Fully Soft-Switched Three-Stage AC-DC Converter

Transcription

1 Fully ofwiched Threeage ACDC Converer Yungaek Jang, David L. Dillman, and Milan M. Jovanović Power Elecronics Laboraory Dela Producs Corporaion P.O. Box 12173, 5101 Davis Drive Research Triangle Park, NC 27709, UA Absrac The paper presens a fully sofswiched hreesage acdc power supply for server applicaions. In his hreesage archiecure, he fronend boos PFC is followed by a dcdc boos converer ha serves as a preregulaor o he isolaed dcdc oupu sage. Through magneic inegraion, sofswiching of all semiconducor swiches in he PFC boos converer and dcdc boos preregulaor is achieved in addiion o a reduced number of magneic componens. Generally, he oupu dcdc sage can be implemened wih any isolaed opology. In his developmen, zerovolageswiching halfbridge dcdc converer is employed for he hird sage. The performance of he proposed hreesage approach was evaluaed on a 700W / 12, universallinerange acdc prooype. I. INTRODUCTION A fronend powerfacorcorreced (PFC) boos converer followed by a dcdc oupu sage is he archiecure ha has been almos exclusively used in offline power supplies for compuer and elecom applicaions. The fronend boos recifier is employed o reduce he linecurren harmonics and o provide compliance wih various worldwide specificaions governing he harmonic limis of he line curren in offline power supplies, whereas he dcdc oupu power sage is employed o provide galvanic isolaion and igh oupu volage regulaion. Alhough for many years he performance of his wosage archiecure, shown in Fig. 1(a), has been coninuously improved o mee he challenges of everincreasing powerdensiy and efficiency requiremens, is poenial o mee fuure powerdensiy and efficiency requiremens of power supplies which mus also mee a holdup ime requiremen may be limied. The major performance limiaion of he wosage archiecure sems from a subopimal design of he dcdc isolaion sage. Namely, for power supplies wih holdup ime requiremen, he dcdc oupu sage mus be designed wih an inpuvolage range ha is wide enough o regulae oupu volage during a line dropou by discharging he energysorage (bulk) capacior. ince here is a srong radeoff beween he size of he bulk capacior and he inpuvolage range of he dcdc oupu sage, he inpuvolage range of he dcdc oupu sage in highdensiy power supplies is relaively large, i.e., i is ypically from 300 o 400. Because he dcdc oupu sage needs o be design for a relaively wide inpuvolage range, is performance is no opimal. pecifically, he increased inpuvolage range requires ha he urns raio of he ransformer is reduced, which increases he conducion losses on he primary side and increases volage sresses on he semiconducor componens on he secondary side. Moreover, he increased secondaryside volage sresses usually mandae he use of componens wih higher volage raings ha are ypically less efficien. To obain he opimal performance of he isolaed dcdc sage, is inpu volage should be consan. This ideal inpuvolage requiremen can be achieved by resoring o he hreesage archiecure shown in Fig. 1(b). In his hreesage archiecure, he fronend PFC sage is followed by a dcdc sage which serves as a preregulaor o he dcdc isolaed oupu sage. Because of he preregulaor, he inpu volage o he oupu sage is always kep consan, even during he holdup ime. Alhough he hreesage archiecure resolves he radeoff beween he bulk capacior size and performance of he isolaed dcdc sage in an opimal way, i is sill no clear if and in wha applicaions he overall powerdensiy and efficiency performance of he hreesage approach is beer han ha of he wo sage approach. The main reason for his ambiguiy is he fac ha he dcdc preregulaion sage inroduces losses