Enabling High-Frequency Operation of MMC-Based Chargers: Frequency-Decoupled Modulation Strategy and Soft Switching

The demand for ultrafast charging of electric heavy-duty vehicles drives the development of megawatt-level interfaces with the medium-voltage grid. Modular multilevel converter (MMC)-based chargers offer a promising solution, utilizing a single medium-frequency transformer (MFT) to enhance power density. This article proposes a novel modulation strategy for ac/ac MMCs with rectangular-wave output that leverages the separation of low-frequency and medium-frequency components within the modulation stage. This separation reduces the scaling factor between the switching frequency and the MFT's operating frequency, enabling the use of higher operating frequencies for the MFT. This, in turn, facilitates the size reduction of the MFT without compromising the efficiency of the MMC. The findings demonstrate significant improvements in the modulator, including higher time resolution of waveform synthesis, and reduced harmonic distortion in the MMC's terminal voltages and currents. This, in turn, enables soft-switching operation in the power converters. Experimental results are given to validate the theoretical analysis and practical feasibility of the proposed modulation and soft-switching strategies.