Optimized commutation circuit for improved utilization of SiC

Abstract

Silicon carbide MOSFETs have increasingly been recognized as advantageous compared to conventional silicon IGBTs; however, they face significant challenges due to a high defect density in the raw material. To optimize semiconductor efficiency, it is critical to minimize both conduction and switching losses. Conduction losses are influenced by on-state resistance and device breakdown voltage, while switching losses are dependent on switching speed, which is constrained by turn-off voltage overshoot and system stray inductance. Therefore, the reduction of stray inductance is paramount for decreasing both types of losses. This work presents a comprehensive analysis of the current state of the art, outlining essential device characteristics and limitations. It identifies the distribution of stray inductance in the DC-link and power module as significant constraints. While DC-snubber circuits are discussed as a potential solution, their effectiveness decreases at higher currents due to increased damping losses. An active controlled snubber is introduced, and its operational principles in conjunction with a half-bridge configuration are described. This setup facilitates a substantial reduction in stray inductance during MOSFET and diode turn-off, resulting in negligible losses during turn-on. The analytical description of the operational mechanism reveals opportunities for zero-current switching of the auxiliary switch. The derived model is validated through empirical measurements and is employed to predict behavior in subsequent switching events. Comparative analysis with a similar DC-snubber configuration indicates that the active snubber exhibits alternating yet stable voltage behavior, with a maximum snubber voltage much lower as with a conventional DC-snubber. Three-phase inverters designed for switching frequencies of 10 kHz and 30 kHz demonstrate a potential increase in output power of 35% for non-optimized semiconductors and 59% for optimized MOSFETs. Furthermore, the introduced configuration is utilized to actively drive a small LC-filter, evaluating its applicability under constraints on maximum voltage slope. A dependency between snubber voltage and the timing of filter pulses has been identified. In comparison to a state-of-the-art IGBT setup with similar output power, a reduction in switching losses by a factor of 20 was observed, accompanied by a concurrently lower voltage slope. However, the time constants of the snubber circuit limit the minimization of the LC-filter.