Active thermal protection and lifetime extension in 3L-NPC-inverter in the low modulation range

The switches of multilevel inverters in high-power applications are mounted on dedicated heatsinks and there is no heat sharing among the devices. If one semiconductor device is exposed to excessive thermal stress, then the lifespan of the affected device is reduced significantly. This thermal overload may arise due to degradation of the cooling system, failures in firing or in the drivers, inappropriate placing of the semiconductor devices etc. In this paper, a control scheme is devised that can reduce the thermal stress of a particular semiconductor device; the benefit is that the reduction of the lifetime of this device is avoided. Thus, after detecting of the occurrence of a local thermal overload by temperature sensing on the particular heatsink, the control scheme modifies the pulse width modulation in such a way, that the power losses of the stressed device are redistributed among the other semiconductors without limitations of the inverter current. In previous works, this method was already presented for the higher modulation index range; it is now extended to the low modulation range and is validated by experiments.