Power supplies. EE328 Power Electronics Assoc. Prof. Dr. Mutlu BOZTEPE Ege University, Dept. of E&E

Transcription

1 Power supplies EE328 Power Electronics Assoc. Prof. Dr. Mutlu BOZTEPE Ege University, Dept. of E&E

2 EE328 POWER ELECTRONICS Outline of lecture Introduction to power supplies Modelling a power transformer Analysis method of converters including a transformer Steady-state analysis of Flyback dc-dc converter Forward dc-dc converter Push-pull dc-dc converter Full-bridge dc-dc converter Half-bridge dc-dc converter 2

3 Electrical isolation requirement A basic disadvantage of the dc-dc converters (buck, boost etc.) is the electrical connection between the input and the output. If the input supply is grounded, that same ground will be present on the output. A way to isolate the output from the input electrically is with a transformer. If the dc-dc converter has a first stage that rectifies an ac power source to dc, a transformer could be used on the ac side. However, not all applications require ac to dc conversion as a first stage. Moreover, a transformer operating at a low frequency (50 or 60 Hz) requires a large magnetic core and is therefore relatively large, heavy, and expensive. 3

4 Electrical isolation requirement A more efficient method of providing electrical isolation between input and output of a dc-dc converter is to use a transformer in the switching scheme. The switching frequency is much greater than the ac power-source frequency, enabling the transformer to be small. Additionally, the transformer turns ratio provides increased design flexibility in the overall relationship between the input and the output of the converter. With the use of multiple transformer windings, switching converters can be designed to provide multiple output voltages. 4

5 Ideal transformer model (a) Transformer; (b) Ideal model 5

6 Real transformer model (c) Complete model 6

7 Most used transformer model The leakage inductances L1 and L2 are usually not crucial to the general operation of the power electronics circuits described in this chapter, but they are important when considering switching transients. Magnetic core reset is important! The average voltage of Lm must be zero! Otherwise the transformer saturates! 7

8 THE FLYBACK CONVERTER Note the transformer winding direction!!! 8

9 Assumptions for the analysis 1. The output capacitor is very large, resulting in a constant output voltage Vo. 2. The circuit is operating in the steady state, implying that all voltages an currents are periodic, beginning and ending at the same points over one switching period. 3. The duty ratio of the switch is D, being closed for time DT and open (1-D)T. 4. The switch and diode are ideal. 9

10 Analysis for the Switch ON On the source side of the transformer 10

11 Analysis for the Switch OFF 11

12 Output voltage Since the net change in inductor current must be zero over one period for steady-state operation 12

13 Switch withstand voltage Note that vsw, the voltage across the open switch, is greater than the source voltage. 13

14 Average magnetizing current Substituting Substituting 14

15 Min&max value of I Lm Continues current operation requires that I Lm,min >0 At the boundary between CCM and DCM; I Lm,min =0 15

16 Output voltage ripple The output configuration for the flyback converter is the same as for the buck-boost converter, so the output ripple voltages for the two converters are also the same. Buck-boost 16

17 EXAMPLE 7-1 Flyback Converter 17

18 EXAMPLE 7-2 Homework!! Flyback converter design 18

19 FORWARD CONVERTER Note that transformer winding direction!!! 19

20 Analysis for the Switch ON 20

21 Analysis for the Switch OFF 21

22 Output voltage 22

23 Transformer reset When switch is on 23

24 Transformer reset When switch is off Slope for 24

25 Transformer reset When switch is on When switch is off By combining these two equations, time duration of Tx can be found as 25

26 Transformer reset For proper resetting of transformer it should be t 0 <T Result in For example, if the ratio N3/N1=1 (a common practice), then the duty ratio D must be less than

27 Some waveforms 27

28 Some waveforms 28

29 Output voltage ripple The circuit configuration on the output of the forward converter is the same as that for the buck converter, so the output voltage ripple based on an ideal capacitance is also the same. EXAMPLE 7-4 AND 7-5 ARE HOMEWORK The equivalent series resistance of the capacitor often dominates the output voltage ripple. The peak-to-peak voltage variation due to the ESR is 29

30 THE PUSH-PULL CONVERTER 30

31 THE PUSH-PULL CONVERTER 31

32 Sw1 is ON 32

33 Sw2 is ON Same with the previous one 33

34 Both switches are OFF 34

35 Output voltage the net change in inductor current over one period must be zero for steady state operation, Solving for Vo 35

36 Output voltage ripple Ripple voltage on the output is derived in a manner similar to the buck converter. The output ripple for the push-pull converter is EXAMPLE 7-6 ARE HOMEWORK As with the other converters analyzed previously, the equivalent series resistance of the capacitor is usually responsible for most of the voltage output ripple. Recognizing that and using 36

37 No reset winding!!! THE FULL BRIDGE CONVERTER 37

38 Note that the maximum voltage across an open switch for the full-bridge converter is Vs, rather than 2Vs as for the push-pull and single-ended forward converters. 38

39 THE HALF BRIDGE CONVERTER 39

40 40

41 Multiple output flyback 41

42 Multiple output forward 42