Three-level SNPC commutation cell: Features and control

The unequal distribution of losses among the semiconductors represents one major disadvantage for 3L-SC (stacked cells) and 3L-NPC (neutral point clamped) commutation cells. The paper studies the loss distribution problem for 3L-SC and 3L-NPC topologies and proposes the 3L-SNPC (stacked NPC) commutation cell to overcome this drawback. A new PWM strategy for 3L-SNPC commutation cell is also presented. It has numerous advantages: (a) natural doubling of the apparent switching frequency without using the flying-capacitor concept, (b) dead times do not influence the operating mode at 50% of the duty cycle, (c) operating at both high and small switching frequencies without structural modifications and (d) better balancing of loss distribution in switches. The PSIM simulation and experimental results are shown in order to validate the proposed PWM strategy and the analysis of the switching states.