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A high efficiency photovoltaic module integrated converter with the asymmetrical half-bridge flyback

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Available online at www.sciencedirect.comSolarEnergy84(2010)1376–1381

www.elsevier.com/locate/solener

Ahigheﬃciencyphotovoltaicmoduleintegratedconverter

withtheasymmetricalhalf-bridgeﬂybackconverter

HeejeKima,JongrakKima,HosungKima,b,KyungjunLeea,b,JonghyunKimb,

DongwookYoob,DongsulShina,*bDepartmentofElectricalEngineering,PusanNationalUniversity,Jangjeon,Geumjeong,Busan609-735,RepublicofKoreaNew&RenewableEnergySystemResearchCenter,KoreaElectro-technologyResearchInstitute,28-1,Sungju-dongChangwon-si,

Kyungsannam-do,1-120,RepublicofKorea

Received17October2009;receivedinrevisedform14April2010;accepted19April2010

Availableonline26June2010

Communicatedby:AssociateEditorNicolaRomeo

aAbstract

Amoduleintegratedconverter(MIC)foraphotovoltaic(PV)cellisimportantpartofpowerconditioningsystem(PCS).ItperformsmaximumpowerpointtrackingofaPVcelltogeneratethepowerasmuchaspossiblefromsolarenergy.ThereareseveralmethodsforconnectionbetweenthePVmodulesandtheMICs.Inordertoavoidpartialshadingeﬀects,converter-per-moduleapproachwaspro-posed.TheMICthatperformsmaximumpowerpointtracking(MPPT),ifitisloweﬃciency,isnouse.TheMICwhoseoutputiscon-nectedtotheoutputofPVmodulewasproposedforhigheﬃciency.However,therearesomeproblems.Inthisstudy,anasymmetricalhalf-bridgeﬂybackconverterisproposedinsteadoftheoriginalﬂybackconverterwithsamemethodtosolvetheproblems.TheproposedMICwasbuilttoverifytheperformance.Thenewtopologyusingsoftswitchingtechniqueshowedgoodperformancefortheeﬃciency.Atthehigherpower,theeﬃciencyoftheproposedconverterishigherthanexistingconverter.Ó2010ElsevierLtd.Allrightsreserved.

Keywords:Solarenergy;Photovoltaic;Moduleintegratedconverter;Asymmetricalhalf-bridgeﬂybackconverter;Softswitchingtechnique;Higheﬃci-ency

1.Introduction

Themosteﬀectiverenewableenergysourcetogenerateelectricitywouldbesolarenergyderivedbyusingphoto-voltaic(PV)cells.Apowerconditioningsystem(PCS)aswellasaPVmodulemustbehighlyeﬃcientintransferringthemaximumpowerfromsourcestoabatteryoragrid.APVmoduleoperatesatalowDCvoltagethusisnotsuit-ableforpowerconversion.So,eachmodulemustbecon-nectedinserieswitheachothertogettheappropriatevoltageandcurrentlevels.Itisessentialthataninputvolt-ageofinverterislargeenoughtomake220VACingrid-connectionapplication.

*Correspondingauthor.Tel.:+82515102770;fax:+82515130212.E-mailaddress:jirubak762@nate.com(D.Shin).

Maximumpowerpointtracking(MPPT)isrequiredbecauseamaximumpowerpointofaPVmoduleischangedbytemperatureandirradianceinrealtime.AlsomismatchesofeachPVmodule,whicharecausedbymanufacturer’stolerancesincellcharacteristics,environmentalstressesandshadowproblem,makesigniﬁcantlosses(KaushikaandRai,2007).Theseproblemsaresolvedbyacascaded

´netal.,DC/DCconverterwiththePVmodule(Roma

2008).ItistrendnowadaysthataDC/DCconverterforMPPTisintegratedintoaPVmoduleanditiscalledasmoduleintegratedconverter(MIC).