and requires space for implemenaion. According o resuls presened in [1], for applicaions where he dc inpu volage variaions are more han 10%, he combined efficiency of a dcdc preregulaor and isolaed dcdc oupu sage can be higher han ha of he isolaed dcdc oupu sage alone. In fac, commercially available high RECTIFIER AND INPUT FILTER RECTIFIER AND INPUT FILTER REC REC FRONT END PFC TAGE FRONT END PFC TAGE (a) BULK CAPACITOR BULK CAPACITOR (b) DCDC CONERTER nd (2 TAGE) H WIDERANGE IOLATED DCDC CONERTER NARROWRANGE IOLATED DCDC CONERTER Fig. 1. Block diagram of a ypical acdc power supply. DC1 DC2 DCn DC1 DC2 DCn

2 L B D D D i D D D i 2 D D REC N 1 L D 1 1 L D C B D C F R i N 1 L D 1 1 L D N 1 n = 2 L M D i D D Fig. 2. ofswiched power supply ha inegraes a PFC boos converer and a dcdc boos converer. powerdensiy, highefficiency, mulisage, dcdc converers which uilize a dcdc buck preregulaor and wo inerleaved forward converers exis oday [2]. Neverheless, more comprehensive and horough research and evaluaion is needed o undersand he performance of acdc power supplies implemened wih a hreesage approach compared o hose implemened wih convenional wosage approach. I also should be noed ha besides he described hreesage acdc power supply archiecure where he second sage employs a nonisolaed opology and he hird sage isolaed opology, he hreesage archiecure can be implemened wih an isolaed second sage and nonisolaed hird sage. In his implemenaion, he secondsage isolaed converer operaes as a dcdc ransformer and he nonisolaed hird sage provides regulaion. Generally, any nonisolaed or isolaed opology can be used o implemen corresponding sages. However, o reduce swiching losses and improve EMC performance, i is desirable o employ sofswiching opologies. While many sofswiching, isolaed dcdc opologies are available [3][5], nonisolaed sofswiching opologies ha do no suffer from undue complexiy are sill no available. As a resul, nonisolaed dcdc sages in [1] and [2] are implemened wih convenional hard swiched opologies. This paper presens a fully sofswiched hreesage acdc power supply ha employs a dcdc boos converer as he secondsage preregulaor. of swiching of his second sage boos converer is achieved by is inegraion wih sofswiched boos PFC recifier described in [6] hrough magneics inegraion similar o ha described in [7] for he flyback converer inegraion. The proposed magneically inegraed PFC boos and dcdc boos converers feaure sofswiching of all semiconducors. The boos swiches in boh converers are urned on while he swich body diode is conducing, i.e., zero volage swiching (Z), whereas he acivesnubber swich of he PFC boos converer urns off afer is drain curren has reached zero, i.e., zero curren swiching (ZC). In addiion, he boos recifiers in boh Fig. 3. implified circui diagram along wih reference direcions of key currens and volages. converers are urned off sofly wih a conrolled di/d rae so ha reverserecoveryrelaed losses of he boos recifiers are virually eliminaed, enabling he use of slower and cheaper fasrecovery recifiers. In his developmen, he dcdc isolaed oupu sage was implemened wih Z halfbridge dcdc converer. II. OFTWITCHED PFC BOOT CONERTER WITH INTEGRATED ECONDTAGE BOOT CONERTER The proposed sofswiched boos converer magneically inegraed wih a dcdc boos converer is shown in Fig. 2. REC The boos converer consiss of volage source IN, boos inducor L B, main swich, boos recifier D, energysorage capacior C B, and he acive snubber circui formed by auxiliary swich 1, winding N 1 of coupled inducor L D, snubber inducor L, and blocking diode D 1. The secondsage boos converer consiss of swich D wih an associaed aniparallel diode, winding of coupled