A Model-Free Capacitor Voltage Balancing Method for Multilevel DAB Converters

Capacitor voltage balancing is of importance in the neutral-point-clamped based dual-active-bridge converters. Most of traditional voltage balancing methods adopt the transformer current models in the balancing process, as the direction of the neutral-point current is affected by the transformer current polarity. However, this approach requires heavy and repetitive offline precalculation for the transformer current polarity under various operating modes, and thus, leading to complicated implementation. To overcome this, a model-free voltage balancing scheme based on a fixed-switching-state method is proposed in this letter. Two switching states are employed during the voltage balancing process in the proposed method, where the direction of the neutral-point current is independent on the transformer current polarity. Hence, the implementation can be simplified without the precalculation of transformer current polarity. Furthermore, the model-free feature of the proposed method is more robust against parameter variations or operating mode changes. Experimental tests are performed, which verify that the proposed method can achieve voltage balancing in terms of easy implementation and fast dynamics.