ApplicationNoteUSCi_AN0011–August2016

Turn-OffCharacteristicsofSiCJBSDiodesLarryLi

USCi_AN0011–August2016 Turn-OffCharacteristicsofSiCJBSDiodes 1UnitedSiliconCarbide

Abstract

SiC junctionbarrier schottky (JBS)diodes,asmajority carrierdevices,haveverydifferent turn-off characteristicsfromconventionalSiPiNdiodes.Thespecificationdatapresentedinthedatasheetsarenotenoughtofullycovertheturn-offcharacteristicsofSiCJBSdiodes.Thisapplicationnotepresentscomprehensiveexperimentalresultstorevealtheturn-offbehaviorofSiCJBSdiodesandservesasasupplementtothedatasheets.

Table of Contents

Abstract.........................................................................................................................................................................1

1 Introduction...........................................................................................................................................................2

2 ExperimentSetup..................................................................................................................................................23 ExperimentalResults.............................................................................................................................................3

3.1 QCatDifferentForwardCurrentsIF.............................................................................................................3

3.2 QcatDifferentJunctionTemperaturesTj....................................................................................................33.3 QcatDifferentdiF/dtRates..........................................................................................................................3

3.4 SwitchingTimetC.........................................................................................................................................4

4 Summary...............................................................................................................................................................5

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1 Introduction

SiCJBSdiodesaremajoritycarrierdevicesandhavenominoritycarriersinjectedintoandstoredinthedriftlayerduringnormalforwardoperation.Therefore,unlikeSiPiNdiodes,SiCJBSdiodeshavenostoredchargeorstoredchargerelatedreverserecoverytimeandcanbeturnedoffmuchfaster.

Likeanyothersemiconductordevice,SiCJBSdiodesmustdevelopadepletionregioninthedriftlayerinordertosupportahighvoltageduringoff-state.Thedepletionregionformsajunctioncapacitorinthedevice,whichmeanstheturn-offprocessoftheSiCJBSdiodesisessentiallythechargingprocessofthejunctioncapacitor.Thejunctioncapacitorisfullydeterminedbythedesignwhichmakestheturn-offprocessoftheSiCJBSdiodesindependentoftemperatureandforwardcurrentlevel.

Duringtheturn-offtransient,areversecurrentmustbedevelopedtochargethejunctioncapacitor.Therequiredtotal charge, Qc, is provided in the datasheets. Qc can be measured or more accurately can be calculated byintegrating the Cj vs. Vr curve of the JBS diode. The charge Qc is completely determined by the JBS design,independentof temperature, forwardcurrent level,anddi/dt rate. Itwillbeshownthat thisparameteralone issufficienttodescribetherecoverytransientoftheSiCJBSDiode.

This application note presents comprehensive experimental results to verify the above conclusions and gives acompletepictureoftheturn-offbehaviorofSiCJBSdiodes.

2 ExperimentSetup

Fig.1showstheexperimentsetupusedtomeasurethecapacitivechargeQcofaSiCJBSdiode.Itisadouble-pulsetest setupwith theswitchplacedon thehigh-sideso thatawidebandwidthsingle-endedvoltageprobecanbeusedtomeasurethediodevoltageaccurately.ASiCcascodeisusedastheswitch.AllexperimentsareperformedataDCbusvoltage(VBUS)of800V.ThecurrentoftheJBSdiodeismeasuredwithacurrenttransformerandtheQc is obtained by integrating the measured current waveform. For comparison, Fig. 2 presents the capacitivechargeQcofUSCi’s1200VSiCJBSdiodescalculatedbyintegratingthetypicalCjvs.Vrcurve.

Fig.1:ExperimentsetupformeasuringthecapacitivechargeQCofaSiCJBSdiode.

Fig.2:CapacitivechargesQCofUSCi’s1200VSiCJBSdiodesobtainedbyintegratingthetypicalCjvs.Vrcurves.

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3 ExperimentalResults

3.1 QCatDifferentForwardCurrentsIFFig.3ashowsthemeasuredturn-offwaveformsofthe1200V-15ASiCJBSdiodeUJ2D1215TattheforwardcurrentIFequalto6Aand16A.ItisseenfromFig.3athatthereversecurrentoscillationisslightlylargerandtheturn-offspeedisslightlyfasteratIF=6Abecausetheswitch(UJC1206K)turnsonfasteratalowercurrent.TheswitchingtimetcoftheJBSdiodecanbedefinedasthetimeintervalbetweenthetimeofzero-crossingofIFtothetimeofVrreaching90%ofVBUS.Thus,theswitchingtimetcofUJC1215Tis33nsatIF=6Aand36nsatIF=16A.

Fig.3bshowsthemeasuredcapacitivechargeQcbyintegratingthereversecurrentwaveformsstartingatt=55nswheretheforwardcurrentsIFcrosszero. It isseenthatthemeasuredQc isbasicallythesameatIF=6AandIF=16A,confirmingthatthecapacitivechargeQcisindependentoftheforwardcurrentlevel.ItalsocanbeseenfromFig.3bthatthemeasuredQcisveryclosetotheQcobtainedbyintegratingthetypicalCjvs.Vrcurve,indicatingthe“reverserecoverycharge”ofSiCJBSdiodesiscapacitivechargeinnature.

