Comparison of the DC/DC-Converters for Fuel Cell Applications

Transcription

1 nternatnal Jurnal f Electrcal and Electrncs Engneerng : 007 Cmparsn f the DC/DC-Cnverters fr Fuel Cell Applcatns Oleksandr Krykunv Abstract he surce vltage f hgh-pwer fuel cell shws strng lad dependence at cmparatvely lw vltage levels. n rder t prvde the vltage f 750 n the DC-lnk fr feedng electrcal energy nt the mans va a three phase nverter a step-up cnverter wth a large step-up rat s requred. he utput vltage f ths DC/DC-cnverter must be stable durng varatns f the lad current and the vltage f the fuel cell. hs paper presents the methds and results f the calculatn f the effcency and the expense fr the realzatn fr the crcuts f the DC/DC-cnverter that meet these requrements DC FC - 75 DC Grd Keywrds DC/DC-cnverter, calculatn, effcency, fuel cell.. NRODUCON HS paper presents the methds and results f the cmparsn f crcuts f the DC/DC-cnverter fr the utput pwer f 5 kw and the utput vltage f 750. n the Sectn the used structures f the 5kW-fuel cell systems are shwn. Sectn prvdes the mathematcal analyss f the chsen crcuts f the DC/DC-cnverter, desgn f these crcuts. n Sectn the methds and results f cmparsn f the DC/DC-cnverters are presented.. SRUCURES OF HE EECRCA FUE CE SYSEM n [] the typcal structures f the electrcal fuel cell system are presented. Fr the reasns f the cntrllablty, relablty and the lw cst the structure, whch s shwn n Fg. s chsen. he DC/DC-cnverter wth galvanc slatn prvdes the slatn f the fuel cell ptental frm the grd DC 750 DC Grd FC Fg. he structure f the electrcal fuel cell system fr the DC/DCcnverter wth galvanc slatn. n the case f the DC/DC-cnverter wthut galvanc slatn the structure, that s shwn n Fg., shuld be used. he cnfguratn shwn n the Fg. s n [] descrbed. he pwer that flws thrugh each DC/DC-cnverter s a half f the nput pwer. Fr DC/DC-cnverter wthut galvanc slatn, the lsses n the ne f the DC/DC-cnverters must be calculated fr the utput pwer f.5kw and must be dubled. Manuscrpt receved Aprl 0, 007. he authr thanks Prf. Dr.-ng. A. ndemann frm the fr the Ott-vn-Guercke-Unversty fr help n the wrk. O. Krykunv s wth the Ott-vn-Guercke-Unversty, Magdeburg MD 906, Germany (crrespndng authr t prvde phne: ; e-mal: leksandr.krykunv@e-technk.un-magdeburg.de. -75 DC Fg. he structure f the electrcal fuel cell system fr the DC/DC-cnverter wthut galvanc slatn. he result s a ttal lss fr tw DC/DC-cnverters. MAHEMACA ANAYSS OF HE CRCUS OF DC/DC- CONERERS FOR HE FUE CE SYSEM A. Chce f the nvestgated crcuts he DC/DC-cnverter fr fuel cell applcatns must be ft nt the requrements: hgh step-up cnversn rat hgh stablty f the utput vltage durng the varatns f the utput current and the nput vltage hgh effcency lw cst he frst requrement excludes all crcuts wthut a transfrmer. Hgh step-up cnversn rat s n these crcuts prvded nly at extreme hgh duty cycles, whch are nt acceptable []. he secnd requrement excludes all crcuts fr whch a cntrller cannt be desgned fr a stable utput vltage [4]. A transferred pwer f 5kW s anther key value fr chsng the crcuts fr the nvestgatn. n sme crcuts the desgn f the transfrmer fr ths pwer results n a large sze f the transfrmer, anther crcuts have prblems wth the saturatn f the transfrmer [5]. hese crcuts wll nt be nvestgated. he fllwng factrs can result n hgh effcency and lw cst f the cnverter [6]: lw current r vltage stresses f the semcnductrs utlzatn f the f the B-H-characterstc f the transfrmer n all quadrants lw nput current rpple lw number f cmpnents smple desgn f the transfrmer All the crcuts, that cmbne sme f these benefts, wll be nvestgated and are shwn n the Fg.. he crcuts were nvestgated fr the deal transfrmer. he leakage nductances f the transfrmer were nt cnsdered. 7

