A Resonant Switched-Capacitor Converter for Neutral-Point Voltage Balancing of Three-Level Inverters

Neutral-point voltage unbalancing is a common problem in three-level inverters. The original neutral-point voltage balancing circuit is the buck-boost converter. Since the inductor carries low frequency components, the power density is limited. At the same time, the efficiency is low because the soft switching cannot be realized. In this article, a neutral-point voltage balance circuit based on resonant switched-capacitor converter (RSCC) is proposed. The proposed converter uses a series resonant inductor and resonant capacitor to transfer the unbalanced energy between two dc-link capacitors. Based on phase shift modulation, the RSCC circuit controls the output current to compensate the neutral-point current of the inverter to achieve the suppression of neutral-point voltage ripple. To achieve the minimum loss, a step-by-step optimal design is presented and the optimal switching frequency, resonant inductance, and resonant capacitance can be derived. The proposed circuit has the advantages that all switches of RSCC can realize the zero voltage switching, and the magnetic volume can be greatly reduced because only high frequency components flow through the inductor. A 400 V-10 A prototype of RSCC and the inverter is built, and main experimental results are provided to verify the time-domain analysis and the effectiveness of the proposed RSCC voltage balance circuit.