Boost Mode Analysis and ZVS-Guaranteed Control Strategy for Frozen Leg Operation of Three-Phase Dual Active Bridge Converters

The frozen leg method is an effective fault-tolerant approach for addressing open-circuit faults in three-phase dual-active-bridge converters, requiring no additional hardware. This method isolates the faulty leg by disabling its two switches so that the converter can continue operating at reduced power levels. Prior literature on the frozen leg method primarily focuses on cases with unity voltage gain or buck mode. However, in boost mode where a higher voltage difference must be overcome, the faulty phase may enter discontinuous current mode (DCM), introducing additional complexity and necessitating a new mathematical framework. Thus, this article presents the first comprehensive investigation of boost mode frozen leg operation, proposing a novel analytical framework for the DCM. The analysis of boost mode is divided into five distinct cases, with corresponding derivations for power, current, and voltage equations. The findings show significant differences in operation compared to buck and unity gain modes. Additionally, this research derives the maximum power transfer achievable in boost mode and conducts a detailed soft-switching analysis. Based on this analysis, a ZVS-guaranteed control strategy is proposed to ensure that the operating points remain within the high-efficiency ZVS region. Experimental results are provided to validate the theoretical analysis.