A symmetric carrier technique of CRPWM for voltage balance method of flying capacitor multi-level inverter

This paper presents a simple carrier symmetric method for the voltage balance of flying capacitors in FCMLI (flying capacitor multi-level inverter). The CRPWM (Carrier-Redistribution PWM) method was reported as a solution for the voltage balance but it has the drawback at the transition of voltage level. To achieve the voltage balance of flying capacitors, the utilization of each carrier must be balanced during a half-cycle of the switching period such as PSPWM (Phase-Shifted PWM). However, the CRPWM method causes the fluctuation of flying capacitor voltages because the balanced utilization of carriers is not achieved. Moreover, it does not consider that the load current change has an influence on flying capacitor voltages by assuming that the current flows into the load. Therefore, the charging and discharging quantity of flying capacitor voltages do not become zero during the switching period. To overcome the drawbacks of CRPWM, it is modified by the technique that carriers of each band are disposed symmetrically at every fundamental period. Firstly, the CRPWM method is reviewed and the theory on voltage balance of flying capacitors is analyzed. The proposed method is introduced and is verified through the simulation and experiment results.