leakage energy induces high volage sress, large swiching losses, and severe EMI problems. Eiher a snubber circui or an aciveclamp scheme is necessary o handle he leakage energy. The use of a RCD snubber is a simple soluion, bu all of he leakage energy is dissipaed in he snubber circui. An aciveclamp circui can recycle he leakage energy, bu a he cos of opology complexiy and some losses relaed o he clamp 19

35 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers circui [B16]. Alhough he aciveclamp Flyback converer can achieve he highes efficiency, as compared o oher soluions, his approach does no ake advanage of he fac ha isolaion is no required. This chaper presens a family of highefficiency, high sepup clampmode converers derived from he nonisolaion coupledinducor converers by aking advanage of he lack of an isolaion requiremen. The nonisolaion coupledinducor buckboos converer is firs derived from he Flyback converer. Then, a novel and simple clamp scheme for recycling he leakage energy is proposed. The operaion of he proposed converer is similar o ha of is aciveclamp counerpars, bu wih beer performance. In he proposed converer, only one diode and a small clamp capacior are added o he coupled inducor converer. The addiional diode serves as he body diode of he aciveclamp swich, and he coupled winding and oupu recifier serve he same funcion as he aciveclamp swich [B16]. Topologies wih one acive swich have significanly reduced cos and circui complexiy, as compared o hose using he aciveclamp scheme. Therefore, he reliabiliy of he converer could be dramaically increased. The proposed clampmode coupledinducor converer can use a lowvolageraed acive swich for he conducion loss reducion. The clamp circui recovers he leakage energy wih a lower circulaing curren. High efficiency is achieved because of he low R Don of he swich and he efficien leakage energy recycling. The concep is he hen generalized and exended o oher opologies. 20

36 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers 2.2. Topology Evaluaions for HID Ballas Converer The operaion of a power converer under exreme duy raios can resul in high volage and curren sresses incurred o achieve high sepup volage gain. For he 36W HID fronend converer, he buckboos converer (negaive oupu is required), as shown in Fig. 2.1, is he simples nonisolaion opology. Unforunaely, 500Vraed devices are necessary o generae he 400V opencircui volage. Furhermore, he high oupu volage and shor pulse curren, wih high ampliude hrough he oupu recifier due o he exreme duy raio, induce high levels of swiching loss. The high conducion loss ha occurs due o he high R Don of he swich, and he high swiching loss resuling from he shor pulse curren wih high ampliude, dramaically degrade he efficiency. The resuls of loss analysis are shown in Fig. 2.1(b)(c). D o V in L C o V o Efficiency (a) Loss(W) Buckboos :IRFP460A, Do:MUR860 V in =12 V, Vo=80V P o =36 W, F s =100 KHz 0.75 Buckboos 0.7 V in =12 V, P o =36 W, F s =100 KHz 0.65 V o (V) _cond _sw Do P_inducor Toal (b) (c) Fig Buckboos converer under exreme duy raio: (a) buckboos converer; (b) calculaed efficiency; and (c) loss breakdown. 21

37 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers The nonisolaion converers [B13] can provide high sepup volage gain wihou incurring an exreme duy raio. For example, he volage gain of he hirdorder woswich converer is given by Eq. 21: Vo Vi d = 1 2 d, Eq. 21 where he d is he duy raio. The volage gain is infinie when he duy is equal o 0.5 for he hirdorder woswich converer. The basic idea for his group of circuis is o sore sufficien energy in he inducors by puing he inpu and oupu volage sources in series during he swichon ime. However, having he oupu volage source charge he inducor inroduces high circulaing curren. Anoher problem wih his idea is he high volage sress of he acive swich, which is 2VoVi. For HID applicaions, he volage raing of he swich needs o be higher han 900V. The high R Don of he swich and he huge circulaing curren make i exremely difficul o achieve decen efficiency wih hese converers. Alhough his converer can avoid exreme duy raio for high sepup volage gain, i is neiher a coseffecive nor an efficien soluion. The conducion loss will degrade he efficiency o lower han 70%. The cascade sepup DCDC converer is anoher soluion for avoiding exreme duy raios. Fig. 2.2(a) shows a cascade srucure wih boos converer and a buckboos converer (for negaive oupu). By uilizing he cascade srucure, boh sages can realize sepup funcions wihou exreme duy raio, and hus he conducion loss and swiching loss are significanly reduced. The inermediae DC bus is designed as 36 o 40V so ha he exreme duy raio can be avoided for boh sages. Fig. 2.2(b)(c) shows he calculaed efficiency and he loss breakdown 22

38 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers ( 1 :IRF24N, 2 :IRFP460A, D o2 :MUR860). For he firs sage, he volage raing of he device could be 55V in order for he conducion loss of he swich o be very low even hough he inpu curren is high. For he second sage, he inpu curren is low due o he boosed volage, and hus he conducion loss is reduced. As can be seen, boh conducion loss and swiching loss are dramaically reduced. However, form an efficiency sandpoin, i is preferable ha he energy no have o be processed wice. L 1 D o1 2 D o2 V in 1 C o1 L 2 C o2 V o (a) Efficiency 0.95 Loss(W) Buckboos Cascade Cascade rucure 12 9 V in =12 V, Vo=80V P o =36 W, F s =100 KHz BuckBoos 0.65 V in =12 V, P o =36 W, F s =100 KHz V o (V) _cond _sw Do P_inducor Toal (b) Fig Analysis he feasibiliy of he cascade srucure: (a) opology srucure; (b) calculaed efficiency; and (c) loss comparision. (c) Insead of nonisolaion converers or cascade nonisolaion DCDC converers, converers wih coupled inducors, such as he Flyback or isolaion EPIC converers, could be used 23

39 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers [B4][B5][B14][B15]. aeofhear soluions are he Flyback or coupled inducor EPIC converers for his lowpower high sepup applicaions [B4][B5]. Converers wih coupled inducors can easily achieve high sepup volage gain uilizing he ransformer funcion of a coupled inducor. Therefore, exreme duy is avoided and low R Don swiches could be used. The conducion and swiching losses are dramaically reduced. However, he leakage energy of a coupled inducor induces no only high volage sress bu also high losses. A leakage recycling circui mus be used. Due o is simpliciy, he RCD snubber is adoped in he HID ballas. Unforunaely, he RCD snubber dissipaes all he leakage energy in he resisor afer he swich urns off. The equivalen circui of he Flyback converer afer he swich urns off is shown in Fig. 2.3(a). Fig. 2.3(b) shows he curren waveforms of he leakage and he refleced oupu curren. Afer he swich urns off, he leakage inducor curren decreases and he magneizing curren increases. There is an overlapping ime period c for he curren ransfer. Lm V ref Ilk_pk ILk Io V c nubber Io ILk Lk on c T s (a) Fig Equivalen circui of he Flyback converer and he curren waveforms: (a) he equivalen circui and (b) waveforms for loss calculaion. (b) 24

40 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers The overlap ime of c can be calculaed as follows: c I = V Lk _ pk k. Eq. 22 c V L ref The loss relaed o he leakage inducance is hen given by: P Loss _ Lk = ( I Lk _ pk Lk I Lk _ pk Vref c ) Fs. Eq. 23 Fig. 2.4 shows only he loss ha occurs due o he leakage inducance in a Flyback converer wih an RCD snubber. No only all of he leakage energy bu also some of he oupu energy is dissipaed in he snubber circui. There are wo ways o reduce he loss relaed o he leakage inducance. Firs, a good coupling inducor wih less leakage inducance is an effecive soluion. econd, increasing he clamp capacior volage can shoren he overlapping ime period c. Therefore, he loss can be reduced. However, he leakage inducance is always presen, and increasing he clamp capacior volage increases he swich volage sress. Loss (%) (Due only o L k ) K = K=0.95 K=0.96 K= V C /V ref L m L m L k (Fs=100KHz) Fig Loss relaed o he leakage inducance. 25

41 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers The calculaed efficiency of a Flyback converer wih an RCD snubber for he HID ballas applicaion is abou 86%, which is far away from he argeed 90%, as shown in Fig. 2.5(b)(c). D o L m N 1 N 2 C o V o V in L k (a) Efficiency 0.95 Loss(W) Flyback w/rcd Cascade rucure BuckBboos 3 V in =12 V, P o =36 W, F s =100 KHz 0.65 V o (V) (b) (c) Fig Flyback converer wih RCD snubber: (a) circui diagram; (b) prediced efficiency; and (c) loss breakdown Buckboos Cascade Flyback w /RCD _cond _sw Do P_inducor P_Lk Toal An aciveclamp circui is an effecive and efficien mehod for recycling leakage energy. The aciveclamp Flyback converer, as shown in Fig. 2.6(a), can recycle leakage energy wih minimal volage sress o he main swich because he aciveclamp scheme reses he leakage inducor during he whole urnoff ime period. The drawbacks are ha adding he aciveclamp swich and is associaed floaing gae drivers increase he cos, complexiy and he losses relaed o he clamp circui [B16]. Any accidenal overlap of he main and aciveclamp swich gaedrive signals could lead o a faal failure of he circui. The efficiency improvemen is limied 26

42 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers because he high curren hrough he aciveclamp swich can induce a high level of conducion loss. Fig. 2.6(b)(c) shows ha a 3% efficiency improvemen can be achieved by using he aciveclamp scheme insead of he dissipaive RCD snubber. D o C c L m N 1 C o N 2 D s2 V in L k Vo 2 1 Efficiency Flyback w/ AciveClamp V in =12 V, P o =36 W, F s =100 KHz Flyback w/rcd Cascade rucure BuckBoos (a) V o (V) Loss(W) 18 (b) (c) Fig Aciveclamp flyback converer: (a) circui diagram; (b) prediced efficiency; and (c) loss breakdown V in =12 V, Vo=80V P o =36 W, F s =100 KHz Buckboos Cascade Flyback w /RCD Aciveclamp Flyback _cond _sw Do P_inducor P_Lk Toal However, he Flyback converer wih eiher an RCD snubber or an acive clamp is adoped in many applicaions ha require isolaion. However, he HID sepup converers do no have his requiremen. A simple way needs o be found o effecively recycle he leakage energy in a coupled inducor wihou he isolaion requiremen. 27

43 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers As discussed above, leakage inducance can cause severe volage sress and exra loss. An effecive leakagerecycling scheme should be used o make coupledinducor converers simple and efficien. In applicaions wih large sepup conversion raios, he imporan quesion peraining o he use of coupledinducor converers is how o effecively recycle he leakage energy wih a novel leakagerecycling scheme Advanages and Disadvanages of he AciveClamp Flyback Converer The buckboos converer is no a good candidae for he HID ballas applicaion because of is exreme duy raios as well as is high conducion and swiching losses. Coupledinducor converers, such as he Flyback and he isolaion EPIC converer, have been used for his applicaion. The volage gain of a Flyback converer is given by Eq. 24: V V d d N N o s =, Eq. 24 in 1 p where V o is he average value of he oupu volage; V in is he average value of he inpu volage; d is he duy raio of he converer; N p is he number of primary urns for he coupled inducor; and N s is he number of secondary urns for he coupled inducor. Uilizing a coupled inducor can easily achieve high sepup volage gain wihou incurring exreme duy raios. However, he leakage inducor of he coupled inducor no only induces high volage spikes bu also dramaically degrades efficiency. In pracical applicaions, an auxiliary circui mus be used o handle he leakage energy. This auxiliary circui can eiher dissipae he leakage energy or recover he leakage energy o he inpu or oupu. 28

44 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers An RCD snubber circui dissipaes he leakage energy o reduce he volage sress of he swich, bu he loss relaed o he leakage inducance is dramaic, as analyzed in Chaper 1. The coupledinducor converer wih RCD clamp canno achieve decen efficiency as compared o he scheme involving lossless leakage energy recovery. The aciveclamp flyback converer, as shown in Fig. 2.7, recovers he leakage energy and minimizes he volage sress of he swich. The aciveclamp scheme uilizes he enire urnoff ime o rese he leakage inducance. Therefore, he volage sress is minimized. Compared o he Flyback wih RCD snubber, he aciveclamp Flyback converer can achieve a 3% higher efficiency. The drawbacks of he aciveclamp soluion are he opology complexiy and he high loss relaed o he clamp circui. The aciveclamp approach requires wo swiches and wo isolaed gae drivers. The curren hrough he aciveclamp swich is he primary curren wih large magniude, which can induce high conducion losses in he aciveclamp circui. The average curren of he posiive par of curren I Cc induces conducion loss in diode D s2. The roomeansquire (RM) value of he negaive porion of curren I Cc induces conducion loss in swich 2. Alhough he leakage energy is recycled, he loss in he aciveclamp circui is no small. From his sandpoin, a clamp scheme wih simple opology and less circulaing energy is a beer choice han he aciveclamp Flyback converer. 29

