A carrier-rotation strategy for voltage balancing of flying capacitors in flying capacitor multi-level inverter

This paper proposes a carrier-rotation (CR) PWM technique that is a new solution for the voltage unbalancing problem of flying capacitors in the flying capacitor multi-level inverter (FCMI). The proposed PWM technique equalizes the utilization of phase leg voltage redundancies corresponding to the charging and the discharging state of individual flying capacitors during each switching period of all the switches. Therefore, the charging and the discharging quantity of flying capacitors are equal, which makes the average variation of flying capacitor voltages become zero and keeps their voltage stable during minimum specified period. It also has the same switch utilization and the reduced harmonic contents of output voltage since the switching frequency of each switch is identical and all the carrier signals are in phase. Hence, it is more suitable for the FCMI compared with the phase-shift PWM method. The proposed PWM technique is analyzed precisely in flying capacitor 3-level inverter and then it has expanded to the FCMI (N-level, N>3). Experimental results on the laboratory prototype flying capacitor 3-level inverter confirm the validity of the proposed PWM technique.