Optimal Control of Multilevel DAB Converters for Soft-Switching and Minimum Current Stress

Multilevel dual-active-bridge (DAB) dc-dc converters offer higher voltage-blocking capability and larger step-up ratios, as well as increased number of degrees of freedom (DoFs) compared with the traditional two-level DAB converter. Thus, it has potential to be used in medium-voltage dc systems. However, the control complexity of the multi-level DAB converters also increases along with the number of DoFs. When soft-switching is considered as an optimization objective together with other performance indices (e.g., peak or root-mean-square values of the inductor current), the analytical solutions will be difficult to derive due to the increased number of control variables and complicated soft-switching constraints. To address this issue and improve the efficiency of the DAB converter, this article proposes a control strategy for a multilevel DAB converter to achieve both soft-switching and minimum-current-stress simultaneously. The analytical solutions are calculated by improved Karush-Kuhn-Tucker (KKT) conditions, where the numerical solutions are used to locate the operating modes and determine the zero-voltage-switching (ZVS) boundaries. This can simplify the Lagrangian function and inequality constraints, and then, the analytical solutions can be obtained by the simplified KKT conditions. In addition, a comprehensive comparison between the quasi-ZVS and strict-ZVS control is carried out, so that the converter efficiency, implementation complexity, and smooth transition between different power levels can be compromised for practical applications. Finally, the performances of the proposed control strategy in terms of current stress minimization and soft-switching operation are validated by experimental tests.