Design and Experimental Analysis of a 10kW 800V/48V Dual Interleaved Two - Transistor DC/DC Forward Converter System Supplied by a VIENNA Rectifier I

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1 Deign nd Experimentl nlyi of 0kW 800V/48V Dul Interleved Two - Trnitor D/D Forwrd onverter Sytem Supplied by VIENN Rectifier I JOHNN MINIBÖK, JOHNN W. KOLR, HNS ERTL Technicl Univerity Vienn, Dept. of El. Drive nd Mchine, ower Electronic Group Guhutre 7/E37, -040 Wien, USTRI/Europe hone: Fx.: emil: hkolr@iem.tuwien.c.t btrct. Thi pper preent the development of the 800V/48V output D/D converter tge of 0kW telecommuniction power upply module. In order to mke poible the ppliction of ft witching power emiconductor with limited blocking cpbility (600V) nd/or for keeping the witching loe low the ytem i relized by n input erie connection of two prtil ytem. The econdry circuit of the prtil converter re connected in prllel t the freewheeling diode. By interleving minimum ize of the output inductor being common for both prtil ytem i chieved. The blncing of the input voltge of the prtil converter i by n equl but invere chnge of the converter duty cycle which i et by the output voltge/current controller nd by mutul coupling of the prtil converter vi blncing nd/or clmp winding. Experimentl reult derived from lbortory model of the ytem how n efficiency of 94% t rted power for prtil converter witching frequency of 5kHz without pecil meure for minimizing the trnitor witching loe. Furthermore, the ytem how high power denity (760W/dm 3 nd/or 3W/in 3 ) nd low weight (kg nd/or 830W/kg) depite only unidirectionl mgnetiztion of the trnformer mgnetic core. Therefore, in the ce t hnd the ppliction of oft-witching nd/or of full bridge converter ytem of coniderbly higher complexity i not required nd/or jutifible if the reulting reduction of the ytem relibility nd the incree of the embling effort re tken into ccount. Introduction The ubject of thi pper i the dimenioning nd prcticl reliztion of the D/D converter output tge of telecommuniction power upply module for the following opertionl prmeter: Input voltge: 800V Output voltge: 48V (46V...56V) Output power: 0kW. The ytem i upplied by unity power fctor VIENN Rectifier [] from the 480V (rm line-to-line voltge) threephe min. The min im i to determine chrcteritic figure for the efficiency, power denity (W/dm 3 ) nd converter weight (W/kg) which cn be chieved for minimum complexity of the power circuit (i.e. hrdwitching nd/or omiion of non-diiptive nubber or of oft-witching opertion fcilitted by uxiliry witche etc.) high immnent ytem relibility (no poibility of turtion of trnformer mgnetic core or of hortcircuiting of the input voltge in ce of control mlfunction of power trnitor) nd ppliction of ltet power emiconductor technology (i.e. reliztion of the power trnitor by 600V Wrp Speed IGBT which lo hve been employed uccefully in the VIENN Rectifier) S, D, N, converter D L + u + M' D, S, b D, b S, N, b N, D F, D b u i u b D F, b u u b D, b S, b N, b converter b u N, R Fig.: Bic tructure of the power circuit of high-power telecommuniction power upply module for reliztion of the input tge by unity power fctor VIENN Rectifier nd reliztion of the output tge by two interleved prtil converter ytem.

