Working at high temperature for a long time will lead to accelerated decay of the LED display and shorten the service life of the LED display, and the probability of failure rate of electronic devices will also increase rapidly. As a result, the reliability of the display screen is reduced. Therefore, solving the heat dissipation problem of LED display screens is a very important part of the production of LED display screens. So what kind of design can reduce costs and improve efficiency? the
1. Relevant knowledge of heat dissipation design
There are two basic laws of heat transfer: heat flows from the high temperature area to the low temperature area; the heat emitted by the high temperature area is equal to the heat absorbed by the low temperature area.
There are three basic ways in which heat is transferred: conduction, convection, and radiation.
Heat conduction: Gas heat conduction is the result of gas molecules colliding with each other when they move irregularly. Heat conduction in metal conductors is mainly accomplished by the movement of free electrons. Heat conduction in nonconductive solids is achieved through vibrations in the lattice structure. The heat conduction mechanism in the liquid mainly depends on the action of elastic waves. the
Convection: Refers to the heat transfer process caused by the relative displacement between the various parts of the fluid. Convection occurs only in fluids and is necessarily accompanied by conduction. The heat exchange process that occurs when a fluid flows over the surface of an object is called convective heat transfer. The convection caused by the different density of the hot and cold parts of the fluid is called natural convection. If the movement of the fluid is caused by an external force (fan, etc.), it is called forced convection. the
Radiation: The process by which an object transfers energy in the form of electromagnetic waves is called thermal radiation. Radiant energy transfers energy in a vacuum, and there is a conversion of energy forms, that is, thermal energy is converted into radiant energy and radiant energy is converted into thermal energy.
When choosing a heat dissipation method, the following factors should be considered: heat flux density, volume power density, total power consumption, surface area, volume, and working environment conditions (temperature, humidity, air pressure, dust, etc.) of the LED display. the
According to the heat transfer mechanism, there are natural cooling, forced air cooling, direct liquid cooling; evaporative cooling; thermoelectric cooling; heat pipe heat transfer and other heat dissipation methods. the
Several common heat dissipation methods are compared as follows:
Free cooling Forced air cooling Direct liquid cooling Evaporative cooling
Heat dissipation capacity W/cm? (When the temperature rise is 40°C) 0.040.30.61.1
Easier to achieve Easier more difficult Difficult
weight or volume, high, low, low
No high or low noise or vibration
There is no difference in power consumption
The cost is low, medium and high
Maintainability Easier Easier Difficult Difficult
It can be seen from the above table that the heat dissipation effect of natural cooling is relatively small, while the heat dissipation effect of evaporative cooling is relatively large. The human body sweats to cool down, using the heat dissipation method of evaporative cooling.
Second, the heat dissipation design method of led display
It can be seen from practical application that at present, the interior of the LED display generates more heat, and the electronic components that generate more heat are: LED, driver IC, and switching power supply. Therefore, it is necessary to carry out a heat dissipation design for the LED display to provide a low thermal resistance path between the heat source and the external environment to ensure the smooth transfer of heat.
When the temperature of the object is lower than 1800°C, the meaningful thermal radiation wavelength is between 0.38 and 100 μm, and most of the energy is in the range of 0.76 to 20 μm in the infrared band. In the visible light band, the proportion of thermal radiation energy is not large. Therefore, various colors can be freely painted inside the led display. The exterior of the LED display that is exposed to direct sunlight needs to be coated with a light color to avoid visible light absorption.
Considering the use of LED display screens, natural cooling and heat dissipation are mostly used for rental screens and Indoor Soft LED Mesh Screen fixed installation screens, and forced air cooling is mostly used for outdoor fixed installation screens.
When installing LED display screens outdoors, heat dissipation design must be considered when installing the entire screen. Restricted by the installation location, with the reduction of power consumption of the LED display, more and more customers will bare the LED display outdoors without other auxiliary heat dissipation measures. For large LED display screens, there is only natural cooling and heat dissipation, and the heat dissipation capacity is relatively poor. Therefore, the heat dissipation design of the LED display cabinet is particularly important. From multiple perspectives such as the reliability of the LED display box and maintenance costs, using a fan for forced convection cooling is a better way to dissipate heat.
The heat exchange area between the heating electronic components and the cold air, and the temperature difference between the heating electronic components and the cold air directly affect the cooling effect. This involves the design of the air volume entering the LED display box and the design of the air duct. When designing ventilation ducts, try to use straight ducts to convey air, and avoid sharp bends and curved ducts. Ventilation ducts should avoid sudden expansion or contraction. The extended opening angle should not exceed 20? , the contraction cone angle should not be greater than 60? . Ventilation ducts should be as tight as possible and all overlaps should be in the direction of flow.
When designing the LED display box, there are several points to be noted: the air intake hole should be set on 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. Vents should be located near the upper side of the box. The air should circulate from the bottom of the box to the top, and special air intake or exhaust holes should be used. Cooling air should flow through the heat-generating electronic components while preventing short-circuiting of the air flow. The air inlet and air outlet should be equipped with filter screens to prevent debris from entering the box. The design shall be such that natural convection facilitates forced convection. When designing, it is necessary to ensure that the air inlet and the exhaust port are far away. Avoid reusing cooling air. Considering the volume expansion of air when heated, the area of the air outlet should be 1.5 to 2 times the area of the air inlet. Switching power supplies and other electronic components that generate high heat should be placed as close as possible to the air inlet. Make sure that the direction of the radiator slots is parallel to the wind direction, and that the radiator slots cannot block the air path.
