Electrothermal characterisation of silicon and silicon carbide power devices for condition monitoring

Condition monitoring in power electronics is increasingly becoming a critical component of reliable power electronics both in traditional discrete and DBC packages as well as in pressure-packages. Condition monitoring involves on and off-line assessment of the state of health of the power module in an effort benchmark the reliability performance of the module (if off-line) or to prolong its useful operating life (if on-line). One widely acknowledged method of condition monitoring involves the use of temperature sensitive electrical parameters (TSEPs) to estimate the junction temperature and hence, the junction to case thermal impedance as well as on-state electrical resistance. However, for TSEP based condition monitoring to become reality, a significant amount of electrothermal characterisation of modern power devices is necessary and that is where this thesis makes a valuable contribution, especially for new SiC power devices with relatively unknown thermal characteristics in alternative packages like press-packs. TSEPs including diode/transistor on-state voltage drop, turn-on current switching rate, gate current/voltage switching transients etc., depend on the physics of the power devices and can vary from device to device depending on whether they are bipolar, unipolar, silicon or SiC. The on-state voltage drop of some devices exhibits a negative temperature coefficient, while others exhibit a positive one depending on the value of the Zero Temperature Coefficient (ZTC) point exhibited in the forward characteristics. The Zero temperature coefficient results from the interaction between junction voltages which reduce with temperature (due to increased intrinsic carrier concentration from bandgap narrowing) and parasitic series resistances which increase with temperature (due to mobility and ambipolar diffusion length reduction with temperature). This thesis shows that SiC power MOSFETs have the unique property of switching on faster at higher temperatures whereas the converse is true for silicon MOSFETs and IGBTs. Hence, the turn-on current switching rate has been proposed as a TSEP in SiC MOSFETs. The impact of parasitic inductance on the temperature sensitivity of the turn-on switching rate is also investigated and recommendations are made for the use of intelligent gate drivers with alterable gate driver impedances for implementing condition monitoring. This thesis also investigates the impact of junction temperature variation in parallel connected power devices on the accuracy of the TSEP for the entire module and how the gate driver could be used to improve the temperature sensitivity of determined electrical parameters. In the context of pressure-packaged assemblies, where the thermal impedance is inextricably linked with the electrical impedance, this thesis presents the electrothermal characterisation of SiC Schottky diodes. Both a single chip and a multichip press-pack prototypes have been designed and tested, including the evaluation of different intermediate contact materials, namely Aluminium Graphite and molybdenum. The impact of pressure imbalance and device temperature characteristics (ZTC point) on electrothermal stability of parallel devices are explored in this thesis.