A New ZVS-PWM Full-Bridge Converter

Transcription

1 New ZV-PW Full-ridge onverer Yungaek Jang and ilan. Jovanović Dela Producs orporaion Power Elecronics Laboraory P.O. ox 73, 50 Davis Dr. Research Triangle Park, N 7709, U... Yu-ing hang DELT Elecronics Inc. 3, Tung Yuan Road, hungli Taiwan, R.O.. bsrac full-bridge converer which employs a coupled inducor o achieve zero-volage swiching of he primary swiches in he enire line and load range is described. ecause he coupled inducor does no appear as a series inducance in he load curren pah, i does no cause a loss of duy cycle or severe volage ringing across he oupu recifier. The operaion and performance of he proposed converer is verified on a 670- W prooype.. Inroducion The full-bridge (F) zero-volage-swiching (ZV) PW converer shown in Fig. is he mos widely used sofswiched circui in high-power applicaions, []-[4]. This consan-frequency converer feaures ZV of he primary swiches wih relaively small circulaing energy. The conrol of he oupu volage a consan frequency is achieved by he phase-shif echnique. In his echnique he swiching ransiion of swiches in he - leg of he bridge is delayed, i.e., phase shifed, wih respec o he swiching ransiion of corresponding swiches in he 3-4 leg. Wih no phase-shif beween he legs of he bridge, no volage is applied across he primary of he ransformer and, consequenly, he oupu volage is zero. On he oher hand, if he phase shif is 80, he maximum vol-second produc is applied across he primary winding, which produces he maximum oupu volage. In he circui in Fig., ZV of he lagging-leg swiches and is achieved primarily by he energy sored in oupu filer inducor L F. ince he inducance of inducor L F is relaively large, he energy sored in inducor L F is sufficien o compleely discharge oupu parasiic capaciance and of swiches and and o achieve ZV even a very ligh load currens. However, he discharge of parasiic capaciances 3 and 4 of leading-leg swiches 3 and 4 is done by he energy sored in leakage inducance L LK of he ransformer because during he swiching of 3 or 4 he ransformer primary is shored by he simulaneous conducion of recifiers D R and D R which carry he oupu filer inducor curren. ince leakage inducance L LK is small, he energy sored in L LK is also small so ha ZV of 3 and 4 canno be achieved even a relaively high oupu currens. The ZV range of he leading-leg swiches can be exended o lower load currens by inenionally increasing he leakage inducance of he ransformer and/or by adding a large exernal inducance in series wih he primary of he ransformer. If properly sized, he exernal inducance can sore enough energy o achieve Fig V I P V D D L LK DTs/ V - N P DTs/ L LK TR Deff Ts/ Ts N N D R D R V O 3 4 V - L F onvenional full-bridge ZV converer and is key waveforms. F D 3 D 4 R L 3 4 V o

2 ZV of he leading-leg swiches even a low currens. However a full load a large exernal inducance also sores excessive energy ha produces large circulaing currens, which adversely affecs he sress of he semiconducor componens as well as he conversion efficiency. One of he major limiaions of he circui in Fig. is a loss of duy cycle which is indicaed by gray color. Generally, a large leakage and/or exernal inducance exends he ime ha is needed for he primary curren o change direcion from negaive o posiive, and vice verse, as shown in Fig.. This exended commuaion ime resuls in a loss of duy cycle on he secondary of he ransformer, which decreases he conversion efficiency. Namely, o provide full power a he oupu, he secondary-side duy-cycle loss D mus be compensaed by reducing he urns raio of he ransformer. Wih a smaller ransformer s urns raio, he refleced oupu curren ino he primary is increased, which increases he primary-side conducion losses. In addiion, since a smaller urns raio of he ransformer also increases he volage sress on he secondary-side recifiers, he recifiers wih a higher volage raing ha ypically have higher conducion losses may be required. noher major limiaion of he convenional F ZV converer in Fig. is a severe parasiic ringing a he secondary side of he ransformer caused by he resonance of he recifier s juncion capaciance wih he leakage inducance of he ransformer and/or he exernal inducance during he urn-off of a recifier. To conrol he ringing, a lossy snubber circui is required on he secondary side which may significanly lower he conversion efficiency of he circui. The ZV range of he leading-leg swiches in he F ZV- PW converer in Fig. can be exended o lower load currens