The technical difficulties currently faced by LED street lamp design

To increase the output power and luminous flux, it is necessary to further improve the basic levels such as epitaxial technology and chip technology of high-power white light LEDs . In terms of secondary optical design, the radiation forms of LEDs include Lambertian, side-emitting, bat-wing and concentrating types. In the field of road lighting, according to design experience, Lambertian and bat-wing types are more suitable. Through secondary optical design, the illumination range and photometric curve of LEDs meet the needs of road lighting.

At present, the most common form of LED used in road lamps is mainly in lamps. On an almost flat mounting surface (also a reflective surface), a matrix LED is installed. This design method is impossible to obtain good lamp light distribution.

The biggest feature of LEDs for lighting is that they have the function of directional light emission, because almost all power LEDs are equipped with reflectors, and the efficiency of such reflectors is significantly higher than the reflector efficiency of lamps. In addition, the efficiency of the LED's own reflector is already included in the detection of the light efficiency of LEDs. Road lamps using LEDs should make full use of the directional light emission characteristics of LEDs, so that each LED in the road lamp can directly emit light to each area of ​​the illuminated road surface, and then use the auxiliary light distribution of the lamp reflector to achieve a very reasonable comprehensive light distribution of the road lamp. It should be said that if road lamps are to truly meet the illumination

