A Variable Switching Frequency PWM Control Method for Single-Phase MMC

Traditional constant switching frequency pulse-width-modulation (CSFPWM) would cause high electromagnetic interference (EMI) peak ratio and high switching loss. This letter introduces a method to make the carrier frequency of the single-phase modular multilevel inverter (MMC) change with the output current by using variable switching frequency PWM (VSFPWM). By adjusting the carrier frequency of single-phase MMC, the switching period of PWM pulse based on carrier signal comparison is adjusted to control the switching frequency of single-phase MMC driven by PWM pulse. When the load current is zero, the switching frequency is maximum, and when the load current is maximum, the switching frequency is minimum. In addition, the average switching frequency is equal to the fixed switching frequency in the control cycle. This method aiming at switching loss optimization in high power application also has benefit in reducing system conduction EMI because of the wider current harmonic spectrum distribution.