2 nternatnal Jurnal f Electrcal and Electrncs Engneerng : 007 Bst cnverter wth tapped nductr ( D v v D D C Extended frward cnverter ( D D D C D D D C c C Cc Current-fed full-brdge cnverter ( 4 D v D D D C ltage-fed full-brdge cnverter (5 C D v D D D C v D D 4 4 D D 4 Current-fed push-pull cnverter (4 / D v D D D D / C v D 4 Asymmet-rcal brdge duble frward cnverter (6 C D D v D C D 6 v D 7 D 5 v D D D D D 4 4 Fg. nvestgated step-up DC/DC-cnverter crcuts he parameter f each crcut (turns rat f the transfrmer, duty cycle has a great nfluence n the effcency f each crcut. he fllwng values change wth the varatn f the duty cycle: rms and average values f the swtch and dde currents blckng vltage f the swtches and ddes sze f the transfrmer Hence the ptmal duty cycle fr each crcut exsts, at whch the effcency f the crcut s maxmum. Fr ths duty cycle the abve-named values must be calculated. Fr sme crcuts the estmatn f ths duty cycle s qute cmplex. hs paper prvdes sme crtera fr the chsng f the ptmal duty cycle. Wth the help f these crtera the duty cycle fr each crcut wll be determned. A. Bst cnverter wt tapped nductr he bst cnverter wth tapped nductr prvdes n galvanc slatn an hence shuld be cnnected t the fuel cell as shwn n the Fg.. Each f the bst cnverter must prvde a maxmum step-up rat f,5 fr varatn f the nput vltage he maxmum nput current ccurs at the mnmum nput vltage and hence the effcency f the cnverter s mnmum at the mnmum nput vltage. hs effcency wll be calculated. he maxmum duty cycle s determned as (Fg.4: t t D ( Wth - swtchng perd, and fr the turns rat f the tapped nductr ü W / W 4 can be calculated [6]: /,5 D 0,7 ( / W / W,5 4 t was shwn n [7] that the lsses n the clampng-crcut are at ths duty cycle mnmum. he smulated wavefrms f the bst cnverter wth tapped nductr fr these parameters are shwn n the Fg. 4. he rms values f the swtch current, dde current and the nput current can be calculated regardng t the functns ( t, D( t n [6]: D D,mn, D,mn D, (,max ( ü (,max( ü ( D ( ( ( ü (4 (,max( ü ( D ( B ( ( ü (5,max ( ü, wth D B ( D D,mn,mn,mn ( t 0 - mnmum swtch current, 7

3 nternatnal Jurnal f Electrcal and Electrncs Engneerng : 007,max D ( t - maxmum dde current. he average value f the nput current and the dde current can be calculated regardng t the functns ( t, D( t n [6]: Fg. 4 Smulated wavefrms f the bst cnverter wth tapped nductr D, avg 0.5( D ( ( ü ( D, max (6, avg 0,5 B ( D,max, mn D Wth B ( D ( D ( ü D (8 he blckng vltages f the swtch and the dde were calculated n [6] as fllws: ( /( W W (9 D ( /( W W ( W / W (0 t can be seen frm the Fg. 4 that the swtch and the dde are turned n and turned ff at ther full blckng vltage. At the tme t the dde D s swtched ff and the swtch s swtched n. he current f the dde D falls t zer and the current f the swtch rses. hs cmmutatn phase ccurs n cnsderatn f the leakage nductances f the tapped nductr and results n the hard turn n f the swtch. he reverse-recvery current f the dde D at turn ff results n the ver current at the swtchng n f the swtch. hs ver current can be Δ W W ( (, n RM, D / B. Extended frward cnverter he extended frward