wihou a significan increase of he circulaing energy and loss of duy cycle by using a saurable exernal inducor insead of a linear inducor [5]. If he saurable inducor is designed so ha i sauraes a higher load currens, he inducor will no sore excessive energy a high loads. he same ime, a low load currens, when he inducor is no sauraed, i will have sufficienly high inducance o sore enough energy o provide ZV of he leading-leg swiches even a very ligh loads. While i was demonsraed ha a properly designed saurable inducor can improve he performance of he F ZV-PW converer, he circui requires a relaively large-size magneic core o implemen he inducor, which increases he cos of he circui. Generally, a larger core is required o eliminae he hermal problem ha is creaed by excessive core loss, since he saurable core is placed in he primary circui and is flux swings beween he posiive and negaive sauraion levels. In his paper, a new isolaed, consan-frequency, F ZV converer which employs a coupled inducor on he primary side o achieve ZV in a wide range of load curren and inpu volage wih reduced circulaing energy and conducion losses is described. ecause in he proposed circui he energy required o creae ZV condiions does no need o be sored in he leakage inducance, he leakage inducance of he ransformer can be minimized. This virually eliminaes he duy cycle loss and also significanly reduces he energy of he secondary-side ringing caused by a resonance beween he leakage inducance and juncion capaciance of he recifier. s a resul, he proposed circui exhibis an increased conversion efficiency.. New F ZV onverer wih oupled Inducor Figure shows a circui diagram of he proposed isolaed, dc/dc F ZV converer ha employs a coupled inducor on he primary side o exend he ZV range of he primary swiches wih a minimum circulaing energy and conducion loss. The primary side of he converer consiss of wo bridge legs - and 3-4 conneced hrough wo capaciors and o he series connecion of coupled inducor L and ransformer TR. The wo primary side capaciors are used o preven he sauraion of he coupled inducor and ransformer cores by blocking he flow of any dc curren hrough L and TR. Generally hese capaciors are seleced large enough so ha heir volages are approximaely consan during a swiching cycle. To regulae he oupu volage agains load and/or inpu volage changes a a consan swiching frequency, he circui requires a phaseshif conrol. I should be noed ha in Fig., he oupu side of he converer is implemened wih a full-wave recifier wih a apped secondary. However, any oher implemenaion of he secondary side recificaion sage is possible. Fig.. D D N N P TR N L N N D R D R Proposed full-bridge ZV converer wih coupled inducor. 3 4 L F F D 3 D 4 R L 3 4 V o

3 To faciliae he explanaion of operaion of he circui in Fig., Fig. 3 shows is simplified circui diagram. In he simplified circui i is assumed ha he inducance of oupu filer L F is large enough so ha during a swiching cycle he oupu filer can be modeled as a consan curren source wih a magniude equal o oupu curren. lso, i is assumed ha he capaciance of blocking capaciors and is large enough so ha he capaciors can be modeled as consan volage sources. ecause he average volages of he coupled inducor windings and he ransformer windings during a swiching cycle are zero and he pair of swiches in each bridge leg operae wih 50% duy cycle, he magniude of volage sources V and V in Fig. 3 are equal o, i.e., V =V =. To furher simplify he analysis, i is also assumed ha he resisance of conducing semiconducor swiches is zero, whereas he resisance of he non-conducing swiches is infinie. In addiion, he leakage inducances of coupled inducor L and ransformer TR, as well as he magneizing inducance of ransformer TR are negleced because heir effec on he operaion of he converer is negligible. agneizing inducance of coupled inducor L and oupu capaciances 4 of primary swiches are no negleced in his analysis, since hey play a major roll in he operaion of he circui. In Fig. 3, coupled inducor L is modeled as an ideal ransformer wih urns raio n L = and wih parallel magneizing inducance L. The number of urns of each of he windings of L is N. Fig. 3. V i V P - V TR L - D R D R V N N L n L n TR i i implified circui diagram of proposed converer showing reference direcions of currens and volages. V - Finally, o furher faciliae he analysis, Fig. 4 shows he opological sages of he converer during a swiching cycle, whereas Fig. 5 shows he key waveforms. s shown in Fig. 5, a ime =T O, swich