Select the right heat sink to keep LEDs cool

Heat is one of the primary factors that determines the life of an LED lamp or array, since operating at elevated temperature can drastically reduce the lifetime of LEDs. The trouble is that LEDs themselves generate heat as a result of inefficiencies in the process, which releases light energy from the semiconductor material. It’s therefore a careful balance between driving the LED hard enough to get maximum brightness out of it, while making sure it doesn’t get too hot so that operating life is not compromised beyond that which is acceptable in the application.

Effective thermal management and packaging design is required to get heat energy away from the LED as fast as possible to keep it cool. Managing waste heat starts with selecting the right LED heat sink. These specially designed heat sinks usually have extruded ‘fins’ to help dissipate heat as quickly as possible, though they come in many different configurations.

Since LEDs, LED emitters and sub-assemblies come in many different shapes and sizes, heat sinks for LEDs are just as diverse.  

A lot of LED products have corresponding ranges of heat sinks available that are specially designed to fit the LED or array. Then it’s a matter of trading off the drive current against the operating temperature and heat sink dimensions. Or, to put it another way, it’s a matter of trading off the brightness desired against the lifetime expected and the space available.

For example, consider a design that uses Intelligent LEDs’ Olson 4 PowerStar series array, which is an IR array for surveillance systems. This array may be driven by currents in the range 100 to 1000mA. Heat sinks available from the manufacturer to fit this array come in various sizes and shapes (pictured below). Larger heat sinks obviously provide more surface area and are better at getting heat away from the LED, but the amount of space available may be constrained by the design.

 

The Olson 4 PowerStar IR array (left) with corresponding heat sinks.Clockwise from top: 50x20mm and 50x80mm star configuration, 78x46x25mm and 70x70x55mm heat sinks.

Checking the data sheet for the heat sinks reveals that they are suitable for use under different operating conditions (see below). If operating at 350mA is enough for the application, the Olson 4 PowerStar doesn’t require a heat sink as it will operate at a temperature below the maximum LED junction temperature, but this is still above what the manufacturer recommends for long life. Adding the smallest heat sink (50x20mm star shape) in this situation, if space and budget allows, will extend the life of the array. Similarly, if the same array is driven at the maximum allowable current (1000mA), the star shaped heat sinks are not sufficient to keep the array below the temperature the manufacturer recommends; of the two larger square heat sinks is required.

This chart in the heat sinks’ data sheet specifies what heat sink is appropriate for what situation.

If there is no corresponding heat sink available for the LED lamp under consideration, you may decide to look at general purpose heat sinks. Fischer Elektronik makes a popular range of LED heat sinks for general purpose use.

These heat sinks come in different cross sections/shapes and diameters, and as they are extruded, they can be made in different heights as well. In order to select from this range, you’ll need to know how much space is available in your application for the heat sink, as well as the thermal resistance required. Thermal resistance of a system is a measure of how easy it is for heat to escape. Adding a heat sink that has low thermal resistance can help reduce the overall thermal resistance of the system and help to keep the LED cool.

To work out the thermal resistance required for the heat sink, use the following formula:

 θsa is the thermal resistance between the heat sink and the ambient air.

Tj is the maximum temperature we want for the LED. This might be the maximum allowable junction temperature, or we might choose to run it more conservatively at the high end of the recommended operating temperature range. Ta is the temperature of the ambient air. Pd is the amount of power that needs to be dissipated, and θjc is the thermal resistance between the LED chip and its case or the outside of the module, which is usually specified in the LED’s data sheets. 

θcs is the thermal resistance between the LED module and the heat sink, also called the interface resistance. The interface resistance is the thermal resistivity of the interface material, times the thickness of the layer, divided by the surface area that’s in contact with the heat sink. Something like RS silicone grease may be used – it has thermal conductivity of 2.9W/m.K (its thermal resistivity is the reciprocal of that).

This should allow you to estimate the required thermal resistance for the heat sink. You can then study the heat sink’s data sheets to work out whether it’s suitable. As an example, the cross section and thermal resistance for the Fischer SK584 heat sink is shown below. These heat sinks are extruded so their thermal resistance depends on the height you specify (many different cross section shapes are available). According to the graph, if you choose a 50mm extrusion of the SK584 shape, the thermal resistance would be 1.0 K/W.

 

 

Cross section of the Fischer SK584 and its thermal resistance versus thickness.

A sensible next move would be to make a prototype and check the heat sink is adequate for your application. If it isn’t, a bigger heat sink may be selected, or in extreme cases, forced air cooling from a fan may be incorporated.