inducor L D, oupu recifier D D and oupu capacior C F. To faciliae he explanaion of he circui operaion, Fig. 3 shows a simplified circui diagram of he proposed converer in Fig. 2. In he simplified circui, energysorage capacior C B and oupu filer capacior C F are modeled by volage sources and by assuming ha he values of C B and C F are large enough so ha he volage ripples across he capaciors are small in comparison o heir dc volages. In addiion, boos inducor L B is modeled as consan curren source by assuming ha he inducance of L B is large so ha during a swiching cycle he curren hrough L B does no change significanly. In his analysis, he leakage inducance of he coupled inducor is negleced since i does no have a significan effec on he operaion of he circui. Moreover, since snubber inducor L and primary winding N 1 of coupled inducor L D are conneced in series, he leakage inducance of he coupled inducor is absorbed by L. As a resul, coupled inducor L D is modeled by magneizing inducance L M and he wowinding ideal ransformer. Finally, i is assumed ha in he on sae, he semiconducors exhibi zero resisance,

3 2 C D 2 D 2 i D i D D C O D C OD i i D i i D (a) [T T ] C DD (e) [T T ] 4 5 D 2 (i) [T T ] 8 9 D 2 i D D C OD I D O IN i D i i D i (b) [T 1 T 2] 2 (f) [T 5 T 6] 2 D (j) [T 9 T 10] D 2 i D I D C O IN O B i D i i D i (c) [T T ] (g) [T T ] 6 7 D 2 (k) [T T ] i RR i D I D IN i D i i D (d) [T T ] 3 4 (h) [T T ] 7 8 Fig. 4. Topological sages during a swiching period. (l) [T T ] i.e., hey are shor circuis. However, he oupu capaciance of he swiches, as well as he juncion capaciance and he reverserecovery charge of he boos recifier are no negleced in his analysis. To furher faciliae he analysis of operaion, Fig. 4 shows he major opological sages of he circui in Fig. 2 during a swiching cycle, whereas Fig. 5 shows is key waveforms. The reference direcions of currens and volages ploed in Fig. 5 are shown in Fig. 3. As can be seen from he iming diagrams in Figs. 5(a), (b) and (c), he urn on of PFC boos swich and of secondsage boos swich D are synchronized, whereas auxiliary swich 1 is urned on prior o he urn on of swiches and D. In addiion, auxiliary swich 1 is urned off before PFC boos swich or secondsage boos swich D is urned off, i.e., he proposed circui operaes wih overlapping gae drive signals for he acive snubber swich and he converer swiches. Prior o he urn on of swich 1 a =T 0, all swiches are open. As a resul, he enire inpu curren flows hrough boos recifier D ino energysorage capacior C B in he boos power sage, while magneizing curren flows hrough oupu recifier D D in he secondsage boos converer as shown in Fig. 4(l). Because oupu recifier D D is conducing during his period, he induced volage across winding N 1 of coupled inducor L D is n( ), i.e., v 1 =nv 2 =n( ),

4 where n=n 1 /. Afer swich 1 is urned on a =T 0, he volage of energysoragecapacior and induced volage n( ) are applied across snubber inducor L so ha curren sars o increase linearly, as illusraed in Fig. 5(g). The slope of curren is di1 B v1 B n( O B ) = =. (1) d L L As curren sars flowing hrough winding N 1 of coupled inducor L D, magneizing curren begins o diver from oupu recifier D D o winding, i.e., = n, as shown in Fig. 4(a) and Fig. 5(l). Curren decreases unil i becomes zero and oupu recifier D D urns off a =T 1. As curren linearly increases, oupu recifier curren linearly decreases a he same rae since he sum of n and is equal o magneizing curren ha is relaively consan, i.e., n =. Therefore, in he proposed circui, he urnoff rae of he boos recifier d ( ) B n O B n (2) d L can be conrolled by he proper selecion of he inducance value of snubber inducor L and urns raio