What should be paid attention to when using LED light sources for road lighting fixtures
Yu Anqi
National Electric Light Source Quality Supervision and Inspection Center (Shanghai) National Lighting Quality Supervision and Inspection Center ( 200012)
Abstract: Due to the influence of the industrial layout of my country's planned economy, the lack of popularization of LED professional knowledge and comprehensive knowledge of lighting appliances among practitioners has resulted in actual design
and In applications, there are no applications that exploit the strengths and avoid weaknesses of LEDs based on their characteristics, and many unreasonable and unqualified application products have emerged. This article analyzes the current problems in the application of LED lighting fixtures on roads
and also provides ideas and examples for solving the problems for reference by the industry.
Keywords: LED light source road lighting design focus
As a novel solid lighting source, LED has almost no pollution in the production process and products, is not afraid of vibration, and can easily achieve 0 ~100 continuous dimming, can work under safe extra-low voltage, can work continuously in the switching state, and its light output is directional - many unique advantages, which are currently a hot topic in the lighting industry in my country and even the world. . Through the continuous efforts of practitioners, the current system efficiency of white light LEDs for lighting (including drive circuits) under normal lighting working conditions has reached 40lm/w. In addition, the manufacturing cost of lighting LEDs has dropped significantly, so the white light for lighting has LED already has the commercial conditions for application in some lighting fields.
However, due to the situation caused by the industrial structure distribution for many years since the era of my country's planned economy, the current industrial groups that manufacture lighting LEDs often lack understanding of the basic knowledge and requirements of lighting appliances, and traditional lighting The lighting production group also often lacks an in-depth understanding of the characteristics of LEDs for lighting. They do not fully utilize the characteristics of LEDs in design and application and use their strengths and avoid weaknesses. This results in many unreasonable errors when lighting LEDs are applied to lighting devices. wrong design. This article puts forward the following insights on some of the main issues currently encountered in the application of LED lighting in road lighting fixtures.
1. Features of LED used in road lighting fixtures
1. The current status and defects of ordinary road lighting fixtures
The light sources commonly used in road lighting fixtures are high-pressure sodium lamps or metal halide lamps. The biggest feature of these two light sources is that the arc tube that emits light is small in size and consists of The small-sized arc tube produces a large light output and has a high light efficiency. The former including supporting electrical appliances can reach 110lm/w, and the latter including supporting electrical appliances can reach 80lm/w. However, when this type of light source is used in road lighting fixtures, only about 40% of the light reaches the road surface directly through the glass cover, and the other light is projected out of the fixture through the reflector of the fixture. Currently, traditional fixtures with this type of light source are commonly used. Basically, there are two shortcomings. First, the illumination in the direction of direct illumination of the lamp is very high, which can reach more than 50lx on the secondary road. This area is obviously over-illuminated, and the illumination at the intersection of the two lamps is only The lower illumination is 40~20. Second, the efficiency of the reflector of this type of lamp is generally only 50-60, so about 60% of the light output is in the lamp, and is projected onto the road after losing 40-30%.
The overall efficiency of such lamps is generally around 70 (only the efficiency of such road lamps from internationally renowned brands can reach 80), and the excessive illumination in the above-mentioned over-illuminated areas is also a waste. Comprehensive analysis of the above-mentioned traditional road lighting fixtures has a comprehensive effective lighting efficiency (excluding excessive lighting) of only 50. Because of the above reasons, the current lighting effects (road illumination and illumination uniformity) of my country's secondary arterial roads basically fail to meet the requirements of CJJ45-2006, CIE31 and CIE115 standards.
2． The current light distribution status of road lighting fixtures using LED light sources
At present, there are two main types of the most common forms of LED used in road lighting fixtures. One is to use the traditional road lighting fixture shell, which is only used in the fixtures. Inside, a matrix LED is installed on an almost flat mounting surface (also a reflective surface). It is impossible to obtain good light distribution of the lamp with this design method. The other type is to integrate multiple LEDs in a circular area (the area diameter is about 30mm ~ 40mm), so that the light output density of this small area is close to that of a high-intensity gas discharge lamp, and then use the lamp to reflect it for light distribution. , but the distribution luminosity of lamps with this design method will not be better than that of traditional road lighting lamps, and due to the integration of high-density LEDs in a small area, the heat dissipation of the LEDs is obviously poor, which not only affects the performance of the LEDs Luminous efficiency also often affects the service life of LEDs.
