Current Status and Progress of LED Lighting Cooling Technology

LEDs are characterized by their small size, low power consumption, environmental protection, ruggedness and rich color of light source. It is favored by the majority of users. However, one of the bottlenecks facing the development of LED lighting is heat dissipation. This paper will analyze the importance of heat dissipation technology in LED lighting by analyzing the influence of heating problems in the lighting process, and will do the current and future cooling technology. Generalization and analysis.

First, the heating problems and influences in LED lighting

1. The heating problem of LED lighting

In the process of using LED lighting, as with conventional lighting, electrical energy needs to be converted into light energy. However, in either of these two ways, none of them can completely convert electrical energy into light energy, and only a small part of the electrical energy can be converted into light energy, and most of the remaining electrical energy (60% - 70%) in LED illumination, illumination In the process of conversion into heat. Especially for high-power LED devices and lighting fixtures, as the power continues to increase, the temperature of the LED internal chip will gradually increase, and the performance of the LED internal chip and other devices will decrease with the temperature rise, even Invalid. Eventually the LED device will not work. Fundamentally, the rise in junction temperature reduces the probability of PN junction luminescence recombination. In the light source, the brightness of the light is reduced, and saturation occurs. Therefore, the problem of heat generation is an urgent problem to be solved in the development of LED.

2. The effect of heating on LED

As mentioned above, the heating problem not only affects the life of the LED device, but also affects the brightness of the light. Experience has shown that the life of LEDs, especially high-power LEDs, depends mainly on the junction temperature of the chip, and the higher the temperature. The lower the reliability, the shorter the working life. Therefore, it is not only necessary to comprehensively design LED devices from the aspects of LED materials, fabrication methods, package structures and illumination principles, but more importantly, to solve the heat dissipation problems existing in LED devices and lamps, to select a suitable package structure, a reasonable heat dissipation method, and Applied to LED lighting.

Second, LED lighting technology status

In view of the limitations of LED devices and lamps in converting electrical energy into light energy, the concept of heat dissipation technology is proposed. The heat dissipation is designed to solve the part of the light energy converted into electrical energy during the LED lighting process, and the heat converted by the electrical energy has an effect on the internal chip of the LED (so that the performance of the chip is degraded, aged or even ineffective).

1. Main factors affecting LED heat dissipation

The main factors affecting heat dissipation are material properties (thermal conductivity), package structure, package material, chip size, chip material, current density on the chip, and so on. In general, LED lighting devices and luminaires are composed of a chip, a circuit substrate, an external heat sink, and a driver. Therefore, there are currently two thermal design schemes: one is to reduce the conversion of LED devices from thermal energy into thermal energy, and the process needs to improve the internal quantum efficiency of the LED internal devices, thereby improving the light-emitting efficiency of the LEDs, thereby internally solving the LED illumination (using). The heat dissipation problem generated in the process; the two sides are from the external design considerations, by changing the LED device and the packaging material or packaging method of the lamp to reduce the thermal resistance of the package, sometimes it is necessary to configure a suitable heat sink to solve the problem. The junction temperature problem, in turn, extends the life of the LED device.

2. The current heat dissipation method

Due to technical limitations, the external design of the LED lighting device or the use of a heat sink has been adopted to solve the heat dissipation problem. There are many kinds of heat dissipation methods for LED lighting devices, which can be divided into package-level heat dissipation methods and lamp-level heat dissipation methods. The package-level heat dissipation method, as its name implies, achieves the effect of reducing the thermal resistance of the package by optimizing the internal package structure and materials of the LED, and is mainly divided into a silicon substrate flip chip (FCLED) structure and a metal circuit board structure in the package structure. The material selection principle based on the substrate material and the adhesive material.