45 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers I 1 I 2 V in I Cc C c N 1 N 2 C o D s2 L k V o D o 2 1 (a) a 1 1 I 1 I Cc I (b) Fig Aciveclamp Flyback converer: (a) opology and (b) key waveforms ClampMode CoupledInducor High epup DCDC Converers In his secion, he ransformaion of he isolaion coupledinducor converers o he nonisolaion version was inroduced. The ransformaion allows he wo windings o share he common par of he ransformer, while he elecrical characerisics of he erminals remain 30

46 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers unchanged. Then, he aciveclamp scheme is applied o he nonisolaion coupledinducor converers. The analysis shows ha he secondary winding and he oupu recifier provide an exising discharge pah for he clamp capacior. Therefore, he swich providing he discharge pah of he clamp capacior could be eliminaed. A simple and efficien high sepup DCDC converer is hen proposed for he HID ballas applicaion NonIsolaion CoupledInducor DCDC Converers The coupled inducor is used no o provide isolaion bu o achieve high volage gain. The saeofhear soluion is he Flyback converer, bu i does no ake advanage of he lack of an isolaion requiremen. By uilizing nonisolaion coupledinducor converers, he magneic srucure is simplified by using less copper. I will be furher demonsraed ha he leakage energy recovery can be more efficien wih a simple auxiliary circui. Fig. 2.8(a) shows a ransformer wih urns raio N 1 :N 2 (N 1 <N 2 ). I is equivalen o he auoransformer version, as shown in Fig. 2.8(b). I 1 I 2 I 1 I 2 1 V 1 N 1 V 1 N 1 N 2 V 2 2 V 2 N 2 N 1 (a) (b) Fig Equivalence of isolaion ransformer and auo ransformer: (a) isolaion ransformer and (b) auo ransformer. 3 31

47 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers For a Flyback converer wih N 1 <N 2 for he sepup funcion, he coupled inducor can be modeled as he combinaion of a magneizing inducor and an ideal ransformer, as shown in Fig. 2.9(a). Changing he isolaion ransformer ino he nonisolaion version by using he rule shown in Fig. 2.8, we can obain he nonisolaion coupledinducor buckboos converer, as shown in Fig. 2.9(b). V in L m N 1 N 2 D o C o V o (a) V in L m N 1 C o V o N 2 N 1 D o (b) Fig Transformaion of Flyback converer o he coupledinducor converer wihou isolaion : (a) Flyback converer and (b) nonisolaion coupledinducor converer. The nonisolaion version has a urns number ha is lower han ha of he isolaion version. The advanage of his nonisolaion version over he Flyback converer could no be jusified if boh converers had an ideal coupled inducor. However, for pracical applicaions, here mus be an auxiliary snubber or clamp circui. The advanage of he nonisolaion version is ha he 32

48 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers simple and effecive leakage energy recovery scheme can be adoped. Therefore, a coseffecive and highperformance DCDC converer for he HID ballas can be found Proposed CoupledInducor High epup DCDC Converers imilar o he Flyback converer, he leakage energy mus be handled properly o reduce he volage sress of he swich and o improve he efficiency. The aciveclamp scheme is used from he performance sandpoin. The resuling coupledinducor buckboos converer wih aciveclamp scheme is shown in Fig Compared o he aciveclamp Flyback converer, his converer works in a similar way, excep ha he primary winding curren is differen when he urns raio of he wo converer has he following relaionship: N p = N 1, and Eq. 25 N s = N 2 N 1. Eq. 26 The difference is ha he secondary winding curren now goes hrough he primary winding in he reverse direcion, which helps o reduce he RM curren of he primary winding. In he nonisolaion coupledinducor buckboos converer wih aciveclamp scheme, diode D a is used o provide a charge pah o absorb he leakage energy, while swich a is used o provide a discharge pah for he clamp capacior C a. As can be seen from Fig. 2.10, he swich a can be urned on any ime afer he main swich urns off and before he curren hrough he clamp capacior reverses is direcion. 33

49 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers When N s is equal o N p, hen he curren decrease slope of he primary curren is equal o he curren increase slope of he secondary curren when a ripplefree magneizing curren is assumed. As can be seen from Fig. 2.11(a), he primary curren is always posiive if he aciveclamp swich urns on immediaely afer he main swich urns off. This means ha he discharge curren of he clamp capacior comes from he secondary curren, which hins ha here is an exising discharge pah for he clamp capacior. The aciveclamp diode and swich can be shifed o he charge and discharge branch, as shown in Fig. 2.11(b). This swich shif does no change he circui operaion. ince diode D o and swich a have he same urnon and urnoff imes, swich a is redundan and is herefore removed. The resuling proposed circui is shown in Fig Fig shows he proposed soluion o achieve he funcion of he acive clamp by adding only diode D c and clamp capacior C c, when N s is equal o N p. The primary leakage energy charges he clamp capacior, while he secondary coupled inducor serves as he swich o provide a discharge pah for he clamp capacior. I works he same as he aciveclamp version when N s is equal o N p. The new soluion proposed here is based on a simple principle: The curren of winding N s is used o discharge he clamp capacior. In oher words, he discharge pah already exiss in he nonisolaion coupledinducor buckboos converer. In high sepup applicaions, N s is much larger han N p. Furher analysis shows ha he proposed soluion can achieve a funcion similar o ha of he aciveclamp scheme, bu wih much less curren circulaing in he clamp circui. 34

50 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers V in I p N p C c I Cc c D c L k N s D o C o V o I Do (a) Gae a I p I Ca I Do (b) Fig Coupledinducor converer wih aciveclamp scheme: (a) opology and (b) key waveforms. 35

51 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers Gae a I p I Cc I Do V in I p I Cc L m N p L k D c C c N s D o C o V o I Do c Fig Derivaion of he clampmode coupledinducor converers wih N s =N p. The new converer requires only one addiional clamp capacior C c and one diode D c. The converer can achieve a level of operaion comparable o ha of he aciveclamp scheme. The basic idea for his opology is ha he clamp capacior C c and he added diode Dc funcion as he aciveclamp charging pah, while he induced curren in he secondary winding N s of he coupled inducor is used o discharge he clamp capacior C c. Therefore, he secondary winding of he coupled inducor serves he same funcion as he aciveclamp swich, and is differen from 36

52 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers he socalled magneic swich [D13]. The leakage energy is recovered and he swich volage sress is reduced. Therefore, he performance of he converer is significanly improved. L m N p C c C o V o V Vin L k D c N s D o Fig Proposed clampmode coupledinducor buckboos converer Operaion Analysis Fig illusraes he six opological sages in one swiching cycle for he proposed clampmode coupledinducor buckboos converer. The coupled inducor is modeled as a magneizing inducor Lm, an ideal ransformer wih a urns raio of N p :N s, and a leakage inducor L k. The converer is redrawn o faciliae he comparison wih he aciveclamp Flyback converer. Fig shows he key waveforms of he operaion modes corresponding o he differen opological sages shown in Fig The six operaion modes are briefly described as follows. 37

53 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers [ 0, 1 ]: wich is on, and he oupu recifier D o is reversebiased. Boh he magneizing inducor and he leakage inducor are linearly charged by he inpu volage source V in. [ 1, 2 ]: wich urns off a 1. The parasiic capacior of he swich is charged by he magneizing curren in an approximaely linear way. [ 2, 3 ]: A 2, he parasiic capacior of swich is charged o he volage of V in V Cc. Clamp diode D c conducs. Almos all of he magneizing curren begins o charge clamp capacior C c. [ 3, 4 ]: A 3, V Cc is charged o he poin ha oupu diode D o is forwardbiased. The refleced volage from he secondary winding N s of he coupled inducor clamps he primary winding N p. Leakage inducor L k and clamp capacior C c begin o resonae. [ 4, 5 ]: A 4, he resonan curren reaches zero. All of he magneizing curren is refleced o he secondary winding N s from he primary winding N p. The clamp capacior is hen discharged by he oupu recifier curren I Do (I Lm /N). [ 5, 0 ]: wich urns on a 5. The leakage inducor L k is quickly charged by he sum of inpu volage V in and he refleced volage (V o V c )/N unil he leakage inducor curren I Lk is equal o he magneizing curren I Lm. The oupu recifier D o is reversebiased. Then, he nex swiching cycle begins. I is should be poined ou ha he ime periods [ 1, 2 ] and [ 2, 3 ] are much shorer han hose shown in Fig. 2.14, which were enlarged in order o clearly show he waveform variaions. 38

54 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers I Lm N p C c V Cc C o V o I Lm N p C c V Cc C o V o V in I Lk D o V in I Lk D o D c N s I Do D c N s I Do (a) (b) I Lm N p C c V Cc C o V o I Lm N p C c V Cc C o V o V in I Lk D o V in I Lk D o D c N s I Do D c N s I Do (c) (d) V in I Lm N p I Lk C c V Cc D o C o V o V in I Lm N p I Lk C c V Cc D o C o V o D c N s I Do D c N s I Do (e) (f) Fig Operaion modes of he clampmode coupledinducor buckboos converer: (a) [ 0, 1 ], (b) [ 1, 2 ], (c) [ 2, 3 ], (d) [ 3, 4 ], (e) [ 4, 5 ] and (f) [ 5, 0 ]. 39

55 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers Gae I Lm I Lk I Cc V Cc V Dc V s I Do Fig Key waveforms of he clampmode coupledinducor buckboos converer. 40

56 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers As can be seen from he analysis, he leakage energy sored in L k is recovered by he clamp capacior such ha he volage of he swich is clamped. The clamp capacior is discharged by he oupu recifier curren I Do, which is equal o he refleced secondary curren from he primary ransformer winding. The primary ransformer curren equals he difference beween he magneizing curren and he leakage inducor curren. Fig shows he relaionship beween he clamp capacior charge and discharge currens by assuming ha he magneizing curren is ripplefree. The clamp capacior needs o mainain a balance beween charge and discharge.. ILk ILm 3 Charge Cc c1 4 IDo Discharge Cc c2 5 ILm N (1D)Ts Fig Curren relaionship. 41

57 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers By making he charge area equal o he discharge area in Fig. 2.15, he following relaionship is found: c = 2 1 (1 D Ts N 1 ), Eq. 27 where N is he urns raio N s / N p. To make he following derivaion simple, define K as: K L L L m =. Eq. 28 m k By applying he volsecond balance of he magneizing inducor and he leakage inducor, he volage gain and he clamp capacior volage of he converer are given by Eq. 29 and Eq. 210, respecively. The ime needed for he leakage inducor o rese is represened by c1. V Vi d (1 K) = ( N 1) Eq d 2 o d (1 K) (1 K) N Vc = Vi Eq d 2 The volage of he oupu recifier is given by Eq. 211: V = d ( N K N 1 1 d ). Eq. 211 Do Vi 42

58 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers The proposed converer is able o provide a wide range of sepup volage gains using various urns raios for he coupled inducor. High sepup volage gain is achieved wihou requiring an exreme duy raio Curren ress Comparison wih he AciveClamp Counerpar On one hand, he aciveclamp scheme uilizes he enire offime of he main swich o rese he leakage inducor, as shown in Fig Therefore, he average volage of he clamp capacior is minimized. The average volage V a of he clamp capacior C a is given as follows. V d =. 1 d Ca Vi Eq. 212 On he oher hand, here is a large circulaing curren going hrough eiher he aciveclamp swich body diode or he aciveclamp swich. The reason is ha N p acs as a volage source refleced from N s afer D o conducs. The leakage inducor L k resonaes wih he clamp capacior C a. The difference beween he proposed converer and is aciveclamp counerpar is ha he clamp capacior is linearly discharged by he secondary refleced magneizing curren (I Lm /N) during [ 4, 5 ], as shown in Fig The secondary refleced magneizing curren is much smaller han he primary magneizing curren because N>>1. The discharge period [ 4, 5 ] in he proposed converer is longer han half of he [ 3, 5 ] period in he aciveclamp converer. A longer [ 4, 5 ] period makes he charge period [ 3, 4 ] shorer in he proposed converer han i is in he aciveclamp scheme. Therefore, he proposed converer has less energy circulaing in he addiional clamp circui. 43

