I've had this question come up a lot when deciding between top-side and bottom-side cooled devices - what is the general recommendation for what type of cooling methodology is used? Devices that are considered are LMG3522, LMG3422.
"What is the difference between top side and bottom side cooling? When should I use one instead of the other?"
Thanks,
John
Hi John,
To give some background on the top side and bottom side cooling. From a package standpoint the packages have the same thermal resistances from junction (where the heat is produced) to case (thermal pad). For the 30mΩ devices for example It is 0.28 °C/W for top side cooled devices and 0.33 °C/W for bottom side cooled devices. The differences come in when the device is implemented in a system. The top side cooled device’s path for heat goes out the thermal pad, through the thermal interface material (TIM) and through the heat sink. The bottom side cooled device’s path for heat goes out the thermal pad, through the PCB, through the TIM, then through the heat sink. In the case such as our EVMs where the TIM is the same thickness and the heat sinks are the same, you will see better thermal performance from the top side cooled device because the path for heat has less thermal resistance because it doesn’t have to go through the PCB also.
Bottom side cooled:
Top side cooled:
In our EVMs the TIM is the same thickness for both types of devices. In order to be able to use the same thickness TIM our top side cooled devices are place on the back side of the PCB with no other components around. Giving the GaN device this much room is not always feasible for customer designs, sometimes other components, with a higher z-height, must be placed in the area that would be covered by the heat sink and TIM. To accommodate this the heat sink has to sit higher off the PCB and the TIM has to be thicker to account for the larger gap between the top side of the device and the heat sink. Depending on the thickness of the TIM this may mean the bottom side cooled devices would be a better solution for cooling.
This is a long way of saying it depends on the system the device is going in. There may not be a definitive solution to which one is better than the other, but by offering both top side and bottom side cooled devices this allows the customer to have the most flexibility to chose which solution works and fits best with their system.
Best,
Kyle Wolf
