A Series-Arm Modular Multilevel DC/DC Converter With Variable Duty Cycle Quasi-Square-Wave Modulation for ZVS Operation

A series-arm modular multilevel dc/dc converter is proposed in this article, which is suitable for dc transformer applications in medium-voltage (MV) dc systems. The proposed converter utilizes the series-arm configuration on the MV side, eliminating the MV terminal concentrated capacitors and reducing the number of submodules. Transmission power regulation can be achieved by adopting the variable duty cycle quasi-square-wave modulation and phase-shifted control with higher ac link frequency. A comprehensive analysis is conducted on the operational principle, and the soft switching characteristic of the converter is analyzed. Based on this, the parameter design procedure and output voltage control strategy are presented, which can effectively combat the degradation of soft switching characteristics caused by the inherent dc circulating current in modular multilevel structures. Hence, both MV-side SM and low-voltage-side full-bridge switches can realize zero voltage switching throughout the whole operating range. Finally, the theoretical analysis and control strategies of the proposed SAMMDC are verified by a 4kW SAMMDC prototype.