1、 ApplicationNoteAPT0202Rev.ASeptember3,2002DANCEDOWERXHNOLOGY # # #ParallelConnectionofPowerElectronicDevicesJonathanDodge,P.E.SeniorApplicatioasEngineerAdvancedPowerTeclmology405S.W.ColumbiaStreetBend.OR97702 # IntroductionThereareseveralreasonsforconnectingmultiplepowersenuconductordeucesinparalle

2、l,suchascost,performance,andphysicallayout.Costoftentakesprecedenceoverotherconsiderationssuchasperformanceandnianufactiirability.Thetotalcomponentcostforparalleleddiscretedevicesisusuallylessthanforasinglepowermoduletliatcoulddothejob.Apowermoduleusuallyprovidesbetterpeifbrmancethanmultiplediscrete

3、devicesmtermsofstrayinductances,circuitcomplexity,andtotalthermalresistance.Sohowwouldmultipledevicesperformbetterthanasinglepowermodule?Powermodulestendtohavehighheatdensity.Insomecaseswhereheatdissipationislimited、suchasinenclosedareaswithlimitedairflow,multipledevicesspreadheatdissipationoveralar

4、gerarea,reducingpeaktemperatures.Thesmallsizeofdiscretedevicesmayalsoprovideflexibilityinphysicallayout,whichcanbeadvantageousformountinginvariouspatternsoraroirndobstacles.Therearedrawbacksandpotentialproblemswithparalleling.Thisapplicationnoteexplainssomecommonproblems,providessomerelevantbackgrou

5、ndondeviceandcircuitcharacteristics,andoutlinessomeguidelinesforsuccessfullyparallelingIGBTs,powerMOSFETs,anddiodes.RisksofParallelingDevicesTheriskswithanypowerelectiomcapplicationarefailuresduetooverheatingandexceedingsafeoperatingconditions.Ailadditionalriskwhenparallelingisparasiticoscillationof

6、powerMOSFETsandIGBTs.Overheatingcanbetheresultofexcessivecurrentimbalancebetweendevices,bothwhileconductingaswellasduringswitching.Thechancesofexceedingsafeoperatingconditionsandthesusceptibilityforparasiticoscillationcanbeslightlyincreasedduetovariationbenveendevicesbutmoresignificantlybyproblemsin

7、thegatedriveandpowercircuits.Ifonedevicefails,oftentimesalltheothersgetdestroyedaswell.Anumberofmechanismscancausethis4dominofailureeffect.Failureofonedevicecouldresultinexcessivecurrentthroughothers,causmgtheirsubsequentfailure.Afailurecancauseanexcessivelylargeorfasttransientmthepowercircuit,leadi

8、ngtootherfailuresduetoo-er-voltage,excessived7dt,orrupturingthegateoxide.Finally,ifgatedrivecircuitryexperiencesahighvoltagetransient,itmayrupturethegateoxideofeachdevicedrivenbythatgatedrivecircuitry.Multiplefailuremechanismsmaymakefindingtherootcausechallenging,smcefailurescouldbeinducedbyfactorst

9、hathavenothingtodowithpaiallelmg.Anexamplewouldbeinadequatecapacitance(possiblyduetonormalcapacitordegradationovertune)resultinginexcessiveripplecurrentandconsequentover-voltagetransients.NumberofDevicesThereisbasicallynolimittothenumberofdeucesthatcanbepaialleled,againprovidingthatnodeviceoverheats

10、,andsafeoperatingconditionsaremaintained.SomeDCmotordrivesusemorethan30devicesinparallel.Ofcoursecostisthereasonbehindsomanypaialleleddevices,andperformancecouldprobablybeimprovedbyusingapowermoduleinstead.Therealquestionishowmanydevicesshouldbeparalleled.Theamountofcurrentthateachdeviceiscapableofc

11、onductingmustbegreaterthanwhateachdeviceisrequiredtoconductintheapplication.Inotherwords,currentderatingisrequired.Sothenumberofdevicesisequaltothetotalcurrentdividedbydelateddevicecurrent.Forexample,supposethetotalcurrentis90Amps,eachdevicecansafelyhandle30Amps(overtherangeofoperatingconditions),an

