The technical problems involved in LED luminaires are many and complex, mainly system reliability issues, including the reliability of LED chips , packaged devices, drive power modules, heat dissipation and luminaires. These issues are analyzed separately below:
1. LED lamp reliability related content introduction
Before analyzing the reliability of LED lamps, first introduce some basic contents related to LED reliability, which will help to analyze the reliability of LED lamps.
(1) Essential failure, subordinate failure
LED device failures are generally divided into two types: intrinsic failure and dependent failure. Intrinsic failure refers to the failure caused by the LED chip, which is divided into electrical drift and ion thermal diffusion failure. Subordinate failure is generally caused by the package structure material and process, that is, the package structure and the epoxy, silica gel, conductive paste, phosphor, solder, lead, process, temperature and other factors.
(2) Ten degree rule
Some electronic devices are reduced by half (or down 1/4) for every 10 °C increase in temperature over a range of temperatures. Practice has proved that when the temperature of the LED device heat sink is between 50 ° C and 80 ° C, the LED lifetime value basically conforms to the ten-degree rule. Recently, there have been reports in the media that the life of LED devices has dropped by 10% for every 2 °C rise. When the temperature rises from 63 °C to 74 °C, the average lifetime is reduced by 3/4. This phenomenon is entirely possible due to the different packaging process of the device.
(3) meaning of life
LED lifetime is the operating time at which the optical output power or luminous flux decays to 70% of the initial value under specified operating conditions, while the chromaticity change remains within 0.007.
The meaning of the average life of the LED is the average of the working time before the LED product fails. It is expressed by MTTF, which is the most commonly used reliability parameter for electronic devices.
Reliability testing includes reliability screening, environmental testing, and life testing (long-term or short-term). What we are discussing here is only the life test, and other projects are not considered for the time being.
(4) Long-term life test
In order to confirm whether the life of LED lamps reaches 35,000 hours, long-term life test is required. The current practice basically forms the following consensus: Since the output optical power of GaN-based LED devices is unstable, electrical aging is required according to the regulations of the American ASSIST Alliance. After 1000 hours, the measured optical power or luminous flux is an initial value. After adding the rated current for 3000 hours, the measured luminous flux (or optical power) attenuation is less than 4%, and the current is added for 3000 hours. The luminous flux attenuation is less than 8%, and then the power is applied for 4000 hours for 10,000 hours. The measured luminous flux attenuation is less than 14 %, that is, the luminous flux reaches 86% or more of the initial value. At this point, it can be proved that the LED life is up to 35,000 hours.
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