10kV SiC-based bidirectional soft-switching single-phase AC/DC converter concept for medium-voltage Solid-State Transformers

State-of-the-art PWM AC/DC converters often suffer from high switching losses due to hard-switching and thus are limited in their efficiency and power density. In this paper, the concept of the integrated Triangular Current Mode (iTCM) operation is introduced, which enables soft-switching over the entire AC mains period and can overcome the limitation of PWM, resulting in a higher system performance. Thereby, an LC-circuit is added to the well-known full-bridge-based PWM AC/DC converter, which internally superimposes a high triangular current to the AC mains current in order to reverse the current direction in the semiconductors in each switching cycle and by this enabling soft-switching operation of all devices. In this paper, the iTCM concept is presented in detail and its performance is compared to the standard PWM and TCM approaches. Furthermore, the proposed iTCM concept is applied to a 10 kV SiC-MOSFET-based bidirectional 25kW single-phase AC/DC converter operated from the 6:6 kV medium-voltage AC grid. In this case, compared to PWM, the iTCM concept allows to increase the switching frequency by almost a factor of five while the total semiconductor losses are reduced by more than 40%.