SG3525推挽式升压电路

SG3525推挽式升压电路2009-08-06下午05:23ForewordThiscontributedprojectisaresultofconsiderablecollaborationbetweenSergioandmyself,andshouldnotbeseenasnecessarilyacompleteprojectinitself,butasteppingstonetounderstandingswitchingpowersupplies,howtheywork,andwhatyoucandowiththem.Bewarned-thereisconsiderablerisk.Becauseoftheextremelyhighcurrentavailablefromacarbattery,atinymistakemayeasilyleadtocatastrophicfailure.Allelectroniccomponentsaresaidtocontainsmoke(wirecontainsanenormousamount),andaslipofthesolderingironcanliberateanunbelievablequantity.Seriouslythough,theriskofsevereburnsandthepossibilityofcausingafireinyourcarareveryreal,andshouldnotbeunderestimated.300Afromacarbatterycandoavastamountofdamageinafewmilliseconds-shouldthefusenotblow(youwilluseafuse,won'tyou?),thenthedamagecanbeextensive.AtvariouspointsinSergio'spartofthearticle,Ihaveaddedsomeofmyowninformation.Thisisshowninindentedsmallfonttext.Pleaseseethespecialnoteattheendofthisarticleforimportantinformationabouttheproject.IntroductionThedifficultiesofinstallinganHI-FIsysteminacararemany,althoughthereisnodoubtthatthemostimportantisthelimitationofthevehiclesupplyvoltage.Asmostreadersalreadyknow,thenominalvoltageofacarbatteryis12V,reachingabout13.8Vwhencharging(i.e.,enginerunning).ThemaximumRMS.audiopowerfromagivenvoltageVissomewhatlessthan:Pmax=(V/(2x2))2/RThus,fora13.8Vsystem,thispowerislimitedtoabout6Wona4Ohmload.Notethatthelowertheresistanceofthespeaker,thehigherthemaximumpower(thisisthereasonmostaudiospeakershavea4Ohmnominalimpedanceinsteadofthemorecommon8Ohminhomesystems).Thismaybesimplifiedtosomeextent...P=(V/3)2/RLandatypicalcalculationbasedona13.8VsupplygivesP=(13.8/3)2/4P=4.62/4=5.29WattsThisallowsforstandardlosses,andisacceptablyaccurateatthisvoltage-theonlyrealwaytoknowistomeasuretheamp,sincethelossesvarydependingonthetopologyoftheoutputstageinparticular.Poweroutputcanbeincreasedbyafactorofnearly4byusingbridgingtechniques,explainedinmoredetailinESPproject14,sowecanobtainuptoabout24Wona4Ohmspeaker.Thiscanbeenoughforthemidrangeandhighfrequencies,butisobviouslyverylimitedforasubwooferapplication,forexample.(moral:distrustof4x45Wheadunitsiswelladvised,fortheycertainlyaren'ttalkingaboutRMSpower).So,whatcanbedonetoincreaseavailableaudiopower?Theanswerisasimplederivationoftheaboveformula-eitherdecreaseloadimpedanceorincreasesupplyvoltage.Thelowertheimpedance,themorecurrentisneeded,makingtheconstructionoflowimpedanceoutputstagesmoredifficult(therearesomeotherpracticallimits),solet'sincreasesupplyvoltage.SwitchModePowerSupplyBasicsThevastmajorityofhigh-poweredaudioamplifiersuseSMPS(SwitchModePowerSupplies)togeneratehighervoltagesfromtheavailable12(13.8)volts.Anextensivetheoreticalexplanationonhowthesethingsworkisbeyondthescopeofthisarticle,butthesearesomefundamentalideasyoushouldknowaboutswitchmodepowersupplies(SMPS)forcaramps:TheDCvoltageatthebatteryhastobeswitchedinsomeformtogenerateanACwaveformsuitableforatransformer.Asyoualreadyknow,atransformerbasicallyconvertstheACvoltageinitsprimarytoascaledversionofitinitssecondary,thescalefactorbeingtheturnsratiooftheprimarytothesecondary.(Again,takethisasanextremesimplification).Atransformerdoesn'tallowDCvoltagestopass,andthereiselectrical(galvanic)isolationbetweenbothwindings.TheACwaveformisusuallyasquarewavethatisrelativelyeasyandefficienttogenerate.Thefrequenciesusuallyfallbetween25kHzand100kHzormore,thusallowingsmallertransformersthantheusedinmainappliances(itsconstructionisalsodifferent,theircoresarenotlaminated,butmadefromferritesorironpowder).Theswitchingelementshavetobecapableofhighcurrentsandmustalsobefastandhavelowswitchinglosses.Usually,powerMOSFETsorhighspeedbipolartransistorsareused(someSMPSdesignsuseSCRsbuttheseareintheminority).Oncethiswaveformisstepped-upbythetransformer,ithastoberectifiedagainandfilteredbacktoDC,sincethatiswhatwewant.Foraudioapplications,weusuallyneedasymmetricalsupply,+/-35V,forexample.Therectificationisdonewithadiodebridge,asitwouldbeusingaconventionaltransformerat50or60Hz.Notethatforthefrequencieswearetalkingabout,fastorultra-fastdiodesareneeded.Ifweneedaregulatedpowersupply,somekindoffeedbackmustbeprovidedfromtheoutputrailstoacontrollerthatcanchangesomeparametersoftheACwaveformattheprimaryofthetransformer.ThisisnormallyaccomplishedwithPWM(pulsewidthmodulation).Wewillexplainthislater,intheregulationparagraph.Alwayskeepinmindthatnoenergyiscreated…givena(total)railstobatteryvoltagesratio,thecurrentdrawnfromtheoutputwillbe(atleast)bemultipliedatthe12Vinputbythesameratio,thusthetotalpowerstaysthesame(assuming100%efficiency,andthatisneverthecase).AgenerictransformertransformsthevoltagebyafactorofTr,currentbyafactorof1/Tr,andimpedanceatthesecondarybyafactorof1/sqr(Tr),Trbeingtheturnsratio.Impedanceisoflittleimportanceinthiscontext.AwellbuiltSMPScanreach90%efficiency.So,ifyouexpecttoproduce+/-35Vat6A(perrail)supply(thissupposes35x6+35x6=360W)thenbepreparedtodrawmorethan30Afromthebattery!Fortunately,whentalkingaboutaudioampsreproducingmusic,powerrequirementsarealwaysmuchlowerthanw