A Modular Multilevel DC-DC Converter with Self Voltage Balancing and Soft Switching

Evolving from the popular modular multilevel ac-dc converter, modular multilevel dc-dc converter (MMDC) is attractive for medium-and high-voltage applications. However, exploiting ac circulating power to balance the submodule energy, the existing MMDC topologies inevitably suffer from high circulating current through the arms, large filter inductor at the low voltage side and complex submodule voltage balancing algorithms. For overcoming these issues, this paper presents a new power transfer mechanism to balance the submodule energy automatically by reconstructing the half-bridge submodule into a circuit quasi-resonant. Based on this submodule structure, a novel MMDC topology for step-up applications is proposed. Compared with the prior-art MMDC, this topology offers the following merits: 1) common-mode circulating current through the lower and upper arms is avoided. 2) self-balancing of the capacitor voltages is ensured by the proposed modulation method to insert and bypass the adjacent submodules in a complementary manner. 3) soft-switching operation is achieved for the majority of switches to alleviate the switching losses and electromagnetic interference. 4) voltage stress across the low-voltage side inductor is limited to the submodule voltage, thus reducing the size of the inductor significantly. Experimental results verify the performance of proposed MMDC.