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1 ENGINEERING FOR RURAL DEVELOPMENT Jelgv, VOLTAGE BALANCE CONTROL OF TWO-LEVEL DC-DC CONVERTER Ksprs Krocs, Ugs Srmels, Vesturs Brzs Insttute of Physcl Energetcs; Rg Techncl Unversty Abstrct. The multlevel DC-DC converters hve ntges of low voltge stress of the swtches nd dodes nd reducton of the flter sze. Therefore, such converters re becomng more populr n wde rnge of pplcton. Whle multlevel topology offers mny new fetures, t lso necessttes blnce control of the cpctors. In the pper method of cpctor voltge blncng for multlevel solted DC-DC converters s proposed. The voltge blncng nlyss s crred out for two level converter wth nput seres output prllel structure. The pper descrbes the opertng prncples of the blncng crcut, nlyzes the fundmentl reltonshps nd compres ths method wth the trdtonl ones. Keywords: DC-DC converter, multlevel converter, voltge blncng Introducton In hgh nd medum voltge pplcton, the multlevel converters re good choce for AC-DC, DC-AC or DC-DC converson s n such converters the voltge cross the semconductors s reduced nd therefore MOSFET trnsstors nsted of IGBT cn be used hvng better swtchng nd conducton performnce compred to the swtches rted on the full blockng voltge. Therefore, the overll performnce of the converter, ncludng the cost nd effcency, cn be better compred to conventonl topology of converters. Often the voltge of the DC bus s ded by usng cpctve voltge der. For some resons voltge of the cpctors cn chnge, for exmple, durng the strt of the converter, vrtons of nput voltge, lght dfference between the drve sgnls of the swtches or due to the dfference of the chrcterstcs for ech ndul component. As result, the voltge on trnsstor swtches cn ncrese to n unsfe vlue. Therefore, blnce control s necessry to connect ndul lods t ech cpctor, n order to dschrge them nd to blnce the cpctor voltges. The cpctors must hve ther voltges blnced t nomnl vlue for proper operton of the converter. Most multlevel topologes re blnced by usng the ctve voltge blncng strtegy [-5] tht conssts of voltge blncng control loop ncorported n the mn control system. Such voltge blnce control system cn be mplemented n the multlevel converters only f dgtl control of the converter s used. Furthermore, ths control loop ddtonlly requres clculton cpcty of the mcrocontroller or FPGA. The control system becomes complcted nd more expensve. An nlogue control method hs fst response, precson, better resoluton, so t remns stll ttrctve n mny pplctons. In ths pper scheme tht llows blncng voltge wthout dgtl pplnces s proposed. Theoretcl nlyss Fgure shows smple cpctve voltge der tht s used to obtn two voltge levels. If the system s blnced, the voltge of the cpctor s equl to / nd the current N s zero. S + C C IN N C C IN - S Fg.. Smplfed crcut of the voltge der used n theoretcl nlyss 40

