LED Thermal Design Challenges: Tips and Techniques

The latest in LED lighting offers high luminous flux, efficiency, CRI, and reliability, suitable for most lighting applications, including architectural, accent, HID, and down lighting. LEDs save energy, reduce waste, are free of mercury and lead, and convert energy to light more efficiently than common light sources used today. 

The majority of LED failures are caused by excessive or long-term high temperature. Elevated junction temperatures can cause a reduction in light output, degradation of chromaticity performance, life expectancy, and reliability. In this article, we will discuss methods to predict the scale of the thermal challenges a design represents, along with ways to ensure the performance of the LED light engine. Included is a review of the basic heat transfer modes, guidelines for heat sink design, the LED thermal model, example calculations, and other considerations that must be taken into account. 

There are three basic modes of heat transfer: conduction, convection, and radiation. Conduction is the transfer of heat by direct contact of particles of matter. Metals are generally the best conductors of thermal energy due to a large amount of free-moving electrons that are able to transfer thermal energy rapidly. Air is a poor conductor of heat, and eliminating air gaps between materials, even at the microscopic level, can drastically improve the efficiency of the conduction heat transfer. Using a thin layer of a thermal compound will help improve the conduction heat transfer from the LED to the heat sink. Convection is the transfer of thermal energy by the movement of molecules from one part of the material to another. As the fluid or air motion increases, the convective heat transfer increases. Radiation is the transfer of heat energy through empty space. No medium is necessary, since it is transferred via far infrared electromagnetic waves. 

In LED heat sink design, each heat transfer mode plays a role in transferring heat away from the LED junction to the ambient environment. We will focus on the convection mode of heat transfer. In most LED designs, the majority of the heat is transferred from the LED package to the ambient air using convection. A realistic expectation of the air temperature surrounding the device needs to be considered, including the heat generated by the driver source if it is contained in the same space. 

The material used should have a high thermal conductivity for best heat transfer. Typically, aluminum is used due to its relatively low cost and good thermal conductivity. Convection transfer takes place at the surface of the heat sink. Therefore, heat sinks should be designed to have a large surface area. More surface area can be obtained by using fins, increasing the size of the heat sink itself, or changing the heat sink’s orientation.