Fully ZVS, Minimum RMS Current Operation of the Dual-Active Half-Bridge Converter Using Closed-Loop Three-Degree-of-Freedom Control

This paper discusses how duty ratios of the primary and secondary half-bridge legs of a dual-active half-bridge converter can be used in conjunction with phase-shift control to operate the converter with the least possible transformer rms current at a given power. Following introduction of all possible operational modes arising due to such three-degree-of-freedom control, results from a numerical-optimization-based approach are used to identify the best mode. Using information gleaned from careful observation of simulation results corresponding to the minimum current trajectory in this mode, key insights regarding circuit conditions to be satisfied for optimal operation are obtained. This information is used to propose the framework of a 3-D modulation strategy, which results in globally minimum rms current operation and is remarkably also found to satisfy the necessary conditions for zero-voltage-switching operation of all devices. The proposed strategy is implemented through closed-loop control and, hence, does not have the demerits associated with a lookup-table-based approach involving offline precomputation of optimal duty ratios. Simulation and experimental results on a 625-W laboratory prototype demonstrate the significant efficiency advantages of the proposed modulation scheme compared to simple phase-shift (1-D) control or 2-D asymmetric control, particularly at light loads in applications with widely varying voltage ratios.