I. Introduction
The modular multilevel converter (MMC) has been the main converter in medium-voltage (MV), high-voltage (HV), and high-power applications with many excellent properties, such as high modularity, scalability, and less harmonics performance [1], [2], [3]. However, just like cascaded H-bridge (CHB) converter [4], MMC consumes abundant semiconductor devices and capacitors whether the submodule (SM) is half-bridge (HB) SM or full-bridge (FB) SM, where the capacitor volume occupies the main volume and weight of MMC [5], [6], [7], which leads to high cost, conduction loss, and bulky footprint. Therefore, in order to settle these challenges, many innovative works have been carried out.