The limited usability of a ZTC-point in tracking IGBT degradation

Abstract:
As a method for IGBT condition monitoring, it has been argued that the point of the zero temperature coefficient (ZTC) in the semiconductor output characteristic is of particular value. It offers temperature independence, and it provides an efficient single-point read-out of the on-state and forward voltages, in the case of diodes. However, there is no published data to show this consistently in aged IGBTs, neither in benchtop experiments nor in converter operation. We have investigated the impact of aging the semiconductor package on the ZTC-derived voltage levels for three degradation mechanisms. From the results for IGBTs packaged both in power modules and TO discretes, we conclude that the feasibility of the single-point read-out cannot be confirmed, and we observe a small increase of collector-emitter voltage VCE for both package types stressed in stages. The ZTC point remains well defined with aging. This stability is valid with either power or thermal cycling or damaging by severing their wirebonds. Our results imply that temperature independence of a method based on the on-state or forward voltages, like the ZTC point-use, must be established by a different means.