外文翻译关于PWM的外文翻译.docx

Pulse-widthmodulationPulse-widthmodulation(PWM)isamodulationtechniquethatconformsthewidthofthepulse,formallythepulseduration,basedonmodulatorsignalinformation.Althoughthismodulationtechniquecanbeusedtoencodeinformationfortransmission,itsmainuseistoallowthecontrolofthepowersuppliedtoelectricaldevices,especiallytoinertialloadssuchasmotors.Inaddition,PWMisoneofthetwoprincipalalgorithmsusedinPhOtOVOltaiCsolarbatterychargers,Theaveragevalueofvoltage(andcurrent)fedtotheIOadiscontrolledbyturningtheswitchbetweensupplyandloadonandoffatafastpace.Thelongertheswitchisoncomparedtotheoffperiods,thehigherthepowersuppliedtotheloadis.ThePWMswitchingfrequencyhastobemuchfasterthanwhatwouldaffecttheload,whichistosaythedevicethatusesthepower.Typicallyswitchingshavetobedoneseveraltimesaminuteinanelectricstove,120Hzinalampdimmer,fromfewkilohertz(kHz)totensofkHzforamotordriveandwellintothetensorhundredsofkHzinaudioamplifiersandcomputerpowersupplies.ThetermdutyCyeIedescribestheproportionof'on'timetotheregularintervalor,period*oftime;alowdutycyclecorrespondstolowpower,becausethepowerisoffformostofthetime.Dutycycleisexpressedinpercent,100%beingfullyon.ThemainadvantageofPwMisthatpowerlossintheswitchingdevicesisverylow.Whenaswitchisoffthereispracticallynocurrent,andwhenitison,thereisalmostnovoltagedropacrosstheswitch.Powerloss,beingtheproductofvoltageandcurrent,isthusinbothcasesclosetozero.PWMalsoworkswellwithdigitalcontrols,which,becauseoftheiron/offnature,caneasilysettheneededdutycycle.PWMhasalsobeenusedincertainCommUniCationSyStemSwhereitsdutycyclehasbeenusedtoconveyinformationoveracommunicationschannel.HistoryInthepast,whenonlypartialpowerwasneeded(suchasforaSeWingmachinemotor),arheostat(locatedinthesewingmachine,Sfootpedal)connectedinserieswiththemotoradjustedtheamountofcurrentflowingthroughthemotor,butalsowastedpowerasheatintheresistorelement.Itwasaninefficientscheme,buttolerablebecausethetotalpowerwaslow.Thiswasoneofseveralmethodsofcontrollingpower.Therewereotherssomestillinuse-suchasvariableautotransformers,includingthetrademarked'Autrastat'fortheatricallighting;andtheVariac,forgeneralACpoweradjustment.Thesewerequiteefficient,butalsorelativelycostly.Foraboutacentury,somevariable-speedelectricmotorshavehaddecentefficiency,buttheyweresomewhatmorecomplexthanconstant-speedmotors,andsometimesrequiredbulkyexternalelectricalapparatus,suchasabankofvariablepowerresistorsorrotatingconvertersuchasWardLeOnarddrive.However,inadditiontomotordrivesforfans,pumpsandroboticservos,therewasagreatneedforcompactandlowcostmeansforapplyingadjustablepowerformanydevices,suchaselectricstovesandlampdimmers.OneearlyapplicationofPWMwasintheSinCIairX10,a10Waudioamplifieravailableinkitforminthe1960s.AtaroundthesametimePWMstartedtobeusedinACmotorcontrol.DTT gDF n QZx)9) Yf OT4D9)Fig. 1: a PUlSe wave, showing the definitions of Vmni UnIZLTand D.>s*4Pulse-widthmodulationusesarectangularPUISCWaVCwhosepulsewidthismodulatedresultinginthevariationoftheaveragevalueofthewaveform.Ifweconsiderapulsewaveform/"),withperiodT,lowvalueUmin,ahighvalueVmandadulyCyCICD(seefigure1),theaveragevalueofthewaveformisgivenby:1ry=f必/JnAs/(t)isapulsewave,itsvalueisVmarfor0<fe).TandVminfor力.TVfVT.Theaboveexpressionthenbecomes:=DVnx+(1。)