Foreword
In China's total electricity consumption, lighting electricity accounts for about 13%. Urban public lighting, especially road lighting, has a large share of the entire lighting sector. In recent years, with the development of the national strategy of building a conservation-oriented society, as a new energy-efficient new light source, LED has become an inevitable trend in the field of road lighting based on its continuous improvement in production technology and performance.
1, the relationship between LED junction temperature and performance
At present, there are two main bottlenecks restricting the development of LED road lighting: one is cost. At present, the price of LED light source is still relatively high compared with the traditional light source. However, with the advancement of production processes and the extensive participation of domestic companies, LED costs will gradually decline. Another bottleneck is heat dissipation. Since LEDs are semiconductor light-emitting devices, the characteristics of semiconductor devices vary significantly with their own temperature. For LEDs, an increase in junction temperature can result in changes and attenuation in all aspects of the device. This change is mainly reflected in three aspects:
(1) reducing the external quantum efficiency of the LED;
(2) shorten the life of the LED;
(3) The main wavelength of the light emitted by the LED is shifted, resulting in a shift in the color of the light source.
Among them, the external quantum efficiency of the device is directly related to the LED light efficiency, and the decrease of the external quantum efficiency will directly lead to the reduction of the LED light effect. As the temperature of the PN junction of the device rises, the dominant wavelength of the white LED chip moves toward the long wavelength. Statistics show that at 100 ° C, the wavelength can be red-shifted by 4 to 9 nm, resulting in a decrease in the absorption rate of the YAG phosphor, which also leads to a decrease in the luminous flux of the LED output. Figure 1 shows the relative light output and junction temperature of the X2Lamps series (C) produced by CREE and the LUXEONK2 series (b) produced by LUMILEDS. These two products are high-power white LEDs with more mature technologies. It can be seen from Fig. 1 that when the junction temperature of the device rises to 65 ° C, the light output of the device will decrease by about 10% with respect to room temperature.
Figure 1: LED relative light output as a function of junction temperature (click on image to enlarge).
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