Cause Analysis and Countermeasure of LED Junction Temperature

1. What is the junction temperature of the LED?

The basic structure of an LED is a semiconductor P-N junction. Experiments have shown that when current flows through the LED component, the temperature of the P-N junction will rise. In the strict sense, the temperature of the P-N junction region is defined as the LED junction temperature. 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, 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 P-N junction cannot be extremely perfect, the injection efficiency of the component 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 Injecting a charge (electron) into the P region, in general, 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 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%) fail to overflow the interface smoothly, and the total reflection between the chip and the dielectric interface returns to the inside of the chip and is eventually absorbed by the chip material or substrate through multiple internal reflections, and is changed 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 the epoxy adhesive is a low thermal conductivity material, the heat generated at the P-N junction is difficult to dissipate into the environment through the transparent epoxy, and most of the heat passes through the substrate, the silver paste, the envelope, and the epoxy bonding layer. The PCB and the heat sink diverge downward. Obviously, the thermal conductivity of the relevant material will directly affect the heat dissipation efficiency of the component. A common type of LED, the total thermal resistance from the P-N junction region to ambient temperature is between 300 and 600 ° C / w. For a power LED component with 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 up to the watt level or higher.

3. 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

The input power of the LED is the only source of thermal effect on the component. A portion of the energy becomes radiant light energy, and the rest is eventually turned into heat, which raises the temperature of the component. Obviously, the main method to reduce the effect of LED temperature rise is to try to improve the electro-optical conversion efficiency of the component (also known as external quantum efficiency), so that as much input power can be converted into light energy. Another important way is to improve the components. The heat dissipation ability causes the heat generated by the junction temperature to be dissipated into the surrounding environment through various channels.

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