毕业论文英文翻译

CUILINUNIVERSITYOFELECTRONICTECHNOLOGY毕业论文英文翻译资料题目直流开关稳压电源的保护技术院（系）： 电子电器工程系 专业： 电气工程及其自动化学生姓名： 班级： 学号： 指导教师： 2012年12月10日DCSwitchingPowerSupplyProtectionTechnologyAbstract:TheDCswitchingpowersupplyprotectionsystem,protectionsystemdesignprinciplesandmachineprotectionmeasures,ananalysisofswitchingpowersupplyintherangeofprotectedcharacteristicsanditsdesignmethodology,introducedanumberofpracticalprotectioncircuit.Keywords:switchingpowersupplyprotectioncircuitsystemdesignA、 IntroductionDCswitchingregulatorusedinthepriceofmoreexpensivehigh-powerswitchingdevices,thecontrolcircuitisalsomorecomplex,Inaddition,theloadswitchingregulatorsaregenerallyusedalargenumberofhighlyintegratedelectronicsystemsinstalleddevices.Transistorsandintegrateddevicetoleraneeelectricity,lessheatshocks.SwitchingRegulatorsthereforeshouldtakeintoaccounttheprotectionofvoltageregulatorsandloadtheirownsafety.Manydifferenttypesofcircuitprotection,polarityprotection,introducedhere,theprogramprotection,over-currentprotection,over-voltageprotection,under-voltageprotectionandover-temperatureprotectioncircuit.Usuallychosentobesomecombinationofprotection,constitutesacompleteprotectionsystem.B、 PolarityprotectionDCswitchingregulatorinputisgenerallynotregulatedDCpowersupply.Operatingerrorsoraccidentsasaresultofthesituationwilltakeitswrongpolarity;switchingpowersupplywillbedamaged.Polarityprotectionpurposes,istomaketheswitchingregulatoronlywhenthecorrectpolarityisnotconnectedtoDCpowersupplyregulatortoworkat.Connectingasingledevicecanachievepowerpolarityprotection.SincethediodeDtoflowthroughswitchingregulatorinputtotalcurrent,thiscircuitappliedinalow-powerswitchingregulatormoresuitable.Powerinthelargeroccasion,whilethepolarityprotectioncircuitasaproceduretoprotectalink,savethepowerrequiredforpolarityprotectiondiodes,powerconsumptionwillbereduced.Inordertoeasytooperate,makeiteasiertoidentifythecorrectpolarityornot,collectthenextlight.C、 ProcedurestoprotectSwitchingpowersupplycircuitisrathercomplicated,basicallycanbedividedintolow-powerandhigh-powerpartofthecontrolpartoftheswitch.Switchisahigh-powertransistors,fortheprotectionofthetransistorswitchisturnedonoroffpowersafety,wemustfirstmodulator,amplifierandotherlow-powercontrolcircuit.Tothisend,theboottoensurethecorrectprocedures.SwitchingRegulatorsgenerallytaketheinputofasmallinductor,theinputfiltercapacitor.Momentintheboot,filtercapacitorwillflowalotofsurgecurrent,thesurgecurrentcanbeseveraltimesmorethanthenormalinputcurrent.Suchalargesurgecurrentmaycontactthegeneralpowerswitchorrelaycontactmelting,andtheinputfuse.Inaddition,thecapacitorsurgecurrentwilldamagetoshortenthelifespanofprematuredamage.Tothisend,thebootshouldbeaccesstoacurrentlimitingresistor,throughthecurrentlimitingresistortocapacitorcharging.Inordernottomakethecurrentlimitingresistorexcessivepowerconsumption,thusaffectingthenormalswitchingregulator,andthetransientprocessinthebootafterashortperiodthenautomaticallyrelaysittoDCpowersupplydirectlytotheswitchingregulatorpowersupply.Thiscircuitswitchingregulatorcalleda"softstart"circuit.Switchingregulatorcontrolcircuitofthelogiccomponentsrequiredorop-ampauxiliarypowersupply.Tothisend,theauxiliarypowersupplymustbeintheswitchcircuit.Thiscontrolcircuitcanbeusedtoensuretheboot.Normalbootprocessis:toidentifythepolarityofinputpower,voltageprotectionprocedures