owersupply功率补偿.ppt

阅读教材380 1Whataretheonewordanswerstothefollowingquestions Itisoneofthemostpopulartypesofdiode Whenforwardbiasedwithcurrentflowingthroughthejunction lightisproduced Thediodescanproduceavarietyofcolours althoughtheoriginalcolourwasred Lightemittingdiodes Thistypeofdiodeisnotthesameastheordinarylightemittingdiodebecauseitproducescoherentlight TheyarewidelyusedinmanyapplicationsfromDVDandCDdrivestolaserlightpointersforpresentations Theyalsohavealimitedlife Laserdiode Itisusedfordetectinglight ItisfoundthatwhenlightstrikesaPNjunctionitcancreateelectronsandholes Thesediodesareoperatedunderreversebiasconditionswhereevensmallamountsofcurrentflowresultingfromthelightcanbeeasilydetected Theycanalsobeusedtogenerateelectricity Forsomeapplications Photodiode Itisaveryusefultypeofdiodeasitprovidesastablereferencevoltage Itisrununderreversebiasconditionsanditisfoundthatwhenacertainvoltageisreacheditbreaksdown Ifthecurrentislimitedthrougharesistor itenablesastablevoltagetobeproduced Thistypeofdiodeisthereforewidelyusedtoprovideareferencevoltageinpowersupplies Zenerdiode Itmaybethoughtofasthenormalorstandardtypeofdiodeinusetoday Thesediodescancomeassmallsignaltypesforuseinradiofrequency orotherlowcurrentapplicationswhichmaybetermedassignaldiodes PNJunctiondiode ThistypeofdiodehasalowerforwardvoltagedropthanordinarysiliconPNjunctiondiodes Atlowcurrentsthedropmaybesomewherebetween0 15and0 4voltsasopposedto0 6voltsforasilicondiode Toachievethisperformancetheyareconstructedinadifferentwaytonormaldiodeshavingametaltosemiconductorcontact Theyarewidelyusedasclampingdiodes inRFapplications andalsoforrectifierapplications Schottkydiodes 2whatarechargecarriers Identifythechargecarriersinmetallicsubstances semiconductingsubstances andconductiveliquids Chargecarriers areanyparticlespossessinganelectricalcharge Differenttypesofsubstanceshavedifferentchargecarriers Metals free conduction band electronsSemiconductors electronsandholesLiquids ions 3Whichtypeofmaterialisthebestconductorofelectricity Whichtypeofmaterialistheworstconductorofelectricity Metalsarethebestconductorsofelectricity Insulatorsaretheworstconductorsofelectricity 4Describethedifferencebetweenanintrinsicandanextrinsicsemiconductingsubstance An intrinsic semiconductingmaterialisabsolutelypure An extrinsic semiconductingmaterialhasdopant s addedforenhancedconductivity 5Inextrinsicsemiconductors whataremajoritycarriersandhowdotheydifferfromminoritycarriers N typesemiconductorThenumberoffreeelectrons Thenumberofholes Freeelectronsaremajoritycarriers Holesareminoritycarriers P typesemiconductorThenumberoffreeelectrons Thenumberofholes Holesaremajoritycarriers Freeelectronsisminoritycarriers 6ThecharacteristiccurveforanormalPNjunctionrectifyingdiodelookssomethinglikethis Reversecharacteristic反向特性 Forwardcharacteristic正向特性 Barrierpotential死区 Breakdownvoltage击穿电压 Reversecut off反向截止 Reversebreakdown反向击穿 Reversesaturationcurrent反向饱和电流 Characteristiccurve特性曲线 Ge diode锗管Si diode硅管 7Howisitpossibletodeterminethepolarityofarectifyingdiode whichterminalistheanode andwhichterminalisthecathode fromitsphysicalappearance thereisastripe similartoacolorbandonaresistor closertooneendofthediodethantheother 8The 1N400 x seriesofrectifyingdiodesareverypopularforlow currentapplications By 1N400 x Imeanthe1N4001 1N4002 1N4003 1N4007 Onlyoneparameterdiffersbetweenthesedifferentdiodemodels Whatparameteristhis Onlythereverse