Fixed-Frequency K+2D Modulation of Modular Multilevel Resonant DC/DC Converter for All Submodule ZVS Operation

Modular Multilevel Resonant DC-DC Converters (MMRDCs) are increasingly recognized as competitive solutions for medium-voltage to low-voltage (MV-LV) conversion with wide input ranges. However, these systems often face challenges of low conversion efficiency due to the inability to achieve sufficient soft-switching or excessive conduction loss. This paper proposes a novel fixed-frequency K+2D control strategy, which combines submodule (SM) number K and 2 SMs adjusting width D. This approach ensures zero-voltage switching (ZVS) on for all SMs, thereby reducing switching losses. It also effectively reduces the loop resonant current, leading to lower conduction losses. The fixed-frequency control method simplifies the magnetic components design. Simulations are conducted to demonstrate the performance under conditions of 9 kV to 18 kV input, 200 kW power, and 750 V output. Experimental results from a prototype further validate the feasibility and effectiveness.