Estimation and measurement of junction temperatures in a three-level voltage source converter

The design of a power converter must guarantee that the operating junction temperatures v_j of all devices do not exceed their limits under all specified operating conditions. Usually, this is ensured by a simulative or analytical junction temperature estimation based on simple electrical and thermal models and semiconductor datasheet values. This paper discusses the difficulties and quantifies the limitations of this approach on the example of a three-level NPC VSC with IGBTs. The calculations are compared to the results of direct junction temperature measurements with an infrared camera. The paper also provides the experimental proof for the unequal loss and junction temperature distribution in the three-level NPC VSC.