2 nd/or to provide bi for the etimtion of the reduction of the converter loe or of the ytem weight nd volume which could be expected by n extenion of the bic converter topology to oft-witching or by increing the witching frequency. The D/D converter topology which h been elected in ccordnce with the bove-lited requirement i hown in Fig.. In order to mke the ppliction of power emiconductor with blocking cpbility of only 600V poible the ytem i formed by n input erie connection of individul converter nd b which i connected to the output voltge of the VIENN Rectifier. The converter output re connected in prllel due to the high output current [, 3]. The converter trnformer T nd T b re operted with unidirectionl mgnetiztion nd do not require center tp of the econdry winding ( oppoed, e.g., to full bridge converter topologie). The ytem employ only ingle output inductor [4, 5], therefore, no pecil meure for blncing the output current between the prtil ytem hve to be provided. The ripple of the output current (nd/or the ize of the output filter) i minimized by operting the prtil ytem 80 out of phe in witching frequency (ripple cncelltion). In the following in ection the dvntge of employing common output inductor for both prtil converter ytem re verified by comprion to conventionl reliztion with n individul output inductor for ech prtil ytem. In ection 3 the ytem control i dicued briefly nd in ection 4 the dimenioning of the power component i treted nd the ditribution of the loe to the ytem component nd the dependency of the efficiency on the ytem output power re hown in grphicl form. Section 5 how reult of n experimentl nlyi of 0kW prototype of the ytem. Finlly, in ection 6 n outlook towrd the plnned further development of the converter i given. Selection of the onverter Topology n output-ide prllel opertion of prtil converter ytem cn biclly be chieved by direct prllel connection of the output terminl of the individul ytem or by prlleling the ytem t the freewheeling diode. In thi ce dvntgeouly only ingle output filter tge h to be provided. In the following the tree on the power component nd the inherent tbility of the potentil of the cpcitive input voltge center point M' will be briefly nlyzed nd compred for both ytem topologie. lredy mentioned the prtil converter ytem hre ingle output inductor L nd re witched with phe-hift of 80 in witching frequency, in order to double the frequency nd the duty cycle of the voltge being pplied to the output filter. For given pek-to-pek vlue,p-p of the output current ripple nd/or of the output cpcitor current i,p-p =,p-p the required inductnce of the output choke i: uo ( ) L = i f L, p p In order to chieve n equl vlue i,p-p for two output inductor converter one get the reltion: () i = i L, p p, p p. () onequently, the inductnce rtio of ingle-inductor converter compred to two-inductor ytem how vlue: Li = L (3) (i =, b) independent of the operting prmeter. Since the verge inductor current of the two-inductor converter i hlf the vlue given for the ingle-inductor converter, the totl mgnetic energy torge of both converter nd/or the totl inductor ize i bout the me. min drwbck of the two-inductor converter i the higher inductor current ripple which i trnferred to the primry ide of the trnformer nd cue higher current (turn-off) nd higher voltge tree on the power trnitor. E.g. for duty cycle of = 0.4 the inductor ripple current i three time higher compred to the ingleinductor converter; ince the verge inductor current i hlf the totl output current the rtio of the pek-to-pek ripple current,p-p to the verge inductor current i 6 time higher. So the primry turn-off current incree from 05% of the trnformed inductor current to 30%. When uing lminted or iron-powder core lo the core loe due to the lternting mgnetiztion will incree over-proportionl in comprion to the decree of the loe due to the lower frequency. The dvntge of the two-inductor converter i the lower current tre on the rectifier diode D nd D b. In Tb. for the ingle-inductor nd for the two-inductor converter formul for the inductnce, the pek-to-pek inductor ripple current, nd the voltge nd verge current tree on the rectifier nd free-wheeling diode re given nd correponding typicl numericl vlue re lited. ccording to Tb. the freewheeling diode of the ingle output inductor converter could be relized with 00V Schottky Output inductnce Output Inductor current Output inductor Ripple current Rectifier diode Voltge tre Free-wheeling diode Voltge tre Rectifier diode urrent tre, vg Free-wheeling diode urrent tre, vg Single output inductor L p 0 µh i O 00 0, p i O i O ( ) 0 V 60 V 80 0 Two output inductor L i O i, p p 0 µh V 0 V i O 40 i O ( ) Tb.: Inductnce vlue, inductor current, econdry ide diode blocking voltge nd verge current tree nd typicl vlue for 0kW 48V/00 converter (input voltge: 800V) with = 0.4 for the ingle-inductor nd two output inductor converter (witching overvoltge neglected). 60