59 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers D o C a L m N 1 C o N 2 D V a in L k Vo a 1 (a) DT s T s I m I k I ca V ca V 1 I Do T0 T1 T2 T3 (b) T4 T5 T0 Fig Aciveclamp flyback converer: (a) circui diagram and (b) key waveforms. 44

60 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers Fig compares he proposed clampmode coupledinducor buckboos converer shown in Fig wih he corresponding aciveclamp scheme shown in Fig afer he main swich urns off. The leakage inducor begins o charge he clamp capacior in boh he proposed converer and in is aciveclamp counerpar, wih he iniial curren acing as he magneizing curren. For he aciveclamp converer, he charge curren, represened by he doed area A1 in Fig. 2.17(a), goes hrough he clamp swich D a. This curren is independen of he urns raio. Bu for he proposed circui, he curren going hrough he clamp diode D c, represened by he doed area B1 in Fig. 2.17(b), is much smaller han i is in he aciveclamp converer. Unlike he aciveclamp circui, he curren going hrough he clamp capacior in he proposed circui depends on he urns raio: The larger he urns raio, he smaller he curren. Anoher advanage of he proposed circui can be observed afer he charge curren decreases o zero. In he aciveclamp converer, he reversed discharge curren goes hrough he aciveclamp swich a, which is a high primaryside curren when he inpu volage is low, as shown in Fig. 2.17(a). This par of he circulaing energy resuls in a high conducion loss in he aciveclamp swich. In he proposed circui shown in Fig. 2.17(b), he curren goes direcly o he oupu filer hrough he oupu recifier D o. The loss relaed o he clamp swich is zero. Table 2.1 summarizes hese comparisons. Table 2.1. Loss comparison. Reason for Loss in D a or D c Reason for Loss in a AciveClamp Converer Average of A 1 RM of A 2 Proposed Converer Average of B 1 N/A (=0) Comparison A 1 > B 1 A 2 >> 0 45

61 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers I Ca I Lm A A 2 I Lm (a) I Cc I Lm B 1 B 2 3 I lm /N 5 4 (b) Fig Comparison of circulaing energy: (a) aciveclamp counerpar and (b) clampmode buckboos converer. The benefi of his clamp scheme is quanified. Fig illusraes he loss breakdown [A4] comparison of he proposed converer and he aciveclamp converer. The operaion condiions are: V in =12V, V o =90V, P o =36W, and F s = 100KHz. As can be seen, he aciveclamp Flyback 46

62 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers converer has less swiching loss due o is zerovolageswiching (ZV) urnon. Bu he high primary curren can resul in large conducion losses if he aciveclamp swich is used. Alhough he clampmode coupledinducor converers have higher swiching losses, he loss in he clamp swich is much less. Therefore, he efficiency is higher in he clampmode coupledinducor converers han in he aciveclamp Flyback converers. Loss (W) A_Flyback Clamp_Buckboos CBuckBoos R cond sw Do Cu a and Da Toal Fig Loss breakdown Volage ress Comparison wih he AciveClamp Counerpar The proposed converer has much less clamp circuirelaed loss. The reason is ha he clamp capacior volage is higher han i is in he aciveclamp soluion. The verical axis in Fig shows he clamp capacior volage of he proposed converer, normalized o he clamp capacior volage of is aciveclamp counerpar. The difference beween he clamp capacior volage and 47

63 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers he primary winding volage reses he leakage inducor. The proposed soluion uilizes par of he urnoff ime given by Eq. 27 o rese he leakage inducance. Therefore, he clamp capacior volage is higher han in he aciveclamp scheme. Forunaely, he leakage inducor is a small inducor, so he exra volage needed o rese his small inducor does no need o be high. As can be seen from Fig. 2.19, he volage sress increase is less han 5% when he coefficien of coupled effec is around In fac, he exra volage sress in he proposed converer does no have o change he device volage raing as i does in he aciveclamp converer. Vcc d 1 d ( V i ) Clamp Capacior Volage Primary Winding Volage N=5 N=3 N= K Fig Normalized clamp capacior volage. 48

64 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers 2.5. Design Guidelines The key design sep is o deermine he urns raio ha allows a lowvolageraed device o have a sufficien safey margin, and ha enables he converer o operae wihou an exreme duy raio. The key design equaion o calculae he urns raio is as follows: Vo _ max N = 1. Eq. 213 VD Vi _ min Fig is he design graph argeed a DCDC fronend converers for HID ballass wih 9 16V inpu volage and 60100V oupu volage (sarup volage 400V). The horizonal axis and he lef verical axis of Fig show he relaionship beween swich volage sress and inpu volage using differen urns raios. In he same graph, he relaionship beween duy raio and inpu volage under differen urns raios is shown on he horizonal axis and he righ verical axis. Afer he urns raio is defined, he corresponding duy raio can be found from Fig The volage raing of he oupu recifier and he clamp capacior can be easily calculaed. The design mus make a radeoff beween he swich and recifier volage sresses. The opimal design can be achieved by carrying ou he process shown in Fig The loss analysis mechanism is based on he swichingcycle performance [A4]. 49

65 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers V D (V) 90 K=0.98 Duy Raio N= N= N= V in (V) Fig Design curves. elec a swich wih specific volage raing Deermine he urns raio wih Equaion 213 elec L m ha keeps CCM operaion for he enire inpu volage range Calculae he inpu and oupu curren, and perform he loss analysis Repea above process, and opimize he efficiency Find opimal design Fig Opimizaion procedure. 50

66 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers 2.6. Experimenal Verificaion As shown in Fig. 2.22, a prooype converer argeed a he HID lamp ballas applicaion is buil. Because he converer needs o generae 400V opencircui volage o ignie he HID lamp, he volage raing of he seleced acive swich is 100V. The maximum volage sress is designed o be 70V when he 400V opencircui volage is generaed. The prooype has he following parameers: N 1 =14 T, N 2 =82 T, and C c =1.36 µf; is IRF1310, D o is MUR860, and D c is 3A, 100V choky diode. Fig Prooype of he HID ballas. 51

67 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers Fig shows he experimenal waveforms of he proposed circui. The waveforms agree wih he analysis, and he volage of he swich is effecively clamped. V c 10 V/div 0 V D 20 V/div I Do 1 A/div Vin =12 V Vo =100 V Po =36 W = 2 µs/div Fig Experimenal waveforms of he DCDC converer for use in HID ballass. Fig shows ha he volage sress is abou 70 V when he 400V sarup oupu volage is generaed. The 100V MOFET has sufficien safey margins. A 100Vvolageraing MOFET has much lower R Don han does a 500V MOFET. V o 200 V/div 0 V D 50 V/div = 5 µs/div Fig Experimenal waveforms when generaing 400V opencircui oupu volage. 52

68 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers By adoping he analysis approach based on he deailed dynamic swiching performance [A4], he loss analysis is conduced for hree differen sepup converers; hese resuls are given in Fig For he 36W DCDC fronend converer of he HID ballas, he proposed clampmode coupledinducor buckboos converer has higher efficiency han does he aciveclamp Flyback converer. The primary magneizing curren of he DCDC converers for HID lamp ballass in auomobiles is fairly high because of he lowinpu baery volage. When he clamp capacior and he leakage inducor resonae, he high primary curren hrough he aciveclamp swich will resul in high losses in ha swich. Therefore, he proposed converer has an even greaer efficiency improvemen han is achieved by he aciveclamp Flyback. Furhermore, he proposed converer is more coseffecive and more reliable han he aciveclamp Flyback converer for hose applicaions no requiring isolaion. The experimenal resuls are also shown by he solid line in Fig The heoreical and measured efficiency are closely mached. Efficiency Experimenal Efficiency 0.92 Proposed converer Converer Vin=12V Po=36W Fs=100KHz Aciveclamp Clamp Flyback Flyback Prediced Efficiency Fig Prediced and measured efficiency. V o (V) 53

69 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers 2.7. Generalized Concep The buckboos is no a good candidae for high sepup applicaions. The nonisolaion coupledinducor buckboos converer can achieve high sepup volage gain wihou exreme duy raio. A hreeerminal coupledinducor swiching cell can be idenified. ince he buck and boos converers have he same hreeerminal cell as is used in he buckboos converer, he swiching cell in he coupledinducor buckboos converer can also exend o buck and boos converers. epup and sepdown converers wihou exreme duy raios are unified. In pracical applicaions, he leakage energy mus be properly handled. The proposed clamp scheme can be exended o oher high sepup applicaions ThreeTerminal Cell of CoupledInducor Converers Buck, boos and buckboos converers have a hreeerminal swiching cell [B13] as shown in Fig. 2.26(a). These opologies encouner an exreme duy raio for high sepup or down power conversions. The nonisolaion coupledinducor converer can achieve a high conversion raio wihou incurring an exreme duy cycle. In ha converer, a hreeerminal cell in he sepup coupledinducor sepup power conversion can be idenified, as shown in Fig. 2.26(b). 54

70 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers 1 1 L m C o V o L m N p C o V o V in D o V in N s D o (a) (b) L L p L s N p N s 2 2 (c) (d) Fig Threeerminal cell and coupledinducor swiching cell. As discussed in oher work [B13], he cell has differen connecions o he inpu volage source and oupu sink, as shown in Fig There are six possible connecions, as lised in Table 2.2. Inpu Converer Cell Oupu V in V o Common Fig Converer cell. 55

71 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers Table 2.2. Connecion of he converer cell o he inpu and oupu. A B C D E F Common Inpu Oupu The six converers are shown in Fig The volage gains of he converers are also given. The converers shown in Fig. 2.28(a) and (b) are he buckboos and he boos versions of coupledinducor converers, which can achieve high sepup volage gain wih a proper urns raio. 56

72 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers N s D o N s D o V in Np V o V in N p V o 1 1 M = N s N N p p d 1 d (a) High sepup buckboos converer M = N p N p N s d 1 d (b) High sepdown buckboos converer N p N s D o N s N p V in V o V in D o V o M = 2 N N s p d 1 1 d 1 d (c) High sepup boos converer M = N p 2 N N s p d N s d (d) High sepdown buck converer N p Do N p V in N s V o V in N s V o 3 D o 3 M 1 = 1 d N s N N s p d 1 d M = N s d N N s p N p d (e) Coupledinducor boosype converer (f) Coupledinducor buckype converer Fig ix opologies wih coupledinducor swiching cell. 57

73 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers chemes for Leakage Energy Recovery The leakage inducance no only degrades he efficiency bu also increases he swich volage sress. To handle he leakage energy, eiher a dissipaive snubber or a lossless clamp circui could be used. The dissipaive snubber resuls in loower efficiency. The aciveclamp scheme can minimize he swich volage sress and improve he efficiency, bu wih he drawbacks of increased circui complexiy and he loss in he clamp circui. A novel clamp scheme is proposed for he high sepup DCDC converers, as shown in Fig This clamp scheme operaes much he same way as he aciveclamp scheme, bu he opology is far simpler and he loss in he clamp circuis is significanly reduced. 1 1 L m N p C o V o L m N p C c C o V o V in L k N s D o V in L k D c N s D o (a) (b) I Cc 1 N p 3 L k N s Do Cc 1 3 N p L k D c N s D o 2 (c) Fig Leakage handling for he sepup converers: (a) nonideal coupledinducor converer; (b) clampmode coupledinducor converer; (c) swiching cell of he converer in (a) and (d) swiching cell of he converer in (b). (d) 2 Boh he coupledinducor buckboos and he boos versions shown in Fig (a) and (c) can achieve high sepup volage gains. Applying he proposed leakage energyrecycling scheme 58

74 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers and he capaciorshifing rule leads o oher new opologies, as shown in Fig The leakage energy is efficienly recycled by he added diode and capacior, and is hen discharged direcly o he oupu by he secondary coupled winding. Compared o he aciveclamp schemes, he proposed soluions use only one acive swich o achieve he same clamp funcion, while dramaically reducing losses relaed o he clamp circui. C c D c D o 3 1 N p N s V in Vo 2 Fig The cell can be applied o he high sepup boos converer. 59