cnverter was prpsed and mathematcally analysed n []. hs cnverter prvdes n galvanc slatn an hence shuld be cnnected t the fuel cell as shwn n the Fg.. Each f the bst cnverter must prvde a maxmum step-up rat f,5 fr varatn f the nput vltage t was shwn n [] that the utput vltage n steady-state f the vltage rpple f the capactrs s lw can be calculated as fllws: (7 ( ü ü D üd C Cc ( D he blckng vltages f the ddes D, D, D and the blckng vltage f the swtch can be calculated as n [7]: D ü ( D D ü (4 he vltage Cc s the blckng vltage f the swtch. t can be seen frm ( that ths vltage strng ncreases wth the ncrease f the duty cycle. mnmze the blckng vltage f the swtch the pssbly mnmum duty cycle must be chsen. Frm (, (4 t can be shwn that the desgn f the crcut fr the duty cycles lwer than 55% results n the use f hgh-blckng vltage ddes (>, k.fr these reasns the turns rat f the transfrmer ü 8 at the maxmum duty cycle f 0,57 at the nput vltage f 0 was determned. he smulated wavefrms f the extended frward cnverter fr these parameters are shwn n the Fg. 5. Fg. 5 Smulated wavefrms f the extended frward cnverter he average and rms values f the dde currents, swtch current and the nput current can be ( üd D D, avg,mnd (5 ( D üd üd ( D D, avg,mn ( D (6 ( ( D ( ü D D, avg ( D,mn,mn (7 D, avg,mn,mnü( D D D ü (8 D,mn D.,mn ü D,,mn D D D ( ü D ü ü D (9 D, ü D D D D ü üd,mn,mn üd üd, mn (0 7

4 nternatnal Jurnal f Electrcal and Electrncs Engneerng : 007 ü Dü D ü D ( ü,mn,mn ( D,mn,mn,mn,mn, ( ( ü Dü D reduce the blckng vltage f the transstr the maxmum,mn,mnü D (,mn,mnü ( D turns rat f the transfrmer s needed. he slutn f (7 fr the turns rat s: D, D ümax ( Dmn /, max 750 ( 0,5 / 60 ( ( he smulatn results fr these parameters are shwn n the Fg. 6. üd üd,mn ( üd,mn üd D, üd ( Wth,mn ( t 0 - mnmum swtch current,,max ( t - mnmum dde current. he blckng vltage f the swtch can be Cc (4 D he swtch and the dde are turned n and turned ff at ther full blckng vltage. t s mprtant fr the calculatn f the swtchng lsses f the swtch and the dde. he chce f semcnductrs fr the swtch must be dne recrdng t the maxmum blckng vltage f the swtch:, max, mac (5 D t can be seen frm the Fg. 5 that the same cmmutatn phase as fr the bst cnverter wth tapped nductr at turn n f the swtch ccurs. he ver current at turn n f the swtch can be Δ, n RM, D (6 C. Current- fed full-brdge cnverter he current-fed full-brdge cnverter was mathematcally analysed n [8]. hs cnverter prvdes a galvanc slatn and hence shuld be cnnected t the fuel cell as shwn n the Fg.. he step-up cnversn rat can be, as shwn n [8], ü ( tv / (7 wth t v t t t4 t ( D 0,5, D>0,5 (8 -tme perd, where all transstrs are n, t 4 t0 -swtchng perd.he blckng vltages f the transstr and the secndary ddes can be, as shwn n [8], / ü (9 D (0,,mn 4,mn 4 v ( v ( ( ( Fg. 6 Smulated wavefrms f the current-fed full-brdge cnverter t can be seen frm Fg. 6 that the swtch s swtched ff at the tme t 0 at the full blckng vltage. At the tme t the swtch s swtched n. he current f the swtch rses and the current f the swtch falls untl the nput current dstrbutes equally between these swtches. hs cmmutatn phase results n the hard turn n f the swtch. At the tme t the dde D s swtched ff. he reverse-recvery current f the dde D results n the ver current at the swtchng n f the swtch. hs ver current can be Δ, n 0, 5 RM, Dü ( wth RM,D reverse-recvery current f the dde D he tme perd, where the swtches, 4 r, are n, s defned as fllws: t t0 t t ( D ( Cnsderng (8, (, the rms values f the swtch current, dde current and the nput current can be calculated[see (4, (5 at the bttm f the page]: ü,mn ( ü ( D ( D 0,5 ( ü ( D ( ü ( ü ü ( D,mn ü ü (4 ( ü 74