3 N L L b = + u = u u u u u M' N N '' '' u b M'' i M q q b u = ub = u u b b N t µ = 0 T T T () b (c) (e) N L L u b M' b N N N L b uo '' u b '' B L b b u = u b b u = u + u q q b,b u = u u t µ = 0 T T T (b) b (d) (f), Fig.: Bic tructure of the power circuit of the D/D converter for () employing common output inductor or (b) individul inductor for the prtil ytem nd b. (c) nd (d): equivlent circuit for omiion of the trnformer ioltion; the indicted connection of the terminl nd B in Fig.(d) hould only clerly how the formtion of the totl input voltge by the prtil converter input voltge. (e) nd (f): time behvior of the totl output current nd/or of the prtil output current for the ytem hown in () nd (b) for the umption of n ymmetry = ½ u u, u b = ½ u + u of the prtil input voltge occurring in tep like mnner; q nd q b denote the electric chrge being drwn from the input cpcitor nd b within ubequent pule hlf period. diode to reduce their conduction loe (in the relized experimentl converter ytem 00V diode re employed).. Input Voltge Blncing In order to chieve high relibility of the converter ytem nd/or for limiting the blocking voltge tre on the power emiconductor one h to enure ymmetric ditribution of the VIENN Rectifier output voltge to the erie connected input of the prtil converter nd b. In the following the inherent tbility of the cpcitive center point M' of the totl input voltge u (cf. Fig.) hll be invetigted for the ingle output inductor converter (cf. Fig.()) by nlyzing the effect of n ymmetry of the prtil input voltge u = u u ub = u + u (4) on the current drwn from the input cpcitor nd b in comprion to the known ytem behvior for individul output inductor of the prtil converter [6] (cf. Fig.(b)). i obviou by conidering the equivlent circuit Fig.(c) nd the time behvior of the prtil input current (Fig.(e)) we hve for the ytem hown in Fig.() independent of u u N = ( u + u ) (5) N O b ( = b = denote the reltive turn-on time of the power trnitor of the converter nd b). For the locl (relted to pule period) verge vlue i M,vg of the current i M drwn out of the input voltge center point M' there reult i M, vg = 0. (6) Fig.(e) clerly how, due to the common output inductor equl electric chrge quntitie q = q b re tken form nd b, therefore, the potentil of M' doe not how inherent tbility but cn be hifted without direct influence on the ytem behvior. On the contrry, n unblnce of the prtil input voltge ccording to Eq.(4) would reult in continuou decree of the prtil current, nd/or correponding incree of,b for the ytem hown in Fig.(b); only the totl output current how time behvior identicl with Fig.(e). n incree/decree of prtil voltge therefore i connected with n incree/decree of the current drwn from the correponding input cpcitor. Therefore, the center point potentil how inherent tbility nd no pecil meure for tbilizing the input voltge ditribution re required. In ummry, for the ytem hown in Fig.() controller G() h to be provided for blncing the prtil input voltge nd u b (cf. Fig.3). In ce voltge unblnce u i preent, the controller output lightly incree/ decree the reltive turn-on time ( = ±, b = m ) nd/or the current conumption of the ytem being connected to the higher/lower prtil voltge wht reult in ytem behvior given for individul output inductor nd/or in correction of the voltge unblnce [7]. cn be verified by imple clcultion, there the voltge trnformtion rtio till i defined by Eq.(5); therefore, the correction of n unblnce of nd u b in firt pproximtion doe not tke influence on the control of the ytem output voltge. Furthermore blncing winding N 3, nd N 3,b with equl number of turn N 3, = N 3,b = N N re provided (cf. Fig. nd Fig.4), which rechrge the input cpcitor howing

4 lower voltge out of the higher prtil voltge within the correponding converter turn-on time for voltge ymmetrie being higher thn u N u 4 N (becue the chrging i in the forwrd mode reitor R nd R b re provided for limiting the compenting current). Therefore, lo the mximum voltge unblnce occurring without controller ction i defined by Eq.(7). 