Explore the reasons for the white LED lamp luminous decay

1.       Phosphor performance at higher temperatures decline

There is sufficient to prove the fact that the temperature rises, does affect the performance and life of the phosphor. Phosphor manufacturers have done a test at a temperature of 80 degree, the phosphor excitation efficiency is reduced by 2%, and restoration after cooling. And this is just a very short time test. This test proof that as the temperature rise, the phosphor performance will decline. As unrecoverable performance degradation, it is a cumulative process that takes some time.

We also often encounter such a thing, after a period of using or aging white LED lamp, it will become brighter. At present, the situation for small power LED, usually occur at 1000 hours. For small power package of the LED, this situation is likely to remain to 2000 hours. This situation may be generated by the following conditions:

1.1   Phosphor and mixed plastic effect, so that the phosphor degradation, under the   temperature of the early stages, the phosphor performance recovery.

1.2   Phosphor and mixed-adhesive effect, improve the performance of the phosphor.

1.3   The initial period of time blue chip performance enhancements.

   In the experiments, at the initial period of time, the white LED luminous flux both increase and decline in the beginning. This situation also have occurred when use the same red chip package by different manufacturers. Therefore, only a short time is difficult to determine the phosphor experimental problems, or packaging materials and packaging process problems. However, in low-power blue LED life test found that the flux rising in the initial period of time is prevalence phenomenon, generally flux increased in 200 hours. While the plug-in white luminous flux increase in 100 hours generally. From this we can infer that the white LED phosphor performance should be the first recession. Small power white LED after a brief luminous flux increase period, the situation is not very optimistic. High-power white LED, is generally in about 100 hours Luminous flux increased, then in less than 6000 hours the state is not steady, as the time goes by, some products have a more substantial flux rise and fall 6,000 hours later, basically started down firmly all the way. general power white LED products have reach the end of life in 15,000 ~ 20,000 hours(luminous decay 50%).

2.       Blue light LED rapid decline

Compared the chip with other LED lights, the blue LED's life is the shortest. Small power plug blue LED at work under the 20mA, the life is between 7,000~10,000 hours. While the small power plug red LED working 8,000 hours under 50mA is not occurred luminous decay. Red LED consume power 1.8 times than Blue LED at the same package, the performance does not deteriorate. Yellow, green LEDs are much longer than the lifetime of 10,000 hours.

From the point of view on the chip material, Blue and green LED chips, mainly different from doped epitaxial materials. Therefore, the extension part of the material structure determines their ability to withstand the high temperature. This should be the focus of the chip to improve.

3.       LED package base and other materials of poor thermal conductivity

For low-power LED, chip fixing bracket materials are generally iron, from the perspective of heat, ferrous materials is bad. Meanwhile, part of the lead frame outside, cross-sectional area is small, this also increases the resistance. White LED light recession, mainly due to heat, and LED thermal path-related materials should be considered.

4.       The impact of UV radiation on the LED

Effects of UV on the LED, mainly impact on the chip material, phosphor and encapsulant. most affected are encapsulated colloidal. General LED will not face to sunlight, only some diffuse reflectance ultraviolet rays will go into the internal LED chip. Therefore, the UV is not a strong role to impact the chip and phosphor.