Controller for an Active Balancing Circuit for Divided DC-Link Capacitors in Multilevel Inverters

In this paper, an active balancing circuit is analyzed and a controller is proposed that is designed to balance the voltage between two series-connected capacitors that are in turn connected to a single DC voltage source. Such series-connected capacitors are found, for example, in the split DC-link commonly used in multilevel inverters such as NPC, ANPC or T-type inverters. The circuit is simple, requiring only two power switches and one inductor. The active balancing circuit is modeled and a controller is proposed that achieves regulation and stability to ensure a constant voltage difference across the two series capacitors, even with different characteristics or capacitances and regardless of the application. The proposal can also achieve asymmetric voltage balancing. Advantages of the proposal are the modular design and the implementation with a single H-bridge leg. It can be easily added or removed from inverter topologies without modifying the inverter. The circuit and its control are evaluated through simulations on a three-phase, three-level, NPC inverter with an unbalanced AC load. The results demonstrate good performance and effective voltage balance regulation

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