1 Phosphor degradation performance at higher temperatures
There seems to be no relevant information on the relationship between the excitation efficiency of phosphors for LEDs and temperature. However, there are sufficient facts to prove that the temperature rise does affect the performance and life of the phosphor.
Fluorescent powder manufacturers have tested that at 60 °C, the phosphor's excitation efficiency is reduced by 2%, and it is restored after cooling. And this is just a test for a short time. It has been shown that the temperature rises and the performance of the phosphor decreases. As for the unrecoverable performance degradation, it is a cumulative process that takes a certain amount of time.
We also often encounter such things, use or aging for white LEDs for a while, and find that the LEDs are brighter, see Figure 1. At present, this situation occurs for low-power LEDs within 1000 hours (here, the 1000-hour luminous flux may be greater than the initial value, and the product and patch products in the middle of 2008 may be close to or reach 1000 hours). For low power packaged LEDs, this condition can be maintained for up to 2000 hours. This condition may result from the following:
A: The effect of the phosphor and the mixed gel reduces the performance of the phosphor, and restores the performance of the phosphor under the initial action of the temperature;
B: Phosphor and mixed gel action to improve the performance of the phosphor.
C: The performance of the Blu-ray chip is enhanced for the initial period of time.
It was found in the experiment that during the initial period of time, the luminous flux of the white LED rises from the beginning and falls from the beginning. This situation also occurs when the same red chip is packaged in different manufacturers. Therefore, it is difficult to determine whether it is a problem of phosphors or a problem of packaging materials and packaging processes in only a short period of time.
However, in the life test of low-power blue LEDs, it is found that there is a phenomenon that the luminous flux rises in the initial period of time, and the general luminous flux rises in about 200 hours. The luminous flux of the plug-in white light is generally about 100 hours. It can also be inferred that the white LED should be the first degradation of the performance of the phosphor.
After the short-lived light flux boost period of the low-power white LED, the situation is not optimistic. Starting with an airplane without wings, the result will be self-evident. High-power white LEDs generally have a luminous flux increase of about 100 hours, and then are not stable until 6000 hours. Over time, the luminous flux of some products has a large rise and fall. After 6,000 hours, it basically began to steadily slide down all the way. At present, the general high-power white light products reach the end of life (50% of light decay) in 1.5~20,000 hours.
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