A New SofSwiched PFC Boos Recifier wih Inegraed Flyback Converer for Sandby Power Yungaek Jang, Dave L. Dillman, and Milan M. Jovanović Dela Producs Corporaion Power Elecronics Laboraory P.O. Box 273, 50 Davis Drive Research Triangle Park, NC 27709 Absrac The paper presens a magneic inegraion approach ha reduces he number of magneic componens in a power supply by inegraing he some magneic componens in wo conversion sages. Specifically, in he proposed approach, a single ransformer is used o implemen he coninuousconducionmode boos PFC converer and he dcdc flyback converer. The inegraed boos and flyback converers offer sof swiching of all semiconducor swiches including a conrolled di/d urnoff rae of he boos recifier. The performance of he proposed approach was evaluaed on a 50kHz, 450W, universalline range boos PFC converer wih 2/2.2A inegraed sandby flyback converer.. Inroducion The majoriy of oday s acdc power supplies used in modern daa processing and elecom equipmen have a boos powerfacorcorreced (PFC) fron end and a lowpower sandby power supply. The block diagram in Fig. shows he ypical srucure of an offline power supply for hose 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 acdc power supplies. On he oher hand, he main purpose of he sandby power supply is o provide housekeeping power and o ensure sysem funcionaliy when he sysem is in lowpower (sandby or sleep) mode. The majoriy of sandby power supplies are implemened wih a flyback converer due o is low par coun and is abiliy o operae efficienly in a wide inpuvolage range. To mee he challenges of he everpresen requiremen o decrease he size of power conversion equipmen, power supplies operaing a higher swiching frequencies and uilizing advanced packaging and hermal managemen echniques have been inroduced. Specifically, in recen years, significan effors have been made o reduce swiching losses of coninuousconducionmode (CCM) boos converers since he CCM boos converer is he preferred opology for implemenaion of a fron end wih PFC over he range from medium o high power. So far, a number of sofswiched boos converers and heir variaions have been proposed [] [6]. All of hem employ an auxiliary acive swich wih a few passive componens o form an acive snubber ha is used o conrol he di/d rae of he recifier curren and o creae condiions for zerovolage swiching (ZS) or zerocurren swiching (ZCS) of he main swich. A furher size reducion can be achieved by minimizing he number of componens hrough componen inegraion. As already have been demonsraed, he number of componens can be reduced by inegraing semiconducor swiches wih drive, conrol and/or supervisory circuis and/or by inegraing magneic componens such as ransformers and inducors on he same core. This paper presens a new magneic inegraion approach where he reducion of he number of magneic componens in a power supply is achieved by uilizing he same magneic componen in wo conversion sages of he power supply. Specifically, in he proposed approach, a single ransformer is used o implemen he inegraion of he CCM PFC boos converer and he flyback sandby converer. The proposed magneically inegraed boos and flyback converers feaure sofswiching of all semiconducors. The boos swich and he primary side flyback converer swich are urned on a zero volage, whereas he acivesnubber swich of he boos converer urns off a zero curren. In addiion, he boos recifier is urned off sofly wih he conrolled di/d rae so ha reverserecoveryrelaed losses of he boos recifier are virually eliminaed. IN RECTIFIER AND INPUT FILTER REC IN FRONT END PFC STAGE DCDC OUTPUT STAGE STANDBY POWER Fig.. Block diagram of a ypical acdc power supply. DC DC2 DCn 2 n 0780382706/04/$7.00 (C) 2004 IEEE
