SG3525推挽式升压电路0001Word文档格式.docx
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SG3525推挽式升压电路0001Word文档格式.docx
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Foreword
ThiscontributedprojectisaresultofconsiderablecollaborationbetweenSergioandmyself,andshouldnotbeseenasnecessarilyacompleteprojectinitself,butasteppingstonetounderstandingswitchingpowersupplies,howtheywork,andwhatyoucandowiththem.
Bewarned-thereisconsiderablerisk.Becauseoftheextremelyhighcurrentavailablefromacarbattery,atinymistakemayeasilyleadtocatastrophicfailure.Allelectroniccomponentsaresaidtocontainsmoke(wirecontainsanenormousamount),andaslipofthesolderingironcanliberateanunbelievablequantity.Seriouslythough,theriskofsevereburnsandthepossibilityofcausingafireinyourcarareveryreal,andshouldnotbeunderestimated.300Afromacarbatterycandoavastamountofdamageinafewmilliseconds-shouldthefusenotblow(youwilluseafuse,won'
tyou?
),thenthedamagecanbeextensive.
AtvariouspointsinSergio'
spartofthearticle,Ihaveaddedsomeofmyowninformation.Thisisshowninindentedsmallfonttext.
Pleaseseetheattheendofthisarticleforimportantinformationabouttheproject.
Introduction
ThedifficultiesofinstallinganHI-FIsysteminacararemany,althoughthereisnodoubtthatthemostimportantisthelimitationofthevehiclesupplyvoltage.Asmostreadersalreadyknow,thenominalvoltageofacarbatteryis12V,reachingabout13.8Vwhencharging(i.e.,enginerunning).ThemaximumRMS.audiopowerfromagivenvoltageVissomewhatlessthan:
Pmax=(V/(2x2))2/R
Thus,fora13.8Vsystem,thispowerislimitedtoabout6Wona4Ohmload.Notethatthelowertheresistanceofthespeaker,thehigherthemaximumpower(thisisthereasonmostaudiospeakers
havea4Ohmnominalimpedaneeinsteadofthemorecommon8Ohminhomesystems).
Thismaybesimplifiedtosomeextent...
P=(V/3)2/RL
andatypicalcalculationbasedona13.8Vsupplygives
P=(13.8/3)2/4
P=4.62/4=5.29Watts
Thisallowsforstandardlosses,andisacceptablyaccurateatthisvoltage-theonlyrealwaytoknowistomeasuretheamp,sincethelossesvarydependingonthetopologyoftheoutputstagein
particular.
Poweroutputcanbeincreasedbyafactorofnearly4byusingbridgingtechniques,explainedinmoredetailinESPproject14,sowecanobtainuptoabout24Wona4Ohmspeaker.Thiscanbeenoughforthemidrangeandhighfrequencies,butisobviouslyverylimitedforasubwooferapplication,forexample.(moral:
distrustof"
4x45W"
headunitsiswelladvised,fortheycertainlyaren'
ttalkingaboutRMSpower).
So,whatcanbedonetoincreaseavailableaudiopower?
Theanswerisasimplederivationoftheaboveformula-eitherdecreaseloadimpedaneeorincreasesupplyvoltage.Thelowertheimpedanee,themorecurrentisneeded,makingtheconstructionoflowimpedaneeoutputstages
moredifficult(therearesomeotherpracticallimits),solet'
sincreasesupplyvoltage.
SwitchModePowerSupplyBasics
Thevastmajorityofhigh-poweredaudioamplifiersuseSMPS(SwitchModePowerSupplies)togeneratehighervoltagesfromtheavailable12(13.8)volts.Anextensivetheoreticalexplanationonhowthesethingsworkisbeyondthescopeofthisarticle,butthesearesomefundamentalideasyou
shouldknowaboutswitchmodepowersupplies(SMPS)forcaramps:
TheDCvoltageatthebatteryhastobeswitchedinsomeformtogenerateanAC
waveformsuitableforatransformer.Asyoualreadyknow,atransformerbasicallyconvertstheACvoltageinits"
primary"
toascaledversionofitinits"
secondary"
thescalefactorbeingtheturnsratiooftheprimarytothesecondary.(Again,takethisasanextremesimplification).Atransformerdoesn'
tallowDCvoltagestopass,andthereiselectrical(galvanic)isolation
betweenbothwindings.
TheACwaveformisusuallyasquarewavethatisrelativelyeasyandefficienttogenerate.Thefrequenciesusuallyfallbetween25kHzand100kHzormore,thusallowingsmallertransformersthantheusedinmainappliances(itsconstructionisalsodifferent,theircoresarenotlaminated,butmadefromferritesor"
ironpowder"
).Theswitchingelementshavetobecapableofhighcurrentsandmustalsobefastandhavelowswitchinglosses.Usually,powerMOSFETsorhighspeedbipolartransistorsareused(someSMPSdesignsuseSCRsbutthese
areintheminority).
Oncethiswaveformisstepped-upbythetransformer,ithastoberectifiedagainandfilteredbacktoDC,sincethatiswhatwewant.Foraudioapplications,weusuallyneedasymmetricalsupply,+/-35V,forexample.Therectificationisdonewithadiodebridge,asitwouldbeusingaconventionaltransformerat50or60Hz.Notethatforthefrequencieswe
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