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Single-Stage DAB-LLC Hybrid Bidirectional Converter With Tight Voltage Regulation Under DCX Operation

Working as a dc–dc transformer (DCX), which operates at the resonant frequency, is a suitable solution to achieve the maximum conversion efficiency of the LLC converter. However, because the fixed-frequency modulation is applied, the voltage regulation performance is limited. To achieve tight voltage regulation and fully utilize the advantages of LLC-DCX operation under bidirectional isolated power application, in this article, a single-stage dual active bridge (DAB) LLC hybrid bidirectional converter based on the sigma converter structure is proposed. In the proposed converter, the main power flows through the LLC converter, and DAB is used for the power flow direction control and output voltage regulation. In addition, a mathematical model of the proposed converter is derived for the controller design, which is verified by circuit simulation. Based on the model, the design of the controller and the circuit parameters are discussed with the goal of fast dynamics. Finally, the experimental results of a 1200-W prototype verify the effectiveness and advantages of the proposed topology and controller design.