Soft-switching techniques for medium-voltage isolated bidirectional DC/DC converters in solid state transformers

Soft switching techniques are very attractive and often mandatory requirements in medium-voltage and medium-frequency applications such as solid state transformers. The effectiveness of these soft switching techniques is tightly related to the dynamic behavior of the internal stored charge in the utilized semiconductor devices. For this reason, this paper analyzes the behavior of the internal charge dynamics in high-voltage semiconductors, giving a clear base to understand the previously proposed zero-current-switching techniques for IGBT-based resonant dual-active-bridges. From these previous approaches, the two main concepts that allow switching loss reduction in high-voltage semiconductors are identified: 1) shaping of the conducted current in order to achieve a high recombination time in the previously conducting semiconductors and 2) achieving ZVS in the turning-on device. The means to implement these techniques in a triangular current mode dual-active-bridge converter together with the benefits of the proposed approaches are analyzed and experimentally verified with a 1:7 kV IGBT-based NPC bridge. Additionally, the impact of the modified currents in the converter's performance are quantified in order to determine the benefits of the introduced concepts in the overall converter.