The Hybrid Bidirectional DC/DC Converter: Mutual Control and Stability Analysis

A hybrid bidirectional DC/DC converter (BDC) is proposed as the fundamental DC/DC module in solid-state transformers, which combines a bidirectional LLC converter and a dual-active-bridge (DAB) converter. Integrated with a mutual control scheme, both parts of this hybrid BDC can be unified into an interdependent community. In this hybrid BDC, the LLC converter supports the output voltage and improves stability by working at the resonant frequency mode and the DAB converter enhances the BDC power capability by controlling the LLC output current constant. The BDC can achieve the full-load-range soft switching of all active switches by designing the auxiliary inductor of LLC and the minimum output current of DAB. By comparing to the single DAB, the proposed BDC has the higher phase and gain margin which means the BDC improved the relative stability based on Nyquist criterion. To solve the bidirectional power control problem, a dead-band voltage control logic is adopted which can determine the BDC's power direction based on the output voltage change. A 200 V experimental system has verified the aforementioned features and functions of the BDC.