一: What are the reasons for the LED junction temperature?
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LED junction temperature reasons and solutions
When the LED is working, there are five conditions that can cause the junction temperature to rise to different degrees:
a. The electrode structure with poor components, the material of the window layer substrate or the junction region, and the conductive silver paste have a certain resistance value, and these resistors are mutually added to form a series resistance of the LED element. When a current flows through the P-N junction, these resistors also flow through, causing Joule heat, which causes an increase in chip temperature or junction temperature.
b. Since the PN junction cannot be extremely perfect, the efficiency of the component injection will not reach 100%, that is to say, in the operation of the LED, in addition to the injection of charges (holes) into the N region, the N region will also be P. Injecting a person's charge (electron), in general, the latter type of charge does not produce a photoelectric effect, but is consumed in the form of heat. Even if the useful part of the injected charge does not become all of the light, some of it combines with the impurities or defects of the junction and eventually becomes hot.
c. Practice has proved that the limitation of light extraction efficiency is the main reason for the rise of LED junction temperature. At present, advanced material growth and component manufacturing processes have enabled most of the input electrical energy of LEDs to be converted into optical radiant energy. However, due to the much larger refractive index of the LED chip material compared to the surrounding medium, the poles generated inside the chip are generated. Most photons (>90%) do not overflow the interface smoothly, but totally reflect on the chip and media interface, return to the inside of the chip and be absorbed by the chip material or substrate through multiple internal reflections, and change in the form of lattice vibration. Heating causes the junction temperature to rise.
d. Obviously, the heat dissipation capability of LED components is another key condition for determining the junction temperature. When the heat dissipation capability is strong, the junction temperature drops. Conversely, when the heat dissipation capability is poor, the junction temperature will rise. Since epoxy glue is a low thermal conductivity material, the heat generated at the PN junction is difficult to dissipate into the environment through the transparent epoxy. Most of the heat passes through the substrate, silver paste, shell, epoxy bonding layer, PCB and The heat sink diverges downward. Obviously, the thermal conductivity of the relevant material will directly affect the heat dissipation efficiency of the component. A common type of LED has a total thermal resistance from PN junction to ambient temperature between 300 and 600 ° C / w. For a power LED component with a good structure, the total thermal resistance is about 15 to 30 ° C / w The large difference in thermal resistance indicates that ordinary LED components can only operate normally with a small input power, while power components can dissipate power as large as watts or higher.
二: What are the ways to reduce the LED junction temperature?
a. reduce the thermal resistance of the LED itself;
b. a good secondary heat dissipation mechanism;
c. reducing the thermal resistance between the LED and the installation interface of the secondary heat dissipation mechanism;
d. control the rated input power;
e. reduce the ambient temperature