  The junction temperature is a very important parameter for monitoring power
electronics converters based on MOSFET transistors. They offer the possibility
of switching at relatively higher frequencies than other transistors like
IGTBTs. However, the electrical parameters of MOSFETs are highly thermally
dependent. The thermo-dependence of MOSFET electrical parameters is rarely
taken into consideration when implementing control strategies, for many
technological reasons, such as the difficulty of measuring the junction
temperature. In practice, the junction temperature of transistors is
inaccessible for direct measurement. The presence of a gel covering the chips,
that provides electrical and thermal insulation, makes measurement by infrared
thermography impossible. Furthermore, direct thermocouple measurement cannot be
implemented due to the electromagnetic disturbances in the environment. Several
researchers have attempted to correlate chip temperature with thermosensitive
electrical parameters. In the present work, a thermal convolutive model has
been developed to estimate the junction temperatures of two MOSFET transistors
belonging to the same electronic circuit from external temperature measurements
in two well-chosen locations (far away enough from the junction to avoid
electromagnetic interference), using also the measured power dissipated on each
chip. The thermal coupling between the two transistors has been considered in
the form of mutual transmittances. The model was first calibrated using
three-dimensional numerical simulations in COMSOL Multiphysics, followed by an
experimental study. The results are very promising, illustrating the robustness
of the convolutional model.
