Download Recommended External Circuitry for Transphorm GaN FETs

RecommendedExternalCircuitry
forTransphormGaNFETs
ZanHuang
JasonCuadra
ApplicationNoteAN0009
Rev.1.0
November22,2016
RecommendedExternalCircuitryforTransphormGaNFETs
AN0009
TableofContents
1
Introduction
3
2
Sustainedoscillation
3
3
Solutionstosuppressthesustainedoscillation
5
4
Verifyingthesolution
9
5
Conclusion
10
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RecommendedExternalCircuitryforTransphormGaNFETs
AN0009
1
Introduction
Transphormgalliumnitride(GaN)FETsprovidesignificantadvantagesoversilicon(Si)superjunction
MOSFETs,offeringlowergatecharge,fasterswitchingspeeds,andsmallerreverserecoverycharge.GaN
FETsexhibitin-circuitswitchingspeedsinexcessof150V/ns,comparedtocurrentsilicontechnologyat
lessthan50V/ns.ThefastswitchingofGaNdevicesreducescurrent-voltagecross-overlossesand
enableshighfrequencyoperationwhilesimultaneouslyachievinghighefficiency.
However,theaccompanyinghighdi/dtduringswitching,combinedwithparasiticinductances,
generatesnoisevoltagesinthecircuit.ThisnoisecaninterferewiththeVGSofthedevice,andinthe
worstcase,createssustainedoscillationwhichmustbepreventedforsafeoperationofthecircuit.In
thisapplicationnote,wesuggestaddingadrainandagateferritebeadtopreventtheoscillation.
Knowngoodvaluesofferritebeadsareprovidedforeachofourdevicesinahard-switchinghalf-bridge
circuit,whichisthemostpronetosustainedoscillation.
2
Sustainedoscillation
Inahalf-bridgecircuitwithhighspeeddevicesonboththehighandlowside,therearethreestepsto
yieldthesustainedoscillationonthehighsideduringlow-sidedeviceturn-on,andviceversa.
STEP 1: VGS CHANGE DUE TO HIGH DV /DT
Considertheturn-oneventofthelow-sidedevice(Q2)inFigure1.Thehalf-bridgeoutputnodefalls
fromVDCto0,atsomerateofdVDS/dt.Thevoltageacrossthehigh-sidedeviceQ1risesfrom0toVDCat
thesamerate.Thehigh-sidedevice(Q1)gatecapacitanceCgsischargedthroughCgdandthefast-rising
dv/dt.ThisvoltageincreasesVGSofthehigh-sidedevice,reducingoff-voltagemargin(VGSgetscloserto
VTH).
Figure1.Low-sidedeviceturn-ontransient
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RecommendedExternalCircuitryforTransphormGaNFETs
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STEP 2: VGS CHANGE DUE TO HIGH DI/DT
Whenthelow-sideGaNFET(Q2)turnson,thehigh-sideGaNFETdraincurrentdecreasesatsomerate
ofnegativedi/dt.Thishighnegativedi/dtmultipliedbystrayinductanceLsinthePCBlayoutproducesa
voltageVLSwhichalsoreducestheoff-voltagemarginoftheFET(Figure2).
Lpcb
Ld
Coss
Rg
Vg_off
Vgs
Lrtn
Cb
iL
Ls + VLs
Lf
Ld
Coss
Ls
Lpcb
Figure2.Highdi/dtanlargeLscausetheFEThavelessoffbiasvoltagemargin
Theequationoffirstandsecondstepscanbedescribedbythebelowequation:
VGS_FET=Vg_off+F*VDS-VLs
=Vg_off+F*VDS+(-Ls*di/dt)
WhereFisthefeedbackfactorofVDStoVGS.
STEP 3: OSCILLATIONS DUE TO LARGE VOLTAGE RINGING
TransphormGaNdevicesaredesignedwithalowratioofCgdtoCgs,minimizingtheMillereffect.The
oscillationusuallycannotbesustainedunlessVGSispulledclosetothethresholdvoltageVTH.However,if
VTHisreached,thedevicewilloperateinthelinearregionwithtransconductancegm:
ID=(VGS-VTH)*gm
ThelargegaininthefeedbackloopthatincludesexternalstraycapacitanceCgd_ext(Figure3),coupled
withthenearly180degreephaseshiftofVGSandVDS,cancauseasustainedoscillation.
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RecommendedExternalCircuitryforTransphormGaNFETs
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Figure3.CircuitfeedbackloopthatincludesCgd_ext
3
Solutionstosuppressthesustainedoscillation
Toavoidsustainedoscillation,itisimportanttoguaranteethatthegatestaysoffwhenthe
complementarydeviceisturningon,and/oranypotentialoscillationis“de-Q’d”(theQofanyresonant
tankiskeptoverdamped).Thiscanbeachievedwiththreerequiredstepsandacombinationofthree
options:
REQUIRED: OPTIMIZE PCB LAYOUT tominimizeCgd_ext,whichstronglyaffectsparasiticturn-on
duetodv/dt.NOTE:Thisisrequiredevenforsingle-endednon-half-bridgedesigns.
