The first thing to consider when designing LED street lights is to make full use of the limited luminous flux to the effective illumination range. The requirements of street lights are the lighting effect of the road surface, and the open space beyond the road surface is not an area illuminated by street lights. Therefore, the distribution range of the light is effectively controlled, so that the light emitted by the LED becomes a long strip of light that spreads along the road surface, and at the same time, the control of glare must be taken into account. The light intensity at the angle of 90° and 90° shall not exceed 30cd/10001m and 10cd/10001m respectively, and regardless of the luminous flux of the light source, the maximum value of the light intensity at the 90° angle shall not exceed 1000cd. With strict control, strong glare can be produced. At present, the standard of LED street light is generally:
(1) The lens used for LED street light, the spot is rectangular, the material is PMMA optical material, the transmittance is ≥93%, the temperature resistance is -38~+90℃, and the anti-UV yellowing rate is 30000h without change.
(2) The average illuminance of road illuminance uniformity is 0.48. The light class ratio is 1:2.
(3) Comply with road illumination. The actual 1/2 center light spot reaches 251x, the 1/4 center light intensity reaches 151x, the minimum light intensity at 16m is 4lx, and the overlapping light intensity is about 61x.
(4) Deep dimming is possible, and the color and other characteristics will not change due to dimming.
(5) Adapt to humidity: ≤95%.
(6) Quality assurance: 2 years.
- The structure of lamp A
(1) Composition structure. The LED street light is composed of aluminum alloy die-cast lamp body, LED module, tempered glass light-transmitting cover, DC/DC constant current driver, electrical room cover, and light pole, as shown in Figure 1.
(2) Functional structure. The functional structure of the LED street light consists of three parts: the heat dissipation lamp body, the light source room, and the electrical room, as shown in Figure 2.
- LED street light heat dissipation
Designing the heat dissipation of LED street lamps is one of the key problems to be solved in the design of props. It is not only directly related to the luminous efficiency of LEDs in actual work, but also because of the high brightness requirements and large heat generation of high-power LED street lamps, due to the harsh outdoor use environment. , If the heat dissipation is not good, it will directly lead to the rapid aging of the LED, and the stability will be reduced. Therefore, the heat dissipation surface is located on the side to facilitate the natural convection heat dissipation of the air. In addition to the need for good heat dissipation of the power LED itself, the road lamps used in outdoor applications should have the necessary level of dustproof and waterproof protection (IP). Good IP protection often hinders the heat dissipation of LEDs.
Solving these two conflicting but both problems is an important aspect that should be paid attention to in the design of road lighting.
(1) Selection of heat dissipation materials
At present, the materials used for LED street lamp radiators are basically metal materials, which are mainly due to three considerations:
1) Good thermal conductivity. Compared with other solid materials, metals have better thermal conductivity.
2) Easy to process. Good ductility, relatively stable at high temperature, and various processing techniques can be used.
3) It is easy to obtain and the price is relatively low.
Naturally, the higher the thermal conductivity, the better, but at the same time, the mechanical properties and price of the material need to be taken into account. Gold and silver with high thermal conductivity cannot be widely used due to their soft texture, high density, and high price, while iron cannot meet the performance requirements of high thermal density applications due to low thermal conductivity, and is not suitable for high-performance production. heat sink. The thermal conductivity of copper is also very high, and it can be used less in heat sinks due to unfavorable conditions such as insufficient hardness, high density, slightly high cost, and difficult processing. Aluminum is favored due to its high thermal conductivity, low density and low price; however, due to the low hardness of pure aluminum, various formula materials are usually added to make aluminum alloys in various application fields. It does not have the characteristics, and it becomes an ideal choice for heat sink processing materials.
