Airflow and Determining Heatsink Fins

Designing Heatsinks and Thermal Solutions for Xilinx Devices (XAPP1377)

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With the exception of liquid cooled systems and those deployed in a vacuum like space, most systems will use air to transport the heat away from the device either by forced or natural convection. In most cases this necessitates fins, pins, or other extrusions being used either directly attached to the heatsink base or indirectly through conduction from the device to where the heat can be dissipated to the air. To get a suitable exchange of heat within the physical constraints of the system to the air, the size, number, attachment, position, and orientation of the fins is dependent on many system-specific parameters. These include ambient temperature, power dissipation, airflow speed, air pressure, turbulence, manufacturability, and even gravity. Thus, the specific design of the fins is particular to the application and open to you to find the right combination for your conditions and constraints. Most Xilinx evaluation boards use fan sinks, as airflow is not guaranteed by other means. Fansinks provide an effective and controlled means to ensure a minimum airflow to the heatsink. More typical to most systems, forced air is provided by fans in the case or chassis for higher power systems and natural convection for lower power systems. In any of these cases, analysis of the maximum power dissipation with maximum ambient conditions can be used as a first order indication of fin effectiveness. But as covered in a later section, thermal simulation tools should be used to evaluate and refine the fin effectiveness for a given design.

Figure 1. Example Thermal Solutions for Differing Demands