反激的典型波形

反激變換器的例子Analysisofbasicwaveforms基本波形分析TheanalysisofthebasicwavfOrmswillbedoneonasimulatedexamplefaf§6ac冬converteroperatingindiscontinuousconductionmode.Typicaldrain-sourcevoltagewavefarmoftheprimarysideswitchisshowninFig.16.在電感電流斷續(xù)模式下運(yùn)行的反激變換器的典型一次側(cè)漏源極開關(guān)電壓波形見圖/5。Fig.16Typicaldrain-sourcevoltageoftheMOSFETinaffyba氐圖7，反激變換器的典型漏源極電壓Thesedrain-sourcevoltagewaveformscanbetheoreticallydistinguishedintotypicalelements.Differentphysicalphenomenainfuencethewaveformatgiventimeinterval.Fig.17andTab.4demonstratethemainelementsofthevoltagewavefOrm.Thesuperpositionofalltheseelementsresultsinatypicaldrain-sourcevoltageshowninFig.16.這些漏源極電壓波形能用典型的理論來描述。各個(gè)時(shí)間段有不同物理現(xiàn)象影響這些波形。圖/7和平臺4描述了電壓波形的主要原理。把這些原理按時(shí)序整合呈現(xiàn)出圖/5所示的典型漏源極電壓。Fig.17Mainelementsof血drain-sourcevoltage圖77漏源極電壓的主要原理

Element1:voltagefallduringturningon600迎■WO：00-；00332610^S.?2S，尸：0^原理1：開通期間的電壓下降過程

332610^S.?2S，尸：0^Element2:parasiticoscillationduringturningonduetocurrentspike原理2:在開通期間因寄生震蕩產(chǎn)生的電流尖刺Element3:voltageriseduringturningoff600SOO：000七-'_?.3I："'5.?>：『-3」［廠5.45-lO"原理3:關(guān)斷期間的電壓上升

Element4:clampingvoltageofsnubber\r100sotsojL;:40」-2005.610~?.S.0~原理4:緩沖電路的鉗位電壓Element5:parasiticoscillationafterclampinginvolvingmainlytheoutputcapacitanceoftheMOSFETandtheleakageinductanceofthetransformer原理5,,鉗位過程結(jié)束后主要由場效應(yīng)晶體管輸出電容和變

壓器漏感引起的寄生振蕩Element6:parasiticoscillationafterflybackphaseinvolvingmainlytheoutputcapacitanceoftheMOSFETandthemagnetizationinductanceofthetransformer■2.SLii-6332ItT5原理6：磁芯存儲磁能釋放完畢后主要由場效應(yīng)晶體管輸出

電容和變壓器電感引起的寄生振蕩Element7:reflectedvoltageduringtheflybackphaseElement81mainrectangularsignalwithbusamplitude笠iiii.ITOC\o"1-5"\h\zi碰三；；三Ii!；-Ii皿-i「11QJ11111:~r§,濕11^1.51<T4如、15L尸3航15成17￡原理7：反激變換器釋放磁能期間的反射電壓

原理3：與直流母線電壓等幅的主要方波Tab.4Mainelementsofthedrain-sourcevoltage

平臺4漏源極電壓的主要原理Thespectrumofthewholedrain-sourcewavfbrm(Fig.16)ispresentedinFig.18.圖/5所示的漏源極電壓呈現(xiàn)的電磁干擾頻譜見圖/3。Fig.18Spectrumof血drain-sourcevoltage(asshowninFig.16)ffi/r圖，6所示的漏源極電壓呈現(xiàn)的電磁干擾頻譜Thespectracfthemainelementsofthedrain-sourcevoltagecan6efoundinFig.20.Fig.19isexact。thesamea^sFig.17andhasbeenrepeatedhereforbetterunder-standing.^20描述了漏源極電壓主要原理產(chǎn)生的電磁干擾頻得。為便于理解，將囹/7映射成囹/夕。Fig.19Mainelementsofthedrain-sourcevoltage(repeated,sameasFig.17)圖79漏源極電壓的主要原理(正確重復(fù)圖77)