3.2 QcatDifferentJunctionTemperaturesTjFig.4presentsthemeasuredturn-offwaveformsofthe1200V-15ASiCJBSdiodeUJ2D1215TatTj=25°Cand150°C.It is seen that there is almost no change in the turn-off current and voltage waveforms when the junctiontemperature is increasedfrom25°Cto150°C,confirmingthattheturn-offcharacteristicsoftheSiCJBSdiodes isindependentofthejunctiontemperature.

3.3 QcatDifferentdiF/dtRatesFig.5 shows the measured turn-off waveforms and the capacitive charge Qc of the 1200V-15A SiC JBS diodeUJ2D1215TatdifferentdiF/dtrates.ThediF/dtrateismeasuredatthezero-crossingpointoftheforwardcurrentIF.WhenthediF/dtrate is increased,thepeakreversecurrent isalso increasedcorrespondently.ButthemeasuredcapacitivechargeQchasnochangewiththeincreaseofthediF/dtrate.

(a)(b)Fig.3:Measuredturn-offwaveformsofthe1200V-15ASiCJBSdiodeUJ2D1215Tatdifferentforwardcurrents(a)andmeasuredcapacitivechargeQcbyintegratingthereversecurrentwaveform(b).

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3.4 SwitchingTimetCTheswitchingtimetcofSiCJBSdiodesisessentiallythetimeittakestochargethejunctioncapacitorCjfrom0VtotheDCbusvoltageVBUS.Thiscapacitorchargingprocessisstronglydependentonthetestingsystemsetup,suchasthesystemRCtimeconstantandtheswitchingspeedoftheswitch,etc.Therefore,theswitchingtimetcisnotagoodparameterfordescribingtheswitchingperformanceofSiCJBSdiodesandisnotprovidedinthedatasheetsofUSCi’sSiCJBSdiodes.

Fig.6showsthemeasuredturn-offwaveformsandcapacitivechargeofUSCi’s1200VSiC JBSdiodesUJ2D1215T,UJ2D1210T,andUJ2D1205T.TheUSCi’s1200VSiCcascodeUJC1206K isusedas theswitch. It isseenthat thesethree SiC JBS diodes display basically the same switching time tc of about 33ns even though they have verydifferent capacitive chargesQc and current ratings. This indicates the switching time tc is pinnedby the testingsystem or the switchUJC1206K. When the status of the testing system is changed, for example the switchingspeed or diF/dt rate is increased, the switching time tc will change accordingly. Fig.7 the switching time tc of

(a) (b)Fig.6:Measuredturn-offwaveforms(a)andcapacitivechargeQc(b)ofthe1200VSiCJBSdiodesUJ2D1215T,UJ2D1210T,andUJ2D1205TatTJ=25°C.Switch:UJC1206K.

Fig.4:Measuredturn-offwaveformsandcapacitivechargeQcofthe1200V-15ASiCJBSdiodeUJ2D1215TatTJ=25°Cand150°C.

Fig.5:Measuredturn-offwaveformsandcapacitivechargeQcofthe1200V-15ASiCJBSdiodeUJ2D1215TatdifferentdiF/dtrates.

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UJ2D1215TatdifferentdiF/dtrates.WhenthediF/dtrate is increasedfrom600A/�sto1,400A/�s,theswitchingtime tc isdecreased from44ns to36ns.Fig.8compares the turn-offwaveformsof theSiC JBSdiodeUJ2D1215Twithdifferentswitchesinthetestsetup.Itisseenthat,underaboutthesamediF/dtrate,theswitchingtimetcisdecreasedfrom37nsto33nswhentheswitchinthetestingsystemischangedfromUJC1206KtoUJC1210K.ThisisbecauseUJC1210KhassmallercapacitancesthanUJC1206Kandturnsonfaster.

4 Summary

SiCJBSdiodesaremajoritycarrierdeviceshavingnostoredcharge,andcanbeturnedoffmuchfasterthanSiPiNdiodes.ThekeyfeaturesofSiCJBSdiodesarelistedbelow:

- Turn-offprocessisthechargingprocessofthejunctioncapacitor;- CapacitivechargeQcisindependentofthejunctiontemperatureTj;- CapacitivechargeQcisindependentoftheforwardcurrentlevelIF;- CapacitivechargeQcisindependentofthediF/dtrate;- CapacitivechargeQcissolelydeterminedbythedevicedesign;- Switchingtinetcismainlydeterminedbythetestsystem.

Fig.7:MeasuredswitchingtimetcoftheSiCJBSdiodeUJ2D1215.TJ=25°C,VBUS=800V,IF=16A,Switch:UJC1206K.

Fig.8:Comparisonoftheturn-offwaveformsoftheSiCJBSdiodeUJ2D1215Twithdifferentswitches.