REC IN L B S D S D TR C B N Fig. 2. Proposed sofswiched power supply ha inegraes a boos converer and a flyback converer. 2. SofSwiched PFC Boos Converer wih Inegraed Flyback Converer The proposed sofswiched boos converer magneically inegraed wih a sandby flyback dcdc converer is shown in REC Fig. 2. The boos converer consiss of volage source IN, boos inducor L B, main swich S, oupu recifier D, energysorage capacior C B, and he acive snubber circui formed by auxiliary swich S, winding N of ransformer TR, snubber inducor, and blocking diode D. The sandby flyback converer consiss of swich S D wih an associaed aniparallel diode, isolaion ransformer TR, and he secondary side circui ha consiss of recifier D R 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 is modeled by volage source by assuming ha he value of C B is large enough so ha he volage ripple across he capacior is small in comparison o is dc volage. In addiion, boos inducor L B is modeled as consan curren source by assuming ha he inducance of L B is large so S D D D D R i TR M i 2 i DR S N N I L M 3 3 IN N n = Fig. 3. Simplified circui diagram of he proposed converer shown in Fig. 2 along wih reference direcions of key currens and volages. D S D S D N 3 D R C F R ha during a swiching cycle he curren hrough i does no change significanly. In his analysis, he leakage inducance of he ransformer is negleced since i does no have a significan effec on he operaion of he circui. Moreover, he effec of he leakage inducance on he operaion of he circui can be accouned o he effec of snubber inducor since is conneced in series wih he leakage inducance of winding N. As a resul, ransformer TR is modeled by magneizing inducance L M and hreewinding ideal ransformer. Finally, i is assumed ha in he on sae, semiconducors exhibi zero resisance, 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), and (c), he urn on of boos swich S and of flyback swich S D are synchronized, whereas auxiliary swich S is urned on prior o he urn on of swiches S and S D. In addiion, auxiliary swich S is urned off before boos swich S or flyback swich S 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 S 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 refleced magneizing curren R =( /N 3 ) flows hrough oupu recifier D R in he flyback power sage as shown in Fig. 4(l). Because oupu recifier D R is conducing during his period, he oupu volage is induced across winding N of ransformer TR, i.e., v =(N /N 3 ). Afer swich S is urned on a =T 0, he volage of energysoragecapacior plus induced volage (N /N 3 ) is applied across snubber inducor so ha curren sars o increase linearly, as illusraed in Fig. 5(g). The slope of curren is d B v B no = =, () d LS LS where n = N /N 3. As curren sars flowing hrough winding N of ransformer TR, he curren in winding N 3 sars o decrease, i.e., R =( /N 3 ) (N /N 3 ), as shown in Fig. 4(a) and Fig. 5(k). Curren R decreases unil i becomes zero a =T, i.e., oupu recifier D R urns off. Since he curren hrough winding N 3 is zero afer he urnoff of D R, he increasing curren in winding N makes curren i 2 in winding larger han magneizing curren. This excessive curren discharges he oupu capaciance of swich S D, as illusraed in Fig. 4 and Fig. 5(d). During his period, volage v 2 across winding of ransformer TR sars o increase. Afer 0780382706/04/$7.00 (C) 2004 IEEE
C D R C OSS I S IN (a) [T T ] 0 (e) [T T ] 4 5 (i) [T T ] 8 9 D I S IN R [T T ] 2 (f) [T T ] 5 6 (j) [T T ] 9 0 I IN R D (c) [T T ] 2 3 (g) [T T ] 6 7 (k) [T T ] 0 i RR S I S IN R D (d) [T T ] 3 4 (h) [T T ] 7 8 (l) [T T ] 2 Fig. 4. Topological sages during a swiching period of he proposed circui. he oupu capaciance of swich S D is fully discharged a =T 2, curren D coninues o flow hrough he aniparallel diode of swich S D, as shown in Fig. 4(c) and Fig. 5(i). To achieve ZS of S D, swich S D should be urned on while is aniparallel diode is conducing. To simplify he conrol circui iming diagram, he urnon of swich S D is synchronized wih he urnon of boos swich S. When he aniparallel diode of swich S D is conducing, volage v 2 across winding is equal o so ha induced volage v on winding N is N v = B = nb, (2) where i is required ha urns raio n=n / should be less han 0.5 for proper operaion of he proposed circui. 0780382706/04/$7.00 (C) 2004 IEEE
