Model Predictive Active Thermal Control Strategy for Lifetime Extension of a 3L-NPC Converter for UPS Applications

The neutral point clamped (NPC) converter topology suffers from unequal thermal stress distribution of the power devices, which limits the maximum output power of the converter and also reduces the lifetime. By applying a control strategy that can maintain an equal stress distribution between the inner and outer devices, the lifetime of the converter can be prolonged. In this paper, positive effects of a predictive active thermal control strategy on the lifetime of the NPC converter are presented applied for an uninterruptible power supply (UPS) application. A mission profile based lifetime is estimated for a typical UPS loading profile. The obtained results show that for the presented application the control algorithm can maintain the difference between the inner and outer device junction temperature within 1 degrees C. Thus, the converter lifetime can almost be doubled by using the predictive active thermal control strategy. Moreover, a comparison of estimated B-1 lifetime for different mission profiles is also presented to illustrate how different ratings of the nominal and standby load affect the converter lifetime.