Active Clamp Converter with Full Resonant Switching

The active clamp DC-DC converter topology efficiently resets the flux in the power transformer with an active switch and a clamp capacitor and provides low peak-inverse voltage for the output rectifiers over a wide operating range of input voltage. But like other isolated power conversion topologies, this topology loses zero-voltage switching (ZVS) for one of the switching transitions, requiring a switch to close across a low impedance with significant voltage. The loss of ZVS is analyzed, and it is shown that including a small inductor in series with the primary winding of the transformer and two clamping diodes can restore ZVS at any load current. A balancing resistor coupled to the clamp capacitor recovers nearly all the recycled energy from the added inductor