SG3525推挽式升压电路.docx

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