Development of high power density flying capacitor multi-level converters with balanced capacitor voltage

Among the various multi-level topologies, flying capacitor converters are promising from the view point of the high power density because the size of the capacitors, which is a major concern, is expected to be reduced. Although the voltages of the capacitors theoretically can stay in the balanced condition, they tend to be unbalanced under some practical conditions. So far, it has been clarified quantitatively that the capacitor voltage becomes unbalance due to inequality in switching delay of power devices. In this paper, some countermeasures to reduce the unbalance in the capacitor voltages are investigated to realize simple multi-level converters. The effectiveness of the initial charging resistors of the capacitors on the voltage balance is clarified analytically. Based on the investigations, a prototype is constructed to demonstrate attainable power density of the converters with balanced capacitor voltages. Some experimental investigations confirm that high efficiency and high power density can be realized.