Capacitor Voltage Balancing Control Scheme for 2/3-Level DAB Converters

Two-three (2/3)-level dual-active-bridge (DAB) DC-DC converters have high potential to be applied in the high-voltage (HV) applications due to their higher voltage blocking capability compared to the two-level DAB converters. However, capacitor voltage balancing will be a crucial issue for the neutral-point-clamped (NPC) bridge in the HV side. In order to regulate the capacitor voltages, this paper proposes a complementary-small-vector (CSV) balancing control scheme. According to the relationships among the polarity of the neutral-point current, the small vectors, and the polarity of the transformer current, two CSV pairs are obtained. In the proposed scheme, if a small vector is diagnosed as an adverse small vector for the capacitor voltage balance, it will be replaced by its CSV. After that, the polarity of the neutral-point current can be changed to increase the required charge injected into or drawn from the neutral point. Meanwhile, the waveforms of the current and voltage during the balancing process will remain unchanged. Therefore, the power fluctuation and current overshoot can be avoided. Furthermore, the implementation of the proposed scheme under different operating modes are demonstrated. Finally, the simulation results verify the performances of the proposed balancing control scheme.