How to extend the life of LED

LED light failure and its junction temperature is related to the so-called junction temperature is the temperature of the semiconductor PN junction, the junction temperature is higher the sooner the light failure, that is, the shorter life expectancy. Can be seen from the figure, if the junction temperature of 105 degrees, brightness down to 70% of the life of only 10,000 hours, 95 degrees there are 20,000 hours, while the junction temperature down to 75 degrees, life 50,000 hours, 65 degrees can be extended to 90,000 hours. So the key to extending life is to reduce the junction temperature, but these data are only suitable for Cree LED, not suitable for other companies LED.

So we buy LED lamps (all not specifically refers to the LED lighting), we must look at its thermal design is good.

From the figure it can be concluded that the key to extending its life is to reduce its junction temperature. The key to lowering the junction temperature is to have a good radiator, can be timely to the heat generated by the LED out.

Here we are not prepared to discuss how to design the radiator problem, but to discuss which a radiator cooling effect is relatively good problem. In fact, this is a junction temperature measurement problem, if we can measure any of the radiator can reach the junction temperature, then not only can compare the various radiator cooling effect, but also know that after using this radiator Can achieve LED life.

We know that LED is a semiconductor diode, it and all the same diode has a volt-ampere characteristic, and all semiconductor diodes, this volt-ampere characteristic has a temperature characteristic. The characteristic is that when the temperature rises, volt-ampere characteristics of the left. The temperature characteristics of the volt-ampere characteristics of the LEDs are plotted.

Assuming constant current supply to the LED to Io, at junction temperature T1, the voltage is V1, and when the junction temperature rises to T2, the entire volt-ampere characteristics of the left, the current Io unchanged voltage V2. The temperature difference between the two voltage is removed, you can get the temperature coefficient to mV / oC said. For a typical silicon diode, this temperature coefficient is about -2mV / oC. However, most of the LED is not made of silicon material, so its temperature coefficient should also be measured. Fortunately, most of the various manufacturers of LED data sheet gives its temperature coefficient. For example, Cree's XLamp7090XR-E high-power LED, the temperature coefficient of -4mV / oC. Than ordinary silicon diode 2 times larger. As for the United States Purui array LED (BXRA) to give more detailed data.

However, they give the data, the scope is too generous, so that the value of the loss of use. In any case, as long as the temperature coefficient of the LED is very easy to measure the forward voltage from the LED to calculate the junction temperature of the LED.

From the junction temperature to estimate life expectancy should be very simple, just check the curve in Figure 1, we can know that corresponds to 95 degrees when the life of the junction temperature can be LED life of 20,000 hours. However, this method for indoor LED lamps have a certain degree of credibility, if applied to outdoor LED lamps, especially high-power LED lights, where there are many uncertainties.

The biggest problem is the heat dissipation efficiency of the LED street light radiator decreases with time. This is due to the accumulation of dust, bird feces and make its cooling efficiency. But also because the outdoor has a very strong UV, LED will also reduce the life expectancy. UV is mainly on the aging of packaging epoxy play a significant role, if the use of silica gel, can be improved. Ultraviolet light on the aging of the phosphor also has some bad effects, but not very serious.

However, this method is relatively used to compare the two kinds of heat sink cooling effect is more effective. Obviously, the smaller the volt-ampere characteristic of the heat sink left, the better the cooling effect. In addition, for the prediction of indoor LED lamp life still has a certain degree of accuracy.