Effects of Parasitics on An Active Clamp Assisted Phase Shifted Full Bridge Converter Operation

Abstract

The phase-shifted full bridge (PSFB) converter is widely used for isolated DC-DC applications due to simple control and zero voltage switching (ZVS) capability. The ZVS is achieved with the help of primary bridge device capacitances and transformer leakage inductance. The secondary diode parasitic capacitances cause high voltage stress and affect the converter voltage gain. In literature, several snubber circuits are presented to clamp the voltage overshoot, but the change in voltage gain due to secondary diode capacitances is ignored. In this paper the operation of PSFB with an active clamp is analysed considering all circuit parasitics including transformer leakage inductance, primary device and secondary diode capacitances. A closed form expression of the voltage gain is analytically derived. It is observed that the voltage gain is independent of snubber clamp voltage. Other than the known duty cycle loss due to leakage inductance, the output voltage has a duty cycle gain due to secondary diode capacitance. The paper also presents an estimation technique of key parasitics through experiment. These findings are essential for the design of a PSFB particularly for high-step-up application. A prototype of 1.5kW with input 400V and output 1.25kV is built and tested. Key experimental results are shown to verify the presented analysis of PSFB considering all parasitics.