电气自动化控制电路设计Word下载.docx

电气自动化控制电路设计Word下载.docx

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I.INTRODUCTION

Insuringstabilityofaproposedpowersupplysolutionisoftenoneofthemorechallengingaspectsofthedesignprocess.Nothingismoredisconcertingthantohaveyourlovinglycraftedbreadboardbreakintowildoscillationsjustasitsbeingdemonstratedtothebossorcustomer,butinsuringagainstthisunfortunateeventtakessomeanalysiswhichmanydesignersviewasformidable.Pathstakenbydesignengineersoftenemphasizeeithercut-and-tryempiricaltestinginthelaboratoryorcomputersimulationslookingfornumericalsolutionsbasedoncomplexmathematicalmodels.Whilebothoftheseapproachabasicunderstandingoffeedbacktheorywillusuallyallowthedefinitionofanacceptablecompensationnetworkwithaminimumofcomputationaleffort.

II.STABILITYDEFINED

Fig.1.Definitionofstability

Fig.1givesaquickillustrationofatleastonedefinitionofstability.Initssimplestterms,asystemisstableif,whensubjectedtoaperturbationfromsomesource,itsresponsetothatperturbationeventuallydiesout.Notethatinanypracticalsystem,instabilitycannotresultinacompletelyunboundedresponseasthesystemwilleitherreachasaturationlevel–orfail.Oscillationinaswitchingregulatorcan,atmost,varythedutycyclebetweenzeroand100%andwhilethatmaynotpreventfailure,itwillsultimatelimittheresponseofanunstablesystem.

AnotherwayofvisualizingstabilityisshowninFig.2.Whilethisgraphicallyillustratestheconceptofsystemstability,italsopointsoutthatwemustmakeafurtherdistinctionbetweenlarge-signalandsmall-signalstability.Whilesmall-signalstabilityisanimportantandnecessarycriterion,asystemcouldsatisfythisrtquirementandyetstillbecomeunstablewithalarge-signalperturbation.Itisimportantthatdesignersrememberthatallthegainandphasecalculationswemightperformareonlytoinsuresmall-signalstability.Thesecalculationsarebasedupon–andonlyapplicableto–linearsystems,andaswitchingregulatoris–bydefinition–anon-linearsystem.Wesolvethisconundrumbyperformingouranalysisusingsmall-signalperturbationsaroundalarge-signaloperatingpoint,adistinctionwhichwillbefurtherclarifiedinourdesignprocedurediscussion。

Fig.2.Large-signalvs.small-signalstability

III.FEEDBACKCONTROLPRINCIPLES

Whereanuncontrolledsourceofvoltage（orcurrent,orpower）isappliedtotheinputofoursystemwiththeexpectationthatthevoltage（orcurrent,orpower）attheoutputwillbeverywellcontrolled.Thebasisofourcontrolissomeformofreference,andanydeviationbetweentheoutputandthereferencebecomesanerror.Inafeedback-controlledsystem,negativefeedbackisusedtoreducethiserrortoanacceptablevalue–asclosetozeroaswewanttospendtheefforttoachieve.Typically,however,wealsowanttoreducetheerrorquickly,butinherentwithfeedbackcontrolisthetradeoffbetweensystemresponseandsystemstability.Themoreresponsivethefeedbacknetworkis,thegreaterbecomestheriskofinstability.Atthispointweshouldalsomentionthatthereisanothermethodofcontrol–feedforward.Withfeedforwardcontrol,acontrolsignalisdevelopeddirectlyinresponsetoaninputvariationorperturbation.Feedforwardislessaccuratethanfeedbacksinceoutputsensingisnotinvolved,however,thereisnodelaywaitingforanoutputerrorsignaltobedeveloped,andfeedforwardcontrolcannotcauseinstability.Itshouldbeclearthatfeedforwardcontrolwilltypicallynotbeadequateastheonlycontrolmethodforavoltageregulator,butitisoftenusedtogetherwithfeedbacktoimprovearegulator’sresponsetodynamicinputvariations.

ThebasisforfeedbackcontrolisillustratedwiththeflowdiagramofFig.3wherethegoalisfortheoutputtofollowthereferencepredictablyandfortheeffectsofexternalperturbations,suchasinputvoltagevariations,tobereducedtotolerablelevelsattheoutputWithoutfeedback,thereference-to-outputtransferfunctiony/uisequaltoG,andwecanexpresstheoutputasyGu

