元器件交易网

SGLS371–JANUARY2007

RectifierSelectionCriteria Rectifierbreakdownvoltage

Therectifierhastowithstandthemaximuminputvoltagewhich,inthiscase,is16V.Toallowforswitchingtransientsthatcanapproachtheswitchingvoltage,a30-Vrectifierwasselected. Diodesize

TheimportanceofpowerlossesfromtheSchottkyrectifier(D2)isdeterminedbythedutycycle.Foralowduty-cycleapplication,therectifierisconductingmostofthetime,andthecurrentthatflowsthroughittimesitsforwarddropcanbethelargestcomponentoflossintheentirecontroller.Inthisapplication,thedutycyclerangesfrom20%to40%,whichintheworstcasemeansthatthediodeisconducting80%ofthetime.Whereefficiencyisofmajorimportance,chooseadiodewithaslowaforwarddropaspossible.Inmorecost-sensitiveapplications,sizemaybereducedtothepointofthethermallimitationsofthediodepackage.Thedeviceinthisapplicationislarge,relativetothecurrentrequiredbytheapplication.Inamorecost-sensitiveapplication,asmallerdiodeinaless-expensivepackageprovidesaless-efficient,butappropriate,solution.

Thedeviceusedhasthefollowingcharacteristics: Vf=0.3Vat3A

Ct=300pF(Ct=theeffectivereverse-voltagecapacitanceofthesynchronousrectifier,D2)ThetwocomponentsofthelossesfromthediodeD2are:

IRIPPLEöæ

÷+1-D)= 653 mWPCOND=Vf´çIçOUT÷´(4èø(11)Where:

D=DutycycleIRIPPLE=RipplecurrentIOUT=OutputcurrentVF=Forwardvoltage

PCOND=Conductionpowerloss

TheswitchingcapacitanceofthisdiodeaddsanACloss,givenby:

PSW+1[C (VIN)Vf)2 f]+6.8mW

Thisadditionallossraisesthetotallossto:660mW.

Atanoutputvoltageof3.3V,theapplicationrunsatanominaldutycycleof27%,andthediodeisconducting72.5%ofthetime.Astheoutputvoltageismovedupto5V,theontimeincreasesto46%,andthediodeisconductingonly54%ofthetimeduringeachclockcycle.Thischangeindutycycleproportionatelyreducestheconductionpowerlossesinthediode.Thisreductionmaybeexpressedas:

æ0.54ö660ç÷=491mW

è0.725ø(13)forasavingsinpowerof660–491=169mW.

Toillustratetherelevanceofthispowersavings,thefull-loadmoduleefficiencywasmeasuredforthisapplicationat3.3Vand5V.The5-Voutputefficiencyis92%versus89%forthe3.3-Vdesign.Thisdifferenceinefficiencyrepresentsa456-mWreductioninlossesbetweenthetwoconditions.This169-mWpower-lossreductionintherectifierrepresents37%ofthedifference.

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