fbootfauxiliarypowersupplycircuitandthroughcurrentlimitingresistorRoftheswitchingregulatorinputcapacitorCfchargemodulationswitchingregulatorcircuit,fshort-circuitcurrentlimitingresistorstabilityswitchingregulator.Intheswitchingregulator,themachinesjustbecausetheoutputcapacitanee,andchargetotheratedoutputvoltagevalueoftheneedforacertainperiodoftime.Duringthistime,samplingtheoutputamplifierwithlowinputvoltagesampling,closed-loopregulationcharacteristicsofthesystemwillforcetheswitchingofthetransistorconductiontimelengthened,sothatswitchingtransistorduringthisperiodwilltendtocontinuousconduction,andeasilydamaged.Tothisend,therequirementsofthisparagraphintheboottime,theswitchtoswitchtheoutputmodulationcircuittransistorbasedrivesignalofthepulsewidthmodulation,canguaranteetheswitchingtransistorbythecut-offswitchesarebecomingmoreandmorenormalstate,thereforetheprotectionofthesettingupofaboottotieinwiththesoftstart.D、Over-currentprotectionWhentheloadshort-circuit,overloadcontrolcircuitfailureorunforeseencircumstances,suchaswouldcausetheflowofswitchingvoltageregulatortransistorcurrentistoolarge,sothatincreasedpowertubes,fever,ifthereisnoover-currentprotectiondevice,highpowerswitchingtransistormaybedamaged.Therefore,theswitchingregulatorintheover-currentprotectioniscommonlyused.Themosteconomicalwayistousesimplefuse.Asaresultoftheheatcapacityofsmalltransistors,generalfuseprotectioningeneralcannotplayaroleintherapidfusecommonfuse.Thismethodhastheadvantageoftheprotectionofvulnerable,butitneedstoswitchtransistorinaccordaneewithspecificsecurityrequirementsoftheworkareatoselectthefusespecifications.Thisdisadvantageisover-currentprotectionmeasuresbroughtaboutbytheinconvenienceoffrequentreplacementoffuses.Linearvoltageregulatorcommonlyusedintheprotectionandcurrentlimitingtoprotectthecut-offintheswitchingregulatorcanbeapplied.However,accordingtothecharacteristicsofswitchingregulators,theprotectioncircuitcannotdirectlycontroltheoutputtransistorswitches,andovercurrentprotectionmustbeconvertedtopulseoutputcommandstocontrolthemodulatortoprotectthetransistorswitch.Inordertoachieveover-currentprotectionaregenerallyrequiredsamplingresistorinseriesinthecircuit,thiswillaffecttheefficiencyofpowersupply,somoreforlow-powerswitchingregulatorofoccasions.Inthehigh-powerswitchingpowersupply,bytakingintoaccountthepowerconsumptionshouldbeavoidedasfaraspossibleaccesstothesamplingresistor.Therefore,therewillusuallybeconvertedtoover-currentprotection,andunder-voltageprotection.E、Over-voltageprotectionSwitchingregulator'sinputover-voltageprotection,includingover-voltageprotectionandoutputover-voltageprotection.SwitchingregulatorisnotusedinDCpowersupplyvoltageregulatorandrectifier,suchasbatteryvoltage,iftoohigh,soswitchingregulatorisnotworkingproperly,orevendamagetointernaldevices,therefore,itisnecessarytousetheinputover-voltageprotectioncircuit.Usingtransistorsandrelaysprotectioncircuit.Inthecircuit,whentheinputDCpowersupplyvoltagehigherthanthevoltageregulatordiodebreakdownvoltagevalue,thebreakdownvoltageregulatortube,acurrentflowingthroughresistorR,sothatVturn-ontransistor,relay,normallyclosedcontactoffopen,cutofftheinput.VoltageregulatorvoltageregulatorwhichcontrolsthevalueofVs.=Earwax-UBE.Thepolarityofinputpowerwiththeinputprotectioncircuitcanbecombinedwithover-voltageprotection,polarityprotectionconstituteadifferentialcircuitandovervoltageprotection.Outputover-voltageprotectionswitchingpowersupplyisessential.Inparticular,forthe5Voutputoftheswitchingregulator,itisalotofloadonahighlevelofintegrationofthelogicdevice.Ifatwork,switchingregulatorsuddendamagetotheswitchtransistor,theoutputpotentialmaybeincreasedimmediatelytotheimportationofnon-regulatedDCpowersupplyvoltagevalue,causinggreatlossinstantaneous.Commonlyusedmethodisshort-circuitprotectionthirsted.Thesimplestover-voltageprotectioncircuit.Whentheoutputvoltageistoohigh,theregulatortubebreakdowntriggeredthirstierturn-on,theoutputshort-circuit,resultinginover-currentthroughthefuseorcircuitprotectivedevicetocutofftheinputtoprotecttheload.Thiscircuitisequivalenttotheresponsetimeoftheopeningtimeofthirstierisabout5~10 卩s.Thedisadvantageisthatitsactionisfixedvoltage,temperaturecoefficient,andactionpointsofinstability.Inaddition,thereisavoltageregulatorcontrolparametersofthediscrete,modelover-voltagestart-upthesamebuthasdifferentvalues,difficulttodebug.Escasuddenincreaseinoutputvoltage,transistorsV1,V2conduction,thethrusterconduction.RefereneevoltageVs.bytype.F、 Under-voltageprotectionOutputvoltagebelowthevaluetoreflecttheinputDCpowersupply,switchingregulatoroutputloadinternalorunusualoccurrenee.InputDCpowersupplyvoltagedropsbelowthespecifiedvaluewouldresultinswitchingregulatoroutputvoltagedrops,theinputcurrentincreases,notonlyendangertheswitchingtransistor,butalsoendangertheinputpower.Therefore,inordertosetupduetovoltageprotection.Duetosimplevoltageprotection.Whennovoltageregulatorinputnormal,ZDbreakdownvoltageregulatortube,transistorsVconduction,therelayaction,contactpull-in,power-switchingregulator.Whentheinputbelowtheminimumallowablevoltagevalue,theregulatortubeZDbarrier,Vcut-off,contactKai-hop,switchingregulatorcannotwork.Internalswitchingregulator,asthecontrolswitchtransistorcircuitdisordersorfailurewilldecreasetheoutputvoltage;loadshort-circuitoutputvoltagewillalsodecline.Especiallyinthereversed-phasestep-uporstep-upswitchingregulatorDCvoltageoftheprotectionduetoover-currentprotectionwithcloselyrelatedandthereforemoreimportant.ImplementationofSwitchingRegulatorsintheterminationoftheoutputvoltagecomparators.Normally,thereisnocomparatoroutput,oncethevoltagedropsbelowtheallowablevalueinthecomparatorontheflip,drivealarmcircuit;alsofedbacktotheswitchingregulatorcontrolcircuit,sothatswitchingtransistorcut-offorcutofftheinputpower.G、 