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1、PowerIntegrations-PowerPuzzler1POWERpuzzlerUnderstandingtheEffectsofDiodesbyPeterVaughanManagerofProductApplicationsPowerIntegrationsTheschematicbelowshowsaFlybackpowersupplybuiltwithaTOPSwitch®-GXpowerconversionIC.Thefollowingquestionsconcerntheselectionoftheoutputdiode(D3)andth
2、eclampdiode(D1).Question1:beginnerWhichdiodeordiodesfromthefollowinglistaresuitableforuseasD3andwhy?a.80V,5A,Schottky(e.g.SB580)b.600V,3A,Ultrafast(e.g.UF5406)c.50V,3A,Rectifier(e.g.1N5400)d.100V,8A,Ultrafast(e.g.BYV29-100)e.45V,7.5A,Schottky(e.g.MBR745)f.1000V,2.5A,Fast(e.g.FR25
3、7)ClicktoseetheAnswerInput:90-375VDCOutput:12V,2.5Ahttp://www.powerint.com/powerpuzzler050308/（第1／8页）2007-8-92:08:09PowerIntegrations-PowerPuzzler1Answer:aanddSuitabilityofDiodeParametersDiodeDiodeParametersPeakReverseAverageForwardReverseRecoveryVoltage(VRRM)Current(IAVE)Time(tr
4、r)(a)SB58080V,5AXX*X(b)UF5406600V,3A,75nsX--(c)1N540050V,3A,2000ns---(d)BYV29-100100V,8A,25nsXXX(e)MBR74545V,7.5A-XX(f)FR2571000V,2.5A,500nsX--*WithbenchverificationoftemperatureriseWhy?:Inselectingtheoutputrectifierthreekeyparametersshouldbeconsidered;voltagerating,currentrating
5、andreverserecoverytime.Consideringeachoftheseinturnwecanseewhichofthediodesissuitable.Maximumrepetitivereversevoltage(VRRM)IdeallythediodeselectedshouldhaveaVRRMrating1.25·VR,whereVRisthereversevoltageseenbythediode.The1.25de-ratingfactorprovidesmargintotakeaccountofleakageinduct
6、ancegeneratedvoltagespikes,andAClinetransientsthatincreasetheDCbulkvoltagetemporarilyplusreducingthedevicestressimprovesreliability.NormallywherepeoplegetintotroubleisdeterminingthevalueofVR.You’dbeforgivenforthinkingthatthereversevoltageseenbythediodeisjusttheoutputvoltage,here1
7、2Vsoanythingabovea15Vdiodewouldbefine.HowevertheanodeofD3isconnectedtoaFlybacktransformer.ThismeansthatwhentheMOSFETinsideU1turnson,undermaximuminputlineof375VDC(265·VACx√2)isseenacrosstheprimary.Thisvoltageistransformedbytheprimarytosecondaryturnsratioandduetothephaseofthewindin
8、gsdrivestheanodeoftheoutputdiodenegative
