Scienific Journal of Riga Technical Universiy Power and Elecrical Engineering Curren-fed Sep-up DC/DC Converer for Fuel Cell Applicaions wih Acive Overvolage Clamping Aleksandrs Andreiciks, Riga Technical Universiy, Ingars Seiks, Riga Technical Universiy, Oskars Krievs, Riga Technical Universiy Absrac. In order o use hydrogen fuel cells in domesic applicaions eiher as main power supply or backup source, heir low DC oupu volage has o be mached o he level and frequency of he uiliy grid AC volage. Such power converer sysems usually consis of a DC-DC converer and a DC-AC inverer. A double inducor sep-up push-pull converer is invesigaed in his paper, presening simulaion and experimenal resuls for passive and acive overvolage clamping. The prooype of he invesigaed converer is elaboraed for 1200 W power o mach he raed power of he proon exchange membrane (PEM) fuel cell locaed in hydrogen fuel cell research laboraory. Keywords: fuel cell sysem, high frequency power converers, ZVS converers I. INTRODUCTION The research of he hydrogen energy has gained a growing ineres in he recen years. The hydrogen fuel cells are fully ecological, aking ino accoun ha hea and waer are he only by-producs, which are excreed ino he environmen [1]. In order o uilize he elecrical energy, produced by hydrogen fuel cells, characerized by slow dynamic response, low oupu volage and large volage variaions, various saic power elecronic converers are researched widely. The fuel cells used as main power supply or backup source in domesic applicaion need o be conneced o he grid. Such power converer sysems usually consis of a DC-DC converer and a DC-AC converer. Because of he comparaively high inpu and oupu volage difference mos frequenly converers wih high frequency ransformer are acknowledged as he opimal soluion for he DC-DC sage [1]-[4]. There are many known ransformer isolaed dc-dc converer opologies, which could be suiable o perform he necessary volage boos from he fuel cell volage level o he inverer dc link volage. Such converers are he full-bridge, half-bridge, he flyback, he forward and he push-pull basic opologies, as well as a number of heir derived opologies [5], [6] These can be divided ino wo groups volage fed converers and curren fed converers. In his paper a curren fed opology is preferred, since i is characerized by low inpu curren ripple, which is more appropriae for proon exchange membrane fuel cells modules [3],[4]. A double inducor boos push-pull converer invesigaed in he paper, presening simulaion and experimenal resuls. The converer is elaboraed for 1200 W power, since such is he raed power of he proon exchange membrane (PEM) fuel cell in hydrogen fuel cell research laboraory of Riga Technical Universiy. II. THE SELECTED CONVERTER TOPOLOGY Considering he necessiy of high volage boosing funcion wih low inpu curren ripple, he mos appropriae converers are curren fed full-bridge and push-pull configuraions. Since he converer efficiency can be considerably improved by reducing he coun of he primary swiches and implemening a ransformer of a simple srucure (wihou spli windings), a double inducor push-pull converer (DIC) was seleced and analyzed in his paper (Fig.1.). (c) il1 il2 V FC L1 L2 S1 S2 Fig. 1. Curren fed double inducor push-pull converer opology. The hard swiching converers have a drawback of volage overshoos a urn-off due o he energy sored in he parasiic inducances. These volage spikes are no only dangerous o he ransisors bu hey subsanially increase he swiching losses. There are wo basic ways o proec he ransisor swiches from being damaged by he overvolage. The firs is using ransisors wih blocking volage raings ha exceed hese sresses. This, however, resuls in poor uilizaion of he ransisors, since on sae resisance of he MOSFET ransisors increases dramaically wih increased blocking volage. The oher way is o limi he sresses wihin safe levels using snubber circuis. The highes volage spikes across he swiches in he opology under consideraion appear a urn-off, when he overvolage occurs due o he ransformer leakage inducance. Therefore wo clamping circui opologies ogeher wih convenional RC snubber circuis were implemened o limi he volage during urn-off - a passive and an acive volage clamping circui. 1:n D3 D4 i S C ic R ir VDC - 115
