Puzzler 15.docx
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Puzzler 15.docx
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Puzzler15
Puzzler15
Testyourpowersupplyprintedcircuitboardlayoutdesignskillsbytryingyourhandatansweringthethreequestionsbelow.
Thecircuitbelowshowsa12V,1ApowersupplyusingaTinySwitch-IIIpowerconversionIC.CapacitorsC1andC2formthebulkcapacitanceforthisset-up.
Figure1.12WUniversalinputflybackpowersupply
Question1
Whatistherelationbetweentheterminalvoltageofacapacitorandtheenergystoredinit?
1.Energystoredinthecapacitorissameastheterminalvoltage
2.Energystoredisindependentofterminalvoltage
3.Energystoredisproportionaltotwotimestheterminalvoltage
4.Energystoredisproportionaltosquareoftheterminalvoltage
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Answer1
Theanswerisd.
Energystoredinacapacitorchangesbysquareoftheterminalvoltage.Acapacitorhas4timeshigherenergystoredinitiftheterminalvoltageisdoubled.
Therelationshipbetweenenergystoredinacapacitoranditsterminalvoltageisgivenbythefollowingequation:
EEnergystoredinjoule
CCapacitorvalueinfarad
VCapacitorterminalvoltageinvolt
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Question2
Thepowersupplyshowninfigure1operatesoveranuniversalinputrange(85-265VAC,50Hz/60Hz)andhasafullloadefficiencyof75%at85VAC.AssumingthesupplycanoperatetoaminimumDCvoltageof75Vandthereisnohold-uprequirement,whatistheminimumsizeoftheinputDCbusfiltercapacitorrequiredforeverywattofoutputpowerdelivered?
a.1.5µF/W
b.3.0µF/W
c.2.5µF/W
d.4.5µF/W
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Answer2
c2.5µF/W
Specificationofatypicaluniversalinputpowersupplydemandssatisfactoryoperationoveraninputvoltagerangeof90to265VAC.Toensuretroublefreeoperationduringsupplyvoltagesags,whichmaylastfrom8msto50ms,someequipmentmanufacturersreducethelowerlimitto85VAC.
TheinputDCbuscapacitorsupportstheoperationofthepowersupplybystoringenergyduringthepeakoftheinputsinusoidandsupplyingthestoredenergytotheconvertercircuitduringintervaltd.(Seefigure.2)
Figure2.Capacitivefilterforfullwaverectifier
Calculationofthecapacitorvaluecanbemadeusingtheequation:
Po=Converteroutputpower
?
=Converterefficiency
td=Capacitordischargeinterval
Vpk=Minimumpeakvoltageseenbythepowersupplyatlowline
Vmin=MinimumDCvoltageforsatisfactoryoperation
ThesetofcurvesinFigure3showtheresultofcalculationgraphically,fordifferentconverterefficienciesandchoiceofminimumDCbusvoltage.
Figure3.Capacitorselectioncurvesfor47Hzfrequencyuniversalinputapplication.(a)Noholduprequirement,(b)4msholduprequirementand(c)holdupofonehalfcycleofsupplyfrequency)
Figure4.Relativecomparisonofrequiredcapacitorvaluefor47Hzand50Hzoperation.Noticetheslightreductionfora50Hzonlydesign
Arobustdesignmadeforworst-caseoperationwouldbeabletooperatesatisfactorilyforavoltageaslowas85VAC,linefrequencyaslowas47Hz,worstcasebulkcapacitor,minimumdevicecurrentlimit,minimumprimaryinductanceandminimumswitchingfrequency.
Statisticallytheprobabilityofthese6independentvariablesbeingattheirabsoluteworst-casevaluesislow.InpracticeformostapplicationsitissufficienttodesignforminimumACinputvoltage,typicalcapacitorvalues,typicallinefrequency(50Hz)minimumcurrentlimit,switchingfrequency(orI²fforPIproducts)andprimaryinductance.