2 ENGINEERING FOR RURAL DEVELOPMENT Jelgv, To understnd the cpctor voltge behvor t s necessry to study the cpctor current whle the cpctor voltge nd current re drectly relted. Anlyzng the current through the cpctors of the crcut shown n Fg., the followng equtons cn be obtned: =, () By substtuton (4) nto (3), t s obtned: C IN S =, () C IN S =, (3) N C C C C = CIN. (4) C C N = C C. (5) As the nput voltge s the sum of the cpctor voltges, t follows: dvin C C = +. (6) The nput voltge s consdered constnt, free of osclltons, so ts dervtve s equl to zero, then the equton (6) cn be smplfed: =. (7) C C Consderng C = C nd replcng (7) n (6), the reltonshp s obtned between N nd v C, s follows: By rerrngng of (8), we obtn: C C N =. (8) C N =. (9) C On the other hnd, current N cn be expressed through the lod current s follows: =. (0) N S S From these equtons t cn be concluded tht the dfference n currents through trnsstors ( S, S ) cuses msblnce of the cpctors voltge. As the feedbck system regultes the lod current, ths current cn be consdered s unchnged through the swtchng perod. The dfference of currents of both legs S, S cn be clled unequl nductnce of the nductors, trnsformers, unlke prmeters of semconductors or dfferent duty cycle, especlly n the trnsent process. Msblnce n the voltge cuses lrger current n the leg wth hgher voltge, so slow process of stblzton wll tke plce but mostly t s not enough to hold the voltge of the cpctor n the desred level. Therefore, some soluton s necessry to prevent the stuton when the voltge of the cpctor reches dngerous vlue nd dmges semconductors or pssve elements. The soluton must compenste current N thus preventng msblnce of the cpctor voltges. The scheme must be suffcently smple wth s smll s possble power losses. The pssve voltge blncng The smplest method to blnce voltges of the cpctors s usng voltge der, whch conssts of two resstors R B, such crcut s shown n Fgure. For prctcl pplcton the vlue of these resstors must be selected such tht the expected msblnce current N s smll n comprson to the current through the resstors by equl voltges on the cpctors der to cheve voltge devton U s close s possble to zero. Even more, the resstors R B must be selected tkng nto ccount the worst-cse msblnce current N. If the ctul vlue of N s smller, losses n the blncng resstors 403

3 ENGINEERING FOR RURAL DEVELOPMENT Jelgv, stll remn permnently hgh. As dfferences of the lekge currents of the both cpctnces re smll ( C C ) n comprson to the msblnce current N, t follows: N =. () compenston + der der + compenston R B compenston R B - compenston C C IN C S N C IN S + U U Fg.. Pssve voltge blncng crcut The voltge devton of the cpctor cn be expressed s follows: V = compenston RB = N RB. () The normlzed devton by the nomnl voltge / of the cpctors s equl to: N R V B v= = = V R IN N der B der If the devton of the voltge of the cpctor v shll be less thn 0 percent not to brekdown trnsstors nd cpctors, vlue of the R B hs to be selected to ensure der >.5 N. Then the losses of the blncng resstors cn be clculted s: (3) RB PB = der RB + N = RB der ( + u ) (4) The msblnce current N = 00 ma t = 600 V DC results n blncng losses of P B = 56 W. If the converter s used, often t leds to very lrge loses of the electrcl energy. Ths suggests the development of n nced blncng method. + S R C C IN + U + - compenston C N R C IN U - S Fg. 3. Actve voltge blncng 404

4 ENGINEERING FOR RURAL DEVELOPMENT Jelgv, The ctve blncng method shown n Fgure 3 nd proposed n [6] llows to vod permnent losses n the resstors, the losses of the ctve blncng cn be clculted: Pblncng = VIN N (5) But stll losses remn hgh s the nput voltge s hgh, ddtonlly, ths blncng crcut requres hgh-voltge opertonl mplfer whch s expensve. The proposed blncng method In ths pper, the voltge blncng crcut for the nput seres-output prllel connected converter confgurton s shown n Fgure 5 for medum-voltge power converson pplctons but such crcut cn be used n other solted confgurtons, too. The nput-seres output-prllel (ISOP) confgurton conssts of two modulr DC-DC converters connected n seres t the nput nd n prllel t the output, enblng the use of hgh swtchng frequency metl oxde semconductor feld effect trnsstors (MOSFETs) wth low voltge rtngs, whch leds to hgh power densty nd hgh converson effcency. As the output current rpple frequency s twce of the swtchng frequency, whch s shown n Fgure 5, the sze nd costs of the output flter cn be reduced. The control of the converter s relzed by usng the nlogue crcut relzed n n nlogue chp. Fg. 5. DC-DC converter nd current n the nductor Fgure 6 shows bsc schemtc of the converter nd blncng crcut. The blncng crcut conssts of n ddtonl of the trnsformer wth number of s equl to the number of the s of the prmry sde of the pulse trnsformer. In the seres wth ths the resstor R s connected, whch lmts the blncng current nd dode VD 3 to prevent power losses n the resstor f the voltge of the cpctor s hgher thn of the blncng. + VD VT T VD7 L +V OUT C IN VD VT Blncng C OUT C IN R VD3 R Blncng VD4 T VD8 L VD5 VT3 -V OUT - VD6 VT4 Fg. 6. Input seres output prllel dc-dc converter wth blncng crcut 405

