控制电路设计外文翻译文献.docx
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控制电路设计外文翻译文献.docx
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控制电路设计外文翻译文献
控制电路设计外文翻译文献
(文档含中英文对照即英文原文和中文翻译)
外文:
DesigningStableControlLoops
Theobjectiveofthistopicistoprovidethedesignerwithapracticalreviewofloopcompensationtechniquesappliedtoswitchingpowersupplyfeedbackcontrol.Atop-downsystemapproachistakenstartingwithbasicfeedbackcontrolconceptsandleadingtostep-by-stepdesignprocedures,initiallyappliedtoasimplebuckregulatorandthenexpandedtoothertopologiesandcontrolalgorithms.SampledesignsaredemonstratedwithMathcadsimulationstoillustrategainandphasemarginsandtheirimpactonperformanceanalysis.
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]L
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