Analysis and Design of Active Snubber of A Step-up Phase Shifted Full Bridge DC-DC Converter Considering Parasitics

In this paper design of an active snubber of a step-up phase shifted full bridge (PSFB) DC-DC converter is presented. In a step-up PSFB, parasitics like transformer leakage inductance and secondary rectifier diode capacitances impacts the converter operation significantly. Rectifier output voltage overshoot due to these parasitics at the end of free-wheeling to power transfer state transition is well known. To reduce the voltage stress of the rectifier diodes, active snubbers are employed. The snubber switch incurs significant loss. This paper proposes a switching strategy of an active snubber, consists of a switch in series with a capacitor, where the device is low frequency switched and hence reducing loss. Moreover zero voltage turn ON of the snubber switch is achieved. A systematic procedure is presented to select the snubber switch frequency, clamp voltage and snubber capacitance. A 1.5 kW, 400V-1.25V step-up PSFB is built and tested with the active snubber. Key experimental results are presented to verify the proposed design and analysis of the active snubber.