| Abstract |
To increase the blocking voltage level of switching device to improve the voltage class of power converters, series-connection of power semiconductor devices is an attractive solution. But it suffers voltage unbalance problem leading to reliability concerns. To avoid the direct series-connection, this paper presents an alternative solution for two series-connected SiC MOSFETs: quasi-two-level(Q2L) modulation based on the neutral-point-clamped (NPC) three-level (3L) converter. The proposed solution is very attractive for the recent 10 kV SiC MOSFETs due to several reasons: 1) the blocking voltage could be increased to 20 kV, suitable for most medium voltage applications; 2) with NPC 3L structure, the voltage balancing of series-connected devices is avoided; 3) with Q2L modulation, the total loss on clamping diodes is significantly reduced so the clamping diodes don't significantly increase converter volume. In this paper, the basic modulation strategy and its impact on the voltage balancing of dc bus capacitors will be discussed. With Q2L modulation, series-connected 3.3 kV SiC diodes with much smaller volume are chosen as an example as the clamping diodes to showcase the benefit of Q2L modulation. Meanwhile, the detail comparison of Q2L modulation is presented between NPC 3L converter and flying capacitor (FC) 3L converter. Single-phase pump-back test is conducted to demonstrate the Q2L modulation and its impact on clamping diode loss. |
