High temperature will lead to the rapid increase of the failure probability of electronic components, resulting in the decline of the reliability of LED display. In order to control the temperature of electronic components inside the LED display screen so that it does not exceed the specified maximum allowable temperature under the working environmental conditions of the LED display screen, the heat dissipation design of the LED display screen is required. How to design the heat dissipation of LED display screen with low cost and high quality is the content of this paper.1 Knowledge of heat dissipation designTwo basic laws of heat transfer: heat flows from high temperature zone to low temperature zone; The heat emitted by the high temperature zone is equal to the heat absorbed by the low temperature zone.
There are three basic ways of heat transfer: heat conduction, convection and radiation.Heat conduction: gas heat conduction is the result of collision between gas molecules when they move irregularly. Heat conduction in metal conductors is mainly completed by the movement of free electrons. Heat conduction in non-conductive solids is realized by the vibration of lattice structure. The heat conduction mechanism in liquid mainly depends on the action of elastic wave.Convection: refers to the heat transfer process caused by the relative displacement between various parts of the fluid. Convection occurs only in the fluid and is inevitably accompanied by heat conduction. The heat exchange process that occurs when a fluid flows through the surface of an object is called convective heat transfer. The convection caused by the different density of the cold and hot parts of the fluid is called natural convection. If the movement of fluid is caused by external force (fan, etc.), it is called forced convection.
Radiation: the process by which an object transmits its ability in the form of electromagnetic waves is called thermal radiation. Radiant energy transfers energy in vacuum and has energy form conversion, that is, heat energy is converted into radiant energy and radiant energy is converted into heat energy.When selecting the heat dissipation mode, the following factors shall be considered: heat flux density, volume power density, total power consumption, surface area, volume, working environment conditions (temperature, humidity, air pressure, dust, etc.) of LED display screen.According to the heat transfer mechanism, there are natural cooling, forced air cooling and direct liquid cooling; Evaporative cooling; Thermoelectric refrigeration; Heat pipe heat transfer and other heat dissipation methods.
The comparison of several common heat dissipation methods is as follows:Natural cooling forced air cooling direct liquid cooling evaporative coolingHeat dissipation capacity w / cm? (when the temperature rise is 40 ) 0.040.30.61.1
It is easier to implement than difficultWeight or volume high, medium and lowNo low noise or vibration
No high or low power consumptionLow cost, medium high costMaintainability is easy and difficult
As can be seen from the above table, the heat dissipation effect of natural cooling is relatively small, and that of evaporative cooling is relatively large. The heat dissipation method of evaporative cooling is used to cool the human body by sweating.2 Heat dissipation design method of LED display screenFrom the practical application, it can be seen that there is more heat inside the LED display screen, and the electronic parts with more heat are led, drive IC and switching power supply. Therefore, it is necessary to design the heat dissipation of LED display screen to provide a low thermal resistance path between the heat source and the external environment to ensure the smooth transfer of heat.
When the object temperature is lower than 1800 , the meaningful thermal radiation wavelength is 0.38 100 M, and most of the energy is in the infrared band of 0.76 20 In the range of M, the proportion of thermal radiation energy is not large in the visible band. Therefore, the interior of the LED display can be coated with various colors at will. The exterior of LED display screen with direct sunlight shall be coated with light color to avoid visible light absorption.Considering the use of LED display screen, natural cooling and heat dissipation are mostly used for rental screen and indoor fixed installation screen, and forced air cooling and heat dissipation are mostly used for outdoor fixed installation screen.
Outdoor fixed installation of LED display screen, heat dissipation design should be considered when installing the whole screen. Due to the limitation of installation location, with the reduction of power consumption of LED display screen, more and more customers install LED display screen outdoors without other auxiliary heat dissipation measures. For the large LED display screen, there is only natural cooling and heat dissipation, and the heat dissipation capacity is relatively poor. Therefore, the heat dissipation design of LED display box is particularly important. Considering the reliability and maintenance cost of LED display box, forced convection cooling with fan is a better heat dissipation method.
The heat exchange area between heating electronic parts and cold air and the temperature difference between heating electronic parts and cold air directly affect the heat dissipation effect. This involves the design of air volume and air duct entering the LED display box. In the design of ventilation pipes, straight pipes shall be used to transport air as far as possible, and sharp bends and bends shall be avoided. Ventilation ducts shall avoid sudden expansion or contraction. The expansion angle shall not exceed 20 ?, The cone angle of contraction shall not be greater than 60 ?. Ventilation ducts shall be sealed as far as possible, and all laps shall follow the flow direction.
When designing the LED display box, there are several points to pay attention to: the air inlet hole should be set at the lower side of the box, but not too low, so as to prevent dirt and water from entering the box installed on the ground. The exhaust hole shall be set on the upper side close to the box. The air shall circulate from the bottom to the top of the box, and special air inlet or exhaust holes shall be used. The cooling air shall flow through the heating electronic parts, and the air flow short circuit shall be prevented. Filter screen shall be set at air inlet and outlet to prevent sundries from entering the box. Natural convection shall be designed to contribute to forced convection. The design shall ensure that the air inlet and exhaust are far away from each other. Avoid reuse of cooling air. Considering the expansion of air heating volume, the area of air outlet shall be 1.5-2 times of that of air inlet. Switching power supply and other electronic parts with large heating shall be close to the air inlet as far as possible. Ensure that the radiator slot direction is parallel to the wind direction, and the radiator slot cannot block the wind path.
The fan is installed in the system. Due to structural constraints, the air inlet and outlet are often blocked by various obstacles, and its performance curve will change. According to practical experience, the air inlet and outlet of the fan should be 40mm away from the barrier. If there is space restriction, it should also be at least 20mm.The selection of fan is generally limited by the inlet and outlet air temperature of the fan. In the case of air extraction, because the fan draws out hot air, it will have a serious impact on the service life of the fan. For fan manufacturers, 60 is generally used as the condition for calibrating the MTBF of fan life. If the ambient temperature of fan application is higher than 60 , the fan life will be reduced by half every 5 increase in temperature.When considering whether to use air extraction or air blowing, you can refer to the comparison of air blowing and air extraction in the following table.
1. The air supply is uneven, the blowing has certain directionality, and the local heat exchange is strong. It is suitable for the situation where the heating devices are relatively concentrated. The air supply is uniform, which is suitable for the situation that the distribution of heating devices is relatively uniform and the air duct is relatively complex.2 the air flow near the fan outlet is mainly turbulent flow, and the flow entering the fan is mainly laminar flow.3. The fan operates at a lower air temperature and has a long service life. The service life of the fan will be affected when the air flow at the air outlet works at high temperature.
4 positive pressure is formed in the box during blowing, which can prevent the dust in the gap from entering the box. Negative pressure is formed in the cabinet, and the dust in the gap will enter the box.The heat dissipation design of the module in the box also needs to be considered. Poor heat dissipation design will cause poor display effect and color spots. When placing heating components on the PCB, try to consider the uniform distribution of heat, and do not gather heating components in a part of the PCB.The following figure is the flow chart of heat dissipation design
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