ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 1 Lectures 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

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

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

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 4 Lectures 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.

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 5 Lectures 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

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 6 Lectures 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

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 7 Lectures 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) = 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

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 8 Lectures 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 = peak v ds = 570 V which is considerably less than 700 V

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

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 10 Lectures 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)

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 11 Lectures 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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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