Soft-Switched Bidirectional Buck-Boost Converters

Transcription

1 SofSwiched Bidirecional BuckBoos Converers Yungaek Jang and Milan M. Jovanović Dela Producs Corporaion Power Elecronics aboraory 5101 Davis Drive, Research Triangle Park, NC, USA Absrac A bidirecional buckboos converer wih a new sofswiching acivesnubber cell ha reduces swiching losses is inroduced. Sofswiching cell consiss of an acive snubber swich, a snubber inducor, and a wowinding ransformer wih associaed magneizing curren rese circui. The sofswiching cells enable he buck and boos recifier o urn off wih a conrolled urnoff rae of heir curren o minimize corresponding reverserecovery losses. In addiion, in he inroduced sofswiching cell, he powerconrolling buck and boos swiches urn on wih zerovolage swiching (ZVS) and he snubber swiches urn off wih zerocurren swiching (ZCS). The performance of he proposed bidirecional converer was evaluaed on a 5kW prooype exchanging energy beween a 400V bus and a baery wih volage range beween and 300 V. The 100kHz prooype circui exhibis he maximum fullload efficiency of 99.1% in he boosmode and 98.2% in he buckmode operaion. I. INTRODUCTI Bidirecional converers are increasingly being used in power sysems wih energysorage capabiliies, such as smargrid and auomoive applicaions where hey are employed o condiion charging and discharging of energysorage devices, such as baeries and supercapaciors. For example, in auomoive applicaions, isolaed bidirecional dcodc converers are used in elecric vehicles (EVs) o provide bidirecional energy exchange beween he highvolage (HV) baery and he lowvolage (V) baery, while nonisolaed dcdc bidirecional converers are ypically employed o opimize he racion inverer performance by preregulaing is inpu volage and providing energy regeneraion. Because a baery s operaing volage range depends on he baery s sae of charge, achieving high efficiency across he enire operaing volage range of he baery is a major design challenge in bidirecional converer designs. Nonisolaed bidirecional dcdc converers are almos exclusively implemened by buckboos converer opology [1][14]. A higher power levels, he coninuousconducionmode (CCM) operaion is preferred over disconinuousconducionmode (DCM) operaion because of beer performance. As described in [15], he major limiaions of CCM operaion of he highvolage, highpower buck and boos converers a high frequencies are relaed o swiching losses caused by he reverse recovery of recifiers and capaciive urnon swich loss of swiches due o heir hard swiching. Generally, in unidirecional buck and boos converer, he reverserecoveryrelaed losses can be virually eliminaed by using SiC or GaN recifiers insead of more cos effecive fasrecovery Si recifiers. Since in bidirecional buckboos converer boh swiches consiss of a combinaion of a conrollable swich and an aniparallel recifier, SiC or GaN recifier can only be used if i is copackaged wih an IGBT, or by employing emerging SiC and GaN MOSFET swiches [16]. The IGBT implemenaion is limied o a relaive low frequency due o a limied swiching speed of IGBTs, which increases he size of he converer; whereas, for he ime being, he SiC or GaN swich implemenaion is no aracive primarily due o increased cos. Today s mos cos effecive highfrequency implemenaion ha employs highvolage Si MOSFETs is only possible if he reverserecoveryrelaed losses of he slow parasiic aniparallel body diode are significanly reduced. In his paper, acivesnubber mehods ha offer reduced swiching losses of semiconducor swiches in he bidirecional buckboos converer are described. The major feaure of hese acivesnubber mehods ha are exensions of he acivesnubber mehod described in [17] is sof swiching of all semiconducor swiches. Specifically, he recifiers are urned off wih a conrolled urnoff rae of heir curren o minimize heir reverserecovery losses, he powerconrolling swich is urned on wih zerovolage swiching (ZVS), and he auxiliary swich in he acive snubber is urned off wih zerocurren swiching (ZCS). Because of fully sofswiched operaion, proposed circuis exhibi improved efficiency and EMI performance compared o heir convenional hard swiched counerpars and also enable he employmen of semiconducor swiches wih a relaively slow aniparallel diode (recifier) a high frequencies. II. SOFTSWITCHED BIDIRECTIA BUCKBOOST CVERTERS Figure 1 shows he proposed sofswiched bidirecional buckboos converer. As shown in Fig. 1, he converer includes wo sofswiching cells, each consiss of acive swich S A, inducor S, isolaion ransformer TR, and resevolage circui D AC RR RD B. In Fig. 1, swiches and are unidirecional curren swiches ha can carry curren only in one direcion. If bidirecional curren swiches such as MOSFETs are used, han a recifier in series wih he swich needs o be employed o preven conducion of is aniparallel diode. Sources and can be any kind of DC power sources or heir combinaions ha can deliver and sore (receive) elecric energy /17/$ IEEE 287

