Predict LED life through LED light decay [Illustration]

Everything has a process of occurrence, development and demise. LED is no exception and has a certain life span. The early LEDs were just gifts such as flashlights and desk lamps. The time spent was not long and the life problem was not outstanding. But now LEDs have begun to be widely used in outdoor and indoor lighting, especially high-power LED street lights, which have high power, high heat, long working hours, and long life problems. In the past, the myth that LED life must be 100,000 hours seems to have completely shattered. So where is the problem?

If the failure of the power supply and the drive is not considered, the life of the LED is manifested by its light decay, that is, the time is long, and the brightness becomes darker and darker until it is finally extinguished. A time of 30% attenuation is usually defined as its lifetime.

So can the life of LEDs be predicted? This question cannot be answered simply and needs to be started from the beginning.

1, LED light decay

Most white LEDs are obtained by illuminating a yellow phosphor with a blue LED. There are two main reasons for the LED light decay. One is the light decay of the blue LED itself. The light decay of the blue LED is much faster than the red, yellow and green LEDs. Another is the light decay of the phosphor, which is very attenuated at high temperatures. The light decay of various brands of LEDs is different. Usually LED manufacturers can give a standard set of light decay curves. For example, the light decay curve of Cree in the United States is shown in Figure 1.

Figure 1: Cree's LED light decay curve

It can be seen from the figure that the light decay of the LED is related to its junction temperature. The so-called junction temperature is the temperature of the semiconductor PN junction. The higher the junction temperature, the earlier the light decay, that is, the shorter the lifetime. As can be seen from the figure, if the junction temperature is 105 degrees, the lifetime of brightness reduction to 70% is only 10,000 hours, there are 20,000 hours at 95 degrees, and the junction temperature is reduced to 75 degrees, and the life expectancy is 50,000 hours. It can be extended to 90,000 hours at 65 degrees. So the key to extending life is to lower the junction temperature. However, these data are only suitable for Cree's LEDs. Not suitable for LEDs of other companies. For example, the light decay curve of Lumiled's Luxeon K2 is shown in Figure 2.

Figure 2: Light decay curve of Lumiled's LuxeonK2

When the junction temperature is raised from 115 ° C to 135 ° C, the lifetime is reduced from 50,000 hours to 20,000 hours. The light decay curves of other companies should be available from the original factory.

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