The thermal structure design of the lamp body is a problem that cannot be ignored in the production of LED lamps. Although the LED is a cold light source and is not a hot body during operation, the resistance heat generated when the current flows through the LED will still cause the lamp body to heat up, causing the LED to heat up. It will age rapidly at high temperature and the light efficiency will decrease. In order to slow down the light decay of the LED and make the LED lamp have a long service life, the junction temperature of the LED chip must be reduced. To reduce the junction temperature, the temperature of the lamp body must be reduced, and the thermal resistance between the LED chip and the lamp body must be reduced. , which requires that the heat dissipation problem of LED lamps must be solved in the design.
Solving the problem of heat dissipation mainly depends on a reasonable lamp body structure. One solution is to use a 2~3mm aluminum substrate, and the high-power LEDs are directly mounted on the aluminum plate, and the LEDs are connected by leads. For low-power LEDs, holes with the same diameter as the outer diameter of the LEDs can be drilled on the aluminum plate according to the number of LEDs used, and then the LEDs are tightly fitted to the aluminum plate, and the LED pins are connected behind the aluminum plate. The shell of the lamp is also made of metal material. The aluminum plate with the LED installed is tightly assembled with the metal shell. When the metal shell is exposed to the air, the heat can be dissipated by radiation and convection. The surface area of metal enclosures exposed to air should be considered as approximately 50m² per watt. In order to reduce the volume of the lamp and ensure a larger heat dissipation area, the lamp housing should be a ribbed heat sink structure.
The temperature difference between the LED chip and the package caused by the maximum junction temperature and thermal resistance of the LED is the most important consideration in thermal design. For high-power LEDs, the thermal resistance of a 1W LED is about 20°C, that is to say, for the nominal If the power of 1W LED inputs 1W electrical power, the LED junction temperature is 20°C higher than the temperature of the package case. The thermal resistance of the 3W LED is about 15°C, and the 3W LED input 3W electrical power, the LED junction temperature is higher than the package case temperature. Therefore, to make the LED junction temperature of lamps made of 3W LEDs and lamps made of 1W LEDs the same, the temperature of lamps made of 3W LEDs should be lower than that of lamps made of 1W LEDs. Conversely, if the lamp temperature is the same, the LED junction temperature of the lamp made with 1W 1.ED is lower than the LED junction temperature of the lamp made with 3W LED.
In this sense, using three 1W LEDs to make 3W LED lamps is more conducive to reducing the junction temperature of LEDs than using one 3W LED, and the luminous flux emitted by three 1W LEDs is more than that of one 3W LED. The luminous flux emitted by the LED is high, therefore, the power of the LED should be reasonably selected to make lamps with high-power LEDs.
In order to reduce the heat generated by the LED, LEDs with high luminous efficiency should be used to make high-power LED lamps, because when a certain electric power is input, the LEDs with high luminous efficiency emit high light energy and emit less heat energy, which can reduce the energy consumption. Small heat sink area.
Both the LED chip and the coated phosphor are produced at a high temperature of several hundred degrees, and they have a certain temperature resistance. However, there is a thermal resistance between the LED package and the chip. This thermal resistance causes a temperature difference between the package and the chip when the LED is in use, and the temperature of the LED chip will be higher than the package case temperature.
Due to the advancement of LED production technology, the internal thermal resistance of high-power LEDs is getting lower and lower. At present, the thermal resistance of 1W LEDs is generally below 15°C/W. The temperature of the case is only 15°C higher. According to the current temperature resistance level of LED chip materials, LED chips can work safely for a long time when the temperature does not exceed 150°C. In this way, the package case temperature can be safely used when the temperature is 135 ℃. However, due to the limitation of packaging materials, the temperature of the package shell in actual use should preferably not exceed 70 °C, so that the temperature of the LED chip is only 85 °C, and the transparent packaging material of the LED will not age rapidly, and can work stably for a long time. Therefore, it is not necessary to reduce the working temperature of the LED lamp very low, but it is necessary to reduce the thermal resistance between the LED chip package shell and the lamp body shell, so that the stable performance can be produced with a relatively small volume and a relatively low cost. of LED lamps.