n of coupled inducor L D. Typically, for oday s fasrecovery recifiers, he urnoff rae di D /d should be kep around 100 A/μs. Wih such a seleced urnoff rae, he reverserecovery curren of he recifier and he relaed power losses and EMI problems are minimized. ince oupu recifier curren is zero afer =T 1, he increasing curren in winding N 1 makes he curren in winding larger han magneizing curren. This excessive curren discharges oupu capaciance C OD of swich D and charges oupu capaciance C DD of recifier D D, as illusraed in Fig. 4(b) and Fig. 5(d). During his period, volage v 2 across winding of coupled inducor L D sars o increase. Afer he oupu capaciance of swich D is fully discharged a =T 2, curren i D coninues o flow hrough he aniparallel diode of swich D, as shown in Fig. 4(c) and Fig. 5(i). To achieve Z of D, swich D should be urned on while is aniparallel diode is conducing. To simplify he conrol circui iming diagram, he urnon of swich D is synchronized wih he urnon of boos swich. When he aniparallel diode of swich D is conducing, volage v 2 across winding is equal o so ha induced volage v 1 on winding N 1 is N1 v 1 = B = nb. (3) ince v 1 is consan, volage applied across snubber inducor L is also consan so ha curren increases linearly wih a slope of di1 B v1 B nb B = = = (1 n). (4) d L L L During he same period, magneizing inducance increases wih a slope given by 1 D v D v 1 v i i D i 2 i D v D (1n) L n L T ONDCDC T ONPFC N n = 1 n ( ) n ( ) n L L M d conrolled d di D conrolled = (1n) d L reverserecovery charge (a) (b) (e) d B =. (5) d LM As curren linearly increases, boos recifier curren i D linearly decreases a he same rae since he sum of and i D is equal o consan inpu curren, i.e., i D =. Therefore, in he proposed circui, he urnoff rae of he boos recifier di D B = (1 n) (6) d L can be also conrolled by he proper selecion of he inducance value of snubber inducor L and urns raio n of coupled inducor L D. The opological sage in Fig. 4(c) ends a =T 3 when he curren of boos recifier D becomes zero. The ime period T reverserecovery charge L M (c) (d) (g) (h) (f) (i) (j) (l) (m) (n) T 0 T 1 T T 3 5 T 6 T 7 T 8 T 9 T 10 T 11 T 12 T T 2 4 Fig. 5. Key waveforms. (k)

5 beween =T 3 and =T 5, he reverserecovery curren of boos recifier D flows hrough snubber inducor L. Afer =T 4, curren sars o discharge he oupu capaciance of boos swich and charge juncion capaciance C D of boos recifier D, as shown in Fig. 4(e). If he urns raio of coupled inducor L D is seleced so ha n<0.5, he energy sored in L is sufficien o compleely discharge he oupu capaciance of boos swich regardless of he load and line condiions. Once he capaciance is fully discharged a =T 5, curren i coninues o flow hrough he aniparallel diode of boos swich, as shown in Fig. 4(f) and Fig. 5(h). During his period, volage v 1 is applied in he negaive direcion across snubber inducor L. Therefore, curren sars o decrease linearly a he rae given by di1 nb =, (7) d L as illusraed in Fig. 5(g). The curren in auxiliary swich 1 also sars o decrease, whereas boosswich curren i sars o increase from he negaive peak value, as shown in Figs. 5(g) and (h). To achieve Z of boos swich, i is necessary o urn on boos swich before is curren becomes posiive a =T 6, i.e., during he period when curren i sill flows hrough he aniparallel diode of swich, as illusraed in Fig. 5(h). As shown in Fig. 5(g), curren coninues o decrease unil i reaches zero a =T 7. horly afer =T 7, auxiliary swich 1 is urned off o achieve ZC. Afer swich 1 is urned off, he enire inpu curren flows hrough boos swich. As a resul, he fronend boos converer sage is compleely decoupled from he secondsage boos converer, as shown in Fig. 