Heat dissipation is a major factor affecting the lighting intensity of LED lamps. The heat sinks can solve the heat dissipation problem of low-illuminance LED lamps. But a single heat sink cannot solve the heat dissipation problem of 75W or 100W LED lamps. In order to achieve the ideal lighting intensity, active cooling technology must be used to resolve the heat released by the LED luminaire components. Some active cooling solutions such as fans have less lifespan than LED lamps. In order to provide a practical active cooling solution for high-brightness LED lamps, the heat dissipation technology must be low energy consumption and can be applied to small lamps, its life span should be similar to or higher than the lamp.
To improve the luminous efficiency and service life of LEDs, a good LED heat dissipation technology is the most important issue.
Generally speaking, according to the way the heat is taken away from the radiator, the radiator can be divided into active heat dissipation and passive heat dissipation. The passive heat dissipation means that the heat source from the LED light source is naturally dissipated into the air through the heat sink. The heat dissipation effect is proportional to the size of the heat sink, but because the heat is naturally dissipated, the effect is of course greatly reduced. Active heat dissipation is forced by cooling devices such as fans. It is characterized by high heat dissipation efficiency and the small size of the equipment.
Active heat dissipation, subdivided in terms of heat dissipation, can be divided into air cooling, liquid cooling, heat pipe cooling, semiconductor cooling, chemical cooling, and so on. Air-cooled heat dissipation is the most common way of heat dissipation, and in comparison, it is also a cheaper way. Air cooling is essentially the use of a fan to take away the heat absorbed by the radiator. It has the advantages of relatively low price and convenient installation. However, the dependence on the environment is relatively high, for example, the heat dissipation performance will be greatly affected when the temperature rises and overclocking.
Liquid cooling heat dissipation is the forced circulation of the liquid driven by the pump to take away the heat of the radiator. Compared with air cooling, it has the advantages of quietness, stable cooling, and less dependence on the environment. The price of liquid cooling is relatively high, and installation is relatively troublesome. For cost and ease of use considerations, liquid-cooled heat dissipation usually uses water as the heat-conducting liquid, so liquid-cooled radiators are often referred to as water-cooled radiators.
The heat pipe is a kind of heat transfer element. It makes full use of the principle of heat conduction and the fast heat transfer properties of the refrigerant. It transfers heat through the evaporation and condensation of the liquid in the fully enclosed vacuum tube. It has extremely high thermal conductivity and good isothermal properties. The heat transfer area on both sides of the cold and heat can be changed arbitrarily, heat can be transferred over a long distance. The temperature can be controlled. And the heat exchanger composed of heat pipes has high heat transfer efficiency, compact structure, small fluid resistance. Its thermal conductivity has far exceeded the thermal conductivity of any known metal.
Semiconductor refrigeration is to uses a special type of semiconductor refrigeration chip to produce a temperature difference when it is energized. As long as the heat of the high-temperature end can be effectively dissipated, the low-temperature end will be continuously cooled. A temperature difference is generated on each semiconductor particle, and a refrigeration sheet is formed by connecting dozens of such particles in series, thereby forming a temperature difference on the two surfaces of the refrigeration sheet. Using this temperature difference phenomenon, combined with air cooling/water cooling to cool the high-temperature end, an excellent heat dissipation effect can be obtained. Semiconductor refrigeration has the advantages of low cooling temperature and high reliability. The cold surface temperature can reach below minus 10, but the cost is too high, and it may cause short circuits due to the low temperature, and the current semiconductor refrigeration technology is not mature and not practical enough.
Chemical refrigeration is to use some ultra-low temperature chemical substances and uses to absorbs a lot of heat when they melt to reduce the temperature. In this regard, the use of dry ice and liquid nitrogen is more common. For example, using dry ice can reduce the temperature to below minus 20, and someone uses liquid nitrogen to reduce the CPU temperature below minus 100 (theoretically). Of course, due to the high price and the short duration, this method is often used. Seen in the laboratory or extreme overclocking enthusiasts.