in - leg and swich 3 in 3-4 leg are closed and currens and flow hrough he corresponding swich, blocking capacior, and winding of coupled inducor L ino he primary of ransformer TR, as can be seen from he equivalen circui in Fig. 4(a). he same ime, oupu curren flows hrough he upper secondary of he ransformer so ha primary curren = = /n TR, where n TR =N P /N is he urns raio of he ransformer, N P is he number of primary-winding urns, and N is he number of secondary-winding urns. From Fig. 4(a) i can be seen ha during his opological sage volage v mus be zero since volage sources V and V are conneced in opposiion hrough closed swiches and 3. Furhermore, because of he coupled inducor winding orienaion (do posiions in Fig. 4(a)), v =v v =0 can only be mainained if he volages across he coupled inducor windings are zero, i.e. only if v =v =0. Therefore, since in his opological sage he volage poenial of poins,, and in Fig. 4(a) mus be he same, primary volage v P = -=, as shown in Fig. 5(j). I also should be noed ha in his opological sage, magneizing curren of he coupled inducor i is consan because v =v =0, i.e., he volage across he windings of L is zero, as illusraed in Fig. 5(i). In addiion, because he urns raio of he windings of L is uniy (n L =), curren flowing hrough winding is equal o curren flowing hrough winding, i.e. =. Finally, from Fig. 3, i can be seen ha = i =( i )/ and =( -i )/. When a =T swich is urned off, curren is divered from he ransisor of swich o is oupu capaciance, as shown in Fig. 4(b). In his opological sage, curren charges capacior and discharges capacior a he same rae since he sum of he capacior volages is equal o consan volage, as illusraed in Figs. 5(e) and (f). s a resul, he poenial of poin sars decreasing causing a decrease of volages v and v P. Namely, volage v decreases from zero oward negaive, whereas volage v P decreases from oward zero, as illusraed in Figs. 5(i) and (j). fer capacior is fully discharged, i.e., when volage v reaches zero, curren sars flowing hrough aniparallel diode D of swich, as shown in Fig. 4(c). Due o negaive volage applied across winding of coupled inducor L, is magneizing curren i decreases wih a rae of /(L ). ince during his opological sage primary curren does no change, i.e. i says consan a /n TR, curren =( i )/ decreases while curren =( - i )/ increases a he same rae. To achieve zero-volage urnon of swich, i is necessary o urn-on swich while is aniparallel diode D is conducing. In Fig. 5, swich is urned on immediaely afer volage v has fallen o zero.

4 3 i 4 3 i 4 3 i i 4 3 i 4 (a) [T 0 - T ] (d) [T 3 - T 4 ] (g) [T 6 - T 7 ] (j) [T 9 - T 0] 3 i 4 i i 4 3 i i (b) [T - T ] (e) [T 4 - T 5 ] (h) [T 7 - T 8 ] (k) [T 0 - T ] i i 4 3 i 4 3 i 4 3 i 4 (c) [T - T 3 ] (f) [T 5 - T 6 ] (i) [T 8 - T 9 ] (l) [T - T ] Fig. 4. Topological sages of proposed converer power sage. agneizing curren i reaches zero a =T 3 and i coninues o increase in he negaive direcion, as shown in Fig. 4(d). s a resul, curren coninues o decrease, whereas curren coninues o increase, as seen from waveforms (m) and (n) in Fig. 5. =T 4 swich 3 is urned off so ha curren is divered from he ransisor of swich 3 o is oupu capaciance 3, as shown in Fig. 4(e). ecause during his ransiion 3 is charging, while 4 is discharging a he same rae, volage v 3 increases from zero oward, whereas volage v 4 decreases from o zero, as illusraed in Figs. 5(g) and (h). ince during his opological sage he poenial of poin decreases from oward zero, while poenial of poin is consan a, volage v increases from oward zero. he same ime, primary volage v P increases in he negaive direcion from zero o forcing he commuaion of he load curren from he upper secondary o he lower secondary. If he inerconnec inducances and he leakage inducances of ransformer TR and coupled inducor L were zero, his commuaion would be insananeous. However, due o he ineviable exisence of various parasiic inducances on boh he primary and secondary side, he commuaion of he load curren from one secondary o he oher when he primary volage changes sign is no insananeous, as shown in Fig. 5. In fac, when primary volage v P becomes negaive, he load curren is carried by boh secondary windings, as shown in Fig. 4(e), i.e., he ransformer windings are effecively shored. ecause during his commuaion period, curren in he upper secondary i decreases, primary curren =(i -i )/n TR changes direcion a he momen he curren in he lower secondary i becomes larger han curren in he upper secondary i. =T 6 he load curren complees he commuaion from he upper o he lower secondary, as shown in Fig. 4(g). During he opological sage in Fig. 4(g), currens i,,, and are consan and flow in he negaive direcion. To achieve ZV of swich 4, i is necessary o urn on swich 4 while curren is posiive, i.e. while i sill flows hrough aniparallel diode