According to different needs, various aluminum alloy materials can obtain various characteristics by adjusting the composition and proportion of the formula materials, which are suitable for different forming and processing methods and applied in different fields. Among different aluminum alloys: AA6061 and AA6063 have good thermal conductivity and processability, suitable for extrusion forming process, and are widely used in heat sink processing. ADC12 is suitable for die-casting, but the thermal conductivity is low, so AA1070 aluminum alloy is usually used instead in the processing of heat sinks, but its mechanical properties are not as good as ADC12. thin fins
(2) Adopt integrated aluminum alloy die-casting heat dissipation shell
In order to solve the heating problem of LED, the shell and radiator of the lamp are designed in an integrated manner. Generally, aluminum or aluminum alloy, copper or copper alloy, and other alloys with good thermal conductivity are selected. According to the functional requirements of the road lighting fixtures and the regulations of the protection safety level of the lamps, the whole lamp is divided into three parts to design. The heat dissipation effect of the heat sink mainly depends on the design of the heat absorbing liner and the heat sink in the contact part between the heat sink and the heating object. A radiator with excellent performance should meet three requirements: fast heat absorption, small thermal resistance, and fast heat removal.
1) Absorbs heat quickly
That is, the thermal resistance between the heat-absorbing liner and the LED module is small, and the heat generated by it can be quickly absorbed. In order to achieve this effect, it is required to combine the heat-absorbing liner with the LED module as closely as possible, so that the metal material is in direct contact with the LED module, and it is best not to leave any space.
The overall thermal resistance of the heat sink is accumulated layer by layer from the contact surface with the L.ED module, and the thermal conduction resistance inside the heat absorbing liner is a part that cannot be ignored. In order to effectively conduct the absorbed heat to as many scales as possible, the heat-absorbing lining plate needs to have good lateral heat conduction capability. When designing lamps, the heat-absorbing lining plate should have sufficient thickness first, and the LED mold should be considered at the same time. Reinforcing the installation holes to enhance the integrity and mechanical strength of the lamps.
2) Small thermal resistance
In order to improve the heat absorption capacity, it is hoped that the heat sink and the LED module are closely combined without leaving any gaps, and the die-cast surface cannot be achieved. There must be a certain gap between the heat-absorbing liner and the LED module. If the air with high thermal resistance is in the gap, good thermal conductivity cannot be obtained. Therefore, materials with lower thermal resistance and better adaptability should be used to fill The gap in it, the solution to this is to use a thermal pad. However, the thermal resistance of thermal paste is always higher than that of metal materials, so the use of thermal lining is not the best method. In order to fundamentally improve the heat absorption capacity of the heat sink and the heat absorbing lining, the flatness of the bottom surface must be improved. The flatness is measured by the maximum drop height of the surface. Usually, the bottom of the heat sink can be less than 0.1mm after a little treatment, and it can reach 0.03mm with a milling machine or multi-pass wire drawing. Smaller is better for heat absorption, but increases manufacturing cost.
3) go to heat fast
Since the heat-absorbing liner of the LED module and the heat-dissipating fins are die-cast into one body, the heat absorbed from the LED module can be quickly conducted to the fins, and the entire lamp body and the upper part of the heat-dissipating fins are exposed in the air. Moreover, the direction of the fins is parallel to the road, and the hot air to be dissipated is in the same direction as the air flow. It will not be stagnant due to the eddy current formed by the air flow, which is then smoothly taken away by the flowing air flow and dissipated, dissipating the heat at the fastest speed. disseminate.
Figure 3(a) is the design structure diagram of the upper heat sink of the lamp body, and Figure 3(b) is the heat dissipation distribution diagram on the side of the lamp body. Due to the design of the lamp body, the upper part of the whole lamp body does not have any openings, and the whole is processed by one-time die-casting of aluminum alloy. There is no water seepage and dustproof problem in the upper part of the lamp body. It is enough to protect the room and the electrical room, which makes the protection problem simple. Only the silicone rubber strip is used for waterproof and dustproof, and the protection level of IP65 can be achieved. The above-mentioned integrated LED street lamp holder has the advantages of fast heat absorption, small thermal resistance, smooth heat dissipation, high protection level, and good mechanical strength, and can control the junction temperature of the LED module below 60°C.
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