75 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers V B N 1 C c C c N D V V 1 c N L 2 D o B k o N2 L k D c Do V in V in V o V in N 1 L k D c N2 V B C c Do V o V in N 1 L k D c N 2 C c V B D o V o V in N 1 V N o 2 L k D c Cc D2 VB V B N 1 L k D c C c N 2 D o V in V o (a) (b) Fig Topology variaions by applying capacior shif rule: (a) buckboos version and (b) boos version. Fig and Fig.2.33 compare he buckboos and boos versions of high sepup DCDC converers. As saed in he capaciorshifing rule, only he clamp capacior volage and he curren hrough he inpu or he oupu volage source will change. Thus, a opology wih he minimum sress can be idenified. In he nex secion, he applicaion of he derived converers will be demonsraed 60

76 Chaper 2. HighEfficiency High sepup CoupledInducor DCDC Converers Buckboos version I in I N N 1 Co I in I 1 Co N 1 V C B c V o V N o 2 V in L k N2 I Co D c Do V o V in I in L k D c C c N2 V B Do V in L k D c Cc V B D2 I Cc I Cc I Cc I in I in I in I do I do I Do V Cc T 0 T 1 T 3 T 4 T 5 T 0 V Cc V in T 0 T 1 T 5 T 3 T 4 T 0 V Cc T 0 T 1 T 3 T 4 T 5 T 0 V o Gain T 2 V V i T 2 d (1 K ) = ( N 1) 1 d 2 o T 2 V cc d (1 K ) (1 K ) N V c = V i 1 d 2 d (1 K ) (1 K ) N Vc = V i 1 d 2 d (1 N ) (1 K ) N V c = V i 1 d 2 Remark Minimum sress Fig Topology variaions and comparison of he buckboos version. 61

77 N N Qun Zhao Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers V in I in V B C c 1 L k D c N 1 N 2 D o I Co V o V in Boos version C C c V Cc C V o V in o o C o I in N 1 L D c D k N 2 o I Co I in L k D c V Cc C c D o N 1 N 2 I Co V o I Cc I Cc I Cc I in I in I in I do I do I Do V Cc T 0 T 1 T 3 T 4 T 0 T 5 V Cc V in T 0 T 1 T 3 T 4 T 5 T 0 V Cc T 0 T 1 T 5 T 3 T 4 T 0 V o Gain V Cc T 2 1 (1 K ) (1 K ) N V c = V i 1 d 2 T 2 V o 1 (1 K ) = ( N 1) V i 1 d d (1 K ) (1 K ) N 2 V c = V i V i T 2 1 (1 N ) (1 K ) N V c = V i 1 d 2 Remark Minimum sress Fig Topology variaions and comparison of he boos version. 62

78 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers For he sepdown converer, he leakage energyrecycling scheme is proposed by auhors a CPE [B18]. Fig shows he deails for applying he concep o he sepdown applicaions. N p 1 N p 3 L k N s Do 1 L k N s 3 D o 2 2 (a) (b) N s Do V in V o N s D o N p V in N p V o Fig Leakage handling for he sepup converers. I has been demonsraed ha his clamp circui can achieve he swich volage sress clamping and leakage energy recovery. The deailed analysis and verificaion are carried ou for a 12V VRM wih 1.5V and 50A oupu [B18] Applicaion Example of he New Family of TopologiesDCBackup Power upply New opologies wih he proposed clamp concep can be derived. These circuis have a wide applicaion range for achieving high sepup volage gain. The new boos coupledinducor converer is a good candidae for replacing he cascade DCDC converers in DCbackup for 63

79 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers neworking applicaions. The emerging DCbackup converer is a simple and efficien soluion compared wih he UP soluion, since i uses only a DCDC converer insead of an inverer. Fig. 2.35(b) shows a dualinpu fronend boos converer. The convergence of he compuer and elecommunicaions indusries makes he welldefined 48V DC baery plan a naural choice o offer hours of reserve ime during ouages of he AC mains [B7][B9]. A nonisolaion DCDC converer insead of a UP inverer (AC UP), boh of which would be powered by he 48V DC power plan, is more efficien and much less complex [B8][B11]. A highefficiency DCDC converer ha can generae 380V of DC bus from a 48V DC plan is he mos imporan par of he DCbackup sysem. I should be poined ou ha isolaion is no necessary for he ACDC or DCDC fronends, since he isolaion is provided by he subsequen DCDC sages. The DCinpu fronend converer mus provide approximaely en imes he volage gain. The saeofhear soluion is he cascade boos converer. CCM operaion of a power converer is preferred because of he low curren sress o he swich. However, he oupu recifier reverse recovery can resul in high urnon loss for he swich. Anoher major concern of high sepup DCDC converers for his applicaion is he exreme duy raio and he recifier reverserecovery problem. Eliminaion of exreme duy raios and alleviaion recifier reverserecovery problem can significanly improve he performance of he power conversion. 64

80 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers 90 o 264V AC ACDC Fronend 380 o 400 V DC Load # 1 DCDC Converer Load # 2 Load # 3 DCAC Fronend V DC_baery 90 o 264V AC ACDC Fronend (a) 380 o 400 V DC Load # 1 DCDC Converer Load # 2 Load # 3 48 o 72 V DC DCDC Fronend (b) Fig Differen backup schemes: (a) ACUP and (b) dualinpu fronend converers [B9]. Boh acive and passive approaches adop an auxiliary circui o diver he recifier curren o a desired pah. Uilizing coupled inducor o realize ripple curren seering [A7][A12] and swiching loss reducion [C20][C23]. Curren seering using coupled inducors is a passive approach, which needs only a coupled winding of he exising magneic componen and few 65

81 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers passive componens. Uilizing he curren seering characerisic of coupled inducors o realize sof swiching can simplify he opology. Each family of circuis has a circui wih minimal volage and curren sress. In his secion, he uilizaion of he converers is demonsraed. For he DCinpu fronend converer, he boosype coupledinducor converer can be used because he oupu and he inpu have he same ground. The proposed soluion, shown in Fig. 2.36, has minimum clamp capacior volage sress. I can be used for he sepup backup power supply. This converer has an inheren curren seering characerisic. During he swich urnoff ime period, he curren is seered from he loop wih he solid line o he loop wih he dashed line. Before swich urns on again, he curren hrough D c is already zero and D c is naurally recovered. All oupu curren goes hrough D o and when swich urns on, he leakage inducance of he coupled inducor conrols he curren decrease rae of he recifier. The recifier reverserecovery problem is hus alleviaed. 66

82 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers V Cc N p C c D c N s D o I Dc I Do V in Vo (a) on off I Dc I C I Do Conrolled di Do /d (b) Fig Curren seering of he proposed soluion: (a) circui diagram and (b) key waveforms. 67

83 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers Fig shows he design graph for he DCbackup power supply. The oher design seps are similar o hose for he high sepup converer for use in he HID ballas. V D (V) Duy Raio d 160 K= N=3 0.6 N=4 120 * A N= V in (V) Fig Design graph for DCbackup power supply. A 1kW DCinpu fronend converer argeing 750W power supplies for highend server sysems [C1] was also buil. The circui opology is a clampmode coupledinducor boos converer. Fig shows he opology and circui parameers. Because he leakage inducor of he coupled inducor and he parasiic capacior of he oupu diode resonae afer boos swich urns on, a proper snubber circui is necessary in order o reduce he oupu recifier peak volage. The inpu volage is 4875V and he oupu volage is 380V. wich is implemened wih hree paralleled IRFP254s (250V, 23A, 0.14Ω, To247) from IR (Inernaional Recifier). The coupled inducor is implemened wih one Kool Mµ oroidal core 77110A7 from Magneics. 68

84 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers The primary winding is 40 urns wih four srands of 175/40 liz wire in parallel. The secondary winding is 165 urns wih 100/40 liz wire. C c : 2.2uF L P L V in 48V dc o 75V dc N p : 40T D c : N s :165T RURG1540 IRF254*3 D o : RHRP30100 C o V o 380V Fig Experimenal es se up. Fig shows he curren waveform hrough primary inducor I Lp, he volage of clamp diode V DC, and he volage of acive swich V D. Because he oupu diode is in series wih he secondary winding of he coupled inducor, he leakage inducor limis he diode curren decrease rae di/d. The reverserecovery problem of he oupu diode is significanly lessened, alhough he converer has high oupu power and high oupu volage. As can be seen from Fig. 2.40, no only is he reverserecovery curren reduced, i is also delayed. There is no overlap beween he swich volage and he reverserecovery curren. 69

85 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers I LP 20 A/div V Dc 100 V/div V D 100 V/div V in = 60 V V o = 380 V P o = 1 kw = 2 µs/div Fig Experimenal waveforms of inpu inducor curren and swich volage. I Lp 20 A/div I Do 2 A/div 6 A/µs V D 100 V/div V in = 60 V V o = 380 V P o = 1 kw = 200 ns/div Fig Experimenal waveforms of inpu inducor curren, oupu diode curren, and swich volage. 70

86 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers Table 2.3 shows he measured efficiency of he prooype converer. As can be seen, he converer achieves more han 90% conversion efficiency under nominal operaion condiions. Table 2.3. Measured efficiency of he prooype converer. Vin 48V 60V 75V Duy Raio Efficiency 90.8% 91.9% 92.4% The loss breakdown shown in Fig compares he proposed soluion wih he cascade boos converer. The proposed converer has lower loss han ha of he cascade boos converer. The reason is ha he oupu recifier reverserecovery problem is alleviaed in he proposed converer. In realiy, he efficiency of he cascade boos converer could also be improved if he oupu recifier reverserecovery problem could be alleviaed. However, alleviaing he recifier reverse recovery in he second boos converer requires an exra circui. The acive approach will make he circui more complex. 71

87 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers Loss (W) Casecade /w H Proposed V in = 60 V V o = 380 V P o = 1 kw F s =100KHz _cond on _off Do Cu Core Oher Toal Fig Loss breakdown. No only does he proposed converer have an efficiency ha is abou 1% higher han ha of he hardswiching cascade boos converer, bu i is also much simpler han he cascade opologies. Table 2.4 compares he device componens of he wo soluions. In he proposed converer, hree IRFP254s (250V, 23A, 0.14Ω, To247) are used. In he cascade converer, hree IRFP3415 (150V, 43A, 0.042Ω, To220) and wo IRFP22N50A (500V, 22A, 0.23Ω, To247) are used. In he proposed converer, only one se of conroller and magneic componens are necessary, while he cascade soluion needs wo ses. Table 2.4. Componen lis. Mosfe Diode Magneics Conroller Proposed Cascade

88 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers The proposed soluion can seer he curren by uilizing he coupled inducor, and has a simple srucure. The leakage inducance of he coupled inducor can be used o conrol he di/d of a recifier. Compared o eiher he acive or oher passive approaches, his soluion is easy o realize wihou inroducing volage and curren sresses. One observaion is ha he cascade boos converer can also achieve significanly performance improvemen if he recifier reverserecovery loss could be eliminaed, as shown in Fig Alhough he exreme duy raio is no a concern for he cascade boos converer, he recifier reverserecovery problem sill can cause exreme losses. There are also many similar applicaions, such as he fronend PFC converers. Adding an exra circui o alleviae he recifier reverse recovery will make he circui more complex. Uilizing he coupled inducor o alleviae he recifier reverserecovery problem is a simple and effecive soluion, as demonsraed in his chaper. How o alleviae he recifier reverserecovery problem by using he coupled inducor for CCM boos converers moivaes he sudy of he nex chaper. Loss (W) Casecade /w H Proposed Cascade w/o Qrr V in = 60 V V o = 380 V P o = 1 kw F s =100KHz _cond on _off Do Cu Core Oher Toal Fig ignifican reverserecovery loss in he cascade converer. 73