5 ENGINEERING FOR RURAL DEVELOPMENT Jelgv, Ferrte core Prmry Blncng Secondry Fg. 7. Trnsformer wth ntegrted blncng Fgure 7 shows the structure of the proposed trnsformer, whch conssts of round ferrte core nd three s. The number of turns of the secondry nd blncng re equl to 7. For the blncng wre wth the cross sectonl re equl to 0, mm s used. For the secondry ltz wre wth cross sectonl re equl to 7 mm s used. Fgure 8 explns the opertonl prncple of the proposed crcut. If the voltge of the cpctor s hgher thn tht of the blncng, the current of the correspondng trnsstor flows from the blncng to the cpctor nd prtly compenstes the current N thus grdully equlzng the voltge levels of the both cpctors. The voltge levels cn not be equlzed to equl vlues but t s not necessry n ths cse s the mn tsk of the blncng s to prevent dngerous vlues of the voltge, ddtonlly, there s relzed crcut tht swtches off the converter f the voltge of one of the cpctors becomes crtclly hgh. compenston C N Blncng + U C C IN U VD 3 R compenston s - Fg. 8. Opertonl prncple of the blncng crcut The power losses n the resstor cn be expressed s follows: P blncng < U. (6) The power losses n ths crcut n comprson to the prevous ones re sgnfcntly lower, therefore, the compct blncng resstor cn be used wth power of severl wtts, ddtonl blncng s cn be creted by usng wre wth smll cross-sectonl re. The expermentl results demonstrte tht the voltge of both cpctors n ll opertng modes stys delly equl; therefore these oscllogrms re not presented n the pper. The mesured temperture of the blncng resstors s less thn 45 degrees Celsus. Ths mens tht blncng losses re smll nd the proposed method s sutble for prctcl pplctons. Of course, t would be necessry to test the blncng crcut n longer tme perod; therefore, n prctcl pplctons t s sble to use n ddtonl protectve crcut gnst overvoltge of the cpctors. N 406

6 ENGINEERING FOR RURAL DEVELOPMENT Jelgv, Conclusons The voltge blncng of the cpctors of the multlevel converter cn be bg chllenge. In the pper method to solve ths problem wthout mplementton of the dgtl control s proposed. The crcut s smple nd chep, t llows prevent rsng of the cpctor voltge to the dngerous voltge wth neglgble power losses. The proposed crcut s useful for the solted multlevel converter. Acknowledgment Ths reserch work hs been supported by the Ltvn Councl of Scence (Project Nr. 673/04). References. Km J. W., You J. S., Cho B. H. Modelng, control, nd desgn of nput-seres-output-prllelconnected converter for hgh-speed-trn power system, IEEE Trns. Ind. Electron., Vol. 48, No. 3, 00, pp Ayynr R., Gr R., Mohn N. Actve nput-voltge nd lod current shrng n nput-seres nd output-prllel connected modulr DC-DC converters usng dynmc nput-voltge reference sheme, IEEE Trns. Power. Electron., Vol. 9, No. 6, 004, pp Run X., Chen W., Cheng L., Tse C. K., Yn H., Zhng T. Control strtegy for nput- seresoutput-prllel converters, IEEE Trns. Ind. Electron., Vol. 56, No. 4, 009, pp Rodrguez J., L J.S., Peng F.Z. Multlevel nverters: survey of topologes, controls, nd pplctons Industrl Electroncs, IEEE Trnsctons on Volume 49, Issue 4, Aug. 00 Pge(s), pp Ntrjn S. P., Anndh T. S Control of nput seres output prllel connected DC-DC converters, Journl of Power Electroncs, Vol. 7, No. 3, 007, pp Ertl H., Wesnger T., Kolr J. W., Zch F. C. A smple ctve method to vod the blncng losses of dc lnk cpctors, Proceedngs of the 4th conference PCIM, 003, pp