4(h). For he res of he swiching cycle, he PFC boos and secondsage boos converers coninue o operae as convenional boos converers. Afer secondsage boos swich D is urned off a =T 8, magneizing curren sar o charge he oupu capaciance of swich D, as shown in Fig. 4(i). When volage v D reaches a =T 9, diode D D sars o conduc, which forces he commuaion of he magneizing curren from swich D o oupu diode D D, as shown in Fig. 4(j). A he same ime, he rese of coupled inducor L D is iniiaed by applied volage v 2 = across winding. During he rese ime of coupled inducor L D, volage v D across he secondsage boos swich is equal o, whereas he volage across auxiliary swich 1 is v 1 =n( ) due o he magneic coupling of windings N 1 and, as illusraed in Figs. 5(d) and (e). Afer boos swich is urned off a =T 10, volage across swich sars increasing linearly because consan inpu curren sars charging he oupu capaciance of boos swich, as shown in Fig. 4(k). The increasing boosswich volage causes an equal increase of volage v 1 across auxiliary swich 1. When boosswich volage v reaches a =T 11, boos diode D begins o conduc, as shown in Fig. 4(l). A he same ime, auxiliaryswich volage v 1 reaches is maximum value of v 1 = n( ). A small clamping diode can be conneced from he drain of auxiliaryswich 1 o oupu volage o eliminae any undesired ringing. The circui says in he opological sage shown in Fig. 4(l) unil he nex swiching cycle is iniiaed a =T 12. In summary, he major feaure of he proposed circui in Fig. 2 is he sofswiching of all semiconducor devices. pecifically, PFC boos swich and secondsage boos swich D are urned on wih Z, whereas auxiliary swich 1 is urned off wih ZC. In addiion, PFC boos diode D and secondsage boos diode D D are urned off wih conrolled urnoff raes of heir currens. Because all semiconducor componens of he proposed converer operaes wih sof swiching, he overall swiching losses are minimized, which maximizes he conversion efficiency. In addiion, sof swiching has a beneficial effec on EMI and may resul in a smaller size inpu filer. However, i should be noed ha complee Z of secondsage boos swich D can only be achieved if inpu curren (which is being commuaed o winding N 1 when auxiliary swich 1 is closed) is large enough o produce a negaive curren hrough primary winding and discharge he oupu capaciance of swich D, as shown in Fig. 4(b). According o Fig. 4(b), o have a negaive curren flowing hrough winding afer =T 1, refleced curren ino winding has o be greaer han magneizing curren. If his condiion is no me, swich D operaes wih parial Z. This mode of operaion ypically occurs near he zero crossing of he line volage in a PFC boos converer. ince he inpu curren is proporional o he line volage, inpu curren is small near he zero crossing of he line volage. However, by adding an exra capacior across boos swich, swich D can achieve complee Z near he zero crossing of he line volage. Due o he Z of he PFC boos swich and he secondsage boos swich, he mos suiable implemenaion of he circui in Fig. 2 is wih he PFC boos swich and he secondsage boos swich consising of MOFET (Meal Oxide emiconducor Field Effec Transisor) devices. imilarly, due o he ZC of auxiliary swich 1, an IGBT (Insulaed Gae Bipolar Transisor) is suiable for he auxiliary swich. In he proposed circui, he volage sresses on PFC boos swich and boos recifier D are idenical o he corresponding sresses in he convenional PFC boos converer wihou a snubber. However, he volage sress of he secondsage swich D and oupu diode D D are equal o oupu volage ha is approximaely 10% higher han energysorage capacior volage. The peak volage of auxiliary swich 1 is v 1(MAX) = n( )< (8) because n is smaller han 0.5.

