Capacitor Voltage Balancing for Multilevel Dual-Active-Bridge DC-DC Converters

Capacitor voltage balancing is a critical issue for neutral-point-clamped-based converters, including the two/three-level dual-active-bridge dc-dc converters. The unbalanced capacitor voltage will increase the voltage stress on power devices and negatively affect the reliability of the converters. Two typical problems during the capacitor voltage balancing process should be solved, i.e., power fluctuation minimization and determination of the transformer current polarity. Accordingly, this article proposes a balancing control scheme based on the complementary switching-state (CSS) method. In the CSS method, the switching states, which are identified to be adverse for balancing, will be replaced by their corresponding CSSs to control the neutral-point current while keeping the voltage and current waveforms unchanged. By doing so, the power fluctuation and current overshoot during the balancing process can be significantly reduced. Moreover, since the dynamic voltage and current waveforms are identical to the steady-state waveforms, the transformer current polarity during the balancing process can be identified based on the steady-state power and current models. Therefore, the determination of the transformer current polarity can be simplified compared to the traditional balancing control scheme. Finally, experimental results are carried out to verify the performance of the proposed balancing control scheme.