Reexamining Phase-shifted Full-bridge Converter with PWM Ripple Current

Phase-shifted full-bridge (PSFB) converter is a widely popular topology in dc-dc conversion applications due to its attractive features of isolation, constant frequency operation and soft-switching of the primary devices without requiring additional support circuitry. Typically, a ripple-free filter inductor current is assumed for simplifying the analysis, which is valid only for large sized output filters. In high-power-density converters, particularly in the soft-switched high-frequency dc-dc topologies, small-sized output filters are desirable. In such systems, where there is notable PWM ripple current, such a simplifying assumption for converter analysis is not valid. In this paper, this aspect is explored and impact of the PWM ripple on PSFB design and soft-switching operation are quantitatively analysed. Detailed analysis of the dependency of output voltage and soft-switching characteristics on PWM ripple and other circuit parameters such as leakage inductance and switching frequency and load are presented. The deviations in behavior from the conventional ripple-free assumption are presented. Simulation results for a 2kW, 400V to 48V converter along with experimental results on a reduced-rated PSFB converter are presented to validate the proposed analysis.