Other technical advantages of leds over traditional light sources include long life, fast response, potentially high light efficiency, small size, and narrow spectrum. But in essence, among the many advantages, the three most important are the potential high efficiency, small size and narrow spectrum of leds, which set them apart from traditional light sources and broaden their applications in a variety of fields. However, it is precisely because of its small size, high light efficiency characteristics, so that LED still exists the application of obstacles – heat dissipation problem.
Key words
LED heat sink
Cooling into LED development must solve the problem
Internal and external heat dissipation interactions determine LED heat dissipation performance
Micro heat pipe array has high efficiency heat absorption
Article contents
With current semiconductor manufacturing technology, high-power leds convert only about 15 percent of their input power into light, and the other 85 percent into heat. And poor heat dissipation will lead to accelerated aging of the chip and reduce the life of LED. If there is no good heat dissipation method, the heat of the chip can not be dispersed, which will make the chip failure. Therefore, LED chip heat dissipation problem has become an obstacle to the application of LED technology.
If the heat of the LED chip cannot escape, it will accelerate the aging of the chip, and may lead to the melting of the solder, making the chip fail. LED luminescence is generated by electron transition between energy bands, and its spectrum does not contain infrared light. The heat of LED cannot be radiated out, so LED is called cold light source.
LED is generally encapsulated by epoxy resin, which has a very poor thermal conductivity and can only be dissipated by pins under the chip. Traditional brightness LED because of the small luminous power, heat is not big, so there is no heat dissipation problem. The power type LED used in lighting requires a plurality of LED light source module to achieve the required luminous flux.
For high-power devices, the input power is ≥1W, and the chip size is between lmm×lmm~2.5mm×2.5mm. The power density of the chip is very high, so it must handle extremely high heat in a small LED package. At present, the light harvesting efficiency of LED can only reach 10%~20%, and 80%~90% of the energy is converted into heat. If the heat of the LED chip can not be dispersed, it will accelerate the aging of the chip, but also may lead to the melting of the solder, so that the chip failure, the specific performance is: first, the intensity of light reduction. With the increase of chip junction temperature, the luminous efficiency of chip will also decrease. The higher chip junction temperature is, the faster the luminous intensity will decrease. Second, the main wavelength of luminescence is offset, resulting in a decrease in the efficiency of light conversion. Three is to accelerate the LED light decay, seriously reduce the life of LED.
Therefore, the heat dissipation problem of power type LED chip becomes the obstacle of the application of LED technology in lighting engineering. In order to ensure the normal operation of power type LED, it is necessary to ensure that the working junction temperature of LED is within the allowable temperature range through effective heat dissipation design. The stronger the heat dissipation capacity, the lower the junction temperature. Heat energy needs to be transmitted through heat conduction, heat convection and heat radiation. Because the LED light source itself has no infrared, ultraviolet, so the LED light source itself has no radiation heat dissipation function, the heat dissipation way of the LED lighting lamps can only be exported through the radiator closely combined with the LED lamp bead board. Only by exporting the heat as soon as possible can we effectively reduce the cavity temperature of the LED lamp, to protect the power supply from working in a permanent high temperature environment, to avoid the premature aging of the LED light source due to long-term high temperature work. The heat dissipation problem of LED lighting system mainly has two aspects: one is the heat dissipation (heat conduction) in the LED power chip, which involves the packaging technology of the device; The second is the external heat dissipation of the LED power chip, which mainly involves the heat conduction of the substrate, the fin radiator and the convection heat transfer with the ambient air.
At present, the main methods used to solve the heat dissipation problem of power type LED lighting system are: adjust the LED spacing, natural convection heat dissipation, install a fan or water cooled forced heat dissipation, heat pipe and loop heat pipe heat dissipation. Under the development trend of LED integration with high density and high heat production and flow rate, it becomes very necessary to realize rapid heat dissipation by means of efficient heat transfer of heat pipe. In addition, the existing heat dissipation device emphasizes the heat conduction link and ignores the convection heat dissipation link, although many manufacturers have considered a variety of measures to improve the heat conduction link: Such as the use of heat pipe, plus thermal conductive silicone grease, but do not realize that the heat ultimately depends on the outer surface of the lamp away, ignore the balance of heat transfer, if the fin temperature distribution is seriously uneven, will lead to some of the fin (low temperature part) efficiency is greatly reduced. The existing cooling devices for LED lighting are still limited to low power LED lighting components, and the effect is not obvious, high cost, not easy to be applied to the actual production.
If the heat of the LED chip cannot escape, it will accelerate the aging of the chip, and may lead to the melting of the solder, making the chip fail. LED luminescence is generated by electron transition between energy bands, and its spectrum does not contain infrared light. The heat of LED cannot be radiated out, so LED is called cold light source. LED is generally encapsulated by epoxy resin, which has a very poor thermal conductivity and can only be dissipated by pins under the chip. Traditional brightness LED because of the small luminous power, heat is not big, so there is no heat dissipation problem. The power type LED used in lighting requires a plurality of LED light source module to achieve the required luminous flux. For high-power devices, the input power is ≥1W, and the chip size is between lmm×lmm~2.5mm×2.5mm. The power density of the chip is very high, so it must handle extremely high heat in a small LED package. At present, the light harvesting efficiency of LED can only reach 10%~20%, and 80%~90% of the energy is converted into heat. If the heat of the LED chip can not be dispersed, it will accelerate the aging of the chip, but also may lead to the melting of the solder, so that the chip failure, the specific performance is: first, the intensity of light reduction. With the increase of chip junction temperature, the luminous efficiency of chip will also decrease. The higher chip junction temperature is, the faster the luminous intensity will decrease. Second, the main wavelength of luminescence is offset, resulting in a decrease in the efficiency of light conversion. Three is to accelerate the LED light decay, seriously reduce the life of LED.