Since v is consan, volage applied across snubber inducor is also consan so ha curren increases linearly wih a slope of di B v B nb B = = = ( n). (3) d LS LS LS During he same period, magneizing inducance increases wih a slope given by d B =. (4) d L M As curren linearly increases, boos recifier curren linearly decreases a he same rae since he sum of and is equal o consan inpu curren, i.e., =. Therefore, in he proposed circui, he urnoff rae of he boos recifier d B = ( n) (5) d LS can be conrolled by he proper selecion of he inducance value of snubber inducor and urns raio n of ransformer TR. Typically, for oday s fasrecovery recifiers, he urnoff rae d /d should be kep around 00 A/µs. Wih seleced urnoff rae as such, he reverserecovery curren of he recifier and he relaed power losses and EMI problems are minimized. The opological sage in Fig. 4(c) ends a =T 3 when he forward curren in boos recifier D becomes zero. Unil =T 4, he reverserecovery curren of boos recifier D flows hrough snubber inducor. Afer =T 4, curren sars o discharge he oupu capaciance of boos swich S and charge he juncion capaciance of boos recifier D, as shown in Fig. 4(e). If he urns raio of ransformer TR is seleced so ha n<0.5, he energy sored in is sufficien o compleely discharge he oupu capaciance of boos swich S regardless of he load and line condiions. Once he capaciance is fully discharged a =T 5, curren coninues o flow hrough he aniparallel diode of boos swich S, as shown in Fig. 4(f) and Fig. 5(h). During he period when he volage across boos swich S is zero, volage v is applied in he negaive direcion across snubber inducor. Therefore, curren sars o decrease linearly a he rae given by di nb =, (6) d LS as illusraed in Fig. 5(g). The curren in auxiliary swich S also sars o decrease, whereas boosswich curren sars o increase from he negaive peak value, as shown in Figs. 5(g) and (h). To achieve ZS of boos swich S, i is necessary o urn on boos swich S before is curren becomes posiive a =T 6, i.e., during he period when curren sill flows hrough he aniparallel diode of swich S, as illusraed in Fig. 5(h). As shown in Fig. 5(g), curren coninues o decrease unil i reaches zero a =T 7. Shorly afer =T 7, auxiliary swich S is urned off o achieve ZCS. Afer swich S is urned off, he enire inpu curren flows hrough boos swich S. As a S S S D v SD v S v S D R v D T ONPFC (n) n n n T ONDCDC N n 2 2 = N 3 d conrolled = d reverserecovery charge (a) (e) resul, he fronend boos converer sage is compleely decoupled from he sandby flyback converer sage, as shown in Fig. 4(h). For he res of he swiching cycle, he flyback converer sage coninues o operae as a convenional flyback converer. Afer flybackconverer swich S D is urned off a =T 8, magneizing curren sar o charge he oupu capaciance of swich S D, as shown in Fig. 4(i). When volage v SD reaches n 2 a =T 9, diode D R sars o conduc, which forces he commuaion of he magneizing curren from swich S D o oupu diode D R, as shown in Fig. 4(j). A he same ime, he rese of he ransformer is iniiaed by applied oupu volage T S N N n 3 O n n = N n = L M (n) n 2 B L M (c) (d) (g) (h) (f) (i) (j) (k) (l) (m) T 0 T T T 3 5 T 6 T 7 T 8 T 9 T 0 T T 2 T 2 T 4 Fig. 5. Key waveforms of he proposed converer. 0780382706/04/$7.00 (C) 2004 IEEE
