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IDescriptionThisblogintroducesapulsewidthmodulationtechnologywithTL494asthecontrolcore.AnditisappliedtoDCmotorcontrolsystem.Theworkingprincipleofthesystem,therealizationcircuitandthestructureandspecificapplicationofthePWMcontrolchipareanalyzedindetail.CatalogIDescriptionIIWorkingPrincipleandRealizationofControlSystem2.1HowSystemWorks2.2SelectionofPWMControlChip2.3RealizationCircuitofSystemIIISimulationResultsIVConclusionFAQOrdering&QuantityIIWorkingPrincipleandRealizationofControlSystem2.1HowSystemWorksThebasicdesignideaofthiscontrolsystemistouseastep-downchoppercircuitasshowninFigure1.Figure1.Step-downChopperCircuitanditsWaveformInthefigure,theDCpowersupplyisUd,andtheloadisamotor(M).Whentheswitchingdevice(VT)istriggeredandturnedon,theDCvoltageisappliedtothemotorforadurationoft1.Whenthefieldswitchingdeviceisturnedoff,thevoltageontheloadiszeroandlastsfort2time.IfwedefinethedutycycleT=t1+t2,andthedutycyclek=t/T,thewaveformdiagramandtheprincipleoftheDCchoppercircuitareasfollows:Theaveragevalue(Uo)ofthesystemoutputvoltageis:Theeffectivevalue(U)ofitsoutputvoltageis:Thepulsewidthmodulation(PWM)workingmodeadoptedbythissystemkeepsTunchangedandt1changes.TheprincipleblockdiagramofthecontrolsystemisshowninFigure2.Figure2.ControlSystemBlockDiagramThepowersupplyinthissystemmakesthemotorworkthroughthepowerdrivecircuit.Theon-offofthepowerdrivecircuitiscontrolledbythePWMcontrolchip.ThesystemsamplesthemotorcurrentfeedsitbacktothePWMcontrolchipandcomparesitwiththecurrentvalueofthecurrentcomparisoncircuit.TocontrolthePWMsignaloutputtoachievethepurposeofspeedregulation.ThesystemalsosamplesthepowersupplyvoltageandfeedsitbacktothePWMcontrolchip.Inthisway,itcanbecomparedwiththevoltagevalueofthevoltagecomparisoncircuittocontrolthePWMsignaloutputandachievetheeffectofUndervoltageprotection.WeusethecommonBUCKcircuitforthepowerdrivecircuitandPowerMOSFETfortheswitchtube.Theregulationofitsoutputvoltageisrealizedbycontrollingtheturn-ontimeofthedevice.Consideringtheinfluenceoftheinductanceofthemotor,theoutputcurrentisrelativelystableandtheenergyconsumptionislow.2.2SelectionofPWMControlChipInthecontrolcircuitofthemotorPWMcontrolsystem,theTL494chipisselected.TL494chiphasthefeaturesofstronganti-interferenceability,simplestructure,highreliabilityandlowprice.TheinternalcircuitofTL494(Figure3)consistsofthefollowingparts:Thereferencevoltagegeneratingcircuit;Oscillationcircuit;Intermittentadjustmentcircuit;Twoerroramplifiers;Pulsewidthmodulationcomparator;Outputcircuit;...Figure3.TL494InternalStructureAmongthem:Forpins1and2,theyarethenon-invertingandinvertinginputterminalsoferroramplifier1.Forpin3,itisphasecorrectionandgainscontrol.Forpin4,itisanintermittentperiodofconditioning,andthecut-offtimecanbechangedfrom2%to100%whenavoltageof0~3.3Visappliedtoit.Forpins5and6,theyareusedtoconnectexternaloscillationresistorRTandoscillationcapacitorCTtodeterminethefrequencyfoscofthesawtoothwavegeneratedbytheoscillator.Where:ThevaluerangeofRTandCT:RT=5~100kQ,CT=0.001~0.1F.Pin7isthegroundterminal;Forpins8,9and11,10,theyarethecollectorandemitterofthetwofinaloutputtransistorsinsideTL494;For12