A Current Allocation Strategy Based Balancing Technique of Voltage Source String in Switch-Ladder Inverter and Its Switched-Capacitor Variety

A charge balancing method for series voltage source feeding multilevel inverters (MLI) based on current allocation control is presented. Unlike those MLIs based on isolated dc sources, the series-connected elements in a voltage source string, such as a battery or supercapacitor stack, cannot be by-passed individually. This brings the problems of source imbalance and limited redundant switching states for current allocation control at high modulation index. In this article, the source balance in the switch-ladder inverter is achieved by allocating the current distribution. Furthermore, the balancing effectiveness is enhanced by a novel configuration combining a switched-capacitor (SC) circuit. The working principle and modulation index limit for the current allocation method is elucidated with a flow diagram and mathematical representation. The feasibility and operation of the proposed method and topology are successfully validated by a simulation study and the experimental results.