Transient DC Bias Suppression Strategy and Zero-Voltage Switching Boundary Analysis for Three-Level Dual-Active-Bridge Converter

The dual-active-bridge (DAB) converter is widely used in DC-DC applications due to its high efficiency and power density. However, the modulation of the DAB converters presents challenges in terms of efficiency and reliability. One key issue is the requirement for a wide zero-voltage-switching (ZVS) range to minimize switching losses, which significantly impact efficiency. Another is the occurrence of transient DC-bias currents in the transformer, which can lead to core saturation and component damage during dynamic operations such as load changes or power reversals. This article addresses the challenges of operating three-level dual-active-bridge (DAB) converters by introducing strategies for ZVS optimization and transient DC-bias suppression. The ZVS boundaries are analyzed using a harmonic decomposition method, which leads to an optimized phase-shift strategy for achieving the widest possible ZVS range. The paper also presents a method to eliminate transient DC-bias by balancing the volt-second integrals during dynamic shifts. This integrated control method can be implemented by using a unified optimization method without the need for additional auxiliary hardware. The effectiveness of these methods is confirmed through simulation and experiments.