2 A1 A1 CE #1 I 2 i D1 N P1 V R1 C R1 R R1 CE #1 TR 1 S1 B1 V2 B1 B2 i i S V S2 B2 TR 2 TR 2 N P2 N S2 V R2 C R2 R R2 i S2 V P2 N P2 N S2 V R2 C R2 R R2 CE #2 CE #2 A2 A2 Fig. 1. Proposed sofswiched bidirecional buckboos converer. In he bidirecional buckboos converer in Fig. 1, swich and diode are boos swich and recifier, respecively, whereas swich and diode are buck swich and recifier, respecively. In his mode, boos swich is conrolled o provide regulaion of he boos oupu, i.e., regulae volage and/or curren I 2 and/or power I 2. Similarly, in he buck mode, buck swich is conrolled o provide regulaion of he buck oupu, i.e., regulae volage and/or curren and/or power. In boh modes, he swich in parallel wih corresponding recifier, i.e., swich in he boos mode and swich in he buck mode, can be eiher kep coninuously open or preferably conrolled as synchronous recifiers o improve efficiency. When he circui in Fig. 1 operaes in he boos mode, i.e., when power is ransferred from o, swich is urned off so ha only sofswiching cell #2 is acive, as illusraed in Fig. 2 by removing (no shown) he componens of sofswiching cell #1. To faciliae he explanaion of operaion, Fig. 2 shows key waveforms of he circui in Fig. 2 during a swiching cycle assuming, for simpliciy, ha buck swich is kep coninuously off during he boosmode operaion. As can be seen from Fig. 2, before swich is urned on a T 0, all swiches are off. As a resul, during his period he enire inducor curren i flows hrough aniparallel diode of swich, because curren i S drawn by sofswiching cell #2 is zero. To reduce he reverserecovery curren of recifier, swich is urned on slighly prior o urning on boos swich. Afer swich is urned on, curren i S sars flowing because volage is impressed across he series connecion of inducor and primary winding N P2 of ransformer TR 2. Curren i S flowing hrough primary winding N P2 induces a curren in secondary winding N S2 ha flows hrough diode o source. As long as he curren flows hrough he secondary winding, he volage across secondary winding N S2 is clamped o, i D1 i S i S2 i D1 d ZVS i S d V P2 V P2 Fig. 2. Circui diagram of acive par of sofswiched buckboos converer in Fig. 1 when i operaes in boos mode, i.e., ransfer power from source o source, where > and key waveforms. inducing consan primary volage ( ), where n 2N P2/N S2 is he urns raio of ransformer TR 2. As a resul, afer swich is urned on a T 0, consan volage is applied across inducor causing is curren i S2 i S o increase linearly. Because he curren of a large buckboos inducor can be considered approximaely consan during a swiching cycle, he curren in recifier mus decrease a he same rae, i.e., ( ). By conrolling he T 0 T 1 T 2 T 3 T 4 T 5 T 6 T T S 288

3 A1 CE #1 CE I 2 i S1 V P1 N P1 V R1 C R1 R R1 N P1 V R1 C R1 R R1 TR 1 TR 1 S1 B1 V2 S i i S B2 TR 2 i D2 CE #2 N P2 N S2 V R2 C R2 R R2 A2 i D2 i S i S1 i D2 d ZVS i S d T V P1 S1 V P1 S1 Fig. 3. Circui diagram of acive par of sofswiched buckboos converer in Fig. 1 when i operaes in buck mode, i.e., ransfer power from source o source, where > and key waveforms. recifier curren urnoff rae, he reverserecovery losses can be minimized. The curren urnoff rae in he circui in Fig. 1 can be adjused by a proper selecion of urns raio n 2 of ransformer TR 2 and value of inducor. Afer recifier curren i D1 reaches zero a T 1, i.e., afer recifier is urned off, inducor curren i S sars discharging oupu capaciance of swich and charging he oupu capaciance of swich, no shown in Fig. 1, by resonance beween he oupu capaciance of he swiches T S T 0 T 1 T 2 T 3 T 4 T 5 T 6 Fig. 4. Proposed sofswiched buckboos converer wih common snubber inducor S. and inducor. As illusraed in Fig. 2, during his resonance, volage v S2 across swich decreases while volage v S1 increases since he sum of he swich volages is consan, i.e., v S1v S2. To achieve ZVS, swich should be urned on when or shorly afer is oupu capaciance is fully discharged. As shown in Fig. 2, swich is urned on when swich volage v S2 reaches zero a T 2. When swich is urned on, a negaive primarywinding volage is impressed across inducor, i.e., v S2 V P2, causing a linear decrease of inducor curren i S wih slope. Afer inducor curren is rese o zero a T 3, swich can be urnedoff wih ZCS, as shown in Fig. 2. When swich is urned off a T 4, he magneizing curren of he ransformer, no shown in he figures, which was flowing hrough closed swich is divered hrough diode o he resevolage circui comprising capacior C R2 in parallel wih resisor R R2. For proper operaion, he value of rese volage V R2 needs o be se by selecing he value of resisor R R2 so ha he magneizing curren reaches zero before a new cycle is iniiaed a T 6 by urning on swich. Since volage sress on swich is given by he sum of volage and rese volage V R2, i.e., by V R2, i is desirable o use he minimum rese volage so ha he sress of swich is also minimized. Figures 3 and show he acive pars of he buckboos circui in Fig. 1 and is key waveforms, when he circui operaes in he buck mode, i.e., when power is ransferred from o. In he buck mode, swich is urned off so only sofswiching cell #1 is acive, as illusraed in Fig. 3 by omiing he componens of sofswiching cell #2. Since he operaion in he buck mode is idenical o ha in he boos mode, he waveforms of he corresponding componens in boh circuis are idenical, as can be seen from comparing waveforms in Figs. 2 and

