1、AvalanchecharacteristicsandratingsofPowerMOSFET # #AvalancheCharacteristicsandRatingsofPowerMOSFETGiovanniPriviteraProduct&ApplicationengineeringPowerMOSFETDivisionSTMicroelectronicsCataniaItalyAvalanchecharacteristicsandratingsofPowerMOSFET # #AvalanchecharacteristicsandratingsofPowerMOSFET #nSIntr

2、oductionBackinthemidSOs,powerMOSFETmanufacturersstartedtoclaimanewoutstandingfeature:theAvalancheRuggednessSuddenly,newfamiliesofdevicesevolved,allwiththis“newfeature.Theimplementationwasquitesimple:tlieverticalMOSFETstructurehasanintegralbodydraindiode,whichcannotbeeliminatedSo,bychangingsomeproces

3、sandlayoutparameters,itispossibletoguaranteetheuseoftheclampingcapabilityofthisdiodeinwithstandingaccidentalvoltage/powersurgesbeyondthenominaldrainsourcevoltage,Ofcoursetlieconfusionaboutthemeaningofruggedness,andhowtoratetinsinthedatasheetwassohuge,coupledwiththepoortlieoreticalknowledgeofit.Despi

4、tethis,allPowerMOSFETmanufacturersstartedtoproduceavalancherateddevices,proposedatasheetratings(altliougliimperfect),toprotectthemselvesandtheendusers,fromthisincompleteknowledge.Today,knowledgeondevice*sbehaviordiinngavalancheconditionsisenhancedAlotof卻plicationnotesandpaperswereissuedwithdifferent

5、approachestoexplainratingsandavalanchebehaviorThescopeofthisnoteistobneflyreviewtheMOSFETphysicsonavalanchetosupplydesignerswithtoolsandhintstodealwithavalancheissues.MOSFETfundamentalsThebasicandsimplifiedverticalstructureofaMOSFETisdrawninFigure1.TheactualMOSFETisaninfiniteparallelofthesemicroscop

6、ic1structuresthatworktogethersharingthesameDRAIN,withalltheGATEsconnectedtogetherbyadeposedpolysiliconmeshandalltheSOURCESlinkedbytlietopmetal.SourceGateDrainFigure1MOSFETverticalstructureandparasiticelementsFigure1referstothewellknownSTpatentedhiglivoltageMOSFETstructure,MESHOVERLAYexceptsomeproces

7、soptimizationoftheshapeofthebody-drainjunctionandotherimportantimprovementsintlieMESHoverlaydesignTheconceptofthisverticalstnicturecouldbeconsideredvalidalsoforvanousoldercellularorotherteclinologiesDuringonstate,wlulethegatesourcevoltageisabovethethreshold,tlieconductioncurrentislocalizedinthedrama

8、ndintheregionbelowthegate(channel).DuringoffstatetlieVoltagedropacrossdrainandsourceissustainedbythePNjunctionatreversebias,andaverysmallcurrent(leakage)flowstlirouglithejunctionIftlievoltageincreasestoomuchandtheelectricalfieldreachesthecriticalvalue,thejunctiongoestobreakdown,andcurrentstartstoflo

9、wthroughthebodyregion.So,ifanovervoltageisappliedtothejunction,acurrentflowstliroughitwhiletheMOSFETlimitstheactualdrain一sourcebreakdownvoltageThebreakdownmechanismitself,isnotdestructiveforap-njunctionHowever,overheatingcausedbythelargebreakdowncurrentandhiglibreakdownvoltagedamagesthePNjunctionunl

10、esssufficientheatsinkingisprovided.LookingatthestructureoftheMOSFET,onecanseethatthePNjunctionisnotasimplenorperfectdiodeThediodeoftheMOSFETisthecollectorbasejunctionofaBJT(BipolarJunctionTransistor,alsocalledtheparasitictransistor)madebytheN+regionofsource,P/P*regionofthebodyandN*regionofthedrain,w