3． Features and applications of lighting LEDs
The biggest feature of current lighting LEDs is their ability to directionally emit light, because most current power LEDs are equipped with reflectors, and the efficiency of such reflectors is obvious Higher than the reflector efficiency of the luminaire. In addition, the efficiency of the LED's own reflector has been included in the light efficiency detection of the LED. Road lighting fixtures using LEDs should make full use of the directional light emitting characteristics of LEDs, so that each LED in the road lighting fixtures directly emits light to each area of ??the illuminated road surface, and then uses the auxiliary light distribution of the reflector of the lamp to To achieve a very reasonable comprehensive light distribution of road lighting fixtures. It should be said that road lighting fixtures must truly meet the illumination and illumination uniformity requirements of CJJ45-2006, CIE31 and CIE115 standards. The function of three light distributions should be included in the fixtures to achieve better implementation. LEDs with reflectors and reasonable beam output angles have good primary light distribution functions. In the lamps, good secondary light distribution function can be achieved by designing the installation position and the direction of emitted light of each LED according to the height of the street lamp and the width of the road surface. The reflector in this type of lamp is only used as an auxiliary three-dimensional light distribution method to ensure better uniformity of road illumination.
In late September 2006, the author participated in the acceptance inspection of the test road using LED light source lamps on the secondary roads in Weihai City organized by the Shanghai Illuminating Society. The lamps and layout of the test road were designed using the above principles. of. Judging from the on-site measurement results, this test road section made full use of the appropriate directional lighting function of LED and adopted a three-dimensional light distribution method. The main lighting direction prevents local over-illumination. Due to the full use of the characteristics of LED and the reasonable light distribution method, the light output of LED is fully utilized, thus making up for the shortcomings of LED itself that the light efficiency is not as good as HID light source, so that the entire road unit area can meet the standard requirements. The energy consumption is only about 70% of the energy consumption of the original lighting design using high-pressure sodium lamps, achieving obvious energy-saving effects.
Currently, governments at all levels are vigorously promoting energy conservation and emission reduction. In indoor lighting, lighting power density (LPD) limits have been enforced. my country's technical standard for road lighting energy-saving certification "Technical Requirements for Energy-saving Certification of Road Lighting Lamps" will be completed early next year. The technical requirements propose assessment requirements for road lighting power density (LPD). The core part is to meet the illumination and lighting conditions of the road surface. Under the premise of illumination uniformity, the lighting power density (unit: W/m2) can be reduced as much as possible to achieve the purpose of energy saving.
The above-mentioned design ideas for road lighting fixtures using LEDs cater to the assessment requirements of the "Technical Requirements for Energy Saving Certification of Road Lighting Fixtures". At present, the Canadian government has implemented its road lighting energy-saving work in accordance with the requirements of road lighting power density (LPD). It is estimated that in the near future, more countries will implement their road lighting energy-saving work in accordance with LPD requirements.
In the actual design of road lighting fixtures, each LED can be fixed on the fixture with a spherical universal joint on the premise of basically setting the projection direction of each LED. When the fixture is used in different When adjusting the height and illumination width, the spherical gimbal can be adjusted to achieve satisfactory results in the illumination direction of each LED. When determining the power and beam output angle of each LED, the basic selection parameters of each LED can be calculated based on E(lx)=I(cd)/D(m)2 (the inverse square law of light intensity and illumination distance). The power that the beam output angle should have, and the light output of each LED can reach the expected value by adjusting the power of each LED and the LED drive circuit output to each LED with different power. These adjustment methods are unique to road lighting fixtures that use LED light sources. Taking full advantage of these characteristics can reduce the lighting power density and achieve energy saving while satisfying the illumination and illumination uniformity of the road surface.
2. Other problems and solutions in the design of lamps using LED
1. The heat dissipation working conditions and IP protection of LEDs
The working conditions and heat dissipation of LEDs are not only directly related to the luminous efficiency of the LEDs during actual operation, but also to the service life of the LEDs. Because road lighting used outdoors should have a certain level of dust-proof and waterproof IP, good IP protection often hinders the heat dissipation of LEDs. Solving these two contradictory but necessary problems is an important aspect that should be paid attention to when designing road lighting fixtures. In this aspect, there are the most unqualified and unreasonable cases when LEDs are used in road lighting in China. The current unqualified and unreasonable situations in domestic use basically include:
A radiator is used for LEDs, but the design of the LED connection terminals and radiator cannot reach IP45 or above, and cannot Meet the requirements of GB7000.5/IEC6598-2-3 standard.
