1. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 1 Lectures 39-40 Zero-voltage transition converters The phase-shifted full bridge converter Buck-derived full-bridge converter Zero-voltage switching of each half- bridge section Each half-bridge produces a square wave voltage. Phase-shifted control of converter output A popular converter for server front- end power systems Efficiencies of 90% to 95% regularly attained Controller chips available

2. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 2 Lectures 39-40 Actual waveforms, including resonant transitions

3. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 3 Lectures 39-40 Issues with this converter It’s a good converter for many applications requiring isolation. But… 1.Secondary-side diodes operate with zero-current switching. They require snubbing or other protection to avoid failure associated with avalanche breakdown 2.The resonant transitions reduce the effective duty cycle and conversion ratio. To compensate, the transformer turns ratio must be increased, leading to increased reflected load current in the primary-side elements 3.During the D’Ts interval when both output diodes conduct, inductor Lc stores energy (needed for ZVS to initiate the next DTs interval) and its current circulates around the primary-side elements—causing conduction loss

4. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 4 Lectures 39-40 Result of analysis Basic configuration: full bridge ZVT Phase shift  assumes the role of duty cycle d in converter equations Effective duty cycle is reduced by the resonant transition intervals Reduction in effective duty cycle can be expressed as a function of the form FP ZVT (J), where P ZVT (J) is a negative number similar in magnitude to 1. F is generally pretty small, so that the resonant transitions do not require a substantial fraction of the switching period Circuit looks symmetrical, but the control, and hence the operation, isn’t. One side of bridge loses ZVS before the other.

5. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 5 Lectures 39-40 Effect of ZVT: reduction of effective duty cycle

6. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 6 Lectures 39-40 Phase-shifted control Approximate waveforms and results (as predicted by analysis of the parent hard- switched converter)

7. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 7 Lectures 39-40 Diode switching analysis

8. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 8 Lectures 39-40 Diode commutation: intervals 3 and 4

9. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 9 Lectures 39-40 Waveforms: ZCS of D6

10. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 10 Lectures 39-40 Approaches to snub the diode ringing (a) conventional diode snubber

11. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 11 Lectures 39-40 Approaches to snub the diode ringing (b) conventional passive voltage-clamp snubber

12. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 12 Lectures 39-40 Approaches to snub the diode ringing (c) simplify to one passive voltage-clamp snubber

13. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 13 Lectures 39-40 Approaches to snub the diode ringing (d) improvement of efficiency in voltage-clamp snubber

14. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 14 Lectures 39-40 Approaches to snub the diode ringing (e) active clamp lossless snubber

15. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 15 Lectures 39-40 Approaches to snub the diode ringing (f) primary-side lossless voltage clamp

16. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 16 Lectures 39-40 Another application of the ZVT: DC Transformer Operate at a fixed conversion ratio with high duty cycle, leading to high efficiency—avoids the problems of circulating currents Use other elements in the system to regulate voltage PFC AC line ZVT DC-DCLoad DC-DC Load DC-DC Load isolation 350 V 5 V1 V

17. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 17 Lectures 39-40 Active clamp circuits Can be viewed as a lossless voltage-clamp snubber that employs a current-bidirectional switch See Vinciarelli patent (1982) for use in forward converter Related to other half-bridge ZVS circuits Can be added to the transistor in any PWM converter Not only adds ZVS to forward converter, but also resets transformer better, leading to better transistor utilization than conventional reset circuit

18. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 18 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

19. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 19 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:

20. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 20 Lectures 39-40 Approximate analysis: ignore resonant transitions, dead times, and resonant elements

21. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 21 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.

22. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 22 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

23. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 23 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

24. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 24 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

25. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 25 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

26. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 26 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