Basicintroduction
PWMswitchingpowersupplyworksinhardswitchingmode(thevoltagedrop/riseandcurrentrise/fallwaveformsoverlapduringtheon/offprocess),sotheswitchinglossislarge.Althoughhighfrequencycanreducethevolumeandweight,theswitchinglossisevengreater.Forthisreason,itisnecessarytostudythetechnologyofswitchingvoltage/currentwaveformsthatdonotoverlap,thatis,theso-calledzerovoltageswitching(ZVS)/zerocurrentswitching(ZCS)technology,orsoftswitchingtechnology.Theefficiencyoflow-powersoft-switchingpowersuppliescanbeincreasedto80%.~85%.Theresonantswitchingpowersupplylaidthefoundationofsoftswitchingtechnologyinthe1970s.Subsequently,newsoftswitchingtechnologiescontinuedtoemerge,suchasquasi-resonant(mid-1980s)full-bridgephase-shiftedZVS-PWM,constantfrequencyZVS-PWM/ZCS-PWM(late1980s)ZVS-PWMactiveclamp;ZVT-PWM/ZCT-PWM(early1990s)full-bridgephaseshiftZV-ZCS-PWM(mid1990s)andsoon.mycountryhasappliedthelatestsoft-switchingtechnologyto6kWcommunicationpowersupplywithanefficiencyof98%.
Technicalapplications
Thekeyrequirementsforbuckregulatorsareusuallysizeandefficiency.Becausetheprintedcircuitboardareaissoprecious,nodesigneriswillingtoallocateextraspacetothepowerdesignscheme.Inaddition,duetothecontinuousinnovationofsingle-chipmicrocomputersanddigitalsignalprocessors(DSP),circuitboarddesignschemesarealsoconstantlyupgraded.Althoughthepowerhasincreased,theproductsizecannotbeincreased.Therefore,high-densityregulatorscontinuetodevelopwiththeimprovementofthelatestICintegration,theimprovementofMOSFETtechnologyandtheimprovementofpackagingtechnology.Evenso,theseregulatorsstillcannotmeettheapplicationrequirementsofthenewsystem.Especiallythepowerdensityinsidethesystemisincreasingdaybyday.ThemainreasonisthatswitchinglosseshindertheinternalperformanceoftheregulatorMOSFET.Ifyoudonotfundamentallysolvetheselossproblems,youcanonlyexpectsomeminorperformanceimprovements.
Reasonsforselection
Themainreasonsforswitchinglossare:First,hardswitching.Today,mostnon-isolatedbuckregulatortopologieshavelargeswitchinglosses.Thereasonisthatduringtheturn-onandturn-offperiod,theMOSFETbearshighcurrentandhighvoltagestressatthesametime.Whentheswitchingfrequencyandinputvoltageincrease,theselossesincreaseatthesametime,limitingthemaximumoperatingfrequency,efficiencyandpowerdensitythatitcanachieve.Thesecondisthegatedriveloss.DuetothehighpowerconsumptionoftheMillerchargeinthegatedrivecircuit,thegatedrivelossresultingfromthehard-switchingtopologyisalsohigh.Thethirdisthatthebodydiodeconducts.Whenthehigh-levelMOSFETisturnedonandoff,thehigh-pulsationcurrentflowsthroughthebodydiodeofthelow-levelMOSFET.Thelongerthebodydiodeisturnedon,thehigherthereverserecoverylossandthebodydiodeconductionloss.Bodydiodeconductioncanalsocausedestructiveovershootandringing.Theswitchinglossalsolimitstheswitchingfrequencyoftheregulator.Thehighertheswitchingfrequency,thelongertheMOSFETswitchingtimeandthegreatertheloss.Iftheswitchcannotbeswitchedatahighfrequency,itwilllimittheuseofsmallerpassivecomponents(resistors,capacitors,andinductors),therebyaffectingthedensityoftheregulator.Manyelectronicdesignershopetousezerovoltageswitching(ZVS)atthepointofload.Inresponsetotheaboveproblems,Picorintroducedahigh-performance,highlyintegrated,soft-switchingbuckregulatorplatformthatcanworkathighfrequencies,greatlyreducingswitchinglossesandimprovingefficiency.