6 L B RHRP1560 D RHRP1560 EMI filer 2x58930A2 50T, u= uh BR 2xPP 20N605 N 1 L D 1 RHRP L D 77312A7 13T 3.3 uh C B 470 uf /450 IRG4BC40W 58930A2 N1=28T : N2=79T 116 uh : 978 uh 380 PP 20N605 D C BY26C D D H R D 1.5 C D uF 450 C T1 2.2uF 250 R1 C T2 2.2 uf 250 TR 2 x FDP047 AN08AD PJ 40/263C96 17T : 1T : 1T 2 x FDP047 AN08AD 3 x 2200 uf A2 2T, 1uH C O R2 L O T1 IRFB 16N60L T2 IRFB 16N60L Fig. 6. chemaic diagram of he experimenal prooype circui. The conrol of he proposed circui can be performed by wo independen conrollers ha are synchronized. pecifically, one conroller is used o regulae he oupu volage of he fronend boos sage, i.e., volage across he energysorage capacior C B. The oher conroller is used o regulae oupu volage of he secondsage boos converer. Any conrol sraegy can be used o conrol hese wo volages, including muliloop conrol sraegies such as various currenmode conrol implemenaions. Recenly, Texas Insrumen (TI) has developed wo combo conrollers (UCC28521 and UCC28528) for his applicaion. III. EXPERIMENTAL REULT The performance of he proposed sofswiched converer was evaluaed on a 700W hreesage prooype circui ha was designed o operae from a universal acline inpu (90 RM 264 RM ) and deliver up o 58.3 A a a 12 oupu. The fronend PFC boos converer wih he inegraed secondsage boos converer was designed o deliver 430, 750W power o he inpu of he hird sage dcdc isolaed converer as shown in Fig. 6. The firs sage and second sage converers operae a 80 khz swiching frequency. As shown in Fig. 6, a convenional halfbridge converer wih synchronous recifiers is implemened as he hird sage converer ha operaes a 125kHz swiching frequency. I should be noed ha any isolaed dcdc converer opology can be used for he hird sage. Moreover, he hird sage converer can be well opimized because i has a very narrow inpu range. Figure 6 shows he schemaic diagram and componen informaion of he experimenal prooype circui. ince he drain volage of boos swich is clamped o bulk capacior C B, he peak volage sress on boos swich is approximaely 380. The peak curren sress on swich, which occurs a full load and low line, is approximaely 13.3 A. Therefore, wo PP20N605 MOFETs ( D = 600, I D25 = 20 A, R D = 0.19 Ω) from Infineon were used for he boos swich. The maximum drain volage of secondsage boos swich D is D(MAX) = = 430, as shown in Fig. 5(d). ince he peak curren sress on secondsage boos swich D is approximaely 2.1 A, an PP20N605 MOFET was used for D. Finally, a high speed IRG4BC40W IGBT ( RRM = 600, I F = 40 A) from IR was used as auxiliary swich 1 since is maximum drain volage is (MAX) = n( ) = (430380) = 398, as shown in Eq. (8). Moreover, diode D C clamps auxiliary swich 1 o capaciors C T1 and C T2, as shown in Fig. 6. ince boos diodes D mus block he bulk volage and mus conduc he peak inpu curren, which is approximaely 13.3 A, an RHRP1560 diode ( RRM = 600, I FAM = 15 A) from Fairchild was used as boos diode D. An RHRP1560 diode was used for diode D 1 and anoher RHRP1560 for D D. To obain he desired inducance of boos inducor L B of approximaely 780 μh, he boos inducor was buil using wo parallel oroidal highflux core (58930A2) from Magneics and 71 urns of magne wire (AWG #16). Exernal snubber inducor L was conneced in series wih winding N 1 of coupled inducor L D, as shown in Fig. 6. The required inducance is approximaely 3.3 μh. nubber inducor L was buil using a oroidal Koolμ core (77312A7) from Magneics and 13 urns of magne wire (AWG #16). Coupled inducor L D was buil using a oroidal highflux core (58930A2), 28 urns of magne wire (AWG# 17) for winding N 1, and 79 urns of magne wire (AWG# 19) for winding. Magneizing inducance L M measured across winding of coupled inducor L D is approximaely 978 μh. A high volage aluminum capacior (470 μf, 450 DC) was used for bulk capacior C B o mee he holdup ime requiremen. Two film capaciors (2.2 μf, 250 DC) are used for halfbridge capaciors C T1 and C T2, respecively. A small RC damping circui ha consiss of capacior C D and

