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The progress of advanced thermal technology of high-power LED
Date:2018.8

Introduction: LED is the abbreviation of light-emitting diode,which is a light-emitting material based on semiconductor die. With the maturity of this technology,large-scale LED has been widely used. LED has low energy consumption but high illumination intensity and long service life. They can be designed in different sizes according to specific needs and have been widely used in interior decoration, automobiles, roads, urban lighting and so on. In real world,the power of the LED cannot reach 100%. Nearly 80% of the power to be converted into heat loss. With the application of large-scale,matrix-shape LED lamps,it will consume more and more energy. If the heat dissipation problem cannot be solved in time,the heat will be accumulated in the P-N junction of the diode,which will reduce the LED lamp's service life and burn the LED lamp.

1. Thermal performance required by high-power LED

Stronger thermal and soaking performance:
The overall thermal resistance of the heat sink is obtained by layer-by-layer accumulation of all the contacting surfaces of the LED can't be neglected when it comes to the overall thermal resistance. The smaller the thermal resistance between it and the LED lamp,the more quickly the thermal energy generated by the LED will be absorbed and transferred to the fins.

Overall, less thermal resistance:
To improve the heat absorption capacity of the heat conducting device and the heat sink device effectively,the heat-conducting device or heat sink should be combined with the LED module as close as possible,even reach seamless. However,in real world, there always will be small seam between the heat dissipation surface of the heat-conducting device/heat sink and the LED module. Therefore,the material with smaller thermal resistance and better adaptability should be chosen to fill the seam. This material is the thermal paste,which can effectively reduce thermal contact resistance and improve efficiency of heat dissipation.

Rapid heat dissipation
During processing,through brazing the heat-absorbing surface and the fins,the heat energy absorbed can be transferred to the fin. In addition,the gap of the fins should be consistent with the air flow direction to prevent the air flow from forming vortexes and causing heat retention.

2. The progress of advanced thermal technology of high-power LED

2.1 Air cooling
This technique(air-cooling) usually uses air concection for heat dissipation,which mainly can be divided into two methods: natural convection heat dissipation and forced convection heat dissipation. Air-cooling mainly takes air as coolant and increases air flow surrounding the components for heat dissipation. This technology and the structure of LED components are relatively simple,so it's easy to be packed and has lower cost. It always functions well,and this technique is relatively mature. However,the efficiency is relatively low. Therefore,generally it can only be used in low-power LED heat dissipation systems. When it comes to the application of natural convection for heat dissipation,using a heat sink can increase the heat dissipation area,improve the thermal performance and also reduce the correlation coefficients of the LED substrate components. There are many factors that can affect the thermal performance,such as the thermal conductivity of the substrate, the convective heat transfer coefficient of the device and so on. The heat dissipation area of the heat sink directly affects the heat dissipation of the LED,and the shape of the heat sink can lead to significant difference as well. Since the power of LED chips keeps increasing,the higher thermal performance is needed. When the natural convection caonnot meet the requirement,fans are needed to accelerate the air convection of the heat dissipation area of the LED device,which therefore can increase heat conductivity. In addition,to reduce the noise produced by the fan,the fan will be designed to have smaller volume and even sometimes be integrated in the LED system.

2.2 Water cooling
LED water cooling system is one of the commonly used heat dissipation technologies in high-power LED equipment. The water cooling system is maninly composed of water pump,substrate,water pipe and other components. It uses the flow of water in the water pipe for heat dissipation,and the main medium is deionized water. Also,the heat pipe can increase surface area, and transfer the heat genetated by the LED chip to the substrate,and then the water will take the thermal energy away. The main function of the pump is to provide the power so that the deionized water can flow in a loop. The system can achieve heat dissipation quickly,though there's still some noise during the process. Therefore,it has been widely used in some small and medium-sized LED devices for the advantages of quietness and fewer dependence on the environment.

2.3 Heat pipe for heat dissipation
Use heat pipe for heat dissipation is also common for LED device. It uses thermal phase transition to improve the thermal performance of the components. The heat pipe heat is mainly composed of the shell,wick and end cap. As for the heat pipe, one end is designed as an evaporation section (heating section) to absorb the thermal energy emitted by the LED device. The other end is designed as a condensation section (cooling section), whose function is to cool the steam and turn it to liquid again. High-power LED arrays are usually adhered to the tube wall of the evaporation section by thermal paste to ensure the effective heat dissipation. When the heat pipe is working,the liquid in the tube will absorb heat and then evaporate in the evaporation section to take thermal energy away. When the steam goes to the condensation section,will release the thermal energy and turn to liquid. The cooled liquid will go back to the evaporation section because of the capillarity. Through this loop,the purpose of heat dissipation can be achieved. However,the disadvantages include the complicated manufacturing process of the equipment,the large volume of the equipment,the relatively high cost and the dissatisfying stability.

2.4 Thermoelectric Effect for heat dissipation
The principle of this technique is thermoelectric effect,and transfers the heat generated by the LED array when it lights up through thermal conductive material. The heat will be absorbed by the condenser and finally achieve the purpose of heat dissipation. Not only the thermoelectric effect,other effects (e.g. the joule heat loss) are applied as well to achieve the heat dissipation. The main advantage of this technique is that structure of the system can be closer. Compared with other traditional heat dissipation methods,the use of thermoelectric radiators can quickly lower the temperature of LED devices (more than 36%). Moreover,this technique can be optimized through choosing the conductive material with higher thermal performance and improving the structure of the device.

2.5 Thermoacoustic Effect for heat dissipation
This technique mainly uses the Thermoacoustic Effect to achieve the purpose of heat dissipation. The principle is to transfer the thermal energy generated by the LED to the dense sound wave area,release it when the sound wave is sparse, and then the energy of sound waves will increase. Because when sound waves propagate in the air,pressure fluctuations or displacement fluctuations will be generated. Through the fluctuations,the heat can be dissipated. The advantages of this technique are fewer components,relatively low cost,and relatively simple structure.

Conclusion:
Technique of heat dissipation is the key to improve the power and performance of LED lamps. There are many heat dissipation methods for LEDs to deal with various situations,including the LED packaging and peripheral equipment. Now,for high-power LED devices,the active cooling method,nano-carbon coating and carbon nanotube thermal interface material are the most suitable. Also,liquid cooling has the best heat dissipation effect and is suitable for ultra-high-power LED device. In order to improve the heat dissipation effect,we can combine those existing methods. For example,liquid cooling system and heat pipe can be combined,substrate and thermal interface material can be improved and nano-carbon coating can be added to achieve outstanding thermal performance. In the future,the target of LED devices must will be higher power and smaller volume,so researching for better heat dissipation techniques or combining various heat dissipation techniques will be a necessary path.

Reference:
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