Withtheadditionoffeedback（actuallythesubtractionofthefeedbacksignal）

yGuyHG

andthereference-to-outputtransferfunctionbecomes

y/u=G/1+GH

IfweassumethatGH__1,thentheoveralltransferfunctionsimplifiesto

y/u=1/H

Fig.3.Flowgraphoffeedbackcontrol

NotonlyisthisresultnowindependentofG,itisalsoindependentofalltheparametersofthesystemwhichmightimpactG（supplyvoltage,temperature,componenttolerances,etc.）andisdeterminedinsteadsolelybythefeedbacknetworkH（and,ofcourse,bythereference）.NotethattheaccuracyofH（usuallyresistortolerances）andinthesummingcircuit（erroramplifieroffsetvoltage）willstillcontributetoanoutputerror.Inpractice,thefeedbackcontrolsystem,asmodeledinFig.4,isdesignedsothatG__HandGH__1overaswideafrequencyrangeaspossiblewithoutincurringinstability.WecanmakeafurtherrefinementtoourgeneralizedpowerregulatorwiththeblockdiagramshowninFig.5.Herewehaveseparatedthepowersystemintotwoblocks–thepowersectionandthecontrolcircuitry.Thepowersectionhandlestheloadcurrentandistypicallylarge,heavy,andsubjecttowidetemperaturefluctuations.Itsswitchingfunctionsarebydefinition,large-signalphenomenon,normallysimulatedinmoststabilityanalysesasjustatwostateswitchwithadutycycle.Theoutputfilterisalsoconsideredasapartofthepowersectionbutcanbeconsideredasalinearblock.

Fig.4.Thegeneralpowerregulator

IV.THEBUCKCONVERTER

Thesimplestformoftheabovegeneralpowerregulatoristhebuck–orstepdown–topologywhosepowerstageisshowninFig.6.Inthisconfiguration,aDCinputvoltageisswitchedatsomerepetitiverateasitisappliedtoanoutputfilter.ThefilteraveragesthedutycyclemodulationoftheinputvoltagetoestablishanoutputDCvoltagelowerthantheinputvalue.Thetransferfunctionforthisstageisdefinedby

tON=switchon-time

T=repetitiveperiod（1/fs）

d=dutycycle

Fig.5.Thebuckconverter.

Sinceweassumethattheswitchandthefiltercomponentsarelossless,theidealefficiencyof

Thisconversionprocessis100%,andregulationoftheoutputvoltagelevelisachievedby

controllingthedutycycle.ThewaveformsofFig.6assumeacontinuousconductionmode（CCM）

Meaningthatcurrentisalwaysflowingthroughtheinductor–fromtheswitchwhenitisclosed,

Andfromthediodewhentheswitchisopen.Theanalysispresentedinthistopicwillemphasize

CCMoperationbecauseitisinthismodethatsmall-signalstabilityisgenerallymoredifficult

toachieve.Inthediscontinuousconductionmode（DCM）,thereisathirdswitchconditioninwhichtheinductor,switch,anddiodecurrentsareall5-4zero.Eachswitchingperiodstartsfromthesamestate（withzeroinductorcurrent）,thuseffectivelyreducingthesystemorderbyoneandmakingsmall-signalstableperformancemucheasiertoachieve.Althoughbeyondthescopeofthistopic,theremaybespecializedinstanceswherethelarge-signalstabilityofaDCMsystemisofgreaterconcernthansmall-signalstability.

ThereareseveralformsofPWMcontrolforthebuckregulatorincluding,

•Fixedfrequency（fS）withvariabletONandvariabletOFF

•FixedtONwithvariabletOFFandvariablefS

•FixedtOFFwithvariabletONandvariablefS

•Hysteretic（or“bang-bang”）withtON,tOFF,andfSallvariable

Eachoftheseformshavetheirownsetofadvantagesandlimitationsandallhavebeensuccessfullyused,butsinceallswitchmoderegulatorsgenerateaswitchingfrequencycomponentanditsassociatedharmonicsaswellastheintendedDCoutput,electromagneticinterferenceandnoiseconsiderationshavemadefixedfrequencyoperationbyfarthemostpopular.

Withtheexceptionofhysteretic,allotherformsofPWMcontrolhaveessentiallythesame

small-signalbehavior.Thus,withoutmuchlossingenerality,fixedfSwillbethebasisforourdiscussionofclassical,small-signalstability.

Hystereticcontrolisfundamentallydifferentinthatthedutyfactorisnotcontrolled,perse.Switchturn-offoccurswhentheoutputripplevoltagereachesanuppertrippointandturn-onoccursatalowerthreshold.Bydefinition,thisis

alarge-signalcontrollertowhichsmall-signalstabilityconsiderationsdonotapply.Inasmallsignalsense,itisalreadyunstableand,inamathematicalsense,itsfastresponseisduemoretofeedforwardthanfeedback.

REFERENCES

[1]D.M.Mitchell,“DC-DCSwitchingRegulatorAnalysis”,McGraw-Hill,1988,

DMMitchellConsultants,CedarRapids,IA,1992（reprintversion）.

[2]D.M.Mitchell,“Small-SignalMathcadDesignAids”,（Windows95/98version）,e/j

BLOOMAssociates,Inc.,1999.

[3]GeorgeChryssis,“High-FrequencySwitchingPowerSupplies”,McGraw-HillBook

Company,1984.

[4]RayRidley,“AMoreAccurateCurrent-ModeControlModel”,UnitrodeSeminar

Handbook,SEM-1300,AppendixA2.

[5]LloydDixon,“ControlLoopDesign”,UnitrodeSeminarHandbook,SEM-800.

[6]LloydDixon,“ControlLoop