Discontinuous Conduction Mode Operation of the Current-Shaping Modular Multilevel DC–DC Converter

The current-shaping modular multilevel dc–dc converter (CS-MMC) is a recently proposed class of modular multilevel dc–dc converters for dc distribution grid applications consisting of a single string of cascaded voltage-source submodule (VSM) cells, a current-source submodule (CSM), and, notably, no series inductor. In the CS-MMC, since it is the CSM that shapes the string current and there is no series inductor, this frequency can readily be in the medium-frequency range, enabling VSM cells to be realized with low cell capacitance. In the previous work, only continuous conduction mode (CCM) operation of the CS-MMC had been considered, which led to several limitations, including elevated switching losses and low utilization of the semiconductor devices in low output voltage applications. In this work, the discontinuous conduction mode (DCM) operation is proposed for the CS-MMC that addresses these limitations. In traditional dc–dc converters, DCM is determined by the inductor current ripple. However, unique to the CS-MMC, DCM is determined by the VSM capacitor voltage ripple. In this work, the proposed DCM operational approach is presented along with its analysis and control. Both simulation and experimental results from a laboratory-scale converter system are provided for validation.