Switching Transition Analysis and Optimization for Bidirectional CLLC Resonant DC Transformer

The demand for an isolated bidirectional dc transformer (DCX) is driven by the fast development of the energy storage system, data center power supply, and transportation electrification. Due to zero-voltage switching (ZVS) and small rms current, the open-loop CLLC resonant converter operating at the resonant frequency is considered a promising candidate for DCX with a constant voltage transfer ratio. To solve unsmooth bidirectional power flow and current distortion in the traditional CLLC -DCX with synchronization rectification (SR) modulation, a dual-active-synchronization (DAS) modulation is adopted with identical driving signals on both sides. First, the switching transition of this modulation is fully analyzed with the consideration of large device output capacitances. After comparison of different transitions, a so-called "Sync-ZVS" transition is found more desirable with ZVS, no deadtime conduction loss, and almost load-independent voltage gain. In order to achieve this switching transition, an "Axis and Center Symmetric" (ACS) method is proposed. Based on this method, an overall design procedure of CLLC -DCX with DAS modulation is also proposed. Finally, the "Sync-ZVS" transition and the proposed "ACS" method are both verified by three 750-V/375-V or 750-V/750-V 18-kW 500-kHz prototypes with a 98.7% peak efficiency. This article is accompanied by one video demonstrating the load-changing test.