Dual-bridge series-resonant DC/DC converters (DBSRC) are an excellent choice for applications in which a bidirectional power flow is required, such as the battery-to-bus or battery-to-battery interfaces in ESS. The key features of DBSRCs include symmetrical structure, soft-switching, and step-up or step-down operation. Also, the resonant capacitor can filter the DC current and limit fault currents under abnormal operation.

For high power density and high-efficiency applications, soft switching of all active power devices is mandatory to provide a high efficiency, because hard switching causes additional switching losses. Furthermore, soft-switching reduces the electromagnetic interference by suppressing the rate of the voltage change (dv/dt) and enhances the converter reliability to avoid high-voltage ringing on power transistors.

Soft-switching implementation of DBSRC is especially challenging in the presence of wide I/O voltage variations and load ranges. With the traditional constant frequency phase-shift modulation (PSM), DBSRC are known to experience severe hard switching at light-to-intermediate load, even though optimized design methods are employed. To overcome this problem, advanced techniques, either hardware- or software-based, have been proposed to extend the soft-switching operation range. Hardware-based designs modify the resonant tank to provide the extended zero-voltage switching (ZVS) commutation current by adding extra passive or active auxiliary circuits. These designs require additional components and even gate drive circuitry, leading to an overall increase in the system cost and complexity.

This design proposed varying frequency modulation (VFM) plus phase-shift modulation (PSM) to provide full input range and full load range ZVS.