Reliability-Oriented Asymmetric Power Device Size Selection of Neutral-Point Clamped Inverter Considering Mission Profiles

One of the major drawbacks of the neutral-point clamped (NPC) inverter is unequal power loss distribution among power devices, which leads to unequal thermal loadings. Therefore, certain power devices experience higher temperature stress, and thus, both the lifetime and power rating of the NPC inverter are limited by the most stressed power devices. In this article, a reliability-oriented asymmetric IGBT chip size selection method is proposed to balance the lifetimes among the power devices of the NPC inverter and also to assist the designer in achieving the necessary system-level lifetime requirements. The thermal distribution of the power devices is analyzed based on a 30 kW NPC inverter as a case study. Then, analytical power loss and thermal impedance models are derived. Finally, by using the derived models, the junction temperature of the power devices as a function of the chip size is discussed, and proper chip size is selected not only for an even temperature distribution at a fixed operating condition but also for a given target lifetime of the NPC inverter under specific mission profiles.