VminThislatterexpressioncanbefairlysimplifiedinmanycaseswhereUmin=OaSQ=DUma.Fromthis,itisobviousthattheaveragevalueofthesignal(U)isdirectlydependentonthedutycycleDFig.2:AsimplemethodtogeneratethePWMpulsetraincorrespondingtoagivensignalistheIntersectivePWM:thesignal(heretheredsinewave)iscomparedwithasawtoothwaveform(blue).Whenthelatterislessthantheformer,thePWMsignal(magenta)isinhighstate(1).Otherwiseitisinthelowstate(0).ThesimplestwaytogenerateaPWMsignalistheintersectivemethod,whichrequiresonlyaSaWtoOthoratrianglewaveform(easilygeneratedusimpleOSCiHatOr)andacomparator.Whenthevalueofthereferencesignal(theredsinewaveinfigure2)ismorethanthemodulationwaveform(blue),thePWMsignal(magenta)isinthehighstate,otherwiseitisinthelowstate.TimeproportioningManydigitalcircuitscangeneratePwMsignals(e.g.,manyIniCroContrOIlCrShavePwMoutputs).TheynormallyuseaCOImterthatincrementsperiodically(itisconnecteddirectlyorindirectlytotheClOCkofthecircuit)andisresetattheendofeveryperiodofthePWM.Whenthecountervalueismorethanthereferencevalue,thePWMoutputchangesstatefromhightolow(orlowtohigh).3Thistechniqueisreferredtoastimeproportioning,particularlyastime-proportioningcontrol4-whichproportionofafixedCyeIetimeisspentinthehighstate.Theincrementedandperiodicallyresetcounteristhediscreteversionoftheintersectingmethod,ssawtooth.Theanalogcomparatoroftheintersectingmethodbecomesasimpleintegercomparisonbetweenthecurrentcountervalueandthedigital(possiblydigitized)referencevalue.Thedutycyclecanonlybevariedindiscretesteps,asafunctionofthecounterresolution.However,ahigh-resolutioncountercanprovidequitesatisfactoryperformance.PWMsamplingtheoremTheprocessofPwMconversionisnon-linearanditisgenerallysupposedthatlowpassfiltersignalrecoveryisimperfectforPWM.ThePWMsamplingtheorem6showsthatPWMconversioncanbeperfect.ThetheoremstatesthatAnybandlimitedbasebandsignalwithin÷0.637canberepresentedbyapulsewidthmodulation(PWM)waveformwithunitamplitude.ThenumberofpulsesinthewaveformisequaltothenumberofNyquistsamplesandthepeakconstraintisindependentofwhetherthewaveformistwo-levelorthree-level.,PowerdeliveryPWMcanbeusedtocontroltheamountofpowerdeliveredtoaloadwithoutincurringthelossesthatwouldresultfromlinearpowerdeliverybyresistivemeans.Potentialdrawbackstothistechniquearethepulsationsdefinedbythedutycycle,switchingfrequencyandpropertiesoftheload.Withasufficientlyhighswitchingfrequencyand,whennecessary,usingadditionalpassiveCICCtroniCfilters,thepulsetraincanbesmoothedandaverageanalogwaveformrecovered.HighfrequencyPWMpowercontrolsystemsareeasilyrealisablewithsemiconductorswitches.Asexplainedabove,almostnopowerisdissipatedbytheswitchineitheronoroffstate.However,duringthetransitionsbetweenonandoffstates,bothvoltageandcurrentarenonzeroandthuspowerisdissipatedintheswitches.Byquicklychangingthestatebetweenfullyonandfullyoff(typicallylessthan100nanoseconds),thepowerdissipationintheswitchescanbequitelowcomparedtothepowerbeingdeliveredtotheload.ModernsemiconductorswitchessuchasMOSFETsorInSUIated-gatebipolartransistors(IGBTs)arewellsuitedcomponentsforhighefficiencycontrollers.FrequencyconvertersusedtocontrolACmotorsmayhaveefficienciesexceeding98%.Switchingpowersupplieshavelowerefficiencyduetolowoutputvoltagelevels(oftenevenlessthan2Vformicroprocessorsareneeded)butstillmorethan70-80%efficiencycanbeachieved.Variable-speedfancontrollersforcomputersusuallyusePWM,asitisfarmoreefficientwhencomparedtoaPOtentiOmeterorrheostat.