11、iththebaseshortedtotheemitterbythe丘ontmetal.ThecapabilityofaMOSFETtowithstandavalancheconditiontakesintoaccounttliesetwoconcerns.Infact,twokindsoffailureanse:onerelatedtocurrent,andtheothertopowerdissipation.Intheformer,failureiscausedbytlielatchingoftheparasiticbipolarduetothecurrentthatflowsthroug

12、hitsbaseresistance,multipliedbythegainTliesecondisreachedwhenthetemperatureofthejunctionrisestoacriticalvaluetliatprovokestheformationofhotspotswithaveragetemperaturesabout650Candpeakofapproximately1000Ccausedbyregenerativethermalrunaway,causingtheextremelyrapiddestructionoftliedevice1.2.1FailureMod

13、eDescriptionshasbeentliefirststepfortheimprovementoftheMOSFET,followedbyothermoresubtleoptimizations.ThepowerthatisdissipatedintheMOSFETcausesanincreaseinjunctiontemperature.Iftlietemperatureincreasestoacriticalvaluesetbythepropertyofthesilicon2,thefailure,withoutthecontnbutionoftheparasiticbipolar,

14、occursbecauseofthecreationofthermallygeneratedearnersintheepitaxyal/bulkregionandsothecreationofhotspots.Thecriticaltemperaturetohavethisphenomenonisbeyondthemaximumjunctiontemperatureofthedevicesandisrelatedtotheintrinsictemperatureofdopedsilicon,towhichtheconcentrationofthebulkequalstheoneofthethe

15、rmalgeneratedcarriers.Thetemperatureincreasedunngavalanchephenomena,duetothermalcapacitanceofthesilicon,isnotinstantaneous.Hence,tinskindoffailureshouldbedistinguished&omthatcausedbycurrentasthedeviceholdsthebreakdownvoltageforafinitetimebeforedestruction.TestingtheAvalancheRuggednessAvalanchecharac

16、teristicsandratingsofPowerMOSFET # #AvalanchecharacteristicsandratingsofPowerMOSFET Aspreviouslydiscussed,theintegraldiodeofaMOSFETistliecollectorbasejunctionoftheparasitictransistor.Ifthecurrentflowslaterallytliroughregionp,theincreaseinthevoltagedropacrosstheemitterbaseresistancecausestheBJTtoturn

17、-on.Tlieinitialavalanchecurrentisconcentratedmainlyinthediodelocalizedinthedeepzoneofpassoonastliecurrentgrows,itbeginstointerestalsothep,lighterdoped,regions.Since,bydesign,thevalueoflateralresistanceRpisluglierthantheoneoftheverticalresistanceoftheheavydopedp+region,andthecurrentisconcentratedinth

18、eregionp+,sotheBJTshouldnotturnonAssoonasthecurrentbeginstointerestthepregion,causingasufficientdropofvoltagetoequaltheVBEoftlieBJT,thecurrentofthebase,lb,inconjunctionwithtlie卩ofthetransistorwillcausetlieBJTturn-on.VBEhasanegativetemperaturecoefficientconsequentlyleadingtotliermalrunawayandfinally,

19、thedestructionofthedeviceduetothesecondarybreakdownoftlieparasiticBJT.TheadoptionofastronglydopedP*region,determiningtliereductionofthegainofthetransistorandthebaseresistanceTheAvalanchecapabilityoftliedeviceisclassicallyevaluatedbyacircuitthatperformsanUnclampedInductiveSwitcliing(UIS)liketheonedes

20、enbedinFigure2.Figure2UISreferencediagramTheoperationisthefollowing,atzerotimetliedeviceswitcheson,closingthecircuit.Duetothepresenceofaninductance,(considenngsomeresistanceduetothelayoutandtheONresistanceoftheMOSFET)thecurrentincreasesfollowinganexponentiallaw,asafunctionoftheL/Rcharactenstiesoftli

