Full Range Operation of Simplified Flying-Capacitor Three-Cell Inverters With Five to Eight Output Voltage Levels

Compared with the conventional flying capacitor (FC) multicell converters, the simplified FCMC converter family has a significantly reduced number of FCs with a much higher power density. However, such simplification results in a limited number of redundant switching states, which makes the balancing of FC voltages a very challenging task that confines their valid operation range. In this article, a modified carrier-overlapped PWM is proposed for simplified FC three-cell (SFC3C) converters with five to eight output voltage levels to achieve full modulation index and power factor range operation. For SFC3C converters operating under five to seven-level cases, where one or more degrees of switching state redundancy exist, the FC voltages are regulated by adjusting the correspondence between carriers and switches for optimal state selections. For SFC3C converters operating as a eight-level case with no redundancy, the FC voltages can be balanced naturally with the fixed relationship between the carriers and switches. An active control by dwell time adjustment is used for better regulating the FCs. Experimental results verify the validity of the proposed method.