5 nternatnal Jurnal f Electrcal and Electrncs Engneerng : 007 ü ( D,mn ü ü,mn (5 D, ü ü ü ü ü ü transstr and the secndary ddes can be, as shwn n [9], D avg D D,,mn ( ( (6 ü ü (4 Frm the Fg. 6 t can be seen that the swtch s turned ff at the full blckng vltage. Durng tme perds t t and t t 4 D,max (44 all swtches cnduct, the energy s nt transferred t the utput. All secndary ddes dn t cnduct. he ddes D, D and D,D 4 are cnnected seral t the utput vltage. he blckng vltage f all ddes at turn ff durng tme perds t t and t t 4 can be hence D, ff / (7 D. Current-fed push-pull cnverter he current-fed full-brdge cnverter was mathematcally analysed n [8]. hs cnverter prvdes a galvanc slatn an hence shuld be cnnected t the fuel cell as shwn n the Fg.. he step-up cnversn rat can be, as shwn n [8], ü ( tv / (8 he blckng vltages f the transstr and the secndary ddes can be, as shwn n [7], / ü (9 D, mqx (40 reduce the blckng vltage f the transstr the maxmum turns rat f the transfrmer s needed. he slutn f (8 fr the turns rat s: ümax ( Dmn /, max 750 ( 0,5 / 60 4 (4 he wavefrms f ths cnverter are dentcal wth the current-fed full-brdge cnverter. he blckng vltage f the swtch s twce as hgh as n full-brdge cnverter. he equatns (4-(6 are vald fr the current-fed push-pull cnverter wth ü W / W. E. ltage-fed full brdge cnverter he vltage-fed full brdge cnverter was mathematcally analysed n []. hs cnverter prvdes a galvanc slatn an hence shuld be cnnected t the fuel cell as shwn n the Fg.. he mnmum turns rat f the transfrmer []: 750 ümn 8 (4,mn Dmax 0 0,5 he maxmum duty cycle cannt be ncreased abve 50%. Decreasng f the duty cycle results n ncreasng f the turns rat f the transfrmer and ncreasng f rms and maxmum value f the swtch current. he blckng vltages f the t can be seen frm (4 that the blckng vltage f the swtch des nt depend n duty cycle and turns rat f the transfrmer. he cnclusn f these thughts s that the effcency wll be lwer wth the decreasng f maxmum duty cycle. he duty cycle f 50% s practcally unrealzable. Hence a lttle bt smaller duty cycle f 45% was determned. he smulatn results fr these parameters are shwn n the Fg. 7. Fg. 7 Smulated wavefrms f the vltage-fed full-brdge cnverter Durng tme perds t t and t t 4 all swtches are turned ff, the energy s nt transferred t the utput. he swtches, 4 and, are cnnected seral t the nput vltage. he blckng vltage f all swtches durng tme perds t t and t t 4 can be hence (45, ff, n t was cnsdered durng the calculatn f the swtchng lss that all swtches are turned n and ff at the half f the nput vltage. he ddes are turned ff at the vltage, whch s equal t the utput vltage. he tme equatns f the wavefrms n [] fr the tme perds t 0 t and t t are the same as fr the extended frward cnverter fr the tme perd t 0 t. Cnsderng (4, the rms, average values f the swtch current, dde current and the nput current can be calculated [see (47, (48at the bttm f the page]: ( ü D D D, avg,mn D (46 75