4 CE B I 2 BYM26C 400 V B 47N60CFD V PQ20163C96 iz 0.1mm x 170 6T, 1.5 uh S TR GW35HF60WD FR305CT (600 V, 3A) B A S N P1 TR C R S B2 S B1 V R R R C A7 wo cores AWG#14 37T, 331 uh 2x560uF /450 V 47N60CFD TR N P1 PQ32303C96 N110 urns iz 0.1mmx120 N240 urns iz 0.1mmx50 M_N2 1.9mH K_N1 1.6uH GW35HF60WD S B1 S B2 C R 47nF R /400V R 30 kω /2W GW35HF60WD FR305CT (600 V, 3A) BYM26C C 2 4x 560uF /450 V C Fig. 6. Experimenal prooype circui. Fig. 5. Proposed sofswiched buckboos converer wih single sof swiching cell. Many variaions of he circui are possible depending how sofswiching cells are conneced and how resevolage circui is implemened. Because inducors S1 and do no operae a he same ime, inducors S1 and in Fig. 1 can be implemened as a coupled inducor, i.e., by having boh inducor windings on a single core, or by implemening he circui wih a common inducor S for boh sofswiching cells as shown in Fig. 4 Furher reducion of componens can be achieved by using a single ransformer and resevolage circui, i.e., by employing he same swiching cell in boh he boos and he buck mode as shown in Fig. 5 wih wo addiional swiches S B1 and S B2. These swiches are used o provide proper polariy of he resevolage for he boos and buck mode. In he boos mode, swich S B1 is urned on and swich S B2 is urned off so he rese volage V R is posiive. In he buck mode, swich S B2 is urned on and swich S B1 is urned off so ha he rese volage is negaive. and, implemened wih FR305CT (600 V, 3 A), are added in series wih swiches and o eliminae unnecessary reverse curren caused by parasiic ringing beween snubber indicor S and oupu capaciances of swiches and, respecively. To obain he desired inducance of filer inducor B of approximaely 330 μh, inducor B was buil by winding 37 urns of magne wire (AWG #14) on wo common oroidal coolmu cores (77258A7) from Magneics. Transformer TR was buil using a pair of ferrie cores (PQ3230, 3C96) wih 10 urns of iz wire (Φ 0.1mm, 120 srands) for he primary winding and 40 III. EXPERIMENTA RESUTS The performance of he proposed converer wih he proposed sofswiching cell shown in Fig. 5 was evaluaed on a 5kW prooype circui ha was designed o exchange energy beween a 400V bus and a baery wih volage range beween and 300 V. Figure 6 shows he experimenal prooype circui wih he full descripion of he employed power componens. I should be noed ha he urns raio (1:4) of ransformer TR is chosen o provide full ZVS over he enire volage and load ranges. As shown in Fig. 6, he prooype is buil using SPW47N60CFD MOSFETs (V DS 600 V, R DS Ω, C OSS2.2 nf, Q rr2 μc) from Infineon for swiches and and STGW35HF60WD IGBTs (V DS 600 V, V CE(SAT) 1.65 V, C OSS235 pf, Q rr90 nc) from ST for swiches and. I should be noed ha wo addiional diodes Fig. 7. Measured gaevolage waveforms of swiches when i operaes in boos mode. Time scales are 2 μs/div. and 400 ns/div. 290

