SLUP412 February   2022 LMG3522R030-Q1


  1. Introduction
  2. Comparing Different Technologies
  3. Advantages of Integrating the Driver With GaN FETs
  4. The GaN-Based 6.6-kW OBC Reference Design
  5. PFC Stage
  6. DC/DC Stage
  7. DC/DC Topology Selection
  8. Frequency Selection
  9. Core Loss
  10. 10Loss of ZVS
  11. 11Dead Time
  12. 12ISR Bandwidth
  13. 13Overall
  14. 14Resonant Tank Design
  15. 15Thermal Solution
  16. 16Layout Best Practices
  17. 17Control-Loop Considerations
  18. 18Conclusions
  19. 19References
  20. 20Important Notice

Thermal Solution

Figure 15-1 shows the complete solution with the cold plate attached. The solution uses liquid cooling with a maximum coolant temperature of 65°C. All hot components access the cold plate from the back side of the printed wiring board (PWB).

GUID-20220218-SS0I-BTGS-042G-TQ20WMXXLW6V-low.png Figure 15-1 3D image of the complete solution.

Figure 15-2 shows a cross-section of this solution illustrating how the GaN FETs interface to the PWB and cold plate. Using a thermal interface material (TIM) will facilitate heat transfer from all hot spots.

Figure 15-2 also shows the placement of the TIM relative to the cold plate and GaN FETs. The FETs themselves have a θJunction-Case of 0.15°C/W. The TIM has a thermal impedance of 1.2 to 1.25°C/W. Overall, θJunction-ColdPlate is 1.35 to 1.5°C/W.

GUID-20220218-SS0I-4LNB-NT6L-4JG1SPPMC0GS-low.png Figure 15-2 Magnified cross-section of the 6.6-kW OBC reference design PWB.

Figure 15-3 shows the bottom side of the PWB, while Figure 15-4 shows the top side of the cold plate. By comparing these images, you can see how the cold-plate features provide access to the thermally significant components highlighted in Figure 15-3.

GUID-20220218-SS0I-F8BJ-SMMZ-HDP98W9JLXJK-low.png Figure 15-3 Bottom view of the 6.6-kW OBC reference design PWB.
GUID-20220218-SS0I-CMQ3-9ZPQ-DSRFP4SZBFSJ-low.png Figure 15-4 Top view of the liquid-cooled cold plate.

Figure 15-5 is a thermal scan of the top side of the board. Given that the significantly hot components are located on the bottom side, there is very little of interest to see here.

GUID-20220218-SS0I-4B4P-FM8Z-KM3LVPM8FBPD-low.png Figure 15-5 Thermal scan of the top side of the 6.6-kW OBC.