Heat dissipation of LED tube
People pay more and more attention to the heat dissipation of LEDs. This is because the light decay of LEDs or their lifetime is directly related to its junction temperature. Lowering 10°C will extend the life by 2 times. It can be seen from the relationship 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! This is the longevity that people dream of, but can it really be achieved? Yes, as long as the heat dissipation problem can be dealt with seriously, it is possible to do it! Unfortunately, the actual heat dissipation of LED lights is far from this requirement! As a result, the life of the LED tube has become a major issue affecting its performance, so it must be taken seriously!
Figure 1. The relationship between light attenuation and junction temperature
Moreover, the junction temperature of the LED tube not only affects the long-term life, but also directly affects the short-term luminous efficiency. For example, the relationship between the luminous output of Cree's XLamp7090XR-E and the junction temperature is shown in Figure 2.
Figure 2. The relationship between junction temperature and light emission
If the luminescence at a junction temperature of 25 degrees is 100%, then when the junction temperature rises to 60 degrees, the luminescence 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 it is very important to improve the heat dissipation and control the junction temperature.
In addition, the heat of the LED will cause its spectrum to move; the color temperature increases; the forward current increases (when power is supplied with constant voltage); the reverse current also increases; the thermal stress increases; the phosphor epoxy resin aging accelerates, etc. There are various problems, so the heat dissipation of LED is the most important problem in the design of LED tube.
The first part of the heat dissipation of the LED chip
1. How the junction temperature is generated
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, 100% of photons cannot be generated. It is usually called "current leakage" that 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 at present, the 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 does not The problem of heat dissipation.
2. Heat dissipation from the LED chip in the LED tube to the bottom plate
The characteristic of the LED chip is that it generates extremely high heat 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 out 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 heat conduction 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.
Figure 3. LED structure diagram of sapphire and silicon carbide substrate
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.
Substrate material thermal conductivity W/(m·K) expansion coefficient (x10E-6) stability thermal conductivity cost ESD (antistatic)
Silicon carbide (SiC) 490-1.4 good high good
Sapphire (Al2O3) 461.9 is generally 1/10 of SiC
Silicon (Si) 1505-20 is good, 1/10 of sapphire is good
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 a different thermal resistance. The thermal resistance of several types of LEDs are as follows:
Type straw hat tube piranha 1W surface glow
Thermal resistance oK/W150-200508-155
Early LED chips were led to the outside of the chip mainly by two metal electrodes, the most typical one was called ф5 or F5