3 Sytem ontrol 3. Output Voltge ontrol The control of the ytem i relized in two-loop tructure, i.e., by n verge-mode output current control with uperimpoed output voltge controller. The output current reference vlue i defined minly by the output cpcitor current i which i meured uing imple current trnformer. With thi, pre-control of the output current nd n increed phe mrgin of the output voltge control (i i equivlent to the output of D-type component of the output voltge controller) cn be chieved with low dditionl reliztion effort. onequently, the output voltge controller R() only h to gurntee the ttionry ccurcy of the output voltge. Furthermore, the meurement of i give ignl which llow direct detection of n output hort circuit condition [8]. lterntively, the current control could lo could be relized pek current mode control bed on, e.g., ening of the primry trnformer current. However, drwbck of thi control method compred to verge current mode control re the increed enitivity to noie nd the fct tht the control nd limittion of the output current i performed only indirectly which impir the output hort circuit behvior (cf. Fig.0 in [3]). Therefore, in the ce t hnd direct verge mode output current control i preferred becue the power circuit topology in principle void trnformer turtion nd/or direct meurement of the (7) trnformer primry current i not necery in principle. The ctive blncing of the prtil voltge i performed by n invere diplcement of the duty cycle of the two prtil ytem by the controller G() (cf. Fig.4). If u, nd u,b re not of equl mgnitude the duty cycle of the ytem howing the higher/lower prtil voltge i increed/decreed. Thi reult in higher/lower verge current drwn from the input cpcitor howing the higher/lower prtil voltge which finlly led gin to u, = u,b = ½ u. 3. Synchroniztion to the VIENN Rectifier The output cpcitor of the VIENN rectifier nd the input cpcitor of the D/D converter hve been eprted by common-mode choke in order to void priori diturbnce of the D/D converter control circuitry by witching frequency common-mode output voltge component of the rectifier. ( common-mode component with witching frequency i preent in principle in the output voltge of ll three-phe WM rectifier ytem.) Thi furthermore give the poibility of performing the tbiliztion of the potentil of the input voltge center point M without conidertion of detil of the control of the VIENN rectifier blncing the prtil output voltge u + nd u -. It h to be noted, however, tht direct prllel rrngement of +, - nd, b would mke poible ignificnt reduction of the cpcitor current tre by ynchroniztion of the witching of the rectifier nd of the D/D converter (extended electrolytic cpcitor ervice life nd/or reduction of the minimum cpcitnce vlue required). Thi cn be explined by the fct tht + i chrged by the VIENN Rectifier ymmetriclly t the beginning nd t the end of pule intervl T nd the chrging of - i hifted in time by ½T. The D/D converter drw current t the beginning of pule period from nd hifted in time by ½T from b (ccording to the phe hifted opertion of the converter nd b). Therefore, for equl witching frequency nd proper phe diplcement [9] the current conumption of the D/D converter cn be provided directly by the rectifier voiding the cpcitor by-p. With thi, depending on the u b G( ) * R( ) i i * L H( ) u * 0 0,,, b, b logic logic,,,,, b, b, b, b ync f Fig.3: Block digrm of the ytem control. By the combintoril logic circuit connected in erie with the comprtor, e.g., the witching ignl, i pplied to the power trnitor S, nd S, (cf. Fig.4) lterntively, i.e., for turning on the trnitor S,, S, remin in the turn-off tte nd vice ver. Therefore, ech power trnitor of the prllel rrngement of S i,, S i, nd S i,b, S i,b (i=,) i operted with only hlf the prtil converter witching frequency nd the problem of imultneou witching of two power trnitor (equl hring of the witching loe) nd of ymmetric ditribution of the totl current nd/or of the conduction loe between device howing negtive temperture coefficient of the forwrd voltge drop i priori voided.

5 D, S, S, D 3, b N, D N 3, b D F, N, D, S, S, Fig.4: Detiled tructure of the power circuit of the prtil converter compriing the prllel rrngement of witche S, nd S, nd/or S, nd S, with ocited RD-nubber circuit. The econdry ide rectifier diode D re lo equipped with RDnubber, the blocking voltge of the freewheeling diode D F, i clmped by men of Zener-type circuit. On the input the blncing winding N 3,b correponding to converter b i hown. modultion indice of both unit, reduction of the totl cpcitor current tre of typiclly 50% cn be chieved. more detiled nlyi of the optimum phe diplcement of the prtil ytem nd of the reidul cpcitor current tre will be given in future publiction. 