7 Efficiency 91% 90% 89% 88% 87% 86% 85% hardswiching wih i diode, Tswich=83 oc hardswiching wih ic diode, Tswich=93 oc 84% Oupu Power in Was sofswiching wih ic diode, Tswich=58 oc sofswiching wih i diode, Tswich=62 o C Fig. 7. Measured efficiencies of he experimenal hreesage converer delivering 12 oupu from 90 AC inpu. resisor R D is conneced across capaciors C T1 and C T2 o eliminae inpu oscillaions of he hird sage converer. The hirdsage half bridge converer was implemened wih an IRFB16N60L MOFET from IR for each of bridge swiches T1 and T2, wo parallel FDP047AN08AD MOFETs from Fairchild for each of synchronous recifier swiches R1 and R2. Transformer TR was buil using a pair of ferrie cores (PJ 40/263C96) wih 17 urns of magne wire (AWG# 16) for he primary winding, 1 urn of copper foil (10 mil, 15 mm) for each of he secondary windings. Oupu filer inducor L O was buil using a oroidal high flux core (77312A7) from Magneics and 2 urns of magne wire (4 AWG #17). Three low volage aluminum capacior (2200 μf, 16 DC) was used for oupu capacior C F. The performance of he proposed sofswiched converer was verified by comparing he efficiencies of he prooype circui wih and wihou he acive clamping circui. To measure he efficiency of he experimenal converer wihou he acive snubber circui, swich 1, diode D 1, inducor L, and winding N 1 of he prooype circui shown in Fig. 6 are disconneced. Moreover, he efficiencies of he prooype circui using recenly developed CD10060 siliconcarbide (ic) diodes from CREE for PFC boos diode D and secondsage boos diode D D are also measured. Figure 7 shows he measured efficiencies of he experimenal converer wih (solid lines) and wihou (dashed lines) he acive snubber circui as funcions of he oupu power of PFC fron end. The figure also shows he measured efficiencies wih silicon diodes or silicon carbide diodes. As can be seen in Fig. 7, he acive snubber improves he conversion efficiency in he enire measured power range. The efficiency improvemen is more pronounced a higher power levels where he reverserecovery losses are greaer. The prooype circui wihou acive snubber canno deliver more han 550 W because he juncion emperaure of he PFC boos swich exceeds is limi. By using silicon carbide diodes for D and D D, he prooype circui wihou acive snubber can deliver up o 650 W which is sill lower han full power. I should be noed ha he efficiencies of he prooype circui using silicon diodes or silicon carbide diodes are no much differen as long as he proposed acive clamp circui is incorporaed. Figure 7 also shows he imporan advanage of he inegraed magneic approach uilized in he proposed converer. By using coupled inducor L D as shown in Fig. 6, magneic energy can be sored in coupled inducor L D by boh swiches 1 and D. During he period when auxiliary swich 1 is on, he coupled inducor provides a direc energy pah ha bypass bulk capacior C B and boos diode D. As a resul, he conducion losses in he PFC boos diode D and secondsage boos swich D are reduced. Figures 8 and 9 show he oscillograms of key waveforms in he experimenal converer when i delivers full power from he low line inpu volage. As can be seen from he corresponding waveforms in Fig. 5, here is a good agreemen beween he experimenal and heoreical waveforms. As can be seen from Figs. 8 and 9, swiches and D are urned on G1 [20 /div] G [20 /div] [10 A/div] [250 /div] GD [20 /div] i 2 [5 A/div] i D [10 A/div] D [250 /div] (a) (b) ZC conrolled di/d Z Z Fig. 8. Measured waveforms of he proposed circui a =90, =380, =430, DC=12, P DC= 700 W. Time base: 2 μs/div.