89 Chaper 2. HighEfficiency, High epup CoupledInducor DCDC Converers 2.9. ummary Emerging applicaions call for highefficiency, high sepup DCDC converers. Basic opologies suffer from exreme duy raios and severe recifier reverserecovery problems. This chaper presens he derivaion, heoreical analysis, pracical design and experimenal resuls for a family of highefficiency, high sepup, clampmode coupledinducor converers. The operaion of he proposed converers is similar o ha of heir aciveclamp counerpars, bu he new converers uilize one addiional diode and one coupled winding insead of an acive swich in order o realize he clamp funcion. By adding a small capacior, he leakage energy is recovered in such a way as o generae only a low level of circulaing energy, and he swich volage sress is significanly reduced. When he oupu volage and power reach a cerain level, he recifier reverserecovery problem can cause severe loss. The saeofhear soluion is o conrol he di/d of he recifier during is urnoff. The circui realizaion involves an auxiliary circui and makes he soluion complex. The proposed opology has an inheren curren seering capabiliy so ha no exra circui is necessary o alleviae he recifier reverserecovery problem. The proposed concep is generalized as a coupledinducor swiching cell and is applied o oher DCDC converers. The experimenal resuls, demonsraed in a 36W converer for HID ballas and a 1KW converer for DCbackup power supply, closely mach boh he heoreical analysis and he efficiency predicion. The recifier reverserecovery problem causes severe loss for some applicaions such as fronend CCM boos converers for elecommunicaions and server applicaion. How o alleviae he recifier reverse recovery wih coupled inducors will be addressed in he nex chaper. 74

90 3.1. Inroducion Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery The oupu recifier reverserecovery problem becomes a severe concern when he oupu volage reaches a cerain level and no choky diode is available. The reverserecovery curren of he oupu recifier has derimenal effecs on he performance of converers. High swiching loss resuling from he reverserecovery curren dramaically deerioraes he hermal condiion of he swiches. The saeofhear soluion is o conrol he curren decrease rae di/d of he recifier during is urnoff. To accomplish his funcion, eiher a simple auxiliary circui wih an acive swich or a complex nework consising only of passive componens is needed. The soluion wih he acive swich increases he circui complexiy. The problem wih he mehod ha uses only passive componens is high volage sress and/or curren sress. The high sepup converers proposed in Chaper 2 have an inheren currenseering characerisic o alleviae he recifier reverserecovery problem. Uilizing coupled inducors insead of an acive swich or a complex circui o seer he curren can resul in a simple opology. In his chaper, such use of a coupled inducor is sudied. Exension of he currenseering approach ino he CCM boos converer is demonsraed. A simple and effecive soluion for alleviaing he recifier reverserecovery problem is proposed, analyzed, and experimenally verified. Then he concep is also exended o oher basic opologies. 75

91 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery 3.2. A Brief Review of aeofthe Ar oluions When a boos converer is operaed in CCM, he reverserecovery curren of he oupu recifier D o has a derimenal effec on he performance of he converer. The adverse effecs relaed o he recifier reverserecovery problem are illusraed in Fig ) During he urnon ransiion of swich, he reverserecovery curren I rr induces exra urn on loss due o he overlapping of he high curren and volage. 2) The reverserecovery curren increases he curren sress of swich. 3) The parasiic inducance L k in loop O can increase he volage sress of he oupu recifier D o. The maximum volage sress of he oupu recifier is given by: V = Vo Lk di( rec) M d, Eq. 31 Do / where di(rec)m/d is a parameer o quanify he sofness of a recifier during is urnoff. 4) The high level of reverserecovery curren may increase he elecromagneic inerference (EMI) noise. 76

92 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery L m I in I rr D o V in I O C o V o (a) ON I I rr I in V D (b) Fig Derimenal effecs of recifier reverserecovery on CCM boos converers: (a) a CCM boos converer and (b) waveforms showing reverserecovery effec. To avoid hese adverse effecs, he use of a disconinuous curren mode (DCM) boos converer is an alernaive. By paralleling wo or more DCM boos converers wih a phaseshif conrol sraegy, he inpu curren ripple is dramaically reduced. Alhough his approach can be exended o highpower applicaions [C4], he complicaed power sage and conrol circuis are no desirable. CCM boos converers are he bes fronend converers for acive inpu curren shaping. Reducing he operaion frequency of CCM boos converers is an opion for improving 77

93 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery he efficiency. However, reducing he operaing frequency is no a good soluion in erms of power densiy and cos. The GaAs recifiers can significanly improve he efficiency and reduce boh device sress and EMI noise. Unforunaely, he GaAs recifiers are expensive, and he hermal problems sill exis alhough he GaA recifiers performance is almos independen of he juncion emperaure [C5]. The ic diode is similar [C6]. The efficiency of a CCM boos converer would be significanly improved if he oupu recifier could be sofly urned off o reduce he adverse effecs of reverserecovery curren. aeofhear echnology o alleviae he silicon recifier reverserecovery problems involves sofly urning off he recifier by conrolling he curren decrease raes di/d of he recifier during is urnoff. A snubber inducor L s can be insered ino parallel branch A of he boos swich, or ino he loop (B or C) ha passes he recifier reverserecovery curren, as shown in Fig This snubber inducor L s is used o conrol he curren decrease raes of he recifier as roughly V o /L s during is urnoff. An auxiliary circui is necessary o provide rese o he snubber inducor. The rese circui can use eiher all passive lossless componens or he combinaion of passive componens and an acive swich. When he rese circui includes an acive swich, i is called an acive approach; oherwise i is a passive approach. 78

94 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery L m D o V in C B A Auxiliary circui C o V o Fig General srucure for alleviaing recifier reverserecovery problem. The acive approach has wo advanages. No only can he recifier reverserecovery problem be alleviaed, bu also zerovolageswiching (ZV) operaion of he main swich can be achieved. The wellknown circuis in he acive approach caegory [C7][C14] are he baby boos and he aciveclamp scheme. The baby boos circui involves shifing he oupu recifier curren o a new parallel branch wih an exra acive swich. The boos swich urns on under a ZV condiion. The new branch uilizes a small inducor Ls o conrol he recifier curren decrease raes during is urnoff. Because he added small inducor is essenially in parallel wih he branch of he boos swich, he boos swich suffers no exra volage or curren sress. The conversion efficiency can be dramaically improved, because he power dissipaion in he added branch can be much smaller han he exra power loss incurred when he new auxiliary circui is no used. However, adding an auxiliary circui wih an acive swich increases he complexiy of boh he circui opology and conrol. The oupu capacior of he auxiliary swich and he small inducor can induce severe undesired resonance. Alhough adding a recifier and a saurable inducor can reduce he resonance, he opology becomes complex and he cos increases. The 79

95 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery aciveclamp approach [C10][C14] insers a snubber inducor L s ino he loop ha passes he recifier reverserecovery curren. The recifier curren decrease rae di/d during urnoff can be conrolled as roughly V o /L s. Meanwhile, an acive swich and a small capacior are also necessary in order o rese he snubber inducor. Despie he advanages of his approach, is disadvanage is ha here is a large amoun of circulaing energy in he auxiliary circui, which may induce high conducion loss. Almos all of hese circuis require an isolaion gae driver [C10][C13]. Overlapping he driver signals of he main swich and auxiliary swich will lead o a faal circui failure. The circui proposed by Jovanović, e al [C14] does no require an isolaion gae driver, and he converer provides beer performance, bu he leakage inducor is a possible concern a high power levels. The exra aciveclamp swich and is associaed conroller are no desirable from boh cos and reliabiliy sandpoins. I has been demonsraed in Chaper 2 ha he sepup DCDC converer can realize inheren curren seering. The curren in he original oupu recifier is seered o a new branch consising of an added diode and an inducive impedance so ha he recifier curren of he added diode is conrolled. The recifier reverserecovery problem can be alleviaed by using he currenseering funcion of a coupled inducor Alleviaed ReverseRecovery Problem wih Coupled Inducors Topology Derivaion The coupledinducor swiching cell in Fig. 3.3(b) is derived form he swiching cell shown in Fig. 3.3(a). These wo cells are equivalen. The new swiching cell is symmerical. Therefore, here are hree converers derived from his cell, as shown in Fig

96 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery N 1 N 1 N 2 N N (a) Fig Derivaion of equivalen swiching cell: (a) he cell discussed in he previous chaper and (b) a new equivalen cell. 2 (b) N p N s D o ame as (a) V in V o M = N N s p d 1 d (a) coupledinducor buckboos converer (b) V in N s D o N p V o V in V o N s N p D o M = N s N N p p d 1 d 1 1 d M = N s N p d ( N N ) d p s (c) coupledinducor boos ame as (c) (e) (d) coupledinducor buck converer ame as (d) (f) 81

97 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery Fig Curren seering of a coupled inducor. These converers all operae in he same way as he convenional buck, boos or buckboos converer when N s is equal o N p. In order o alleviae he recifier reverserecovery problem, i is necessary for he leakage inducance of he coupled inducor o conrol he currenseering rae. However, he leakage energy mus be handled properly. The way ha he leakage energy is discharged o he oupu can also be exended o oher opologies. Fig. 3.5(a) shows he new coupledinducor swiching cell. In pracical applicaions, he leakage inducance can be used o conrol he currenseering process and hen o conrol he di/d of he diode during he urnon of swich. However, he leakage energy can induce swich sress and loss for swich when i is urned off. A soluion ha discharges he leakage energy o he oupu has been verified. Therefore, he pracical cell can be shown in Fig. 3.5(b). Applying his cell o he converer shown in Fig. 3.4 generaes he new converers shown in Fig. 3.5(c)(d)(e). The wo converers have basically he same opology. The leakage inducor is uilized on one hand o conrol he di/d of he oupu recifier; on he oher hand, he leakage energy mus be handled properly o make he circui work. As discussed in Chaper 2, eiher a dissipaive snubber or an aciveclamp scheme can be used. The previously proposed clamp scheme does no apply o his case since he discharge pah has been shifed. Bu he aciveclamp scheme can sill be applied. The clamp volage is slighly higher han he oupu volage; his volage difference drives he curren seering. This aciveclamp scheme resuls in a complex opology. A simple soluion o handle he leakage energy in his converer is o discharge he leakage energy direcly o he oupu. 82

98 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery N 1 N C c 1 3 N p D c N s Do 2 (a) 2 1 N 1 N 2 3 N s D o 1 3 N 1 2 N p D c (b) 2 N s D o 1 3 N p Dc V in V o 2 (c) Fig. 3.5 Derivaion of boos converer wih alleviaed recifier reverse recovery: (a) previous leakage energyrecovery scheme; (b) new leakagerecovery scheme; and (c) coupledinducor boos converer. The converer shown in Fig. 3.5 can achieve curren seering. If boh windings are conneced o he same volage source, hen he currenshifing condiion can be me by changing he urns 83

99 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery raio. We have found ha he condiion is ha he secondary winding mus be larger han 1/K, which means ha he secondary winding urns number mus be higher han ha of he primary. Because he urns raio is larger han one, he inpu curren is differen from ha in a CCM boos converer. There is a small slope change, which will be discussed in he operaion analysis. D o 2 3 D c V in N p N s V o 1 (a) N p 2 1 N s V in D c D o V o 3 (b) Fig The concep as applied o oher coupledinducor converers: (a) buck converer and (b) buckboos converer. 84

100 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery Operaion Analysis The proposed boos converer employing a coupled inducor reduces he reverserecoveryrelaed loss. Compared o he convenional boos converer, he proposed converer has a new branch ha consiss of a recifier D o and a new coupled winding for he original boos inducor. The added winding has a number of urns ha is slighly larger han ha of he original boos inducor. The basic idea of he circui is simply o conrol he curren decrease rae hrough he oupu recifier D o using he leakage inducor of he coupled inducor when swich urns on. During he swich urnoff ime period, he coupled inducor will seer he curren of he recifier D c o he recifier D o. The curren hrough he clamp boos recifier D c can be reduced o zero before swich urns on. To analyze he circui operaion, he coupled boos inducor is modeled as a combinaion of he magneizing inducor L m, an ideal ransformer wih urns raion N s :N p, and a leakage inducor L k, as shown in Fig The reference direcions for currens and volage are also given in he same graph. N s T(N s > N p ) D o IDo I k N p T D a I Da I in V in L m I m C o R V o Fig Analysis model. 85

