Time-Domain Analysis and Optimization of a Three-Phase Dual-Active-Bridge Converter With Variable Duty-Cycle Modulation

The duty-cycle control (DCC) modulation scheme for the three-phase dual-active-bridge (3p-DAB) dc–dc converter is a promising three degree-of-freedom modulation scheme which can extend the converter's soft-switching range and reduce conduction losses under partial loading and wide voltage variations. However, the prior suggested methods to implement DCC in 3p-DABs have drawbacks, such as requiring a multifrequency approximation and offline optimization process, or achieving less than optimal efficiency. To overcome these challenges, this article first proposes an optimal DCC modulation strategy (OMS) for the 3p-DAB based on a novel piecewise time-domain analysis (TDA) and optimization process that obtains the optimal control parameters for minimum rms phase current. Second, this article proposes a novel closed-form minimum current stress optimization (MCSO) DCC scheme based on the theoretical findings of the TDA optimization. The MCSO reduces the transformer phase currents and extends soft-switching operation under partial loading and wide voltage variations. Experimental results show that the proposed closed-form MCSO DCC scheme has virtually identical efficiency as the OMS, making this the first article to provide a closed-form DCC modulation scheme for a 3p-DAB that achieves efficiency results equivalent to a fully optimized offline scheme, but without the drawbacks of the offline optimization process.