Over-temperatureprotectionSwitchingregulatorandthehighlevelofintegrationoflight-weightsmallvolume,withitsunitvolumegreatlyincreasedthepowerdensity,powersupplycomponentstoitsworkwithintherequirementsoftheambienttemperatureisalsoacorrespondingincrease.Otherwise,thecircuitperformaneewilldeteriorateprematurecomponentfailure.Therefore,inhigh-powerswitchingregulatorshouldbesetupover-temperatureprotection.Relaysusedtodetectthetemperatureinsidethepowersupplytemperature,whentheinternallygeneratedpowersupplyoverheating,thetemperatureoftherelayontheaction,sothatwholecircuitinawarningalarm,andtherealizationofthepowersupplyover-temperatureprotection.Temperaturerelaycanbeplacedinthevicinityoftheswitchingtransistor,thegeneralhigh-powertubeshelltoallowthemaximumtemperatureis75°C,adjustthetemperaturesettingto60C.Whentheshellafterthetemperatureexceedstheallowablevaluetocutoffelectricalrelayontheswitchprotection.Semiconductorswitchingdevicethermal"hotthirstier,"intheover-temperatureprotection,playedanimportantrole.Itcanbeusedasdirectedcircuittemperature.Underthecontrolofp-hot-gatethirstier(TT102)characteristics,byRTvaluetodeterminethetemperatureofthedeviceturn-on,RTgreaterthetemperaturethelowertheturn-on.Whenplacednearthepowerswitchingtransistororpowerdevice,itwillbeabletoplaytheroleoftemperatureinstructions.Whenthepowercontrolthetemperatureoftheshellortheinternaldevicetemperatureexceedstheallowedvalue,theheatconductionthirstieron,sothatLEDwarninglight.Iftheoptcouplerwith,wouldenablethewholecircuitalarmactiontoprotecttheswitchingregulator.Itcanalsobeusedasapowertransistorastheover-temperatureprotection,crystalswitchthebasecurrentbyn-typegatecontrolthirstierTT201thermalbypass,cut-offswitchtocutoffthecollectorcurrenttopreventoverheating.H、ThefuturedevelopmentofrelayprotectionProtectiontechnologytofuturetrendsiscomputerized,network,intelligent,protectionandcontrol,measurementanddatacommunicationsintegrationdevelopment.ComputerizedAlongwiththerapiddevelopmentofcomputerhardware,microcomputerprotectionanddevelopmentinhardware.Formernorthelectricityinstituteofcomputerlineprotectionhardwarehasexperieneedthreedevelopmentstages:fromeightsingleCPUstructureofthemicrocomputerrelayprotection,lessthan5yearstimetodevelopmoreCPUstructure,andtodevelopthelargedon'tbusmodule,performaneegreatlyimprovemodulestructure,widelyused.Huazhonguniversityofscieneeandtechnologydevelopmentofmicrocomputerprotectionfrom8bitsCPU,isthecorepartinpolymerizing-kettledevelopmentbasedon32-bitmicrocomputerprotection.Nanjinginstituteofelectricalautomationbegana16-bitCPUisdevelopedonthebasisofcomputerlineprotection,hasbeenwidespreadinthestudy,alsoprotectthehardwaresystem32bits.Thedevelopmentofsoutheastuniversitycomputerhardwareandequipmenttoprotectandimprovethroughalot.Tianjinuniversityisastartwithmorethan16bitsCPUdevelopedonthebasisofcomputerlineprotection,whichbeganin1988study32-bitdigitalsignalprocessor(DSP)fortheprotectionandcontrol,measuringdevices,microcomputerintegratedwithZhuhaijinelectricalautomationequipmentcompanydevelopedintoafullyfunctional32-bitbigmodules,amoduleisasmallcomputer.Using32-bitmicrocomputerchipisnotonlybecausetheaccuracyandprecisionoftheA/Dconverterbytheresolutionlimit,morethan16whenconvertingspeedandcostisunacceptable,Moreimportantisthe32-bitmicrocomputerchipwithhighintegrity,highfrequencyandcomputingspeedandlargespace,richaddressingthecommandsystemandmoreinputoutput.TheCPUregisters,databus,theaddressbusare32-bitmemorymanagementfunction,andmemoryprotectionfunctionandtasks,andtheconversionfunctionwillCache(Cache)andfloating-pointcomponentsareintegratedintheCPU.NetworkPowersystemofmicrocomputerprotectionrequirement,besidesimprovingthebasicfunctionofprotection,alsoshouldhavethecapacityofthefaultinformationanddatastoragespace,long-termfastdataprocessingfunction,strongcommunicationability,andotherprotectionandcontroldevicesandthewholesystembysharingnetworkdispatchingdata,informationandnetworkresources,theabilitytoprograminahigh-levellanguage,etc.ThisrequiresmicrocomputerprotectiondeviceisequivalenttoaPC.Intheearlydevelopmentofcomputertoprotect,oncethoughttouseasmallcomputerfinishrelayprotectiondevice.Duetothelargesize,thenminicomputershighcostandreliabilityofthisideaisnotreality.Now,withmicroprocessor-basedprotectionfunctionofsimilarsize,speed,work-managingmachinegreatlyexceedthestoragecapacityofminicomputer,therefore,makeuseoftherelayprotectionsetspolymerizing-kettletimeisripe,thisisthedevelopmentdirectionofthemicrocomputerrelayprotection.TianjinUniversityhasdevelopedwithmicroprocessor-basedprotectiondevicewithanidenticalstructurereformpolymerizing-kettleofrelayprotectiondevice.Thisdevicehasseveraladvantages:(1)486PCmachinecanmeetallthefunctionsofthecurrentandfuturedemandsofmicrocomputerprotectionfunction.(2)dimensionandstructureandthecurrentprotectiondevice,exquisiteworkmanship,shockproof,preventoverheating,preventingelectromagneticinterfereneeabilityisstrong,canruninverybadworkingenvironment,costcanbeaccepted.(3)usingSTDbusorPCbus,hardwaremodular,fordifferentprotectioncanbearbitrarilychoosedifferentmodules,flexibleconfiguration,easyextension.Themicrocomputerrelayprotectiondevice,computerizedisirreversibletrend.Buthowtobettersatisfypowersystem,howtofurtherimprovethereliabilityoftherelayprotection,andhowtoobtainmoreeconomicbenefitandsocialbenefit,stillmustspecificallyin-depthresearch.820networksThecomputernetworkinformationanddatacommunicationtoolhasbecomethemainstayoftheinformationage,makehumanproductiontechnologyandsociallifeofthefundamentalchangeshavetakenplace.Itisdeeplyaffectallindustries,aswellasvariousindustryprovidespowerfulmeansofcommunication.Sofar,inadditiontothedifferentialprotectiondisoperationandprotectionoftherelayprotectiondevice,allcanonlyprotecttheelectricityinstallationreaction.Protectionfunctionislimited,reduceaccidentsresectionfaultcomponents.Thisismainlyduetothelackofdatacommunications.StrongForeignhadputforwardtheconceptofprotectionsystem,whichwasmainlyreferstothesafetyautomaticdevice.Becauseoftherelayprotectionfunctionnotonlyfaultcomponentsandlimitresectionimpactaccident(thisistoensurepriority),thesafeandstableoperationofthewholesystem.ThisrequireseachprotectionunitcanbealtogetherI、ConclusionDiscussedaboveintheswitchingregulatorofavarietyofconservation,andintroducesanumberofspecificwaystoachieve.Ofagivenswitchingpowersupplyisconcerned,butalsoprotectionfromthewholetoconsiderthefollowingpoints:TheswitchingregulatorusedintheswitchingtransistorintheDCsecurityrestrictionsontheworkofregionalwork.ThetransistorswitchesselectedbythemanualavailabletransistorsgetDCsafeworkingarea.Accordingtothemaximumcollectorcurrenttodeterminetheinputvalueofover-currentprotection.However,theinstantaneousmaximumvalueshouldbeconvertedtotheaveragecurrent.Atratedoutputcurrentandoutputvoltageconditions,theswitchofthedynamicloadlinedoesnotexceedasafeworkingareaDCmaximuminputvoltage,inputover-voltageprotectionisthevoltagevalue.Theswitchingregulatoroutputlimitgivenbythetechnical indicatorswithin.Workwithintherequiredtemperaturerange,theswitchingregulator'soutputvoltage,thelowerlimitoftheoutputisoff,duetothevoltagevalueofvoltageprotection.Over-currentprotectioncanbebasedonthemaximumoutputcurrenttodetermine.Falsealarminordernottoprotectthevalueofacertainmargintoremainappropriate.Fromtheabovetwomethodstodeterminetheprotectionafterthepowersupplydeviceinaccordaneewiththeneedsofmeasurestodeterminethealarm.Measuresthegeneralalarmsoundandlightalarmtwopolice.Voiceofthepoliceappliedtomorecomplexmachines,powersupplypartsanddonotstandoutinaplace,itcangivestaffaneffectivewarningoffailure;opticalPoliceinstructionscanbeeye-catchingandfaultalarmandpointedoutthatthefaultlocationandtype.Protectionmeasuresshouldbeprotectedastodeterminethelocation.Inthehigh-power,multi-channelpowersupply,alwayspaying,DCcircuitbreakers,relays,etc.high-sensitivityauto-protectionmeasures,tocutofftheinputpowersupplytostopworkingthesystemfromdamage.Throughthelogiccontrolcircuittomaketheappropriateprogramcut-offswitchtransistorissensitiveitisconvenientandeconomic.Thiseliminatedlarge,longresponsetime,thepriceofyourhigh-powerrelayorcircuitbreaker.Thepowerofputtingintheprotectioncircuitwillbeaffectedafterthereliabilityofthesystem,forwhichwanttoprotectthereliabilityofthecircuititselfishigherinordertoimprovethereliabilityoftheentirepowersystem,therebyincreasingitsownpowersupplyMTBF.Thisrequirestheprotectionofstrictlogic,thecircuitissimple,atleastcomponents,Inadditiontotheprotectioncircuitshouldalsobeconsideredafailureofmaintenanceoftheirdifficultyandtheirpowertoprotectthedamage.Therefore,wemustbecomprehensiveandsystematicconsiderationofavarietyofswitchingpowersupplyprotectionmeasurestoensurethenormaloperationofswitchingpowersuppliesandhigh-efficiencyandhighreliability.