With the continuous evolution of LED materials and packaging technology, the brightness of LED products is continuously improved, and the application of LEDs is becoming more and more widely. The use of LEDs as backlights for displays is a hot topic recently, mainly due to different kinds of LED backlight technologies. The color, brightness, life, power consumption and environmental appeal are more advantageous than the traditional cold cathode tube (CCFL), which attracts the active investment.
The initial single-chip LEDs were not very powerful, had limited heat generation, and had little thermal problems, so the package was relatively simple. However, with the continuous breakthrough of LED material technology in recent years, the packaging technology of LED has also changed. From the early single-chip gun-type package, it has gradually developed into a flat, large-area multi-chip package module; its operating current is from early 20mA. The low-power LEDs on the left and right have progressed to the current high-power LEDs of 1/3 to 1A. The input power of a single LED is as high as 1W or more, and even the 3W and 5W package modes are more evolved.
Since the heat problem caused by the high-brightness and high-power LED system will be the key to affecting the function of the product, to quickly discharge the heat generated by the LED component to the surrounding environment, it is necessary to start with the thermal management of the package level (L1 & L2). At present, the industry practice is to connect the LED chip to a heat spreader with solder or thermal paste, and reduce the thermal impedance of the package module through the heat spreader. This is also the most common LED package module on the market, mainly from Lumileds. LED internationally renowned manufacturers such as OSRAM, Cree and Nicha.
Many terminal applications, such as mini projectors, automotive and lighting sources, require more than a thousand lumens or tens of thousands of lumens for a given area. Single-chip package modules are clearly inadequate. Going to the multi-chip LED package and directly bonding the chip to the substrate is a future development trend.
The heat dissipation problem is the main obstacle in the development of LEDs for lighting objects. The use of ceramic or heat pipes is an effective way to prevent overheating. However, the heat management solution increases the cost of materials. The purpose of high-power LED heat management design is to effectively reduce The heat dissipation between the chip and the final product, R juncTIon-to-case is one of the materials-based solutions that provide low thermal resistance but high conductivity, through chip attachment or hot metal methods to transfer heat directly from the chip to The outside of the package housing.
Of course, the heat dissipation components of LEDs are similar to the heat dissipation of CPUs. They are mainly air-cooled modules composed of heat sinks, heat pipes, fans and thermal interface materials. Of course, water cooling is also one of the thermal countermeasures. In the current hottest large-size LED TV backlight module, the 40-inch and 46-inch LED backlights have input powers of 470W and 550W respectively, 80% of which are converted into heat, and the required heat dissipation is about 360W and 440W or so.
So how do you take this heat away? At present, there is water cooling in the industry for cooling, but there are doubts about high unit price and reliability. It is also useful to use heat pipes with heat sinks and fans for cooling. For example, the 46-inch LED backlight LCD TV of Japanese manufacturer SONY, but the fan consumes power and Problems such as noise still exist. Therefore, how to design a fanless cooling method may be an important key to determine who will win in the future.
Here are some materials for heat dissipation and heat dissipation.
Heat dissipation method
In general, the heat sink can be divided into active heat sink and passive heat sink according to the way heat is removed from the heat sink. The so-called passive heat dissipation means that the heat of the heat source LED light source is naturally radiated into the air through the heat sink, and the heat dissipation effect is proportional to the size of the heat sink, but because of the natural heat dissipation, the effect is of course greatly reduced, and is often used in those space spaces. In equipment that is not required, or used to dissipate heat for components that generate less heat. For example, some popular motherboards also adopt passive heat dissipation on the North Bridge. Most of them adopt active heat dissipation. Active heat dissipation is forced by heat sinks such as fans. The heat emitted by the heat sink is taken away, which is characterized by high heat dissipation efficiency and small size of the device.
Active heat dissipation, subdivided from the heat dissipation method, can be divided into air cooling, liquid cooling, heat pipe cooling, semiconductor refrigeration, chemical refrigeration and so on.
Air-cooled air-cooled heat is the most common form of heat dissipation, and in comparison, it is also a cheaper way. Air cooling is essentially a fan that takes away the heat absorbed by the heat sink. It has the advantages of relatively low price and convenient installation. However, it is highly dependent on the environment, such as rising temperatures and overheating performance.
Liquid cooling
The liquid cooling heat is forced to circulate away the heat of the radiator by the pump under the driving of the pump. Compared with the air cooling, it has the advantages of quietness, stable temperature stability and low dependence on the environment. The price of liquid cooling is relatively high, and the installation is relatively cumbersome. At the same time, install it as much as possible according to the instructions in the instructions to get the best heat dissipation. For reasons of cost and ease of use, liquid cooling heat dissipation usually uses water as a heat transfer liquid, so liquid cooled heat sinks are often referred to as water cooled heat sinks.
Heat pipe
The heat pipe belongs to a heat transfer element, which fully utilizes the principle of heat conduction and the rapid heat transfer property of the refrigerant medium, and transfers heat by evaporation and condensation of the liquid in the fully enclosed vacuum tube, and has extremely high thermal conductivity and good isothermal temperature. of heat transfer area can be arbitrarily changed both hot and cold, the heat transfer can be remote, can control the temperature of a series of advantages, and is composed of heat exchanger tubes having a high heat transfer efficiency, compact structure, small resistance loss of the fluid Etc. Its thermal conductivity has far exceeded the thermal conductivity of any known metal.
Semiconductor refrigeration
Semiconductor refrigeration uses a special semiconductor refrigeration chip to generate a temperature difference for cooling when energized. As long as the heat at the high temperature end can be effectively dissipated, the low temperature end is continuously cooled. A temperature difference is generated on each of the semiconductor particles, and a cooling sheet is formed by connecting dozens of such particles in series to form a temperature difference on both surfaces of the refrigerant sheet. By using this temperature difference phenomenon, the air cooling/water cooling can be used to cool the high temperature end, and an excellent heat dissipation effect can be obtained. Semiconductor refrigeration has the advantages of low refrigeration temperature and high reliability. The cold surface temperature can reach below 10 °C, but the cost is too high, and it may cause short circuit due to low temperature. Now the process of semiconductor refrigeration film is not mature enough. practical.
Chemical refrigeration
The so-called chemical refrigeration is the use of some ultra-low temperature chemicals, which use them to absorb a large amount of heat during melting to lower the temperature. It is more common to use dry ice and liquid nitrogen in this regard. For example, using dry ice can lower the temperature to below minus 20 °C, and some more "perverted" players use liquid nitrogen to lower the CPU temperature below minus 100 °C (in theory), of course, because of the high price and short duration, this Methods are more common in the laboratory or extreme overclocking enthusiasts.
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