Operating temperature effects on LED lifetime

LED operating temperature and lifetime

LED lamps, modules and fixtures can be damaged by high operating temperature, with mechanical failure or significant drop of performance being the most common.

How a LED behaves when functioning at higher operating temperatures is directly related to its quality. High quality LEDs, like Nichia or Cree, will function within parameters at high temperatures too, while low quality LEDs will break down, change their color, loose brightness or a combination of these.

For LED technology, it is a must to avoid operating at temperatures beyond those specified by the manufacturer. Failure to do so while lead to at least one of the following:
  1. complete failure of the LED
  2. light output is decreased permanently (Lumen Degradation) even if the issue with high temperature is solved
  3. light output is decreased temporally while the LED functions at high temperature
  4. the color temperature of the white LED changes

1. Complete failure of the LED

Most materials that compose a LED do not resist at high temperature and when the "weakest link" reaches the breaking point, the LED will fail. Then can also be seen visually, in many cases, below a 4 chip damaged LED.

 

2. LED light output is decreased permanently (Lumen Degradation)

Before the "weakest link" inside a LED reach the breaking point (as above) it, and the other materials, will suffer, gradual and permanent changes. These will lower the light output of the LED, permanently. We can easily understand this with an analogy with the tires on our automobile.  As the materials that make up a tire are subjected to wear and tear, our car will not break as effectively or have difficulty with gripping the road.

As we must replace tires while they still allow the car to move about and not wait for a catastrophic failure, the LED lamps or fixtures must be replaced when their light output has degraded by a certain amount.

Usually, LED should be replaced when it lost 30% of the initial light output. An equivalent statement is "the light output is at 70% of the initial value" and has the acronym "L70". This is what LED lifetime actually means and not the moment the LED fails completely. In the image below, the LED to the left has 35.000 hours lifetime and the one to the right (Nichia) 100.000 hours lifetime, both at a temperature of 85 °C.

The speed at which the materials inside an LED degrade to the point we need to replace it (lifetime L70, 30% light loss) is very dependent on the operating temperature, the quality of the materials and of the design of the LED package.  LEDs could lose 30% of light in a few thousands hours or in over 200.000 hours, as the two graphs show.

To the surprise of some, LED lifetime can be as little as 2000 hours in some cases.

3. Light output is decreased temporally while the LED functions at high temperature

Temperature does not only have long term influence on LED lifetime, it also has immediate influence on its performance. To continue the analogy with tires and automobiles, winter tires have a gradually decreasing performance (grip, breaking) as the outdoor temperature increases beyond 7°C while summer tires have more and more issues as the outdoor temperature goes below 7°C and into negative territory. The 7°C temperature is not chosen at random but industry standard based on testing and the properties of the rubber used in summer, all season or winter tires.

LED performance has a similar relationship with temperature with one important difference: there is no standard temperature value, no "7°C threshold". Instead, the quality of the materials and of the design of the LED package determine it, with quite extreme differences for one LED brand to another.

For example at 75°C the performance of a LED can be 5%-70% less than what we would expect from it (what is written in the datasheet).

4. The color temperature of the white LED changes

Besides lifetime and performance, temperature also influences the color of the white light (color temperature). Poor quality materials or deficient design can change in such a way when exposed to high temperatures that the white light of a LED can become almost blue, permanently.

 

An example that shows effects 2, 3 and 4:

With the above in mind we have selected the Nichia LEDs as the LED of choice for most our strips. The proprietary Nichia design of the 757G has the best resistance to high temperature on the market and thus the best performance in most applications.

LumiFlex Nichia 757G Flexible LED Strips

 

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