Simple Active Capacitor Voltage Balancing Method without Cost Function Optimization for Seven-Level Full-Bridge Flying-Capacitor-Multicell Inverters

Flying-capacitor (FC)-based multicell inverters have recently drawn more attentions due to their outstanding performances. In this article, a new approach to actively control the FC voltages in seven-level full-bridge FC multicell (FCM) inverters is presented. In the proposed method, first, the output current's direction and voltage of the FCs are measured and are interpreted into logic variables. Next, these logic variables are fed into a set of equations, named as logic equations, to achieve two goals: 1) to adjust the voltage of the FCs at their reference values; 2) to generate the required voltage level at the output which is determined by pulsewidth modulation unit. Since the proposed method does need neither the optimization of cost functions nor the utilization of proportional-integral controllers, it is straightforward and simple to be implemented, specifically for the FC-based industrial inverters. To verify the performance and feasibility of the proposed logic equations-based active voltage balancing method, the simulation and experimental results obtained from a seven-level full-bridge FCM inverter are provided. The proposed method is validated under the transient conditions such as variations of modulation index and the dynamic changes of dc-link voltage to demonstrate its robustness and performance.