4 Dimenioning of the ower ircuit The converter power loe re clculted for prtil converter. Thi men tht the input voltge i 400V, the output power i O,i = 5kW; only the output inductor being common to both prtil ytem i clculted bed on the totl output power of 0kW. 4. Trnformer The trnformer re relized uing two core et of type E70 ech. The witching frequency i f = 5kHz; the duty cycle h to be limited to mx < 0.5. In order to provide fety mrgin the mximum duty cycle i et to mx = The turtion limit of the ferrite mteril N67 (SIEMENS) ued i bout B t = 00. So mximum mgnetic induction of B mx = 300mT cn be choen. Bed on thee umption one get reitnce of the trnformer primry winding of R N = 7mΩ in ce of uing high frequency litz wire. The D reitnce of foil-type winding would be lower but experiment hve hown didvntge of thi olution concerning the HF loe due to kin nd proximity effect. The mximum primry rm current i I N,rm =.5, with the umption of n incree of the winding temperture of T = 75 the primry-ide copper loe mount to: v, N = IN, rm RN ( + α T) = 0.4 W. (8) The power lo of the econdry winding i obtined tking into ccount reitnce R N = 488µΩ nd current of I N,rm = 9: v, N = IN, rm RN ( + α T) = 9.9 W. (9) The core lo denity t B pk = B mx / = 50mT nd 5kHz uing ferrite mteril N67 mount to p v,fe 50kW/m 3, with core-volume of *0000mm 3 thi reult in core loe of v,t,fe = 0.W. In ummry the totl loe of one trnformer dd up to: v, T = 30.5 W. (0) 4. Output Inductor onidering the phe-hifted opertion of the prtil converter nd/or the correponding doubling of the effective frequency nd duty cycle of voltge u ( compred to prtil convert output) nd with the umption of pek-topek ripple current,p-p = 0.I O =0.8 the required inductnce of the output inductor = 0.7µH. hooing n llied Signl owerlite Metgl lloy S -ore M 0 with n ir-gp of mm reult in winding reitnce of R L = 476µΩ nd/or in copper lo of: v, L, u = IO, rm RL ( + α T) = 7. W. () The core lo denity t B pk = 5mT nd 50kHz for llied Signl powerlite metgl mteril mount to p v,fe 5W/kg, conidering the weight the mgnetic core of 0.35kg thi reult in core loe of v,l,fe = 5.3W. The totl output inductor loe therefore mount to: v, L = 3.5 W. () 4.3 ower Trnitor The power trnitor re relized uing 600V Wrp-Speed IGBT IRG450W (Interntionl Rectifier). Overheting of the witche S nd S i voided be employing prllel rrngement of two IGBT for ech witch nd by operting the individul IGBT lterntively [0] (hlving of the conduction nd witching loe for the individul device). The conduction loe of one converter leg mount to:

6 Fig5: Switching behvior of the witch S,; (): turn-on behvior, (b): turn-off behvior; IGBT current i S, (5/div), IGBT voltge u S, (00V/div) nd product u S,* i S, (kw/div) which cn be ued for etimting the witching loe (there, the time-dely of current probe h to be conidered). = I U = 3.9 W. (3) v, S, c S, vg E, t Due to the try-inductnce of the trnformer in principl no turn-on loe of the IGBT do occur; therefore, the witching loe re minly due to turn-off (the turn-on lo due to the RD nubber i conidered in ection 4.6). With meured turn-off energy of E off = 0.5mJ (Fig.5 (b)) t hlf the nominl lod (I = 8, junction temperture 50 ) the witching loe for one leg reult t full lod to: v, S, IS, off = Eoff f = 0.9 W. (4) I E onequently, the totl IGBT loe per converter re: = ( + ) = 69.6 W. (5) v, S v, S, c v, S, 4.4 Output Diode The output diode re of type Motorol MUR000T. The loe of one converter reult with forwrd voltge drop of U D = 0.9V nd n verge forwrd current of I D,vg = 04 to: = U I = 93.6 W. (6) v, D D D, vg () (b) 4.5 uxiliry ower Supply The uxiliry power upply provided for upplying the control bord, the IGBT gte drive nd the fn i relized by two flybck converter connected to the prtil input voltge (primry ide erie connection, econdry ide prllel connection). One converter ct mter converter nd i controlled for getting contnt vlue of output voltge; the econd converter i controlled with equl duty cycle in order not to cue n unblnce of the prtil input voltge. The totl power conumption of the uxiliry upply i: v, H = 0 W. (7) 4.6 Snubber For ech of the IGBT RD nubber with S,S =.nf i provided (cf. Fig.4, [,]), which reult with witching frequency f = 5kHz nd turn-on voltge of U S,on = 00V (ee Fig.5 ()) in power lo: v, S, on = 4 S, S U S, on f = 4.4 W. (8) lo, RD-nubber i connected in prllel to ech rectifier diode D i (i =, b); with S,Di = 4.7nF nd f = 5kHz nd turn-on voltge of U Di,on = 30V the reulting power lo i: v, Di, on = S, Di U Di, on f =.0 W. (9) The nubber of the free-wheeling diode D F,i i relized by Zener-clmp-type circuit (cf. 4) in order to hndle the overhoot occurring for dicontinuou output inductor current mode (cf. Fig.6). The nubber power lo i umed to be comprble with the lo of the nubber of the rectifier diode, i.e.: v, D, on =.0 W. (0) 4.7 Lo Ditribution, Efficiency The ditribution of the totl converter power lo to the different component i hown in Fig.6. Bed on thi digrm the efficiency of the D/D converter ytem reult in: Trnformer Output ind. IGBT Diode ux. power Snubber 6.8 Wiring dd. loe ower lo / W Fig.6: Lo ditribution, ll min lo contribution of component of the D/D converter ytem re hown, the loe of the IGBT re divided into conduction nd witching loe.

