Abstract: Scientific evaluation of LED light output degradation and color change to ensure the reliability of LED use requires not only the establishment of predictive models, but also the need to guarantee a certain test time. This paper mainly introduces several prediction models of output light intensity degradation caused by LED current accelerated aging experiments, and analyzes its limitations. At the same time, the results of predicting the lumen maintenance of power LEDs by Weibull distribution function are discussed. It is hoped that through these studies, the industry will be able to propose more predictive models to more objectively describe the changes in light output and color versus time of the lighting industry for the lighting industry.
1 Introduction
With the continuous rapid development of LED technology, especially the power and efficiency of white light, semiconductor lighting devices represented by high-power LEDs have been rapidly developed in recent years, and in the past few years in different lighting systems (for example, export Signal lights, traffic lights, channel letter symbols and other display forms are increasingly used. Compared with traditional lighting sources, LEDs have many advantages such as long life, low driving voltage and fast response time. LED efficacy is generally 80 to 90 lm/W [1, 2], which is significantly higher than the efficiency of 100W incandescent lamps (17 lm/W), although there are reports that the efficiency of incandescent lamps can reach 30 lm/W by 2010. [3], still far lower than the current LED efficacy. Compared with fluorescent light (85 ~ 105 lm / W), the best white LED (90 lm / W 100W) is still slightly lower than the efficacy of fluorescent lamps. However, with the rapid improvement of the efficiency of solid-state light sources, especially the improvement of material growth technology and the improvement of design level, it is inevitable to produce more efficient illumination sources. For example, 131lm/W LED (8W) has been reported here [ 4] and 150 lm/W (9 lm) [2] efficiency, it is expected that by 2020, LED light source will penetrate into the general lighting market, the efficacy of white LED is expected to exceed 200 lm / W [5].
LED In addition to the many advantages mentioned above, there are also many factors that affect the quality of its efficacy, such as the optical initial variables, temperature and electrical properties in the same batch of LEDs, and the inconsistencies in their associated variables over time. Behavior, these factors need to be considered in the structural design, production and maintenance of LED lighting products. For example, for a new batch of LEDs, the initial optical properties may vary due to defects in the growing material and manufacturing processes. Changes in junction temperature or external environment will affect the LED's light output characteristics and electrical parameters, and the brightness and color of the LED will also decay as the ignition time increases. As a result, different LEDs from the same batch may have different light output intensity degradation rates and color changes. If multiple LEDs are integrated into one lighting system, this change will inevitably result in LED array space color and luminous intensity. Non-uniformity, which can be a big problem for large color displays, especially for various imaging systems.
This paper summarizes several predictive models of current output light intensity degradation based on current accelerated aging reported. The limitations were analyzed and the results of the LM-80 lumen maintenance of the power LEDs predicted by the Weibull distribution function were analyzed. It is hoped that through these analyses, the industry will be able to propose more predictive models to more objectively describe the changes in light output and color versus flash time for the lighting industry.
2 Current aging test for light intensity decay assessment
The degradation of the LED spectrum can significantly affect the performance of LED products. The degradation of the LED spectrum, especially the variability of the decay process of the same batch of LEDs, such as differences in package thermal resistance, current load and working substrate temperature. Since the factors mentioned above may affect the quality of LED products, it is necessary to find an objective method for objectively evaluating the quality and performance of LED products. There are many reports of causes of spectral degradation, such as an increase in the internal junction temperature of the LED due to high applied current stress, and an increase in the non-radiative recombination center of the illuminating region, causing spectral degradation [6-8]. Some scholars believe that the excessive conversion of heat from the PN junction results in a decrease in the efficiency of phosphor conversion, which leads to the degradation of the LED spectrum [9]. In addition, the degradation of encapsulated epoxy resin is also considered to be the cause of LED spectral intensity degradation [10,11], because the use of epoxy packaging materials to limit the LED operating temperature can not exceed 120 degrees. Because of the variety of white LEDs on the market, whether from the growth of the front chip or the packaging of the back channel, the heat transfer capability from the chip to the surrounding environment is different. Therefore, it is reasonable to estimate that different products have different decay rates for junction temperatures.
Currently, the best LEDs will have a lifetime of up to 10,000 hours, and it is somewhat impossible to verify the spectral intensity as a function of ignition time under normal operating conditions. The use of accelerated aging quality assessment methods can quantitatively evaluate the important factors of LED quality in the shortest possible time. Junction temperature is an important reason that affects LED light attenuation. The rise of junction temperature will cause LED light to decay quickly. LEDs generate more heat under high current operating conditions, which accelerates aging. There have been many reports on temperature and LED electrical, optical (intensity and color) characteristics. Among them, the mode of decline of LED intensity with the ignition time is the most interesting and often debated focus. At present, many prediction models for output light intensity degradation based on current accelerated aging are successively proposed. Below, we will mainly discuss the following four evaluation models.