SMPS电源技术.docx

SMPS电源技术.docx

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SMPS电源技术

SwitchModePowerSupplyBasicsTutorial

-summaryortutorialaboutthebasicsofswitchmodepowersupplies,theirdesign,operationandadvantagesanddisadvantages.

SMPSTUTORIALINCLUDES

∙Switchmodepowersupply

∙Switchmoderegulatorfundamentals

∙Stepdown'Buck'converter

∙Stepup'Boost'converter

∙Buck-Boostconverter

Switchmodepowersuppliesarewidelyusedbecauseoftheadvantagestheyofferintermsofsize,weight,cost,efficiencyandoverallperformance.

Asaresultoftheirperformance,switchmodepowersuppliesareusedinallbutthemostexactingapplicationstoprovideanefficientandeffectivesourceofpowerformostformsofelectronicsystems.

Switchmodepowersupplieshavebecomeanacceptedpartoftheelectronicssceneandareoftenreferredtoasswitchmodepowerconverters,orjustswitchers.

Switchmodepowersupplyterminology

Switchmodepowersupply,SMPS,technologymaybereferredtobyanumberofsimilarterms.Whiletheyalllookatthesamebasictechnology,theyrefertodifferentelementsoftheoveralltechnology:

∙Switchmodepowersupply,SMPS:

  Thetermswitchmodepowersupplyisgenerallyusedtoindicateanitemthatcanbeconnectedtothemains,orotherexternalsupplyandusedtogeneratethesourcepower.Inotherwordsitisacompletepowersupply.

∙Switchmoderegulator:

  Thistypicallyrefersjusttotheelectroniccircuitthatprovidestheregulation.Aswitchmoderegulatorwillbepartoftheoverallswitchmodepowersupply.

∙Switchmoderegulatorcontroller:

  Manyswitchmoderegulatorintegratedcircuitsdonotcontainheseriesswitchingelement.Thiswillbetrueifthecurrentorvoltagelevelsarehigh,becauseanexternalseriesswitchingelementwouldbeabletobetterhandlethehighercurrentandvoltagelevels,aswellastheresultantpowerdissipation.

Switchmodepowersupplybasics

ThebasicconceptbehindaswitchmodepowersupplyorSMPSisthefactthattheregulationisundertakenbyusingaswitchingregulator.Thisusesaseriesswitchingelementthatturnsthecurrentsupplytoasmoothingcapacitoronanoff.

Switchingregulatorconceptusedinswitchmodepowersupply

Thetimetheserieselementisturnedoniscontrolledbythevoltageonthecapacitor.Ifitishigherthanrequired,theseriesswitchingelementisturnedoff,ifitislowerthanrequired,itisturnedon.Inthiswaythevoltageonthesmoothingorreservoircapacitorismaintainedattherequiredlevel.

Switchmodepowersupplyadvantages/disadvantages

Theuseofanytechnologyisoftenacarefulbalanceofseveraladvantagesanddisadvantages.Thisistrueforswitchmodepowersupplieswhichoffersomedistinctadvantages,butalsohavetheirdrawbacks.

SMPSadvantages

∙Highefficiency:

  Theswitchingactionmeanstheseriesregulatorelementiseitheronoroffandthereforelittleenergyisdissipatedasheatandveryhighefficiencylevelscanbeachieved.

∙Compact:

  Asaresultofthehighefficiencyandlowlevelsofheatdissipation,theswitchmodepowersuppliescanbemademorecompact.

∙Flexibletechnology:

  Switchmodepowersupplytechnologycanbesuedtoprovidehighefficiencyvoltageconversionsinvoltagestepupor"Boost"applicationsorstepdown"Buck"applications.

SMPSDisadvantages

∙Noise:

  Thetransientspikesthatoccurfromtheswitchingactiononswitchmodepowersuppliesareoneofthelargestproblems.ThespikescanmigrateintoallareasofthecircuitsthattheSMPSspowerifthespikesarenotproperlyfiltered.AdditionallythespikesortransientscancauseelectromagneticorRFinterferencewhichcanaffectothernearbyitemsofelectronicequipment,particularlyiftheyreceiveradiosignals.

∙Externalcomponents:

  Whileitispossibletodesignaswitchmoderegulatorusingasingleintegratedcircuit,externalcomponentsaretypicallyrequired.Themostobviousisthereservoircapacitor,butfiltercomponentsarealsoneeded.Insomedesignstheseriesswitchelementmaybeincorporatedwithintheintegratedcircuit,butwhereanycurrentisconsumed,theseriesswitchwillbeanexternalcomponent.Thesecomponentsallrequirespace,andaddtothecost.

∙Expertdesignrequired:

  Itisoftenpossibletoputtogetheraswitchmodepowersupplythatworks.Toensurethatitperformstotherequiredspecificationcanbemoredifficult.Ensuringtherippleandinterferencelevelsaremaintainedcanbeparticularlytricky.

∙Costs:

  Carefulconsiderationofthecostsofaswitchmodepowersupplymustbemadebeforedesigningorusingone.Beyondthebasicpowersupply,additionalfilteringmayberequiredandthiscanaddtothecost.