and illumination uniformity requirements of CJJ45-2006, CIE31 and CIE115 standards , the lamp should contain a three-way light distribution function to achieve better results. , and LEDs with reflectors and reasonable beam output angles have good primary light distribution functions. In the lamp, the installation position and light emission direction of each LED can be designed according to the height of the street lamp and the width of the road surface to achieve a good secondary light distribution function. The reflector in this type of lamp is only used as an auxiliary three-way light distribution method to ensure better uniformity of road illumination. In the actual design of road lighting fixtures, each LED can be fixed to the fixture with a spherical universal joint on the premise of basically setting the emission direction of each LED. When the fixture is used at different heights and illumination widths, the spherical universal joint can be adjusted to achieve satisfactory results for the illumination direction of each LED. When determining the power and beam output angle of each LED, the power that each LED should have when the beam output angle is basically selected can be calculated based on E (lx) = I (cd) / D (m) 2 (the law of inverse square of light intensity and illumination distance), and the light output of each LED can be adjusted by adjusting the power of each LED and the different power output of the LED drive circuit to each LED to achieve the expected value. These adjustment methods are unique to road lighting fixtures using LED light sources . By making full use of these characteristics, it is possible to reduce the lighting power density while meeting the illumination and illumination uniformity of the road surface, thereby achieving the purpose of energy saving. 2. Heat dissipation of LED street lights Heat dissipation of LED street lights is one of the key issues that need to be addressed. It is not only directly related to the luminous efficiency of LEDs during actual operation, but also because LED street lights have high brightness requirements, high heat generation, and the outdoor use environment is relatively harsh. If the heat dissipation is not good, it will directly lead to rapid aging of LEDs and reduced stability. Because road lamps used outdoors should have a certain level of dust and water resistance (IP), good IP protection often hinders the heat dissipation of LEDs. Solving these two contradictory but must-solve problems is an important aspect that should be paid attention to when designing road lamps. This aspect is also the most unqualified and unreasonable situations when LEDs are applied to road lamps in China. The unqualified and unreasonable situations that have occurred in domestic use are basically as follows: (1) A heat sink is used for the LED, but the design of the LED connection terminal and the heat sink cannot reach IP45 or above, and cannot meet the requirements of the GB7000.5/IEC6598-2-3 standard. (2) Using an ordinary road lamp housing and a matrix LED inside the light-emitting surface of the lamp, this design can meet the IP test, but due to the lack of ventilation in the lamp, the temperature of the lamp cavity will rise to 50℃~80℃ during operation. Under such high working conditions, the luminous efficiency of the LED is not high, and the service life of the LED will be greatly reduced. In fact, there is an obvious unreasonable situation. (3) An instrument fan is used in the lamp to dissipate heat for the LED and radiator. The air inlet is designed below the lamp to prevent rain from entering, and the air outlet is designed around the downward-emitting LED light source. This can also effectively prevent rain from entering. In addition, the radiator and LED (light source cavity) are not in the same cavity. If this design is done well, it can pass the IP test requirements of the lamp smoothly. This solution not only solves the heat dissipation problem of the LED, but also meets the IP level requirements. However, this seemingly good design actually has obvious unreasonable situations. Because in most of the occasions where road lamps are used in China, the amount of flying dust in the air is large, sometimes reaching a large amount (such as sandstorms). After such lamps are used for a period of time under normal conditions (about three months to half a year), the gaps in the internal radiator will be filled with dust, which greatly reduces the effect of the radiator. Finally, the service life of the LED will be significantly shortened due to the high working temperature. The disadvantage of this solution is that it cannot be used for a long time. To take into account the heat dissipation and IP protection of LEDs in road lamps, the more reasonable design guiding ideology is a. Use heat conduction plates at key heat dissipation positions. The heat conduction plate is a metal plate with thin conduits for the flow of refrigerant evenly distributed inside the plate, and the refrigerant is filled in the thin conduits. When a certain part of the heat conduction plate is heated, the refrigerant in the thin conduits will flow quickly and the heat will be quickly conducted. The thermal conductivity coefficient of a good heat conduction plate can reach 8 to 12 times that of a copper plate of the same thickness. Although the price is higher, if it is used in key positions, it will have a multiplier effect on the heat dissipation of LEDs. b. Design the outer shell of the lamp into a radiator shape. At present, most road lamp housings are made of aluminum. Directly using the outside of the lamp housing as a heat sink can not only meet the IP protection level requirements, but also obtain a large heat dissipation area. In addition, the heat sink composed of the lamp housing can be washed by natural wind and rain when there is dust, thereby ensuring the continuous effectiveness of the heat sink. 3. The power supply system of LED street lamps The power supply system of LED street lamps is also different from that of traditional light sources. The constant current drive power supply required by LEDs is a cornerstone to ensure its normal operation. Simple switching power supply solutions often cause damage to LED devices. How to make a group of tightly compressed LEDs work safely and reliably is also an indicator for evaluating LED street lamps. The requirement of LED for the drive circuit is to ensure the constant current output characteristics, because the relative change area of ​​the junction voltage is very small when the LED is working in the forward direction, so ensuring the constant LED drive current basically ensures the constant LED output power. Given the current situation of unstable power supply voltage in China, it is very necessary for the drive circuit of road lamp LEDs to have constant current output characteristics, which can ensure constant light output and prevent over-power operation of LEDs.



























In order to make the LED drive circuit present constant current characteristics, the output internal impedance must be high when looking 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 plus a DC constant current source circuit or a general switching power supply plus a resistor circuit, it must also consume a lot of active power. Therefore, the efficiency of these two types of drive circuits is impossible to be high under the premise of basically meeting the constant current output. The correct design scheme is to use active electronic switch circuit or high -frequency current to drive the LED. The above two schemes can make the drive circuit have a high conversion efficiency while maintaining good constant current output characteristics.

At present, China's road lamps basically use the mode of HID light source plus trigger and inductor ballast. Although this mode has the problems of low energy efficiency and flicker, its reliability is very high. When LED lamps using electronic drive circuits are used in outdoor lighting occasions, an important aspect that threatens their reliability is the problem of lightning induction. As we all know, lightning in the sky emits a wide spectrum of radio waves, and the overhead road lamp power supply line is a good receiver of radio waves. The radio waves emitted by the same lightning received by two power lines are common-mode interference signals for the drive circuit. This common-mode interference can reach hundreds to thousands of volts to the ground, which can easily break through the EMC grounding capacitor in the drive circuit or the smaller electrical gap to the ground (to the shell), causing damage to the drive circuit.