REQUIRED: USE A FERRITE BEAD IN THE GATE.Atableofrecommendedferritebeadsisprovided
inTable1.RecommendedBOMforTransphormdevicesinhard-switchingbridgesetup.Thisbeadmust
bemountedveryclosetothegateleadofthedevice.NOTE:Thisisrequiredevenforsingle-endednonhalf-bridgedesigns.
HIGHLY RECOMMENDED: ADD A DRAIN FERRITE BEAD(Figure4andFigure5).Adrainferritebead
“de-Q’s”theresonanttankformedbytheCossofthedevicesandtheinductanceinthe“powerloop”
(drain,source,andbypasscapacitorPCBtraceinductance),byactinglikeaseriesdampingresistanceat
thetankresonantfrequency.Typicalresonantfrequencyforthetankrangesfrom50MHzto200MHz
becauseparasiticinductancevariesaccordingtoPCBlayout.Theferritebeadimpedanceat100MHzis
theimportantspecification.NOTE:Thisisrecommendedevenforsingle-endednon-half-bridge
designs.
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RecommendedExternalCircuitryforTransphormGaNFETs
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Figure4.InsertingadrainferritebeadineachTO-220orTO-247deviceinahalf-bridge
EitherathroughholeferritebeadslippedontothedrainleadoranSMDferritebeadcanbeused.
PartnumbersforbothtypesareshowninTable1Table1.RecommendedBOMforTransphormdevicesinhardswitchingbridgesetup.
Figure5.Insertinga“drain”ferritebeadineachSMDdevice
TherecommendedpartnumbersareSMDbeads.NoteinthecaseofSMDFETs,the“drain”beadinthehigh-side
deviceisplacedinthesourcesothelarge“Draintab”,wheretheheatcomesout,isconnectedtoanuninterrupted
PCBcopperpourforgoodheatdissipation.(Thelow-sidedeviceisa“sourcetab”forlowEMI).
Notealsothatthehigh-sidegatedriveisreturneddirectlytothesourcepin.
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RecommendedExternalCircuitryforTransphormGaNFETs
AN0009
TheferritebeadpartnumbersinTable1havebeenextensivelytestedandareverifiedtoprevent
oscillationwithoutexcessivevoltageovershoot,uptoeachdevice’sfullratedcurrent.Notethat
Tranpshormdeviceshavearepetitivepeakdrainvoltagerating“VTDS”whichis150Vgreaterthanthe
maximumcontinuousdrainvoltage(formore,seetheDrainVoltageandAvalancheRatingsapplication)
note.)Thetestcircuitisahalf-bridgesetupdrivenbyaSiLabsSI8273highspeed,high-andlow-side
driver.
Table1.RecommendedBOMforTransphormdevicesinhard-switchingbridgesetup
Device(s)
Package
TPH3202Px
TO-220
TPH3202Lx
PQFN88
TPH3206Px
TO-220
TPH3206Lx
PQFN88
TPH3208PS
TO-220
GateFerriteBead(Ω)
DrainFerriteBead(Ω)
60Ω(MMZ1608Y600B)
Notrequired
7427007141(Wurth)
120Ω(MMZ1608Q121BTA00)
220Ω(MPZ1608S221ATA00)
330Ω(MPZ1608S331ATA00)
MPZ2012S300ATD256A(TDK)
BLM21SN300SN1D8.5A(Murata)
BLM31PG330SH1L6A(Murata)
74270012(Wurth)
2661-000101(FairRite)
330Ω(MPZ1608S331ATA00)
MPZ2012S300ATD256A(TDK)
BLM21SN300SN1D8.5A(Murata)
BLM31PG330SH1L6A(Murata)
TPH3208Lx
PQFN88
TPH3212PS
TO-220
180Ω(MMZ1608S181ATA00)
74270012(Wurth)
2661-000101(FairRite)
TPH3205WS1
TO-247
IncludesinternalFB
ExternalFB(40-60Ω)optional
74270011(Wurth)
MPZ2012S300ATD256A(TDK)
BLM21SN300SN1D8.5A(Murata)
BLM31PG330SH1L6A(Murata)
TPH3207WS1
TO-247
IncludesinternalFB
ExternalFB(40-60Ω)optional
MPZ2012S300ATD256A(TDK)
BLM21SN300SN1D8.5A(Murata)
BLM31PG330SH1L6A(Murata)
Notes:
1. Therecommendeddrainbeadvalueis≤4mΩDCR,15~30Ω@100MHzFB;ifSMDbeadRMScurrentisexceededand
morecurrentcapabilityisneeded,use2or3pieces8.5AFBinparallel
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RecommendedExternalCircuitryforTransphormGaNFETs
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OPTION 1: ADD NEGATIVE VG_OFF VOLTAGE Vg_off=-2Vto-5V.AsimpleAC-couplednegative
gatedrivecircuitisshowninFigure6.
