In the actual design of road lighting fixtures, each LED can be fixed on the fixture with a spherical joint on the premise of basically setting the projection direction of each LED. When the fixtures are used in different heights and illumination widths , You can achieve satisfactory results by adjusting the spherical gimbal to make the irradiation direction of each LED. When determining the power and beam output angle of each LED, according to E(lx)=I(cd)/D(m)2 (the inverse square law of the distance between light intensity and illuminance), the basic selection of each LED can be calculated separately. The power that the beam should have at the output angle, and the light output of each LED can reach the expected value by adjusting the power of each LED and the different power output by the LED drive circuit to each LED. These adjustment methods are unique to road lamps that use LED light sources. Taking full advantage of these characteristics can reduce the lighting power density and achieve the purpose of energy saving on the premise of satisfying the illuminance and illuminance uniformity of the road surface.
1. Heat dissipation and protection structure design:
According to the equivalent relationship between luminous flux (lumens) and radiant flux (watts), 1W of radiant flux can produce 683 lm of luminous flux in the most ideal case (blackbody radiation). Therefore, even if the luminous efficiency of LEDs is constantly improving (it has reached more than 120 lm/W), all the energy cannot be converted 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. The heat dissipation of LED street lamps is one of the key problems that need to be solved. It is not only directly related to the luminous efficiency of LEDs in actual work, but also because LED street lamps have high brightness requirements, large heat generation, and harsh outdoor use environments. If the heat dissipation is not good, it will directly Leads to rapid aging of the LED and reduced stability. Road lamps used outdoors should also have a certain level of dust and water resistance (IP),
Good IP protection tends to hinder the cooling of LEDs. Solving these two contradictory but both problems is an important aspect that should be paid attention to in the design of road lighting.
In the use of most road lamps in our country, the amount of flying dust in the air is relatively large, sometimes reaching a large amount (such as sandstorms). ), the gaps of the internal radiator will be filled with dust, which will greatly reduce the effect of the radiator, and will eventually shorten the service life of the LED due to the high operating temperature.
To take into account the heat dissipation and IP protection of LEDs in road lamps, a more reasonable design concept is:
a. In key heat dissipation positions, use thermal conductive plates. The heat-conducting plate is inside the metal plate, evenly distributed with thin tubes for refrigerant flow, and the thin tubes are filled with refrigerant. When a certain part of the heat-conducting plate is heated, the refrigerant in the thin tubes will flow rapidly and make the heat rapidly. ground conduction. The thermal conductivity of a good thermal plate can reach 8 to 12 times that of a copper plate of the same thickness. Although the price is high, if it is used in key parts, it will have a multiplier effect on the heat dissipation of the LED.
b. Design the shell of the lamp as a radiator. At present, most road lamps and lanterns shells are made of aluminum. Directly using the outside of the lamp shell as a radiator can not only ensure the requirements of IP protection level, but also obtain a large heat dissipation area. , can be washed by natural wind and rain, so as to ensure the continuous effectiveness of the radiator work.
2. Secondary light distribution optical design
Secondary optical design is an important technology that determines the light distribution curve, output light efficiency, uniformity, and glare index of LED street lamps. In terms of secondary optical design, the radiation forms of LEDs include Lambertian, side-emitting, bat-wing and concentrating. In the field of road lighting, Lambertian type and bat-wing type are more suitable according to design experience. Through secondary optical design, the illumination range and luminosity curve of LED can meet the needs of road lighting.
At present, the most common form of LEDs used in road lamps is mainly in the lamps. On an almost flat mounting surface (also a reflective surface), a matrix LED is installed. This design method is impossible to obtain good results. Light distribution.
The biggest feature of LEDs for lighting is the function of directional emission of light, because at present, almost all power LEDs are equipped with reflectors, and the efficiency of such reflectors is significantly higher than that of lamps and lanterns. In addition, the efficiency of the self-reflector has been included in the light efficiency detection of the LED. Road lamps using LEDs should make use of the directional light emission characteristics of LEDs as much as possible, so that each LED in the road lamps directly emits light to each area of the illuminated road, and then uses the auxiliary light distribution of the lamp reflector to achieve A very reasonable comprehensive light distribution of road lamps.
3. Constant current drive circuit design
The power supply system of LED street lights is also different from traditional light sources. The constant current drive power required by LEDs is a cornerstone to ensure their normal operation. Simple switching power supply solutions often cause damage to LED devices. How to make a group of LEDs tightly compressed together work safely and reliably is also an indicator for investigating LED street lights.
The requirement of the LED for the drive circuit is to ensure the characteristics of constant current output. Because the junction voltage changes relatively small area when the LED is working in the forward direction, so the constant LED drive current is guaranteed, which basically guarantees the constant LED output power. For the current situation of unstable power supply voltage supply in our country, it is very necessary for the driving circuit of the road lamp LED to have a constant current output characteristic, which can ensure the constant light output and prevent the over-power operation of the LED.
In order to make the LED drive circuit exhibit constant current characteristics, the output internal impedance must be high when looking inward from the output end of the drive circuit. When working, the load current also passes through this output internal impedance. If the drive circuit is composed of a step-down, rectifier and filter followed by a DC constant current source circuit or a general switching power supply and a resistance circuit, it must also consume a lot of active power. Therefore, the efficiency of these two types of driving circuits cannot be high under the premise of basically satisfying the constant current output. The correct design scheme is to use an active electronic switch circuit or a high-frequency current to drive the LED. The above two schemes can make the drive circuit have high conversion efficiency under the premise of maintaining good constant current output characteristics.
To sum up the discussion: whether LED street lamps can truly achieve the advantages of energy saving, environmental protection and long life in practical applications, the key is to solve the three aspects of heat dissipation and protection structure design, constant current drive circuit design and secondary light distribution optical design.
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