12、dthedevicecmrentisderatedby30%.Theminimumnumberofdevicesis90A=4.3,whichmustberoiuidedup.So30A(1-30%)theniHiiniumnumberis5,whereaswithnoderating,3deviceswouldjustbeenough.TemperatureCoefficientAnegativetemperancecoefficient(negativetenipco)meansthatatagivencurrentthroughthedevice,asthedeviceheatsup,t

13、hevoltagedropacrossitscurrentcartyingterminalsdecreases.Conversely,foragivenvoltageacrossthedevice,thecurrentincreaseswithincreasingtemperanire.Apositivetempcoisjusttheopposite:thecurrentdecreasesasthetemperatureincreasesforagivenvoltageacrossthedevice.Apositivetempcocausesdecreasmgcurrentthroughade

14、viceasitstemperatureincreasesabovethatofotherparalleleddevices.Therefore,apositivetempcoisdesirableforpaiallelingsinceitavoidsapotentialtliemialrunawayconditionwhereahotterdevicehogscurrent,resultingineveiincreasingcurrentandheatinguntildestruction.However,apositivetempconeitlierensuresthateachdevic

15、econductsequalcurrent,nortliatthetemperatureofeachdeviceisthesame.Evenwiththepotentialriskofthennalrunaway,itisacoininonnusconceptionthatnegativetempcodevicescannotbeparalleled.Therearetwobalancmgmechanismsthatenableparalleling,evenofnegativetempcodevices.Thesemechanismsare:HeatsharingIncreasingtemp

16、cowithincreasingcurrent(meaningthetempcoincreasesinnumericalvalueandnotnecessarilyinniagnitiidesinceitcouldbeanegatn-enumber)Heatsharingtlirouglitheheatsinkreducesthedifferencebetweendevicejiuictiontemperatures,whichiswhatreallymatters.Heatsharingisespeciallyimportantfornegativetempcodevices.However

17、、evenforpositivetenipcodevices,heatsharingdoesnotsignificantlyforcecurrentsharing2,3.Negativetempcodevicesmustshareaconiinonheatsink.Ifpossible,positivetempcodevicesshouldbemountedonacommonheatsinkaswell.Ifmultipleheatsinksmustbeused,theirheatdissipationcapacitiesshouldbematched(sametyp亡ofheatsink,s

18、amecoolantflow,samecoolanttemperatures,etc.)Inotherwords,thethermalimpedancefromdietoambientforeachdeviceshouldbeaboutthesame.DiodesandIGBTsthathavehighervoltagediopatagivencurrentandtemperaturehavehigherswitchingspeedthandeviceswithlessfoltagedrop.Ageneralizationcanbemadethatthetempcoincreases(mval

19、ue)withdecreasingdevicespeed.However、thisisonlynoticeablebetweentechnologies,suchasbetweenPTandNPTIGBTs.Thereisnosignificantcorrelationbenveentempcovalueanddevicespeedforlikedevices.Thusvanationmtempcoisanegligiblefactor,unlessyouparalleldeviceswithdifferentpartnumbersordevicesfromdifferentmanufacmr

20、ers.Ofcoursetinsshouldbeavoided.On-StateCurrentSharingWhenparalleleddevicesareallconducting,thesteadystatevoltageacrosseachparalleldeviceisforcedtobethesame.Therefore,excessiveimbalanceincurrentandoiheatdissipationcancauseoverheatinginoneormoredevices.Somecausesofon-statecurrentimbalanceare:variatio

21、ningatedrivevoltageforIGBTsandMOSFETsvariationinRdSOnforMOSFETsvariationmcurrentbetweendevicesatagivenVCELlJa:200100100.30.50.70.91.11.315VcrvSOURCE-TO-DRAINVOLTAGE(VOLTS)Figure2MOSFETBodyDiodeForwardVoltageBeforethecrossoverpomt,thediodeexhibitsanegativetempcochaiacteristic;atcurrentabovethecrossov

22、erpomt,ithasapositivetempco.Thecrossoverpointiscausedbytheresistanceofsilicon,whichgoesupwithincreasingtemperature.PowerMOSFETsandIGBTsPowerMOSFETRdSOnalwayshasapositix-etemperaturecoefficientregaidlessofcurrentbecauseoftheresistanceofsilicon,andthefactthatthereisnoconductivitymodulation(MOSFETsarem

