LED Heat and Thermal Design

One item that comes up often in forums that we do like seeing, is that thermal is discussed! As many of you know, thermal control is very important for not only LED life, but also the optical output, color temperature, and overall reliability. This is also why we opted for a downward firing reflector as opposed to a typical TIR or reflector bowl since it would allow for more a more optimal thermal pathway from the LED chip to the incoming airflow.

The main LED board is separate from the control board and is monitored using a thermistor that is located near the chip. We don’t expect that when the light is in use that it will reach a thermal step-down, but of course need to plan for the worst if we want to have a reliable product. Many thermal simulations were run to decide on the final design.  The beam pattern has even been designed such that it forces the user to partially aim the light “down” just slightly so that incoming air is hitting the back of the LED heat sink directly. Combine this design decision, with large die cut thermal interface material (not just globs of paste) that are sandwiched together by the magnesium die casting, and you end up with a very well designed, thermally optimized heat sink that is not only thermally efficient, but also very lightweight due to the magnesium material choice.

When using the included large thick silicone strap for mounting on the handlebar, you will notice that the mount itself has an integrated air scoop that helps force more cooling air into areas of the heat sink that otherwise would be hard for air to reach in a conventional manner. Again this scoop was thermally optimized for overall height, fins, and more to help achieve the final design.

LED's themselves exhibit decreased behavior when the thermal load increases. This is often found on cheaper type of lights (across all industries, not just bikes) where after a while the color gets noticeably blue, or the brightness decreases. This is because the temperature at the LED die itself is getting far too hot and operating outside it's designed temperature range.  We are talking about temperatures in excess of 300°F! When we take apart cheaper lights we often find a very small thermal pathway exists for the heat to go from the LED to the outside of the case. Or the case itself does not have enough surface area to properly dissipate the heat.

So what to look for when choosing a light? In respect to thermal, look for a light that has fins that act as heat sinks (such as the Trail Evo that has advanced cooling built in), and if possible try and find pictures of the light taken apart and see if there is a large metal surface for the PCB board that has the LED on it. You'll want to see thermal paste to help fill the imperfections between the surfaces and maximize the thermal transfer. When it comes to bright lights, it's not just about the lumen number, but the proper thermal design will see to it that the light will work reliably for years to come.