TheeﬃciencyoftheDC/DCconverterthatperformsMPPTisthemostimportant.Alsothecostissigniﬁcantlyimportant.Toachieveahighlyeﬃciencyconverter,there

´naremanycomponentstobetakenintoaccount.Roma

etal.(2008)describethepartsoftheDC/DCconverterto

0038-092X/$-seefrontmatterÓ2010ElsevierLtd.Allrightsreserved.doi:10.1016/j.solener.2010.04.019

H.Kimetal./SolarEnergy84(2010)1376–13811377

beconsideredfortheeﬃciency.ThemostcomponentsoftheDC/DCconverterareconcernedwiththeeﬃciency.Anditisimportantthattheconverterishowtobedesigned.InordertoﬁndasuitabletheDC/DCconverterforMPPT,manyresearchersintheworldhaveconductedexperimentsandsimulationfortheMIC.ThebasictopologiesoftheDC/DCconverter,whicharebuckconverter,boostcon-verterandbuck-boostconverter,wereassessedthroughthe-oreticalmethod.Throughthisstudy,itwasconﬁrmedthatabuck–boostconverterissuitablefortheMICwhichworkswellregardlessoftemperature,irradianceandaloadcondi-tion(Enriqueetal.,2007).Aboostconverterwithaturn-onandturn-oﬀsnubbercircuitforhigheﬃciencywasexperi-mentedwithP&OMPPTalgorithm(Santosetal.,2006).Intheotherpapers,itwasshownthatabuckconverteristhebestconverterfortheMICwithlowcost,lowcomplex-ityofstructureandhigheﬃciency(WalkerandSernia,2004).Inthisstudy,anovelasymmetricalhalf-bridgeﬂy-backconverterisproposedfortheMIC.Theoutputofthisconverterisseries-connectedtothePVmodule.However,theMICwhoseoutputisconnectedtotheoutputofPVmodulewasproposedforhigheﬃciencywithsomeprob-lems(Kimetal.,2009).Thesoftswitchingtechniqueisusedtosolvetheproblemsforallswitches.Thisreducesswitch-inglossesintherangefromno-loadtofull-load.2.ComparisonbetweenanexistingtopologyandanewtopologyfortheMICAnasymmetricalhalf-bridgeﬂybackconverterhasalreadybeenproposedinseveralreferences(ChenandChen,2002;Tso-MinChen,2002;Xuetal.,2004).Inthisstudy,anewtypeasymmetricalhalf-bridgeﬂybackconverterisachievedbyconnectingaPVmoduletoinputandoutputoftheMIC.Inotherpaper,anoriginalﬂybackconverterwhoseoutputwasconnectedtoaPVmodulewasalreadyproposed.Thereareafewdrawbacks.Asthediﬀerencebetweentheinputandoutputvoltagebecomelarger,theeﬃ-ciencyofconverterbecomelowerwithahardswitchingtech-nique.Also,astressofswitchisincreased.Apowerratingbarelyreaches30W(Kimetal.,2009).Toimprovetheeﬃ-ciencyandtheoutputpower,theasymmetricalhalf-bridgeconverterthatcouldsolvethoseproblemsisproposedwiththesamemethod.

3.Explanationofanasymmetricalhalf-bridgeﬂybackconverter

Fig.1showsanexistingasymmetricalhalf-bridgeﬂy-backconverter.Thistopologyhasgreatabilitytoreducethestressandtoincreasetheeﬃciencyofswitch.Anorigi-nalﬂybackconverterusingRCDclamphasthehighervolt-agestresswhentheswitchisoﬀ.However,thistopologyhasthevoltagestresscorrespondingtotheinputvoltage.VDSsw¼Vin

ð1Þ

Andtheswitchingeﬃciencyisimprovedbyzero-voltageswitching(ZVS).ZVSisachievedbyaresonantcapacitance(Cr)andaresonantinductance(Lr).WhenSw1isonandSw2isoﬀ,theILrispositivecurrent.Theresonantcomponentsandmagnetizinginductancearebeingcharged.Andthen,whenSw1isturnedoﬀ,ILrﬂowsthroughabodydiodeofSw2.Themagnetizinginductanceistransferredtothesecondaryside.Inthatmoment,ZVSisachievedbyturningonSw2.WhileSw2isonandSw1isoﬀ,theresonantcapacitor(Cr)andtheresonantinductance(Lr)resonate.Theresonantfrequency(fr)ispﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃfr¼1=2pLrÂCrð2ÞFig.1.Anexistingasymmetricalhalf-bridgeﬂybackconverter.1378H.Kimetal./SolarEnergy84(2010)1376–1381

