1. SWITCH-MODE POWER SUPPLIES AND SYSTEMS Silesian University of Technology Faculty of Automatic Control, Electronics and Computer Sciences Ryszard Siurek Ph.D., El. Eng. Lecture No 7

2. ZZpZZp ZZSZZS T DD1DD1 C RR0RR0 U0U0 U IN C IN Flyback converter  T transfer ratio  exceptional topology comprising transformer and output choke in one magnetic component component  topology with lowest compenet count – cheapest solution IpIpIpIp ITITITIT IDIDIDID I0I0I0I0 ICICICIC ZZpZZp ZZSZZS DD1DD1 C RR0RR0 U0U0 U IN C IN  T IDIDIDID I0I0I0I0 ICICICIC T ITITITIT L Compare to flyback switching regulator

3. Flyback converter basic relations analysis Cycle I - transistor T is ON ZZpZZp ZZSZZS T DD1DD1 C RR0RR0 U0U0 U IN ITITITIT I0I0I0I0 ICICICIC  ITITITIT I pmax B H Magnetic energy stored in the core By the end of cycle I BSBSBSBS Cycle II - transistor T is OFF ZZpZZp ZZSZZS T DD1DD1 C RR0RR0 U IN I T =0 I0I0I0I0 ICICICIC UpUp nU 0  IDIDIDID I Dmax B H BSBSBSBS U0U0 IDIDIDID ’’’’ T Magnetic energy recovered from the core by the end of cycle II

4. From energy balance : (1) (2) U0U0 Ro II0II0 ~ UCUC LSLS I I Dmax i i D (t) From equation (1) : Al - core constant hence: U 0 : U 0 may be calculated also from energy balance: valid only in case of discontinuous flux (current) flow, it means  ’ < T -  At the point of  ’ - I D (  ’) = 0, hence: (3) (4)

5. From equation (4) : IDIDIDID I Dmax ’’’’T U 0 (R 0 ) U’ 0 (R’ 0 < R 0 ) I 0kr  =  0,5  > 0,5 I0I0I0I0  < 0,5 Compare to flyback regulator U0U0 for R 0 I 0cr ) the flux in the core does not decay to 0 – so called „continuous flux flow” starts IDIDIDID ITITITIT   T I Tmax I Dmax I Dmin I Dmax = nI Tmax I Dmin = nI Tmin

6. Real diagrams of flyback converters ZZpZZp ZZSZZS ZZaZZa T D C RR0RR0 U0U0 U IN C IN DDdDDd Z Z p =Z a CsCsCsCs RsRsRsRs DsDsDsDs snubbar circuit for dumping overvoltage spikes and reducing transistor power losses recovery of energy stored in the leakage inductance Advantages: Energy stored in leakage inductace is recovered, transitor voltage does not exceed 2U IN Disadvatages: Complicated and expensive transformer

7. ZZpZZp ZZSZZS T D C RR0RR0 U0U0 U IN C IN CsCs RsRs UpUp U T =U p +U IN 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 2U IN Advantages: Cheaper transformer, lack of extra overvoltage spikes due to residual leakage inductance This topology often used in low power converters up to 100W This topology often used in low power converters up to 100W LLLLLLLL Leakage inductance measurement method LLLLLLLL LLLLLLLL

8. Multi - output coverters ZZpZZp Z Z S1 T DD1DD1 C1C1C1C1 R R 01 U 01 U IN C IN I D1 I 01 Z Z S2 DD2DD2 C2C2C2C2 R R 02 U 02 I D2 I 02 U S1 U S2 Flyback topology In II cycle In II cycle U 01 = U S1 U 02 = U 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. Feedback loop

9. ZZpZZp Z Z S1 ZZaZZa T DD2DD2 DD1DD1 L1L1L1L1 C R R 01 U 01 U IN C IN DDaDDa Z Z S2 DD4DD4 DD3DD3 L2L2L2L2 C R R 02 U 02 Forward converter This relation only valid in case of cotinuous magnetic flux (current) flow in L 1 & L 2

10. ZZpZZp Z Z S1 ZZaZZa T DD2DD2 DD1DD1 L1L1L1L1 C1 R R 01 U 01 U IN C IN DDdDDd Z Z S2 DD4DD4 DD3DD3 L2L2L2L2 C2 R R 02 U 02 Coupled output inductors L1L1L1L1 U 01 U 02 L2L2L2L2 Equivalent output circuit valid for cycle I :

11. Equivalent output circuit valid for cycle II: L1L1L1L1 U 01 U 02 L2L2L2L2 To achieve proper relation between output voltages the following condition must be satisfied : 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.

12. ZZpZZp ZZwZZw ZZdZZd T DD1DD1 DD2DD2 L C RR0RR0 U0U0 U IN C IN DDaDDa Z Z p =Z a DD2DD2 C2C2C2C2 RR2RR2 U2U2 L C2C2C2C2 RR2RR2 U2U2 L2L2L2L2 L2L2L2L2 L C2C2C2C2 RR2RR2 L2L2L2L2 U2U2 U0U0 U0U0 z2z2z2z2 zLzLzLzL The other way of obtaining auxilliary regulated output voltages with low load requirements :