5 urns of iz wire (Φ 0.1mm, 50 srands) for he secondary winding. The measured magneizing and leakage inducances a he primary winding are 119 μh and 1.6 μh, respecively. To achieve he desired urnoff rae of he recifier curren o approximaely 150A/μsec, he inducance value of snubber inducor SV O/(i S/d) of 3 μh was seleced. Since he leakage inducance of ransformer TR is in series wih snubber inducor S, i.e., i is a par of inducor S, he inducance of an exernal inducor should be approximaely 1.5 μh. The inducor was buil using a pair of ferrie cores (PQ2016, 3C96) wih 6 urns of iz wire (Φ 0.1mm, 170 srands) and approximaely 3.08 mm gap. iz wire was used o reduce he fringingeffecinduced winding loss near he gap of he inducor core. The volage regulaion for boh direcions was implemened by a TMS320F28027 microconroller wih 32 bi CPU from TI. A simple wopole wozero compensaor is implemened o he oupu volage conrol loop. The bandwidh of he volage conrol loop is se o approximaely 5 khz which is abou weny imes lower han ha of he swiching frequency. Figures 7 and 8 show measured gaevolage waveforms of swiches when he prooype operaes in boos and buck mode, respecively. As shown in Fig. 7, swich is disabled for boos mode operaion and operaes as a synchronous recifier. Figure 7 shows expanded waveforms of he gae signals. The dead ime beween swiches and is se o be 900 ns o preven any accidenal overlapping of he gae signals. The delay ime beween he urnon ransien of swich and ha of swich is se o be 500 ns. As shown in Fig. 8, swich is Fig. 9. Measured waveforms of proposed converer when i operaes in boos mode. Time scales are 500 ns/div. and 2 μs/div. Fig. 8. Measured gaevolage waveforms of swiches when i operaes in buck mode. Time scales are 2 μs/div. and 400 ns/div. Fig. 10. Measured waveforms of proposed converer when i operaes in buck mode. Time scales are 500 ns/div. and 2 μs/div. 291

6 disabled for buck mode operaion and operaes as a synchronous recifier. Figure 8 shows expanded waveforms of he gae signals. The dead ime and delay ime are 900 ns and 500 ns, respecively. Figure 9 shows he measured waveform of snubber inducor curren i S and he gae waveforms of swiches and of he experimenal circui when i operaes in boos mode and delivers full power from 200V inpu o 400V oupu. As i can be seen from he waveforms, swich urns on wih ZVS and swich urns off wih ZCS. Figure 10 shows he measured waveform of snubber inducor curren i S and he gae waveforms of swiches and of he experimenal circui when i operaes in buck mode and delivers full power from 400V inpu o 200V oupu. The waveforms show ZVS of swich and ZCS of swich. Figures 11 and 12 show measured efficiencies and losses of he prooype converer wih and wihou he proposed sofswiching cell when i operaes in boos and buck modes, respecively. I should be noed ha he converer wih sof swiching exhibis much higher efficiency han he converer wih hard swiching. In fac, because of high losses, he oupu power of he implemenaion wihou he proposed sofswiching acivesnubber cell is hermally limied o Efficiency [%] oss [W] V 300 V BAT 300 V Hard Swiching V BAT V BUK 400 V Oupu Power [W] Sof Swiching 68 deg 66 deg T SW 36 deg 42 deg V BUK 400 V Oupu Power [W] Hard Swiching Sof Swiching Fig. 11. Measured efficiencies and losses of experimenal prooype wih and wihou proposed sof swiching circui as funcions of oupu power when i operaes in boos mode. Efficiency [%] oss [W] V BAT 300 V 300 V Sof Swiching V BUK 400 V V BAT V BUK 400 V Hard Swiching Sof Swiching Oupu Power [W] 85 deg 81 deg Hard Swiching T SW 48 deg 55 deg Oupu Power [W] Fig. 12. Measured efficiencies and losses of experimenal prooype wih and wihou proposed sof swiching circui as funcions of oupu power when i operaes in buck mode. around 2 kw. The prooype circui exhibis he maximum fullload efficiency of 99.1% a boos mode operaion and 98.2% a buck mode operaion wih he swiching frequency of 100 khz. IV. SUMMARY In his paper, a bidirecional buckboos converer wih new sofswiching acivesnubber cells ha reduce swiching losses has been inroduced. The sofswiching cells enable he buck and boos recifier ha are he bodydiode of he corresponding swiches o urn off wih a conrolled urnoff rae of heir curren o minimize corresponding reverserecovery losses. In addiion, in he inroduced sofswiching cell, he powerconrolling buck and boos swiches urn on wih ZVS and he snubber swiches urn off wih ZCS. The performance of he proposed bidirecional converer was evaluaed on a 5kW prooype exchanging energy beween a 400 V bus and a baery wih volage range beween 200V and 300 V. The 100kHz prooype circui exhibis he maximum fullload efficiency of 99.1% in he boosmode operaion and 98.2% in he buckmode operaion. 292

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