23、ajoritycarrieronlydevices).alsoincreaseswithcurrent.VGE=15V.2&PULSETESTUJo中ol&oloLUTlooAswithdiodes,themcreasemtenipcowithincreasingcurrentandheatsharingaiethebalancmgmechanismsthatenableparallelingnegativetenipcoIGBTs.Foralltypesofdeucesbutespeciallynegativetempcodevices,heatsharingiscritical.Curre

24、ntderatingisalsorequired,evenforpositivetempcodevices,sincevacationindevicespeedandheatdissipationleadstovacationincurrentandjunctiontemperaturesbetweenparalleleddevices.CurrentSharingDuringSwitchingSomecausesofcurrentimbalanceduringswitchingare:variationingatedrzevoltagevariationingatedriveimpedanc

25、e,includmgstiayinductancesvariationingatedrzetinmigvariationinpowercircuitimpedances,especiallystrayinductancesvariationindevicespeedvariationinthresholdvoltagevariationinheatdissipationSomeofthesecausesarethesameasforon-statecurrentunbalance,andthesameguidelinesapply.Toreiterate:Multiplegatedrn-ers

26、/buffersareOK,butgatedrruevoltageandgatednvecurrentcapacityshouldbethesameforeachdevice.Mountdevicesonacommonheatsink.DiodesFortransientanalysis,paralleldiodeshavebeenmodeledasaladdernenvoikin4and5asshowninFigure5.Althoughtheworkin4and5wasforhighcurrentlinerectifierswithhighercurrentandlargergeometr

27、ies,theconceptsstillapplytoswitchmodeconvertersbecausecurrentslewratesareveryhigh.Evenwithdiodesofidenticalfonxrarddropchaiactensticsandperfectlymatchedinductancesandresistances,theladdereffectcancausevariationindiodecurrentsduetomutualmductance.Figure5DiodeLadderNehvorkVariationmstrayinductancescan

28、causevanationininducedvoltagesthatdwarfthespreadinforwardvoltageatratedcurrent.Theeffectisgreaterduringtunionduetotherelativelylowvoltageacrossthediodetosuppressunbalancedtransients.Buslayoutshouldbesynmietrical.Higherresistanceinthediodepaththaninthebuspathhelpsreducecurrentimbalance,sothebusimpeda

29、ncesshouldbelninunized.Keepthebussesasclosetogetheraspossibleanddiodecurrentpathsasshortaspossibletoreducetheniagnitiideofstrayinductances.IftherearefewerdiodesthanIGBTs(orMOSFETs),placetheIGBTs(orMOSFETs)closetogether,andspacethediodesevenlyapart.Tinsreducesmutualinductance,butmoremiportantlymimini

30、zesbusmipedancesandresultsmasyinnietncallayout.PowerMOSFETsandIGBTsTheconmientsconcerningtheladdereffectfordiodesappliesequallywelltoalltypesofpowersemiconductordevices.TinsdiscussionfocusesonIGBTs,howeveritappliesaswelltopowerMOSFETs.Simplysubstitutedrainforcollectorandsourceforemitter.Figure6shows

31、apairofparallelIGBTs,althoughtherecouldbeanynumberoftheminparallel.Strayinductancesareshown.LcandLearetheinductancesofthepowerbusses(rails).Lci,LC2,Lei,andLe：aretheinductancescreatedbytheconnectionsbenveentheIGBTsandthepowerbusses.Inanutshellstraycollectorinductancecausesovershootsandringing;strayem

32、ittermductanceslowsdownswitching.Actually,collectorandemitterinductancesbotheffectvoltageandcurrentduringswitching.However、emittermductancehasastronginfluenceonswitchingspeedandconsequentlyoncurrentbetweendevicesbecauseofitsimpactontheeffectivegate-eniittervoltage.GatesupplyFigure6ParallelIGBTStrayI

33、nductancesAilinductanceresistschangesincurrentbygeneratinganopposingvoltage.Thisopposingvoltageacrossstrayemittermductanceraisesthepotentialoftheemitterduringtiirn-onandlowersitduringslirinkingthevoltageavailabletodrivethegate.Thustheeffectofsttayemittermductanceisveryanalogoustoaferritebeadonthegat