6 nternatnal Jurnal f Electrcal and Electrncs Engneerng : 007 ( D ( ü Dü,mn,mn ( ü Dü ü ( D ( D ( ü Dü,mn,mn ( ü Dü ü ( D, (47, (48 ü( D D,mn,mn D, (49 ü( D ( ü ( D D, avg,mnd (50 E. Asymmetrcal brdge duble frward cnverter he asymmetrcal full brdge cnverter was mathematcally analysed n []. hs cnverter prvdes a galvanc slatn an hence shuld be cnnected t the fuel cell as shwn n the Fg.. he maxmum duty cycle can be determned at same way as fr vltage-fed full-brdge cnverter. he mnmum turns rat f each transfrmer can be, as shwn n [], 750 ümn 8 (5,mn Dmax 0 0,5 he smulatn results fr these parameters are shwn n the Fg. 8. he blckng vltages f the transstr and the secndary ddes can be, as shwn n [], (5 he transfrmer f the frward cnverter wth a swtch s magnetzed durng the tme perd t 0 t. he demagnetzatn f the transfrmer ccurs durng the tme perd t t. Durng the tme perd t t 4 the prmary vltage f the transfrmer s null. Hence the swtches and are cnnected seral t the nput vltage. he swtchng ff vltage f the swtch s, ff 0, 5 (5 Durng the tme perd t t the frward cnverter wth a swtch transfers the energy t the utput. n ths peratnal state the sum f secndary vltage f tw transfrmers s a blckng vltage f dde D 5 : D5 ü (54 Durng the tme perds t t, t t 4 all tw frward cnverters dn t transfer the energy t the utput. he secndary vltage f each transfrmer s a blckng vltage f dde D and D. he vltage f the ddes at ther turn ff can be hence ü (55 D, ff D, ff he free-wheelng-dde D desn t cnduct durng the tme perds: t t0 t t D (56 Fg. 8 Smulated wavefrms f the asymmetrcal brdge duble frward cnverter he secndary vltage f the transfrmer s the blckng vltage f the free-wheelng dde. hs vltage can be thus D ü (57 he free-wheelng dde s turned ff at the full blckng vltage. he swtches are, as n the case f full-brdge cnverter, turned n at the tme, where n current flws. he swtch n current s nt equal null, but the current rses very slwly at the tme, where the blckng vltage s null. he swtchng n lss can be then neglected.. MEHODS AND RESUS OF COMPARSON OF DC/DC- CONERERS A. Calculatn f the effcency Fr the calculatn f the effcency the semcnductrs fr the swtches and ddes f the crcuts must be defned. hs defntn was dne regardng t the calculated maxmum vltage stress at the nput vltage 60 and maxmum current stress at the nput vltage 0. Fr the ddes f all crcuts was the dde f type DSEP 9-06CR used. smplfy the calculatn, MOSFE f the same type fr the swtches f all DC/DC-cnverter was chsen. he nresstance f the MOSFE was fr each crcut calculated recrdng t the frmula []:,6 R DS, n ~,max (58 he cnductn lsses f the swtch can be calculated []:, P, cnd( RDS, n (59 N 76

7 nternatnal Jurnal f Electrcal and Electrncs Engneerng : 007 Wth, rms value f the swtch current, N number f parallel cnnected MOSFE fr ne swtch. he swtchng lsses f the MOSFE culd be calculated regardng t the measured swtchng lsses, Fg. 9. he measured swtchng lsses can be cnverted nt the values fr certan values f the ver current and swtchng n and ff vltages. smplfy the calculatn, t was cnsdered that 50% f the n swtchng lss s drect prprtnal t the ver current and anther 50%,n Δ Fg. 9 Measured n and ff swtchng lsses at hard turn n and turn ff as a functn f n and ff swtchng current fr the MOSFE XH88N0P s drect prprtnal t the swtchng n vltage. he swtchng ff lss s drect prprtnal t the swtchng ff vltage. he cnductn lss f the dde can be calculated []: P, cnd( D rd D, / N D 0 D, avg (60 Wth r d dfferental resstance, 0 threshld vltage, N D number f parallel cnnected ddes. he swtchng lss f the ddes was calculated regardng t the apprxmated wavefrms, whch are shwn n Fg. 0. (t v(t u F Fg. 0 Used fr the calculatn apprxmated wavefrms f