(Neitherofthelatterispracticaltooperateelectronically;theywouldrequireasmalldrivemotor.)1.ightdimmersforhomeuseemployaspecifictypeofPWMcontrol.Home-uselightdimmerstypicallyincludeelectroniccircuitrywhichsuppressescurrentflowduringdefinedportionsofeachcycleoftheAClinevoltage.Adjustingthebrightnessoflightemittedbyalightsourceisthenmerelyamatterofsettingatwhatvoltage(orphase)intheAChalfcyclethedimmerbeginstoprovideelectricalcurrenttothelightsource(e.g.byusinganelectronicswitchsuchasatriac).InthiscasethePWMdutycycleistheratiooftheconductiontimetothedurationofthehalfACcycledefinedbythefrequencyoftheAClinevoltage(50Hzor60Hzdependingonthecountry).VoltageregulationMainarticle：SWitchednK)dePoWerSUPpIyPWMisalsousedinefficientVOltageregulators.Byswitchingvoltagetotheloadwiththeappropriatedutycycle,theoutputwillapproximateavoltageatthedesiredlevel.Theswitchingnoiseisusuallyfilteredwithaninductorandacapacitor.Onemethodmeasurestheoutputvoltage.Whenitislowerthanthedesiredvoltage,itturnsontheswitch.Whentheoutputvoltageisabovethedesiredvoltage,itturnsofftheswitch.AudioeffectsandamplificationPWMissometimesusedinsound(music)synthesis,inparticularSUbtraCtiVeSynthesis,asitgivesasoundeffectsimilartochorusorslightlydetunedoscillatorsplayedtogether.(Infact,PWMisequivalenttothedifferenceoftwoSaWtOOthWaVeSwithoneoftheminverted.1)TheratiobetweenthehighandlowlevelistypicallymodulatedwithaIOWfrequencyoscillator.Inaddition,varyingthedutycycleofapulsewaveforminasubtractive-synthesisinstrumentcreatesusefultimbralvariations.Somesynthesizershaveaduty-cycletrimmerfortheirsquarewaveoutputs,andthattrimmercanbesetbyear;the50%point(truesquarewave)wasdistinctive,becauseeven-numberedharmonicsessentiallydisappearat50%.Pulsewaves,usually50%,25%,and12.5%,makeuptheSOUndtraCkSOfCIaSSiCViCIeogames.newclassofaudioamplifiersbasedonthePWMprincipleisbecomingpopular.Called"ciassT)amplifiers”,theyproduceaPWMequivalentoftheanaloginputsignalwhichisfedtotheIoUdSPeakerviaasuitablefilternetworktoblockthecarrierandrecovertheoriginalaudio.Theseamplifiersarecharacterizedbyverygoodefficiencyfigures(90%)andcompactsize/lightweightforlargepoweroutputs.Forafewdecades,industrialandmilitaryPWMamplifiershavebeenincommonuse,oftenfordrivingservomotors.Field-gradientcoilsinMRImachinesaredrivenbyrelativelyhigh-powerPWMamplifiers.Historically,acrudeformofPWMhasbeenusedtoplaybackPCMdigitalsoundonthePCspeaker,whichisdrivenbyonlytwovoltagelevels,typically0Vand5V.Bycarefullytimingthedurationofthepulses,andbyrelyingonthespeaker*sphysicalfilteringproperties(limitedfrequencyresponse,self-inductance,etc.)itwaspossibletoobtainanapproximateplaybackofmonoPCMsamples,althoughataverylowquality,andwithgreatlyvaryingresultsbetweenimplementations.Inmorerecenttimes,theDir。CtStrCamDigitalsoundencodingmethodwasintroduced,whichusesageneralizedformofpulse-widthmodulationcalledPUlSedensitymodulation,atahighenoughsamplingrate(typicallyintheorderofMHz)tocoverthewholeacousticfrequenciesrangewithsufficientfidelity.ThismethodisusedintheSACDformat,andreproductionoftheencodedaudiosignalisessentiallysimilartothemethodusedinclassDamplifiers.