Abstract

Using series-connected SiC mosfets to enable high-speed switching units is desired to meet the growing demand for high-density medium-voltage power converters. Due to its fast-switching speed, the voltage sharing of series-connected SiC mosfets is more sensitive to the surrounding parasitic components and gate-signal mismatch, which results in more challenges for the voltage balancing control. In this article, the gate turn-off delay-time control is first used as an example to analyze the limitations of the existing active voltage balancing (AVB) control methods: 1) AVB control has a limitation of adjusting the delay time accurately under ac current conditions; and 2) the voltage imbalance of the body diodes cannot be solved by AVB control. To achieve voltage balancing control of series-connected SiC mosfets for ac current, this article proposes a new hybrid approach: 1) passive dv/dt compensation: one small compensation capacitor is applied to balance the measured nonuniform distribution of parasitic capacitors; and 2) active gate signal turn-off time adjustment: a digital closed-loop delay time control is applied to compensate the gate signal mismatch of mosfets. To verify the proposed balancing approach, a single-phase pump-back test is conducted to show the improvement of voltage sharing of both mosfets and body diodes.