LLC-MMC Resonant DC-DC Converter: Modulation Method and Capacitor Voltage Balance Control Strategy

DC-DC converter plays an important role in high-voltage direct-current (HVDC) transmission and medium-voltage DC (MVDC) distribution system. Dual-active bridge topology with modular multilevel converter (DAB-MMC) is a preferred choice due to the small transformer size and high voltage handling ability. However, DAB-MMC cannot achieve soft-switching over entire load conditions. Meanwhile, the traditional LLC resonant converter, which is widely used for low voltage application, features inherent ZVS soft-switching because of the resonant network. Combine the DAB-MMC and traditional LLC resonant converter, this paper proposes a LLC resonant converter with MMC structure (LLC-MMC). The pulse frequency modulation (PFM) method with quasi-square-wave is utilized to achieve low dv/dt stress. Besides, capacitor voltage balance control strategy executed in dead time is proposed to maintain the balance. LLC-MMC guarantees full ZVS soft-switching over a wide load resistance range. Finally, the proposed modulation method and balance strategy are verified by the sufficient simulation and experimental results.