Analysis of an Active Clamp Forward Converter

This paper studies the detailed circuit operation, mathematical analysis and design example of the active clamp forward converter with synchronous rectifier. There is one auxiliary switch and one clamp capacitor used in the forward converter to recycle the energy stored in the transformer leakage in order to minimize the spike voltage at the transformer primary side. Therefore the voltage stress of main switch can be reduced. The active clamped circuit can also help the main switch to turn on at zero voltage switching using the switch output capacitor and transformer leakage inductance. The synchronous rectifier is used at the secondary side to reduce the conduction losses at the rectifier switches. Therefore the system efficiency can be increased. First the circuit operation and mathematical analysis are discussed. The design example of active clamp forward converter is presented. Finally the experimental results based on the ac input voltage of 110V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</inf> and dc output voltage of 5V prototype operating at rated output load of 20A and a switching frequency of 100kHz are provided to verify the zero voltage switching at turn-on.