Like traditional light sources, semiconductor light-emitting diodes ( LEDs ) also generate heat during operation, depending on the overall luminous efficiency. Under the action of external electric energy, the radiation of electrons and holes recombines to produce electroluminescence, and the light radiated near the PN junction needs to pass through the semiconductor medium of the chip itself and the encapsulating medium to reach the outside (air). Comprehensive current injection efficiency, radiation luminescence quantum efficiency, external light extraction efficiency of the chip, etc., finally only 30-40% of the input electrical energy is converted into light energy, and the remaining 60-70% of the energy is mainly caused by non-radiative recombination lattice vibration. Form converts heat.
The increase in the temperature of the chip enhances the non-radiative recombination and further weakens the luminous efficiency. Because people think that high-power LEDs have no heat, in fact, there are. A lot of heat, so that problems occur during use. In addition, many people who use high-power LEDs for the first time do not know how to solve the heat problem effectively, which makes product reliability a major problem. So, is there any heat generated by the LED? How much heat can you produce? How big is the heat generated by the LED?
At the forward voltage, the electrons receive energy from the power source. Under the driving of the electric field, the electric field of the PN junction is overcome, and the N region transitions to the P region. These electrons recombine with the holes in the P region. Since the free electrons drifting to the P region have higher energy than the P region valence electrons, the electrons return to the low energy state during recombination, and the excess energy is released in the form of photons. The wavelength at which the photons are emitted is related to the energy difference Eg. It can be seen that the illuminating region is mainly near the PN junction, and the luminescence is the result of the combination of electrons and holes releasing energy. A semiconductor diode, in which electrons encounter resistance throughout the entire distance from the semiconductor region to the semiconductor region. In principle, the physical structure of the semiconductor diode is simply from the principle that the electrons emitted from the negative electrode of the physical structure of the semiconductor diode and the number of electrons returning to the positive electrode are equal. Ordinary diodes, in the recombination of electron-hole pairs, are due to the difference in energy level Eg, and the released photon spectrum is not in the visible range.
When electrons are inside the diode, power is consumed due to the presence of a resistor. The power consumed is in accordance with the basic laws of electronics:
P =I2 R =I2(RN ++RP )+IVTH
Where: RN is the N-body resistance
VTH is the turn-on voltage of the PN junction
The heat generated by the power consumed by the RP is the P-region resistance 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 is consistent with the above description. The electric power it consumes is:
P LED = U LED Ã— I LED
Where: U LED is the forward voltage across the LED source
The I LED is the current flowing through the LED. These consumed electrical power is converted into heat release:
Q=P LED Ã— t
Where: t is the power-on time
In fact, the energy released by the electrons in the P region and the holes are not directly supplied by the external power source, but because the electrons are in the N region, when there is no external electric field, its energy level is better than that of the P region. The valence electron energy level is higher than Eg. When it reaches the P region, it recombines with the holes to become the valence electrons of the P region, and it releases so much energy. The size of the Eg is determined by the material itself and is independent of the external electric field. The effect of the external power supply on the electrons is only to push it to do directional movement and overcome the role of the PN junction.
The heat production of LEDs has nothing to do with light efficiency; there is not a few percent of electrical power to produce light, and the remaining few percent of electrical power creates a thermal relationship. Through the understanding of high-power LED heat generation, thermal resistance, junction temperature concept and theoretical formula derivation and thermal resistance measurement, we can study the actual package design, evaluation and product application of high-power LED. It should be noted that thermal management is a key issue in the current stage of low luminous efficiency of LED products . It is the bottom line to fundamentally improve the luminous efficiency to reduce the generation of thermal energy, which requires chip manufacturing, LED packaging and application product development. Advances in technology at all stages.
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