Phase-Shift Control with Unified PWM/PFM for Improved Transient Response in a Bidirectional Dual-Active-Bridge DC/DC Converter

Determining how to efficiently obtain superior transient response under both unidirectional power-changing and power-reversing operations in a bidirectional dual-active-bridge (DAB) dc/dc converter is a challenging problem. The traditional direct phase-shifting scheme would induce a transient dc current bias in the high-frequency link of the DAB, and therefore lead to some adverse issues, such as magnetic saturation and increased current stress. In addition, the ac transient can propagate to the dc terminates and give rise to a worse transient response in a closed-loop control system. In this study, a fast phase-shift control is proposed which can execute all the possible power changes in a unified manner. By properly operating the primary bridge with pulsewidth modulation (PWM) and the secondary bridge with pulse frequency modulation (PFM), the DAB converter can complete the phase-shifting operation fast without a dc current bias in the ac link. A closed-loop control based on the hybrid PWM/PFM is further designed to regulate the primary dc current with improved transient performance. Since both PWM and PFM are standard, the whole control algorithm can be implemented efficiently in practical applications. Experimental results are provided to show that the power of the DAB converter can be governed bidirectionally with the proposed phase-shift control. The dc terminal current can be changed to both polarities with fast transient and no apparent overshoot or oscillation.