7 η = = 94.5 %. () + v, tot O The meured individul efficiencie of the D/D converter nd of the VIENN Rectifier nd the overll efficiency re hown in Fig.7. The mximum overll efficiency i chieved t bout hlf the nominl lod. The efficiency of the D/D converter decree more thn expected t full lod, thi could be due to the over-proportionl incree of the copper loe which occur due to under-dimenioning of the econdry ide wiring nd the output inductor winding. Efficiency 98% 96% 94% 9% 90% 88% 86% Output ower / kw η VR η D/D Fig.7: Meured overll efficiency η of the power upply module, nd individul efficiencie of the VIENN Rectifier η VR nd of the D/D converter η D/D for lod rnge of.5...0kw. 4.8 Dicuion In ection 4. the witching frequency h been et to f =5kHz. Thi led to reltively lrge volume of the trnformer ( core et nd/or 4 core E70 per trnformer) nd lrger output inductor compred to higher witching frequencie. Doubling the witching frequency would llow to hve trnformer employing 4 core E65, under the umption of equl core lo denity (p v,fe = 50kW/m 3 ). lited in Tb. thi would led to lower core loe (-3%) due to the lower core volume, but would not ignificntly lower the copper loe (the winding reitnce could be reduced by only 5%). η D/D-converter t hlf the rted output current. The opertionl condition for the nlyi of the ytem operting in continuou output inductor current mode (M) re given in Tb.3; Tb.4 detil the opertionl condition for the invetigtion of the ytem behvior for dicontinuou output inductor current (DM). 5. ontinuou Output Inductor urrent The opertionl condition re given in Tb.3. Input voltge: U I = 460V Output voltge: U O = 47.9V Input current: I I = 6.6 Output current: I O = 0 Input power: I = 5.3kW Output power: O = 4.9kW ower fctor: λ = Efficiency: η = 9.7% Tb.3: Opertionl condition for the nlyi of the M. Figure 8 clerly how the bic principle of opertion of the D/D converter by depicting the primry ide trnformer voltge u N,i (i=, b) nd current i N,i which re hifted by 80 in phe. Due to mnufcturing inccurcie the mgnetiztion inductnce i lower for trnformer T thn for Switching frequency f 5kHz 50kHz ore type 4 x E70 4 x E65 Volume 0.56dm 3 0.4dm 3 Weight.kg.0kg ore lo 0.W 7.8W opper lo 0.3W 9.3W Fig.8: Time behvior of trnformer primry voltge u N,i (50V/div) nd trnformer primry current i N,i (0/div) of the prtil converter i=, b. Tb.: omprion of the chrcteritic of the trnformer of prtil ytem for witching frequency f =5kHz nd f =50kHz. The min drwbck of increing the witching frequency i tht lo the IGBT witching loe, the nubber loe nd the gte drive loe incree linerly. Thi men tht for electing f =50kHz one would hve to incree the ize of the hetink by bout 3% (+0.33dm 3 ) nd would hve to ccept decree of the efficiency of 0.5%. 5 Experimentl nlyi The meurement were done uing n indutril reliztion of VIENN Rectifier in connection with prototype of the Fig.9: Output inductor current (40/div), IGBT current i S, nd i S, (0/div) nd correponding drive ignl, nd, (0V/div).

