By integrating current injection efficiency, radiant luminescence quantum efficiency and chip external light removal efficiency, only 30-40% of the input electric energy is converted into light energy, and the rest 60-70% of the energy is mainly converted into heat energy in the form of non-radiation composite lattice vibration.

heat sink
heat sink

However, the increase of chip temperature will enhance the non-radiation recombination and further weaken the luminescence efficiency. Because, subjectively, people think that high-power leds have no heat, and they do. So much heat that problems occur during use. In addition, many people who use high-power LED for the first time do not know how to solve the thermal problem effectively, making the product reliability become the main problem. So does the LED generate heat at all? How much heat is generated? How much heat is generated by leds?

Under the forward voltage of LED, electrons obtain energy from the power source. Driven by the electric field, they overcome the electric field of PN junction and transition from N region to P region. These electrons recombine with holes in P region.

Since the free electrons drifting to the P region have higher energy than the valence electrons in the P region, the electrons return to the low energy state when 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 luminescence region is mainly around the PN junction, and the luminescence is the result of the combined energy release of electrons and holes.

In a semiconductor diode, electrons encounter resistance all the way into and out of the semiconductor region. In simple principle, the physical structure of a semiconductor diode is the same as the number of electrons that come back to the positive electrode from the negative electrode. For ordinary diodes, when electron-hole pair recombination occurs, due to the factor of energy level difference Eg, the photon spectrum released by them is not in the range of visible light.

As electrons travel through the interior of the diode, the resistance consumes power. The power consumed conforms to the basic laws of electronics:

P = I2R = I2 + IVTH RN++ (RP)

Where: RN is the volume resistance in N region

VTH is the opening voltage of PN junction

RP is the bulk resistance in the P region

The heat generated by the power consumed is:

Q = Pt

Where: t is the electrification time of the diode.

Essentially, the LED is still a semiconductor diode. Therefore, when the LED is working forward, its working process conforms to the above description. The electrical power consumed by it is:


Where: ULED is the forward voltage at both ends of LED light source:

ILED means the current flowing through the LED

The electrical power consumed is converted into heat emission:

Q = PLED * t

Where, t is the power-on time

In fact, the energy released by the electron when it recombines with the hole in the P region is not directly provided by the external power source, but because the electron in the N region, when there is no external electric field, its energy level is higher than Eg of the valence electron in the P region. When it gets to the p-zone and recombines with the hole to form a valence electron in the p-zone, it releases that much energy.

Eg is determined by the material itself, independent of the external electric field. The effect of external power source on the electron is only to push it to do directional movement and overcome the effect of PN junction.

LED heat generation is independent of light efficiency; There is no relationship between a few percent of electric power producing light and the other few percent producing heat.

heat sink
heat sink

Through the generation of high-power LED heat, thermal resistance, junction temperature concept understanding and theoretical formula derivation and thermal resistance measurement, we can study the actual packaging design, evaluation and product application of high-power LED.

It needs to be noted that heat management is the key issue at the current stage when LED products are not highly efficient. To fundamentally improve the luminous efficiency to reduce the generation of heat energy is a radical move, which requires technological progress in chip manufacturing, LED packaging and application product development.

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