外文翻译关于PWM的外文翻译.docx
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外文翻译关于PWM的外文翻译.docx
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外文翻译关于PWM的外文翻译
外文原文
Pulse-widthmodulation
Pulse-widthmodulation(PWM)isamodulationtechniquethatconformsthewidthofthepulse,formallythepulseduration,basedonmodulatorsignalinformation.Althoughthismodulationtechniquecanbeusedtoencodeinformationfortransmission,itsmainuseistoallowthecontrolofthepowersuppliedtoelectricaldevices,especiallytoinertialloadssuchasmotors.Inaddition,PWMisoneofthetwoprincipalalgorithmsusedinphotovoltaicsolarbatterychargers,[1]
Theaveragevalueofvoltage(andcurrent)fedtotheloadiscontrolledbyturningtheswitchbetweensupplyandloadonandoffatafastpace.Thelongertheswitchisoncomparedtotheoffperiods,thehigherthepowersuppliedtotheloadis.
ThePWMswitchingfrequencyhastobemuchfasterthanwhatwouldaffecttheload,whichistosaythedevicethatusesthepower.Typicallyswitchingshavetobedoneseveraltimesaminuteinanelectricstove,120Hzinalampdimmer,fromfewkilohertz(kHz)totensofkHzforamotordriveandwellintothetensorhundredsofkHzinaudioamplifiersandcomputerpowersupplies.
Thetermdutycycledescribestheproportionof'on'timetotheregularintervalor'period'oftime;alowdutycyclecorrespondstolowpower,becausethepowerisoffformostofthetime.Dutycycleisexpressedinpercent,100%beingfullyon.
ThemainadvantageofPWMisthatpowerlossintheswitchingdevicesisverylow.Whenaswitchisoffthereispracticallynocurrent,andwhenitison,thereisalmostnovoltagedropacrosstheswitch.Powerloss,beingtheproductofvoltageandcurrent,isthusinbothcasesclosetozero.PWMalsoworkswellwithdigitalcontrols,which,becauseoftheiron/offnature,caneasilysettheneededdutycycle.
PWMhasalsobeenusedincertaincommunicationsystemswhereitsdutycyclehasbeenusedtoconveyinformationoveracommunicationschannel.
History
Inthepast,whenonlypartialpowerwasneeded(suchasforasewingmachinemotor),arheostat(locatedinthesewingmachine'sfootpedal)connectedinserieswiththemotoradjustedtheamountofcurrentflowingthroughthemotor,butalsowastedpowerasheatintheresistorelement.Itwasaninefficientscheme,buttolerablebecausethetotalpowerwaslow.Thiswasoneofseveralmethodsofcontrollingpower.Therewereothers—somestillinuse—suchasvariableautotransformers,includingthetrademarked'Autrastat'fortheatricallighting;andtheVariac,forgeneralACpoweradjustment.Thesewerequiteefficient,butalsorelativelycostly.
Foraboutacentury,somevariable-speedelectricmotorshavehaddecentefficiency,buttheyweresomewhatmorecomplexthanconstant-speedmotors,andsometimesrequiredbulkyexternalelectricalapparatus,suchasabankofvariablepowerresistorsorrotatingconvertersuchasWardLeonarddrive.
However,inadditiontomotordrivesforfans,pumpsandroboticservos,therewasagreatneedforcompactandlowcostmeansforapplyingadjustablepowerformanydevices,suchaselectricstovesandlampdimmers.
OneearlyapplicationofPWMwasintheSinclairX10,a10Waudioamplifieravailableinkitforminthe1960s.AtaroundthesametimePWMstartedtobeusedinACmotorcontrol.
Fig.1:
apulsewave,showingthedefinitionsof
andD.
Pulse-widthmodulationusesarectangularpulsewavewhosepulsewidthismodulatedresultinginthevariationoftheaveragevalueofthewaveform.Ifweconsiderapulsewaveform
withperiod
lowvalue
ahighvalue
andadutycycleD(seefigure1),theaveragevalueofthewaveformisgivenby:
As
isapulsewave,itsvalueis
for
and
for
.Theaboveexpressionthenbecomes:
Thislatterexpressioncanbefairlysimplifiedinmanycaseswhere
as
.Fromthis,itisobviousthattheaveragevalueofthesignal(
)isdirectlydependenton
thedutycycleD
Fig.2:
AsimplemethodtogeneratethePWMpulsetraincorrespondingtoagivensignalistheintersectivePWM:
thesignal(heretheredsinewave)iscomparedwithasawtoothwaveform(blue).Whenthelatterislessthantheformer,thePWMsignal(magenta)isinhighstate
(1).Otherwiseitisinthelowstate(0).
ThesimplestwaytogenerateaPWMsignalistheintersectivemethod,whichrequiresonlyasawtoothoratrianglewaveform(easilygeneratedusingasimpleoscillator)andacomparator.Whenthevalueofthe
referencesignal(theredsinewaveinfigure2)ismorethanthemodulationwaveform(blue),thePWM
signal(magenta)isinthehighstate,otherwiseitisinthelowstate.