8 Fig.0: Trnformer primry voltge u N, (50V/div) nd trnformer primry current i N, (0/div) for prtil converter operting in M. trnformer T b. ccordingly, the reonnt frequencie nd the voltge pek vlue occurring immeditely before freewheeling diode trt conduction re different for T nd T b. Figure 9 how the lternte witching of the prllel connected IGBT S, nd S,. Thi mode of opertion ditribute the conduction nd witching loe eqully to the individul device nd gurntee low junction temperture (60 houing temperture t full lod, hetink temperture: 4 ). In Fig.9 lo the output inductor current i hown in order to demontrte the reltion between IGBT witching frequency nd output inductor current ripple frequency. Figure 0 demontrte the condition of trnformer T in greter detil. Firt, the primry winding N, i connected to the prtil input voltge u vi the witche S, nd S, for t on = T 5µ. fter turn-off of the witche the trnformer core demgnetiztion i vi the diode D, nd D,. The mgnetizing current cn lo be een in Fig.0. t the end of the demgnetiztion intervl the trnformer winding trt to ocillte with reonnt frequency determined by the min inductnce of the trnformer (nd the trnformer try inductnce nd wiring inductnce) nd the pritic cpcitnce of the trnformer, the power trnitor nd the primry nd econdry ide nubber cpcitor. Thi ocilltion i interrupted t the end of the conduction intervl Fig.: Secondry ide rectifier diode voltge u D (50V/div), free-wheeling diode voltge u DF (50V/div), trnformer primry current i N, (0/div) nd inductor current (80/div). of the econd prtil converter b where the free-wheeling diode i forced into conduction (cf. Fig.) nd then continue until the econdry ide diode nubber cpcitor re fully dichrged nd the primry ide IGBT nubber cpcitor re chrged to hlf the prtil input voltge ech. It i dvntgeou in term of turn-on or nubber-loe to llow the trnformer voltge to recover to zero before the beginning of the ubequent conduction intervl (by proper election of the nubber cpcitnce) becue in thi ce the turn-on voltge of ech witch i only hlf the prtil input voltge (00V). The invere voltge i pplied on the rectifier diode in two tep (cf. Fig.). The firt voltge tep occur t the end of the conduction intervl of converter, i.e., when S, nd S, re turned off nd the demgnetiztion of trnformer T trt; the econd voltge tep occur t the beginning of the conduction tte of the econd prtil converter b, i.e., when S,b nd S,b re turned on. The dvntge of pplying the invere voltge in two tep compred to immeditely pplying the full invere voltge in ingle tep i the lower decree of the rectifier forwrd current which reduce the diode revere recovery current. 5. Dicontinuou Output Inductor urrent Output voltge: U O = 48.V Output current: I O = 5 Output power: O = 40W Tb.4: Opertionl condition for the nlyi of the DM. Fig.: Trnformer econdry voltge u N, (50V/div), freewheeling diode voltge u DF (50V/div), trnformer primry current i N, (0/div) nd inductor current (80/div) for prtil converter operting in M. Figure 3 demontrte the ytem behvior for dicontinuou output inductor current (the inductor current reche = 0 t the curor poition). The duty cycle h been reduced to 0.3. Due to the lower lod current the mgnetizing current of the trnformer cn be recognized more clerly compred to Fig.0. When the inductor current reche zero n ocilltion between the pritic cpcitor, the primry IGBT nubber nd econdry diode nubber nd the output inductor i initited (cf. Fig.4 nd Fig.5). Due to thi ocilltion voltge howing n mplitude of bout twice the mximum output voltge (in the ce t hnd *56V=V) would occur cro the free-wheeling diode

9 Fig.3: Trnformer primry voltge u N, (50V/div), trnformer primry current i N, (5/div) nd inductor current (0/div) for prtil converter operting in DM. Fig.5: Secondry ide rectifier diode voltge u D (50V/div), free-wheeling diode voltge u DF (50V/div), trnformer primry current i N, (5/div) nd inductor current (0/div) for DM. Fig.4: Trnformer econdry voltge u N, (50V/div), freewheeling diode voltge u DF (50V/div), trnformer primry current i N, (5/div) nd output inductor current (0/div) for prtil converter operting in DM. D F,i. Thi ocilltion i clmped with Zener-type circuit (cf. Fig.6) which i et to u Z 70V wht in principle would llow to employ 00V Schottky-diode free-wheeling diode. Furthermore, the clmp circuit reduce the mximum voltge tre on the rectifier diode D i (i=,b) which lo would be higher for DM thn for M. 5.3 Reliztion, erformnce Dt hoto of the experimentl ytem re hown