fr^queucy,Hz40140地mli9>fr^queucy,Hz40140地mli9>」竺JdwsoA110*Fig.20Spectraofthemainelementsofthedrain-sourcevoltage圖名。漏源極電壓主要原理產(chǎn)生的電磁干擾頻譜Thismethodallowsassociatingcertainpartsofthespectrumwiththeirrootcauses,i.e.thepea冬at20MHzinthespectrumofthedrain-sourcevoltageiscausedbytheparasiticosciUhtionduetotheoutputcapacitanceoftheMOSFETandtheleakagein血ctanceofthetransformer.這種方法可以確定電磁干擾頻得中某些頻點(diǎn)的來源，也就是說漏源極電壓產(chǎn)生的電磁干擾頻得中的9。兆赫茲峰點(diǎn)是鉗位過程結(jié)束后主要由場效應(yīng)晶體管輸出電容和變壓器漏感引起的寄生振蕩產(chǎn)生的。Theanalysisofthedraincurrentoftheprimaryswitchwillbedoneinthesameway.Fig.21demonstratesatypicaldraincurrentinaD^CMflyba氐對一次側(cè)開關(guān)的漏極電流進(jìn)行分析采用相同的方法。圖N/展示出一個(gè)工作于電感電流斷續(xù)模式反激變換器的典型漏極電流。s■io'7i-it*i5-itf'time,s■io'7i-it*i5-itf'time,s3is-kTmW*Fig.21TypicaldraincurrentinaJly6ac^</Ug-InuThiswavefOrmcanbepresenteda^sasuperpositionofthefollowingelements(Fig.22andTOb.5).ThesuperpositionofaKtheseelementsresultsinatypicaldraincurrentshowninFig.21.這個(gè)波形可以被看作是下列原理的疊加(圖NN和平臺5)。全部這些波形的疊加整合結(jié)果變成圖N7所示的典型漏極電流。currciiLA幽NN挈營仲葬琴土炳飄謠Element2:currentspikeduringturningonduetoparasiticcapacitancesofthecircuit原理2:在開關(guān)開通期間因寄生分布電容引起的電流尖刺Element3:parasiticoscillationafterclampinginvolvingmainlytheoutputcapacitanceoftheMOSFETandtheleakageinductanceofthetransformeriic""注原理M,,鉗位過程結(jié)束后主要由場效應(yīng)晶體管輸出電容和變

壓器漏感引起的寄生振蕩2.S10-32.S10-33濕3210~6TCP-0.06Element4:parasiticoscillationafterflybackphaseinvolvingmainlytheoutputcapacitanceoftheMOSFETandthemagnetizationinductanceofthetransformer0.040.02原理4：磁芯存儲磁能釋放完畢后主要由場效應(yīng)晶體管輸出

電容和變壓器電感引起的寄生振蕩Tab.5Mainelementsofthedraincurrent平臺5漏極電流的主要原理Thespectrumofthewholedraincurrentwavfbrm(Fig.21)ispresentedinFig.23.

全部漏極電流波形產(chǎn)生的電磁干擾頻譜(^21)呈現(xiàn)在圖2SOwwTOvncap=妙切3fiequency,HzwwTOvncap=妙切3fiequency,HzFig.23Spectrumofthedraincurrent(asshowninFig.22)^23漏極電流產(chǎn)生的電磁子攏頻譜(與圖22相同)ThespectracfthemainelementsofthedraincurrentcanbefoundinFig.25.Fig.24isexact。thesamea^sFig.22andhasbeenrepeatedforbetterunderstanding.漏極電流主要原理產(chǎn)生的電磁干擾頻譜見圖25。圖24和圖22相同。

