Modulation and Soft-Switching Optimization Control of Multilevel Dual Active Bridge DC-DC Converters

The dual active bridge (DAB) converter has been recognized as one of the most promising dc-dc topologies. By introducing the multilevel structure, the DAB converter can be applied in dc distribution system of medium-voltage. In addition, the multilevel DAB (ML-DAB) converter has more freedoms to control the power flow and optimize its operating performance, for example, soft-switching performance. However, most of the existing researches only focus on the control and optimization of three-level DAB, due to the explosively-growing complexity as the number of voltage levels rises. Thus, complexity of control and optimization for ML-DAB restricts the practical implementation and application compared with conventional two-level (2L) DAB, which can achieve power flow control and operating performance optimization with only three phase-shifting ratios. Hence, to improve the performance of ML-DAB within feasible complexity, the quasi-two-level (Q2L) modulation and multiphase-shifting (MPS) control suitable for arbitrary n L-DAB are proposed in this article. The characteristics of power flow, inductor current, and soft switching for ML-DAB is analyzed by migrating the characteristics of 2L-DAB properly. Furthermore, an optimization control strategy to achieve full-range soft switching is proposed for ML-DAB. Compared with the existing n L modulation method for ML-DAB, the proposed Q2L modulation and MPS control methods can realize low-complexity control and high-performance operation of ML-DAB. Experimental results of a four-level DAB prototype are also presented for validation.