Isolated Multiple Output Flyback Converter Design …

1 ApplicationReportSLVA666 April2014 IsolatedMultipleOutputFlybackConverterDe signUsingTL494 SrivatsaRaghunathABSTRACTThe TL494is one of the earliestpulse-width-modulation(PWM)contr ollersusedin incorporatesall the functionsrequiredin the constructionof a PWMcontrolcircuiton a power-supplycontrol,this deviceoffersthe flexibilityto tailorthe power-supplycontrolcircuitryto a Multiple -outputlow-cost isolatedflybackconverterwith the magneticsdesignis also successfullyimplemented,the TL494 PWMmodeensuredlow EMI andnoiseoperationdue to its powersupplyis designedfor +5-, +15-,and + is self-containedwith overcurrentand overvoltageprotections.

2 And the PI controlleris designedto limit the the flybackconverterdesignand componentselectionis MathCADfile is availablealongwiththis document,whichhelpsin April2014 IsolatedMultipleOutputFlybackConverterDe signUsingTL494 SubmitDocumentationFeedbackCopyright 2014, NominalinputvoltageVin_nom= 325 V MinimuminputvoltageVin_min= 260 V MaximuminputvoltageVin_max= 390 V Tolerance% = 20 Outputripplevoltage= 50 mV OutputvoltagesVO= +15 V, +15 V, +5 V Maximumload currentIo= A, A, 3 A Minimumload currentIomin= A, A, A MaximumoutputpowerPo= 30 40 knownto startthe flybackmagneticsdesignswitchselectedis the 2SK2605N-Channelenhancementtype MOSFET2SK2605specifications.

3 VDS_max= 800 V IDS_max= 5 A tr= 40 ns tf= 40 ns Rds on = Qgtotal = 34 nC VGS= 10 V2 IsolatedMultipleOutputFlybackConverterDe signUsingTL494 SLVA666 April2014 SubmitDocumentationFeedbackCopyright 2014,TexasInstrumentsIncorporatedorcmrgu uAlulK K K PKLdciII2 in _ minmaxpsVDLi f K K o miniI2 Jo maxLdcmaxn I I1 D K in _ maxmaxin _ maxo _ maxIDIn I K odmaxodin _ minVVnDVVVn Odin _ maxDS _ maxDS _ maxVV8 VVVn100 shownin the MathCADfile and summarizedin the following:Step1: Calculateturnsratio1 safetymarginof 8%:(1)Step2: Calculatemaximumduty equivalentprimaryreferredbuck-boostcircu it:(2)A moreaccurateestimatefor Dmaxis calculatedby consideringa theoreticalefficiencyestimateof 70%as:(3)Step3: inputreferredbuck-boostcircuitis givenby Equation4.

4 (4)This gives i = r ILdc, wherer is the ripplefactor,= to : CCMcheckat Iomin(if:, thenCCMensured)Step3b: FlybackchokedesignInductancevaluein primaryreferredcircuit:(5)Peakinductorcu rrent:(6)Corespecifications:EE-42/21/15c ore Ac= 10 4m2 Aw= 10 4m2 lm= 10 2m Ap= 10 8m4 ur= 2000 Bm= gap:Selectair gap thicknesslgas per storeenergy,the transformercoreneedsanair is the maindifferencein transformerdesignfor a flybackversusa transformerdesignforforwardconvertertype s.(7)3 SLVA666 April2014 IsolatedMultipleOutputFlybackConverterDe signUsingTL494 SubmitDocumentationFeedbackCopyright 2014,TexasInstrumentsIncorporateds _ rmssecIaJ o maxs _ rmsmaxII1 D p _ rmspriIaJ 22maxp _ rmsLdcDiI3 I23 K K mPKpcLIBNAK KppLN primaryand secondaryturns:Equation8 showsthe numberof primaryturns.

5 (8)Equation9 showsthe numberof n Np(9)Checkfor the possibilityof < Bmensuresthat the corewill not saturate.(10) showsthe equationfor primaryRMScurrent.(11)Requiredwire crosssectionalarea:(12)Choiceof showsthe equationfor secondaryRMScurrent.(13)Requiredwire crosssectionalarea:(14)Choiceof wire:SWG30 / 284 IsolatedMultipleOutputFlybackConverterDe signUsingTL494 SLVA666 April2014 SubmitDocumentationFeedbackCopyright 2014,TexasInstrumentsIncorporated KRP= / 2avgKRP= 1 (DCM)KRP< 1 (CCM) windowutilizationfactorof Kw= ,the availablewindowareais Aw Kw= needan areaof Np apri+ Ns apri+ Ns asec< AW transformermagneticdetailsfromthe previouslysubmittedMATLAB designfile for summarize.

