6 Thermal performance

To characterize both the package-level temperature distribution and the effectiveness of the dual-side cooling approach, thermal performance was evaluated under two steady-state operating conditions. The first measurement, shown in Figure 4, is a 50A output current with the heat sink removed, enabling the thermal camera to directly observe the four LMG708B0 packages and surrounding PCB. At this operating point, the four-phase implementation delivers 600W of output power while dissipating approximately 16.6W of total measured power losses. The maximum observed temperature was approximately 97℃, with the highest temperatures concentrated around the power-stage region.

Subsequently, the converter was evaluated at the full 80A load condition with a heat sink installed to maximize the effect of the dual-side cooling structure. The heat sink is made from aluminum, with a base size of 10.72 centimeters (4.22 inches) by 2.70 centimeters (1.06 inches) with a height of 1.61 centimeters (0.63 inches). Additionally, the heat sink has seven 2.54 centimeters (1.00 inch) high fins that extend from the base. The heat sink is designed to have a simple yet effective way of extracting heat away from the IC through the dual-side cooling package. Most applications, notably in the automotive sector, will have either a larger heat sink, such as the housing of the electronic control unit (ECU), or a more optimized cooling structure that will enable better performance. The TIM used between the heat sink and the LMG708B0 devices was the T-WORK9000, which is 1mm thick and has a thermal conductivity of 20W/m-K. At this operating point, the converter delivers 960W, corresponding to approximately 31.2W of total converter power losses. With the heat sink covering the integrated GaN packages, the thermal image shown in Figure 5 primarily illustrates the temperature distribution across the external cooling structure and surrounding converter. The thermal image was taken after letting the converter maintain steady-state operation at 960W for approximately 20 minutes to reach thermal equilibrium.

Together, the two measurements demonstrate the complementary thermal paths incorporated into the converter design. The 50A measurement in Figure 4 provides a direct observation of the power-stage and PCB temperature distribution, while the 80A measurement in Figure 5 evaluates the complete cooling implementation under the maximum operating condition and illustrates the effectiveness of the dual-side cooling structure. These results complement the thermal simulation presented in Figure 2, which modeled the 80A full-load condition. Figure 5 presents a glimpse of the power stage region showing each IC to have an approximate temperature of 101℃. A more accurate case temperature was taken using thermal coupling with the wire leads placed on top of the primary LMG708B0 device but underneath the TIM and heat sink. The case temperature of the primary LMG708B0 device was 105.4℃. Figure 6 shows the setup used for measuring the case temperature of the primary IC.

These results coincide with the thermal simulation results to show the significance of the dual-side cooling structure. Moreover, the use of a larger or more reliable cooling system or adding a fan to provide airflow would achieve lower temperatures and thus enable even better system performance.