101 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery Fig. 3.8 shows he seven opological sages of he converer in one swiching cycle. Fig. 3.9 shows he corresponding key waveforms of he operaion. [T 0, T 1 ]: wich is already on, and he oupu recifier D o and clamp recifier D a are reversebiased. The magneizing inducor is linearly charged by he inpu volage source. During his ime period, he volage sress of D a is he same as ha of he convenional boos, while he volage sress of D o is given by Eq. 32: VDo = Vo Vin ( N 1). Eq. 32 [T 1, T 2 ]: wich urns off a T 1. The parasiic capacior of he swich is charged by he magneizing curren in an approximaely linear way. [T 2, T 3 ]: The oupu recifier D o is forwardbiased a T 2 when V D (volage on parasiic capacior of swich ) is charged o he value meeing condiion Eq. 33: V in ( VD Vin) N > Vo. Eq. 33 [T 3, T 4 ]: A T 3, he parasiic capacior is charged o he level of he oupu volage. Oupu recifier D o conducs. The refleced volage from he winding wih 1T o he winding wih NT is N(V o V in ). The oal volage applied o he winding wih NT is V o V in. Therefore, a posiive volage (V o V in )(N1) is applied o he leakage inducor L k. The curren hrough L k increases linearly. The currenshifing process, from he clamp recifier o he oupu recifier, begins. [T 4, T 5 ]: If he leakage inducor L k is provided wih sufficien charge volage and ime, he oupu curren is compleely shifed o he new branch a T 4. All boos curren goes o he oupu 86

102 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery filer hrough D o. The clamp recifier D a will be naurally recovered. The refleced magneizing curren hrough D a decreases linearly. [T 5, T 6 ]: A T 5, swich urns on again. The volage V o V in (N1) is applied o he leakage inducor L k. The di/d of he recifier D o is conrolled by he leakage inducor L k. The reverserecovery problem of he recifier D o is alleviaed due o he conrolled di/d. [T 6, T 0 ]: The swich curren increases linearly unil i reaches he level of he inpu curren, and he recifier curren i Da falls o zero. However, he recifier curren coninues flowing in he opposie direcion due o he residual sored charge, even hough his charge has been dramaically reduced. 87

103 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery N s T(N s >N p ) I k D o I Do N s T(N s >N p ) I k D o I Do N p T L k D a I Da N p T L k D a I Da I in V in L m I m C o R V o I in V in L m I m C o R V o (a) (b) N s T(N s >N p ) I k D o I Do N s T(N s >N p ) I k D o I Do N p T L k D a I Da N p T L k D a I Da I in V in L m I m C o R V o I in V in L m I m C o R V o (c) (d) N s T(N s >N p ) I k D o I Do N s T(N s >N p ) I k D o I Do N p T L k D a I Da N p T L k D a I Da I in V in L m I m C o R V o I in V in L m I m C o R V o (e) (f) N s T(N s >N p ) I k D o I Do N p T L k D a I Da I in V in L m I m C o R V o (g) Fig Operaion modes of he proposed converer: (a) [T 0, T 1 ]; (b) [T 1, T 2 ]; (c) [T 2, T 3 ]; (d) [T 3, T 4 ]; (e) [T 4, T 5 ]; (f) [T 5, T 6 ]; and (g) [T 6, T 0 ]. 88

104 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery DT s T s I m I s V D I Do I Da I in T0 T1 T3 T2 T5 T0 T4 T6 Fig Key waveforms corresponding o he operaion modes. Recifier D o has a juncion capacior. This parasiic capacior and he leakage inducor form a resonan ank. Due o he undesirable resonance ha occurs beween he leakage inducor and he 89

105 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery parasiic capacior of D o afer swich urns on, a small snubber circui for D o is generally necessary o avoid exra volage sress on D o. I also should be menioned ha he socalled di(rec)m/d of a fasrecovery recifier has a significan effec on he volage sress of D a [C3], since he maximum volage sress of he recifier D o is given by Eq. 34: V = Vo Lk di( rec) M d. Eq. 34 Do / To effecively alleviae he recifier reverserecovery problem in he proposed circui, wo condiions mus be saisfied. Firs, he curren hrough recifier D a mus be reduced o zero before swich urns on so ha D a can be naurally recovered. The decrease rae of I Da is given by Eq. 35: di d Da ( Vo Vin) ( N 1) = N. Eq. 35 Lk A his decrease rae, he forward curren hrough clamp recifier D a can be reduced o zero during he urnoff ime period if he following condiion is saisfied: di d Da ( 1 d) Ts > IDa _ T 2, Eq. 36 where T s is he swiching ime, d is he duy raio, and I Da_T2 is he curren of D a a ime insance T 2. The duy raios of CCM boos converers for PFC vary wih he line variaions. Forunaely, he curren is small when he duy raio is large a he inpu volage near he zerocrossing poin. Alhough swich has a shorer urnoff ime period during which he curren mus be shifed o he new branch, he curren is also small. When he inpu volage is close o he peak area, he CCM boos converer has a large level of recifier forward curren and small duy raios. 90

106 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery Therefore, more ime is available for he curren hrough D a o be shifed. In oher words, no large N is needed o shif he curren. ecion IV will address he design issues in deail. The second condiion ha mus be me is ha, in order o alleviae he recifier reverserecovery problem of he oupu recifier D o, he curren decrease rae of recifier D o mus be conrolled when swich urns on. The conrolled rae is given by Eq. 37: di Do Vo Vin ( N 1) d =. Eq. 37 Lk Generally, i is preferable for his decrease rae o be conrolled wihin 100 A/µ in order o effecively alleviae he recifier reverserecovery problem. A larger L k could offer beer conrol of he di Do /d. However, a larger L k requires a higher rese volage in order o reduce is curren o zero during a given period, which means a larger N is needed. There are wo disadvanages o having a larger N. Firs, he volage sress of D o given by Eq. 32 increases when N increases. I is preferred o be able o use a recifier wih he same volage raing as he original recifier. To minimize he volage sress of recifier D o, he N should be designed o be close o one. econd, here is a small slope change in he inpu curren during he currenseering process. Afer swich urns on, he curren is quickly seered back from he added branch o he original one; he inpu curren has a larger increase slope during [T 5, T 6 ] han during [T 0, T 1 ]. From boh inpu filer and EMI sandpoins, he less he slope changes, he beer, which means a small N is preferred. 91

107 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery Design Issues and Experimenal Resuls for DCDC Applicaions Alhough silicon recifiers have problems wih reverse recovery, which has a significan impac on he performance of a power converer, here is insufficien informaion in he daa shees provided by recifier manufacurers. Table 3.1 shows a comparison of differen hyperfas recovery recifiers from differen manufacurers. As can be seen, he daa shees only provide one poin of es daa under room emperaure, which is far from sufficien o know how o effecively conrol he reverserecovery curren. A circui o es recifier reverserecovery performance as shown in Fig. 3.10, was buil in order o obain an undersanding of how o alleviae he recifier reverserecovery problem. Loop O is minimized o reduce he parasiic parameers. Two probes wih he same properies are used o measure he volage and curren o avoid delay. From he survey and previous experiences, he RHRP860 from Inersil (Fairchild now) is seleced as he oupu recifier because of is low reverserecovery charge and sof recovery characerisic. Fig shows he es waveforms wih differen levels of di/d a room emperaure (25 o ). Fig shows he es waveforms wih differen levels of di/d under an ambien emperaure of 55 o. 92

108 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery Table 3.1. Comparison of differen fasrecovery diodes. Manufacure Device V Res (V) I F (A) V F (V) T rr (n) Q rr (nc) Package ON MUR A 1A NA To220 IXY DEI806A A 8A 200A/us 8A To220AC IXY DEP806B IR HFA08TB A 8A NA NA To220AC 8A 200V To220AC Inersil RURD A 8A 195 To251 Inersil RHRP A To220AC Fig shows he measured reverserecovery charge Qrr of he RHRP860 under differen emperaures. From Fig. 3.13, we can draw he following conclusions: The emperaure of he recifier can srongly affec he reverserecovery charge. Therefore, he oupu recifier ha can resul in he reverserecovery problem should be as cool as possible. The curren decrease rae di/d of he recifier can dramaically affec he reverserecovery charge Qrr. An effecive soluion for alleviaing he recifier reverserecovery problem is o conrol he curren decrease rae di/d so ha i remains under 100 A/µs; his is also 93

109 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery verified in oher work [C12]. No furher significan benefis can be achieved if he di/d is conrolled lower han 40 A/µs. The curren decrease rae di/d of he recifier does no change he snap facor. Low levels of recifier reverserecovery curren also reduce he recifier volage sress. G L m I Lm Loop O R s =25mΩ DUT ID V L a Rese Circui V D I Lm I D Di/d=V s /L a (a) (b) Fig Tes bed for measuring reverserecovery charge: (a) diagram of es circui and (b) waveforms of he wopulse es mehod. 94

110 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery I D (4A/div) I D (4A/div) V D (100V/div) V D (100V/div) di/d=200a/µs di/d=400a/µs V max =518V 50ns /div V max =492V 50ns /div (a) (b) I D (4A/div) I D (4A/div) V D (100V/div) V D (100V/div) di/d=80a/µs di/d=40a/µs V max =472V 50ns /div V max =436V 50ns /div (c) (d) Fig Measured waveforms showing he reverserecovery of PHRP860 (a 25 o C). I D (4A/div) V D (100V/div) di/d=200a/µs I D (4A/div) di/d=80a/µs V D (100V/div) V max =558V 50ns /div V max =496V 50ns /div (a) (b) I D (4A/div) V D (100V/div) di/d=40a/µs V max =456V 50ns /div (c) Fig Measured waveforms showing he reverserecovery of PHRP860 (a 55 o C). 95

111 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery Q rr (nq) T=25 o C 450 I F =12A I F =8A I F =4A I F =2A 50 0 di/d(a/µs) (a) Q rr (nq) T=55 o C I F =12A I F =8A I F =4A I F =2A I F =1A di/d (A/µs) (b) Fig Measured reverserecovery charge: (a) a room emperaure (25 o C) and (b) under an ambien emperaure of 55 o C. For he silicon recifiers, he preferred di/d of he recifier urnoff rae is less han 100 A/µ. Eq. 37 shows ha he di/d of recifier D o during is urnoff is roughly deermined by V o /L k when N is close o one. The oupu volage is generally 400 V for a boos converer wih 96

112 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery universalline inpu. A leakage inducance of abou 5 µh o 10 µh is sufficien o achieve he desired improvemen. Recifier D o has a juncion capacior. This parasiic capacior and he leakage inducor form a resonan ank. Due o he undesirable resonance beween he leakage inducor and he parasiic capacior of D o afer swich urns on, a small snubber circui for D o is generally necessary o avoid exra volage sress on D o. I also should be menioned ha he socalled di(rec)m/d of a fasrecovery recifier has a significan effec on he volage sress of D o [C3], since he maximum volage sress of he recifier D o is given by Eq. 38: V = Vo Lk di( rec) M d. Eq. 38 Do / A 500W CCM DCDC boos converer operaing a 100KHz was buil o verify he proposed concep. The experimenal seup is given in Fig Fig shows he curren waveforms hrough he wo recifiers wih he maximum curren, which is he wors case. As can be seen, he curren begins o shif from recifier D a o he oupu recifier D o. The curren hrough D a is reduced o zero before swich urns on, and D a is naurally recovered before urns on. 97

113 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery 100T 96T D o :RHRP860 V in Lm=0.5mH 125 V Lk=10µH D a :MUR860 dc o 350 V dc C o R IRFP 460 V o =400V Fig Experimenal seup for DCDC boos converer. I Do 2 A/div I Da 2 A/div V D 200 V/div V in = 125 V V o = 400 V I o = 1.25 A P o = 500 W = 2 µs/div Fig Compleed curren shif before swich urns on when V in = 125 V dc Fig shows he significan performance improvemen aained by conrolling he recifier di/d during is urnoff. As illusraed, no only is he reverserecovery peak curren reduced, bu also he ime a which he reverserecovery curren appears is delayed. The volage of swich V D is already zero when he reduced reverserecovery curren appears. Therefore, swiching loss relaed o his reverserecovery curren is dramaically reduced. 98

