The most comprehensive LED heat dissipation problem in history

2020-12-11 17:34:34

The most comprehensive LED heat dissipation problem in history

People pay more and more attention to the heat dissipation of LEDs. This is because the light decay or lifetime of LEDs is directly related to its junction temperature. If the heat dissipation is not good, the junction temperature will be higher and the life span will be short. According to the Arrhenius law Lowering 10°C will extend the life by 2 times. It can be seen from the relationship diagram between light attenuation and junction temperature released by CREE (Figure 1) that if the junction temperature can be controlled at 65°C, the life span of light attenuation to 70% can be as high as 100,000 hours! Can the long-awaited lifespan really be achieved? Yes, as long as the heat dissipation problem can be dealt with seriously! Unfortunately, the actual heat dissipation of LED lights is far from this requirement! As a result, LED lamps The longevity has become a major issue affecting its performance, so it must be taken seriously!

   Moreover, the junction temperature not only affects the long-term lifetime, but also directly affects the short-term luminous efficiency. For example, the relationship between the luminous intensity of Cree’s XLamp7090XR-E and the junction temperature.

   If the luminescence at a junction temperature of 25 degrees is 100%, then when the junction temperature rises to 60 degrees, the light emission will only be 90%; when the junction temperature is 100 degrees, it will drop to 80%; at 140 degrees, it will only be 70%. It can be seen that improving heat dissipation and controlling junction temperature are very important.

In addition, the heat of the LED will cause its spectrum to move; the color temperature increases; the forward current increases (when the constant voltage is supplied); the reverse current also increases; the thermal stress increases; the phosphor epoxy resin aging accelerates, etc. Various problems, so the heat dissipation of LED is the most important issue in the design of LED lamps.

  The first part of the heat dissipation of the LED chip

  一. How does the junction temperature occur?

   The reason why the LED heats up is because the added electric energy is not all converted into light energy, but part of it is converted into heat energy. The luminous efficiency of LED is currently only 100lm/W, and its electro-optical conversion efficiency is only about 20~30%. In other words, about 70% of the electrical energy is turned into heat.

   Specifically, the LED junction temperature is caused by two factors.

   1. The internal quantum efficiency is not high, that is, when electrons and holes are recombined, they cannot generate 100% photons. It is usually called "current leakage" which reduces the recombination rate of carriers in the PN region. The leakage current multiplied by the voltage is the power of this part, which is converted into heat energy, but this part does not account for the main component, because the internal photon efficiency is now close to 90%.

  2. The photons generated internally cannot all be emitted to the outside of the chip and finally converted into heat. This part is the main one, because the current so-called external quantum efficiency is only about 30%, and most of them are converted into heat.

Although the luminous efficiency of the incandescent lamp is very low, only about 15lm/W, it converts almost all electric energy into light energy and radiates it out. Because most of the radiant energy is infrared, the light efficiency is very low, but it eliminates The problem of heat dissipation.

  二. The heat dissipation from the LED chip to the bottom plate

  LED chip is characterized by extremely high heat generation in a very small volume. The heat capacity of the LED itself is very small, so the heat must be conducted out at the fastest speed, otherwise it will produce a high junction temperature. In order to draw the heat to the outside of the chip as much as possible, many improvements have been made in the structure of the LED chip.

  In order to improve the heat dissipation of the LED chip itself, the main improvement is to use a substrate material with better heat conduction. Early LEDs only used Si silicon as the substrate. Later it was changed to sapphire as the substrate. However, the thermal conductivity of the sapphire substrate is not very good (about 25W/(mK) at 100°C). In order to improve the heat dissipation of the substrate, Cree uses a silicon carbide substrate whose thermal conductivity is (490W/() mK)) is nearly 20 times higher than sapphire. And sapphire needs to use silver glue to solidify the crystal, and the thermal conductivity of silver glue is also very poor. The only disadvantage of silicon carbide is that it is more expensive. Currently, only Cree produces LEDs with silicon carbide substrates.

   After using silicon carbide as the substrate, it can indeed greatly improve its heat dissipation, but its cost is too high and it has patent protection. Recently, domestic manufacturers have begun to use silicon materials as substrates. Because the silicon substrate is not restricted by patents. And the performance is better than sapphire. The only problem is that the expansion coefficient of GaN is too different from that of silicon and it is prone to cracking. The solution is to add a layer of aluminum nitride (AlN) in the middle as a buffer.

After the LED chip is packaged, the thermal resistance from the chip to the pin is the most important thermal resistance in the application. Generally speaking, the size of the junction area of the chip is the key to heat dissipation. For different rated powers, corresponding sizes are required The junction area. It also manifests as different thermal resistance.

Early LED chips were led to the outside of the chip mainly by two metal electrodes. The most typical one was a straw hat tube called ф5 or F5. Its heat dissipation was completely led out by two thin metal wires, so the heat dissipation effect was very poor. The thermal resistance is very large, which is why the light decay of this straw hat tube is very serious. In addition, the material used in packaging is also a very important issue. Low-power LEDs usually use epoxy resin as the packaging material, but the absorption rate of epoxy resin to 400-459nm light is as high as 45%. It is easy to cause serious light decay due to the aging after long-term absorption of this short-wavelength light, 50% The life span of light decay is less than 10,000 hours. Therefore, silica gel must be used as a packaging material in high-power LEDs. The absorption rate of silica gel to light of the same wavelength is less than 1%. This can extend the life of the same light attenuation to 40,000 hours.

   It can be seen from the table that the thermal resistance of Cree’s LEDs is at least twice lower than that of other companies because of the use of silicon carbide as the substrate. In order to improve the heat dissipation of high-power LEDs, a solderable copper base plate is usually placed under the base for soldering to the heat sink. These thermal resistances are actually measured on this copper base plate.