in Fig.7. The converter volume i V = 40x30x70mm 3 = 3dm 3 (length x width x height), the weight i m = kg. Thi trnlte into power-denity of ρ V = 760W/dm 3 = 3W/in 3 nd powerweight of ρ = 830W/kg. 6 oncluion hown in thi pper for 0kW D/D output tge of telecommuniction power upply module one cn chieve high efficiency of the energy converion nd high power denity nd low weight η 0.94 (t rted power) ρ = 830W/kg ρ V = 760W/dm 3 3W/in 3 Fig.6: Detil of the time behvior of the free-wheeling diode voltge u DF (50V/div) nd inductor current (0/div) for DM (compre Fig.5). lo for omiion of non-diiptive turn-off nubber [3] of the power trnitor (ppliction of RD-nubber inted of oft-witching or qui-reonnt opertion) nd unidirectionl trnformer mgnetiztion (nd/or voidnce of pecil meure for enuring ymmetric trnformer mgnetiztion required, e.g., for bridgetype circuit) by proper combintion of prtil converter ytem of imple tructure. The ppliction of converter topologie of higher complexity which poibly could how lightly higher efficiency or power denity therefore in generl eem not to be jutifible, epecilly if the reulting reduction of the ytem relibility i tken into ccount. In the coure of further development of the propoed ytem in firt tep the power trnitor nd primry ide diode hll be replced by device in novel iolted pckge (IXYS ISOLUS47, [4]). Beide reduction of the effort for embling the ytem compred to noniolted device with externl ioltion foil thi will provide reduction of the therml reitnce of the power emicon-

10 Fig.7: hoto of the indutril reliztion of the D/D converter ytem, top view (left), bottom view (right). ductor by fctor of.8 (cf. p. 3 in [4]). Therefore, for equl junction temperture the therml reitnce of the hetink cn be reduced nd/or the power denity of the converter will be increed coniderbly. Furthermore, the conducted nd rdited EMI emiion of the converter hll be minimized by proper plcement of the power component nd by inerting Frdy hield between the trnformer primry nd econdry winding [5]. In econd tep reliztion of the propoed ytem nd/or of the ytem hown in Fig.(b) with only ingle trnformer (cf. Fig.8) hll be nlyzed. The reulting ytem tructure could be conidered full bridge circuit where one bridge leg (trnitor S,, S, ) i connected to the upper nd the econd bridge leg (trnitor S,b, S,b ) i connected to the lower prtil input voltge. compred to employing individul trnformer in ech prtil ytem there the wiring nd embling effort cn be reduced (ingle econdry winding, only two output diode) nd higher utiliztion of the mgnetic mteril cn be chieved due to the bidirectionl mgnetiztion of the trnformer mgnetic core. Didvntge conit in the direct coupling of both prtil ytem nd the poibility of trnformer turtion. Furthermore, in order to keep the contruction of the trnformer imple poible nd for chieving high utiliztion of the econdry winding the econdry circuit hll be relized by current doubler rectifier [6]. The topology of thi rectifier cn be thought to be derived by modifiction of the topology of the econdry circuit of the ytem hown in Fig.(b) (cf. Fig.8(b), (c)) nd require only ingle econdry winding which prticipte in the conduction of i O / for poitive nd negtive mgnetiztion of the trnformer (within T ) nd therefore how higher utiliztion compred to the prtil econdry winding N, nd N,b of the circuit ccording to Fig.8(c) (conduction of i O / in T, repectively). oncerning the output inductor L nd L b nd the current tree on the diode the circuit ccording to Fig.8(b) nd (c) do not how difference (e.g., diode D + cn be thought to be relized by prllel connection of the diode D nd D F, ). Therefore, only mrginl improvement of the efficiency compred to the ytem dicued in thi pper cn be expected. Therefore, the reulting incree of the power denity nd reduction of the converter weight will be of pecil interet. S, S, b N D + D S, (b) S, b N N L L b D D b D F, D F, b () Fig.8: Bic tructure of the D/D converter power circuit compriing only ingle trnformer for both prtil ytem nd reliztion of the econdry circuit by current doubler rectifier (cf. ()); (b) nd (c): comprion of the reliztion effort of the econdry circuit of the circuit hown in Fig.(b) with current doubler rectifier. KNOWLEDGMENT The uthor re very much indebted to the Hochchuljubiläumtiftung der Stdt Wien which generouly upport the reerch of the ower Electronic Group of the Deprtment of Electricl Drive nd Mchine t the Technicl Univerity Vienn. (c)

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