114 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery Fig shows ha he maximum volage sress for he recifier D o is 505 V when D o is RHRP860. Therefore, his 600V recifier could safely be used. However, when D o is MUR860, he maximum volage sress is abou 600V because he RHRP860 has smaller di(rec)m/d. I Do 2 A/div (RHRP860) di/d = 270 A/µs V D 200 V/div V in = 125V V o = 400V I o = 1.25A P o = 500W = 25 ns/div (a) I Do 2 A/div (RHRP860) di/d = 36 A/µs I Da 2 A/div (MUR860) V D 200 V/div No reverserecovery V in = 125V V o = 400V I o = 1.25A P o = 500W = 100 ns/div (b) Fig Effec comparisons: (a) severe reverserecovery problem for hyperfas recifier RHRP860 (rr< 30 ns) wihou conrolled di/d; and (b) no only is he reverserecovery curren reduced, bu he curren appearance is also delayed. 99

115 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery I Do 2 A/div V Do 200 V/div V D 200 V/div V in = 125 V V o = 400 V I o = 1.25 A P o = 500 W = 2 µs/div Fig The volage sress of recifier D o. The maximum 2% efficiency improvemen is obained a he minimum inpu volage, where he reverserecovery losses are he mos significan. The efficiency improvemen coninues, bu he improvemen ampliude decreases o 0.5% a he maximum inpu volage, as shown in Fig Efficiency Proposed oluion 0.98 wih N= Prooype N= % V indc (V) Fig Efficiency measuremens of he DCDC converer 100

116 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery 3.4. Improved Topologies for he FronEnd PFC Applicaions The CCM boos converer has been widely used as he fronend PFC converer for elecom and server applicaions. Table 3.2 shows he specificaions of he saeofhear fronend converers. Table 3.2. aeofhear fronend converers. Produc Profile (Inch) Cooling Mehod Po (W) Power Densiy (W/inch 3 ) Inpu Volage (V ac ) Eff (%) Power Facor A 2.6*7.9*13.8 Fan ? >0.99 B 3.35*6.75*11.5 Fan >86 >0.99 C 2.5*7*10 Fan ? >0.99 D 2.74*4.9*12.5 Fan >89 >0.98 E 1U*6*12 Fan ? 0.99@115V 0.95@230V F 2.6*8.7*10.4 Naural V ac) >91 >0.99 As can be seen from he able, some applicaions require consan oupu power wih universal inpu volage, and some applicaions degrade he oupu power a lowline inpu volage. The major challenge for improving he CCM boos performance is hee recifier reverse recovery. In his secion, he proposed converer will be demonsraed o achieve a significan performance improvemen for fronend PFC applicaions. 101

117 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery The oupu power in such applicaions is generally around 1KW o 1.2KW. On one hand, he leakage inducance of he coupled inducor conrols he di/d of he oupu recifier o alleviae he reverserecovery problem; on he oher hand, he leakage inducance and he parasiic capaciance consiue a resonan ank. The resonance induces severe volage sress in he oupu recifier. Alhough uilizing an RCD snubber can reduce he volage sress of he oupu recifier, he loss and hermal handling in he sunbber circui becomes anoher problem. The soluions of his parasiic resonance will be addressed CoupledInducor Converer wih a Lossless nubber In his secion, he proposed converer wih a lossless snubber in he oupu recifier is analyzed and experimenally verified Topology and Operaion Analysis Recifier D o has a juncion capacior. This parasiic capacior and he leakage inducor form a resonan ank. Due o he undesirable resonance beween he leakage inducor and he parasiic capacior of D o afer swich urns on, a snubber circui for D o is generally necessary o avoid exra volage sress on D o. An RCD snubber can be used for applicaions wih relaively low power. For a fronend converer wih abou a kilowa of oupu power, he excessive loss in he RCD snubber deerioraes he hermal handling of he snubber circui. A lossless snubber can be added by using he flying capacior concep [C7]. The lossless snubber has wo diodes and a small capacior, as shown in Fig

118 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery NT (N>1) D o 1T C a D a1 D a2 D c V in C o R V o Fig The proposed converer wih a lossless snubber for he sress reducion of D o. Fig andfig illusrae he opological sages and he key waveforms of he proposed converer wih a lossless snubber. Compared o he original proposed converer, hree new sages (c, d and h) appear. The volage sress of oupu recifier D o is dramaically reduced because of his clamp circui. 103

119 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery I k I Do I k I Do N s T(N s >N p ) L k C a D o N s T(N s >N p ) L k C a D o N p T D a1 D a2 I Dc N p T D a1 D a2 I Dc I in V in L m I m D c C o R V o I in V in L m I m D c C o R V (a) (b) I k I Do I k I Do N s T(N s >N p ) L k C a D o N s T(N s >N p ) L k C a D o N p T D a1 D a2 I Dc N p T D a1 D a2 I Dc I in V in L m I m D c C o R V o I in V in L m I m D c C o R (c) (d) I k I Do I k I Do N s T(N s >N p ) L k C a D o N s T(N s >N p ) L k C a D o N p T D a1 D a2 I Dc N p T D a1 D a2 I Dc I in V in L m I m D c C o R V o I in V in L m I m D c C o R V (e) (f) I k I Do I k I Do N s T(N s >N p ) L k C a D o N s T(N s >N p )) L k C a D o N p T D a1 D a2 I Dc N p T D a1 D a2 I Dc I in V in L m I m D c C o R V o I in V in L m I m D c C o R V Fig Eigh operaion modes of he proposed converer wih a lossless snubber. (a) [T 0, T 1 ]; (b) [T 1, T 2 ]; (c) [T 2, T 3 ]; (d) [T 3, T 4 ]; (e) [T 4, T 5 ]; (f) [T 5, T 6 ]; and (g) [T 6, T 0 ]. 104

120 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery Gae 0 DT s T s I m I s V D I Dc I Do I Da1 I in T 6 T T T1 T3 7 0 T 4 T 6 T 0 T 2 T 5 T 7 Fig Key waveforms of of he proposed converer wih a lossless snubber. 105

121 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery The design procedure for he PFC applicaions is differen from ha of he DCDC applicaions because he line volage and he inpu curren are variable. The design deails will be discussed here Design of he Turns Raio for PFC Applicaions Compared o he DCDC applicaion, he inpu curren changes wih he inpu line volage. Therefore, he design for he PFC applicaions is more complicaed han ha for he DCDC applicaions. The duy raios of CCM boos converers for PFC vary wih he line variaions. Forunaely, he curren is small when he duy raio is large a he inpu volage near he zerocrossing poin. Alhough swich has a shorer urnoff ime period during which he curren mus be shifed o he new branch, he curren is also small. When he inpu volage is close o he peak area, he CCM boos converer has large recifier forward curren and small duy raios. Therefore, more ime is available for he curren hrough D a o be shifed. In oher words, no large N is needed o shif he curren. ecion IV will address he design issues in deail. As in he previous discussion, he value of N is close o one. To simplify he pracical design, he sligh increase of he volage gain of he proposed converer is ignored. Therefore, he curren hrough oupu recifier D a mus be reduced o zero during he urnoff period of swich, which is given by Eq. 39: T off V = (1 d) Ts = in _ pk sin( ω ) Ts, Eq. 39 Vo 106

122 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery where T s is he ime period of a swiching cycle, V in_pk is he inpu peak volage, and V o is he oupu volage. A ime T 2, he curren I Do _ T 2 of he oupu recifier D a is defined by Eq. 310: I Do _ T 2 2Po = sin( ω ), Eq. 310 Vin _ pk where P o is he oupu power. ubsiuing Eq. 35, Eq. 39 and Eq. 310 ino Eq. 36 gives he minimum urns raio N, which can be expressed by Eq. 311: N Vo Vo Vin _ pk sin( ω ) 2Po Lk Fs 2 Vin _ pk, Vo Vin _ pk sin( ω ) Eq. 311 where ω is he angular frequency of he inpu line volage, and F s is he swiching frequency. For a given oupu power, he urns raio is a funcion of leakage inducance L k. The physical meaning of his key design equaion is quie simple: The L k deermines he preferred recifier curren rae di/d during is urnoff, while he minimum N guaranees complee curren shifing so ha he original recifier D a can be naurally recovered. The wors case occurs a he line peak. Fig shows he relaionship beween he N and he line variaions for a 1KW CCM boos converer a low line inpu. This converer requires ha N be a leas 1.07 in order o conrol he di/d, which is abou 100 A/µ. 107

123 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery N L k = 5.0 µh V in = 90 V ac V o = 400 V P o = 1000 W 1.08 L k = 4.5 µh L k = 4 µh π w Fig Turns raio for complee curren seering Conrol of he Leakage Inducance For he proposed opology, he recifier di/d is conrolled by he leakage inducor. The leakage inducance plays a crucial funcion in alleviaing he recifier reverse recovery. Therefore, finding a mehod o conrol he leakage inducance and deermining how he opology is sensiive o he variaions of he leakage inducance are imporan. Fig. 3.23(a) shows a oroidal core wih a pah lengh of l m. Fig. 3.23(b) shows he crosssecion of he core wih wo coupled windings. Fig. 3.23(c) shows he srucure if he core is cu and hen sreched. Assume he area beween he wo coupled windings is A, as shown in Fig. 3.23(b). The leakage inducance of his srucure can be given by: L k = µ 0 N lm 2 A. Eq

124 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery Winding l m Core A: Blue area. (a) (b) Core L k = µ o N l m 2 A l m (c) Fig Facors affecing he leakage inducance of he coupled inducor. I can be found ha A is he only conrollable facor ha can change he leakage inducor of his coupled inducor. Adding isolaion ape is an effecive way o change he leakage inducance. Three coupled inducors are made by hand o check he variaion of he leakage inducance. We ry o ge 4 µh of leakage inducance in all hree inducors, as shown in Fig The primary winding is onelayer wih 48 urns and he secondary winding is 50 urns. As can be seen, he leakage inducance can be varied from 3.65 µh o 4.08 µh. 109

125 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery L = µh Lk = 3.9 µh L = µh Lk = 4.08 µh L = 366 µh Lk = 3.65 µh Fig Three coupled inducors ensiiviy of he Complee Curren eering o he Leakage Inducance The leakage inducance will have some variaion during he manufacure process, so he nex sep is o deermine how sensiive he complee curren seering is o he variaion. Forunaely, he proposed opology has a high olerance o he leakage inducance variaion in order o guaranee he complee curren seering. For he given 48 urns of he primary winding, he secondary nominal urns can be changed from o The secondary winding is 52 urns if he leakage inducance varies from 4 µh o 4.5 µh. The number of secondary urns is no sensiive o he variaion of leakage inducance. 110

126 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery N s / N p N s (N p =48T) L k = 5.0 µh L k = 4.5 µh V in = 90 V ac V o = 400 V P o = 1000 W 52 L k = 4 µh π 50 w Fig ensiiviy o he leakage inducance Experimenal Resuls Based on he design procedure, a 1KW prooype has been buil and esed o verify he performance improvemen of he proposed converer in he fronend PFC applicaions. Fig. 3.26(a) and (b) show he inpu volage and curren waveform a low line and high line inpu volage, respecively. As can be seen from he waveforms, he inpu curren has no disorion. 111

127 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery Vin: 50V/Div Iin: 5A/Div =2ms/div (a) Vin: 100V/Div Iin: 5A/Div (b) =2ms/div Fig Inpu volage and curren waveforms a low line and high line inpu. (a) V in =120V and (b) V in =230V. Fig. 3.27(a) shows he curren of he wo coupled windings. The currenseering process during he swiching urnoff ime period can be observed. The curren is seered from he branch wih N p o he branch wih N s jus before he swich urns on. Recifier D a is recovered naurally. The reverserecovery problem is alleviaed because he curren decrease rae of D o is conrolled a bou 100A/µs, shown in Fig. 3.27(b). 112

128 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery I Np 10A/div I Ns 10A/div V D 200V/div V in = 90 V V o = 395 V P o = 1000 W = 2 µs/div (a) I Np 10A/div I Ns 10A/div di/d=100 A/µ V D 200V/div V in = 90 V V o = 395 V P o = 1000 W = 50 ns/div (b) Fig Curren seering of he coupled inducor: (a) waveforms wih he ime scale 2µs/div and (b) enlarged waveforms wih ime scale 50ns/div. Due o he effeciveness of he snubber, he volage sress has been dramaically reduced o abou 450V, as shown in Fig

