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History of the most complete LED heat dissipation problems

时间:2017-04-06 10:31:09    来源:

LED heat now more and more attention, this is because the LED light or its life is directly related to its junction temperature, the heat is not good as high temperature, short life, in accordance with the law when the temperature is lowered by 10, 'c life will be extended 2 times. Relationship between attenuation and junction temperature released from CREE's (Figure 1) can be seen, if the junction temperature can be controlled in 65 ~ C, then its decay to 70% life can be as high as 100 thousand hours! This is the dream of people's life, but really can achieve? Yes, as long as seriously it is the heat problem processing possible! Unfortunately, now LED lamp radiating and the actual demands! That LED lamp life has become a major problem affecting the performance, so it must be taken seriously!
Moreover, the junction temperature not only affects the long life, but also directly affects the luminous efficiency for a short time, for example, the relationship between the amount of luminescence and junction temperature of Cree's XLamp7090XR-E.
If the junction temperature is 25 degrees, when the luminescence is 100%, then when the junction temperature rises to 60 degrees, the luminous intensity is only 90%; when the junction temperature is 100 degrees, it drops to 80%; the 140 degree is only 70%. So it is very important to improve the heat dissipation and control the junction temperature.
In addition to this LED fever also makes its spectral shift; temperature rise; forward current increases (constant pressure supply); reverse current increase; thermal stress increased; acceleration and so on various problems, aging of the fluorescent powder epoxy resin so that LED cooling is an important issue for the design of LED lamps.
The first part is the heat dissipation of LED chip
I. how is the junction temperature produced?
The reason for LED fever is that the electrical energy added is not converted to light energy, but partly to heat energy. The light efficiency of LED is only 100lm/W at present, and the electro-optic conversion efficiency is only about 20~30%. That is to say, about 70% of the electricity is converted to heat.
Specifically, the generation of LED junction temperature is due to two factors.
1., the internal quantum efficiency is not high, that is, in the electronic and hole composite, and not 100% of all photons, usually referred to as "current leakage", so that the PN region carrier composite rate decreased. The leakage current is multiplied by the voltage, which is the power of this part, which is converted to heat, but this part is not the main component because the internal photon efficiency is already close to 90%.
2. photon generated can be emitted to the outside of the chip and finally converted into heat, which is the main part, because this is called the external quantum efficiency is only about 30%, most of them transformed into heat.
Although the incandescent light efficiency is very low, only about 15lm/W, but it will be almost all electrical energy into light energy and radiation out, because most of the infrared radiation is, so the light efficiency is very low, but with no heat problem.
Two.LED chip in the end of the heat dissipation
The LED chip is designed to produce very high heat in a very small volume. The heat capacity of LED itself is very small, so it must be transmitted at the fastest speed, otherwise it will produce very high junction temperature. In order to heat the chips out of the chips as much as possible, many improvements have been made on the chip structure of LED.
In order to improve the heat dissipation of LED chips, the most important improvement is the use of substrate materials with better thermal conductivity. The early LED only used Si silicon as the substrate. Sapphire was later used as the substrate. But the thermal conductivity of sapphire substrate is not too good, (at 100 C is about 25W/ (m-K)), in order to improve the heat dissipation of the Cree substrate, using silicon carbide silicon substrate, its thermal conductivity (490W/ (m-K)) to nearly 20 times higher than that of sapphire. Moreover, the sapphire uses the silver glue solid crystal, but the silver glue's heat conduction is also very bad. The only drawback is that the cost of silicon carbide is more expensive. Currently, only Cree produces silicon carbide based LED.
The use of silicon carbide as a substrate can indeed greatly improve its heat dissipation, but its cost is too high, but also patent protection. Recently, domestic manufacturers began using silicon material as the substrate. Because the base of silicon material is not limited by patent. And the performance is better than sapphire. The only problem is that the expansion coefficient of GaN and the difference between the silicon is too large and prone to cracking, the solution is to add a layer of aluminum nitride (AlN) buffer.
LED chip package, the thermal resistance from the chip to the pin is a thermal resistance, the most important application in the general area, then the size of the chip surface heat is the key, for different power ratings, requires a corresponding size of the junction area. It also shows different thermal resistance.
Early LED chips mainly by two metal electrodes and leads to the outside of the chip, the most typical is called Phi 5 or F5 strawhat tube, its heat dissipation completely on two of the thin metal wire lead out, so the cooling effect is very poor, large thermal resistance, which is why the straw tube decay very serious reason. In addition, the material used in packaging is also an important issue. Small power LED is usually used as packaging materials of epoxy resin, epoxy resin for 400-459nm but the light absorption rate as high as 45%, it is easy to absorb the aging due to the long short wavelengths produced after the light degradation of life is serious, 50% less than 10 thousand hours. Therefore, silica gel must be used as encapsulation material in high-power LED. Silica gel absorbs less than 1% of the same wavelength light. To extend the life of the same decay to 40 thousand hours.
As shown in the table, the thermal resistance of Cree's LED is due to the use of SiC as the substrate, at least twice as much as the thermal resistance of other companies.

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