When the fan is installed in the system, due to structural limitations, 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 have a distance of 40mm from the barrier, and if there is a space limit, it should be at least 20mm.
The fan selection is generally limited by the air temperature at the fan inlet and outlet. For the situation of drafting, because the fan draws out hot air, it will have a serious impact on the life of the fan. For fan manufacturers, 60°C is generally used as the condition for calibrating the fan life MTBF. If the ambient temperature of the fan application is higher than 60°C, the fan life will be reduced by half for every 5°C increase in temperature.
When considering whether to use ventilation or blowing, you can refer to the comparison of blowing and blowing methods in the table below.
1 The air supply is uneven, the blowing has a certain directionality, and the local heat exchange is strong, which is suitable for the situation where the heating devices are relatively concentrated. The air supply is uniform, which is suitable for the situation where the distribution of heating components is relatively uniform and the air duct is relatively complicated.
2 The airflow near the fan outlet is mainly turbulent flow, and the flow entering the fan is mainly laminar flow.
3 The fan works at a lower air temperature, and the fan life is longer. The fan works under the high temperature of the air outlet, and the service life will be affected.
4. When blowing air, a positive pressure is formed in the box, 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 cabinet.
The heat dissipation design of the modules in the cabinet also needs to be considered. Poor heat dissipation design will cause poor display effect and color spots. When placing heat-generating components on the PCB, try to consider the uniform distribution of heat, and do not gather components that generate more heat in a certain part of the PCB.
The following figure is the flow chart of heat dissipation design
3. Example of heat dissipation design for led display
Let’s discuss the heat dissipation design by taking the actually installed outdoor fixed LED display as an example.
The picture above is an outdoor fixed-installation LED display actually installed by Honghua Optoelectronics in Guangdong. The model P10 outdoor surface-mounted full-color display has a brightness of 5500cd and a maximum power of 923W. The heat dissipation method of the large LED display screen is natural cooling. Considering the cost and quality of heat dissipation, the heat dissipation method of the LED display cabinet is forced cooling by fan air.
Let’s calculate the air convection flow first, the calculation formula is L=Wr/(Cρ(tn-tl))
L: air convection flow, m3/s; Wr: air forced cooling heat dissipation; C: specific heat capacity of air, kJ/(kg℃); ρ: air density, kg/m3; tn: internal temperature of the box, ℃; tl: cold air temperature input by the cooling system,
C
According to the above formula, it can be known that if the air temperature inside the LED display cabinet is to be guaranteed not to exceed tn°C, the cooling system needs to input the air volume of cold air as Wr/(Cρ(tn-tl))m3/
.
Assuming that the heat dissipation required by the LED display box is W, considering that natural cooling and radiation heat dissipation play a very small role in the entire heat dissipation process of the box, W is considered as the forced air cooling heat dissipation. Accurate calculation of the internal heating value of the LED display is not easy to achieve in engineering, and the estimation method is usually used. We know from experience: when playing a still white picture at maximum brightness, the heating power of A1688 is about 300W, that is, Wr=0.3KW. Under normal circumstances, in the LED display, the heat generated by the LED light source accounts for about 45% of the heat generated by the entire screen, the heat generated by the driving part accounts for about 50% of the heat generated by the entire screen, and the controller and connecting wires account for about 5% of the heat generated by the entire scree
.
The specific heat capacity and density of air are related to the temperature, humidity and pressure of the air. Check the local historical climate data, consider the most unfavorable climate conditions, determine C=0.7kJ/(kg℃), ρ=0.7kg/
3.
The internal temperature tn of the box refers to the highest air temperature in the box, usually at the upper part of the box. Check the specifications of electronic components such as switching power supplies and driver ICs, and determine that tn is 60
C.
According to the local climate data and installation characteristics, the cold air temperature tl input by the cooling system is determined to be 40
C.
L=0.3/(0.7*0.7*(60-40))=0.031m
/s
Next, calculate the opening area of the fan and the selection of the f
n.
According to the conditions of the installation site, the heat dissipation method by fan blowing is adopt
d.
The company’s commonly used fans are selected, with an air volume of 50CFM, converted to 0.023m3/s. Therefore, two fans are selected for blasti
.
See the picture above for the design behind the b
x.
After the design is completed, actual test verification is required. The test results are shown in the table bel
w.
Experimental temperature record table (unit ℃, experimental conditions: 40 ℃ high temperature room, brightness 12000nit, all white still pic
)
maximum temper
ture
The air temperature at the tuyere is
40.5
The air temperature at the air outlet is
58.3
Switching power supply 1 temperature
66.1
Switching power supply 2 temperature
66.3
Receiving card temperature
44.5
R lamp driver IC temperatu
e 81
G light driver IC temperature
86.8
B lamp driver IC temperature
79.4
R lamp temperatu
e 82
G lamp temperature
87.8
B lamp temperature
79.9
Through good heat dissipation design, this LED display installed abroad has been running stably and working normally for several y
rs.
4. Concl
sion
High temperature has a great impact on the reliability of the LED display, so heat dissipation design is required. For outdoor fixed-installation LED displays, it is necessary to start with the heat dissipation design of the large screen, and then carry out the heat dissipation design of the cabinet and the heat dissipation design of the module. For heat dissipation design, it is necessary to make a plan first, and then do prototype test verification after the calculation is correct. The test verifies that there is no problem with the LED display, so that there will be no high temperature problems in actual work.