1. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 1 Lectures 39-40 The conventional forward converter Max v ds = 2V g + ringing Limited to D < 0.5 On-state transistor current is P/DV g Magnetizing current must operate in DCM Peak transistor voltage occurs during transformer reset Could reset the transformer with less voltage if interval 3 were reduced

2. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 2 Lectures 39-40 The active-clamp forward converter Better transistor/transformer utilization ZVS Not limited to D < 0.5 Transistors are driven in usual half-bridge manner:

3. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 3 Lectures 39-40 Approximate analysis: ignore resonant transitions, dead times, and resonant elements

4. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 4 Lectures 39-40 Charge balance V b can be viewed as a flyback converter output. By use of a current-bidirectional switch, there is no DCM, and L M operates in CCM.

5. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 5 Lectures 39-40 Peak transistor voltage Max v ds = V g + V b = V g /D’ which is less than the conventional value of 2 V g when D > 0.5 This can be used to considerable advantage in practical applications where there is a specified range of V g

6. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 6 Lectures 39-40 Design example 270 V ≤ V g ≤ 350 V max P load = P = 200 W Compare designs using conventional 1:1 reset winding and using active clamp circuit

7. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 7 Lectures 39-40 Conventional case Peak v ds = 2V g + ringing = 700 V + ringing Let’s let max D = 0.5 (at V g = 270 V), which is optimistic Then min D (at V g = 350 V) is (0.5)(270)/(350) = 0.3857 The on-state transistor current, neglecting ripple, is given by  i g  = DnI = Di d-on with P = 200 W = V g  i g  = DV g i d-on So i d-on = P/DV g = (200W) / (0.5)(270 V) = 1.5 A

8. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 8 Lectures 39-40 Active clamp case: scenario #1 Suppose we choose the same turns ratio as in the conventional design. Then the converter operates with the same range of duty cycles, and the on-state transistor current is the same. But the transistor voltage is equal to V g / D’, and is reduced: At V g = 270 V:D = 0.5peak v ds = 540 V At V g = 350 V:D = 0.3857peak v ds = 570 V which is considerably less than 700 V

9. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 9 Lectures 39-40 Active clamp case: scenario #2 Suppose we operate at a higher duty cycle, say, D = 0.5 at V g = 350 V. Then the transistor voltage is equal to V g / D’, and is similar to the conventional design under worst-case conditions: At V g = 270 V:D = 0.648peak v ds = 767 V At V g = 350 V:D = 0.5peak v ds = 700 V But we can use a lower turns ratio that leads to lower reflected current in Q1: i d-on = P/DV g = (200W) / (0.5)(350 V) = 1.15 A Conclusion: the active clamp circuit resets the forward converter transformer better. The designer can use this fact to better optimize the converter, by reducing the transistor blocking voltage or on-state current.

10. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 10 Lectures 39-40 Active clamp circuits: some examples Basic switch network reduces to: (if the blocking capacitor is an ac short circuit, then we obtain alternately switching transistors—original MOSFET plus the auxiliary transistor, in parallel. The tank L and C ring only during the resonant transitions)

11. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 11 Lectures 39-40 Example: addition of active clamp circuit to the boost converter The upper transistor, capacitor C b, and tank inductor are added to the hard- switched PWM boost converter. Semiconductor output capacitances C ds are explicitly included in the basic operation.

12. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 12 Lectures 39-40 Active clamp circuit on the primary side of the flyback converter

13. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 13 Lectures 39-40 Active clamp to snub the secondary-side diodes of the ZVT phase-shifted full bridge converter

14. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 14 Lectures 39-40 Active clamp forward converter

15. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 15 Lectures 39-40 Waveforms (including L l )

16. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 16 Lectures 39-40 Details: different modes

17. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 17 Lectures 39-40

18. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 18 Lectures 39-40 About L l

19. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 19 Lectures 39-40 Definitions

20. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 20 Lectures 39-40 Subinterval 1

21. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 21 Lectures 39-40 Subinterval 2

22. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 22 Lectures 39-40 Subinterval 2

23. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 23 Lectures 39-40 State plane, subinterval 2

24. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 24 Lectures 39-40 Subinterval 3

25. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 25 Lectures 39-40 Subinterval 3: state plane trajectory

26. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 26 Lectures 39-40 Subinterval 4

27. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 27 Lectures 39-40 Subinterval 5

28. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 28 Lectures 39-40 Subinterval 6

29. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 29 Lectures 39-40 State plane trajectory including intervals 5 and 6

30. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 30 Lectures 39-40 Averaging

31. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 31 Lectures 39-40 Averaging

32. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 32 Lectures 39-40 Averaging

33. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 33 Lectures 39-40 Average output voltage

34. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 34 Lectures 39-40 The system of equations that describes this converter page 1

35. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 35 Lectures 39-40 The equations that describe this converter page 2

36. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 36 Lectures 39-40 Results