across winding N 3. During he rese ime of he ransformer, volages v SD across he flybackconverer swich is equal o ( /N 3 ), whereas he volage across auxiliary swich S is (N /N 3 ) due o he magneic coupling of windings N and, as illusraed in Figs. 5(d) and (e). Afer boos swich S is urned off a =T 0, volage across swich S sars increasing linearly because consan inpu curren sars charging he oupu capaciance of boos swich S, as shown in Fig. 4(k). The increasing boosswich volage causes an equal increase of volage v S across auxiliary swich S. When boosswich volage v S reaches a =T, boos diode D begins o conduc, as shown in Fig. 4(l). A he same ime, auxiliaryswich volage v S reaches is maximum value of (N /N 3 ). The circui says in he opological sage shown in Fig. 4(l) unil he nex swiching cycle is iniiaed a =T 2. In summary, he major feaure of he proposed circui in Fig. 2 is he sofswiching of all semiconducor devices. Specifically, boos swich S and flybackconverer swich S D are urned on wih ZS, whereas auxiliary swich S is urned off wih ZCS. In addiion, boos diode D is urned off wih a conrolled urnoff rae of is curren. 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 ZS of flybackconverer swich S D can only be achieved if inpu curren (which is being commuaed o winding N when auxiliary swich S is closed) is large enough o produce a negaive curren hrough primary winding and discharge he oupu capaciance of swich S D, as shown in Fig. 4. According o Fig. 4, o have a negaive curren flowing hrough winding afer =T, refleced curren in winding has o be greaer han magneizing curren. If his condiion is no me, swich S D operaes wih parial ZS. This mode of operaion ypically occurs in acdc applicaions when hey operae near he zero crossing of he line volage. Since he inpu curren is proporional o he line volage in PFC operaion, inpu curren is small near he zero crossing of he line volage. Due o he ZS of he boos swich and he flyback swich, he mos suiable implemenaion of he circui in Fig. 2 is wih he boos swich and he flyback swich consising of MOSFET (Meal Oxide Semiconducor Field Effec Transisor) devices. Similarly, due o he ZCS of auxiliary swich S, an IGBT (Insulaed Gae Bipolar Transisor) is suiable for he auxiliary swich. In he proposed circui, he volage sresses on swiches S, S D, and boos recifier D are idenical o he corresponding sresses in he convenional boos converer wihou a snubber. However, he volage sress of auxiliary swich S is v = n. (7) S(MAX) B O The conrol of he proposed circui is performed by wo independen conrollers ha are synchronized. Specifically, 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 flyback converer. Any conrol sraegy can be used o conrol hese wo volages, including muliloop conrol sraegies such as various currenmode conrol implemenaions. 3. Experimenal Resuls The performance of he proposed sofswiched converer was evaluaed on a 450W prooype circui ha was designed o operae from a universal acline inpu (90 RMS 264 RMS ) and deliver up o.2 A a 380 oupu. Moreover, he inegraed flyback converer delivers 26W sandby power a 2 oupu. Swiches S, S, and S D operae a 50 khz. Since he drain volage of boos swich S is clamped o bulk capacior C B, he peak volage sress on boos swich S is approximaely 380. The peak curren sress on swich S, which occurs a full load and low line, is approximaely GS GS [0 A/div] [250 /div] GS GSD [0 A/div] [250 /div] ZS ZCS (a) conrolled di/d ZS Fig. 6. Measured waveforms of he proposed circui a IN =90, =380, =2, P DC = 450 W, P AUX =26 W. Time base: µs/div. 0780382706/04/$7.00 (C) 2004 IEEE