vltage and current f the ddes at turn ff he swtchng lss f the dde can be then PREC 0,5 D, ff RMt f f (6 Wth t f fall tme f the reverse-recvery-current, RM maxmum value f reverse-recvery-current, f swtchng frequency. he maxmum value f reverse-recvery-current can be taken frm the datasheet. he rse tme f reverse-recvery-current can be t RM s (6 d / dt F t s t f t rr U D,ff R RM he fall tme f reverse-recvery-current can be t f trr ts (6 After the calculatn f all lsses n the crcut the junctn temperatures f all semcnductrs must be calculated. t was cnsdered that all semcnductrs are placed n the same heat snk. he junctn temperature f n the transstr and the dde D can be nd n Rth( jc, Rth( ck (64 j( a P Rth( ka P ( D R k th( ka P ( R ( k th( ka N N k k nd n Rth( jc, D R th( ck j( D a P D Rth ka P R k th ka P D R ( ( ( ( ( k th( ka N D N D k k Wth R th jc, k (65 ( junctn t case thermal resstance f the MOSFE (can be taken frm the datasheet, R th ( ck,5 K / W case t heat snk thermal resstance, R th(ka heat snk t ambent thermal resstance, n D ttal number f ddes n the crcut, n ttal number f swtches n the crcut, P ttal lss fr ne swtch, P ttal ( k D lss fr ne dde, a ambent temperature, N number f parallel cnnected semcnductrs fr ne swtch, N D number f parallel cnnected semcnductrs fr ne dde. After calculatn f the junctn temperature the new parameter f the semcnductrs fr ths temperature can be calculated. he lss fr ths parameter must be then calculated. he lss fr the new parameter and the lss fr the ld parameter are cmpared. he teratn wll cntnue untl ths dfference has a mnmum value. he gal f the calculatn s t determne the number f transstrs fr each crcut that are needed t keep the junctn temperature f the MOSFEs at the defnte value j 9 C. hs value s dentcal fr all crcuts. Hence the expense fr the semcnductrs fr each crcut can be seen. he results f the calculatn are presents n the able. B. Calculatn f the expense fr flter and fr transfrmer he mnmum cre-crss sectn f the transfrmer that s suffcent t prvde that the transfrmer wll nt saturate can be regardng t [] D 0 AK,mn (66 NΔBmax Wth prmary vltage f the transfrmer, magnetzng nductance, N number f turns at the prmary sde, 0 cnstant cmpnent n the magnetzatn f the transfrmer, ΔB max maxmum magnetc flux swng. he maxmum magnetc flux swng was determned n cnsderatn that the transfrmer wll nt saturate. Fr the crcuts wth the utlzatn f the B-H-characterstc f the transfrmer n all quadrants the magnetc flux can change frm negatve value t pstve. he flux swng can be then duble. he sze f the transfrmer f DC/DC-cnverters s cmpared at the same utlzatn f the transfrmer fr each crcut. he length f magnetc path that crrespnds t the calculated cre-crss-sectn can be then determned wth the ( k 77

8 nternatnal Jurnal f Electrcal and Electrncs Engneerng : 007 help f the datasheets f ferrte cres. he cpper lss n the prmary r secndary wndng n the transfrmer []: P ρl N A (67, CU( m ( (, / CU( Wth l m length f magnetc path, N ( number f turns f the prmary r secndary wndng, A CU( cpper area fr the prmary r secndary wndng, (, rms value f current n the prmary r secndary wndng, -6 ρ,74 0 Ω cm resstance per unt length fr cpper. he cpper area fr the prmary r secndary wndng can be calculated []: (, (68 ACU ( jmax Wth j max 4A/mm maxmum current densty n the wndng. he magnetzatn lsses f the transfrmer P,fe at the defned magnetc nductn B and defned frequency f can be determned frm