5 3 4 v v v 3 v 4 v v P i v i = I n TR = i = ( i )/ i = (i - i )/ P Ts Duy ycle Loss DTs/ L _ i = I ntr DTs/ DTs/ Deff Ts/ T 0 T T T 3 T 4 T 5 T 6 T T 7 8 T 9 T 0 T T T 3 (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) (m) (n) (o) 4 is urned off, which iniiaes swiching ransiion in he 3-4 leg. ecause during his ransiion capacior 3 is discharging and capacior 4 is charging, poenial of poin is increasing from o. ince during his ime poenial of poin is consan a, volage v is decreasing from oward zero, while primary volage v P is increasing from zero oward. s a resul, posiive primary volage forces he commuaion of he load curren from he lower secondary o he upper secondary, as shown in Fig. 4(k). =T capaciance of swich 3 is fully discharged and curren sars flowing hrough aniparallel diode D 3 of swich 3, as shown in Fig. 4(l). To achieve ZV, swich 3 is urned on shorly afer D 3 sars conducing. During he opological sage in Fig. 4(l), primary curren, curren, and curren coninue o increase from negaive values oward posiive, as seen from waveforms in Figs. 5(k), (m), and (n). Finally, a =T, he commuaion of he 3-4 leg is compleed so ha he circui eners he same opological sage as shown in Fig. 4(a), awaiing he nex swiching cycle o be iniiaed by he conroller. I should be noed ha in he proposed circui, he value of he magneizing inducance of coupled inducor L has no effec on commuaion ime of he primary curren from one direcion o he oher. This commuaion ime is proporional o he leakage inducance of he ransformer. Therefore, o minimize he secondary-side duy-cycle loss and opimize he performance of he circui, i is necessary o minimize he leakage inducance of he ransformer. The minimizaion of he leakage inducance also minimizes he secondary-side parasiic-ringing energy, which furher improves he circui performance. Fig. 5. Key waveforms of proposed converer power sage. D 4 of swich 4. In Fig. 5, swich 4 is urned on immediaely afer =T 5, i.e., immediaely afer volage v 4 falls o zero. The second half of a swiching cycle sars a =T 7 when swich is urned off, which iniiaes he charging of capaciance of swich and discharging of capaciance of swich, as shown in Fig. 4(h). During his swiching ransiion volage v increases from zero oward, while primary volage v P increases from o zero. This opological sage ends a =T 8 when volage v across swich reaches zero and aniparallel diode D of swich sars conducing curren, as shown in Fig. 4(i). To achieve ZV of swich, swich needs o be urned on while diode D is conducing. In Fig. 5, swich is urned on immediaely afer volage v has fallen o zero. ecause afer swich is urned off volage v sars increasing, magneizing curren i sars increasing as well, as can be seen from Fig. 5(l). From insan =T 8, his increase is linear since consan volage v =v /= is applied across magneizing inducance L. =T 9, curren i becomes posiive, as shown in boh Fig. 4(j) and Fig. 5(l). Finally, a =T 0, swich 3. Design Guidelines s can be seen from he waveforms in Fig. 5, he commuaion of he swiches in he - leg is iniiaed when curren = i =( i )/ is maximum, i.e. when =( /n TR I )/. lso, he commuaion of he swiches in he 3-4 leg is iniiaed when curren =( -i )/ is maximum, i.e., when =( /n TR I )/. Therefore, in he proposed circui, all primary swiches are commuaed wih he same magniude curren. However, he charging and discharging of he capaciances of swiches and is done by he sum of he energy sored in he oupu filer inducor, which is proporional o ( /n TR ), and he energy sored in he magneizing inducance of coupled inducor L, which is proporional o (I ). On he oher hand, he charging and discharging of he capaciances of swiches 3 and 4 are done by he sum of he energy sored in he leakage inducance of he ransformer and he energy sored in he magneizing inducance of coupled inducor L. Therefore, swiches in he - leg can achieve ZV in a wide range of inpu volage and load curren even wihou assisance from he energy sored in magneizing inducance L of coupled inducor L since pleny of energy is available from filer