orblocking voltageratingdiffersbetweenthesediodemodels 9ApplicationsoftheDiode 1 Rectifying整流 2 Amplitudelimiting限幅 Fig 1 3 Clamping钳位 4 Isolation隔离 10Whatarepowersupplies Allelectroniccircuitsneedapowersourcetowork Forelectroniccircuitsmadeupoftransistorsand orICs thispowersourcemustbeaDCvoltageofaspecificvalue 11Fillintheblanks 1 TheAClineisfirstpassedthroughalowpassfiltershowninfigure ThiseliminatesnoiseontheAClinefrombotheringthepowersupplycircuitsandpreventsunwantedsignalsfromthepowersupplyfrombeingtransferredbackintotheAClinewheretheymightinterferewithotherequipment 2 AtransformeriscommonlyusedtosteptheinputACvoltageleveldownorup MostelectroniccircuitsoperatefromvoltageslowerthantheAClinevoltagesothetransformernormallystepsthevoltagedownbyitsturnsratiotoadesiredlowerlevel Forexample atransformerwithaturnsratioof10to1wouldconvertthe120volt60Hzinputsinewaveintoa12voltsinewave 3 TherectifierconvertstheACsinewaveintoapulsatingDCwave Thereareseveralformsofrectifiersusedbutallaremadeupofdiodes 4 TherectifierproducesaDCoutputbutitispulsatingratherthanaconstantsteadyvalueovertimelikethatfromabattery Afilterisusedtoremovethepulsationsandcreateaconstantoutput Themostcommonfilterisalargecapacitor 5 Thesimplestformofrectifieristhehalfwaverectifiershown Anotherwidelyusedrectifieristhebridgerectifier Itusesfourdiodes 6 Theregulatorisacircuitthathelpsmaintainafixedorconstantoutputvoltage ChangesintheloadortheAClinevoltagewillcausetheoutputvoltagetovary Mostelectroniccircuitscannotwithstandthevariationssincetheyaredesignedtoworkproperlywithafixedvoltage Theregulatorfixestheoutputvoltagetothedesiredlevelthenmaintainsthatvaluedespiteanyoutputorinputvariations 7 MostmodernpowersuppliesalsocontainoneormoreDC DCconvertersModernelectronicsoftendemanddifferentvoltagestofunction ADC DCconverterchangesoneDCvoltagetoanother higherorlowerDCvoltage ADC DCconverterisusedwithapowersupplytopreventtheneedforasecondAC DCsupply 12Maincircuitsinmostpowersupplies 13Howdoesthefilterwork Alargecapacitorisconnectedacrosstheloadresistor Whenthediodeconducts thecapacitorchargesuptothepeakofthesinewave Thenwhenthesinevoltagedrops thechargeonthecapacitorremains Sincethecapacitorislargeitformsalongtimeconstantwiththeloadresistor Thecapacitorslowlydischargesintotheloadmaintainingamoreconstantoutput 14Althoughnotapopulardesign somepowersupplycircuitsaretransformerless DirectrectificationofAClinepowerisaviableoptioninsomeapplications However thisformofAC to DCpowerconversionhassomesignificantlimits Explainwhymostpowersupplycircuitsutilizeatransformerinsteadofdirectlyrectifyingthelinepowerasthiscircuitdoes Transformersprovidevoltage currentratiotransformation andalsoelectricalisolationbetweentheAClinecircuitandtheDCcircuit Theissueofisolationisasafetyconcern asneitheroftheoutputconductorsinanon isolated direct rectifiercircuitisatthesamepotentialaseitherofthelineconductors 15Observethefollowingtwowaveforms asrepresentedonanoscilloscopedisplaymeasuringoutputvoltageofafilteredpowersupply Ifbothofthesewaveformsweremeasuredonthesamepowersupplycircuit atdifferenttimes determinewhichwaveformwasmeasuredduringaperiodofheavier loading a heavier loadbeingdefinedasaloaddrawinggreatercurrent Theleft handwaveformwasmeasuredduringaperiodofheavierloading Ifitisnecessaryforustomaintainalowamountofrippleunderthisheavyloading whatmustwechangeinthepowersupplycircuit 16Whatisripple Thecapacitordoesagoodjobofsmoothing