25”31尸F(xiàn)ig.24Mainelementsofthedraincurrent圖3425”31尸F(xiàn)ig.24Mainelementsofthedraincurrent圖34漏極電流的主要原理<AualunuIio"(u-io-6times2-ifl^Fig.25Spectraofthemainelementsofthedraincurrent<=SHPUGPE-0Asincaseofdrain-sourcevoltagethismethodallowstoassociatetheelementsofthedraincurrentwaveformwithitscontributiontothewholespectrum.Forexample,thepea冬at20MHzinthespectrumiscausedbytheparasiticoscillationduetotheoutputcapacitanceoftheMOSFETandthe(eakageinductanceofthetransformer.就象漏源極電壓的例子那樣，用這種方法也可以找出漏極電流的哪一部分對電磁干擾頻譜產(chǎn)生影響。舉例說明，NO兆赫茲的峰點(diǎn)是鉗位過程結(jié)束后主要由場效應(yīng)晶體管輸出電容和變壓器漏感引起的寄生振蕩產(chǎn)生的。Thismethodofseparatingthewaveformintimedomainintoitsmainelementshepstofindoutwhatpartofthespectruminfrequencydomaincausedbywhatrelatedphysicalphenomena.Theseparationintomainelementsshouldbedoneinrespectofreasonableeventsinthepowercircuitlikeonandoffslopes,oscillations,clamping,snubbering,reflectedvoltage,etc.這種在時(shí)域里對主要原理進(jìn)行拆分的方法有助于找出產(chǎn)生電磁干擾頻段的干擾源。這種離析主要原理的手法有助于合理審視電源電路里諸如變化速率、振蕩、鉗位、緩沖、反射電壓等過程。InthisftybacRexampleonlytheprimaryswitchhasbeenanalyzeda^sactivesourceofelectricalnoise.Therearealsoothers,Ekesecondarysidediodesorsynchronousrectifier,controlIC(especial!yitsgatedrive),etc.Inordertoobtainmorecompleteanalysisalltheseinterferencesourceshavetobeanalyzed.在這個(gè)反激變換器里只對一次側(cè)開關(guān)進(jìn)行電磁噪聲產(chǎn)生的分析。但是還有其他的部分，象二次側(cè)的二極管或同步整流器、控制集成電路（尤其是它們的柵極驅(qū)動）等等。按順序分析將獲得更完善的關(guān)于這些電磁干擾源的解析。However,itisimpossibletopredicttheconductedEMIspectrumusingthisapproachduetothefact,thatonlyinterferencesourcesa^^econsidered.Thereisnoanalysisofthespreadingpathsftheinteferenceinthismethod然而，這種方法不可能預(yù)知用頻譜反映的電磁干擾的實(shí)際行為，僅僅是干擾源被重視起來。在那里沒有對分布參數(shù)產(chǎn)生的干擾進(jìn)行分析的方法。Nevertheless,theassociationfharmonicsrootcausewiththerespectedphysicalphenomenawillreducethefortsofEMIreduction.Theimpactftheidentifiedrootcausecanbereducednotonlybyfiltering,butalsobymeansfinfluencingtherootcauseitsef.不過，重視物理現(xiàn)象并不能成就電磁干擾的降低。降低干擾并不僅僅是濾波，也同樣意味著干擾源自身的影響。Opemtionmo&sofdiscontinuousflybacjconverter電感電流斷續(xù)工作反激式變凝著的運(yùn)行模式Thef§6akconverterrunningindiscontinuousconductionm^odecanbeoperatedinhardswitchingorquasiresonant(orvalleyswitching,orZVS)moderegardingtheprimarysideswitch.Thedifferencebetweenahardswitchingandquasiresonantf§bac冬converteristheturnontimepointoftheprimaryswitch.InahardswitchingmodetheturningonoftheMOSFETisnotsynchronizedwiththedrain-sourcevoltagevalue.Thistypeofconvertersrunsmainlyinfixedfrequencymode.史感電流斷續(xù)工作的反激式變換器一次側(cè)開關(guān)可工作于硬開關(guān)或準(zhǔn)諧振(或谷值開關(guān)或零電壓開關(guān))模式。硬開關(guān)和準(zhǔn)諧振反激變換器之間的差異在于一次側(cè)開關(guān)的開啟時(shí)間點(diǎn)。在硬開關(guān)里場效應(yīng)晶體管的開啟波形拐點(diǎn)并不和漏源極電壓值同步。這種變換器大體上運(yùn)行于固定頻率模式。InaquasiresonantmodetheresonantcircuitdeterminedbytheoutputcapacityoftheMOSFETandtheinductanceofthetranformerwillbeutilizedtoswitchonatlbwestpossiblevalueofthedrain-sourcevoltage.Thiscircuitstartstooscillateattheendofthecurrentflowthroughthe^^^on^^iysideofthetransformer,henceattheendoftheflybac冬phase.TheMOSFETwillbeturnedonattheminimumofthisoscillation.ThequasiresonantapproachusesthisoscillationtoachieveminimumvoltageswitchingduringturnonfortheMOSFETThisoperationmoderunsatavariablefrequency.在準(zhǔn)諧振模式里，由變壓器電感和場效應(yīng)晶體管輸出電容引起的諧振促使開關(guān)的開通時(shí)刻發(fā)生在漏源極電壓的最小值上。這種電路在電流從變壓器二次側(cè)流盡以后（反激回掃過程結(jié)束）開始振蕩。