# Multipoint Thermal Sensing System for Power Semiconductor Devices Utilizing Fiber Bragg Gratings
## 5 Results and Discussion  
### Load Power vs. IGBT Temperature  
Having established the sensitivity of the FBG array after its installation, since both fiber configurations (vertical and horizontal) for FIBER1 and FIBER2 exhibit similar characteristics, we chose the vertical orientation (FIBER1) for our experiment. This choice was made to simplify the installation and minimize the risk of fiber damage that could occur if the fiber were to cross or block the hole used for tightening the IGBT baseplate to the heatsink. In this work, the electronic load \"Chroma 63802\" was set to constant power mode since we were interested in monitoring the temperature of PSDs with respect to the input load. Due to equipment limitations, the load power \(P_{L}\) was adjusted with 0.5 kW intervals from 0 to 4 kW. When the load power is applied to the inverter, the current changes, while the power and conduction losses cause the temperature of the IGBT modules to increase. For the values of \(P_{L}\) presented in Table 2, the temperature gradient measured by the three sensors, \(T_{FBG1}\), \(T_{FBG2}\), and \(T_{FBG3}\), along the IGBT baseplate was estimated using the average Bragg wavelength demodulated from the optical spectrum analyzer. Each measurement was repeated three times for each load power \(P_{L}\) to ensure accuracy.  
As mentioned in Section 3.1, the center wavelengths for FBG1, FBG2, and FBG3 in this experiment are 1530 nm, 1540 nm, and 1555 nm, respectively. Due to equipment limitations, only two FBG spectra can be displayed on the OSA at a time. Therefore, the measurement from the third FBG is obtained by resetting the OSA to the corresponding central wavelength and viewing it at the upper or lower half of the screen. As shown in Figure 8, the Bragg wavelengths correspond to each peak at the upper and lower windows on the OSA. The calculated wavelength shifts between the present and reference measurements are then translated into temperature changes. The wavelength varies depending on the temperature sensed by the FBG sensor at the baseplate of each module, while \(T_{IR}\) is the average surface temperature sensed by the thermal camera for the respective IGBT / rectifier along the baseplate. \(T_{DIF}\) is the difference between the average temperature measured by the sensor and the thermal camera.  
The temperature rises with an increase in load power, as illustrated in Figure 9. This is due to the increase in current flow through the IGBT module. It is evident that the temperature measurement by the sensor varies as the thermal gradients are non-uniform. This is caused by the fluctuations in the thermal spreading resistance, which rely on the IGBT's relative location and surface area in relation to the heatsink. FBG1 and FBG2 were positioned near the two IGBTs located at the center of the heatsink. Meanwhile, FBG3 was affixed to the densely packed rectifier positioned 52 mm away from the IGBTs.  
Each sensor reading, \(T_{FBG1}\), \(T_{FBG2}\), and \(T_{FBG3}\), is the average of three temperature measurements taken from each IGBT and rectifier on the heatsink, validating the potential of FBG array sensors for multipoint thermal sensing. Nonetheless, \(T_{DIF}\) is a result of  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Load Power**} & \multicolumn{8}{c}{**Temperature (\({}^{\circ}\)C)**} \\ \cline{2-10}  & \(T_{FBG1}\) & \(T_{IR}\) & \(T_{DIF}\) & \(T_{FBG2}\) & \(T_{IR}\) & \(T_{DIF}\) & \(T_{FBG3}\) & \(T_{IR}\) & \(T_{DIF}\) \\ \hline
0 & 22.10 & 22.1 & 0.0  
inherent errors within the spectrum analyzer, interface mismatch, and resistance within the tiny airgap across the IGBT-FBG interface.