21、ecircuit.Figure3TypicalUISwaveformsAssoonasthedeviceisswitchedoff,asthemagneticfieldintheinductancecannotinstantaneouslygotozero,thedi/dtcausesanovervoltageonthedrainofthedeviceNaturally,thedeviceispracticallyanopencircuituptoitsownblockingvoltage,thereforetheextravoltageislimitedbytheBVDSSoftheDUTD

22、uringtheavalanche,thecurrentflowstliroughtheDUT,dissipatingtheaccumulatedenergythatwasstoredinthecoilduringthechaigingTable1explainsseveralrelationsforthetav,EavandtliePavgwithseveralcircuitconfigurations.Adeviceiscommonlydefinedrugged,oravalaiiche-rated,ifatsomestatedconditionsofcoilandconductedcur

23、rentitsurvivestinstest.Inthepast,othercircuitsweresuggestedtotesttinsdevicecapabilityliketlieoneinFigure4Thecurrentismaintainedconstantforasettimeeliminatingthedependencyfortheenergytobewitlistoodonthecoil,butactuallytheonlyrecognizedmethod(JEDECstandardNo.24-5,MIL-STD750Dmethod30402)isthecircuitdes

24、cribedinFigure2.AcircuitcommonlyusedtotesttheAvalancheruggednessoftheMOSFETisshowninfigure5.IthasaspecialfeatureofapowerswitchinsenestotheVDDtliatconnectsthevoltagesourcetothecircuitonlydunngthecoilcharging,disconnectingitafewmicrosecondsbeforetheswitch-offandtheavalancheoperationThistechniqueallows

25、toincreasetheVddbeyonditsmaximumratedVDS,speedsupthechargeoftliecoilduringturnon,andconsequentlydecreasestheturnonstatetime.Also,theenergydissipatedisdifferentasonecanreadinthetable1DatasheetRatingsWhenthedeviceisclassifiedasAvalancheRated0,tliedatasheetprovidestheend-usersomeusefulparameters,whichd

26、efinetheratingsofthedeviceduringavalanche-Iar,definedastliemaximumcurrentthatcanflowthroughthedeviceduringtheavalancheoperationswitlioutanyBJTlatclungphenomenonThisMaximumlnxutmustbeconsideredasanabsolutemaximiunratingEvenifthecriticalcurrenttobnngthedevicetofailureishiglierthantheIAR,theproducergua

27、ranteestheoperationofthedevicebelowthislimit.Besides,itisusuallytestedforseveralmicroseconds.AllSTMicroelectronicsHighVoltagePowerMOSFETaretestedaccordingtotheIarAlltheavalancheoperations(singleeventorrepetitive)belowthiscurrentvaluecanbeconsideredsafeunlesspowerdissipationissues.Itisimportanttounde

28、rlinethatforMOSFETsconnectedinparallel,thecurrentthatisswitchedduringtheavalanchephenomenonisnotshared,differentlyfromtlieoperationsinconductionstateInfact,atturnoffonlythedevicewiththelowerbreakdownand/orwiththefasterswitchwillgointoavalanche,withstandingthetotalcurrentthatduringtheonstateissharedw

29、iththeotherMOSFETsinparallelIfsuchcurrentismorethantheIar,thedevicecanfail.Eveniftheenergyassociatedtothateventisverylow,failureisduetotheactivationoftlieMOSFETsparasiticbipolar-EAS(EnergyduringAvalancheforSinglepulse)isdefinedasthemaximumenergythatcanbedissipatedbythedeviceduringasinglepulseavalanc

30、heoperation,(atthecircuitconditionsdescnbedabove),attheIarandatthestartingjunctiontemperatureof25C,tobringthejunctiontemperatureuptothemaximumonestatedintheabsolutemaximumratings.Ofcourse,thisvaluedecreasesasthestartingjunctiontemperatureincreases.UsuallyinsomedatasheetanenergyderatingcurvecalledAva