場效應(yīng)晶體管將在振蕩幅值的最小值開啟（谷值開通）。這種運(yùn)行模式工作在可變的頻率上。HigheramplitudeoftheoscillationresultsinlowerdrainsourcevoltagelevelatwhichtheMOSFETturnsoncorrespondinglylowerswitchinglossesandhigherefficiencyofthesyst^^m^.更高幅值的振蕩導(dǎo)致場效應(yīng)晶體管更低的漏源極開通電壓幅值來產(chǎn)生更低的開關(guān)損耗和更高的系統(tǒng)效率。Toachievehighoscillationpeaksthedesignofthetransformerhastobesettohighreflectedvoltage.Thisincreaseofthereflectedvoltageresultsinahigherdrain-sourcevoltageblockingM^OSFETandlongerdutycycles.要達(dá)到比較高的振蕩電壓峰值，變壓器的反射電壓必須設(shè)置的比較高。增加的反射電壓導(dǎo)致使用更高漏源極擊穿電壓的場效應(yīng)晶體管和更大的開關(guān)占空比。Comparisonofthreedifferentf§bac&solutionshasbeenmade.Allofthemhavebeenoperationat3^0kHz,busvoltageof400V,outputpowerof120W,outputvoltageof16V.Thesedesignincludeddifferentmodesofoperationanddifferentvaluesofrflectedvoltage,resultingindifferentMOSFETsvoltageratings:比較現(xiàn)有的三種反激變換器。它們都工作在300千赫茲，直流母線電壓40。伏特，輸出功率120瓦特,輸出電壓75伏特。這些設(shè)計(jì)包含不同的運(yùn)行模式和反射電壓等級，因此使用不同電壓等級的場效應(yīng)晶體管：Hardswitchingf§6akwithCooMOS600V,reflectedvoltageof100V硬開關(guān)反激變換器使用500伏特C：ooMOS.100伏特反射電壓Quasiresonantf§6ackwithCooMOS600V,reflectedvoltageof100V?準(zhǔn)諧振反激變換器使用500伏特CooMOS.100伏特反射電壓Quasiresonantf§6ackwithCooMOS800V,reflectedvoltageof390V準(zhǔn)諧振反激變換器使用800伏特CooMOS,390伏特反射電壓Theclampingsnu66ercircuitwa^ssettotheratedbreakdownvoltageoftheMOSFET(600Vand800Vrespectively).鉗位緩沖電路被設(shè)定在場效應(yīng)晶體管的額定擊穿電壓上（分別為500伏特和,00伏特）。Fg6a氏inhardswitchingmodewith600VMOS^ET使用6。。伏特場效應(yīng)晶體管的硬開關(guān)反激變換器Thehardswitchingapproach（asshowninFig.26）doesn'tconsidertheminimumdrain-sourcevoltage.TheMOSFETwill6eturnedonhard,inthiscaseatavoltagelevelof500V（attimepoint3.3同.Thedischargeofcircuits'parasiticcapacitancesleadstoahighcurrentspikeduringturningon.硬開關(guān)（?26所示）幾乎不考慮漏源極電壓的最小值。場效應(yīng)晶體管開通應(yīng)力大，在這個(gè)例子里，開通電壓在50。伏特（在，3微秒的時(shí)間點(diǎn)）。由寄生電容引起的泄放電流在開通時(shí)產(chǎn)生很高的電流尖刺。t國Fig.26Drainrsourcevoltageanddraincurrentofhardswitching600Vfy6a氏圖366。。伏特硬開關(guān)反激變換器的漏源極電壓和漏極電流F§6ac七inquasiresonantmodewith600VMOSFET使用6。。伏特場效應(yīng)晶體管的準(zhǔn)諧振反激變換器Thedrain-sourcevoltage(Fig.27)startsoscillatingattheendofthef§6akphaseandreachingtheminimumf300VwhentheMOSFETturnson.漏源極電壓(囹N7)在反射過程結(jié)束后并減小到M。。次特時(shí)場效應(yīng)晶體管導(dǎo)通。Thedutycycleislowercomparedtoan800Vsolutionduetoalowerreflectedvoltageof100V.Shorterdutycycleforthesameoutputpowerresultsinhigherpea冬currentsontheprimaryside.因?yàn)?。。伏特的反射電壓，比較3。。次特解決方案它有更小的占空比。小占空比實(shí)現(xiàn)同樣的功率輸出必須使用更高的一次側(cè)峰值電流。Fig.27Drain-sourcevoltageanddraincurrentofquasiresonant600Vfy6ach圖名76。。伏特準(zhǔn)諧振反激變換器的漏源極電壓和漏極電流Fg6acJinquasiresonantmodewith800VMOSFET使用T。。伏特場效應(yīng)晶體管的準(zhǔn)諧振反激變換器Thedrain-sourcevoltage(Fig.28)startsoscillatingattheendofthef§6ac冬phaseandreachingtheminimumof100VwhentheMOSFETturnson.Theturningoncurrentspikeis(^ow.漏源極電壓(囹NT)在反射過程結(jié)束后并減小到/。。次特時(shí)場效應(yīng)晶體管導(dǎo)通。開通電流尖刺比較低。Thedutycycleishighercomparedtoa600Vsolutionduetoahigherreflectedvoltageof390V.Longerdutycycleforthesameoutputpowerresultsinlowerpea冬currentsontheprimaryside.因?yàn)橛蠱夕。伏特的反射電壓，所以有比600伏特解決方案更大的占空比。更大的占空比實(shí)現(xiàn)同樣的輸出功率可以使用更低的一次側(cè)峰值電流。Fig.28Drain-sourcevoltageanddraincurrentofquasiresonant800Vfy6ach圖名多#00次特準(zhǔn)諧振反激變換器的漏源極電壓和漏

極電流Comparisonfspectra干擾頻譜比較Thespectracfthedrain-sourcevoltagesforcorrespondingf§6a氐design(Fig.26Fig.27andFig.28)areshowninFig.29.相應(yīng)設(shè)計(jì)的反激變換器(囹9