Performance of LED Thermal Management System Based on Thermoelectric Cooling / Liquid Metal

        With the improvement of LED luminous efficiency and the manufacture of high-power chips, high-power LEDs are being used more and more. The chips of high-power LED are usually closely arranged to reduce the size of the LED and increase the power, which may cause serious heat accumulation and excessive temperature rise. Since the optical performance and reliability of LED are greatly affected by the junction temperature, the highest junction temperature of LED operation is below 120-140°C. A high junction temperature will reduce the lifetime and luminous efficiency of the LED, and reduce the color stability. Effective thermal management can ensure the safe and efficient operation of LED and prolong their life.

The thermoelectric cooling device (TEC) transfers the heat from the cold end to the hot end, which can realize rapid cooling of the parts in contact with the cold end. Using TEC in LED cooling system can enhance the performance of the cooling system. Liquid metal cooling is rapidly emerging as a novel and promising heat dissipation solution to meet the requirements of high heat flux optoelectronic devices. Compared with water systems and heat pipes, the liquid metal system exhibits the lowest temperature and the greatest stability.

Both thermoelectric cooling and liquid metal cooling are effective thermal management techniques. Combining the advantages of both is expected to further enhance the thermal management performance of LED.

1. Experimental System

The liquid metal used in the experiment is Ga68In20Sn12, which has the advantages of low melting point, high thermal conductivity, non-flammability, non-toxic activity, low vapor pressure and high boiling point. It is therefore suitable for LED cooling systems. The thermal conductivity of the liquid metal was measured using a thermal constant analyzer HotDisk500. During the test, the probe is vertically inserted into the liquid metal and then rested to prevent convection of the sample, and the measurement temperature is 25°C. Ga68In20Sn12 metal is liquid at room temperature, and its thermal conductivity is more than 20 times that of water, which is beneficial for it to be used as a coolant in the thermal management system of electronic devices.

The experimental platform adopts a closed flow channel and is equipped with a liquid reservoir, which is convenient for injecting liquid metal in the flow channel before the experiment and storing the liquid metal after the experiment. The outlet of the reservoir is located close to the bottom to avoid pumping the oxide layer on the surface during circulation. The experimental platform is composed of LED heat source and thermoelectric cooling-liquid cooling heat management system (Figure 1). The power of the LED heat source is 40W, and the heat dissipation area is 5.2cm×4.6cm. The thermal management system consists of a thermoelectric cooler, a copper liquid-cooled radiator, an air-cooled radiator, a liquid reservoir, and a peristaltic drive pump. The cold end of the TEC is connected to the LED, and the hot end of the TEC is connected to the radiator. Liquid metal is used as the medium to flow through the liquid cooling radiator for effective cooling. When the system is running, the cold end of the thermoelectric cooling chip dissipates heat to the LED, and the hot end of the electric cooling chip is cooled by the liquid cooling radiator. The liquid metal is driven by a peristaltic pump, and the heat is dissipated to the environment through the air cooling radiator. The liquid metal returns to the reservoir after passing through the radiator to complete the cycle. A thin layer of thermal conductive silicone grease is coated between the LED, thermoelectric cooling sheet and liquid cooling radiator to reduce surface roughness and contact thermal resistance between devices. The temperature of the LED substrate and the ambient temperature are measured by thermocouples, and the average value of the data is recorded after the temperature is stable in the experiment.

First, compare the heat dissipation performance of the system when liquid metal and water are used as coolant. Then, the orthogonal experiment method was used to explore the influence of TEC power PTEC, ambient temperature Ta, coolant inlet temperature Ti and pump speed vB on the substrate temperature Ts. Finally, test the thermal performance of the system under extreme conditions. Due to the characteristics of liquid metal, the fluid adopts a closed loop. The effect of different flow rates was studied by changing the pump speed. Orthogonal experiment method is a method of scientifically arranging and analyzing multi-factor experiments by using an orthogonal table, which can evenly select the optimal plan with a small number of experiments. And through the means of analysis of variance, analyze the significance of the impact of each factor. The influence of four factors on the heat dissipation performance is studied through experiments. Since the LED lamp can work at an ambient temperature as high as 65°C, the maximum ambient temperature is 70°C. 

