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SWITCH-MODE POWER SUPPLIES AND SYSTEMS Lecture No 7 Silesian University of Technology Faculty of Automatic Control, Electronics and Computer Sciences Ryszard Siurek Ph.D., El. Eng. Flyback converter Ip UIN Zp t I0 IC C ZS R0 U0 IT CIN D1 ID T T Zp n ZS transfer ratio exceptional topology comprising transformer and output choke in one magnetic component topology with lowest compenet count – cheapest solution L IT T UIN Zp ID ZS D1 I0 IC C R0 U0 CIN t T Compare to flyback switching regulator Flyback converter basic relations analysis Cycle I UIN - transistor T is ON D1 Zp ZS UIN n I0 IC C iT (t) IT UIN t Lp Ipmax U0 R0 t IT B BS T Magnetic energy stored in the core By the end of cycle I Cycle II - transistor T is OFF ID D 1 dUp UIN nU0 2 LpIpmax E1 2 ZS U0 Zp H ID I0 IC C U0 iD (t) IDmax IDmax R0 t U0 t LS T t’ B BS IT=0 T Magnetic energy recovered from the core by the end of cycle II 2 LSIDmax E2 2 H From energy balance : 2 2 LpIpmax LSIDmax E1 E2 2 2 IDmax (1) U iD (t) IDmax 0 t LS LS (2) I0 iD(t) ~ UC Ro U0 From equation (1) : IDmax Ipmax Lp LS hence: IDmax Ipmax L Al z2 but Al - core constant Zp I n ZS pmax U0 may be calculated also from energy balance: 2 LpR 0 LpIpmax U20 T U0 Ipmax 2 R0 2T At the point of t’ U 0 IDmax 0 t' LS t' IDmaxL S U0 ID(t’) = 0, Ipmaxn U0 Lp n2 (3) hence: IpmaxLp nU0 (4) valid only in case of discontinuous flux (current) flow, it means t’ < T - t U0 t' From equation (4) : ID IDmax U0(R0) U0 U’0(R’0 < R0) LpR 0 t’ T t U0 Ipmax for R0 < R0cr (I0 > I0cr) the flux in the core does 2T not decay to 0 – so called „continuous flux U0 flow” starts U IN n 2 g2 2LfI0 Compare to flyback regulator U0 UIN g n 1 g IT g > 0,5 ITmax g = 0,5 t ID g < 0,5 T IDmax = nITmax IDmax IDmin = nITmin I0kr I0 IDmin from the condition : IT n ID U0 UIN g n 1 g Real diagrams of flyback converters recovery of energy stored in the leakage inductance Dd Zp=Za Zp CIN ZS D C R0 U0 Za UIN Cs T Ds Rs snubbar circuit for dumping overvoltage spikes and reducing transistor power losses Advantages: Energy stored in leakage inductace is recovered, transitor voltage does not exceed 2UIN Disadvatages: Complicated and expensive transformer LL D LL Rs Cs Leakage inductance measurement method LL Up Zp ZS C R0 U0 UIN CIN T UT=Up+UIN 2 Up2 LLIpmax T 2 Rs Up Ipmax LLR s 2T Disadvatages: Energy stored in the leakage inductance is dissipated in resistor Rs, lower efficiency, necessity of power resistor utilisation, component heating, possibility of transitor voltage higher than 2UIN Advantages: Cheaper transformer, lack of extra overvoltage spikes due to residual leakage inductance This topology often used in low power converters up to 100W Multi - output coverters Flyback topology ID1 D1 I01 ZS1 US1 ID2 Zp UIN R01 U01 C1 D2 I02 T CIN ZS2 US2 C2 R02 U02 Feedback loop In II cycle U01 = US1 U02 = US2 U01 U02 US1 US2 z S1 z S2 In this topology output voltages are dependent only on the secondary numbers of turns. In case of perfect magnetic coupling only one output voltage may be regulated to obtain the regulation of other outputs. Valid for discontinuous as well as continuous current flow One of the cheapest and simple solution delivering several regulated output voltages. Forward converter D1 Da L1 D2 ZS1 C R01 U01 C R02 U02 Zp CIN D3 Za L2 UIN D4 ZS2 T n1 U01 U02 UIN n1 UIN n2 g g zp z S1 and n2 zp z S2 U01 n z 2 S1 n12 U02 n1 z S2 This relation only valid in case of cotinuous magnetic flux (current) flow in L1 & L2 Coupled output inductors D1 Dd ZS1 L1 D2 C1 R01 U01 C2 R02 U02 Zp CIN D3 Za L2 UIN ZS2 D4 T Equivalent output circuit valid for cycle I : L1 UIN n1 UIN n2 U01 g ' UL1 ' UL2 L2 U02 g UIN n1 UIN n2 ' UIN U UIN g UIN UIN 1 g UL1 01 n1 n1 n1 n1 ' UIN U UIN g UIN UIN 1 g UL2 02 n2 n2 n2 n2 U' z L1 n2 S1 n1 z S2 U' L2 Equivalent output circuit valid for cycle II: L1 U01 U"L1 U"L1 U"L2 L2 g " U02 UL2 UIN U' U z L1 01 n1 n2 S1 U02 g UIN n1 z S2 U' L2 n2 To achieve proper relation between output voltages the following condition must be satisfied : U' U" z z S1 z L1 L1 S1 L1 z S2 z S2 zL2 U' U" L2 L2 In real circuit: diode voltage drop and nonlinear diode characteristics have significant influence on output voltages - - influece of winding resistances - significant influence of leakage (poor winding coupling) Detailed relations of turns number for particular windings are usually set by the way of experiment in practice – equations presented above give only the rough approximation. The other way of obtaining auxilliary regulated output voltages with low load requirements : D2 Da D2 Zp=Za C2 L2 R2 U2 L Zp CIN D1 Zw C R0 U0 Zd UIN T C2 L2 UIN n R2 U2 z2 L2 L U0 zL L C2 U0 R2 U2 U2 z2 zL U0