# Multipoint Thermal Sensing System for Power Semiconductor Devices Utilizing Fiber Bragg Gratings
## 1 Introduction  
Power electronic systems have found widespread applications in various fields, including hybrid electric vehicles and renewable energy systems [1, 2]. These systems are driven by a variety of power semiconductor devices (PSDs), such as IGBTs and silicon carbide semiconductor field effects transistors (SiC-MOSFETs) [3]. In high-power applications where circuit robustness is required, multiple IGBTs are used to control the electrical current in the circuit [4]. As such, the adoption of IGBTs continues to grow as power systems evolve and advance. However, one of the major causes of failure is the high operating temperature within the chip due to thermal cycling [5, 6, 7]. Typically, power systems that exploit IGBTs in their circuitry are likely to experience variations in the electrical input signal, the terminal load, and the switching frequency, where these varying parameters contribute to the increase in IGBTs' junction temperature (\(T_{j}\)) [8]. This in turn accelerates the wear-out and degrades the overall performance and reliability of IGBTs [9, 10]. To prevent overburdening the IGBTs with thermal stress, several studies have reported numerous techniques for monitoring the junction temperature [11, 12, 13, 14]. There are two methods for determining the junction temperature, i.e., electrical- and optical-based techniques. The estimation based on electrical techniques includes temperature-sensitive electrical parameters (TSEPs) [12, 15] and temperature-sensitive electrical devices (TSEDs) [13, 16]. TSEP techniques rely on electrical parameters, such as turn-off delay [17], collector-emitter voltage [18], and on-state resistance [19], to estimate\(T_{j}\). Since these parameters can vary with the age of the IGBT, they may not always be reliable for \(T_{j}\) estimation. On the other hand, the TSED technique uses electrical devices and components like silicon diodes [20], thermistors, and thermocouples [13], which are prone to measurement errors due to thermal contact resistance. The optical methods include temperature-sensitive optical parameters (TSOPs), infrared cameras (IRCs), and the fiber Bragg grating (FBG) technology [16]. TSOPs use the electroluminescence of forward bias silicon devices to estimate \(T_{j}\)[21], yet this method is not useful for commercial IGBTs. An IRC, being involved in the most common optical method, is used in various fields [22] and renders a visual 2D display of the surface temperature distribution. However, it necessitates direct line-of-sight and homogeneity of the targeted surface. On the other hand, the FBG sensing technique offers a unique advantage for \(T_{j}\) sensing due to its miniaturization, immunity to electromagnetic interference, inherent robustness, and multiplexing capability. The multiplexing feature enables the implementation of \(T_{j}\) sensing for multiple devices as opposed to other techniques confined to single-point measurement. Previous studies on FBG techniques have explored the direct placement of sensors on the IGBT's chip or affixed to the bond wires, or forming a groove at a surface of the structure or baseplate [23; 24; 25; 26; 27]. Unfortunately, this arrangement is limited by the encapsulation of the IGBT as it requires dismantling the ceramic casing. Moreover, it is difficult for commercial application and may also affect the thermal distribution and aging of the device or cutting due to altercations in the device's structure. To address these challenges, this paper introduces a multipoint thermal sensing approach utilizing three FBG sensors with distinct wavelengths. These sensors were positioned beneath two IGBTs and a packed rectifier baseplate installed on a common heatsink in a three-phase inverter without a groove at the surface of the baseplate. The study examines the performance of both single-point and multipoint sensing capabilities of FBG sensors using a single optical fiber.