thermal conductivity vs. convection rates


If you are a specifier trying to sort through “good” LED fixtures from “junk” LED fixtures, many of the design + engineering trade-offs in the product are often obfuscated.  Heatsink design is one of those areas.  Sometimes “good” LED fixtures require big-ass heatsinks; sometimes they have no apparent heatsink.  Why?

There was a smartly written post explaining the need to balance thermal conductivity of materials versus the convection rate of the overall heatsink design.  In sum, a highly thermally-conductivity material (i.e. aluminum) does not necessarily help if the overall convection rate of the heatsink does not match the  flow rate of the material (i.e. not enough surface area, or a constricted air flow) .

The post was written by Mikhail Sagal in the Linked-In group “Innovations in Light”.  I will repost here for posterity:

“The total heat transfer in any thermal system, including LED is a combination of conduction radiation and convection cooling. Given that most applications are operating in natural convective situations i.e. there is not a lot of airflow, one does not need the thermal conductivity of aluminum to dissipate the heat. In a good thermal design, if the heat spreading is correctly designed and achieved, then thermally conductive plastics will be more than adequate to dissipate the heat to the ambient air. Convection is independent of thermal conductivity – so no matter how much conduction there is, dissipating the heat from the surface of the part is the problem and the bottleneck. Finding just the right amount of thermal conductivity needed for a given application is key. “

Mikhail continues on to say:

“Designing for the material is critical….[for example] there are plenty of solutions where an extrusion works and works well, die-casting also. In some cases metals are required due to the high power and the conduction required.  For active cooling metal materials become important because conduction begins to become a bottleneck to heat dissipation rather than convection.”

This basic principal helps explain why high-power point sources (floodlights, grazing fixtures, etc.) need large metal heatsinks. They have to move a lot of power fast away from the source, and dissipate it in hot environments.  On the other end, some designs have lots of low- or mid-power LEDs spread over a broad area (a large PCB in a plastic housing); in this case, they don’t have much thermal power to move very far, and already have lots of surface area to dissipate the heat.