Luminance Characteristics of High Power Light Emitting Diodes

• Main Authors: Pei-HsuanLee, 李佩璇
• Other Authors: Jung-Hua Chou
• Format: Others
• Language: en_US
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/11406272960298252389

博士 === 國立成功大學 === 工程科學系碩博士班 === 99 === With the advancement of emitting efficiency and reliability of the high power light-emitting-diode (LED) module, which leads to less environmental pollution, the LED has been applied to a large amount of daily-life illumination and its replacement of traditional incandescent bulbs and fluorescent lamp is believed to be the future trend. However, the electrical-to-optical conversion efficiency of LED is still too low, and most of input power has been dissipated as heat, which results in the increase of junction temperature within the LED chip. The junction temperature of the LED directly and greatly affects its output characteristics and lifetime. Therefore, it is necessary and extremely important that the junction temperature of the LED be measured and evaluated accurately. In this study, via the luminous flux experiment in the integrating sphere, the comparison of measurement results shows that the thermal resistance of LED is not the major determinant of its luminous flux decay. Obviously, the luminous flux decay of LED is dependent on the external quantum efficiency of chip material being used. A 3D numerical simulation, based on commercial software ANSYS 12.0, was applied to carry on transient thermal-conduction analysis, and the measurement of diode forward voltage was implemented to estimate the junction temperature of four different LED samples. The simulated temperature for LED model was found to increase with time, while the slug temperature also showed the same trend in simulation and experiment. The simulated thermal resistance were 5.04°C/W and 6.88°C/W for AlN substrate model (red and amber LED) and sapphire substrate model (white and green LED), respectively. The Electrical Test Method (ETM) (also called the forward voltage method) was used to measure the K factor and junction temperature of LEDs with four different colors (red, green, white and amber). Twenty samples for each color. The experimental results show that the ranges of the estimated thermal resistance for red, green, white, and amber LED samples lie between 3.39 to 9.57°C/W, 6.45 to 7.94°C/W, 6.33 to 7.97°C/W, and 3.98 to 8.20°C/W, respectively. The thermal resistance of an AlGaInP-material-based LED (red and amber) reveals higher variability than that of an InGaN-material-based LED (green and white). In the pulsed bias experiment on the basis of 80% duty cycle of input current and 1k Hz operation frequency, the input current of 445.7mA and 418.4mA is respectively required in the white and red LED samples to provide the equivalent illuminance as in normal operation. Moreover, the values of the LED slug temperature appeared to be lower 1.4°C and 2.0°C as compared to the results in the normal steady operation. It is found that a white LED which is activated by the form of duty cycle having 292 hours longer life time than by the normal operation, and the decay of luminous flux would be reduced 0.32%. The similar condition has been found in red LED, and 658 hours longer life time can be achieved under duty cycle than normal operation, and the decay of luminous flux would be reduced 1.00%. The method developed in this study can be used to measure the thermal resistance and luminous decay of LEDs with different colors, and it is helpful in advanced product designs and module reliability analysis to enhance the light intensity and prolong the lifetime of LEDs. The proposed non-destructive measurement method is not restricted to a particular packaging form, and it can be widely applied to other LED modules as well.