8 G1 [20 /div] G [20 /div] [10 A/div] [250 /div] Z (a) ZC and 9. Wih his di/d rae, peak reverserecovery curren I RR is reduced o approximaely 1.5 A. Figure 10 shows he measured waveforms of bulk capacior volage and oupu volage of he secondsage boos converer. Because he secondsage boos converer can mainain is oupu volage from he peak bulk capacior volage PEAK =380 o near 120, approximaely 90% of he sored energy in C B is uilized for a holdup ime. The measured holdup ime is approximaely 30 msec as shown in Fig. 10. ince all swiches operae wih zerovolage or zerocurren swiching, he recifier reduces swiching losses and is also expeced o improve EMI. GD [20 /div] i 2 [5 A/div] i D [10 A/div] D [250 /div] (b) Fig. 9. Measured waveforms of he proposed circui a =90, =380, =430, DC=12, P DC= 700 W. Time base: 1 μs/div. [100 /div] [100 /div] AC [100 /div] reverserecovery curren conrolled di/d Z Fig. 10. Measured waveforms of he proposed circui a full load. Time base: 10 ms/div. wih Z since heir volages and D fall o zero before gaedrive signals G and GD become high. Moreover, auxiliary swich 1 achieves sofswiching urn off because swich curren becomes zero before auxiliary swich 1 is urned off. I should be also noed ha he slope of recifier curren i D is approximaely di/d = 70 A/μs during he period when boos diode D is urned off. The recifiercurren slope is conrolled by snubber inducance L, as indicaed in Figs. 8 Holdup ime=30 ms AC I. UMMARY A fully sofswiched hreesage acdc power supply for server applicaions has been inroduced. In his hreesage archiecure, he fronend boos PFC is followed by a dcdc boos converer ha serves as a preregulaor o he isolaed dcdc oupu sage. By using a single magneic device which is muually shared by he PFC boos converer and he dcdc boos converer, PFC boos swich and secondsage boos swich D are urned on wih Z, auxiliary swich 1 is urned off wih ZC, and boos diodes D and D D are urned off sofly using a conrolled di/d rae. As a resul, he urnon swiching losses in he boos swiches, he urnoff swiching loss in he auxiliary swich, and reverserecoveryrelaed losses in he boos diodes are eliminaed, which maximizes he conversion efficiency. Zerovolageswiching halfbridge dcdc converer is employed for he hird sage. The performance of he proposed converer was verified on a 700W/12 prooype circui ha was designed o operae from a universal acline inpu. The proposed sofswiching echnique improves he efficiency by approximaely 2% a 700 W in comparison o he efficiency of he convenional hardswiching converer. REFERENCE [1] J. Drobnik, Is cascade connecion of power converers inefficien?, Power Conversion Conf. (PCIM) Proc., pp. 3443, [2] M.F. chlech, High efficiency power converer, U.. Pa. 6,222,742 B1, Apr. 24, [3] L.H. Mweene, D.M. Oen, and M.F. chlech, A highefficiency 1.5 kw, halfbridge converer operaed a 100% duyraio, IEEE Applied Power Elecronics (APEC) Conf. Proc., pp , [4] T. Ninomiya, N. Masumoo, M. Nakahara, and K. Harada, aic and dynamic analysis of zerovolageswiched halfbridge converer wih PWM conrol, IEEE Power Elecronics pecialiss Conf. (PEC) Rec., pp , [5] B. Yang, F.C. Lee, A.J. Zhang, and G. Huang, LLC resonan converer for fron end dc/dc conversion, IEEE Applied Power Elecronics (APEC) Conf. Proc., pp , [6] Y. Jang, M.M. Jovanović, K.H. Fang, and Y.M. Chang, ofswiched highpowerfacor boos converer, Inernaional Telecommunicaion Energy Conf. (INTELEC) Proc., pp , [7] Y. Jang, D.L. Dillman, and M.M. Jovanović, A new sofswiched PFC boos recifier wih inegraed flyback converer for sandby power, IEEE Applied Power Elecronics (APEC) Conf. Proc., pp , 2004.