VIPER22A

September20021/17A.BAILLY-G.AUGUSTONIAN1585APPLICATIONNOTEPCStand-byPowerSupplywithVIPer22AAdesktopPCpowersupplyisgenerallymadeoftwopowersupplies:themainonebuiltaroundaforwardstructureandabletodeliverafewhundredsofWattandswitchedoffinstandbymode;thesecondonewithapowercapabilityofupto15Walwaysoperatestoinsurethe”instanton”featureorsimplythewakingupfromtheoffstate.ThisapplicationnotedescribestheresultsobtainedwhendesigningaVIPer22Ainthestandbysectionofsuchpowersupplies.Aparticularconcernistheconsumptionintheidlemode(0.5Wofoutputpower),wheretheinputpowermustnotexceed1W.Inaddition,abrown-outfeaturemonitorstheinputvoltageinordertoswitchoffthepowersupplywhenitistoolow.Thefigureherebelowshowsthecorrespondingdemoboard.Itsdescriptionisalsoincludedinthisdocument.AN1585-APPLICATIONNOTE2/171.VIPERX2ADESCRIPTIONTheVIPer12AandVIPer22Adevicesarehighvoltageintegratedcircuits,intendedtobeusedinofflinepowersupplyswitchingtakingadvantagefromminimizedpartcount,reducedsize(SO-8packageavailable)andconsumption.TheyarealsoabletomeetthenewEcoStandardswithcosteffectiveness.1.1.GeneralfeaturesTheVIPerX2AfamilyisarangeofPWMcontrollerICtogetherwithahighvoltagepowerMOSFEThousedintheDIP-8andthesmallSMDSO-8packages.Thefeaturesofthesedevicesallowtoreducetheoverallpartscount,leadingtocompactnessandhigherreliabilitywhicharealsoreinforcedbytheautomaticthermalshutdown,thankstothemonolithicstructure.ThisstructureusestheproprietaryVIPowerM0-3HVTechnologywhichcombinesapowerstagewithverticalcurrentflowandalowvoltagecircuitryinaP-typeburiedlayer,asillustratedbyfigure1.Figure1:M0-3technologyTheVIPerX2Afamilydevicesgetabenefitfromthistechnologyandfromsmallsizepackagestoaddresslowpowerapplications,asshownontables1and2.Notethatthesepowercapabilitiescanbeachievedwithadequatethermalconfiguration,suchassufficientcopperplaneareaconnectedtothedrainpinsontheprintedcircuitboard.Table1:Powercapabilitywithwideinputvoltagerange(85-265Vac)Table2:PowercapabilitywithEuropeaninputvoltagerange(180-265Vac)Figure2:BlockdiagramDEVICEPACKAGESO-8DIP-8VIPer12A5W8WVIPer22A7W12WDEVICEPACKAGESO-8DIP-8VIPer12A8W13WVIPer22A12W20WON/OFF0.23VDRAINSOURCEVDDPWMLATCH60kHzOSCILLATORBLANKING+_8/14.5V_+FFSR1R4QR3FBREGULATORINTERNALSUPPLYOVERVOLTAGELATCHOVERTEMP.DETECTOR1kΩ42V_+R2FFSRQ230ΩAN1585-APPLICATIONNOTE3/171.2.BlockdiagramFigure2presentstheinternaldiagramoftheVIPerX2Afamily.ThepowersectionisahighvoltagesenseNtypemosfet,withaminimumguaranteedbreakdownof730V.ItisdrivenbyacurrentmodestructurewithfastcomparatorusingthecurrentdeliveredbytheNmosfetsense,andblankingtime.Theswitchingfrequencyisinternallyfixedat60kHz.Alltheinternalsignalcircuitsaresuppliedwitharegulatorabletoacceptavoltageinexcessof45V.Variousprotectionsareimplemented,suchastheovervoltageontheVDDpinat42V,andthethermalshutdownat170°Ctypical.Asthecontrolstructureisacurrentmode,thedraincurrentislimitedcyclebycycleandhasamaximumvaluecorrespondingtoaFBpinheldtoground.ThefeedbackloopisimplementedbydrivingthisFBpinwithanoptocouplerconnectedtoapositivevoltage.AnhysteresiscomparatormonitorstheVDDvoltagetomanagethestartupcurrentsource.ItisswitchedonforchargingtheVDDcapacitortothestartupthreshold,andmaintainedintheoffstateduringthenormalswitchingoperationtominimizetheinputpowerconsumption.1.3.CurrentmodestructureandburstmodeAfeedbackpincontrolstheoperationofthedevice.UnlikeconventionalPWMcontrolcircuitswhichuseavoltageinput(theinvertedinputofanoperationalamplifier),theFBpinissensitivetocurrent.Thefigurepresentstheinternalcurrentmodestructure.ThePowerMOSFETdeliversasensecurrentIsproportionaltothemaincurrentID.R2receivesboththiscurrentandthecurrentcomingfromtheFBpin.ThevoltageacrossR2isthencomparedtoafixedreferencevoltageofabout0.23V.TheMOSFETisswitchedoffwhenthefollowingequationisreached:ByextractingIsandintroducingthesensemosfetratioGID:ThisformulademonstratesthatthepeakdraincurrentdependslinearlyontheFBpincurrent,andthatthefeedbackcurrentmustbeincreasedforthedraincurrenttodecrease.Forverylowdraincurrents,itiseffectiveaslongasIFBsatisfies:WhereIFBsdisaninternalthresholdoftheVIPerX2A.IfIFBexceedsthisthreshold,thedevicestopsswitching.ThisthresholdontheFBpincorrespondstoabout12%ofthecurrentlimitationofthedevice,i.e.about80mAforaVIPer22A.WhentheoutputloadisdecreasedandtheregulationloopmakestheFBpinreachtheIFBsdthreshold,thedeviceentersaburstmodeoperationbyskippingswitchingcycles.Thisisespeciallyimportantwhentheconverterislightlyloaded,inordertoachieveverylowinputpowerconsumption.Valuesintherangeof100mWofinputpowercanbereachedwithnoloadontheoutput.Figure3:Feedbackandcurrentmodestructure2.PCSTANDBYAPPLICATION2.1.SchematicsFigure4givestheschematicusedtodelivertwosupplyvoltages,the+5Vandthe+12V.Notethatmostofthepowerisdeliveredonthe+5V,withacurrentcapabilityof2Aor3A,dependingontheoutputdiodeD6.Theboardcomeswitha1N5822axialdiodeabletodeliverupto2A,andthePCBfootprintisalsocompatiblewithanSTPS745togoupto3A.Alltheresultsgiven,havebeenmeasuredinthe3Aconfiguration.The+12Vcurrentcapabilityis100mA.The+5VoutputisregulatedthankstoU3andtheresistived