Transformer-Isolated-Gate-Driver-Provides-very-lar

AN-950(v.Int)Transformer-IsolatedGateDriverProvidesverylargedutycycleratios(HEXFET®isthetrademarkforInternationalRectifierPowerMOSFETs)Transformercouplingoflowlevelsignalstopowerswitchesoffersseveraladvantagessuchasimpedancematching.DCisolationandeitherstepuporstepdowncapability.Theyalsoprovidenegativegatebiastoreducetheriskof“dv/dtinducedturn-on”.Unfortunately,transformerscandeliveronlyACsignalssincethecorefluxmustbereseteachhalfcycle.This“constantvoltseconds”propertyoftransformersresultsinlargevoltageswingsifanarrowresetpulse,i.e.,alargedutycycleisrequired(Figure1).NOTE:VOLT-SECONDSPRODUCTINSHADEDAREASMUSTBEEQUAL.THISCAUSESRESETVOLTAGETOBE3TIMESAPPLIEDVOLTAGEE.Figure2.WideDutyCycleHEXFETPowerMOSFETDrivercircuitForthisreasontransformersinsemiconductordrivecircuitsarelimitedto50%,dutycycleorroughlyequalpulsewidthspositiveandnegativebecauseofdrivevoltagelimitationsofthesemiconductorsthemsevles.Forlargedutycycleratiosdesignersmustchooseanalternativetothetransformer,suchasanopticalcouplertoprovidethenecessarydriveisolation.OptoisolatorsforpowerelectronicsrequirehighdV/dtcapabilityandareexpensive.Theyalsorequireadditionalfloatingpowersourceswhichaddcomplexityandcost.Mostofthemrequireabufferstagetohandlethelargegatecapacitances,typicalofpowerdevices.Ifdutycyclesaresuchthatoptoisolatorsaretheonlyalternative,theycanbeusedinamorecost-effectivewayasdriversforaMOS-gatedriver.ThecircuitinFigure2providesalowimpedanceturn-ondrive,andminimumpulsewidths(onoroff)ofapproximately1microsecond;furthermore,itcanhaveanydesiredvoltageratio,andprovideselectricalisolation.InFigure2,Q2isthemainpowerdevice,shownasaMOSFET,providingtheswitchingfunctionforaswitchingpowersupply,motordriveorotherapplicationrequiringisolationbetweenthelowlevellogicandhighpoweroutput.QlisalowpowerHEXFETPowerMOSFETsuchastheIRLML2803,whichisusedtocontrolthedrivesignalstoQ2,andT1isasmall1:1drivertransformerprovidingelectricalisolationfrom,andcouplingto,thelowlevelcircuitry.TT/3E-3EZ1T11:1Q1Q2POWERSWITCH+12V0-12V+12V0-12V+12V0-12V0ABCDLOWLEVELLOGICSIGNALT1WINDINGVOLTAGEVGSPOWERHEXFETQ2VDSPOWERHEXFETQ2Figure1.Constant-Volt-SecondsCharacteristicsofTransformersFigure3.WaveformCharacteristicsofHEXFETPOWERMOSFETDriverCircuitAN-950(v.Int)Figure4.SingleSwitchRegulatorsThewaveformsinFigure3explainthecircuitoperation.WaveformAisthedesiredlogicsignaltobeswitchedbyQ2.WhenthisvoltageisappliedtotheprimaryofT1thewaveformissupportedbychangingcorefluxuntilsaturationoccursasshowninwaveformB.Atthistimethewindingvoltagesfalltozeroandremainsountilthecorefluxisreversedbythenegative-goingportionofwaveformAsaturationwillagainoccurifthenegative-appliedpulseexceedsthevolt-secondscapabilityofthecore.Duringthepositiveportionofthesecondarywaveform,which,ofcourse,hasthesameformastheprimary,theintrinsicdiodeofQlisinforwardconductionandQ2receivesapositivegatedrivevoltagewithasourceimpedanceofZ1plustheintrinsicdiodeforwardimpedance.Inapracticalcircuitthiscanbelessthan10Ohmstotal,withaconsequentturn-ontimeofaround75nsec.WhenT1saturates,theintrinsicdiodeofQ1isolatesthecollapseofvoltageatthewindingfromthegateofthepowerdeviceandtheinputcapacitanceCissofthepowerswitchholdsthegatebiasatthefullyenhancedconditionforatimelimitedonlybythegateleakagecurrentofQ2asindicatedinFigure3c.WhenwaveformAgoes12voltsnegativeQlwillbecomefullyenhanced;andthemainswitchQ2willnowbeturnedoffatapproximately-12VatasourceimpedanceZ1+RDS(ON)ofQ2.Thiswillagainbelessthan10Ohmsandwillyieldaturn-offtimelessthan100nsec.WhenT1againsaturates,duringthenegativehalfcycle,itswindingvoltagesfalltozeroandQ1turnsoff.AsT1voltagecollapses,thegateofQ2alsofollowsthisvoltageandremainsatzerobias.ThedrainvoltageofthepowerHEXFETPowerMOSFETQ2appearsinFigure3d,showingthatitisindeedamirrorimageofwaveformA,thedesiredlowlevellogicsignal.NotethatbecauseT1needonlysupporta12Vsignal,for1msecorless,itisverysmall—andinexpensive.InapracticalcircuitZ1isfrequentlya0.1mFcapacitor,andthesignalsourceisalowimpedancedriversuchasaPWMcontrollerorgatedriver.GROUNDREFERENCELOGICVOVINIRFD1ZOIRF840IRFD1ZOIRF840IRFD1ZOIRF840IRFD1ZOIRF840HEXFETAVALANCEPROTECTIONFigure5.HighVoltage,HighPowerHEXFETPowerMOSFETSwitch(500V,8AperSection)AN-950(v.Int)ItshouldbenotedthatthecircuitinFigure3(b)maynotprovidethenecessarynoiseimmunitywhenthepowerdeviceisoff.Thegate-sourcevoltageofQ2intheOFFstatereturnstozerowhenT1saturatesandtheonlynoiseimmunityisprovidedbythethresholdvoltageofQ2(2VVTH4V).Inmostapplicationsitmaybedesirabletoprovidemorenoiseimmunity,byaddinganothersmallN-ChannelHEXFETPowerMOSFET(typicallyanotherIRLML2803)asshowninFigure7.Thecircuitnowprovides-12VtothepowerMOSFETafterthetransformersaturates,andthisreversebiasremainsuntilthenextpositivehalfcycleofdrive.Thus,aminimumof14Vnoiseimmunityisprovidedwhichshouldbeadequateforallapplica-tions.Thecostandnoiseimmunityofthissolutionismuchlessthanalternativesusingoptoisolatorsandtheirauxiliarysupplies.Figures5and6showtwoapplicationswherethisgatedrivemethodisparticularlyadvantageous.Thefirstisahigh-voltage,high-frequencyswitch.Thesec