Forthisexample,sincetheefficiencyoftheconverteris75%andtheconverterisdesignedtooperatefora75Vbusvoltage,2.5µF/Wminimumisrequired.Highervalueswouldallowoperationtoalowervoltageorprovideholduptimeattheexpenseoflargerandhighercostcapacitors.
Forapowersupplywithapi-filterasshowninFigure.1,thecapacitorvaluecalculatedcanbesplitintwocapacitors.
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Question3
Whydoesremovingamaximumdutycycle(maximumon-time)limitinaflybackconverterincreaseenergytransferfromtheinputcapacitorasitsterminalvoltagereduces?
Whydoesthissignificantlyhelpinapplicationswheredatamustbestoredtonon-volatilememoryonACpowerfailure?
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Answer3
Inaflybackconvertertheenergyisfirststoredintheinductorduringtheontimeoftheprimaryswitchandisthendeliveredtotheloadduringtheswitchofftime.WhentheACinputfails,thebusvoltagefalls,reducingthevoltageappliedacrosstheprimaryinductanceduringtheswitchontime.Thisreducesthedi/dt,requiringalongeron-timetoreachpeakcurrent.
Inatypicalconverteratsomebusvoltagetheon-timeneededtoreachcurrentlimitisgreaterthanallowedbythemaximumdutycyclelimit.Oncethispointisreachedthepeakinductorcurrentreducesasthebusvoltagecontinuestofall.Ineffectdutycycleandnotpeakcurrentnowlimitsthepowerdeliveredtotheload.
Withduty-cycleextensiontheswitchontimeisnotlimited,allowingcurrentlimittobereachedregardlessofthebusvoltage.Thismaximizesenergytransferfromthebulkcapacitortotheload.Foragivenbulkcapacitorvaluethisprovidesalongerhold-uptimecomparedtoapowersupplywithnodutycycleextension.
Mostmicroprocessor-basedcircuitsneedpowersuppliesthathaveadequatehold-uptimeformultiplereasons:
1.Toenableuninterruptedoperationduringsubcyclesupplylinedisturbances.
2.MicroprocessorbasedapplicationsoftenneedseveralmillisecondstostorecriticaldatatoNVRAMorEEPROMmemoriesoncea"supply-fail"conditionisdetected.Outputregulationshouldbemaintainedtoensuregracefulshutdownwithnolossofdata.
Microprocessorbasedcircuitscanreducepowerconsumptionbydisablingnon-criticalloadsonceinputsupplyfailureisdetected.Forareducedpowerlevel,FlybackconverterswithontimeextensioncanmaintainregulationattheoutputtoasignificantlylowerDCbusvoltage.Withtheabilitytomaximizeenergytransfertotheloadtheontimeextensionfeatureenablestheuseoflowerinputcapacitorvalueswhilestillprovidingadequateholduptime.
Figure5.RelationbetweenDCBusVoltagedropandpowerdeliverycapabilityofFlybackpowersupplieswithandwithoutontimeextension.At60%loadregulationislostat77V(a)withouton-timeextensionand43V(b)withon-timeextension.
Figure.3showsthatwithontimeextensionapowersupplydesignedtooperatewithaminimumDCbusvoltageof100Vcandeliverupto60%ofitsratedloadforaDCbusvoltageaslowas43V.AconventionalFlybackconverterunderthesameconditionsbutwithouton-timeextensionwillfalloutofregulationassoonasDCbusvoltagedropsbelow77V.
Thisexplainswhyconverterswithontimeextensionprovidelongerholduptimeascomparedtofixedfrequencysolutionsespeciallyforreducedloadlevelswithoutanyadditionalchangesinthedesign.
Figure4showsanexampleofimprovementinhold-uptimeofthe12V,1Apowersupplyshowinfigure1operatingatfullloadduetouseofontimeextension.
Figure6.Improvementinholduptimewithontimeextension
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