Bootstrap Circuitry Selection for Half Bridge Configurations

1 1 SLUA887 August2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedBootstr apCircuitrySelectionfor half -BridgeConfigurationsApplicationRepo rtSLUA887 August2018 BootstrapCircuitrySelectionfor half -BridgeConfigurationsMamadouDiallo,H ighPowerDriversABSTRACTD rivingMOSFETsin half -bridgeconfigurationspresentmanychal lengesfor thosechallengesis generatingbiasfor the bootstrapcircuittakescareof this ,TI's 620 Vhalf-bridgegatedriverwith interlockto presentthe differentcomponentsin a bootstrapcircuitand how to properlyselectthemin orderto ensurepredictableswitchingof the of to to BootstrapDiodeReverseRecoveryTime(Zoomed Out).

2 55HB_HSRingingEffectson (Rboot= 0 Ohms)..77 VDD/HB-HSFastRampUp (Rboot= ).. of Tables1 IntroductionWhenusinghalf-bridgeconfigur ations,it is necessaryto generatehigh-sidebias to drivethe gateof thehigh-sideFET referencedto the of the mostpopularand cost effectiveway fordesignersto do so is the use of a bootstrapcircuitwhichconsistsof a capacitor,a diode,a resistorand applicationreportwill explainhow this circuitworks,the key componentsof the bootstrapcircuitsand theirimpactin the app notewill put emphasison half -bridgegatedrivesusingdriverswith no built-inbootstrapdiode.

3 Whichgivesdesignersflexibilityand reducespowerdissipationin the gatedriverIC. Additionally,it will discussthe layoutconsiderationsfor the differentcomponentsof this August2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedBootstr apCircuitrySelectionfor half -BridgeConfigurations2 BasicOperationof BootstrapCircuitA bootstrapcircuitis usedin half -bridgeconfigurationsto supplybias to the showsthe chargingpathof a bootstrapcircuitin a simplifiedhalf-bridgeconfigurationusingU CC27710,TI's 620 Vhalf-bridgedriverwith low-sideFET is on ( high -sideFET is off)

4 , the HS pin and theswitchnodeare pulledto ground;the VDDbias supply,throughthe bypasscapacitor,chargesthebootstrapcapac itorthroughthe bootstrapdiodeand BootstrapChargingPathWhenthe low-sideFET is turnedoff and the high -sideis on, the HS pin of the gatedriverand the switchnodeare pulledto the high voltagebus HV; the bootstrapcapacitordischargessomeof the storedvoltage(accumulatedduringthe chargingsequence)to the high -sideFET throughthe HO and HS pins of the gatedriveras shownin August2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedBootstr apCircuitrySelectionfor half -BridgeConfigurations3 BootstrapComponentsSelectionThis sectiondiscusseseachcomponent'srole and its impactin the designperspective,this is the mostimportantcomponentbecauseit providesa low impedancepathto sourcethe high peakcurrentsto chargethe a generalrule of thumb.

5 Thisbootstrapcapacitorshouldbe sizedto haveenoughenergyto drivethe gateof the high -sideMOSFET withoutbeingdepletedby morethan10%.This bootstrapcap shouldbe at least10 timesgreaterthanthegatecapacitanceof the reasonfor that is to allowfor capacitanceshift fromDC biasand temperature,and also skippedcyclesthat occurduringload gatecapacitancecan bedeterminedusingEquation1:(1)Oncethe gatechargedetermined,the minimumvaluefor the bootstrapcapacitorcan be estimatedusingEquation2:(2)Alternatively ,a moredetailedcalculationof the minimumbootstrapcapacitorvaluecan be doneusingEquation3.

6 (3)It is importantto notethat valuesbelowthe minimumrequiredbootstrapcapacitorvalueco uldlead toactivationof the driver'sUVLO thereforeprematurelyturningoff the the flip side,highervaluesof the bootstrapcapacitorlead to lowerripplevoltageand longerreverserecoverytime in someconditions(wheninitiallychargingthe bootstrapcap or with a narrowbootstrapchargingperiod)as well ashigherpeakcurrentthroughthe relatesthe bootstrapcap and the peakcurrentsthroughthe bootstrapdiode.(4)It is generallyrecommendedto use low ESRand ESL surfacemountmulti-layerceramiccapacitors (MLCC)with goodvoltageratings(2xVDD)

7 ,temperaturecoefficientsand August2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedBootstr apCircuitrySelectionfor chargeto replenishthe bootstrapcapacitormustcomefromsomelarger bypasscapacitor,usuallythe a rule of thumb,this bypasscapacitorshouldbe sizedto be at least10timeslargerthanthe bootstrapcapacitorso that it is not completelydrainedduringthe chargingtime ofthe allowsthe bootstrapcapacitorto be 10x ratioresultsin 10%maximumrippleon the VDDcapacitorin worstcaseconditions.(5) orderto minimizelossesassociatedwith the reverserecoverypropertiesof the diodeand groundnoisebouncing,a fast recoverydiodeor Schottkydiodewith low forwardvoltagedropand low junctioncapacitanceis risk associatedwith chargesuppliedbackto the gatedriversupplyfromthe bootstrapcapacitorand , showsthe reverserecoverylosseswhenusingdiodeswith reverserecoverytimeson HB-HS(Ch1).

8 We canobservelargeamountof overand undershooton the HB-HSpin whichcan triggerthe driver'sUVLO andshutdownthe HS pin (switchnode)is pulledto a highervoltage,the diodemustbe able reversebias fastenoughto blockany chargesfromthe bootstrapcapacitorto the bootstrapdiodeshouldbe carefullychosensuchthat it is capableof handlingthe peaktransientcurrentsduringstart-up;ands uchthat its voltageratingis higherthanthe systemDC-linkvoltagewith ReverseRecoveryLossesdue to BootstrapDiodeReverseRecoveryTimeFigure4 belowshowsa reverserecoveryconditioncreated(channel1 ) by settingup the timingtospecificallyforcethe switchnodehigh with the August2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedBootstr apCircuitrySelectionfor half -BridgeConfigurationsFigure4.

9 ReverseRecoveryLossesdue to BootstrapDiodeReverseRecoveryTime(Zoomed Out)Figure5 showsthe effectsof the losseson the HB-HSpin whichcan triggerthe switchnodeandpotentiallydamagethe August2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedBootstr apCircuitrySelectionfor half -BridgeConfigurationsFigure5. HB_HSRingingEffectson role of the bootstrapresistoris to limit the peakcurrentsat the bootstrapdiodeduringstart-up,itshouldthe reforebe carefullyselectedas it introducesa time constantwith the bootstrapcapacitorgivenby Equation6.

10 (6)This time constantoccuringduringthe high -sideoff time explainsthe dependencyon duty dutycyclebeingconstant,the bootstrapresistorand bootstrapcapacitorshouldbe tunedappropriatelytoachievethe bootstrapresistorvalueswill increasethe time constantleadingto ,the bootstrapresistorchosenmustbe able to withstandhigh powerdissipationduringthe firstchargingsequenceof the energycan be estimatedby Equation7:(7)This energyis dissipatedduringthe chargingtime of the bootstrapcapacitorand can be estimatedusingEquation8:(8) August2018 SubmitDocumentationFeedbackCopyright 2018,TexasInstrumentsIncorporatedBootstr apCircuitrySelectionfor half -BridgeConfigurationsThis resistoris essentialin limitingthe peakcurrentsthroughthe bootstrapdiodeat start-upand limitingthe dv/dtof HB-HS( high -sidefloatingsupplyto the returnhigh-sidefloatingsupply).