1. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 1 Lecture 37 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

2. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 2 Lecture 37 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

3. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 3 Lecture 37 The active-clamp forward converter Better transistor/transformer utilization ZVS Not limited to D < 0.5 Transistors are driven in usual half-bridge manner:

4. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 4 Lecture 37 Approximate analysis: ignore resonant transitions, dead times, and resonant elements

5. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 5 Lecture 37 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.

6. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 6 Lecture 37 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

7. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 7 Lecture 37 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

8. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 8 Lecture 37 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

9. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 9 Lecture 37 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

10. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 10 Lecture 37 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

11. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 11 Lecture 37 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)

12. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 12 Lecture 37 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.

13. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 13 Lecture 37 Active clamp circuit on the primary side of the flyback converter

14. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 14 Lecture 37 Active clamp to snub the secondary-side diodes of the ZVT phase-shifted full bridge converter

15. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 15 Lecture 37 Active clamp forward converter

16. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 16 Lecture 37 Waveforms (including L l )

17. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 17 Lecture 37 Details: different modes

18. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 18 Lecture 37

19. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 19 Lecture 37 About L l