6 Thermal Performance of QFN 12x12 Package on Half-Bridge Evaluation Board

An evaluation board (LMG3422EVM-043) incorporating all design tradeoffs was built consisting of two LMG3422R030 devices configured in half-bridge topology. A 35-mm x 50-mm x 20-mm elliptical fin heatsink (UB3550-20B, Alpha Novatech) and a 12-V, 1.68-W DC fan (F-3010H12BIII-16, Cofan) were selected to cool the system. Using four sets of push-pins and springs, a constant 20-psi pressure was applied to press the heatsink onto the back side of PCB with a 0.5-mm-thick gap filler pad TIM (GR80A, Fujipoly) in between.

The test was setup in a synchronous buckconverter configuration, with 400-V bus voltage, 20-A output current and power level at 4 kW. The test waveform is shown in Figure 6-1, with green waveform being the inductor current and pink waveform being the switching node voltage. As the zoomed-in waveform presented in
Figure 6-2 , the active GaN device has turn-on speed of over 120 V/ns with maximum speed of 150 V/ns benefiting from the minimized common-source inductance of this integrated package. Moreover, the optimized power loop allows the overshoot voltage to be less than 50 V in this test at such high switching speed. The thermal performance of the system was measured under this operating condition with the DC fan directly being attached on and blowing air to the heatsink. IR thermal image (Figure 6-3) shows the QFN 12x12 package case temperature of 99.3 °C which stays well below the maximum operating temperature (125 °C) recommended for this LMG3422R030 GaN power stage product. The measured R_θJA is 3.8 °C/W for this bottom-side cooling system.

Figure 6-1 Power Stage Testing Waveform of LMG3422EVM-043

Figure 6-2 Zoomed-In Waveform at Switching Node

Figure 6-3 IR Thermal Image for EVM Under Testing