driver
Figure6.Negativegatedrivecircuit
AsinFigure6,assumethesupplyvoltageofthedriverICis12V.The7.5Vzener(D1)clampsthe“on”
gatevoltageto7.5V+0.7V=8.2V(whichisenoughtoensurefullenhancementofthegate)andcharges
C1toabout-3.8V(12V-8.2V).Theminimum“off”statenegativegatedrivevoltagewillbe-3.8V.A4.7V
Zener(D2)isusedinsteadofa1N4148diodebecauseanyleading-edgespikeorovershoot(ringing)on
theturn-onedgeofthegatedrivewouldcausethevoltageonC1tocreepupateveryturn-onedge,
causingtheon-voltagedrivetodropwellbelow8.2V.The4.7VZenerclampsthenegativedrive(and
thusthecapacitorvoltage)toamaximumof4.7V+0.7V=5.4V.Thisisalsosetsthemaximumvoltage
onC1,bydischargingitslightlyattheturn-offedgeif5.4Visexceeded.12V-5.4V=6.6Vsetsthe
minimum“on”voltage.NotethatthesumofthetwoZenervoltagesplus0.7V,i.e.,4.7V+7.5V+0.7V=
12.9Vinthisexample,mustbegreaterthanthesupplyvoltage(12V)orelsethecapacitorwillbe
chargedordischargedheavilyateveryswitchingcycle.Acombinationofan8.2Vand4.3VZenerwill
alsowork.Fora15Vsupplyuse11Vand4.7V.
Note:DONOTattempttouseareversediodeforgateturnoff.Thisreducesthegateoff-voltage
margin.
R1
driver
D1
Figure7.DONOTusediodeinpathofgatedrivecircuit–
itraisesthe“off”gatevoltage,reducingmarginagainstparasiticturn-on
TL072
X1
8
IRF530
Q1
RecommendedExternalCircuitryforTransphormGaNFETs
AN0009
OPTION 2: REDUCE THE TURN-ON DV/DTtoavoidparasiticturn-onbyloweringthegatedrive
voltage(butstay≥8V),orchooseadriverwithreducedturn-oncurrentdrivei.e.,higherturn-ondrive
impedance.
OPTION 3: ADD AND RC SNUBBER TO DAMP THE RINGING ENERGYThedrainferritebeadis
veryeffectiveinpreventingtheoscillationwithoutaffectingcircuitefficiency.However,itdoescreate
someVDSvoltageovershootandthusinsomecasesisnotapreferredsolution.Undersuch
circumstances,anexternalRCsnubberforeachdevicecanbeusedtodamptheringingenergyand
preventthesustainedoscillation(Table2).Note:thegateferritebeadisalwaysrequiredeitherusinga
drainFBorRCsnubber.
Table2.RecommendedBOMforTransphormdevices
whenusinganexternalRCsnubberinsteadofadrainferritebead
Device(s)
Package
TPH3202Px
TO-220
TPH3202Lx
PQFN88
TPH3206Px
TO-220
TPH3206Lx
PQFN88
TPH3208PS
TO-220
TPH3208Lx
PQFN88
TPH3212PS
GateFerriteBead(Ω)
RCSnubberNetwork
60Ω(MMZ1608Y600B)
None
120Ω(MMZ1608Q121BTA00)
220Ω(MPZ1608S221ATA00)
330Ω(MPZ1608S331ATA00)
None
330Ω(MPZ1608S331ATA00)
47pF+7.5Ω
TO-220
180Ω(MMZ1608S181ATA00)
47pF+7.5Ω
TPH3205WS2
TO-247
IncludesinternalFB
ExternalFB(40-60Ω)optional
47pF/100pF+7.5Ω
TPH3207WS2
TO-247
IncludesinternalFB
ExternalFB(40-60Ω)optional
100pF+10Ω
Notes:
2. 47pFRCsnubberallowsswitchingto36A;forextramarginwithaless-than-ideallayout,usea100pFcapacitor;the
switchinglossdifferenceisverysmall
4
Verifyingthesolution
Inordertoverifymarginagainstoscillation,examinetheVDSwaveformattheturn-onandturn-off
switchingedgesatthemaximumdraincurrentoftheapplication.Thismayoccuratstart-uporat
maximumloadstep.Alternatively,adouble-pulseormulti-pulsetestcanbeperformedintheactual
layout,upto>120%oftheexpectedpeakcurrentoftheapplication.SeeFigure8foranexample.Verify
thattheringingontheVDSwaveformattheedgesaredamped.
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RecommendedExternalCircuitryforTransphormGaNFETs
AN0009
Chokecurrent
Half-bridgeoutputvoltage
(low-sideVDS)
ZOOMofabove
Chokecurrent
Half-bridgeoutputvoltage
(low-sideVDS)
ZOOMofabove
Figure8.Switchingwaveformsshowingunderdamped(withoutdrainbead)anddamped(withdrainbead)VDS;
testwasperformedwithamulti-pulsetestfixture
5
Conclusion
SustainedoscillationcanoccurinswitchingapplicationsusingTransphormhighspeedGaNdevices
whichmustbepreventedforsafeoperation.Byinsertingagateferritebead,adrainferritebead,and/or
anRCsnubberwithrecommendedvalues,TransphormGaNFETscanoperateinahard-switchingbridge
uptotheirfull-ratedcurrentevenwithaless-than-idealPCBlayout.
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