Analysis and implementation of a zero-voltage switching forward converter with a synchronous rectifier

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 V_rms, 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.