Status And Progress Of Led Lighting Cooling Technology

LED is favored by the majority of users because of its small size, low power consumption, environmental protection, durability and rich light source color. But at present, one of the bottlenecks in the development of LED lighting is heat dissipation. This paper will analyze the influence of the heating problem in the lighting process on LED, to lead to the importance of heat dissipation technology in LED lighting, and summarize and analyze the current and future heat dissipation technology.

 
1、 Heating problem and its influence in LED lighting
1). Heating problem of LED lighting
In the process of using LED lighting, it is necessary to convert electric energy into light energy just like traditional lighting. However, in these two ways, none of them can completely convert electric energy into light energy, and only a small part of the electric energy can be converted into light energy. The rest of the electric energy (60% - 70%) is converted into heat energy in the process of LED lighting. Especially for high-power LED devices and lighting fixtures, with the continuous increase of power, the temperature of LED internal chips will gradually rise, and the performance of LED internal chips and other devices will decline or even fail with the rise of temperature. Finally led devices can not work. Fundamentally speaking, the rise of junction temperature reduces the probability of PN junction luminescence recombination. Performance in the light source is the decline in brightness, resulting in a saturation phenomenon. Therefore, the heating problem is an urgent problem to be solved in the process of led development.

2). Influence of heating on LED
In the above heating problem mentioned, heating problem will not only affect the life of LED devices, but also affect the brightness. Experience has proved that the life of LED, especially high-power LED, mainly depends on the junction temperature of the chip. The higher the temperature is, the lower the reliability is, and the shorter the working life is. Therefore, it is not only necessary to comprehensively design LED devices from the aspects of LED materials, manufacturing methods, packaging structure and light-emitting principle, but also more important to solve the heat dissipation problems of LED devices and lamps, select the appropriate packaging structure and reasonable heat dissipation methods, and apply them to LED lighting.

2、 Current status of LED lighting cooling technology
In view of the limitation of LED devices and lamps in the aspect of transforming electric energy into light energy, this paper puts forward the concept of heat dissipation technology. The purpose of heat dissipation is to eliminate the part of light energy converted from electric energy in the process of LED lighting, and the influence of heat converted from electric energy on the internal chip of LED (making the chip performance decline, aging or even failure).
1). Main factors affecting LED heat dissipation
The main factors affecting heat dissipation are material properties (thermal conductivity), packaging structure, packaging material, chip size, chip material, current density on chip, etc. Generally, and lamps are composed of chip, circuit board, external radiator and driver. Therefore, there are two kinds of heat dissipation design schemes: one is to reduce the conversion of LED devices from electric energy to heat energy, the realization process needs to improve the internal quantum efficiency of LED internal devices, so as to improve the light efficiency of LED, and solve the heat dissipation problem of LED in the lighting (use) process from the internal; Second, from the external design considerations, by changing the packaging materials or packaging methods of LED devices and lamps, in order to achieve the purpose of reducing the thermal resistance of packaging, sometimes it is necessary to configure the appropriate radiator to solve the problem of high junction temperature, so as to extend the service life of LED devices.

2)   Examples of several heat dissipation methods
(1) The principle of selecting the best materials
The premise of adopting this heat dissipation method is that the package structure has been determined. The most suitable packaging materials can be selected according to the determined packaging structure to improve the thermal conductivity of the system, thus reducing the thermal resistance of LED lighting devices, and finally achieving the effect of system heat dissipation. Packaging materials can be roughly divided into substrate materials, adhesive materials and packaging materials. As far as the substrate material is concerned, the heat dissipation technology involved in LED lighting devices requires that the substrate material has high electrical insulation, high stability, high thermal conductivity and thermal expansion coefficient of chip matching. The commonly used substrate materials are silicon, metal (aluminum, copper, etc.), ceramic (A1N, SiC) and composite materials.

(2) Liquid cooling
Liquid cooling heat dissipation is a kind of heat dissipation technology, which uses liquid to flow through the radiator surface driven by the pump to dissipate heat. Liquid cooling is a kind of active cooling method. However, the manufacturing process of liquid cooling method is complex and difficult to realize, and the price is high, so it is not suitable for high temperature, vibration and other harsh environment. Moreover, in the application of liquid cooling method in LED lighting devices, the liquid circulation cooling device with high sealing requirements will cause damage to LED devices if it is slightly improper in the production process.

3、 Development of LED lighting heat dissipation technology
With the maturity of LED lighting technology and the popularity of LED lighting applications, the existing cooling technology is not only based on the packaging structure and materials, but also based on the energy transfer process. In recent years, the substrate materials of packaging materials have a new development, and the latest trend points to the research and development of Gan on silicon (GaN—on—Silicon). Compared with the existing sapphire substrate, silicon-based GaN has the following characteristics: it can reduce the coefficient of difference of thermal expansion, enhance the LED luminous intensity, low manufacturing cost and significant heat dissipation effect. Therefore, silicon-based Gan is favored by LED manufacturers.

4、 Conclusion
Compared with the traditional lighting technology, LED has not completely replaced the traditional light source, this is because there are still many key problems in LED lighting technology, one of the main bottlenecks is heat dissipation. Although there are many existing ways of heat dissipation, there are still some limitations, such as difficult implementation, high cost, poor thermal conductivity, high environmental requirements and immature technology. Therefore, the research and development of LED lighting heat dissipation technology is still needed, in order to lay the foundation for the mature development of LED related technology and the wide application of LED.