Asymmetrical Quasi-Two-Level Modulation of a Compact Modular Multilevel DC-DC Converter (MMDC) With Reduced SM Number

As a pivotal role in interconnecting the medium-voltage dc (MVDC) and low-voltage dc (LVDC) grids, the modular multilevel dc-dc converter (MMDC) with compact size over a wide voltage range is a trending concern. This article thus proposes an asymmetrical quasi-two-level (AQ2L) modulation for a compact MMDC, which enables asymmetrical duty cycle control of the submodules (SMs). The equivalent circuit of the compact MMDC is built first, and its Boost-alike dc loop characteristic is revealed, which contributes to the reduction of SM number and converter size. Then, the power transfer characteristic under the AQ2L modulation is derived, and the favorable flatness of the transferred power over a wide voltage range is found. Besides, the proposed AQ2L modulation also achieves zero-voltage switching to ensure low switching losses. Comparison between the proposed AQ2L and conventional Q2L modulations is also demonstrated, and finally, the simulation results at 10 kV/1MW and the experimental results at 2.6 kV/50 kW verify the effectiveness and advantages of the proposed modulation.