Timeproportioning
ManydigitalcircuitscangeneratePWMsignals(e.g.,manymicrocontrollershavePWMoutputs).Theynormallyuseacounterthatincrementsperiodically(itisconnecteddirectlyorindirectlytotheclockofthecircuit)andisresetattheendofeveryperiodofthePWM.Whenthecountervalueismorethanthereferencevalue,thePWMoutputchangesstatefromhightolow(orlowtohigh).[3]Thistechniqueisreferredtoastimeproportioning,particularlyastime-proportioningcontrol[4]–whichproportionofafixedcycletimeisspentinthehighstate.
Theincrementedandperiodicallyresetcounteristhediscreteversionoftheintersectingmethod'ssawtooth.Theanalogcomparatoroftheintersectingmethodbecomesasimpleintegercomparisonbetweenthecurrentcountervalueandthedigital(possiblydigitized)referencevalue.Thedutycyclecanonlybevariedindiscretesteps,asafunctionofthecounterresolution.However,ahigh-resolutioncountercanprovidequitesatisfactoryperformance.
PWMsamplingtheorem
TheprocessofPWMconversionisnon-linearanditisgenerallysupposedthatlowpassfiltersignalrecoveryisimperfectforPWM.ThePWMsamplingtheorem[6]showsthatPWMconversioncanbeperfect.Thetheoremstatesthat"Anybandlimitedbasebandsignalwithin±0.637canberepresentedbyapulsewidthmodulation(PWM)waveformwithunitamplitude.ThenumberofpulsesinthewaveformisequaltothenumberofNyquistsamplesandthepeakconstraintisindependentofwhetherthewaveformistwo-levelorthree-level."
Powerdelivery
PWMcanbeusedtocontroltheamountofpowerdeliveredtoaloadwithoutincurringthelossesthatwouldresultfromlinearpowerdeliverybyresistivemeans.Potentialdrawbackstothistechniquearethepulsationsdefinedbythedutycycle,switchingfrequencyandpropertiesoftheload.Withasufficientlyhighswitchingfrequencyand,whennecessary,usingadditionalpassiveelectronicfilters,thepulsetraincanbesmoothedandaverageanalogwaveformrecovered.
HighfrequencyPWMpowercontrolsystemsareeasilyrealisablewithsemiconductorswitches.Asexplainedabove,almostnopowerisdissipatedbytheswitchineitheronoroffstate.However,duringthetransitionsbetweenonandoffstates,bothvoltageandcurrentarenonzeroandthuspowerisdissipatedintheswitches.Byquicklychangingthestatebetweenfullyonandfullyoff(typicallylessthan100nanoseconds),thepowerdissipationintheswitchescanbequitelowcomparedtothepowerbeingdeliveredtotheload.
ModernsemiconductorswitchessuchasMOSFETsorInsulated-gatebipolartransistors(IGBTs)arewellsuitedcomponentsforhighefficiencycontrollers.FrequencyconvertersusedtocontrolACmotorsmayhaveefficienciesexceeding98%.Switchingpowersupplieshavelowerefficiencyduetolowoutputvoltagelevels(oftenevenlessthan2Vformicroprocessorsareneeded)butstillmorethan70–80%efficiencycanbeachieved.
Variable-speedfancontrollersforcomputersusuallyusePWM,asitisfarmoreefficientwhencomparedtoapotentiometerorrheostat.(Neitherofthelatterispracticaltooperateelectronically;theywouldrequireasmalldrivemotor.)
LightdimmersforhomeuseemployaspecifictypeofPWMcontrol.Home-uselightdimmerstypicallyincludeelectroniccircuitrywhichsuppressescurrentflowduringdefinedportionsofeachcycleoftheAClinevoltage.Adjustingthebrightnessoflightemittedbyalightsourceisthenmerelyamatterofsettingatwhatvoltage(orphase)intheAChalfcyclethedimmerbeginstoprovideelectricalcurrenttothelightsource(e.g.byusinganelectronicswitchsuchasatriac).InthiscasethePWMdutycycleistheratiooftheconductiontimetothedurationofthehalfACcycledefinedbythefrequencyoftheAClinevoltage(50Hzor60Hzdependingonthecountry).
Voltageregulation
Mainarticle:
Switched-modepowersupply
PWMisalsousedinefficientvoltageregulators.Byswitchingvoltagetotheloadwiththeappropriatedutycycle,theoutputwillapproximateavoltageatthedesiredlevel.Theswitchingnoiseisusuallyfilteredwithaninductorandacapacitor.
Onemethodmeasurestheoutputvoltage.Whenitislowerthanthedesiredvoltage,itturnsontheswitch.Whentheoutputvoltageisabovethedesiredvoltage,itturnsofftheswitch.
Audioeffectsandamplification
PWMissometimesusedinsound(music)synthesis,inparticularsubtractivesynthesis,asitgivesasoundeffectsimilartochorusorslightlydetunedoscillatorsplayedtogether.(Infact,PWMisequivalenttothedifferenceoftwosawtoothwaveswithoneoftheminverted.[1])Theratiobetweenthehighandlowlevelistypicallymodulatedwithalowfrequencyoscillator.Inaddition,varyingthedutycycleofapulsewaveforminasubtractive-synthesisinstrumentcreatesusefultimbralvariations.Somesynthesizershaveaduty-cycletrimmerfortheirsquare-waveoutputs,andthattrimmercanbesetbyear;the50%point(truesquarewave)wasdistinctive,becauseeven-numberedharmonicsessentiallydisappearat50%.Pulsewaves,usually50%,25%,and1
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