A study on the multi-carrier PWM methods for voltage balancing of flying capacitor in the flying capacitor multi-level inverter

The voltage control of flying capacitor is very important for safe operation in flying capacitor multi-level inverter (FCMLI). The voltage unbalancing of flying capacitor causes serious problems in safety and reliability of system, and also limits FCMLI´s applications. The voltage unbalancing is occurred due to the difference of each capacitors charging and discharging time applied to FCMLI. This paper investigates and analyzes multi-carrier PWM methods to solve capacitor voltage balancing problem. Among the various PWM methods for the FCMLI, the phase-shift PWM (PSPWM) method, the modified carrier-redistribution PWM (MCRPWM) method and the saw-tooth-rotation PWM (STRPWM) method are discussed and compared with respect to switching state, balancing voltage of capacitors and output waveform. Also, this paper considers the performance of PWM methods. These three PWM methods are analyzed in the flying capacitor three-level inverter. The results are shown through simulation and experiment. Finally, the harmonics about the output voltages of their methods are compared using the harmonic distortion factor (HDF)