2. Experimental Results and Discussion

The cooling performances of water and liquid metal as coolants were compared. The experimental conditions are peristaltic pump speed 50r/min, ambient temperature 30°C, coolant inlet temperature 30°C. As shown in Figure 2, the temperature of the LED substrate decreases with the increase of the thermoelectric cooling power. Under the same heat load, when liquid metal is used as the coolant, the temperature rise of the LED substrate is much lower than when water is used as the coolant. The reason for the above-mentioned difference is the difference in the heat dissipation capability of the hot end of the TEC. On the premise that the heat dissipation condition of the hot end of the TEC is good, the temperature of the cold end of the TEC can be effectively controlled. On the contrary, the temperature of the hot end will rise, even if the working performance of the TEC remains unchanged. Since the thermal conductivity of water is smaller than that of liquid metal, its ability to dissipate heat from the hot end of the TEC is relatively small when water is cooled, resulting in a higher temperature rise of the LED. Based on the same reason, since the electric energy input into the TEC will eventually be converted into heat energy, the heat dissipation of the hot end of the TEC is increased. If the heat dissipation capacity of the hot end is insufficient, the temperature of the hot end of the TEC will increase, which in turn will cause the temperature of the cold end of the TEC and the LED to increase. Therefore, under the water-cooling condition shown in Figure 2, when the TEC power is large, the temperature of the LED increases with the increase of the TEC power. If the heat dissipation of the TEC hot end is not liquid-cooled, the temperature of the hot end will further increase. Due to the higher thermal conductivity of liquid metal, the heat generated by LEDs and TECs can be transported more efficiently. Therefore, when the power of the TEC is high, the temperature of its hot end can still be kept low, and the temperature of the LED can be further reduced accordingly.

As shown in Figure 3, the liquid metal with higher thermal conductivity significantly reduces the system thermal resistance, and the thermal resistance reduction coefficient increases with the increase of TEC power. When the TEC power is 50W, the slope of the thermal resistance reduction coefficient increases slowly, and the thermal resistance at this time is reduced by 79.8% compared with that when water is used as the coolant. 

Orthogonal experiments were carried out according to the level combination of factors, and the experimental data of LED substrate temperature Ts were obtained. Due to the cooling effect of the TEC, the temperature of the cold end of the TEC can be lower than the ambient temperature. When the ambient temperature Ta is high, the LED substrate temperature Ts is even lower than the ambient temperature Ta in some experiments. Experimental results show that the thermal management system combined with liquid metal and thermoelectric refrigeration exhibits good heat dissipation performance.

The analysis of variance found that the liquid metal inlet temperature significantly affects the heat dissipation performance of the hot side of the TEC. The power of the TEC can be adjusted according to the heat dissipation temperature requirements of the LED, and the power of the heat dissipation system can be reduced as much as possible under the premise of meeting a certain heat dissipation effect.

The results of orthogonal experiments show that the coolant inlet temperature and TEC power are the main factors affecting the heat dissipation performance of the thermal management system. In the actual working process of the LED, the inlet temperature Ti of the coolant in the system changes due to the influence of TEC and other factors. At present, the thermal management performance of the system under extreme conditions is mainly studied. For convenience, a higher coolant inlet temperature, a higher ambient temperature, and a lower fluid flow rate were chosen for the study. In other words, take Ti as 50°C, Ta as 70°C, and vB as 50r/min for experiments. If the heat dissipation performance of the system under this extreme condition can meet the requirements, it means that when the values of Ti and other parameters fluctuate in a more gentle direction The heat dissipation performance of the system must also meet the requirements. As shown in Figure 4, when the TEC power does not exceed 50W, the LED substrate temperature decreases as the TEC power increases. Moreover, the magnitude of the reduction decreases as the TEC power increases. When the TEC power is 10W, the LED substrate temperature is the highest value of 64.8℃ under the experimental conditions. This value is lower than the ambient temperature Ta, and far lower than the maximum operating temperature of the LED. This shows that the thermal management system still has good cooling performance under extreme conditions. When the TEC power exceeds 50W, the temperature of the LED substrate increases with the increase of the TEC power. This is because an increase in TEC power will not only increase its ability to absorb heat from LEDs and dissipate heat to liquid-cooled radiators, but also increase the amount of heat it generates. In addition, an increase in TEC temperature leads to lower efficiency. Therefore, there is an appropriate TEC power to make the LED substrate temperature the lowest.

At the same time, the influence of different liquid metal inlet temperatures on the heat dissipation performance of the LED thermal management system under extreme conditions was studied. Under the conditions of higher ambient temperature and lower TEC power and fluid flow rate, the experiment was carried out with Ta as 70°C, PTEC as 10W, and vB as 50r/min. The experimental results are shown in Figure 5. The LED substrate temperature Ts increases approximately linearly with the increase of the liquid metal inlet temperature Ti. When the liquid metal inlet temperature is 50 °C, the LED substrate temperature is the highest value of 64.8 °C under the experimental conditions, indicating that the system has good heat dissipation performance.

3. Conclusion

Under the same conditions, liquid metal cooling can achieve lower LED temperature than water cooling. Under the experimental conditions studied, the maximum reduction in thermal resistance reaches 79.8%. Under the conditions that the experimental environment temperature is 70°C, the liquid metal inlet temperature is 50°C, and the pump speed is 50r/min, the temperature of the LED substrate does not exceed 64.8°C. This shows that the thermoelectric cooling/liquid metal thermal management system can effectively cope with the heat dissipation requirements of LEDs under extreme operating conditions, namely high ambient temperature, high liquid metal inlet temperature and low liquid metal flow rate.

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