# Thermal Mapping of Power Semiconductors in H-Bridge Circuit +
## 4 Thermal Stresses of Power Devices at Various Loading Conditions
### Simulation and Experimental Verification  
Consistent with the converter parameters listed in Table 1, the simulation can be carried out by using PLECS (Piecewise Linear Electrical Circuit Simulation), a software tool for system-level simulations of electrical circuits developed by Plexim [24]. For the inverter and rectifier modes, the power loss and junction temperature of the IGBT and the diode are shown in Figure 9a,b, respectively. In the case of the inverter mode, the IGBT is the most stressed. It can be observed that the average loss dissipation consumes 20.7 W, and the junction temperature fluctuates between 53.8 and 67.7 \({}^{\circ}\)C. Nevertheless, the diode becomes the most stressed under the rectifier mode, with the average power loss of 12.3 W and the temperature swing of 10.5 \({}^{\circ}\)C.  
In respect to the experimental setup, the control scheme of the test and load legs is shown in Figure 10a. By using the open-loop control, the driving signal of the test leg is generated from the required voltage profile. However, the driving signal of the load leg is produced by using a closed-loop control of the loading current. The photo of the experimental setup is shown in Figure 10b. It is worthwhile to mention that the control scheme is realized by means of a TI Digital Signal Processor (DSP) TMS320F28335 [25]. The loading current is measured by using the ADC module, while the PWM signals are generated by using ePWM module. Figure 8: Impacts of various switching frequencies (\(f_{sw}\)) on power module. (**a**) Loss dissipation. (**b**) Mean junction temperature. (**c**) Junction temperature swing. Figure 9: Loss dissipation and junction temperature of IGBT and diode with fundamental frequency of 10 Hz and switching frequency of 10 kHz. (**a**) Inverter mode. (**b**) Rectifier mode. For the junction temperature measurement, the temperature swing is fluctuated within a fundamental period of 100 ms in this case study. The bandwidth of the temperature response needs to be fast, where the fiber-optical thermometer and thermographic camera are typical alternatives. In addition, both of them are capable to provide the electromagnetic isolation, which is preferred by the fast-switching transistors. To prepare the power modules for the thermographic camera, the dielectric gel needs to be removed, and then high-temperature material is painted across the chip surface in order to achieve the consistent emissivity. For the fiber-optical thermometer, the chip surface of the IGBT can be accessed without the removal of the dielectric gel, which reserves the function of the voltage isolation. Based on the temperature dependence of the band gap of the sensing material, a miniature spectrometer provides a spectrum with the position of the band edge, from which the temperature is calculated. A thermal fiber monitoring system from OpSens is applied, and the OTG-F sensor is selected due to the fastest response time of 5 ms [26]. As shown in Figure 11, the converter can be operated in either the inverter or rectifier mode. It is noted that the peak loading current is 20 A, the fundamental frequency is 10 Hz, and the switching frequency is 10 kHz. Besides, the heatsink temperature of the power module is controlled at 20 \({}^{\circ}\)C by using the refrigerated circulator, which indicates that the boundary condition of the experiment is exactly the same with the simulation. In the case of the inverter mode, the loading current is expected to be out of phase with the phase voltage. In respect of the junction temperature, it is noted that the temperature fluctuates with the fundamental frequency of 10 Hz. Moreover, the IGBT is more stressed compared to the diode. In the case of the rectifier mode, it is evident that the loading current is in phase with the phase voltage. Besides, the junction temperature of the diode is more stressed compared to the inverter mode, which is consistent with the analytical calculation.