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 nL-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 nL 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.