The lamp-level heat dissipation method mainly refers to the way of heat dissipation during the transfer process of heat from the package substrate to the external heat sink, which is mainly divided into passive heat dissipation and active heat dissipation. Active heat dissipation refers to driving the LED internal chip through energy outside the system. The heat of the device itself is dissipated, mainly including the addition of a fan for forced heat dissipation, liquid cooling, semiconductor cooling, ion wind cooling, and synthetic jet heat dissipation. Passive heat dissipation refers to the heat sink itself, which will be generated during the LED lighting process. The heat is dispersed to achieve the effect of lowering the junction temperature, mainly including direct natural convection heat dissipation and heat pipe (flat heat pipe, loop heat pipe and finned heat pipe).

3. Examples of several heat dissipation methods

(1) Principles of selection of materials

The premise of adopting this heat dissipation method is that the package structure has been determined, and the most suitable packaging material can be selected according to the determined package structure to improve the thermal conductivity of the system, thereby reducing the package thermal resistance of the LED lighting device, and finally achieving the system heat dissipation effect. . The packaging material can be roughly classified into three types: a substrate material, a bonding material, and a packaging material.

As far as the substrate material is concerned, the heat dissipation technology involved in the LED lighting device requires the substrate material to have high electrical insulation, high stability, high thermal conductivity, and chip-matched thermal expansion coefficient. Commonly used substrate materials are mainly silicon, metal (aluminum, copper, etc.), ceramic (A1N, SiC) and composite materials.

(2) Liquid cooling

The liquid cooling heat dissipation method is a heat dissipation technology that uses a liquid to flow through the surface of the heat sink under the force of the pump. American manufacturer Etemaleds has launched a "water-cooled" LED light - Etemaleds HydraLux. It adopts liquid cooling method, which not only saves the heat pipe, heat sink and fan used to cool the inside of the bulb, but also does not cover the heat sink material in the upper part of the bulb, and its light radiation angle is expanded to 360 degrees.

As can be seen from the above description, the liquid cooling method is an active heat dissipation method. However, the liquid cooling method is complicated and difficult to implement in the production process, and the price is high, and it is not suitable for use in harsh environments such as high temperature and vibration; In the application of LED lighting devices, it is required to seal the liquid circulation refrigeration device with high sealing. If it is slightly improper in the production process, the LED device will be damaged.

Third, the progress of LED lighting cooling technology

With the maturity of LED lighting technology and the popularity of LED lighting applications, the existing cooling technology is not only based on package structure, materials, but also based on energy transfer process. The substrate materials in packaging materials have been newly developed in recent years, and the latest trend points to the development of GaN-on-Silicon. Compared with the existing sapphire substrate, the silicon-based gallium nitride has the following characteristics: the coefficient of thermal expansion difference can be reduced, the luminous intensity of the LED can be enhanced, the manufacturing cost is low, and the heat dissipation effect is remarkable. Therefore, silicon gallium nitride is favored by LED manufacturers.

Fourth, the conclusion

Compared with traditional lighting technology, LED does not completely replace 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 heat dissipation methods, there are limitations such as difficulty in implementation, high cost, poor thermal conductivity, high environmental requirements, and immature technology. Therefore, in the LED lighting and cooling technology, it is still necessary to be deeply researched and developed in order to lay the foundation for the mature development of LED related technology and the wide application of LED.

references

[1] Gou Jun, Liu Zhongliang. LE D lighting cooling technology status and progress [J]. China Lighting Appliance, 2o12(o2)

[2] Huang Lei, Chen Honglin. Research progress of ELD lighting heat dissipation [J]-Guangzhou Chemical Industry, 2012(O8)

[3] Zhao Xueli. Research on heat dissipation technology in LED devices and lamps [D]. China Institute of Metrology. 2012

[4] Zhao Ziwei, Ji Xianbing, Xu Jinliang, Xue Qiang. Zhang Wei. Research progress on the life and heat dissipation technology of high-power LEDs [f]. Microelectronics, 2013 (02)

[5] Experimental study of Sun Zheran's LED cooling technology for assisted illumination [D]. Beijing University of Chemical Technology. 2012

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