Fact or Fiction – LEDs don’t produce heat

Each interface produces a resistance to heat transfer, which means the number of interfaces should be reduced, and the thermal resistance between mating surfaces should be minimized. Surfaces are rarely smooth, for example a metal heat sink might have grooves where it has been milled. If two rough surfaces are mated, most of the heat transfer takes place via point contacts, since air is a poor conductor of heat. This problem can be overcome by filling the gap with a soft, thermally conductive material (see figure 4).

Each interface produces a resistance to heat transfer, which means the number of interfaces should be reduced, and the thermal resistance between mating surfaces should be minimized. Surfaces are rarely smooth, for example a metal heat sink might have grooves where it has been milled. If two rough surfaces are mated, most of the heat transfer takes place via point contacts, since air is a poor conductor of heat. This problem can be overcome by filling the gap with a soft, thermally conductive material (see figure 4).

Calculating thermal resistance
The thermal resistance (units: °C/watt) between two points measures the change in temperature difference between those two points for each watt of input power. Or: temperature change = thermal resistance x LED power (where LED power = voltage x current)

Clearly, running the LED at a higher current results in a higher temperature change. Also, a low thermal resistance is highly advantageous in keeping temperature change to a minimum.

The total thermal resistance (Rth) from junction to ambient is the sum of the individual Rth values. Referring to figure 3:

Rth(junction to ambient) = Rth(junction to case) + Rth(case to ambient)

Rth(junction to case) should be provided by the LED manufacturer's data sheet. Rth(case to ambient) can be calculated by measuring the temperature change from case to ambient. This is generally done using a thermocouple, ideally in a hole drilled in the side of the board directly under the device.

Fixture performance
With the above information, it should be possible to calculate the junction temperature of the LED under different operating conditions, which in turn should allow the lifetime of the LEDs to be estimated in the fixture in question.

It’s imperative that the thermal performance of the entire system is taken in to account. For example, LEDs have been placed into IP-rated fixtures that are used as floodlights. This creates a sealed skin around the LED module, forming an air pocket which prevents efficient thermal transfer to the outside surface. The lack of a radiator is a sign that heat sinking isn't a major consideration in the design – which can lead to significant long-term problems.


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