Despitethedisadvantages,switchmodepowersupplytechnologyisthemajorformofpowersupplytechnologyusedforawholevarietyofapplicationsespeciallythoseincludedincomputers.Forapplicationswhereverylownoisearerequired,linearregulatortechnologyisstillwidelyused.

SwitchModeRegulatorFundamentals

-summaryofthefundamentalsofhowaswitchmoderegulatorworksandthefundamentalsofitsoperation.

SMPSTUTORIALINCLUDES

∙Switchmodepowersupply

∙Switchmoderegulatorfundamentals

∙Stepdown'Buck'converter

∙Stepup'Boost'converter

∙Buck-Boostconverter

Aswitchmoderegulatorisattheheartofanyswitchmodepowersupply.

Theswitchmoderegulatoristhecircuitthatprovidesthevoltageregulation.Itcanbeusedonitsownoraspartofacompletepowersupply.

Switchmoderegulatorscomeinavarietyofforms,buteachonewillbeabletoprovidevoltageregulationbyusingaseriesswitchingelementthatchargesupareservoircapacitorwhenthevoltagefallsbelowapredeterminedlevel.

Switchingregulatorbasics

Thebasisofswitchmoderegulatorrevolvesaroundtheabilityofinductorsandcapacitorstostoreenergy.Thecapacitorsandinductorsareintegralelementsoftheswitchmoderegulatortechnology.

∙Capacitance  Ifacurrentisappliedtoacapacitor,thecapacitorgraduallychargesupandthevoltageacrossitriseslinearlyatarateequaltoI/CwhereistheappliedcurrentandCisthecapacitance.Inthiscasethevoltageacrossthecapacitorcannotchangeinstantly.

Whenaninstantaneouschangeincurrentoccurs,thevoltagechangeslinearly.[Thisassumesacurrentsourcewithaninfinitevoltagecapabilityisused].

∙Inductor:

  :

Foraninductor,itisnotpossiblefortheretobeaninstantaneouschangeincurrent.Instead,whenavoltageisapplied,thecurrentbuildsuplinearlyovertimeatarateequaltoV/LwhereVistheappliedvoltageandListheinductance.

Usingthestandardequationsitispossibletodeterminethecurrentandvoltageprofiles:

Theenergyfromtherisingcurrentisstoredinthemagneticfieldassociatedwiththeinductor.Ifthecurrentflowingthroughtheinductorissuddenlyinterrupted,themagneticfieldreactsagainstthisandproducesaveryhigh"backemf"tocounteractthechange.

Havingseenthefundamentalorbasicconceptsbehindswitchingvoltagesandcurrentstocapacitorsandinductors,thesebasicconceptscanbeappliedtoswitchmoderegulatorsolutionstoprovideavarietyscenariosforvoltagestepupandstepdowncircuits.

Asthetechnologyusesswitchingtechniqueswheretheserieselementisonoroff,thisapproachprovidesmuchbetterlevelsofefficiencythanalinearwherepowerisdissipated.

Capacitorbasedswitchmoderegulation

Thebasicconceptofthecapacitorswitchedmoderegulatorisshowninthediagram.Whentheswitchisclosed,currentisabletoflowintothereservoircapacitorandprovidecharge.Whenthevoltageonthecapacitorisatisrequiredlevel,theswitchopensandtheloadwilldrawcurrentfromthecapacitor.

Asthevoltagefalls,thiswillbesensedbythecontrolcircuitryandtheseriesswitchwillbeturnedonagaintobringthecapacitorvoltageuptotherequiredlevel.

Thiscircuitisnotaseffectiveasmaybethoughtatfirstsight.Althoughtheonlyresistiveelementinthetheoreticalcircuitistheload,thisisnottheonlywayinwhichenergyislostbecausechargingacapacitordirectlyfromavoltagesourceoracapacitordissipatesasmuchenergyasistransferredtothecapacitor.Asaresultofthis,switchingmoderegulatorscannotusecapacitorswitchingtechniquesalone.

Inductorbasedswitchmoderegulation

Itisalsopossibletouseinductorsasanelementinswitchmoderegulators.

Theinductorcanbeusedtotransferenergyfromonevoltagesourcetoanother.Whileasimpleresistorcanbeusedasadroppertodropvoltagewhentransferringfromonevoltagesourceatahighervoltagetooneatalowervoltage,thisisverywastefulintermsofpower.Ifaninductorisused,thenalltheenergyistransferred,assumingaperfectinductor.

Theuseofaninductorhastheadvantagesthatenergycanbetransferredfromonesourcetoanotherregardlessoftherespectivevaluesofvoltageandtheirpolarities.Toachievethistheproperconfigurationisobviouslyrequired.

Whentheswitchesareinthepositionsshownabove,thevoltageV1isappliedacrosstheinductorandthecurrenti1buildsupatarateequaltoV1/L.Thereforethepeakvalueobtainedwillbeproportionaltothetimetheswitchesareinthisposition,i.e.（V1/L）xt

Whentheswitchesarereversed,thecurrentwillcontinuetoflowataratei2whichisequalto-V2/L.

Asanidealinductordissipatesnoenergy,thereisnopowerlossinanidealsystemusinganinductorinthisfashion.Asaresult,itisthismethodofenergytransferthatformsthebasisforallswitchingregulators.

StepDownBuckRegulator/Converter

-summaryortutorialaboutthecircuitando