31、lancheEnergyvsstartingTj*,isprovidedfigure6.AllthesingleeventavalancheoperationsbelowtinsEnergyvalueareconsideredsafeforthedeviceifthejunctionstartingTemperatureis25C,switchingadraincurrentlessorequaltoIarIfTj25C,theendusercanrefertothecurveAvalancheEnergyvsstartingtoapplythenghtderating.Figure6EASv

32、stemperatureplotofSTP9NK80ZFromtheboardanalysis,wefindthatthedeviceforsuchpowersupplyconditioncanexpenenceasinglepulseavalancheoperationThemeasurementshaveshownthattheaveragejunctiontemperatureis100C,thepeakdraincurrentswitchedduringtheavalancheis4Aandtheenergythatisdissipatedduringtliatsingleavalan

33、cheoperationis0.24inJ.Tounderstandifthedeviceisworkingwitluntlieratings,wehavetochecktheswitchedcurrent,comparingittotheIar,becausetheIdpeakvalueis4AandbelowtheIar,thisfirstratingissatisfiedNow,tounderstandifthejunctiontemperatureisbelowtheT)max,wesupposethatbeforetlieavalanchethejunctiontemperature

34、istlieaverageone,100C.LookingattheplotAvalancheEnergyvsstartingTj,tlieenergytobnngthejunctiontemperaturetotlieallowedmaximumrating,starting&om100C,isapproximately50mJ.Sincetheenergymeasuredisbelowtliatvalue,themaximumjunctiontemperaturereachedduringtheavalanchewillbelessthantheTjmax.Consideringtliat

35、boththebonds,currentbelowIarandjunctiontemperaturebelowmaximumtemperaturearewellsatisfied,onecansafelystatetliattliedevice(underthatsingleavalancheevent)isworkingwithintheratings.ItisinterestingtounderstandhowtinsEASvalueissetbySTMioroelectronics,alsobecauseeachMOSFETmanufacturerstateshis/herownappr

36、oachandfindingsInordertoprovideaclearexplanation,theEASvalueisnotsimpletostate,asitisverydifficulttolookatthejunctiontemperaturedunngtheavalancheoperation.SomemanufacturerssettinsvaluebythethermalimpedancestatedinthedatasheetThiscouldbeaninterestingapproach,butsomeconcernswouldhavetobetakenintoconsi

37、derationThedatasheetthermalimpedanceistheresponseofthesystemtoarectangularpowerpulse,maintainingthepackagecaseat25CExample.LetssupposetohaveanSTP9NKS0ZworkingasmainswitchofaDC/DCconverterThedatasheetratingsoftinsdevicesareEas=350inJIar=7.5A,Tjmax=150C=Z%(r)九”Theapplicationoftheabovementionedformulaw

38、itlioutanymodificationcannotgivepreciseinformation,infactconsidenngtwopowerpulseshavingthesamepeak,onerectangular,theothertriangular,weobviouslyfindthatthepeaktemperatureisquitedifferent(figure7).TousetheZth,thetnangularpulsecanbeapproximatedasrectangularwithscaledamplitudeandwidtligoodapproximation

39、inordertocalculatethetemperatureincreasewitlunthedeviceduringtheavalancheoperationiftliepulsehasashortduration.Starting&omtinsmodel,theexperimentalverificationismadelookingtotheVDSshapeduringtheAvalancheInfactthebreakdownvoltageofthedeviceisnotconstantwhenvaryingthetemperatureandthecurrent2Avalanche

40、characteristicsandratingsofPowerMOSFET # #Figuie7ThermalresponseoftiiangularpulseFigure8BVDSSvsIDAvalanchecharacteristicsandratingsofPowerMOSFET Onemoreimportantconcernabouttheuseofthermalimpedance,isthatusuallyitisexperimentallyandtheoreticallycalculatedconsideringtheon-stateofthedeviceandsoapowerd

41、istnbutionwithinthedevicedifferent&omtheoneduringtheavalanche-state.STMicroelectronics*approachtoEASstatementstarts&omatlieoreticalthermalmodeloftliediewithsomeexperimentalverifications.TocalculatethemaximumtemperatureinthejunctionduringAvalanche,anequationliketheonebelowcanbeused5:=食底+1Where切isthea