feet,itisthepowersupplyterminal;Forpin13,itistheoutputcontrolterminal.Whenthispinisgrounded,itisaparallelsingle-endedoutputmode.Whenpin14isconnected,itisapush-pulloutputmode;For14feet,itisthe5Vreferencevoltageoutputterminal,themaximumoutputcurrentis10mA;For15and16pins,theyaretheinvertingandnon-invertinginputterminalsoftheerroramplifier2.2.3RealizationCircuitofSystemTheconcreterealizationcircuitofthiscontrolsystemisshowninFig.4.Thesystemusescurrentnegativefeedbacktotrackthespeedofthemotor.Atthesametime,throughthenegativefeedbackofthevoltageofthepowersupply,thesystemhasthefunctionofundervoltageprotection.Figure4.SystemImplementationCircuitThemotorcurrentisdetectedandfedbacktopin1oftheerroramplifier1oftheTL494,comparedwiththecurrentreferencesignalofpin2tocontrolthePWMoutputofTL494.Soastorealizethefunctionofregulatingthespeedofthemotor.Byadjustingtheresistancevalueoftheadjustableresistor(RES1).Thatis,changethesizeofthesetcurrentreferencesignaltoadjustthedutycycleofthePWMoutputsignal.Soastoachievethepurposeofadjustingthemotorspeed.Bysamplingthevoltageofthepowersupply,itisfedbacktopin15oftheinternalerroramplifier2ofTI494.Thencomparewiththevoltagereferencesignalofpin16tocontrolthePWMoutputofTL494.Soastorealizetheundervoltageprotectionfunctionofthesystem.ThesystempowerisdrivenbyPowerMOSFET,whichhasahighinputimpedanceandcanbedirectlydrivenbyatransistor.Pin13ofTI494isusedtocontroltheoutputmode.Inthissystem,choosetoinputthisendaslowlevel.Atthistime,theflip-flopsQ1andQ2intheTL494donotwork,thetwooutputsarethesame,thefrequencyisthesameastheoscillatorfrequency,andthemaximumdutycycleis98%.IIISimulationResultsInordertoverifytheeffectivenessofthesystem,wecansimulatethecircuitoftheabove-mentionedDCmotorcontrolsystembasedonPWMtechnologybycomputer.WesetthesystemspowersupplyvoltageUcc=12V,DCmotorratedparameters:Un=12V,In=15A,4poles,armatureresistanceRa=0.21Q,momentofinertiaJ=0.57kg㎡.ThroughchangingtheadjustableresistanceRES1ofthecontrolcircuittorealizethedifferentdutyratioofthesystemoutput.Whentheoutputdutyratiois0.2,0.65/0.8,thevoltageandcurrentwaveformsofthemotorareshowninFigure5.Figure5.MotorVoltageandCurrentWaveformItcanbeseenfromFigure5thatwhenoutputtingdifferentdutyratios,thevoltageandcurrentwaveformsofthemotorarestable,whichisconducivetothelong-termstableoperationofthemotor.ThisshowsthattheTL494-basedPWMcontroltechnologyintroducedinthisarticleisfeasibleinpracticalapplicationsandrunsstably.IVConclusionThisblogsummarizesaPWMcontroltechnologybasedontheTL494chip.Thesimulationresultsshowthatthesystemhasasimplestructure,lowenergyconsumption,andstableoperation.AndthecharacteristicsoftheTL494chiphavebeenfullyutilizedsothatthesystemhastheadvantagesofundervoltageprotection.FAQWhatisTL494?TL494isaPWMcontrollerICusedforpowerelectronicscircuits.Itcomprisesofon-chiptwoerroramplifiersanoscillatorwithadjustablefrequencyfeature,anoutputflip-flophavingpulsesteeringcontrol,andanoutputcontrolcircuitwithfeedback.WhatisthedetaileddescriptionofTL494?TheTL494deviceincorporatesallthefunctionsrequiredintheconstructionofapulse-width-modulation(PWM)controlcircuitonasinglechip.Designedprimarilyforpower-supplycontrol,thisdeviceofferstheflexibilitytotailorthepower-supplycontrolcircuitrytoaspecificapplication.TheTL494devicecontainstwoerroramplifiers,anon-chipadjustableoscillator,adead-timecontrol(DTC)comparator,apulse-steeringcontrolflip-flop,a5-V,5%-precisionregulator,andoutput-controlcircuits.Theerroramplifiersexhibitacommon-modevoltagerangefrom0.3VtoVCC2V.Thedead-timecontrolcomparatorhasafixedoffsetthatprovidesapproximately5%deadtime.Theon-chiposcillatorcanbebypassedbyterminatingRTtothereferenceoutputandprovidingasawtoothinputtoCT,oritcandrivethecommoncircuitsinsynchronousmultiple-railpowersupplies.Theuncommittedoutputtransistorsprovideeithercommon-emitteroremitter-followeroutputcapability.TheTL494deviceprovidesforpush-pullorsingle-endedoutputoperation,whichcanbeselectedthroughtheoutput-controlfunction.Thearchitectureofthisdeviceprohibitsthepossibilityofeitheroutputbeingpulsedtwiceduringpush-pulloperation.WhatareTL494productfeatures?CompletePWMPower-ControlCircuitryUncommittedOutputsfor200-mASinkorSourceCurrentOutputControlSelectsSingle-EndedorPush-PullOperationInternalCircuitryProhibitsDoublePulseatEitherOutputVariableDeadTimeProvidesControlOverTotalRangeWhatisPWMIC?TheTL494fixedfrequencyPWMControllercanbeusedforDCtoDCconversionregardlessofbuckorboosttopology....ThisICfeatureanoutputcontrolcircuit,aflipflop,adeadtimecomparator,twodifferenterroramplifiers,a5Vreferencevoltage,anoscillator,andaPWMcomparator.HowdoesPWMICwork?Asitsnamesuggests,pulsewidthmodulationspeedcontrolworksbydrivingthemotorwithaseriesofON-OFFpulsesandvaryingthedutycycle,thefractionoftimethattheoutputvoltageisONcomparedtowhenitisOFF,ofthepulseswhilekeepingthefrequencyconstant.WhichICisbetterforabuckconverter,TL494orUC3843?TheymainlydifferintypeofcontrolTL494=voltagemodecontrol(Oneloopcontrol).whileUC3843usescurrentmodecontrol(Nestedloopcontrol,withainner/fastcurrentloopandanotherouter/slowervoltageloop)Typicallyvoltagemodeareusedinmultipleoutputconverterswithgoodcross-regulation.CurrentmodewhenyouwanttoparallelmultipleconverterstomakeasingleconverterwithhighercurrentratingTL494isaverypopularIC.IfyouhavesimplerequirementsTL494isrecommendedHowdoIproperlysetthefeedbackpinonaTL494SMPSIC?Thefeedbackpinistheoutputofbotherroramplifiers,usedincomparingandadjustingtheoutputpulsewidthtotheDCcontrolvoltage.OnvariouscircuitsIhavelookedup,theop-ampconnectedtopins23areusedtosetthegainofthefeedbackloop,using2resistorswithoneresistorconnectingto2.5Vpotentialdivideron5Vreferencevoltage.Withtheotherconnectingtotheoutput(viasuitableisolation)Thegainappearstobesetat101,usinga51kfeedbackwith510ohmstothe2.5Vreference.Itisusedtocontrolthegainofthefeedbackvoltage.NoliteratureIhaveyetfound,givesanindicationonhowthisgainbeset,exceptagraphshowinganopenloopgainof1000,presumablythegainissetforthebeststability,althoughtherewillalsobeatimeconstant.WhyistherenofrequencycompensationrequiredinTIsTL494examplebuckregulatordesign(operationalamplifier,buckphase,shiftphase,margin,TL494,electronics)?ItsafixedfrequencyPWMcontrollerwithinternaldeadtimetimer.Frequencycompensationisnotrequired.Takealookatthedatasheet.HowtouseTL494?

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