GS GS [0 A/div] [250 /div] GS GSD [0 A/div] (a) ZCS of auxiliary swichs ZS of boos swich S conrolled di/d for boos diode D ZS of flyback swich S D Efficiency [%] 94 93 92 9 90 89 88 PROPOSED SOFT SWITCHING CONERTER HARD SWITCHING CONERTER 50 200 250 300 350 400 450 Oupu Power [W] Boos swich S is hermally unsable Fig. 8. Measured efficiency of he 50 khz, 450W experimenal converer wih (dashed line) hard swiching and (solid line) sof swiching a IN =90 AC and =380 as funcions represening oupu power of PFC boos converer, where he inegraed flyback converer supplies 26 W consan sandby power a 2 oupu volage. [250 /div] Fig. 7. Expanded waveforms shown in Fig. 6 a IN =90, =380, =2, P DC = 450 W, P AUX =26 W. Time base: 0.5 µs /div. 7.8 A. Therefore, an SPP20N60C2 MOSFET ( DSS = 600, I D25 = 20 A, R DS = 0.9 Ω) from Infineon was used for he boos swich. The maximum drain volage of flyback swich S D is (MAX) = n 2 = 380(52/4)2, as shown in Fig. 5(d). As a resul, he peak volage sress on flyback swich S D is approximaely 536. The peak curren sress on flyback swich S D is approximaely.3 A, herefore, an SPAN80C3 MOSFET ( DSS = 800, I D25 = A, R DS = 0.45 Ω) from Infineon was used for he flyback swich. Moreover, a high speed HGTG2N60A4 IGBT ( RRM = 600, I F = 2 A) from Fairchild was used as auxiliary swich S since is maximum drain volage is (MAX) = n = 380(2/4)2 = 46, as shown in Eq. (7). Since boos diodes D mus block he bulk volage and mus conduc he peak inpu curren, which is approximaely 7.8 A, an RHRP560 diode ( RRM = 600, I FAM = 5 A) from Fairchild was used as boos diode D. An RHRP860 diode ( RRM = 600, I FAM = 8 A) was used as diode D. A 6CTQ060 diode ( RRM = 60, I FAM = 6 A) from IRF was used as oupu diode D R. To reduce he conducion losses of he swiches and oupu diodes, devices which have higher curren raings han he designed maximum curren were seleced. To obain he desired inducance of boos inducor L B of approximaely 250 µh a full load, he boos inducor was buil using a oroidal core (MS30060) from Arnold and 7 urns of magne wire (AWG #9). Exernal snubber inducor was conneced in series wih winding N of ransformer TR, as shown in Fig. 2. The required inducance is approximaely 2.4 µh a full load. Snubber inducor was buil using a oroidal core (A89043) from Arnold and 8 urns of magne wire (AWG #9). Transformer TR was buil using a pair of ferrie cores (Philips, ER353F3) wih an air gap (4 mils). Three magne wires (N =2 urns: =52 urns: N 3 =4 urns) were used o obain he desired magneizing inducance. A high volage aluminum capacior (470 µf, 450 DC) was used for bulk capacior C B o mee he holdup ime requiremen. A low volage aluminum capacior (220 µf, 6 DC) was used for oupu capacior C F. Figures 6 and 7 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. 6 and 7, swiches S and S D are urned on wih ZS since heir volages and fall o zero before gaedrive signals GS and GSD become high. Moreover, auxiliary swich S achieves sofswiching urn off because swich curren becomes zero before auxiliary swich S is 0780382706/04/$7.00 (C) 2004 IEEE
urned off. Also, i should be noed ha he slope of recifier curren is approximaely di/d = 80 A/µs during he period when boos diode D is urned off. The recifiercurren slope is conrolled by snubber inducance, as indicaed in Figs. 6 and 7. Wih his di/d rae, peak reverserecovery curren I RR is reduced o approximaely 2 A. Figure 8 shows he measured efficiencies of he experimenal converer wih (solid lines) and wihou (dashed lines) he acive snubber circui as funcions represening oupu power of PFC fron end. As can be seen in Fig. 8, 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 acive snubber improves he efficiency by approximaely 5% a 450 W, which ranslaes ino approximaely 50% reducion of losses. Since all swiches operae wih zerovolage or zerocurren swiching, he recifier reduces swiching losses and hereby improves he specral performance of he recifier for less EMI. 4. Summary A new PFC boos converer wih an inegraed sandby flyback converer ha can achieve