6 inducor L F. However, ZV of he swiches in he 3-4 leg is enirely dependen on he energy sored in he magneizing inducance of coupled inducor L since, for opimal performance, i is desirable o minimize he leakage inducance of he ransformer so ha he secondary-side duycycle loss and he energy of he secondary-side parasiic ringing is also minimized. Generally, o achieve ZV of all bridge swiches in he enire inpu-volage and load range, i is necessary o saisfy L VIN I VIN LVIN TR, () where = 3 = 4 is he capaciance across primary swiches 3 and 4, L is he inerwinding capaciance of coupled inducor L, and TR is he capaciance seen across he primary of ransformer TR ha includes inerwinding capaiance of he ransformer and any refleced capaciance of he secondary-side circui. If capaciances L and TR are negleced, Eq. () simplifies o L I V. () s can be seen from Eq. (), if he value of inducor L is seleced so ha ZV is achieved a no load and maximum inpu volage (max), ZV is achieved in he enire load and inpu-volage range. The value of inducor L required o achieve ZV a no load can be calculaed by observing he waveform during ime inerval T 8 -T 0 in Fig. 5. agneizing curren i changes linearly from maximum negaive value I - =I o maximum posiive value I =I, i.e., i changes for I, due o a posiive volage of induced across he winding of inducor L. ince according o Fig. 5, he ime inerval T 8 - T 0 is approximaely equal o (-D)T /, where D is duy cycle and T is a swiching period, I can be calculaed from VIN I = L, (3) T ( D) as ( D)VIN I =, (4) 8Lf where f =/T is he swiching frequency. ince a no load D 0 because he wo bridge legs mus be ou of phase o reduce vol-sec produc across he primary winding, he ZV condiion a no load and high line from Eqs. () and (4) is VIN(max) L VIN(max) 8Lf. (5) Finally, from Eq.(5), he value of L required o mainain ZV a no load and high line is L. (6) 8f s can be seen from Fig. 3, curren i flowing hrough magneizing inducance L inroduces a curren asymmery in he wo bridge legs. Namely, because of he coupling of IN windings and, curren flowing hrough winding is equal o curren flowing hrough winding so ha = I. Therefore, in he proposed circui, leg - always carries a higher curren han he leg 3-4, he difference being magneizing curren i. To simulaneously achieve ZV a no load and minimize he bridge conducion loss in he proposed circui in Fig., i is necessary o selec he maximum magneizing inducance L deermined from Eq. (6). Furhermore, if for such a seleced magneizing inducance, curren in he 3-4 leg is significanly lower han curren in he - leg, differen size swiches can be seleced for he wo legs, which may reduce he cos of he implemenaion wihou sacrificing he circui performance. Finally, i should be noed ha o achieve maximum efficiency improvemen, he urns raio of he ransformer mus be maximized. In fac, since he duy-cycle loss in he converer in Fig. is negligible due o he minimized leakage inducance of he ransformer, he converer can be designed wih a larger urns raio compared o a converer ha uses he leakage inducance and/or exernal inducance o exend he ZV range. oreover, he minimized leakage inducance grealy reduces he secondary side ringing beween he leakage inducance of he ransformer and he juncion capaciance of he recifier so ha any residual parasiic ringing can be damped by a small snubber circui as, for example, he RD-snubber circui shown in Fig. 6. The conrol of he circui in Fig. is he same as he conrol of any oher consan frequency F ZV converer. In fac, any of he inegraed phase-shif conrollers available on he marke can be used implemen he conrol of he proposed circui. However, i should be noed ha in he circui in Fig. he maximum oupu volage is obained when he bridge legs are operaed in phase, which is he opposie from he behavior of he convenional F ZV converer shown in Fig. ha achieves he maximum oupu volage when he bridge legs are swiched ou of phase. This difference in he conrol characerisic of he converer has a minor effec on he conrol-loop design since a simple conrol-signal inversion in he volage conrol loop circumvens he problem. I also should be noed ha he proposed circui in Fig. can be implemened wih any ype of secondary-side recifier. pecifically, i can be also implemened wih full-wave, fullbridge recifier, or curren-doubler recifier. 4. Experimenal Resuls The performance of he proposed circui was verified on a 670-W experimenal prooype operaing a khz. The experimenal converer was designed o operae from 400-V dc inpu and deliver 4 from a 48-V oupu. The componen values of he experimenal circui are shown in Fig. 6. The phase-shif conrol circui was implemened using he U3875 conroller. For performance comparison purposes, an experimenal prooype of he convenional F