To effectively dissipate heat and reduce the volume and production cost of lamps, lamps must have a reasonable heat dissipation structure. The problem is how to reasonably conduct the heat generated by the LED to the external light, how to effectively increase the contact surface between the external party and the air, and how to facilitate the flow of air on the surface of the shell, which is to be solved by the thermal structure design of the luminaire. question. According to the equivalent relationship between luminous flux (Im) and radiant flux (W), 1w of radiant flux may produce 683Im luminous flux in the most ideal case (black body radiation). Therefore, even if the light efficiency of LED reaches 2001m/W, it cannot convert all the energy into light energy output, and the rest is converted into heat energy. In the long run, the heat dissipation problem of LED lamps will be a long-term problem. In the heat dissipation design of LED lamps, it can be divided into two modes: natural convection heat dissipation and forced convection heat dissipation. For general lighting applications, fans are not recommended due to the noise they produce.
In addition, adding fans will also increase the power consumption of the overall system, so natural convection cooling will be a better choice. At present, the problems existing in the heat dissipation design of LED lamps are:
1) The random setting of the area of the heat dissipation fins makes the arrangement of the heat dissipation fins unreasonable. The arrangement of the heat dissipation fins of the lamps does not take into account the use of the lamps, which affects the performance of the fins.
2) Emphasizing heat conduction and ignoring convection heat dissipation. Although many manufacturers have considered various measures: heat pipes, loop heat pipes, and thermal grease, etc., they do not realize that heat will ultimately rely on the outer surface of the lamps to dissipate.
3) Ignoring the balance of heat transfer, if the temperature distribution of the fins is seriously uneven, it will cause some of the fins (the part with lower temperature) to have no effect or limited effect.
In the design of natural convection heat dissipation fins, the key considerations include: the thickness of the fins, the number of fins, the width between the scales and the fins, and the selection of materials. Too dense or too sparse scales are not the optimal design. Because the contact area between the dense fins and the air is too small, the heat is not easily dissipated, and the sparse fins will make the heat not easily dissipated. At present, the heat dissipation technology of LED lamps and lanterns is generally mostly the heat conduction plate method, that is, a 5mm thick copper plate (temperature equalization plate) is used to uniformly dissipate the heat source; there are also heat sinks installed to dissipate heat, but the disadvantage of this method is that it is too heavy .
Figure 1 is a simulation diagram of the worst natural convection light source heat dissipation of LED lamps at an ambient temperature of 40°C. The external dimensions of the heat sink are 600mm × 440mm × 54mm. At this time, the junction temperature of high-power LEDs is basically guaranteed to be 68.6+20-88.6 ℃<90℃, so the heat dissipation volume and area can ensure sufficient heat dissipation of the high-power LED.
To ensure the safe use of high-power LEDs and ensure the heat dissipation conditions of the light source and power supply, the size of the lamps will be larger, which will increase the difficulty of design and increase the relative cost. Today, due to the limitations of LED color temperature and luminous flux, it is difficult to promote high-power safe production and cost-effectiveness. To design high-performance LED lamps, the first thing to do is to do a good job in the heat dissipation of the lamps. However, heat dissipation and safety protection of the lamps are a contradiction. In the design, research and development should be carried out for this contradiction that requires symbiosis.
If using aluminum alloy profiles to design the cooling system of LED lamps, combine a flat radiator profile with an area of 305mm × 500mm and a stainless steel frame, which is made of AA6063, and paste the LEDs on the aluminum substrate to make a module, at the bottom of the module Apply thermal conductive green and fix it on the aluminum alloy radiator plate with screws. The LED lamps with this structure have good heat conduction and heat dissipation effects, and can quickly conduct the heat generated by the LED to the radiator, and then dissipate into the air by the heat dissipation fins exposed in the air, and the heat is taken away by the flowing air. , but because the whole lamp is composed of multiple parts, the product consistency is poor, and it is extremely unreliable to rely on waterproof glue in terms of anti-seepage safety, so the protection level cannot reach GB 7000.1-2002, GB 7000.5 -12005 requirements.
More importantly, all the LEDs are installed on a flat panel, and it is impossible to carry out a reasonable light distribution for the lamps, so further research and development are needed. If the LED lamps are transformed from traditional lamps, although aluminum profiles are used as radiators, the entire light source and driving circuit are installed in the aluminum alloy lamp housing. Although the problem of the protection level of the lamps is solved, the entire light source is in a closed lamp. In the shell, the heat generated by the working of the lamp cannot be dissipated into the air, which causes the LED module and drive circuit to work in a very harsh environment. Even if it is damaged, the driver is damaged due to excessive temperature, which reduces the reliability and service life of LED lamps.
The heat dissipation and protection of LED lamps and lanterns is an industrial problem. The system optimization design should be carried out in material selection and structural design to solve the contradiction between the heat dissipation and product protection of LED lamps.