反激变压器设计(DCM模式)Flybacktransformer.docx

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1、反激变压器设计DCM 模式Flyback transformer designDCMModeIllustration：Step 1：The turns ratio of primary and secondaryWhen Q is on, the dropped voltage across the primary is V= V - 1 . From theLIohms-lawV= VLI- 1 = L di( t )dtV -1So, the i( t )can be described asi( t ) =ILt + i( 0 )IF the flyback is operated at

2、 discontinuous mode, i( 0 ) = 0 . So,V -1i( t ) =It . It leads toIV- 1=I (m i n )TLPLonP1When Q is off state, the diode at secondary side is turn on and the mirror voltage will reflect to the primary sideNN2(V=P V PNSS. The voltage VS= V + 1 . That is V=oPPV + 1) .NoSSo, the transistor is sustained

3、the voltage stress VmsN(V= VmsI+V= VP+I (m ax)NPV + 1)oSThe turn ratio ofNVNP =ms-VI ( m ax)V + 1SoStep 2：The core must be guaranteed not to saturated. The voltage-time product of“on-time” must be equal to“off-time”. That is()NVT= V + 1P Ti(m i n ) onoNrSand the circuit must be remained in discontin

4、uous mode.T+ T + Tonrtd= T T+ Tonr)(.)N= 0.8T ( VI (m i n )-1 Ton= V +1oP0 8T - TNonS( VI (m i n )-1+ V(oN+1) NPST= (V)ono+1)NPNS0.8T()V + 1oNNP 0.8Tso,T=onVS- 1 + ( VN+ 1) NPI (m i n )oSstep 3: Determine the inductanceLP of primaryWhen the transistor is “on”, the energy storage in the primary is eq

5、ual to1 W1W =L I 2joules .So, the input powerP=L I 22 P PinT2TPPnow, the efficiency is assumed to 80%, we have11 V-121( V- 1) 2P= 1.25P=L I 2 =L I (m i n )T=LI (m i n )T 2ino2TPP2TPLPon 2TPL2on PL= (VI (min) - 1)2 T 2That isPono2.5P TStep 4： The primary wire turns()V= V- 1 = NA DB 10-8VN=I (m ax)-1

6、Ton 108PI (m ax)Pe TonPA DBeStep 5: The secondary wire turnsVI ( m ax)- 1N=PV + 1NoS1i 2 (t)dt TIP3TonTStep 6：The primary rms current and wire size must be calculated.From the rms formula of the primary isI=prms=and therms formula of the secondary isI=1i2 (t)dt TIP3T - TNonpTNssrmsAnd wire size is s

7、pecified by 300500 circular miles per rms ampereIP3TonTF= 300PIP30.8T - TNonPTNSF= 300Sstep 5: considered the skin effect.2837fskin depthS =. So, when the primary and secondary wire miles are largerthan skin depth, more wire numbers will be better a single larger wire sizefix( F s )s1FThe parallel o

8、f primary wire numbers aren=sand the secondary wirefix( F s )s1Fnumbers aren=。sFlyback transformer designCCMMode Illustration：Step 1：The turns ration of primary and secondaryWhen Q is off state, the diode at secondary side is turn on and the mirror voltage will reflect to the primary sideNNV=P V. Th

9、e voltage V= V + 1 . That is V=P ( V + 1) .PNS SSoPNoSSo, the transistor is sustained the voltage stress VmsNV= VmsI+V= VP+I (m ax)P ( VNoS+ 1)Step 2：The core must be guaranteed not to saturated. The voltage-time product of“on-time” must be equal to“off-time”. That is() NVT= V + 1P Ti(m i n ) onoNSo

10、ffand the circuit must be remained in continuous mode. That is, Ton+ T= ToffSo,VT= (V+ 1)NP ( T - T) .i(m i n ) onoNS( V + 1)oonNP TNIt leads toTon=VI (m ax)+ ( Vos+ 1) NPNSstep 3: Determine the inductanceLof primaryPTIcprIcsrTontoffFrom above fig. the output power is givenP = VooIcsrToffT= V IocsrT

11、( 1 -on )That isIT=csrVPoT1-on The input power is giveno T P= 1.25Pino= ( VI (m i n )- 1) IcprTonThat isIT=cpr1.25Po T( V-1) on I (m i n ) T Considered the boundary condition between the DCM mode and CCM mode, the following condition is satisfieddII=pcpr2PThe slop of the rampdIis given as(dI= VI (m

12、i n )-1)TonPLPSo, the inductance of the primary is(VL=I (mi n )-1) 2 T 2onP2.5P TinStep 4：The primary rms current and wire size must be calculated.From the rms formula of primaryIprms= Icpron =TT1.25PToon T T(V- 1)on I (min)And wire size is specified by 300500 circular miles per rms ampereF= 300IPrm

13、sp T From the rms formula of primaryI= Ioff =PToffTormsscsrTT TV 1-on F= 300ISrmssstep 5: considered the skin effect.2837o T fskin depthS =. So, when the primary and secondary wire miles are largerthan skin depth, more wire numbers will be better a single larger wire sizeFThe parallel of primary wire numbers arens = fix(F s ) and the secondary wires1Fnumbers aren= fix(s ) 。sFs1Step 6： The primary wire turnsV= V- 1 = NA DB 10-8(V=NIm ax- 1) Ton108PI (max)Pe TonV+ 1pDB AeandNs= NpoVI (max) -1