Capacitor Voltages Balancing Method for Flying Capacitor Multilevel Converters Based on Overall Priority Index

In this paper, a generalized method for voltage balancing is proposed that can be applied to all multilevel converters that utilize flying capacitors. The proposed method determines a priority index (PI) based on the charging and discharging status of flying capacitors in different redundant switching states. Using the overall priority index (OPI), which represents the sum of PIs in a phase, the proper switching state is selected that will result in the appropriate voltage balancing of flying capacitors. To calculate the OPI, the proposed method employs phase current direction and capacitor voltage deviation, resulting in less hardware and software complexity than methods that use cost functions. This paper presents a systematic method for calculating the OPI. In addition, a simplified OPI calculation method is presented for the classic flying capacitor multicell (FCM) topologies. For a five-level FCM converter, the method is illustrated. Simulated results demonstrate that the proposed method is effective in regulating flying capacitor voltages. Furthermore, the results show a good dynamic response when the operating point changes. Both capacitor voltage ripples and switching frequencies are considered in the comparison. In a laboratory setup, the proposed method is demonstrated to be practicable.