DuetotheresonancebetweenCrandLr,ILrbecomesanegativecurrent.WhiletheILristhenegativecondition,byturningoﬀSw2andonSw1withareasonabledeadtime,Sw1achievesZVS.AlloftheswitchesachievetheZVSandthisreducesswitchinglosses.4.Explanationofanewtopology

Thesoftswitchingandthereducedvoltagestressareimportantfactortoconsiderwhenchoosingtheswitchingcomponentsandtoincreasetheswitchingfrequency.TomakealowPVmodulevoltagehigher,aboostconverterisusedfortheMICthatperformsMPPT.ThereisalwayssomediﬀerencebetweentheoutputvoltageofthePVmod-uleandtheconverter.Itisthereasonthattheoutputvolt-ageofPVmoduleislowerthantherequiredvoltageforinputofinverter.ThepointisthatifthisvoltagediﬀerenceisaddedtothePVmoduleoutputvoltage.AsseeninFig.2,theoutputvoltageofPVmoduleisconnectedtoanoutputcapacitorinaconverter.Then,theoutputvolt-ageisexpressed:VDC¼VCþVPH

ð3Þ

whereVDCistheoutputvoltage,VCisthevoltagepro-ducedbytheasymmetricalhalf-bridgeﬂybackconverterandVPHisthePVmodulevoltage.Theasymmetricalhalf-bridgeﬂybackconvertergeneratesonlythediﬀerenceofvoltagesbetweentheoutputvoltageofPVmoduleandrequiredvoltagefortheinputvoltageofinverter.Cout1con-ductstheroleoftheoutputﬁlterforasymmetricalhalf-bridgeﬂybackconverterthatproducesthevoltageasthediﬀerencebetweeninputvoltageandoutputvoltageofthewholeconverter.Cout2conductstheroleoftheoutputﬁlterforthewholeconverter.Arequiredpowercapacityofthisconverterisdramaticallyreduced.InordertoreducethenumberoftheMICconnectedtoPVmoduletogener-atevoltageashighas220V,largervoltagecapabilityfromtheMICisneeded.Theproposedconverteriscapableofproducingmorepowerthantheexistingconverter.Amax-imumpowerpointofaPVmodulecanbeachievedatitsmaximumoperatingtemperaturepointandamaximumirradiationpoint.Therequiredmaximumpowerofacon-vertercanbecalculatedfromFig.2:IDCTM¼PPHTM=VDC

ð4Þ

PC¼IDCTMÂðVDCÀVPHTMÞ¼PPHTMÂVDCÀVPHTM=VDC

ð5ÞwhereVPHTMisthePVmoduleoutputvoltageatthemax-imumoperatingtemperatureandPPHTMisthePVmoduleoutputpoweratthemaximumoperatingtemperature.Thetotaleﬃciencyismainlyaﬀectedbytheeﬃciencyoftheconverter.Theproposedconvertereﬃciencyisdescribed:gC¼ðPSþPCÂgDCÀDCÞÂ100

ð6Þ

wheregCistheeﬃciencyoftheproposedtopologycon-verter,PBisthepercentageofthebypasspower,PCisthepercentageoftheconverterpowerandgDC–DCistheeﬃ-ciencyoftheasymmetricalhalf-bridgeﬂybackconverter.5.Experimentresults

Theconverterproposedinthispaperisanewtypeofasymmetricalhalf-bridgeﬂybackconverterwith60W.Thepowerratingistwicelargerthantheexistingtopology.Thisconverterisbuilttoverifytheperformance;itsspeci-ﬁcationsareasfollows:(1)(2)(3)(4)Inputvoltage:23–30V.Outputvoltage:30V.Outputcurrent:0–2A.Switchingfrequency:80–120kHz.Fig.2.AseriesconnectionofPVmodulewithoutputcapacitor.H.Kimetal./SolarEnergy84(2010)1376–13811379Fig.3.ThePVmoduleoutputvoltage,theconverteroutputvoltageandthetotaloutputvoltageatVin=24V.Fig.4.ThePVmoduleoutputvoltage,theconverteroutputvoltageandthetotaloutputvoltageatVin=28V.Fig.5.TheMOSFETGatesignalsofSw1andSw2andthecurrentofresonantinductanceatVin=24V.Thecircuitparametersarelistedasfollows:(1)(2)(3)(4)(5)(6)

Switches:MOSFETSTP12NM50Ã2.SchottkyRectiﬁer:40CTQ150Ã2.Inputcapacitor:100lF.

