SG3525推挽式升压电路.docx
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SG3525推挽式升压电路.docx
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SG3525推挽式升压电路
SG3525推挽式升压电路
2009-08-06下午05:
23
Foreword
ThiscontributedprojectisaresultofconsiderablecollaborationbetweenSergioandmyself,andshouldnotbeseenasnecessarilyacompleteprojectinitself,butasteppingstonetounderstandingswitchingpowersupplies,howtheywork,andwhatyoucandowiththem.
Bewarned-thereisconsiderablerisk.Becauseoftheextremelyhighcurrentavailablefromacarbattery,atinymistakemayeasilyleadtocatastrophicfailure.Allelectroniccomponentsaresaidto
containsmoke(wirecontainsanenormousamount),andaslipofthesolderingironcanliberateanunbelievablequantity.Seriouslythough,theriskofsevereburnsandthepossibilityofcausingafireinyourcarareveryreal,andshouldnotbeunderestimated.300Afromacarbatterycandoavast
amountofdamageinafewmilliseconds-shouldthefusenotblow(youwilluseafuse,won't
you?
),thenthedamagecanbeextensive.
AtvariouspointsinSergio'spartofthearticle,Ihaveaddedsomeofmyowninformation.Thisis
showninindentedsmallfonttext.
Pleaseseethespecialnoteattheendofthisarticleforimportantinformationaboutthe
project.
Introduction
ThedifficultiesofinstallinganHI-FIsysteminacararemany,althoughthereisnodoubtthatthemostimportantisthelimitationofthevehiclesupplyvoltage.Asmostreadersalreadyknow,thenominalvoltageofacarbatteryis12V,reachingabout13.8Vwhencharging(i.e.,enginerunning).
ThemaximumRMS.audiopowerfromagivenvoltageVissomewhatlessthan:
2Pmax=(V/(2x2))/R
Thus,fora13.8Vsystem,thispowerislimitedtoabout6Wona4Ohmload.Notethatthelowertheresistanceofthespeaker,thehigherthemaximumpower(thisisthereasonmostaudiospeakers
havea4Ohmnominalimpedanceinsteadofthemorecommon8Ohminhomesystems).
Thismaybesimplifiedtosomeextent...
2P=(V/3)/RL
andatypicalcalculationbasedona13.8Vsupplygives
2P=(13.8/3)/4
2P=4.6/4=5.29Watts
Thisallowsforstandardlosses,andisacceptablyaccurateatthisvoltage-theonlyrealwayto
knowistomeasuretheamp,sincethelossesvarydependingonthetopologyoftheoutputstagein
particular.
Poweroutputcanbeincreasedbyafactorofnearly4byusingbridgingtechniques,explainedin
moredetailinESPproject14,sowecanobtainuptoabout24Wona4Ohmspeaker.Thiscanbe
enoughforthemidrangeandhighfrequencies,butisobviouslyverylimitedforasubwooferapplication,forexample.(moral:
distrustof"4x45W"headunitsiswelladvised,fortheycertainly
aren'ttalkingaboutRMSpower).
So,whatcanbedonetoincreaseavailableaudiopower?
Theanswerisasimplederivationoftheaboveformula-eitherdecreaseloadimpedanceorincreasesupplyvoltage.Thelowerthe
impedance,themorecurrentisneeded,makingtheconstructionoflowimpedanceoutputstages
moredifficult(therearesomeotherpracticallimits),solet'sincreasesupplyvoltage.
SwitchModePowerSupplyBasics
Thevastmajorityofhigh-poweredaudioamplifiersuseSMPS(SwitchModePowerSupplies)to
generatehighervoltagesfromtheavailable12(13.8)volts.Anextensivetheoreticalexplanationonhowthesethingsworkisbeyondthescopeofthisarticle,butthesearesomefundamentalideasyou
shouldknowaboutswitchmodepowersupplies(SMPS)forcaramps:
TheDCvoltageatthebatteryhastobeswitchedinsomeformtogenerateanACwaveformsuitableforatransformer.Asyoualreadyknow,atransformerbasicallyconvertsthe
ACvoltageinits"primary"toascaledversionofitinits"secondary",thescalefactorbeingtheturnsratiooftheprimarytothesecondary.(Again,takethisasanextremesimplification).A
transformerdoesn'tallowDCvoltagestopass,andthereiselectrical(galvanic)isolation
betweenbothwindings.
TheACwaveformisusuallyasquarewavethatisrelativelyeasyandefficienttogenerate.
Thefrequenciesusuallyfallbetween25kHzand100kHzormore,thusallowingsmallertransformersthantheusedinmainappliances(itsconstructionisalsodifferent,theircoresarenotlaminated,butmadefromferritesor"ironpowder").Theswitchingelementshavetobecapableofhighcurrentsandmustalsobefastandhavelowswitchinglosses.Usually,power
MOSFETsorhighspeedbipolartransistorsareused(someSMPSdesignsuseSCRsbutthese
areintheminority).
Oncethiswaveformisstepped-upbythetransformer,ithastoberectifiedagainand
filteredbacktoDC,sincethatiswhatwewant.Foraudioapplications,weusuallyneeda
symmetricalsupply,+/-35V,forexample.Therectificationisdonewithadiodebridge,asitwouldbeusingaconventionaltransformerat50or60Hz.Notethatforthefrequenciesweare
talkingabout,fastorultra-fastdiodesareneeded.