sofswiching of all semiconducor devices in he power sages has been inroduced. By using a single magneic device which is muually shared by he PFC boos converer and he sandby flyback converer, boos swich S and flyback swich S D are urned on wih ZS, auxiliary swich S is urned off wih ZCS, and boos diode D is urned off sofly using a conrolled di/d rae. As a resul, he urnon swiching losses in he boos and flyback swiches, he urnoff swiching loss in he auxiliary swich, and reverserecoveryrelaed losses in he boos diode are eliminaed, which maximizes he conversion efficiency. The performance of he proposed converer was verified on a 50kHz, 450W prooype circui ha was designed o operae from a universal acline inpu and deliver up o.2 A a 380 oupu volage by he PFC fron end and 2.2 A a 2 oupu volage by he inegraed flyback converer. The proposed echnique improves he efficiency by approximaely 5% a 450 W. References [] R. Srei, D. Tollik, High efficiency elecom recifier using a novel sofswiched boosbased inpu curren shaper,'' Inernaional Telecommunicaion Energy Conf. (INTELEC) Proc., pp. 720726, Oc. 99. [2] G. Hua, C.S. Leu, F.C. Lee, Novel zerovolageransiion PWM converers,'' IEEE Power Elecronics Specialiss' Conf. (PESC) Rec., pp. 556, June 992. [3] D.C. Marins, F.J.M. de Seixas, J.A. Brilhane, I. Barbi, A family of dcodc PWM converers using a new ZS commuaion cell, IEEE Power Elecronics Specialiss' Conf. (PESC) Rec., pp. 524530, June 993. [4] J. Basse, New, zero volage swiching, high frequency boos converer opology for power facor correcion,'' Inernaional Telecommunicaion Energy Conf. (INTELEC) Proc., pp. 83 820, Oc. 995. [5] C.M.C. Duare, I. Barbi, An improved family of ZSPWM aciveclamping dcodc converers, IEEE Power Elecronics Specialiss' Conf. (PESC) Rec., pp. 669675, 998. [6] M.M. Jovanović, A echnique for reducing recifier reverserecoveryrelaed losses in highvolage, highpower boos converers, IEEE Applied Power Elecronics (APEC) Conf. Proc., pp. 000007, Mar. 997. [7] Y. Jang, M.M. Jovanović, A new, sofswiched, highpowerfacor boos converer wih IGBTs, Inernaional Telecommunicaion Energy Conf. (INTELEC) Proc., Paper 8 3, 999. [8] J.P. Gegner, C.Q. Lee, Zerovolageransiion converers using a simple magneic feedback echnique, IEEE Applied Power Elecronics (APEC) Conf. Proc., pp. 590596, 994. [9] N.P. Filho,.J. Farias, L.C. de Freias, A novel family of dcdc converers using he selfresonance principle, IEEE Power Elecronics Specialiss' Conf. (PESC) Rec., pp. 38539, 994. [0] G. Moschopoulos, P. Jain, G. Joós, A novel zerovolage swiched PWM boos converer, IEEE Power Elecronics Specialiss' Conf. (PESC) Rec., pp. 694700, 995. [] J.H. Kim, D.Y. Lee, H.S. Choi, B.H. Cho, High performance boos PFP (power facor preregulaor) wih an improved ZT (Zero olage Transiion) converer, IEEE Applied Power Elecronics (APEC) Conf. Proc., pp. 337342, 200. [2] I.Q. Lee, D.Y. Lee, B.H. Cho, High performance boos preregulaor wih improved zerovolageransiion (ZT) converer, IEEE Power Elecronics Specialiss' Conf. (PESC) Rec., pp. 69696, 2003. [3] C.A. Canesin, I. Barbi, Comparison of experimenal loses among six differen opologies for a.6kw boos converer, using IGBT s, IEEE Power Elecronics Specialiss' Conf. (PESC) Rec., pp. 26527, 995. [4] D.M. Xu, C. Yang, L. Ma, C. Qiao, Z. Qian, X. He, A novel singlephase aciveclamped pfc converer, IEEE Applied Power Elecronics (APEC) Conf. Proc., pp. 26627, 997. [5] F.T. Wakabayashi, M.J. Bonao, C.A. Canesin, Novel highpower facor ZCSPWM preregulaors, IEEE Trans. Indusrial Elecronics, vol. 48, no. 2, pp. 322333, Apr. 200. [6] H.S. Choi, B.H. Cho, Zerocurrenswiching (ZCS) power facor preregulaor (pfp) wih reduced conducion losses, IEEE Applied Power Elecronics (APEC) Conf. Proc., pp. 962 967, 2002. 0780382706/04/$7.00 (C) 2004 IEEE