The system analysis, circuit design and implementation of an active-clamp forward converter with a synchronous rectifier is presented. Different to a conventional forward converter, there is an auxiliary switch in the active-clamp forward converter. The clamp circuit is used to reset the energy stored in the leakage inductor in order to minimise the spike voltage at the transformer's primary side. Thus, the voltage stress on the main switch can be reduced. A resonant circuit based on the output capacitor and leakage inductor of the transformer is able to achieve a zero-voltage switching turn-on for both the main and auxiliary switches which will increase the circuit efficiency. A synchronous rectifier is used at the transformer's secondary side to further reduce the conduction losses. The operational principles of the active-clamp forward converter are analysed in detail and the circuit performance is compared with that of a conventional forward converter. The design procedure and an example of an active-clamp forward converter are presented. Finally experimental results are presented for a converter with an AC input voltage of 90∼130 Vrms, an output voltage of 5 V/20 A operating at a switching frequency of 150 kHz which verify the zero-voltage switching at turn-on.
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