Scienific Journal of Riga Technical Universiy Power and Elecrical Engineering III. OVERVOLTAGE PROTECTION OF THE SWITCHES no dissipae he leakage inducor energy on a resisance bu A. Passive Volage Clamping feeds i o he ransformer primary. The heoreical converer operaion waveforms are shown in Fig. 4. The energy from he leakage inducance of ransformer is Acive clamping circui has some very imporan dissipaed on passive clamping circui, which consiss of wo advanages over passive clamping circui, for example, i is clamping diodes, clamping capacior and clamping resisor. able o operae a he zero volage swiching (ZVS) mode boh Fig. 2. shows he schemaic of DIC converer wih passive a urn-on and urn-off for all swiches and simple o clamping circui. The capacior of he clamp circui has o implemen as i conains only a capacior and wo ransisors ensure ha he clamp circui overakes he energy from he conrolled in ani-phase o he main swiches. The basis of he ransformer leakage inducance, wihou increasing volage of operaion of he presened clamping circui is he resonan he swiches beyond safe values and can be calculaed from phenomena beween he leakage inducance of he ransformer he following equaion: (L lk ) and he clamping capacior (C clamp )., (1) C 2 L ( I I ) ( V V ) 2 2 2 clamp = lk FC FC clamp norm il1 L1 Llk 1:n is D3 where: C clamp is he capaciance of he clamp capacior, L lk is he leakage inducance of he ransformer, V clamp is he maximum, bu V norm normal volage of he clamp capacior. The clamp resisor, on he oher hand, has o be calculaed o discharge he clamp capacior o is normal volage and dissipae he energy colleced from he leakage inducance, according o: il2 VFC L2 S1 Sa1 Da1 Ca1 C1 Sa2 Ca2 Cclamp VP VS Da2 C2 S2 C D4 ic R ir VDC - 2 C ( norm FC ) lk R = V V P, (2) ifc Fig. 3. Curen-fed DIC Converer wih Acive Clamping Circui. where 2, (3) 1 IFC IFC Plk = Llk 2 fs 2 2 is he power o be dissipaed in he clamp resisor. Unforunaely such passive clamping circui can be opimized only for one operaion poin, so he maximum load condiion was chosen for calculaion, since i is he wors case. il1 L1 Llk 1:n D3 is QS1, QS1a QS2, QS2a v P ()v lk () i P ()=i lk () QS1 QS2a QS1a QS2 QS1 V clamp QS2a L2 VP V S C ic R ir VDC il2 VFC ifc Rc S1 C1 Dc1 Dc2 C clamp S2 Fig. 2. Curen-fed DIC Converer wih Passive Clamping Circui. C2 D4 - i clamp () v S1a (), v S1 () i S1 () vs1a Vclamp v S1 vs1a B. Acive Volage Clamping A DIC converer wih acive clamping circui, which uses conrolled swiches insead of he diodes and wihou clamping resisor is shown on Fig. 3. In his case, he energy from he leakage inducance is no dissipaed on he clamping resisance bu ransferred o he oupu. The acive clamping circui can improve he efficiency of he converer, as i does 0 1 2 3 4 5 6 7 8 Fig. 4. The converer waveforms according o he operaion of he clamping circui. The value of C clamp should be se so ha one half of he resonan period formed by Cclamp and L lk exceeds he 116
Scienific Journal of Riga Technical Universiy Power and Elecrical Engineering maximum urn-off ime of he main swiches. [4] The The above simulaions can be used o deermine he RMS clamping capacior value is curren of he clamping diodes. 2 2 2 clamp > s (1 ) π lk C T D L. (4) To achieve ZVS mode for he main swich, i mus be urned on afer urn-off he auxiliary swich. This delay should be seleced o be less han one quarer of he resonan period formed by L lk and C 1 capacior [4] T L C π M 1 M 1a = lk 1 2, (5) B. Acive Volage Clamping The schemaic of DIC converer wih acive clamping circui in LTspice environmen and simulaion resuls for he acive clamping opology are presened in Fig. 7. and Fig. 8. I can be noed ha he shape of he auxiliary swich curren is deermined by he resonan circui formed by he L lk and C clamp. where he value for C 1 has been aken from he daashee of he chosen swich. The maximum ime delay beween urning off he main swich and urning on he auxiliary, using he calculaed minimum value for he clamping capacior can be considered by T L C π M 1 M 1a = lk clamp 2. (6) IV. SIMULATION OF THE DIC CONVERTER The simulaion of he DIC converer was done using LTspice simulaion ool. Since a seady operaion poin was o be examined, he FC was modeled by a consan volage source. The core losses of he inducances were negleced. The ransformer was modeled as an ideal ransformer inroducing he leakage inducance in series. The power MOSFET ransisor swiches (IXFN73N30) and acive clamping MOSFET ransisors (IRF740) were modeled using SPICE models provided by he manufacurers. The conrol circui was realized using a field-programmable gae array (FPGA) Sparan 