隔离型直流电源转换器.docx
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隔离型直流电源转换器.docx
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隔离型直流电源转换器
隔離型直流電源轉換器
三種本DC-DC轉換器之架構:
buck,boos,buck-boost適用於:
無隔離情況
輸出、輸入電壓倍數相差不大之情況上述情況若無法滿足時,需使用隔離型轉換器常見隔離刑轉換器有:
馳返式(flyback)
順向氏(forward)
推挽式(push-pull)
半波橋式(halfbridge)
全波橋式(fullbridge)
馳返式(flyback)
flybacktopologyshownas:
Itiswidelyusedforoutputpowersfromabout150downtounder5W
Ithasnosecondaryoutputinductors,suchthatthecostandvolumeoftheoutputinductorsisasignificantadvantage.
TwooperationmodesareusedCCMandDCMmode
CCM:
Thecurrentoftheprimaryisalwayslargerthanzero.
DCM:
Thecurrentoftheprimarywillbereducedtozero
Themajortopicistodesignthetransformerselectatransistordiodeandcapacitor
tosatisfythespec.
Np
Ns
TheoperationofDCMmode:
showsasfollows
Step1:
Theturnsratioofprimaryandsecondary
WhenQison,thedroppedvoltageacrosstheprimaryisVL=V,-1.Fromthe
ohms-law:
Vl7-1-Ldi(t)。
So,thei(t)canbedescribedasdt
VI-1
i(t)ti(0)。
Iftheflybackisoperatedatdiscontinuousmode,i(0)=0.So,
i(t)二
VI-1
t.ForfixedcurrentIp,theminimizeinputvoltageVI(min)willinduce
andthecircuitmustberemainedindiscontinuousmode.%*Tr■Ttd=T
二T.Tr=0.8T
12W12
WbLpIpjoulesSo,theinputpower〒盲Lp|p。
Now,the
efficiencyisassumedto80%,wehave
Step4:
Theprimarywireturns
Step5:
Thesecondarywireturns
Step6:
Theprimaryrmscurrentandwiresizemustbecalculated.
T°nandthe
1i|
FromthermsformulaoftheprimaryisIprms:
一Vi2(t)dt=P.
i2(t)dJ3W
<3\TN
Ts'3kT
rmsformulaofthesecondaryisIsrms
Andwiresizeisspecifiedby300~500circularmilesperrmsampere(itjustonlysuggestion
step7:
consideredtheskineffect.
thanskindepth,morewirenumberswillbebetterasinglelargerwiresize.
Step8:
determinethecapacitor
Duringthetransistorontime,theloadcurrentissuppliedbythecapacitor.Theripple
chargeQis:
:
^^IOTon-T°n=C:
V。
So,thevalueofthecapacitoris
RO
C=,whereVistherippleofoutputvoltage.
DCMtoCCMtransition
■Whentheoutputvoltagedecreas(theoutputloadincreasetheoutputresistancedecreasethepeakcurrentofprimarycurrentIpwillbeincreasedfrompointB”toD”.AndtheontimeofthetransistorwillincreasefrompointC”oE”
■Atabovecondition,theofftimeisreducedfromf'”toL”.Thestartcurrentofofftime”isincreasedfromH”toK”.Fromabovefigure,thedeadtimewillbereducedfromTdttozero.ThiscaseistheboundaryofDCM/CCM.
■Iftheloadisstillincreased,theontimewillbeincreased.So,thepeakcurrentofprimaryIPwillbefurthergrowthandofftimewillbedecreased.Whentheontimeofnextcyclestarts,theprimarycurrentIPcan'reducetozero.TheCCMmodeisoperated.
TheoperationofCCMmode:
showsasfollows
Step1:
Theturnsrationofprimaryandsecondary
WhenQisoffstate,thediodeatsecondarysideisturnonandthemirrorvoltagewillreflecttotheprimarysideVP二NpVs.ThevoltageVS二V。
1.Thatis,
Ns
VP二Np(Vo1).So,thetransistorissustainedthevoltagestresVms
Ns
Vms二V|Vp二V|(max)-NP(Vo1)
Ns
Step2:
determinethemaximizeontime
Thecoremustbeguaranteednottosaturated.Thevoltage-timeproductof
ND
6n-time”mustbeequaltooff-time”.Thatis,Vi(minTon二Vo1PToff
Ns
andthecircuitmustberemainedincontinuousmode.ThatisTonToff=T
NP
So,Vi(minTon=!
Vo1-(T-Ton).Itleadsto
Ns
step3:
DeterminetheinductaneeLPofprimary
亠T
lepr
Ton
|csr
toff
Fromabovefig.theoutputpowerPisequaltotheoutputvoltagetimestheaverage
ToffTon
ofthesecondarycurrentpulsesangivenPo=V°Gr〒=V°Icsr(1-〒).Thatis,
(V|(min)T)
dlDCMmodeandCCMmode,thefollowingconditionissatisfiedIcpr
Step4:
Theprimaryrmscurrentandwiresizemustbecalculated.
