0 Preface
LED lighting applications and industry development prospects are enticing, but LEDs must be solved on a large scale and widely used. There are still many problems to be solved. Heat dissipation is one of the difficulties, key and core issues. At present, the photoelectric conversion efficiency of LED is relatively poor. Only 20% to 30% of the input electrical energy is converted into light energy, and the remaining 70% to 80% of the energy is mainly converted into thermal energy in the form of lattice vibration generated by non-radiative recombination. The LED temperature rises, and the increase in temperature causes the following adverse effects: reduced luminous intensity, dominant wavelength shift of the light, severely reduced lifetime of the LED, and accelerated light decay of the LED. Therefore, the performance and reliability of LED products depend to a large extent on whether there is a good heat dissipation design and whether the heat dissipation measures taken are effective. At present, the heat flux density of LED devices is up to 1W/mm2, which belongs to high heat flux devices. The heat dissipation problem of LED devices is one of the key issues for its large-scale application. Low thermal resistance and good heat dissipation are important manifestations of the future development trend of high-power LEDs and product competitiveness. Based on the analysis of the thermal resistance of the main components of high-power LED heat dissipation, this paper reviews the current research progress of LED heat dissipation technology from two aspects of heat dissipation technology and thermal interface materials.
1 LED thermal resistance analysis
A typical LED chip assembly structure is shown in Figure 1, in which the LED chip is fixed on the substrate by soldering or silicone grease, and the heat is transferred from the chip to the heat sink by the solder layer, the substrate, and the thermal interface material (TIM) by heat conduction. Then, the heat is transferred to the environment by various heat dissipation methods, and the heat sink can be replaced by other cooling schemes, such as forced air cooling, heat pipe, and thermoelectric cooling. According to the principle of thermal resistance analysis, we can know:
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Among them, Tjunction is the LED junction temperature, Ta is the ambient temperature, P is the power converted into thermal energy from the LED input electrical energy, Rj-sp is the thermal resistance of the LED chip to the solder layer, and Rsp-sub is the thermal resistance of the solder layer to the substrate. Rsub-TIM is the thermal resistance between the substrate and the thermal interface material, RTIM-hs is the thermal resistance between the thermal interface material and the heat sink, and Rhs-a is the thermal resistance between the heat sink and the environment. It can be seen from the above formula that the junction temperature of the LED is mainly determined by the thermal load, the ambient temperature and the thermal resistance of each link. To reduce the junction temperature, the thermal resistance of each link should be reduced, especially the rational design of the heat sink and the selection of thermal interface materials.
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Figure 1 LED thermal resistance model
2 LED cooling technology
2.1 Natural Cooling Natural convection heat is usually used for heat dissipation in low-power devices and single high-power devices. Fin heat dissipation is the most common method. Arik et al. compared the fin form and the pin fin form as the performance of the LED heat sink, and compared the heat dissipation performance of different materials, fin numbers and heights. Luo et al. gave the design and optimization method of plate-fin heat sink and used it for thermal management of high-power street lamps. The reliability of the method was verified by experiments. Scheeper et al. compared the detailed 3D LED thermal model with the simplified thermal resistance model. The detailed model takes into account the effects of power dissipation and thermal resistance on the light output. It is pointed out that the simplified thermal resistance model will overestimate the junction temperature. An optimized design method for natural convection heat dissipation of LEDs is given, which can be used to guide the design of external ribbed heat sinks.
2.2 Air cooling
Ordinary fans have low noise, high power consumption, and poor reliability, and the rotating magnetic field around the fan causes the normal operation of the surrounding electronic components due to magnetic leakage or electric spark, so it is rarely used in LED heat dissipation. Acikalin et al. proposed a scheme for applying piezoelectric fans to LEDs, and studied the effects of fan amplitude, distance from heat source, length, and fan resonance frequency offset. It is pointed out that fan resonance frequency offset and fan amplitude are the most important parameters. This method reduces the heat sink temperature from 70.6 ° C to 37.4 ° C under natural convection conditions. Using the piezoelectric principle, Ma et al. developed a piezoelectric rib as an LED heat dissipation structure. The piezoelectric ribs are covered with a thin copper layer. When a voltage is applied to the piezoelectric rib, the ribs will follow the voltage and The change in frequency produces vibration, which destroys the thermal boundary layer and achieves the purpose of increasing the heat transfer coefficient.
Chau et al. use electrohydrodynamics (EHD) to enhance the heat dissipation performance of LEDs, that is, the electrodes are ionized by high-voltage DC to ionize the air between the fins, causing them to generate ion wind to form forced convection, thereby enhancing the heat dissipation performance of the LED; research and testing, at voltage 15 The convective heat transfer coefficient at ~23kV is 7 times that of natural convection and 1.4 times that of ordinary fans. The disadvantage is the large size and high-voltage DC power supply problem.
The synthetic jet is a zero mass jet technique, the principle of which is shown in Figure 2 and the LED lamp cooled by this technique.