129 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery V D 200V/div V Do 200V/div I Ns 10A/div V in = 90 V V o = 395 V P o = 1000 W = 2 µs/div Fig Reduced volage sress of he oupu recifier. The measured efficiency of he proposed converer is shown in Fig The hardswiching version has a hermal runaway problem when he inpu volage is decreased o 110V. The proposed converer wih an RCD snubber can work a volage as low as 100V wih a significan performance improvemen. The proposed converer can achieve universalline operaion wih he bes efficiency. Eff Proposed oluion w/ Lossless nubber 0.96 Hard wiching V o = 395 V 0.91 Proposed oluion P o = 1000 W F w/ RCD nubber s = 100KHz V in (V) Fig Measured efficiency. 114

130 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery CoupledInducor Boos Converer wih Reduced Winding Loss As can been see from Fig. 3.27(a), he magneizing curren alernaes beween he wo coupled windings. The AC componens of he coupled winding can resul in large loss in he coupled inducor a highfrequency operaion. To overcome his drawback, an improved opology srucure wih he same concep is developed, and is shown in Fig. 3.30(a). The major idea is ha he CCM inpu curren can flow hrough he mos urns of winding N 1, while windings N 2 and N 3 (wih much fewer urns of he coupled inducor) are used o alleviae he recifier reverse recovery. Fig. 3.30(b) shows he key waveforms of he operaion of he improved version. I is obvious ha he CCM inpu curren I in flows hrough winding N 1, and only N 2 and N 3 are used for he currenshifing purpose. By uilizing his srucure, he inducor loss can be reduced. As o he design of he boos inducor wih hree windings, he windings on he firs layer are used for winding N 1. Two coupled windings wih much fewer urns on he second layer should be locaed far away from each oher in order o achieve sufficien leakage inducance. The urns raio N in Eq. 311 is defined as Eq. 313 for he improved version: N N 1 3 =. Eq. 313 N 1 N N 2 115

131 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery N 3 T (N 3 >N 2 ) D o I N1 I N3 C a N 1 T N 2 T D a1 D a2 I N2 D c V in C o R V o (a) DT s T s I m I N2 I N3 I N1 T0 T1 T3 T2 T5 T0 T4 T6 (b) Fig Improved version wih he reduced inducor winding loss: (a) circui diagram and (b) key waveforms. 116

132 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery The inducor has he following number of urns: N 1 =45T, N 2 =3T, and N 3 =6T. Fig shows ha a CCM curren flows hrough he winding N1. The loss in he coupled winding is reduced. I N1 10A/div I N3 10A/div V D 200V/div V in = 90 V V o = 395 V P o = 1000 W = 2 µs/div Fig CCM curren in winding N 1. Fig shows he measured efficiency. There is an improvemen of abou 0.6% a 90V inpu volage Eff Improved Version Hard wiching 0.94 Proposed oluion 0.93 w/ Lossless nubber 0.92 V o = 395 V 0.91 Proposed oluion P o = 1000 W 0.9 w/ RCD nubber F s = 100KHz V in (V) Fig Measure efficiency. 117

133 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery CoupledInducor Converer wih Minimal Volage ress The proposed snubber only achieves he urnon snubber funcion by suppressing he diode reverserecovery curren. The urnoff of he device is sill under he hardswiching condiion. Besides ha, here is sill an undesirable resonance beween he oupu capaciance of eiher he acive swich or he diode and he equivalen resonan inducor. The ringing causes he excessive volage sress over he semiconducor devices and affecs he circui operaion. A new lossless snubber configuraion for he boos converer is proposed by Wei Dong [C22]. I furher enhances he concep of he urnon and urnoff snubber funcion. One coupled inducor is used o help reduce boh urnon and urnoff losses. During he swich s off period, he coupled inducor shifs he curren from he original oupu diode o he auxiliary oupu diode. Therefore, he diode reverserecovery loss is much reduced in a way ha is similar o he proposed converer. Afer he swich is on, he coupled inducor can charge he snubber capacior volage o he oupu volage level. Consequenly, he snubber capacior can effecively slow he volage rising ime a he swich s urnoff and reduces he urnoff loss accordingly. The reduced dv/d is also expeced o alleviae he EMI noise. The proposed snubber circui has wo ineresing feaures: Firs, all devices block he volage no exceeding V o and no addiional clamping circui is required in he acual implemenaion; econd, i requires small number of added componens. I adds one coupled inducor, hree auxiliary diodes and one snubber capacior. Alhough here is one more diode volage drop during he swich s off period, as compared wih he convenional hardswiching boos converer, he resulan exra conducion loss is no significan when compared wih he swiching loss reducion. Fig andfig show he operaion modes and key waveforms. 118

134 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery I Do I Da2 I Do I Da2 D o D a2 N s T(N s >N p ) D o D a2 N s T(N s >N p ) N p T I Dc L k I I Da1 k D a1 N p T I Dc L I k I Do k D a1 I in V in L m I m D c C a C o R V o I in V in L m I m D c C s C a C o R V o (a) (b) I Do I Da2 I Do I Da2 D o D a2 N s T(N s >N p ) D o D a2 N s T(N s >N p ) N p T I Dc L I k I Do k D a1 N p T I Dc L I k I Do k D a1 I in V in L m I m D c C s C a C o R V o I in V in L m I m D c C a C o R V o (c) (d) I Do I Da2 I Do I Da2 D o D a2 N s T(N s >N p ) D o D a2 N s T(N s >N p ) N p T I Dc L I k I Do k D a1 N p T I Dc L I k I Do k D a1 I in V in L m I m D c C a C o R V o I in V in L m I m D c C s C a C o R V o (e) (f) I Do I Da2 I Do I Da2 D o D a2 N s T(N s >N p ) D o D a2 N s T(N s >N p ) N p T I Dc I L k I Do k D a1 N p T I Dc I L k I Do k D a1 I in V in L m I m D c C a C o R V o I in V in L m I m D c C a C o R V o (g 1 ) (g 2 ) Fig Operaion modes of he converer wih minimum volage sress: (a) [T 0, T 1 ]; (b) [T 1, T 2 ]; (c) [T 2, T 3 ]; (d) [T 3, T 4 ]; (e) [T 4, T 5 ]; (f) [T 5, T 6 ]; (g 1 ) mode I during [T 6, T 0 ] and (g 2 ) mode II during [T 6, T 0 ]. 119

135 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery Gae 0 DT s T s I m I s V D I Dc I Do I Da1 I in T 6 T T T 1 T T 4 T 6 T 0 T 2 T 5 T 7 Fig Key waveforms. Fig shows he experimenal resuls during he swich urnon and urnoff ime periods. As can be seen, he recifier reverserecovery problem is alleviaed when swich urns on, and he volage increase slope is reduced when urns off. 120

136 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery V Ds 200 V/div nubber Turnon I Np 10 A/div 2 µ/div (a) V D 200 V/div nubber Turnoff V Ca 200V/div I Ns 10A/div 2 µ/div (b) Fig Experimenal resuls. (a) snubber urnon and (b) snubber urn off. The fas recovery diode D o in he alernaive branch only needs o block V o V in. hown in Fig. 3.36(a), including he effec of he parasiics in he circui, D o blocks no more han 300 V afer urning off a high curren levels. D c, wih no curren flowing before urning he swich on, blocks he oupu volage V o when he swich urns on, as can been from Fig (b). Afer D o has recovered, he D c volage sars o slowly decrease. This implies ha D a1 began o block he 121

137 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery volage. Therefore, he D c s series diode, D a1, does no need o be a hyperfas diode, which helps o furher alleviae he effec of he exra conducion loss. V Ds 200 V/div V Do 200 V/div I Np 10 A/div 50 n/div (a) V Ds 200 V/div V Dc 200 V/div I Np 10 A/div (b) Fig Deailed waveforms of he recifier volage sress: (a) D o blocks he volage V o V in and (b) D c blocks he volage V o. Fig shows he measured efficiency. The efficiency improvemen of he converer wih minimal volage sress is similar o ha of he converer wih reduced inducor winding loss. The 122

138 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery major reasons are ha here are wo diodes in series in he curren pah and he urnoff loss is no significan Eff Version w/ Minimum ress Hard wiching Proposed oluion w/ Lossless nubber Improved Version 0.92 V o = 395 V 0.91 P o = 1000 W Proposed oluion F s = 100KHz w/ RCD nubber V in (V) Fig Measured efficiency Developmen of he 1U High PowerDensiy FronEnd Converer CCM boos converers are used in fronend converers o achieve uniy power facor for boh elecommunicaions and compuer servers. I is quie clear ha he lowprofile (1U) and high powerdensiy fronend converer is emerging as a sandard package. To achieve his arge, he recifier reverserecovery problem is he major obsacle for he PFC fronend. In his secion, he developmen of he 1U lowprofile, high powerdensiy fronend converer by uilizing he proposed soluion is demonsraed. The original operaion frequency of he benchmark boos fronend is 60KHz. The downsream DCDC converer is a halfbridge of a fullbridge converer wih an inpuvolage range of 350V o 400V. The reason is ha he fronend converer needs o provide 10ms holdup 123

139 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery ime during an AC ouage. The normal operaion condiion for he DCDC converer will be 400V of DC inpu. A pulse widh modulaion (PWM) converer, such as a halfbridge converer or a fullbridge converer, is unable o achieve opimized efficiency under a wide inpuvolage range. The design radeoff of he wide inpuvolage range sacrifices he efficiency. Therefore, high power densiy of he fronend converer is impossible. The power densiy of he benchmark converer is abou 4.3 W/inch 3. Fig shows he argeed dimensions of he fronend converer. The profile of he converer is 1U, and he nominal oupu power is 1.2KW a V in =150V. The power densiy is abou 11 W/inch 3. The converer uses fan cooling insead of naural cooling. 9.8 inch 1 U 1.75 inch 6.3 inch Fig Targeed dimensions of he fronend converer. To mee hese new challenges, new opologies and design echniques mus be employed. For he boos fronend converer, increasing he operaing frequency of he CCM boos converer wih alleviaed recifier reverserecovery problem is a viable opion for reducing he volume and increasing he power densiy. For he downsream DCDC converer, he holdup ime requiremen imposes a radeoff of he efficiency of PWM converers, because a PWM converer 124

140 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery canno achieve opimized efficiency under nominal operaion condiions wih a wide inpu volage variaion. To overcome his drawback, he LLC resonan converer has been comprehensively invesigaed [C26]. The LLC resonan converer can achieve he highes efficiency a high inpu volage. Therefore, i is easy for he converer o be opimized under nominal operaion condiions. However, he LLC resonan converer is beyond he scope of his research Design Guideline Fig shows he relaionship beween he N and he line variaions for a 1200W CCM boos converer a low line inpu. This converer requires ha N be a leas 1.05 in order o conrol he di/d, which is abou 100 A/µ. N 1.08 L k = 8µH V in = 150 V ac V o = 400 V P o = 1200 W 1.06 L k = 6µH 1.04 L k = 4 µh π w Fig Design graph o deermine he urns raio N. 125

141 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery Kool Mµ power cores are a produc made by Magneics Inc. The high flux sauraion level and low core losses make Kool Mµ cores excellen for use in PFC circuis. Kool Mµ cores provide a higher energysorage capabiliy han can be obained wih gapped ferries of he same size and effecive permeabiliy. Following he design procedure, we can find ha he 77192A7 from Magneics is appropriae for he applicaion. For he fronend converer under sudy, high power densiy is a major concern. No only he profile bu also he fooprin needs o be minimized. Alhough he 77192A7 can mee applicaion specificaions, he large fooprin and small profile are problemaic. The componen profile and he fooprin should reach a radeoff in order o achieve opimal resul. Two 77083A7 Kool Mµ cores are used for his applicaion. Fig. 3.40(a)(b) compares he wo designs from he sandpoins of profile and fooprin. Fig. 3.40(c) shows picures of he wo designs. 126

142 Chaper 3. Uilizing Coupled Inducors o Alleviae Recifier Reverse Recovery A7 Fooprin: 5 inch 2 Fooprin: 2.5 inch A A (a) (b) Fron View Original design New design Top View (c) Fig Comparisons of he wo designs: (a) fooprin by use of 77192; (b) fooprin by use of and (c) comparison of wo inducors. 127