# Junction Temperature Optical Sensing Techniques for Power Switching Semiconductors: A Review
## 3 Junction Temperature Optical Sensing Techniques
### FBG Sensing Technique
A pulse of different widths and duty cycles was applied at the gate of the MOSFET while the oscilloscope measured the input direct current and voltage. The interrogator monitored the wavelength shift to capture the thermal pulses and the thermalsampling frequency was limited to 1 kHz; thus, the optical interrogator captured changes in wavelength every 1 ms. It was observed that the junction temperature increased and accumulated as the duty cycle increased from 2 up to 50% for the same pulse width of 300 s. However, when the pulses ended, a slight temperature rise was initially observed, after which the temperature dropped gradually due to the error introduced by the air gap between the grating portion of the sensor and the die, and the limited sampling rate of the employed interrogator. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Year**} & \multirow{2}{*}{**Device**} & \multirow{2}{*}{**\(\lambda\) (nm)**} & **FS** & **Sensor** & **Sensitivity** & **L (mm)** & **Err** & **\(\Delta\)T (\({}^{\circ}\)C)** & **Key Features** & **Ref.** \\  & & & **(Hz)** & **Casing** & **(pm/\({}^{\circ}\)C)** & **L (mm)** & **Err** & **\(\Delta\)T (\({}^{\circ}\)C)** & **Key Features** & **Ref.** \\ \hline
2022 & IGBT-PI & 1540 & 1 M &
\begin{tabular}{c} Teflon \\ tube \\ \end{tabular} & 11.7 & 3 & 0.03 \({}^{\circ}\)C & 160 & Sensor head interface & [68] \\
2022 & MOSFET & 1510– & 1 k & & 14, 21 & 15 & \(\pm\)0.5 K & - & Thermal model for duty cycles & [112] \\
2022 & IGBT & 1550 & 2 k & - & 13 & 1–5 & \(\pm\)0.1 \({}^{\circ}\)C & 40–67 & Head length influence & [114] \\  & & &
\begin{tabular}{c} Tube \\ \end{tabular} & 10.2, & & & & & & & & \\
2022 & IGBT-PI & 1550 & 100 & \
\begin{tabular}{c} Metal \\ Plate \\ \end{tabular} & 10.4, & - & - & 175 & FBG housing effect & [47] \\  & & &
\begin{tabular}{c} Plate \\ \end{tabular} & 14.7 & & & & & & & \\
2021 & IGBT & 1545 & - &
\begin{tabular}{c} Tube \\ \end{tabular} & - & 15 & - & - & & & \\
2020 & IGBT & 1550 & 2.5 k & \
\begin{tabular}{c} Tube \\ \end{tabular} & 10.9 & 5 & - & 25–95 &
\begin{tabular}{c} Steady and \\ dynamic response \\ \end{tabular} & [115] \\
2017 & IGBT & 1530 & 100 &
\begin{tabular}{c} Teflon \\ tube \\ \end{tabular} & - & 5 & 0.48 \({}^{\circ}\)C & 40–180 & Influence of vibration & [116] \\
2012 & IGBT & - & 2 k & - & - & \(\pm\)0.02 \({}^{\circ}\)C & \(<\)150 & RC model & [113] \\
2011 & IGBT-SI & 1550 & \
\begin{tabular}{c} Metal \\ sheet \\ \end{tabular} & 14 & & & & 26–90 &
\begin{tabular}{c} Established FBG \\ sensitivity threshold \\ \end{tabular} & [117] \\
2011 & IGBT-SI & 1555 & - & \
\begin{tabular}{c} Metal \\ sheet \\ \end{tabular} & - & - & - & 0–80 &
\begin{tabular}{c} Interface conductivity \\ \end{tabular} & [118] \\
2010 & - & 1549 & 10 & & 11.4 & 5 & 30–900 &
\begin{tabular}{c} High-temperature \\ sensing \\ \end{tabular} & [119] \\
2010 & - & 1530 & \
\begin{tabular}{c} Steel \\ Tube \\ \end{tabular} & 21 & & & & & & & \\ \hline \hline \end{tabular}
\begin{tabular}{c} \(\lambda\): wavelength, FS: sampling rate, L: grating length, Err: measurement error, \(\Delta\)T: temperature range, PI: PWM inverter, SI: solar inverter. \\ \end{tabular}
\end{table} 
Table 6: Advancements in FBG sensing technique for junction temperature measurement of switching semiconductor devices. Figure 9: Experimental setup for the FBG-based sensing technique for temperature measurement. [PERSON] et al. [68] proposed direct on-chip thermal measurement for Si IGBT to demonstrate the effect of the sensor bond interface with the chip surface and evaluated the performance of real-time junction temperature measurements. For the direct detection of the die temperature, the ceramic package of the IGBT module was removed and the FBG sensor was placed directly on the chip, as shown in Figure 10a. Measurements from two FBG sensors with different interfaces (air, and solid bond interfaces with IGBT chips) were then examined.