Andwiresizeisspecifiedby300~500circularmilesperrmsampere
:
」P=300lrmsp。
Fromthermsformulaofsecondary
(亍、T,
V。
1上T
\T丿
step5:
consideredtheskineffect.
o2837
skindepthS二.So,whentheprimaryandsecondarywiremilesarelarger
thanskindepth,morewirenumberswillbebetterasinglelargerwiresizeStep6:
Theprimarywireturns
N一(Wmax一1)hn10*p-Ae
ABo
Vp=V|(max)^NPAeT-108
Vo1
Ion
andNs=Np
spV|(max)-1
Step7:
determinethecapacitor
Duringthetransistorontime,theloadcurrentissuppliedbythecapacitor.Theripple
VO-
chargeQis:
:
Q=lOT°n-T°n=CV。
So,thevalueofthecapacitoris
VOTon
VRo
whereVistherippleofoutputvoltage.
順向氏(forward)
Forwardtopologyshownas:
DV
Itwidelyusedforoutputpowerunder200WwhemmaximumDCinputvoltageisintherangeof60Vto200V
Whentheinputvoltagebelow60Vtheprimaryinputcurrentbecomes
uncomfortablylarge.
Whentheinputvoltageabove200Vthemaximumvoltagestressonthetransistorbecomestolarge.
Comparedtothepush-pulltopology,itismoreeconomicalduetoontransistorbeused.
WhenQ1isturnedon:
CurrentandpowerflowsintothedotendofNp1.AllrectifiersdiodeD2toD4areforward-biasandcurrentandpowersflowstotheloadthroughLCfilter.
V°mr=[(Vdc-1)N^'-Vd2
.Np」
Thevolt-secondproductistheareaA=VdcT°n,duetothefluxchange
WhenQ1off:
ThevoltageacrossNrandNpisVdcandthevoltageattheno-dotendofNpandattheQ1isthen2Vdc.
Forthevolt-secondsareaproductA2isequaltoareaA1.
Inthecommonexpress,theresetvolt-secondshasequaltothesetvolt-seconds.
Therectifierdiodesd2,D3,D4areoffstate,
Thefree-wheelingdiodesD5,D6,D7areonstate.
•Themajortopicistodesignthetransformerselectatransistordiodeandcapacitortosatisfythespec
Step1:
Theturnsrationofprimaryandsecondary
WhenQison,thedroppedvoltageacrosstheprimaryisV_=V|-1.
ThevoltageofthecathodeofD2isthemirroroftheprimaryvoltage
Vomr=(Vi-1-Vd2
INp
TheoutputvoltageistheaverageofVomrthroughthe(L,C)filter.So,
|nordertoensurethecoreinthenon-saturation,theontimeshouldbelesstha5Tfortheminimuminputvoltage.Thatis,
Thedropvoltageofthediodeisassumedas1.V
Step2:
Theprimarywireturnsandsecondaryturns
d二
FromtheFaradayslawV=E.dl.Wehave
dt
5
Vo1
2
andNsNp
Vi(min)-1
Step3:
Thepeakflat-topcurrentofprimary
Thedutycycleoftheprimarypulseis—T/T二0.4。
Theefficiencyofforwardis
..2
JJIZIL
5
thefunctionofL,Cfilteristoaveragethevoltageattherectifiercathodes.Theinductorwillguaranteetheconverterinthecontinuousmod(CCM).Thecapacitorisselectedtoyieldaminimumspecifiedoutputripplevoltage.Consideredtheboundarycondition,thefollowingissatisfied
1
IdcdI.So,
2
dI
Vl=Vrk-Vo=L.WhereVrkisthevoltageoftheanodenodeofdiode
—Ton
Fromthefact「thefunctionofL,Cfilteristoaveragethevoltageatthe
rectifiercathodes」
T-
wehaveVo=Vrk21andT°n=0.4T.Theinductoris
T
*Theripplealmostdeterminedbytheequivalentseriesresistance^ofthecapacitor.Thepeak-to-peakrippleVor=Rdl
ThealuminumelectrolyticcapacitorhasthecharacteristicRC。
=6510"
sv/itchingfrequ&ncy
Zerocurrent
推挽式(push-pull)topology
push-pulltopologyshownas
ItconsistsofatransformerT1,withmultiplesecondaries
Twoequalandadjustable-width180outofphasearesuppliedtodrivetheswitchofQ1,Q2FeedbackloopisclosedaroundVsmandToniscontrolledtoregulateditagainstlineandloadchanges.
TheoutputvoltageVm,Vs1,Vs2are
switchingftequency
AsVdcvaries,thenegative-feedbackloopcorrectsToninsuchadirectionaastokeepVmconstant.
Slavesareregulatedagainstlinebutonlypartiallyagainstload.
Half-bridgetopology
Half-bridgetopologyshownas:
ItsmajoradvantageisthatitsubjectstheofftransistortoonlyVdc.
Thecircuitalwaysyieldroughly320VofrectifiedDcvoltagewhetherinputis120or220Vac
WhenQ1isonandQ2off,thedotendofNpis168Vpositivewithrespecttoitsno-dotendandtheoffstressonQ2isonly336V
WhenQ2isonandQ1off,thedotendofNpis168Vnegativewithrespecttoitsno-dotendandQ1is336Vreversed.
Full-bridgetopology:
Fullbridgetopologyshownas:
ThevoltageimpressedacrosstheprimaryoffullbridgeisVdccomparedto0.5Vdcforthehalf-bridge
ThefullbridgecandelivertwicethepowerofthehalfbridgeatsameconditionDiagonallyoppositetransistors(Q2andQ3orQ4andQ1areturnedonsimultaneouslyduringalternatecycle.
Theoutputvoltagesareshown:
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- 关 键 词:
- 隔离 直流电源 转换器