3E board from Digilen. The operaion condiions of he simulaion are adjused o he raed parameers of he converer. Acive load wih a small filer capacior of 10µF was considered. The value of leakage inducance (Llk = 4,8µH) was obained experimenally, by measuring he ransformers muual and self-inducances. I can also be noed, ha he converer operaes as desired, providing 400V oupu volage a raed inpu and load condiions and ha he ransisor volage does no exceed he maximum allowed volage of main power ransisor 300V and maximum allowed curren 73A. A. Passive Volage Clamping The schemaic of DIC converer wih passive clamping circui in LTspice environmen and simulaion resuls for he opology wih passive clamping are presened in Fig.5. and Fig.6. The resuls show ha using he clamping capacior he volage overshoo on he MOSFET is limied o a safe value. Fig. 5. DIC Converer Schemaics wih Passive Clamping Circui in LTSpice environmen. Fig. 6. Simulaed waveforms of he DIC converer wih passive clamping circui: from he op curren hrough ransisor M1, oupu volage, volage of he clamp capacior, ransformer primary volage. I can also be noed, ha he converer operaes as desired, providing 400 V oupu volage a raed inpu and load condiions. 117
Scienific Journal of Riga Technical Universiy Power and Elecrical Engineering Fig. 7. DIC Converer schemaics wih Acive Clamping Circui in LTSpice environmen. Fig. 8. Simulaed waveforms of he DIC converer wih acive clamping circui: from he op curren hrough ransisor M1, oupu volage, volage of he clamp capacior, ransformer primary volage. 118
Scienific Journal of Riga Technical Universiy Power and Elecrical Engineering V. EXPERIMENTAL RESULTS Experimenal esing of acive clamping DIC converer was carried ou as well. The esing was performed using Ballard Nexa PEM fuel cell module wih nominal power 1,2kW and connecing he DIC converer o a resisive load. Measuremens of he inpu curren, inpu volage and load volage and curren were done. Then he efficiency of he converer was calculaed. The efficiency of he DIC converer wih he passive clamping circui is approximaely 88% and wih he acive clamping circui is 93% (Table I.). TABLE I THE MEASURED QUANTITIES Parameer Measuremens Passive clamp ~730W Acive clamp ~760W Vin 23.3 V 26.7 V Vou 411 V 336 V Iin 35.8 A 28.73 A Iou 1.78 A 2.12 A efficiency 88 % 93 % A. Passive Volage Clamping Basing on equaions (1) and (2), he esimaed resisance of he clamping resisor was 670Ω and he capaciance of he clamping capacior 6µF. In case he resisor is oo small and he clamping capacior discharges oo fas, here is he possibiliy o decrease he volage on he clamping capacior below he ransformer primary volage, causing he clamping diodes o work as a recifier of he primary volage which reduces he overall efficiency of he converer. In parallel wih he power ransisors, RC snubber circuis were implemened composed of 2.2nF polyeser film capaciors and 4,7Ω 5W resisors. Experimenal waveforms of he DIC converer wih he passive clamping circui are shown in Fig. 9. The ess were performed below he raed power of he converer approximaely 730W a he oupu, a full load condiions he urn-off volage overshoos are close o he maximum ransisor blocking volage (300V) signifying ha furher opimizaion of he clam and snubber circuis needs o be carried ou if passive clamping circui is used. The inpu curren and volage of he DIC converer prooype are shown on Fig.10. As eviden, he inpu curren ripple is ± 1.5A (less han 5% of he average value of he curren), which is saisfacory. B. Acive Volage Clamping Faser diodes (beer urn-on and urn-off characerisics han inegraed diodes in ransisors) and 2.2nF capaciors were added in parallel wih acive clamping ransisors. In parallel wih he power ransisors, RC snubber circuis were implemened composed of 4,7nF polyeser film capaciors and 4.7Ω 5W resisors. Basing on equaion (4) he capaciance of he clamping capacior was 3,3µF. Fig. 9. Experimenal waveforms of he DIC converer wih he opimized board layou from he op: conrol volage, volage across one power ransisor. Fig. 10. Experimenal waveforms of he DIC converer wih he opimized board layou from he op: inpu curren, inpu volage. Experimenal waveforms of he DIC converer wih he acive clamping circui are shown in Fig.11. Acive clamping and power ransisor conrol volage are shown in Fig. 12. I was possible o es converer a full load as here was no primary swich overvolage problems. VI. CONCLUSIONS The power converer necessary in order o use he hydrogen fuel cell as a main power supply or backup source in domesic applicaions usually consiss of a sep-up DC/DC sage and a DC/AC inverer sage. As an efficien soluion for he DC/DC sage - a double inducor push pull converer wih acive volage clamping circui is analyzed in his paper presening simulaion and experimenal resuls. I was acknowledged ha he DIC converer wih acive clamping circui performs well and he efficiency of he converer is 93%. The efficiency could be furher increased 119
