Choosing Standard Recovery Diode or Ultra-Fast …

1 VinTonLp ApplicationReportSNVA744 October2015 ChoosingStandardRecoveryDiodeorUltra-Fas tDiodein SnubberKeningGao,UlrichB. GoerkeABSTRACTW hileusinga Snubbercircuitis verycommonfor flybackdesign,suppressingthe stressof MOSFET isnot the only designconsiderationfor Snubbercircuitwill also impactthe efficiency,standbypower,and workingprincipleof snubberand illustratesthediodeselectionfor RCD/R2 CDand of and VoltageAcrossthe Cc and VoltageAcrossthe Cc and and (CH1:Vds;CH3:VoltageAcrossTVS)..510 EfficiencyComparisonof UCC28740 EVMU singUS1 Mand of Tables1 SnubberCircuitin FlybackFlybacktopologyis widelyusedin low-powerapplicationsbecauseof its simplestructure,low componentcount,and low ,afterthe MOSFET turnsoff, thereis high voltagestresson the drain-sourceof protectthe MOSFET,a Snubbercircuitis neededto suppressthe is a typicalFlybackcircuitwithouta is turnedon, thecurrentgoingthroughthe primaryof transformerwill primarycurrentwill be equaltowhenthe MOSFETis turnedoff, whereVin is the inputvoltage,Ton is the on time of MOSFET,Lp is the sum of magneticinductanceLm and LeakageinductanceLk.

2 WhenMOSFETis turnedoff, Ip willcontinueto chargeinto Cp + Coss,whereCp is the primarywindingcapacitancein the transformerandCossis the drainto sourcecapacitanceof the MOSFET,until the voltageon the secondary-sideof thetransformerreachesto Vo + Vf, whereVo is the outputvoltage,Vf is the sum of forwardvoltageof rectifierSTMicroelectronicsis a registeredtrademarkof STMicroelectronics, October2015 ChoosingStandardRecoveryDiodeor ultra -FastDiodein SnubberSubmitDocumentationFeedbackCopyri ght 2015,TexasInstrumentsIncorporated+CossLk + Vin IpQ1 LmRs+ VfTransformerN:1Vo+ DrCpRcCc+ Dc+CossLk+ Vin IpQ1 LmRs+ VfTransformerN:1Vo+ DrCpLk(VoutVf )IpVinCpCossN +++ + 12 Lk(Cp Coss)p+RCDand the voltagedropon the resistanceof this moment,the energystoredin the Lm willbeginto transferto energystoredin Lk will resonatewith Cp+Cossat a fixedfrequency. If thereis no snubbercircuit,the voltagestresswill be very peakvaluecouldreachto.

3 This very high spikecouldcausebad EMI orevendestroythe wouldbe worstat maximumIp and suppressthe spike,the usualway is to use an RCDsnubberlike in FlybackCircuitModelWithoutSnubberFigure2 . FlybackCircuitWithRCDS nubber2 RCDand R2 CDSnubberFor the low-powerflybackapplications,moreand moreengineerslike to use the standardrecoverydiodeinsteadan ultra -fastdiodein the theclampdiodeto damposcillationsdue to effectof the standarddiode s reverserecoveryprofileis difficultto calculatequantitativelybecauseits modelis not clearin the benefitsfor efficiencyand EMI energystoredin the clampcapacitorwill be dischargedby the resistorin parallelwith it. Afteran Ultra-Fast recoverydiodestopsconductingcurrent,ther eis still a resonancebetweenLk, Cp, and isa dampedresonanceas in Figure3, for someof the energyis dissipatedin the resistanceof the circuitand someof it is transferredto the secondary[1].

