Boost-Gain Reduction and Resonant-Frequency Limitation on Bidirectional CLLC Resonant Converters Due to Drain-Source Capacitance

The limitation of the drain-source parasitic capacitance of switches on the boost gain of the bidirectional CLLC resonant converters is revealed in this article. It is found that compared with the ideal CLLC resonant converter without considering drain-source capacitance, a tiny drain-source capacitance would lead to a significant reduction in the voltage boost gain within the below-resonant region. Moreover, the influence of drain-source capacitance on boost-gain reduction increases with the increasing resonant frequency and drain-source capacitance of the converter. It indicates that the effect of the drain-source capacitance must be taken into consideration when designing the resonant frequency and resonant tanks of the CLLC resonant converter. This work facilitates the understanding of the operating principle and voltage gain characteristics of CLLC resonant converters, and provides the knowledge base with new rules and methods for parameter design of the CLLC resonant converter. Detailed theoretical analysis has been carried out, and the influence of drain-source capacitance has been verified with both simulation and experimental results. © 2025 IEEE.