42、valanchetime(s)AdieArea(m3)Popeakpower(W)Ksiliconsthermalconstant(WinsxK)Itcomesfromthearrangementofthesolutionofthegeneralequationofheattransmission(Fourierequation),forthespecialcaseofaninfinitemedia(bi-dimensionalcase)subordinatedtoashortrectangularpowersourcepulseuniformlydistributedoveranarea1T

43、heeq1,appliedtoatriangularpowerpulse,isasimpleandThevanationoftlieblockingvoltagewiththecurrentisnearlylinearandthisresistanceisspecificofthedie-sizeofagivensetofdesignrules,epitaxiallayerandpackage.FigureSshowstliemeasuredcharacteristicofBVDSSvstheIdfora700Vdevice.Thevoltagevariationwiththetemperat

44、ureispositivetoo,asshowninthedatasheetplot,andthevariationdependsonthestnicturalcharacteristicsofthesilicondieBytakingtlieseintoaccount,thejunctiontemperaturefiromtheVDSshapecanbeextrapolated.Figuie9VariationofVDS/IDsliapeswiththeCoilduiingtheUISFig9showstherealVDSshapeduringavalanche,wheretheincrea

45、seofVDSobservedisduetothenseofthesilicontemperature.Figure9alsoshowsthewaveformsforthesamedevicesubmittedtodifferentCoils(i.e.differentenergy)atthesamepeakcurrentItisevidentthatthedifferentvdsshapesareduetodifferentmaximumtemperaturesreachedbythejunctionTlielinearrelationshipoftheVdswiththedietemper

46、aturebecomesclearwhencomparingtheVdsquasiparabolicshapetothetemperatureprofileforatnangularpowerpulseinfigure7.EAR(EnergyduringAvalancheforRepetitivepulse)isoftenstatedinthedatasheet.ThisvalueisdefinedastheenergyatIar,duringrepetitiveoperation,andthevalueisclassicallyfoundconsideringalOKHzpowerpulse

47、trainwithadutycycleof50%andthenominalpowerratingofthedevice:Earpoer/fForinstance,iftheavailablepoweroftheMOSFET(Pdinthedatasheet)is150W,theEARshouldbeequalto15mJ.Itiseasytosaythatthisvalueisredundantandnotveryusefulforthedesign.Itgivesverypoorinformationabouttliemaximumenergytobedissipatedtomaintain

48、thejunctiontemperaturebelowtlieTjmax.AquickwaytoevaluatethetemperatureduringavalancheistotakeintoaccounttheextrapowerdissipationintroducedbytheRepetitiveavalanche,tocalculatetheheatsinksizetodissipateitbecauseofatriangularpulseE丄nj2川Ifthefrequencyisf,twthetimeinavalanche,VandI,thepeakvoltageandcurre

49、nt,theadditionalaveragepowertobedissipatedisPtl,=Ej=LntArjInadditiontotheconduction(PC0Ki)andswitclung(P)lossesPtot=Pcond+Psw+PovTomakeanexample,wecanstart&omthepreviousSTW9NK80Z.WeconsiderthattheAvalancheintinscaseisnotasingleevent,butarepetitiveonewithafrequency匸50kHz.Forthesingleeventexample,sinc

50、ethecurrentisbelowIarandthejunctiontemperatureisbelow150C,thedeviceworksinsafeoperatingmode.Now,wehavetocheckonlytliatthedevicesubmittedtorepetitiveavalanchemaintainsthejunctiontemperaturebelow150C.Becauseforeachavalanchepulsewedissipate024mJ,theaveragepowerforavalancheisP“=Ef=0.24nJ-50kH:=121VIfthe

51、devicehas:Rthja=Rthjc+Rthcs+Rthsa=10C/WIftheaverageswitchingandconductionlossescomponentequalsto2W,theaveragetemperatureis:Ptot=Pcond+Psw+Pe=217+1217=1417T)=PtotRthja+Ta=140C+TaSincethisistheaveragetemperatureandthepeaktemperaturedunngtheavalancheshouldbelugher,itsclearthattheavalanchephenomenonisge

