Analysis and Optimal Modulation for 2/3-Level DAB Converters to Minimize Current Stress With Five-Level Control

A two-to-three-level dual-active-bridge (DAB) dc–dc converter has high potential to become a promising solution for medium-voltage high-power applications. However, its switching characteristics, modeling, and optimal modulation schemes have not been comprehensively explored yet. In order to avoid voltage and current distortions and ensure reliable operation, this article analyzes the operating constraints for the neutral-point-clamped (NPC) DAB converters. Based on the obtained operating constraints, the transferred power and current stress models under a five-level control scheme, which is considered as one of the most advantageous control strategies for the NPC DAB converters, are obtained with an equivalent-wave modeling method. Generic control strategies based on analytical solutions have not been fully explored for the multilevel DAB converters due to the increased number of control variables. With the above, this article proposes a minimum-current-stress control strategy employing analytical solutions in the entire power range, which are obtained by combining the Karush–Kuhn–Tucker conditions and numerical-solution analysis. With the proposed modulation scheme, the control variables can be automatically modified online, instead of relying on the offline calculation of the numerical control variables. Therefore, the control complexity can be reduced significantly, especially when the operating parameters change in a wide range. Finally, experimental tests verify the effectiveness of the analysis.