Analysis, Design and Implementation of an Active Clamp Forward Converter with Synchronous Rectifier

This paper presents the system analysis, circuit design and implementation of an active clamp forward converter with synchronous rectifier. Compared with the conventional forward converter, there is one auxiliary switch in the active clamp 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 resonant circuit based on the output capacitance and leakage inductance of the transformer will achieve zero voltage switching turn-on for both main and auxiliary switches to increase the circuit efficiency. The synchronous rectifier is used at the secondary side to further reduce the conduction losses. The operation principles of the active clamp forward converter are analyzed in detail and the circuit performance is compared with the conventional forward converter. The design procedure and example of active clamp forward converter are presented. Finally experimental results are presented for a converter with an ac input voltage of 90~130V_rms, an output voltage of 5 V/20 A and operating at switching frequency of 150 kHz are provided to verify the zero voltage switching at turn-on.