This paper studies the influence of Fiber Bragg Grating (FBG) head length on insulated gate bipolar transistor (IGBT) direct on-chip thermal sensing performance of FBG sensors. To this end the surface of a commercial IGBT chip is thermally simulated and experimentally characterized. Uniform FBG sensors with three head sizes are then tested in two promising thermal sensing locations. The study has found that the large thermal gradients in this application create an additional constraint when using longer head lengths. A distortion in the reflected spectrum of the 5 mm FBG sensor is used to illustrate the underlying physical effect which causes this limitation for IGBT junction temperature measurement. This additionally affects the length of head sizes providing accurate temperature readings of the IGBT surface hotspots, and significantly this limit is location dependent in a given IGBT geometry.

This paper investigates the application of adhesive bonded FBG sensors for insulated gate bipolar transistor (IGBT) direct on-chip thermal sensing. The influence of physical properties of different adhesives on the sensor's calibrated wavelength to temperature characteristic is first investigated. The thermal sensing performance of the bonded FBG sensor is then tested under various current levels in laboratory experiments on a commercial IGBT module. It is found that the bonding adhesive's curing temperature, glass transition temperature, coefficient of thermal expansion, and hardness can modify the sensor's calibration characteristic. The adhesive's glass transition temperature needs careful consideration to ensure an effective match to the IGBT's thermal operating range of interest and the hardness of the solid bond required for a given application. Operation within the adhesive's linear working region is shown to be sufficient to provide accurate sensing performance while ensuring the retention of a solid physical bond between the sensor and the chip.