34、e.ItisveryimportantthatthestrayenutterinductancesLelandLe2bematched,andthatallemitterinductances(Lel,Le2,andLe)beminimized.Gatednveloopareasshouldbelnmunized.Connectthegatedrivecircuitssupplyreturnascloseaspossibletotheenutterpins.Devicesandtheirbusconnectionsshouldbearrangedsynimetrically.Gatedrive

35、circuitshouldalsobearrangedassyimnetricallyaspossible.ItisnormalforMOSgateddevicestoexlubitvanationinthresholdvoltageandcapacitances.Athighswitchingfrequency,thevanationintluesholdvoltageshouldbeacconmiodatedforbysorting,sincelowerthresholddeviceswillturnonbeforeandoffaftertheothers,potentiallycausi

36、ngexcessivecurrentimbalanceduringswitchmg.Tliresholdvoltageshouldbesortedtowithin150to200millivoltsforhighfrequencyapplications.Currentimbalanceduetovanationincapacitancesmustbeacconmiodatedforbyseparategateresistors,oneperdevice6.Theseparategateresistorsalsoabsorbtheenergyofcurrentsthatcanshoottlir

37、ougliparasiticdevicecapacitances,suppressingoscillationwithparasiticmductancesofparalleldevices.Theindividualgateresistanceshouldbeassmallaspossibleforbalancingswitchmgcurrentyetlargeenoughtoprecludeparasiticoscillation.Also,thetotalgateresistancelalueshouldnotbelargerthannecessarytosetvoltageoversh

38、ootandringingwithindesiredlimits.Otherwise,theswitchmglosseswouldincreaseunnecessarily.Reference6recoimnendsthatRq/Nbesetatabout10%ofthetotalgateresistanceRy,whereNisthenumberofparalleledIGBTs(orMOSFETs).Rt=Rgc+菩(I)2_=0.1Rt=Rg=0.1Rt-N(2)Substituting(2)into(1),=0.9RT.ForanexamplecorrespondingtoFigure

39、6,ifthedesiredtotalgateresistanceis15Ohnis、andthenumberofparalleleddevicesis2,thenRG=0.115-2=30,andRGC=0.915=13.5Q.TheresistancevaluesinFigure6areforexampleonlyandnotnecessarilyrecommendedvalues.Infact,thegateresistorsmFigiue6areconceptualreally,sincesometmiesferritebeadsareaddedtocontroloscillation

40、s3,7,andsometunesSchottkydiodesandseparateresistorsareusedtotimetiim-offspeedmdependentlyoftunvonspeed.Turn-onistypicallyaffectedbydiodereverserecoverycurrent,sotiirn-onspeedissettominimizetiim-onlossandnoise8.Tiirn-offspeedissettocontrolvoltageovershootand/orsuppressd7dtinducednirn-oninbridgetopolo

41、gies.Themostmiportantpointisthatthegatesoftheparalleldevicesshouldnotbesmiplydirectlyconnectedtoeachother.Also、tohelpreducecurrentmibalanceduringswitching,thetoleranceforindividualgateresistorsshouldbetight,5%orless.Thetotalchargerequiredtoswitchallparalleldevicesisthesumofwhatisrequiredtoswitcheach

42、device.Alowoutputimpedancegatedrivercircuitshouldbeused.Ifmanydevicesareparalleled,itmaybenecessarytousemultiplegatedriverICs,ortousealowoutputimpedancebufferstage,orboth.AilexampleofagatedrivebuffercircuitisshowninFigure7.NotethatMOSFETscouldbeusedinthebufferstageinFigure7insteadofthematchedNPN,PNP

43、transistorpair.IfseparategatedrrueICsareused,makesurethemaximumdifferenceinpropagationdelaybenveenthemislow,about25nsorless.Donotusemultipleopticallyisolatedgatedrn-eICssincetheytypicallyhavetoomuchvanationinpropagationdelay.Resistanceaddedinseneswitheachemitterhelpsforcecurrentbalanceanddampsoutosc

44、illations,ifany.However,italsoaddsstrayinductance,slowsswitching,anddissipatespower.Therefore,addingseriesenutterresistanceshouldbeavoidedforhighpowerswitchmodeapplications.ForpowerMOSFETsoperatedinthelinearregion.APToffersaseriesofLmearMOSFETsspecificallytailoredtolinearmodeoperation,andreferences3

