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Section5Designlimitations:Themostimportantlimitingfactorsininductordesignare(a)temperatureriseandefficiencyconsiderationsarisingfromcorelossesandacanddcwindinglosses,and(b)coresaturation.Outputfilterinductors(buck-derived)--singleandmultiplewindingsareseldomoperatedinthediscontinuouscurrentmodebecauseoftheaddedburdenthisplacesontheoutputfiltercapacitor,andbecauseitresultsinpoorcross-regulationinmultipleoutputsupplies.Typicallyoperatedinthecontinuousmodewithpeak-peakripplecurrentmuchsmallerthanfullloadcurrent,acwindinglossisusuallynotsignificantcomparedtodcloss.Filterinductors,boostinductorsandflybacktransfonnersareallmembersofthepowerinductorfamily.Theyallfunctionbytakingenergyfromtheelectricalcircuit,storingitinamagneticfield,andsubsequentlyreturningthisenergy(minuslosses)tothecircuit.Aflybacktransfonnerisactuallyamulti-windingcoupledinductor,unlikethetruetransfonn-ersdiscussedinSection4,whereinenergystorageisundesirable.ApplicationConsiderationsDesignconsiderationsforthisfamilyofinductorsvarywidelydependingonthetypeofcircuitapplica-tionandsuchfactorsasoperatingfrequencyandrip-plecurrent.Inductorapplicationsinswitchingpowersuppliescanbedefinedasfollows(seeFig.5-1):.Singlewindinginductors:Outputfilterinductor(buck-derived)BoostinductorFlyback(buck-boost)inductorInputfilterinductor.Multiplewindinginductors:Coupledoutputfilterinductor(R5)FlybacktransformerInductordesignalsodependsgreatlyonthein-ductorcurrentoperatingmode(Figure5-2):.Discontinuousinductorcurrentmode.whentheinstantaneousampere-turns(totaledinallwind-ings)dwellatzeroforaportionofeachswitchingperiod..Continuousinductorcurrentmode.inwhichthetotalampere-turnsdonotdwellatzero(althoughthecurrentmaypassthroughzero).Inthecontinuouscurrentmode,theripplecurrentisoftensmallenoughthatacwindinglossandaccorelossmaynotbesignificant,butinthediscontinuousmode,aclossesmaydominate.5-1Forexample,assumefullloadIdcof10A,andtypicalpeak-peaktriangularripplecurrent20%ofIdc,or2A(worstathighVin).Inthisexample,theworst-casermsripplecurrentis0.58A(triangularwave-formrmsequalsIpp/.Jj2),andrmsripplecurrentsquaredisonly.333,comparedwiththedccurrentsquaredof100.Thus,fortheacfRlosstoequalthedcloss,theRajRdcratiowouldhavetobeaslargeas300(Section3,Fig.3-5).Thisiseasilyavoided.Therefore,acwindinglossisusuallynotsignificant.Also,thesmallfluxswingassociatedwithsmallripplecurrentresultsinsmallcoreloss,withhighfrequencyferritecorematerialoperatingbelow250kHz.Coreutilizationisthenlimitedbysaturation(atpeakshort-circuitcurrent).However,thesmallfluxswingmaypermittheuseoflossiercoremateri-alswithhigherBSAT,suchaspowderediron,Kool-mu@,orlaminatedmetal.Thismayenablereducedcostorsize,butcorelossthenbecomesmoresignifi-cant.Also,distributed-gapmaterialsexhibitroundingoftheB-Hcharacteristics(Sec.2,pg.2-3),resultingindecreasinginductancevalueascurrentincreases.Boostandinputfilterinductorsandsinglewindingflybackinductorsareoftendesignedtooperateinthecontinuousmode.Aswiththebuck-derivedfilterinductorsdescribedpreviously,inductordesignisthenusuallylimitedbydcwindinglossesandcoresaturation.However,manyboostandflybackapplicationsaredesignedtooperateinthediscontinuousmode,becausetherequiredinductancevalueislessandtheinductorphysicalsizemaybesmaller.Butinthedis-continuousmode,theinductorcurrentmustdwellatzero(bydefinition)duringaportionofeachswitch-ingperiod.Therefore,thepeakofthetriangularcur-rentwaveform,andthusthepeak-to-peakripplemustbeatleasttwicetheaveragecurrent,asshowninFig.5-2(a).Thisverylargeripplecurrentresultsinapo-tentiallyseriousacwindinglossproblem.Also,theresultinglargefluxswingincurshighcoreloss.Corelossthenbecomesthelimitingfactorincoreutiliza-tion,ratherthansaturation,andmaydictatealargercoresizethanotherwiseexpected.Thus,thecircuitdesigner'schoiceofoperatingmodemakesasubstantialdifferenceintheinductordesignapproach.Whenflybacktransformersareoperatedinthecontinuousinductorcurrentmode,thetotalampere-turnsofallthewindingsneverdwellatzero(bydefi-nition).However,thecurrentineachwindingofanyflybacktransformerisalwayshighlydiscontinuous,regardlessofinductorcurrentmode.Thisisbecausecurrent(ampere-turns)transfersbackandforthbe-tweenprimaryandsecondary(s)attheswitchingfre-quency.AsshowninFig.5-3,thecurrentineachwindingalternatesfromzerotoahighpeakvalue,eventhoughthetotalampere-turnsarecontinuouswithsmallripple.Thisresultsinlargeacwindingloss,regardlessoftheoperatingmode.However,thecoreseesthetotalampere-turnripple.Thus,corelossbehavesinthesamemanneraswiththesinglewindingflybackinductordiscussedpreviously-smallcorelosswhendesignedandoper-atedinthecontinuousmode,largecorelossinthediscontinuousmode.5-2deredmetalcores,thewinding(s)shouldbelikewisedistributed.Thus,atoroidalcoreshapeshouldhavethewindingsdistributeduniformlyaroundtheentirecore.Withadiscretegap,usedwithlaminatedmetalalloycoresorferritecores,thewindingshouldbedirectlyoverthegap.Forexample,ifapairofCcorehalveshasagapinonelegandthewindingisplacedontheopposite(ungapped)leg,asshowninFig.5-4a,theentiremagneticforceintroducedbythewindingapp