Outputcapacitor1:100lFÃ2+470lFÃ2.Outputcapacitor2:47lFÃ3.

Transformer:Np/Ns=0.71,Np=20turnsonaferritecorePC44PQ32/30Z-12,Lm=388lH,Llk=8.4lH.Figs.3and4representtherelationbetweenVinandVout1.Vout2isalways30V.However,Vout1ischangedwhentheVinischanged.Thepointofthisstudyistoproducethevoltageasthediﬀerencebetweeninputvoltageandoutputvoltageoftheconverter.Itis24VinFig.3and28VinFig.4.Thosewaveformsprovethattheconverteronlyproducesthediﬀer-encebetweentheoutputvoltageofthePVmoduleandthetotaloutputvoltage.Figs.5and6representthesoftswitch-ingofthenewconverter.Figs.5and6showstheMOSFETGatesignalsofSw1andSw2andthecurrentofresonant

1380H.Kimetal./SolarEnergy84(2010)1376–1381Fig.6.TheMOSFETGatesignalsofSw1andSw2andthecurrentofresonantinductanceatVin=28V.Fig.7.Thetotaleﬃciencyversustheoutputpowerinrangefrom10to60Watswitchingfrequency=80.7kHz.inductance.TheredlineisSw1’sgatesignal.TheZVScondi-tionisthatthecurrentoftheresonantinductanceisnegativewhenSw1’sgatesignalisbeingturnedoﬀ.Thisisexperimen-tallyconﬁrmedintherangefromno-loadtofull-loadofinputvoltagesbetween23Vand30V.Theeﬃciencyoftheproposedconverterisveriﬁedwiththediﬀerentswitchingfrequencies.Asswitchingfrequencychanges,theeﬃciencyoftheconverterslightlychanges.Inthisrangeofthefrequencies,thechangesofeﬃcienciesarenegligible.Whenthefrequencyishigherthanthisrange,Fig.8.Thetotaleﬃciencyversustheoutputpowerinrangefrom10to60Watswitchingfrequency=96.7kHz.H.Kimetal./SolarEnergy84(2010)1376–13811381Fig.9.Thetotaleﬃciencyversustheoutputpowerinrangefrom10to60Watswitchingfrequency=120.3kHz.Table1

Theeﬃciencyoftheconverterat30W.Vout1ExistingconverterNewconverter

2(%)95.295.6

3(%)94.695.7

4(%)9495.2

5(%)93.595.1

6(%)–94.8

7(%)–94.2

Acknowledgments

ThisworkwassupportedbyNationalResearchFoun-dationofKoreaGrantfundedbytheKoreanGovernment(KRF-2007-D00283).References

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thedesiredinputvoltagerangecannotbeachievedbecauseinputvoltagerangeisreduced.Andwhenthefrequencyislowerthanthisrange,theeﬃciencyoftheconverterislow-eredbecauseofthesaturationoftransformer.Figs.7–9justshowtheeﬃciencyoftheconverterinthenormalrangethatkeepsthedesiredinputvoltagerange.AsshowninFigs.7–9,thelargerthebypassedenergyis,thehighertheeﬃciencyis.Theoutputpoweroftheproposedcon-verteristwicelargerthantheexistingconverter.Whentheoutputpoweris30Wwhichisthemaximumpoweroftheexistingconverter,theeﬃciencyoftheproposedcon-verterishigherthantheexistingconverter.Thecompari-sonoftheeﬃciencybetweentheexistingconverterandthenewconverterisshowninTable1.6.Conclusion

TheexistingconverterwithitsoutputvoltageconnectedwithPVmoduleisineﬃcientwithlargeload.Inordertoachievethehighereﬃciency,theasymmetricalhalf-bridgeﬂybackconvertercapableofZVSwasproposedinplaceoftheoriginalﬂybackconverter.ExperimentalresultsshowedgoodperformancewithachievedZVSandintheconditionofthelargeroutputpowerrating.SwitcheswithalowerRDSonandSchottkyRectiﬁerswithalowforwardvoltagedroparegoingtomaketheproposedconverterhighlyeﬃcient.

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