45、and9discussaddingsourceresistanceforparalleloperation.DeratingandSortingParallelingIGBTsanddiodeswithmatchedvoltagedrop(switchingspeed)isidealbothforcurrentbalancingaswellasforsyncluomzingswitchingtransients.However、sortingdevicesbasedonswitchmgspeedisnotnecessarilyrequired,especiallyforpositivetemp

46、codevicesandforopeiationatlowtomediumswitchingfrequencies.SortingMOSFETsbasedon&$如orgainisalsounnecessary.Currentderatingontheotherhand,forbothpositiveandnegativetempcodevices,isalwaysrequired.Forhighfrequencyapplications.MOSFETsandIGBTsshouldbesortedforthresholdvoltagewithinawindowof150to200millivo

47、lts.Sortingdevicesallowslessderatingforthesamelevelofreliability.Devicespeed,heatsharing,tempco,anddynamiceffects(straymductancesandladdereffect)determinethebalancingofcurrentandconsequentlytheamountofcurrentderatingrequired.Thereisnotasunpleclosedfonnsolutionforcurrentderating.Ifyouknowthedistribut

48、ionofonvoltageforIGBTsordiodes,orRdvonforMOSFETs(bymeasuringseveralparts),youcanestimatetheon-statecurrentimbalanceandcanderatethecurrentaccordingly.ForIGBTsanddiodes,youcanusedatasheetgraphstoestimatethecurrentimbalance.Dietemperatureswontallbethesame、soaddadditionalguardbandforthat.Ifyoudontknowth

49、edistribution,youcanusetypicalandmaximumratingsfromthedatasheettodetenmnethespreadindevicespeed.Bewarethatdatasheetmaximumsrepresenttheedgeofthedistribution,andmayresultinsomewhatconservativederating.IftypicalvaluesforKkona代notlisted,useatleasta25%spread.Deratingfordynamiceffectsisnotsostraightfbnva

50、rd.However,switchmgcurrentmibalanceforumnatcheddevicesisrelativelysmall3.Theeffectofumnatchedstraymductancesismuchgreater.Becauseofdynamiceffects,mgeneraLthemoredevicesinparallel,themorecurrentderatingmustbemcreased.SummaiwParallelingdiscretesemiconductordevicesisoftendoneasalowercostalternativetopo

51、wermodules.E*ennegatn-etemperaturecoefficientdevicescanbeparalleled.Powermodulesinfactusuallyreachtheirpowerhandlmgcapabilitybyparallelingmultipledice.Ofcourseitiseasierforamodulemanufacturertoparalleldevicessincethediceareeasilymatchedbymanufacturinglotandconnectionsaremadeastightascanpossiblybedon

52、e,bothelectricallyandthermally.Thusmanufactiirersofpowermodulesfollowbestparallelingpracticesforyou,resultinginbetterperfonnancethanispossiblewithparalleleddiscretedevicesmtermsofstrayinductance,heatsharing,heatdissipation,parametermatching,compactsize,andeaseofnianufacmring.Whencomponentcost,heatde

53、nsity,oruniquelayoutrequirementsmandatetheparallelconnectionofdiscretedevices,thefollowingtheguidelinesoutlinedinthisapplicationnotewillhelpensuresuccess.Theseguidelinesaresummarizedhere.Mountthedevicesonthesameheatsink.Thisismandatoryfornegativetenipcodevicesbutisalsomiportantforpositivetempcodevic

54、es.Ifmultipleheatsinksmustbeused,thethermalmipedancefromdietoambientforeachdeviceshouldbematched(sametypeofheatsink,samecoolantflow,samecoolanttemperatures,etc).Allowforsomecurrentderating.Arrangedevices,connectionsandbussessymmetrically.Iimmizebusimpedancesandstrayinductances,especiallyenutter(source)mductancesbykeepingthebussesasclosetogetheraspossibleanddevicecurrentpathsasshortaspossible.Useaseparategateresistor(orresistornetwoik)foreachswitchingdevice.Donottiethegatesdirectlytogether.Useasmallvalueforindividualgateresistors.UsetheRG=0.1-