4 But if the snubberis implementedwith the standardrecoverydiode,it has a relativelylong trr s to several s. The trr is usuallylongerwith standardrecoverydiodewill conductnegativecurrent,the energystoredin the clampcapacitorcan also participatein the resonancewith Lk, Cp, and , we can see the voltageacrossthe clampcapacitordecreasesquickly,causedby the slowreverserecoveryof the this case,mostof energytakespart in the resonanceand someof it will be transferredto the secondarycausingless powerloss thanthat of resistancedischargewith efficiencycouldbe betterwithstandarddiodethanwith an standardrecoverydiodeas the clampdiode,the valueof resistorparalleledwith the clampcapacitorcouldbe muchhigherthanwith an ,the choiceof the resistorvalueshouldensurethat the voltageacrossthe clampcapacitoris alwayshigherthanN(Vo+ Vf), or else it willdissipatesomeenergyfromthe ringingof usingstandarddiodeis betterthanusingultra-fastbecausewith an ultra -fastdiode,theringinghas higheramplitudeand the EMI performanceshouldbe betterwith ultra -FastDiodein SnubberSNVA744 October2015 SubmitDocumentationFeedbackCopyright 2015,TexasInstrumentsIncorporated+CossLk + Vin IpQ1 LmRs+ VfTransformerN:1Vo+ DrCpRcCc+ DcRdHigh OscillationVoltage Across Clamp CapacitorVds of Primary MOSV oltage Across Clamp CapacitorCc takes part in the resonanceDischarged More by the Reverse RecoveryVds of Primary R2 CDSnubberFigure3.

5 Vds and VoltageAcrossthe Cc UsingUS1M-E3 Figure4. Vds and VoltageAcrossthe Cc Using1N4007As previouslyexplained,whenusinga standarddiode,Cc, Lk, Coss,and Cp all participatein secondresistorRd can be addedto dampthe ringingas in Figure5. The structureis choiceof Rd valueis to dampthe Lk-Ccresonancewith a Q that is [1].Figure5. R2 CDSnubber3 SNVA744 October2015 ChoosingStandardRecoveryDiodeor ultra -FastDiodein SnubberSubmitDocumentationFeedbackCopyri ght 2015,TexasInstrumentsIncorporatedVDD1VS2 FB3 GND4CS5 DRV6HV8U1 UCC28740D~+~ FC4D6 SBR10U45SP5-131 FC8270 FC101 FC1277 VAC, 2 AF1 RST 212J1 LINENEUTRALINPUT: 100 VRMS - 265 VRMS, A PEAK, 47 Hz TO 63 FC5100 HIGH VOLTAGE+VOUTNCNC-VOUTOUTPUT: 5V, ohmt RT10R5220 HL1R13R14 SGND11 FC70R80R171Do Not FC2D3 SMBJ120A-13-FD5 SBR10U45SP5-13 JMP31 illustratesthe UCC28740 EVM-525usinga UCC28740 EVM-525 UsingTVSS nubber4 ChoosingStandardRecoveryDiodeor ultra -FastDiodein SnubberSNVA744 October2015 SubmitDocumentationFeedbackCopyright 2015,TexasInstrumentsIncorporatedt0t1t3t 2N(Vo + Vf) someapplications,it is advantageousor evennecessaryto use a TVSsnubberinsteadof an get the TVSsnubber,we changethe clampcapacitorin R2 CDsnubberto TVSand removethe parallelresistor,as in Figure6.

6 The TVSsnubberhas a highercost ,thereis agrowingneedfor the ultra -lowstandbypowerin the market,and a TVSsnubberhas an advantagefor couldhelp achievehigherefficiencyin zeroload and light load,for it will not dissipatepowerbeforethe voltageat its cathodereachesVin + additionto that,the choiceof clampdiodeis very improperchoicecouldcauseanefficiencydrop and improperVds use UCC28740 EVM-525for the test as in Figure6; thecomparisonis testedbetweenStandardrecoverydiode1N4007 and TI s new valley-switchingcontrollerto get high test conditionis toapply230-VDC on the bulk we look at the Vds waveformsin Figure7 and Figure8, we cansee the differencebetweenthe two kindsof a flat Vds Vds and VoltageAcrossTVSU singUS1M-ESFigure8. Vds and VoltageAcrossTVSU sing1N4007 Figure9. WaveformsUsing1N4007(CH1:Vds;CH3:Voltage AcrossTVS)5 SNVA744 October2015 ChoosingStandardRecoveryDiodeor ultra -FastDiodein SnubberSubmitDocumentationFeedbackCopyri ght 2015,TexasInstrumentsIncorporatedOutput Current (A) the QuickDecayingResonantTo understandthe reasonwhy using1N4007losesthe valley , we couldzoomin the , MOSFETis turnedoff; the total draincapacitanceis chargedby ILK.

