1. What is the junction temperature of LED?
The basic structure of LED is a semiconductor PN junction. The experiment pointed out that when the current flows through the LED element, the temperature of the P-N junction will rise. Strictly speaking, the temperature of the P-N junction area is defined as the junction temperature of the LED. Usually because the component chips have a small size, we can also regard the temperature of the LED chip as the junction temperature.
2. What are the reasons for the LED junction temperature?
When the LED is working, the following five conditions can cause the junction temperature to rise to varying degrees:
a. Poor component electrode structure, window layer substrate or junction area material and conductive silver glue, etc., all have certain resistance values, and these resistances add to each other to form the series resistance of the LED component. When the current flows through the P-N junction, it will also flow through these resistors, thereby generating Joule heat, causing the chip temperature or junction temperature to rise.
b. Since the P-N junction cannot be extremely perfect, the injection efficiency of the component will not reach 100%, that is to say, in addition to the charge (holes) injected from the P area to the N area when the LED is working, the N area will also Injecting charge (electrons) into the P area, under normal circumstances, the latter type of charge injection does not produce a photoelectric effect, but is consumed in the form of heat. Even if the useful part of the injected charge, it will not all become light, and some of it will become heat when combined with impurities or defects in the junction area.
c. Practice has proved that the limitation of light extraction efficiency is the main reason leading to the increase of LED junction temperature. At present, advanced material growth and component manufacturing processes can convert most of the input electrical energy of the LED into light radiation energy. However, the LED chip material has a much larger refractive index compared with the surrounding medium, resulting in the extremely high temperature inside the chip. Most of the photons (>90%) cannot escape the interface smoothly, and total reflection occurs at the interface between the chip and the medium, returns to the inside of the chip and is finally absorbed by the chip material or substrate through multiple internal reflections, and changes in the form of lattice vibration. It becomes heat, which causes the junction temperature to rise.
d. Obviously, the heat dissipation capacity of LED components is another key condition that determines the junction temperature. When the heat dissipation capacity is strong, the junction temperature will decrease, and conversely, when the heat dissipation capacity is poor, the junction temperature will increase. Since epoxy glue is a material with low thermal conductivity, it is difficult for the heat generated at the P-N junction to radiate upwards to the environment through the transparent epoxy. Most of the heat passes through the substrate, silver paste, shell, and epoxy adhesive layer. The PCB and the heat sink diverge downward. Obviously, the thermal conductivity of related materials will directly affect the heat dissipation efficiency of the component. A common type of LED, the total thermal resistance from the PN junction area to the ambient temperature is between 300 to 600°C/w. For a power LED component with a good structure, the total thermal resistance is about 15 to 30°C /W. The huge difference in thermal resistance indicates that ordinary LED components can only work normally under a small input power condition, while the power dissipation power of power components can be as large as watts or even higher.
3. What are the ways to reduce the junction temperature of LEDs?
a. Reduce the thermal resistance of the LED itself;
b. Good secondary heat dissipation mechanism;
c. Reduce 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
The input power of the LED is the only source of the element's thermal effect. Part of the energy becomes radiant light energy, and the rest eventually becomes heat, which raises the temperature of the element. Obviously, the main method to reduce the LED temperature rise effect is to try to improve the electro-optical conversion efficiency (also known as external quantum efficiency) of the element, so that as much input power as possible is converted into light energy, and another important way is to try to improve the element The heat dissipation capacity of the heat dissipation capacity enables the heat generated by the junction temperature to be dissipated to the surrounding environment through various channels.