7 400 V IRFP460.uF/50V IRFP460 L 0T:0T 45 uh 3 IRFP840.uF/50V 4 IRFP840 V [00 V/div] [5 /div] Duy ycle Loss V 4 [500 V/div] TTH6003W D R Y6 L F V [500 V/div] TR 5T:5T:5T V 0.uF /50V.k/3W 48V F 000uF/80V (a) D R TTH6003W Fig. 6. Experimenal 670 W, proposed converer power sage wih coupled inducor. V [00 V/div] [5 /div] ZV converer shown in Fig. was also buil. The convenional F ZV converer was designed wih an exernal inducance of 8 µh in series wih he primary winding of he ransformer (8T:5T:5T) o achieve ZV over he load range from 50% o 00%. Figures 7(a) and 7(b) show he oscillograms of key waveforms of he convenional F ZV converer and he proposed F ZV converer, respecively. s can be seen from Fig. 7(a), in he convenional F ZV converer he parasiic ringing caused by he exernal leakage inducance wih he recifier's juncion capaciance is severe even wih a snubber circui which dissipaes approximaely 0 W. oreover, he duy cycle loss is approximaely 0.5 µsec which is more han 8% of he secondary side duy cycle. s can be seen from he corresponding waveforms in Fig. 7(b), he proposed converer has a very small duy cycle loss (< 0. µsec), as well as a very much reduced parasiic ringing because of a minimized leakage inducance of he ransformer ha is less han µh. Figure 8 shows he measured efficiencies of he convenional F ZV converer and he proposed F ZV converer as funcions of he oupu power. s can be seen from Fig. 8, he proposed converer shows a conversion efficiency improvemen in he enire measured power range V [500 V/div] V 4 [500 V/div] Fig. 7. (b) Duy ycle Loss easured key waveforms a P O = 670 W: (a) convenional F ZV converer; (b) proposed F ZV converer. From op o boom: secondary volage V [00V/div]; primary curren [5 /div]; drain-o-source volage V of [500 V/div]; draino-source volage V 4 of 4 [500 V/div]. Time base: µs/div. from 50 W o 670 W. Generally, he efficiency improvemen is more pronounced a ligh loads where he convenional F ZV converer operaes wih hard swiching. pecifically, a ligh loads, he efficiency improvemen is more han 0%. full load, he proposed circui shows a efficiency improvemen of approximaely 3%, which ranslaes ino approximaely 30% reducion of he losses.

8 Efficiency Proposed ZV Full-ridge onverer wih oupled Inducor onvenional ZV Full-ridge onverer Inpu Volage = 400 V Oupu Volage = 48 V wiching Frequency = khz Noice The circui shown in Fig. and is variaions are proeced by U.. Paens 6,356,46 and 6,39,90 as well as relaed foreign paens. References [] O.D. Peerson, D.. Divan, Pseudo-Resonan Full ridge D/D onverer, IEEE Power Elecronics pecialiss onf. Rec., pp , 987. [] R.. Fisher, K.D.T. Ngo, and.h. Kuo, 500 khz, 50 W D- D onverer wih uliple Oupus onrolled by Phase-hifed PW and agneic mplifiers, High Frequency Power onversion Proceedings, pp. 00-0, ay 988. [3] L.H. weene,.. Wrigh, and.f. chlech, kw, 500 khz Fron-end onverer for a Disribued Power upply ysem, IEEE pplied Power Elecronics onf. (PE) Proc., pp , 989. [4] J. abae, V. Vlakovic, R.. Ridley, F.. Lee, and. H. ho, Design onsideraions for High-Volage High-Power Full-ridge Zero-Volage-swiched PW onverer, IEEE pplied Power Elecronics onf. (PE) Proc., pp , 990. [5] G. Hua, F.. Lee,.. Jovanović, n Improved Full-ridge Zero- Volage-wiched PW onverer Using a aurable Inducor, IEEE Power Elecronics pecialiss onf. Rec., pp , Fig Oupu Power easured efficiencies of convenional F ZV converer and proposed F ZV converer as funcions of oupu power. 5. onclusion In his paper, a new isolaed, consan-frequency, F ZV converer which employs a coupled inducor on he primary side o achieve ZV in a wide range of load curren and inpu volage wih reduced circulaing energy and conducion losses has been described. ince his coupled inducor does no appear as a series inducance in he load curren pah, i does no cause a loss of duy cycle or severe volage ringing across he oupu recifiers. The operaion and performance of he proposed circui was verified on a 670-W (48-V/4-) prooype. The measured efficiency improvemen of he proposed circui wih respec o he convenional F ZV converer was 3% a full load and more han 0% a ligh loads. The abiliy of he proposed circui o mainain a high efficiency a ligh loads makes he proposed converer paricularly aracive in applicaions where a number of power converers conneced in parallel share he load curren so ha each converer operaes wih a load which is a fracion of is full load.