Time-Domain Analysis of a Phase-Shift-Modulated Series Resonant Converter with an Adaptive Passive Auxiliary Circuit

This paper presents a comprehensive time-domain analysis of series resonant converters operating with phase-shift-modulated full-bridge above resonance. Closed-form formulas for all quantities are derived using two methods: the commonly used fundamental harmonic approximation as well as a precise time-domain analysis considering the effect of all the harmonics. Detailed analytical method describes steady-state behavior of the converter in three mutually exclusive and collectively exhaustive modes of operation based on continuity of the resonant inductor current: a discontinuous mode and two continuous modes. The difference between two continuous modes is in the existence of natural zero-voltage switching in the leading leg of the full bridge. Quantitative predictions of the key quantities from two methods are compared and the accuracy of the first harmonic approximation is examined. The time-domain approach provides useful insights for design considerations with no need to know the load value. It precisely determines closed-form equations for the boundary conditions of the three operation modes. An adaptive passive auxiliary circuit is suggested to guarantee zero-voltage switching for the entire operating conditions. Experimental results from a 100 W prototype confirm the predicted time-domain behavior and achievement of soft switching using the proposed auxiliary circuit.