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Causes of LED’s Heat Generation

Aug 10, 2020

Causes of LED’s Heat Generation


As with conventional light sources, semiconductor emitting diodes (LEDs) also generate heat during operation, depending on the overall luminous efficiency. Under the action of applied electrical energy, the radiation of electrons and holes recombines to produce electroluminescence, and the light radiated near the P-N junction needs to pass through the semiconductor medium and packing medium of the chip itself to reach the outside (air). Comprehensive current injection efficiency, radiation luminescence quantum efficiency, chip external light extraction efficiency, etc., the final only 30-40% of the input energy into light energy, and the remaining 60-70% of its energy mainly occurs in a non-radiation complex form of dot-matrix vibration conversion heat.

The increase of chip temperature will enhance the non-radiation complex, further weakening the luminous efficiency. Because people subjectively think that high power LEDs have no heat, in fact, they do. A lot of heat easily causes many problems during use. In addition, many people who use high power LEDs for the first time and do not understand how to effectively solve thermal problems, making production reliability become the main problem. So here are some questions let us think of: Does the LEDs have any heat generated? How much heat can it produce? How much heat does the LED generate?

Under the forward voltage of the LED, the electrons obtain energy from the power supply. Under the driving of the electric field, the electric field of the PN junction is overcome, and the transition from the N region to the P region occurs. These electrons recombine with the holes in the P region. Since the free electrons drifting into the P region have higher energy than the valence electrons in the P region, the electrons return to a low energy state during recombination, and the excess energy is released in the form of photons. The wavelength of the emitted photon is related to the energy difference Eg. It can be seen that the light-emitting area is mainly near the PN junction, and light emission is the result of the energy released by the recombination of electrons and holes. In a semiconductor diode, electrons will encounter resistance during the entire journey from the semiconductor zone to the semiconductor zone. Simply from the principle, the physical structure of the semiconductor diode is simply from the principle, the number of electrons emitted from the negative electrode and the electrons returned to the positive electrode of the semiconductor diode are equal. Ordinary diodes, when electron-hole pair recombination occurs, due to the factor of energy level difference Eg, the released photon spectrum is not in the visible range.

On the way inside the diode, electrons consume power due to the presence of resistance. The power consumed conforms to the basic laws of electronics:

P = I2 R = I2 (RN + + RP) + IVTH

Notes: RN is the body resistance of the N zone

VTH is the turn-on voltage of the PN junction

RP is the bulk resistance of the P region

The heat generated by the power consumed is:

Q = Pt

Where: t is the time the diode is energized.

In essence, the LED is still a semiconductor diode. Therefore, when the LED is working in the forward direction, its working process conforms to the above description. The electrical power it consumes is:

P LED = U LED × I LED

Where: U LED is the forward voltage across the LED light source

I LED is the current flowing through the LED

The electrical power consumed is converted into heat and released:

Q=P LED × t

Notes: t is the power-on time

In fact, the energy released when the electron recombines with the hole in the P region is not directly provided by the external power supply, but because the electron is in the N region, when there is no external electric field, its energy level is higher than that of the P region. Valence electron level is higher than Eg. When it reaches the P region and recombines with holes to become valence electrons in the P region, it will release so much energy. The size of Eg is determined by the material itself and has nothing to do with the external electric field. The role of the external power supply to the electron is to push it to move directionally and overcome the role of the PN junction.

The amount of heat generated by an LED has nothing to do with light efficiency; there is no relationship between what percentage of electrical power produces light, and the remaining percentage of electrical power produces heat. Through the understanding of the concepts of heat generation, thermal resistance and junction temperature of high-power LEDs and the derivation of theoretical formulas and thermal resistance measurements, we can study the actual packaging design, evaluation and product applications of high-power LEDs. It should be noted that heat management is a key issue at the current stage of low luminous efficiency of LED products. Fundamentally improving luminous efficiency to reduce the generation of heat energy is the bottom of the kettle. This requires chip manufacturing, LED packaging and application product development. Technological progress in all aspects.

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