currentsnot

byKentWaltersandBobWernerSchottkyrectifiershavebeenusedforover25yearsinthepowersupplyindustry.
Theprimaryadvantagesareverylowforwardvoltagedropandswitchingspeedsthatapproachzerotimemakingthemidealforoutputstagesofswitchingpowersupplies.
Thislatterfeaturehasalsostimulatedtheiradditionaluseinveryhighfrequencyapplicationsincludingverylowpowerinvolvingsignalandswitchingdioderequirementsoflessthan100picoseconds.
TheserequiresmallSchottkydeviceswithlowcapacitance.
ThereverserecoverytimeofSchottkydiodesareextremelyfast(butsoft)recoverycharacteristics.
Whatlittlereverserecoverytimetheymayexhibitisprimarilydictatedbytheircapacitanceratherthanminoritycarrierrecombinationasinconventionalpnjunctionrectifiers.
ThischaracteristicprovidesverylittlereversecurrentovershootwhenswitchingtheSchottkyfromtheforwardconductingmodetothereverseblockingstate.
Thecombinationofvery"fast-softswitchingproperties"ofaSchottkycanalsoeliminatetheneedforsnubbercircuitsinmanyapplicationsthatmayotherwiseberequiredwithfastorultrafastrectifiersdisplayingabruptrecoverycharacteristics.
Thesefeaturesmakeschottkyrectifiersaveryattractivechoiceforlowparasiticswitchinglosses.
DesignconsiderationswithSchottkydevicesarelimitedinsomeapplicationscomparedtopnjunctionrectifiersbecausetheirreverseleakagecurrentsaremanytimeshigher.
AlsoSchottkyrectifiershavemaximumratedjunctiontemperaturestypicallyintherangeof125°Cto175°C,comparedtothetypical200°Cforconventionalpnjunctionswhichfurtherinfluencesleakagecurrentbehavior.
Forsomeapplications,Schottkydevicesarelimitedinavailablereverseblockingvoltageratingscomparedtoconventionalpnjunctionrectifiers.
Neverthelesswithjudiciousselection,manyapplicationsareoptimizedwithSchottkyrectifiersandtheiruniqueoperatingcharacteristics.
Schottkyrectifiersseldomexceed100voltsintheirworkingpeakreversevoltage(VRWM),sincedevicesmoderatelyabovethisratinglevelwillresultinforwardvoltagesequaltoorgreaterthanequivalentpnjunctionrectifiers.
TheSchottkyrectifierpropertiesdescribedaboveareprimarilydeterminedbythemetalenergybarrierheightofmaterialdepositedonthesiliconbythemanufacturer.
Ametalwithalowenergybarrierheightwillminimizeforwardvoltage,butwillalsoberestrictedinitshightemperatureoperatingcapabilityandhaveveryhighreverseleakagecurrents.
Ahighbarriermetalheightselectionwillminimizetemperatureandleakagecurrentsensitivitybutwillincreasetheforwardvoltage.
Dependingontheapplicationrequirements,thesedesignfeaturescanbeusedasatradeoffinproperchoicewhenselectingaschottkyrectifierusingdifferentbarriermetalsfromamanufacturer.
MicrosemiCorporationoffersavarietyofbarriermetaloptionsonann-epitaxiallayeroveralowresistivitysubstrateforoptimizingparametricperformanceinadditiontoaprotectiveguardringandpassivation.
ThisconfigurationisshowninFigure1.
AreliableschottkyjunctionisdesignedwithapnjunctionguardringtoIntroductiontoSchottkyRectifiersMicroNoteSeries401terminatetheperimeterwithadiffusedpregion.
Thisalsoservesasatransientvoltagesuppressorforreverseenergyabsorptionandover-voltageprotectionincloseproximitytotheSchottkyjunction.
ThisperimeterregioneffectivelyisdrivenintoavalanchebreakdownbeforetheSchottkyisdamagedbyexcessiveamountsofreversecurrentflowandenergyduringtransientevents.
Thevoltage-currentdeviceprotectiverelationisillustratedinFigure2alongwiththetypicalelectricalparametersspecifiedforSchottkyrectifiers.
Insomecases,Schottkyrectifiersarealsospecifiedwithareverseavalancheenergytest.
ThiswillhelpensureasafelevelofoperationinveryfastswitchedapplicationsresultinginhighLdi/dtinductive(suchastransformer)voltagekicksorothersourcesofover-voltagetransientsthatcanbrieflydrivetheSchottkybeyonditsmaximumratedVRWM.
Insuchapplications,thesespecialreverseenergyrequirementsshouldberequestedforadditionalscreeningbytheuserwhennototherwisespecified.
MicrosemiCorporateApplicationsKentWalters(602)941-6300SchottkyBarrierMetalFigure1VerticalstructureofSchottkyRectifierDieReverseVoltageVBRForwardVoltageReverseCurrentAvalancheofpnjunctionSchottkywithoutguardringVRWMSchottkyJunction25°C100°Cto150°CForwardCurrentFigure2SchottkyVoltage-CurrentCharacteristicsandTypicalParametersPARAMETERSSPECIFIEDAT25°CVFForwardVoltageatIFIFForwardCurrentVRWMWorkingPeakReverseVoltageIFSMForwardSurgeCurrentMaximum(Specifiednon-repetitive8.
3ms,1/2sine)IRMaximumLeakageCurrentatVRWMVBRNotSpecified(Typically20%to30%higherthanVRWM)IRatVRWM25°CIRatVRWM100to150°CIFVFatIFSeries401SchottkyRectifiersSiO2PassivationFrontMetaln+substraten-epilayerGuardringBackMetal