6 NP = 62T (splitprimaryis used,31T in one layerof 28 SWGwire gauge) NS1(5 V) = 5T (22 SWGwire gauge) NS2(15 V) = 17T (30 SWGwire gauge) NS3(15 V) = 17T (30 SWGwire gauge) NBias(12 V) = 12T (30 SWGwire gauge) LP = 370 H (leakageof < 5 H)Discontinuousmodeof operationis ,the ripplefactorKRF= InductorCurrentRippleFactor5 SLVA666 April2014 IsolatedMultipleOutputFlybackConverterDe signUsingTL494 SubmitDocumentationFeedbackCopyright 2014, 10sVVn K K lkPKsoSLItVVnK practicalmeasurement,the leakageinductancefor the flybackchokewas foundto be Llk= 10 H.

7 Thepowerdissipatedin the snubbercircuitis givenby:PS= VS IPK tS fS(15)where(16)Figure2 revealsa closerVSis to VO/ n, the greaterthe PSvs VSVSis constrainedto be less than:VDS_max Vin_max= 110 ,selectVS 100 resetleakageinductance:(17)Snubberresist ance:(18)6 IsolatedMultipleOutputFlybackConverterDe signUsingTL494 SLVA666 April2014 SubmitDocumentationFeedbackCopyright 2014,TexasInstrumentsIncorporated6omaxop opsn 10 FVf K K K K Vn nFRfV K K :Allowa voltageripple: VS= 8% of VS= VThus,6 OutputCapacitorDesignIn the primaryside referredbuck-boostconvertercircuit,COref lectsas Cop= n2 COAn outputrippleof VO= 50 mV is equivalentto an inputreferredrippleof:(19)This gives:Outputcapacitorsare designedas per the summarize: 5-V outputcapacitors:1000 F (x3) 15-Voutputcapacitors:100 F (x2) 15-Voutputcapacitors:100 F (x2) 12-Vbias outputcapacitors.

8 22 F (x1)7 SoftStartCircuitA soft startcircuitis providedto reducestresson switchingtransistorsat April2014 IsolatedMultipleOutputFlybackConverterDe signUsingTL494 SubmitDocumentationFeedbackCopyright 2014,TexasInstrumentsIncorporatedz _ RHPcrossff800 Hz8 |ESRRHP2pss11wwP(s)Gs1w K DK K z _ Hz2 ESR C 5 KKz _ Hz2 LnD 5 K K Hz2LC 5 K H1 D planttransferfunction:Effectiveinductanc evaluein outputLC filter:(20)This givesdoublepole locationat:(21)Righthalf-planezerolocati on:(22)ESRzerolocation:(23)(24)Planttran sferfunction:(25)Thus,compensatorshoulda dd a 0 at fp.

9 Also,the usermustlimit the BW of the systemwith an open-loopcrossoverof:(26)This is one of the majordrawbacksof voltagemodecontrolwhereinthe transientresponseof the systemis poorfor load and line openloop bodeplot is shownin April2014 SubmitDocumentationFeedbackCopyright 2014, OpenLoopBodeThe compensatorcircuitis shownin Figure4:Figure4. CompensatorCircuitThe compensatorzerois positionedto cancelone of the doublepolesat April2014 IsolatedMultipleOutputFlybackConverterDe signUsingTL494 SubmitDocumentationFeedbackCopyright 2014,TexasInstrumentsIncorporated21OR1 CTRRA Hz2RC 5 K.

10 (27)Rf1= 50 k and Rf2= 10 k in orderto get V from15 V at the referencepin of V at the referencepin of givesCf= 4 k is selectedby takinginto considerationthe minimumbias currentof TL431and maximumforwarddiodecurrentof achievean openloop crossoverfrequencyof fcross= 800 Hz, compensatorgain is requiredto be equalto, whereAO= 800 Hz fromthe ,Equation28 givesR2 4 k consideringa CTR= for the MCT2E.(28) April2014 SubmitDocumentationFeedbackCopyright 2014,TexasInstrumentsIncorporatedminsD1t 100 ns100f K| PNPin drivecircuitensuresprimaryMOSFET gatechargeis removedfor fast resistancein MOSFET drive:The total gatechargerequirement= 63 turn-ontime shouldbe at most2% of total :(29)td(ON)+ tr= 56 ns is the maximumturn on time limit.