7 ILKis the currentin the t1, The parasiticcapacitorof the TVSresonateswith Cp, Coss,and Lk, and its voltagegoesdowntoN(Vo+ Vf) becauseof the longertrr of , the currentin secondarydiodegoesto parasiticcapacitorof TVS,Cp, Coss,Lk, and Lpstartsto t3, becauseof the very low energystoredin TVSbeforet2 (the Cj of SMAJ120 Ais bout30 pf at VR,accordingto its datasheet),and long trr of voltageacrossTVSis resonatedto t4, still in the capacitorof TVSis dischargedto zero,the TVSis in forwardconductionby its Vin takespart into the resonance,and makesthe , the resonancevanishesin a few cycles,and the voltageon Vds staysflat at Vin. For ICs whichhavevalleyswitchingcharacteristics, it losesthe benefitsof efficiencytest was doneon the EVMboardwith thesetwo kindsof can see the efficiencyis lowerwith 1N4007in Figure10. The efficiencywas testedwith 230-VDC EfficiencyComparisonof UCC28740 EVMU singUS1 Mand 1N4007 The workingfrequencyof UCC28740 EVMis about70 kHz at full calculatethe in Figure7 and Figure8 showed,the Vds voltagebeforeMOSFET turn-onis 1N4007,it is 230 V, For US1M-ESit is 200 V.

8 The MOSFET usedis STMicroelectronics 'STU7NM60N,fromits datasheetthe outputcapacitancestoredenergyare J and J. So wecalculatedthe deltaswitchingloss at MOSFET turn on to be about( ) J 70 kHz = (Thisloss will be higherat higherinputvoltages.)Anotherpowerloss differenceis in the energystoredin the resonantcircuitafterthe secondarycurrentgoesto energywith 1N4007is all dissipatedin the circuit,but with US1M-ES,it is only partlydissipatedsincethe resonantoscillationstill existsbeforethe turn-onof ,thepeakcurrenton Rd measuredis about40 mA in the resonancewhichalso goesthroughthe differencesincludethe energysinkedfromVin by the snubberwhenthe TVSis forward-conductingdue to the reverserecoverycurrentof the ultra -FastDiodein SnubberSNVA744 October2015 SubmitDocumentationFeedbackCopyright 2015, followingconclusionsare madebasedon the informationprovidedin this applicationreport:1. In low powerofflineflybackapplication,usingstan dardrecoverydiodein the RCD/R2 CDsnubbercanhelp get higherefficiencyand betterEMI thanusingan A resistorin-serieswith the clampdiodeis suggestedto suppressthe ringingin A TVSsnubberis fit for thoseapplicationswhichneedvery low in thesecases,theclampdiodeshoulduse the ultra -fastdiodeto Liu Shunlin,Cao Xiaoshengand Ma Yibo,Designand Analysison FeedbackEnergyLossof RCDC lampingFlybackConverters, Proceedingsof the CSEE,2010-332.

9 UCC28600 Datasheet(SLUS646J)7 SNVA744 October2015 ChoosingStandardRecoveryDiodeor ultra -FastDiodein SnubberSubmitDocumentationFeedbackCopyri ght 2015,TexasInstrumentsIncorporatedIMPORTA NTNOTICET exasInstrumentsIncorporatedand its subsidiaries(TI) reservethe rightto makecorrections,enhancements,improvement sand otherchangesto its semiconductorproductsand servicesper JESD46,latestissue,and to discontinueany productor serviceper JESD48, latestrelevantinformationbeforeplacingor dersand shouldverifythat suchinformationis semiconductorproducts(alsoreferredto hereinas components ) are sold subjectto TI s termsand conditionsof salesuppliedat the time of warrantsperformanceof its componentsto the specificationsapplicableat the time of sale,in accordancewith the warrantyin TI s termsand conditionsof sale of otherqualitycontroltechniquesare usedto the extentTI deemsnecessaryto supportthis applicablelaw, testingof all parametersof eachcomponentis not assumesno liabilityfor applicationsassistanceor the designof Buyers responsiblefor theirproductsandapplicationsusingTI minimizethe risksassociatedwith Buyers productsand applications,Buyersshouldprovideadequate designand doesnot warrantor representthat any license,eitherexpressor implied,is grantedunderany patentright,copyright,maskworkright,orot herintellectualpropertyrightrelatingto any combination,machine.

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