Topic 9Main contentsLast lecture reviewBasic of Magnetic Forwad Converter
Main contents • Last lecture review • Basic of Magnetic • Forwad Converter Topic 9
1Flyback ConvertersDuring the ton period:the output load is entirelysupplied by the energy storedin the output capacitor.i2=ip=0i(t)=im(t)= Im(O)o<t<ton-(N,/N2)VΦI = Im = I(O)+(10-9)Φ(0)n1D
During the ton period: the output load is entirely supplied by the energy stored in the output capacitor. (10 −9) on m 1 m m L V I ˆ = I ˆ = I (0) + d t o t ton Lm V i1 (t) = im (t) = Im (0) + d t i2 = iD = 0 i D v1 Φ Φ(0) -(N1/N2)Vo Vd toff ton Io t t t D C R T N1 :N2 i1 + Vd - + Vo - i2 + v1 - - v2 + 1 Flyback Converters
During the toff period:the output load is entirely suppliedVaby the energy stored in thetransformer.im(t)= l21Lm(ton<t<T)(10-11)iz(t)=ip(t)(tNi(t) =0-(N/N2)VΦTheflybacktransformerisΦ(0)actually a two-winding inductor,whichstoresandtransfersDenergy
1 (10−11) N2 N L N2 N V ( ) i2 (t) =iD (t) = 1 im (t) (ton t Ts ) m o im (t) = I ˆ − (t − t ) m on i v1 Φ Φ(0) -(N1/N2)Vo Vd toff ton t t D D Io energy. t C R T i1 + Vd - + Vo - i2 N1 :N2 + v1 - - v2 + During the toff period: the output load is entirely supplied by the energy stored in the transformer. i1 (t) =0 The flyback transformer is actually a two-winding inductor, which stores and transfers
In the continuous-conduction modeNi + △i,=0VdVDVa1-DN0A1N2DV.N2(10 -7)VdN, 1- D-(N,/N2)V。dEq. 10-7 shows the dependence (0)of the voltage-ratio on the duty-1ratio D is identical to that in theDBuck-Boost converter
(10 −7) Vd N1 1− D ratio D is identical to that in the i v1 Eq. 10-7 shows the dependence Φ(0) of the voltage-ratio on the duty- Φ -(N /N1 2 )Vo Vd off ton t t t D C R T i1 + Vd - + Vo - i2 N1:N2 + v1 - - v2 + In the continuous-conduction mode i1 + ip =0 = Vd 1− D V1 D 2 D Io Buck-Boost converter. t 1 = 0 N N i1 + i2 Vo = N2 D
TVdN2)V+-Nor-toff-(N,/N2)VΦApplication:Φ(0)low powerlevel below50wD
Application: low power level below 50W. iDΦ Φ(0) -(N 1/N 2)Vo v 1 Vdton t off Io ttt D C R T i 1 +Vd - +Voi2 N1 :N2 +v1 - -v 2+ o d d o N V N NN ) V ) V 12 D , peak 21 T , peak = V + ( V = V + (
ViVIV4LRutontontofontoff水T.TT.VTVTVTUNNIimexiImaxiliImaxi212i212max2mab)Boundarybetweena) discontinuous-c)continuouscontinuousandconductionmodeconductionmodediscontinuousconduction
0 toff Ts 0 N1Uo N2 U + N1Uo d N2 v1 0 a) discontinuousconduction mode Vd 0 Uo N1 N2 U+ N1U d N2 o L1 Ud t L2 I2max − Uot 0 0 b) Boundary between continuous and discontinuous conduction c) continuousconduction mode 0 t 0 L1 Ud t U L2 I2max − o t Ud Uo + N1 N2 Ud 0 0 v1 v1 vT vT vT i1 i1 i2 i1 i2 i2 I2max I1max I1max I1max I2max I2max Ts Ts ton t t on off Vd N1 N Uo 2 on tm toff Vd
2.A practical forward converter→☆we must provide a path for magnetizing current im until core“resets”（磁通复位）to zero fluxeach cycle.Otherwise,im，Bswil“run away"over time and saturate the core.iLLDii2Winding N3-demaanetizingwindings+ VL-V（去磁绕组）本D2Anapproach that allowsthe transformer本 D3magnetic energy to befed back to the inputsupply.Practical forwardconverter
Winding N3 - demagnetizing windings （去磁绕组） An approach that allows the transformer magnetic energy to be fed back to the input supply. + Vd - + Vo - D1 L iL + vL - D2 T N1 N3 N2 i1 i3 i 2 D3 Practical forward converter 2. A practical forward converter → ☆ we must provide a path for magnetizing current im until core “resets”（磁通复位） to zero flux each cycle. Otherwise, im, Bs will “run away” over time and saturate the core
VI本D,NBNVa本D3Practical forward converter01OperationprincipleVDN.(1) During ton period (O<t<ton):D,forward biased and D and D3 reversebiased. Therefore, im and i, increaseClinearlyN2V.1mVi = V.VD2 = V2 =N,0iVL = V2 -V0E-mail:xihuang@bitu.edu'.cn
+Vd - + Vo - D 1 L iL + vL - D 2 TN1 N3 N2 i i 1 3 i 2 D 3 Practical forward converter 1 Operation principle (1) During ton period (0<t<ton): D 1 biased. Therefore, im and i L increase v 10 t d V t on t off S T 0 t 0 tttt t m v T Vo vD2 f o r w ard biase d and D 2 an d D 3 reverse i 10 i m0iL0 − N 1 Vd N 3 N 1 Vd N 2 Vd + N 1 Vd N 3 Vd 2 o E -mail: xjhuang@bjtu.edu 1 .cn 3 3 Vd v L = v − V NN v = Vd N 1 N 2 linearly.v1 = Vd vD2 = v 2 =
+VC2N3N3本 D2aT1本D3VTPractical forwardconverter0VD21mN0ii+Viti2 = i =ILminLNimmN,0iLis= 0ip2=00
v 10 t d V on t t off T S ttttt t m v T Vo 0 vD20i10 im0iL0 d N 3 − N 1 V N 3 N 1 Vd N 2 Vd V + d N 1 V d N Lv i 2 1 T 1 mLmin m i = i = i + N 2 i + L i t 2 = iL = I i3 = 0 iD 2 = 0 t L m = Vd +Vd - +Vo - D 1 L iL + v -L D 2 TN1 N3 N2 i 1 i 3 i 2 D 3 Practical forward converter
(2) During tm period (ton<t<ton+tm): D,reverse biased, D,andD,forwardbiased, and im and i, decrease linearlyN2 1VL =-VV, = -VV2VTNN(10-19)17N3VD2iD2 = iz = ILmaxLiliz = 0i=i=0NIV1mN3V(t-t.omLmmNiL(10-18)E-mail:xihua3ng@bjtu.edu.cn
11 v 10 t d V on t t off T S 0 ttttt t m v T Vo vD20i10 im0iL0 d N 3 − N 1 V N 3 N 1 Vd N 2 Vd V + d N 1 V d ( 2 ) D u rin g t m pe rio d (t o n<t<t o n+t m): D 1 reverse biased, D2 and D 3 forward biased, and im and iL decrease linearly. V NN V L E-mail: xjhua3ng@bjtu.edu.cn N1 Lm (10 -18) d 1 L m i m 2 Lmax on 3 3 L o 3 d 2 1 i 3 = N i m − 3 (t − t o n = ) d t o n iT = i1 = 0 − V o iD2 = i (t - t ) L = I (10 −19) i = 0 NN v = − 1 V N v = − V v = − V v = − N2 V d d