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Figure 2 Synthetic jet LED luminaire and its schematic
The piezoelectric actuator is composed of a metal substrate and a piezoelectric material. The piezoelectric sheet generates an inverse piezoelectric effect under the action of a voltage signal, and converts the input electric energy into vibration kinetic energy, thereby generating an unsteady jet at the opening hole of the exciter. When the gas in the cavity is compressed, the air exits the cavity through the outlet hole; when the cavity expands, the gas in the vicinity of the outer exit hole enters the cavity through the opening, and during the alternate blowing/suction process of the gas, the opening hole The airflow near the slit is strongly sheared to form a vortex pair, which enhances the heat transfer between the air and the heat sink. Song et al. established a life analysis model of LED downlights using synthetic jet cooling. The results show that the temperature rise of the junction is negligible within the expected life (50 000h), and the LED luminous flux can still maintain about 76% of the initial luminous flux after 50 000h. .
2.3 liquid cooling
2.3.1 Water cooling
Ma et al. proposed a water-cooled LED cooling method. As shown in Figure 3, the liquid is carried away by flowing through the underside of the LED. In order to enhance the heat exchange effect, a square needle is arranged directly under the chip, and the square needle acts as a rib to enhance the flow turbulence and thereby increase the heat transfer coefficient. However, the study also pointed out that if the design is not proper, the downstream chip temperature will be higher than the upstream and middle chips, resulting in uneven temperature, which may accelerate the LED thermal runaway problem and affect the reliability of the device.
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Figure 3 Schematic diagram of water-cooled LED
2.3.2 Microchannel cooling
Lu Jiadong introduced silicon-based microchannel refrigeration technology into LEDs, pointing out that microchannel refrigeration has the advantages of high heat transfer coefficient, compact structure and close thermal expansion coefficient of the chip. For the LED heat sink with straight fin microchannel structure, Li Ruchun theoretically analyzed the factors affecting its thermal resistance. It is pointed out that when the size, material and working medium of the heat sink are determined, the thermal resistance mainly depends on the structural parameters and working fluid of the heat sink. The flow rate and pressure; the optimization results show that the total thermal resistance is only 0.063 W/°C when the microchannel width is 70 μm, which is much lower than the heat resistance of air convection heat transfer. In the aspect of microchannel structure design, Li Ruchun and Yuan respectively studied the interstitial microchannel heat sink, which pointed out that the structure can effectively improve the heat transfer coefficient and improve the temperature uniformity of the chip array.
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Figure 4 Schematic diagram of high-power LED structure for micro channel cooling
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Figure 5 Schematic diagram of a staggered structure microchannel heat sink
2.3.3 Micro-jet cooling
The research in this direction is mainly focused on Professor Liu Sheng's research group. The principle is shown in Figure 6. Under the action of the micropump, the fluid in the small fluid chamber enters the flow chamber through the inlet of the micro-injection device, and under the action of the indenter, a strong jet will be formed through the series of nozzles, and the jet directly impacts the lower surface of the chip substrate. The surface produces a strong heat transfer effect. The high heat flow generated by the chip will be reduced by the jet absorption temperature, and the warmed fluid will flow into the small fluid chamber through the outlet of the micro-injection device, releasing heat to the environment under the action of the heat sink and the fan. The temperature drops and completes a cycle. Luo Xiaobing et al. optimized the structure. The results show that under the same conditions, the fluid single-input and double-out structure can make the jet more uniform and the heat dissipation effect is better. Compared with the single-input and single-out structure, the maximum temperature of the chip can be reduced. As much as 23 ° C.
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Figure 6 Schematic diagram of the micro-spray cooling system
2.3.4 Porous micro heat sink cooling
Wan et al. first proposed porous micro-heat sink cooling as the LED heat dissipation system. The principle is shown in Figure 7. The system flow is similar to that of micro-jet cooling. The difference is that the micro-injection array and chamber are replaced by porous micro-heat sinks. The inside of the micro heat sink is a porous medium, which has the advantages of large specific surface area and high heat transfer coefficient. The numerical simulation results show that the heat flux density is 500W/cm2, the average heat transfer coefficient is about 61.1kW/(m2·°C), and the maximum heat sink temperature is 58.4°C, which is much lower than the limit of LED junction temperature. value.
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Figure 7 Schematic diagram of porous micro heat sink cooling system
2.3.5 Liquid metal cooling
Liquid metal cooling technology utilizes good fluidity and high thermal conductivity in liquid media. Figure 8 is a schematic diagram of a system in which liquid metal is used for heat dissipation of the LED. The main heat sink is directly in contact with the LED substrate, and while absorbing its heat, the temperature of the liquid metal rises and flows through the sub-radiator, and the heat is dissipated into the environment, and the liquid metal is circulated back to the main body under the action of the electromagnetic pump. The next cycle takes place inside the heat sink. The test results show that when the input power of the LED lamp reaches 25.7W, the liquid metal heat dissipation system can maintain the temperature of the base at 33.1 °C, which ensures the safe and stable operation of the LED lamp.
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Figure 8 Liquid metal technology for LED heat dissipation schematic