Scienific Journal of Riga Technical Universiy Power and Elecrical Engineering Fig. 11..Experimenal waveforms of he DIC converer wih he acive clamping from he op: volage across one acive clamping ranzisor, volage across one power ransisor. REFERENCES [1] EGG Services Parsons Inc., Fuel cell handbook (6h ediion), Unied Saes Deparmen of Energy, USA, November 2002. [2] G. Genile, S. Meo, F. Esposio, Comparison Among Differen Topologies of DC-DC Converer for Fuel-Cell-Based Inverer Sysem, EPE-PEMC2004, Riga, Lavia, Sepember 2004. [3] W. Choi, P.N. Enjei, J. W. Howze, G. Joung, An experimenal evaluaion of he effecs of ripple curren generaed by he power condiioning sage on a proon exchange membrane fuel cell sack, Journal of Maerials Engineering and Performance, New York, USA, Vol. 13. pp.3257-264, 2004. [4] S. De Caro, A. Tesa, D. Triolo, M. Cacciao, A. Consoli, Low Inpu Curren Ripple Converers for Fuel Cell Power Unis// in Proc. Hard Swiching Converers and Conrol EPE 2005. Germany, 2005. [5] N. Mohan, T. Undeland, W.P. Robbins, Power Elecronics. Converers, Applicaions and Design, John Wiley & Sons, ISBN:0-471-22693-9, 2003. [6] R.W. Erickson, D. Maksimovic, Fundamenals of Power Elecronics, Chapman & Hall, ISBN 0-7923-7270-0, 2001. [7] J.T. Kim, B.K. Lee, T.W. Lee, S.J. Jang, S.S. Kim, C.Y. Won, An Acive Clamping Curren-Fed Half-Bridge Converer for Fuel-Cell Generaion Sysems, 35h Annual IEEE Power Elecronics Specialiss Conference, Aachen, Germany, 2004. [8] F.J. Nome, I. Barbi, A ZVS Clamping Mode - Curren-Fed Push-pull DC-DC Converer, IEEE, 1998. Alexander Andreiciks was born in Riga, Lavia, in 1985. He received he B.sc.ing. and M.sc.ing degree a Riga Technical Universiy, Riga, Lavia in 2006 and 2008, respecively. Currenly he is working owards a PH.D. degree in Riga Technical Universiy, Riga, Lavia He is currenly an scienific assisan in he Insiue of Indusrial Elecronics and Elecrical Engineering, Riga Technical Universiy. His main field of ineres is he design and opimizaion of power elecronic circuis for renewable energy sysems. Fig. 12..Experimenal waveforms of he DIC converer wih he acive clamping from he op: conrol volage of he one acive clamping ransisor, conrol volage of he one power ransisor. basically in wo ways: by decreasing he resisance of he converer s primary circui componens or by reducing he primary curren, which can be achieved by connecing wo or more idenical converers in parallel. The ripple of he inpu curren of he experimenal prooype is below ± 5% of he mean value, which is wihin accepable limis for he fuel cell. The elaboraed prooype of double inducor push-pull DC/DC converer wih acive clamping circui can be used as a background for furher work on clamp circui opimizaion and elaboraion of closed loop curren conrol sysem. DC/DC converers. Ingars Seiks received he B.sc.ing. and M.sc.ing. degree from he Faculy of Power and Elecrical Engineering, Riga Technical Universiy, Riga, Lavia, in 2003 and in 2005 accordingly. Since 2006, he has been a Researcher in he Insiue of Indusrial Elecronics and Elecrical Engineering, Riga Technical Universiy. His main research ineress include modular mulilevel power converer applicaions for fuel cells. Mr. Seiks is a Suden Member of he IEEE Indusrial Elecronics Sociey since 2006. Oskars Krievs has received Bachelor s (2001), Maser s (2003) and Docor s (2007) degrees in he field of elecrical engineering a he Faculy of Power and Elecrical Engineering of Riga Technical Universiy. O. Krievs has been working in Riga Technical Universiy since 2001 and currenly is in he posiions of assisan professor and leading researcher a he Deparmen of Power and Elecrical Engineering of Riga Technical Universiy. His main research fields include acive power filers, frequency converers and ACKNOWLEDGMENT This work has been suppored by he European Social Fund wihin he projec Suppor for he implemenaion of docoral sudies a Riga Technical Universiy. 120