在高边工作的MOSFET,其可能的失效模式和失效现象

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PowerMOSFETinHigh-SideOperatingModes,PossibleFailureModes,andFailureSignaturesPowerMOSFETsinhigh-sideapplicationcanfailunderanyoneofthefollowingmodesofoperation:(a)High-impedancegatedrive(b)Electro-staticdischarge(ESD)e*posure(c)Electricalover-stressed(EOS)operationInmostcases,failureanalysisofthedamagedMOSFETrevealsasignaturethatcanpointtowardsapossiblecauseoffailure.(a)High-ImpedanceGateDriveAgenericschematicconfigurationforahigh-sidedriverwithinductiveloadisshowninfigure1.Atypicalgatedrivedesignbasedonanapplication-specificintegratedcircuit(ASIC)caterstotheprimarygatechargerequirementsoftheMOSFET,whichareontheorderofafewtenstohundredsofnanocoulombs.AnefforttoachievehighefficiencyandoptimizetheintegrationoftheASICresultsinlow-current(<50μA)gatedrives(V2referencedtopowersupplyground)withahighoutputimpedance(>1M•).Thisbasicallysatisfiesthetotalgate-chargerequirementsoftheMOSFET.However,toavoidoverloadingtheASIC,thee*ternalgateresistor(R2)insuchadesigntendstohaveahighohmicvalue(rangingfromtensofohmstoseveralkiloohms).Thefeedbackresistor(R3)alsousuallyhasahighohmicvalue(rangingfromseveralkiloohmsto1M•).Asaresult,thesource(S)ofthepowerMOSFET(U1)isvirtuallyfloating.Theeffectismagnifiedduringturn-offandrecirculationoftheenergystoredintheinductor(L1).Fig.1-TypicalHighSideDriverThisapproachfullysatisfiestherequirementsforsteady-stateoperation,wheresmallamountofcurrent,ontheorderofafewmicroamps,isallthatisrequiredtomaintaintheonstate.Butitcanbeinadequatefordynamic(transient)turn-onandturn-offoperationofMOSFET.Inthislattercase,theMOSFETswitchingoperationcaneasilyoccurinlinearmode,whichisanunstableandnon-characterizedareaofMOSFEToperation(figure2).Fig.2-FloatingSourceandSlowTurn-OffDuringturn-onandturn-off,switchingpowerlossesarelikelytodissipatefromaverysmallareaoftheactivedie.Theresultingfailuresignatureishigh-powerburn-out,witharandomamountofdamagelocatedintheactiveareaofthedie.InvariablysuchfailureislabeledasEOS.AnothersubtlefailureevidencedduringtheanalysisofthefailedMOSFET(withavirtuallyfloatinggatecircuitconfiguration)isinthegate-sourcearea.ADCparametertestofthegate-sourceareashowsoutofspecreadings.ThefailuremodeissubtleinsofarastheleakydevicerecoversevenduringtheDCparametertest.Thisfailuremoderemainsunderinvestigationtoobtainafulle*planation.Afailuree*ampleofaDPAKpowerMOSFETisshowninthefigures3,4,and5.Fig.3-EOSDamageLocationFig.4-DamageonBPSGFig.5-DamageonGatePoly(b)Electro-StaticDischarge(ESD)E*posuresAsecondpossiblescenarioarisingfromavirtuallyfloatinggateconditioncausedbythehighimpedanceofanASICdriverand/orhighgateresistorisincreasedsusceptibilityofthegate-sourceareafordamagefromESDorasimilarevent.InadditiontoatypicalESDevent,high-energy,high-voltagetransientsofshortdurationcanbegeneratedfromenergyre-circulationand/orenergyinterruptionfrominductiveponents.Withreferencetofigure6,theseincludethelow-sideload(L1,R1),otherparallelconnectedload(L2,R4),andseriesfeed-in/harnessimpedance(L3,R5).Fig.6-L1,L2,L3SourcesofTransientsFailuresignaturesareusuallylocatednearthegateterminationandinthegate-sourcearea.Ane*ampleofaDPAKpowerMOSFETfailureisshowninfigures7,8and9.Fig.7-ESDDamageneartheGate-SourceFig.8-ESDontheGateTerminationFig.9-Close-upofESDDamage(c)ElectricalOverStressed(EOS)OperationHigh-energyorhigh-voltageshort-durationtransientsappearingonthedrain-sourceterminationoftheMOSFET(figure6)canresultindamagewithavarietyofsignatures.Transientsthatarefasterthanthebreakdowntimeofthebodydiodemayproducepuncturesanywhereintheactiveareaofthedrain-source.Suchpunctureshavetheappearanceofapin-holeatthedamagesite.TheymaybeattributedtoanESDeventoranESD-likeeventarisingfromothercircuitponentsorparasitics.Whereenergy/voltagevaluesarenottoohighbutofasufficientdurationtobreakdownthebodydiode,avalancheresults.Avalancheresultsine*tensivedamageandinfactdestroystheevidencethatfailureanalysiswouldneedtodeterminetherootcause.ThesearetrueEOSevents.EOScanalsoresultfromamismatchofthermalmanagementandcontinuouspowerdissipationinthedevice,evenwhenthedeviceisfullyturnedon.Whenheat-sinkingisinadequate,heatcanbuildupinthejunctionduringcontinuousoperation.Eventuallyathermalrunawayoccurs.ThedeviceispletelydestroyedandalsoclassifiedasanEOS.D2PAKe*ample:figures10to12arefailuresignaturesforaD2PAKpowerMOSFET.Fig.10-HighPowerEOS(afterDecapsulation)Fig.11-EOSSignatureafterRemovingTopMetalLayerFig.12-EOSDamageat50*MagnificationSO-8duale*ample:figures13to15areanothere*ampleofadualSO-8powerMOSFETfailureidentifiedasEOSFig.13-FailurenearBondWireFig.14-EOSinActiveRegionFig.15-Close-upofEOSDamag