Ifweneedaregulatedpowersupply,somekindoffeedbackmustbeprovidedfromtheoutputrailstoacontrollerthatcanchangesomeparametersoftheACwaveformattheprimaryofthetransformer.ThisisnormallyaccomplishedwithPWM(pulsewidthmodulation).We
willexplainthislater,inthe"regulation"paragraph.
Alwayskeepinmindthatnoenergyiscreated…givena(total)railstobatteryvoltagesratio,thecurrentdrawnfromtheoutputwillbe(atleast)bemultipliedatthe12Vinputbythe
sameratio,thusthetotalpowerstaysthesame(assuming100%efficiency,andthatisneverthecase).Agenerictransformer"transforms"thevoltagebyafactorofTr,currentbyafactorof
1/Tr,andimpedanceatthesecondarybyafactorof1/sqr(Tr),Trbeingtheturnsratio.
Impedanceisoflittleimportanceinthiscontext.
AwellbuiltSMPScanreach90%efficiency.So,ifyouexpecttoproduce+/-35Vat6A
(perrail)supply(thissupposes35x6+35x6=360W)thenbepreparedtodrawmorethan30A
fromthebattery!
Fortunately,whentalkingaboutaudioampsreproducingmusic,power
requirementsarealwaysmuchlowerthanwithpuresinewaves.
Atthispoint,thereadershouldrealisethemagnitudeofthecurrentsinvolvedinahighpower
SMPSforacaramplifier,andthatextremecautionshouldbetakenespeciallywhenconnecting"the
creature"tothecarelectricalsystem.
Thesystem
ThepresentprojectdescribestheconstructionofaflexibleSMPScapableofdeliveringpowersin
theorderof350Wcontinuously,dependingonthetransformerused.Theoutputvoltagedepends
mainlyontheturnsratiooftheprimaryandsecondarywindings,butmaybeadjustedtoasomewhatlowervalueusingregulation.Thisshouldbeenoughtopowera200Wsubwoofer
amplifierplusperhaps2stereoampsforthemidsandhighs.
ItispartofacompletecarampthatIhavebuilt,with6powerstagesbasedonNational'sLM3886OvertureAmplifier.Theycanbecombinedintoone>250W/4Ohmsubwooferchannelplus2x
65W/4Ohmmid+highchannels,alternativelyinto2x120W/4Ohm+2x65Woreventoformamultichannel6x65W/4Ohmamplifier,soitisanextremelyflexibleandhigh-poweredsystem
withoutrenouncingsoundquality.Theparallelbridgingtechniquesneededtodothiswillbe
possiblydescribedinanotherproject.
ConstructionoftheSMPS
ThecompleteschematicoftheSMPSisshownbelow.
Note:
ThisisSergio'soriginalversionofthesupply,theoneshowninFigure9islikelytobe
themostcommonlyused,asitissomewhatsimpler,buthasvirtuallyidenticalperformance.
[esp]
Figure2-SwitchmodeControllerSchematic
Therearethreemainblocksdescribedbelow...
A-SwitchingMOSFETsandtransformer
B-Rectificationandfiltering
C-Controlcircuitry
A-SwitchingMOSFETsandTransformer
Theselectedswitchingtopologyiscalleda"push-pull"converter,becausethetransformerhasadoubleprimary(ora"centre-tapped"one,ifyourprefer).Thecentretapispermanentlyconnectedtothecarbattery(viaanLCfiltertoavoidcreatingpeaksinthebatterylines,whichcouldaffect
otherelectronicequipmentinthecar).Thetwoendsoftheprimaryareconnectedtoapairof
paralleledMOSFETseachthattiethemtogroundineachconductioncycle(Vgsofthe
correspondingMOSFEThigh).
TheseMOSFETsshouldbefast,abletowithstandhighcurrents(inexcessof30Aeachifpossible)andhavethelowestpossibleRds(on).TheproposedOn-Semiconductor’sMTP75N06can
withstand75AmpandhasaRds(on)below10milliohm.Thisisimportant,becausethelowerthisresistanceis,thelesspowertheyaregoingtodissipatewhenswitchingwithasquarewaveform.
AnotheralternativesareMTP60N06,orthemorepopularBUZ11andIRF540.
Althoughtheschematicsshowapreviousbipolarpush-pullstage,youcanalsoconnectthegate
resistordirectlytotheoutputofthecontrollingIC,leavingoutthetransistors,astheSG3525iscapabletodriveupto500mA(theoretically),morethanenoughtoswitchtheMOSFETsfast.
B-RectificationandFiltering
IfonelookstothesecondarysideoftheSMPS,itresemblesexactlytheschemeofatypicalmainsPSU,withonefundamentaldifference-theswitchingdiodeshavetobeFASTorULTRAFAST,if
youuseastandarddiodebridgethesystemwillsimplyblowup(andthiscanbeveryimpressive,
believeme!
)Althoughadiodebridgeisrepresented,itcanbemadewithdiscretediodesaswell.Usehighcurrent(10Aminimumandasuitablevoltagerating)diodes.Irecommendusing4x
TO220doublediodesthatcanbeparalleledtoformasingleoneineachpackage.
Youmaybesurprisedthatthecapacitorsaren'ttoobig.Thisisduetothehighswitchingfrequency.Itisimportantthattheyaregoodqualityonesandmustberatedfor105degrees
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