the datasheet f the cre. he magnetzatn lsses at the anther magnetc nductn B and anther frequency f are []: P P ( B / B ( f f β (69 fe, fe γ / Wth γ,45, β,7 fr ferrte materals []. he calculated lsses are shwn n the Fg. 0. he effcency, calculated n the able, cnsders the ttal semcnductr lss and ttal transfrmer lss. ABE CACUAED CURREN AND OAGE SRESSES, EFFCENCY OF HE DC/DC-CONERERS ( j 9 C, R th(ka 0,K/W, 0, 750, f60khz, 750, P 5kW, N 9, ΔB max 0,; * 75, P,5kW; ** ΔB max 0,6 alue, Nr. f crcut 94,* (D0,7 09* (D0,57 98 (D0,76 4 0, (D0, (D0,45 6 4, (D0,45 D, D, D, D,avg D,avg D,avg,,avg Δ (/ D,RM D,RM D,RM t rr(d t rr(d t rr(d (ns D,ff D,ff D,ff (,n,ff (,* 6,7* 95,5* 85,5* 0,5* 5* 7* 500* 00* 00* 5,4*,8* 09* 84,* 0,8* * 4* 65* 7* 4,6* * * 0* 540* 7* 0,5* 6,7* 8,5* 4* 75* 6,5, 66,7 66, ,5 6,5 6,8, 66,7 66, ,, , ,5, , , 0, Δ,n,max ( 60 ( R DS(n, ( j5 C (mω D,max D,max D,max ( η (% Sarea fr transstrs (% A K,mn (mm 75* 5* 7,5* 65* 90, ,5* 80* 8,5* 080* 7* 00* 94, ,5 5, , 66,7 00,5 ** , , 00 0 ** , 750 9,6 8 4,6 ** , 680 9,4 6 8,5 680 Δ,, avg (70 he hgher current rpple causes the larger nput flter. he calculated values fr the current rpple are shwn n the able. t can be seen that the current-fed crcuts have mnmum nput current rpple. he extended frward cnverter has the hghest current rpple because f nt t hgh duty cycle. he crtern fr the determnatn f the duty cycle was pssble hghest effcency. he requred cre crss sectn f the transfrmer s mnmum fr the vltage-fed full-brdge cnverter. he DC/DC-cnverter wth a qute hgh duty cycle (bst cnverter wth tapped nductr, extended frward cnverter requres a larger transfrmer. Fg. Calculated lsses f the DC/DC-cnverters he relatve nput current rpple can due t [6] be defned as:. CONCUSONS Sx crcuts f the DC/DC-cnverter fr the nput vltage 0, utput vltage 750, the utput pwer P 5 kw and the swtchng frequency f60khz were mathematcally analyzed. he results f the analyss are the rms, average values f the swtch and dde currents, the maxmum vltage 78

9 nternatnal Jurnal f Electrcal and Electrncs Engneerng : 007 stresses f the swtches and ddes, the swtch and dde vltages at turn n and turn ff. hese values were used fr the calculatn f the effcency, relatve nput current rpple and the mnmum sze f the transfrmer. he extended frward cnverter has the hghest effcency f 94,% and the bst cnverter wth tapped nductr has the lwest effcency f 90,%. Fr the extended frward cnverter the lwest number f transstrs s needed. REFERENCES [] E. Sant, D. Franzn, D. Mnt, A. Pattersn, F. Pns, F. Barry A fuel cell based unnterruptable pwer supply EEE Appled Pwer Electrncs Specalst Cnference, 00, Prceedngs n CD-ROM [] O. Krykunv, Analyss f the Extended Frward Cnverter fr Fuel Cell Applcatns, SE, 007 [] Rbert W. Ercksn, Fundamentals f Pwer Electr (Bk style, Unversty f Clrad, 997 [4]. A. Nergaard, J. F. Ferrell,. G. esle, and J. S. a, Desgn cnsderatns fr a 48 fuel cell t splt sngle phase nverter system wth ultra capactr energy strage, EEE Appled Pwer Electrncs Specalst Cnference, 00 [5]. Meyer, DC/DC-Wandler mt klener Engangsspannung für phtvltasche, elektrchemsche und thermelektrsche Zellen, Frtschr.-Ber. D, R., Nr. 88, Düsseldrf, D-erlag, 000 [6] H. Slmecke, Optmerter Strmrchter für Brennstffzellenanlagen Dssertatn, Hagen, 998 [7]. Merfert, O. Krykunv, A. ndemann, Hgh-pwer tapped-nductr bst cnevrter ft Fuel Cell Applcatns, PCM, 005 [8] arter, E. Ralph, Sld-state pwer cnversn handbk, 99 79