Therefore, the heat dissipation problem of power type LED chip becomes the obstacle of the application of LED technology in lighting engineering. In order to ensure the normal operation of power type LED, it is necessary to ensure that the working junction temperature of LED is within the allowable temperature range through effective heat dissipation design. The stronger the heat dissipation capacity, the lower the junction temperature. Heat energy needs to be transmitted through heat conduction, heat convection and heat radiation. Because the LED light source itself has no infrared, ultraviolet, so the LED light source itself has no radiation heat dissipation function, the heat dissipation way of the LED lighting lamps can only be exported through the radiator closely combined with the LED lamp bead board. Only by exporting the heat as soon as possible can we effectively reduce the cavity temperature of the LED lamp, to protect the power supply from working in a permanent high temperature environment, to avoid the premature aging of the LED light source due to long-term high temperature work. The heat dissipation problem of LED lighting system mainly has two aspects: one is the heat dissipation (heat conduction) in the LED power chip, which involves the packaging technology of the device; The second is the external heat dissipation of the LED power chip, which mainly involves the heat conduction of the substrate, the fin radiator and the convection heat transfer with the ambient air. At present, the main methods used to solve the heat dissipation problem of power type LED lighting system are: adjust the LED spacing, natural convection heat dissipation, install a fan or water cooled forced heat dissipation, heat pipe and loop heat pipe heat dissipation. Under the development trend of LED integration with high density and high heat production and flow rate, it becomes very necessary to realize rapid heat dissipation by means of efficient heat transfer of heat pipe.
In addition, the existing heat dissipation device emphasizes the heat conduction link and ignores the convection heat dissipation link, although many manufacturers have considered a variety of measures to improve the heat conduction link: Such as the use of heat pipe, plus thermal conductive silicone grease, but do not realize that the heat ultimately depends on the outer surface of the lamp away, ignore the balance of heat transfer, if the fin temperature distribution is seriously uneven, will lead to some of the fin (low temperature part) efficiency is greatly reduced. The existing cooling devices for LED lighting are still limited to low power LED lighting components, and the effect is not obvious, high cost, not easy to be applied to the actual production.
Micro-heat pipe array can basically solve the heat dissipation problem of various LED because it has the characteristics of high efficiency heat absorption, transmission and heat release. The main method of apparent performance evaluation of heat pipe is to measure the temperature uniformity along the axial direction. The response time of heat pipes depends on the heat capacity of the materials (including metal materials and working medium). In order to evaluate the plate microheat pipe array, the heat pipe with 50cm length was used to measure the temperature uniformity and thermal response time. In the experiment, four T-type thermocouples were arranged in the vertical direction of the heat pipe, respectively located in the evaporation section, adiabatic section and condensing section of the heat pipe.
The experimental data show that the plate microheat pipe array has good temperature uniformity. The temperature difference from evaporation section to condensation section is less than 1℃, and the thermal response time is less than 80s. By testing several different combination forms of power LED lighting device based on flat panel micro heat pipe array, the influence of contact area, heat transport length and contact thermal resistance of flat panel micro heat pipe array and radiator on power LED lighting device based on flat panel micro heat pipe array was analyzed. It includes the influence of the contact area of the tube plate on the heat transfer of the device, the influence of the contact thermal resistance on the heat transfer of the LED heat transfer device of the plate microheat pipe array and the performance of the LED lighting heat dissipation device of the U-shaped plate microheat pipe array.
The micro-heat pipe can be randomly combined into a flat micro-heat pipe array of a certain width, and the micro-heat pipe can be bent arbitrarily, and the heat transfer effect has no obvious change at low heat flux. U-shaped microheat pipe array is a bending form of microheat pipe array which has been proved by experiments to have good heat transfer performance. Micro-heat pipe array can basically solve the heat dissipation problem of various LED because it has the characteristics of high efficiency heat absorption, transmission and heat release, and can be flexibly deformed and combined with fin.
First, the maximum heat transfer capacity of the evaporative heat transfer part of the micro-heat pipe array can reach 200W/cm; Second, high thermal conductivity: it is more than 5000 times of solid aluminum; Third, high reliability: because each micro heat pipe in the plate works independently, even if one or two micro heat pipes are damaged, other micro heat pipes still play a role without affecting the use. Moreover, its pressure bearing capacity is more than 10 times that of the traditional circular heat pipe, so it is difficult to occur mechanical damage. Four is high isotherm: the temperature difference per meter is less than 1℃, can almost be considered as an isotherm; Fifth, large area contact: due to the large heat release area of the micro heat pipe array, the temperature of the aluminum fin, the substrate and the heat pipe can be basically the same, and the “fin effect” can be almost completely eliminated.