52、neratingpowersohighthatthesystemthermalbehaviorcannotdissipateinordertomaintainthejunctiontemperaturebelowtheTmax.Theonlysolutionistoreducethetliermalresistanceofthesystem,changingtheheat-sinkorre-designthe卻plicationtoavoidtlieAvalanchefailure.Inordertoobtainamoreaccuratecomputation,theonlywaytoeval

53、uatethemaximumtemperatureduringrepetitiveavalancheistocalculateitbytheZthofthesystem.DespitealltlieconcernsalreadydiscussedaboutadoptingthepublishedZthfortheavalanche,experienceletsussafelystatethattliecomputationresultsforrepetitiveavalanchearesufficientlyguardbandedforrealworldapplicationsSeveralm

54、ethodscanbeusedtofindthesteadystatemaximumtemperaturedunngsteadystateorafterafinitenumberofavalancheoccurrencesThemostfrequentlyandsufficientlyconservativeequationusedinordertofindthemaximumtemperatureforpenodicrectangularpowerpulsesatsteadystateis:込max話岸陋+(1-月Z也刖“2WherePoisthepeakpower,Tthepenodoft

55、hepulsestrainandtpisthepowerpulsewidthConsideringtheapplicationontherepetitiveavalanche(butitcanbeappliedtoseveraloperativemodes)thecalculationofthepeaktemperaturemustalsotakeintoaccounttheotherlosses,duetoconductionandswitchingFigure10ashowstliepowerprofileofatypicalswitchingItiswellknownthatatrian

56、gularpowerpulsecanbemodeledasarectangularpulseofamplitude07ofthepeakpowerandwidth0.71oftherealone.Moreover,thataramppowershapecanbemodeledasasteppulseofwidth0.56andamplitude0.89oftherealones（Table2）.Figure11showsthesimulatedtemperatureresponseofanarbitrarythennalimpedancetothetnangularpowerpulse（blu

57、eline）comparedtotherectangularapproximation（redline）Sotheaveragepowercanbecalculatedfromeachpeakpower:Bysuperimpositionpnnapleandthemanipulationofequation2,wecanfindthepeaktemperatureduetoeverysinglepowerpulseoffigure10.Forthefirstpulse:AT.maxl=Pm:（Rihu-Z也+以）+0.7Z%（門）Forthesecond:AT；max,=PE（Rthti-Zt

58、hJ+叮）+0.89遲Z傀（fj+0.7丹z也q-如（tni+rjAndforthetlurdone:max嚴(yán)巴沏“+D+切+r；）+0.7RZ%（門）+0.7丹-如+h+&+sJ-Z也以+L+r；+如）+E+0.89遲Z也蟲+f；+如）0；+切）AvalanchecharacteristicsandratingsofPowerMOSFET # #AvalanchecharacteristicsandratingsofPowerMOSFET # #LetsmakearealexampleFigure12showstheswitchingwaveformsofSTP11NM60FP,anSTMi

59、croelectromcMDMesh,workingasmainswitchinanACadapter.ThewaveformsshowthatthedraintosourcevoltageisgoingbeyondtheVdsAbsolutemaximumrating,repetitivelywithaAvalanchecharacteristicsandratingsofPowerMOSFETAvalanchecharacteristicsandratingsofPowerMOSFETAvalanchecharacteristicsandratingsofPowerMOSFETAvalan

60、checharacteristicsandratingsofPowerMOSFETperiodof12us.Figure13showsadetailedwaveformoftheavalanchephenomenon.Tocheckifthedeviceisworkingwitlunthespecificationwehavetocheckthedatasheetratings.Thedatawehave:T=12usTcase=70Cton=4OnsPpeakcn=l60V*1.6A=256Wtof60nsPpeakoff=280V*2.4A=672Wtcond=2.4usPpeakcand