直流开关稳压电源的保护技术摘要：讨论了直流开关稳压电源的保护系统，提出保护系统设计的原则和整机保护的措施，分析了开关稳压电源中的各种保护的特点及其设计方法，介绍了几种实用保护电路。关键字：开关电源；保护电路；系统设计一、 引言直流开关稳压器中所使用的大功率开关器件价格较贵，其控制电路亦比较复杂，另外,开关稳压器的负载一般都是用大量的集成化程度很高的器件安装的电子系统。 晶体管和集成器件耐受电、热冲击的能力较差。因而开关稳压器的保护应该兼顾稳压器本身和负载的安全。保护电路的种类很多，这里介绍极性保护、程序保护、过电流保护、过电压保护、欠电压保护以及过热保护等电路。通常选用几种保护方式加以组合，构成完善的保护系统。二、 极性保护直流开关稳压器的输入一般都是未稳压直流电源。由于操作失误或者意外情况会将其极性接错，将损坏开关稳压电源。极性保护的目的，就是使开关稳压器仅当以正确的极性接上未稳压直流电源时才能工作。利用单向导通的器件可以实现电源的极性保护。最简单的极性保护电路如图1所示。由于二极管D要流过开关稳压器的输入总电流，因此这种电路应用在小功率的开关稳压器上比较合适。在较大功率的场合，则把极性保护电路作为程序保护中的一个环节，可以省去极性保护所需的大功率二极管，功耗也将减小。为了操作方便，便于识别极性正确与否，在二极管之后接指示灯。三、 程序保护开关稳压电源的电路比较复杂，基本上可以分为小功率的控制部分和大功率的开关部分。开关晶体管则属大功率，为保护开关晶体管在开启或关断电源时的安全，必须先让调制器、放大器等小功率的控制电路工作。为此，要保证正确的开机程序。开关稳压器的输入端一般接有小电感、大电容的输入滤波器。在开机瞬间，滤波电容器会流过很大的浪涌电流，这个浪涌电流可以为正常输入电流的数倍。这样大的浪涌电流会使普通电源开关的触点或继电器的触点熔化，并使输入保险丝熔断。另外，浪涌电流也会损害电容器，使之寿命缩短，过早损坏。为此，开机时应该接入一个限流电阻，通过这个限流电阻来对电容器充电。为了不使该限流电阻消耗过多的功率，以致影响开关稳压器的正常工作，而在开机暂态过程结束后，用一个继电器自动短接它，使直流电源直接对开关稳压器供电。这种电路称之谓开关稳压器的“软启动”电路。开关稳压器的控制电路中的逻辑组件或者运算放大器需用辅助电源供电。为此，辅助电源必须先于开关电路工作。这可用开机程序控制电路来保证。一般的开机程序是：输入电源的极性鉴别，电压保护一开机程序电路工作一辅助电源工作并通过限流电阻R对开关稳压器的输入电容器C充电一开关稳压器的调制电路工作，短路限流电阻一开关稳压器稳定工作。在开关稳压器中，刚开机时，因为其输出电容容量大，充到额定输出电压值需要一定时间。在这段时间内，取样放大器输入低的输出电压采样，根据系统闭环调节特性将迫使开关三极管的导通时间加长，这样一来，开关三极管就会在这段期间内趋于连续导通，而容易损坏。为此，要求在开机这一段时间内，开关调制电路输出给开关三极管基极的脉宽调制驱动信号，能保证开关三极管由截止逐渐趋于正常的开关状态，故而要加设开机保护以配合软启动。四、 过电流保护当出现负载短路、过载或者控制电路失效等意外情况时，会引起流过稳压器中开关三极管的电流过大，使管子功耗增大，发热，若没有过流保护装置，大功率开关三极管就有可能损坏。故而在开关稳压器中过电流保护是常用的。最经济简便的方法是用保险丝。由于晶体管的热容量小，普通保险丝一般不能起到保护作用，常用的是快速熔断保险丝。这种方法具有保护容易的优点，但是，需要根据具体开关三极管的安全工作区要求来选择保险丝的规格。这种过流保护措施的缺点是带来经常更换保险丝的不便。在线性稳压器中常用的限流保护和电流截止保护在开关稳压器中均能应用。但是 ，根据开关稳压器的特点，这种保护电路的输出不能直接控制开关三极管，而必须使过电流保护的输出转换为脉冲指令，去控制调制器以保护开关三极管。为了实现过电流保护一般均需要用取样电阻串联在电路中，这会影响电源的效率，因此多用于小功率开关稳压器的场合。而在大功率的开关稳压电源中，考虑到功耗，应尽量避免取样电阻的接入。因此，通常将过电流保护转换为过、欠电压保护。五、 过电压保护开关稳压器的过电压保护包括输入过电压保护和输出过电压保护。开关稳压器所使用的未稳压直流电源诸如蓄电池和整流器的电压如果过高，使开关稳压器不能正常工作，甚至损坏内部器件，因此，有必要使用输入过电压保护电路。在该电路中，当输入直流电源的电压高于稳压二极管的击穿电压值时， 稳压管击穿，有电流流过电阻R，使晶体管V导通，继电器动作，常闭接点断开，切断输入。其中稳压管的稳压值Vz=ESrmax—UBE。输入电源的极性保护电路可以跟输入过电压保护结合在一起，构成极性保护鉴别与过电压保护电路。输出过电压保护在开关稳压电源中是至关重要的。特别对输出为 5V的开关稳压器来说，它的负载是大量的高集成度的逻辑器件。如果在工作时 ，开关稳压器的开关三极管突然损坏，输出电位就可能立即升高到输入未稳压直流电源的电压值 ，瞬时造成很大的损失。常用的方法是晶闸管短路保护。当输出电压过高时，稳压管被击穿，触发晶闸管导通,把输出端短路，造成过电流，通过保险丝或电路保护器将输入切断，保护了负载。这种电路的响应时间相当于晶闸管的开通时间，约为5〜10卩s。它的缺点是动作电压是固定的，温度系数大，动作点不稳定。另外，稳压管存在着参数的离散性，型号相同但过电压起动值却各不相同，给调试带来了困难。六、 欠电压保护输出电压低于规定值时，反映了输入直流电源、开关稳压器内部或者输出负载发生了异常。输入直流电源电压下降到规定值之下时，会导致开关稳压器的输出电压跌落，输入电流增大，既危及开关三极管，也危及输入电源。因此，要设欠电压保护。简单的欠电压保护如图6所示。当未稳压输入的电压值正常时，稳压管ZD击穿，晶体管V导通，继电器动作，触点吸合，开关稳压器加电。当输入低于所允许的最低电压值时，稳压管 ZD不通，V截止，触点跳开，开关稳压器不能工作。开关稳压器内部，由于控制电路失常或者开关三极管失效会使输出电压下降；负载发生短路也会使输出电压下降。特别在升压型或反相升压型的直流开关稳压器中欠电压的保护是跟过电流保护紧密相关的，因而更加重要。实现方法是在开关稳压器的输出端接电压比较器。正常时，比较器没有输出，一旦电压跌落在允许值之下比较器就翻转，驱动告警电路；同时反馈到开关稳压器的控制电路，使开关三极管截止或切断输入电源。七、 过热保护开关稳压器的高集成化和轻量小体积，使其单位体积内的功率密度大大提高，电源装置内部的元器件对其工作环境温度的要求也相应提高。否则，会使电路性能变坏，元器件过早失效。因此在大功率开关稳压器中应该设过热保护。采用温度继电器来检测电源装置内部的温度，当电源装置内部产生过热时