Thermal conductivity (unit: W/mK)
Silver 429;Copper 401;Gold 317;Aluminum 237;Iron 80;Lead 34.8;1070 aluminum alloy 226;1050 aluminum alloy 209;Type 6063 aluminum alloy 201;6061 aluminum alloy 155
Generally speaking, ordinary air-cooled radiators naturally choose metal as the material of the radiator. For the selected material, it is hoped that it has both high specific heat and high thermal conductivity. It can be seen from the above that silver and copper are the best thermally conductive materials, followed by gold and aluminum. But gold and silver are too expensive, so the current heat sink is mainly made of aluminum and copper. In comparison, both copper and aluminum alloy have their own advantages and disadvantages: copper has good thermal conductivity, but it is more expensive, difficult to process, heavy, and copper radiators have a small heat capacity and are easy to oxidize. On the other hand, pure aluminum is too soft to be used directly. The aluminum alloy used can provide sufficient hardness. The advantages of aluminum alloy are low price and lightweight, but the thermal conductivity is much worse than copper. Therefore, in the history of the development of radiators, products with the following materials have also appeared:
Pure aluminum radiator
The pure aluminum radiator is the most common radiator in the early days. Its manufacturing process is simple and the cost is low. So far, pure aluminum radiator still occupies a considerable part of the market. In order to increase the heat dissipation area of its fins, the most commonly used processing method for pure aluminum radiators is aluminum extrusion technology, and the main indicators for evaluating a pure aluminum radiator are the thickness of the radiator base and the Pin-Fin ratio. Pin refers to the height of the fin of the heat sink, and Fin refers to the distance between two adjacent fins. The Pin-Fin ratio is the height of the Pin (excluding the thickness of the base) divided by the Fin. The larger the Pin-Fin ratio means the larger the effective heat dissipation area of the radiator and the more advanced the aluminum extrusion technology.
Pure copper radiator
The thermal conductivity of copper is 1.69 times that of aluminum, so under the same other conditions, the pure copper radiator can quickly remove heat from the heat source. However, the texture of copper is a problem. Many advertised as “pure copper radiators” are not actually 100% copper. In the list of copper, those with a copper content of more than 99% are called acid-free copper, and the next grade of copper is red copper with a copper content of less than 85%. The copper content of most pure copper radiators on the market is somewhere in between. The copper content of some inferior pure copper radiators is even less than 85%. Although the cost is very low, its thermal conductivity is greatly reduced, which affects heat dissipation. In addition, copper also has obvious shortcomings, such as high cost, difficult processing, and large radiator quality, which hinder the application of all-copper heat sinks. The hardness of red copper is not as good as aluminum alloy AL6063, and some mechanical processing (such as slitting, etc.) performance is not as good as aluminum; the melting point of copper is much higher than that of aluminum, which is not conducive to extrusion and other problems.
Copper and aluminum bonding technology
After considering the respective shortcomings of copper and aluminum, some high-end radiators in the current market often use a copper-aluminum combined manufacturing process. These heat sinks usually use copper metal bases, and the heat sink fins are made of aluminum alloy. Of course, In addition to the copper bottom, there are also methods such as using copper pillars for the heat sink, which are also the same principle. With high thermal conductivity, the copper bottom surface can quickly absorb the heat released by the CPU; aluminum fins can be made into the most conducive shape for heat dissipation with the help of complex processing methods, and provide a larger heat storage space and quickly release it. A balance point was found in all aspects.
To improve the luminous efficiency and service life of LEDs, solving the heat dissipation problem of LED lighting products is one of the most important issues. The LED industry has extremely strict requirements on the alignment accuracy of the heat dissipation substrate itself, and it needs to have high heat dissipation and small size. The characteristics of size, good adhesion of metal lines, etc., therefore, the use of yellow light lithography to make thin-film ceramic heat dissipation substrates will become one of the important catalysts to promote the continuous improvement of LEDs to high power.
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