Use an ordinary lamp casing and use a rectangular LED in the light output surface of the lamp. Although this design can meet the IP test, due to the lack of ventilation in the lamp, it will cause the temperature of the inner cavity of the lamp to rise during operation. As high as 50℃-80℃, at such a high temperature, the luminous efficiency of LED cannot be high, and the service life of LED will also be greatly reduced.
An instrument fan is used in the lamp to dissipate heat from the LED and radiator. The air inlet is designed under the lamp to avoid the entry of rainwater, and the air outlet is designed around the down-emitting LED light source. This can also effectively prevent rainwater 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. (One company has adopted the above design ideas, and its products have also passed VDE's IP65 test) This solution not only solves the heat dissipation problem of LEDs, but also meets the requirements of the IP level. However, this seemingly good design actually has obvious irrationalities.
Because in most of the use occasions of road lighting fixtures in our country, the amount of flying dust in the air is relatively large, sometimes reaching a large amount (such as sandstorms). This type of lamps can be used for a period of time under normal conditions. After a period of time (about three months to half a year), the gaps in the internal radiator will be filled with dust, which will greatly reduce the effectiveness of the radiator. Finally, the service life of the LED will be significantly shortened due to excessive operating temperature. The shortcomings of this solution prevent the lamp from being used for a long time and well.
To take into account the heat dissipation and IP protection of LEDs in road lamps, a more reasonable design is the key heat dissipation issue. Use a heat conductive plate, and thin tubes for the flow of refrigerant are evenly distributed inside the metal plate. And the thin tube is filled with refrigerant. When a certain part of the heat transfer plate is heated, the refrigerant in the thin tube will flow quickly to conduct the heat quickly. The thermal conductivity of a good thermal conductive plate can reach 8 to 12 times that of a copper plate of the same thickness. Although the price is higher, if used in key parts, it will have a twice the result with half the effort in heat dissipation of LEDs. The second is to design the casing of the lamp into a radiator shape.
At present, most of the road lamp casings are made of aluminum. Directly using the outside of the lamp casing as a radiator can not only ensure the IP protection level requirements, but also obtain a large heat dissipation area. In addition, the radiator composed of the lamp casing will be used when there is dust. , can be washed by natural wind and rain, ensuring the continued effectiveness of the radiator.
2． Efficiency and output characteristics of LED drive circuit
LED’s requirement for the drive circuit is to ensure constant current output characteristics. Because the relative change area of ??the junction voltage of the LED is very small when the LED is working, the LED drive current is ensured. Constantity basically guarantees the constant LED output power. Due to the unstable power supply voltage in my country, it is very necessary for the drive circuit of road lighting LEDs to have constant current output characteristics, which can ensure constant light output and prevent over-power operation of LEDs.
In order for the LED drive circuit to exhibit constant current characteristics, looking inward from the output end of the drive circuit, its output internal impedance must be high. The load current also passes through this output internal impedance during operation. If the drive circuit is composed of a voltage reduction, rectification and filtering followed by a DC constant current source circuit or a general switching power supply plus a resistor circuit, a large amount of active power must also be consumed on it. , so the efficiency of these two types of drive circuits cannot be high on the premise of basically meeting the constant current output. The correct design solution is to use active electronic switching circuits or use high-frequency current to drive LEDs. Using the above two solutions can make the drive circuit have high conversion efficiency while maintaining good constant current output characteristics.
3. Lightning protection induction of electronic drive circuits in outdoor lighting fixtures
Currently, my country's road lighting fixtures basically adopt the mode of HID light source with trigger and inductive ballast. Although this mode has low energy efficiency and flickering problems, but its reliability is very high. When LED lamps using electronic drive circuits are used in outdoor lighting situations, an important aspect that threatens their reliability is lightning induction.
What controls the emission of lightning is a broad-spectrum radio wave, and the overhead power supply line is a good receiving antenna. The radio waves emitted by the same lightning received by the two power lines are ***-mode interference signals for the drive circuit. This ***-mode interference can reach hundreds to thousands of volts to the ground and is easily broken down. The EMC grounding capacitance in the drive circuit or the small electrical gap to ground (to the shell) causes damage to the drive circuit.
In addition, since my country's power supply line is a three-phase four-wire polarity power supply with the neutral line grounded, at the moment when lightning radio waves are sensed in each section of the two overhead power supply lines, due to The instantaneous impedance of the two power supply lines to the ground is different, causing a differential mode interference voltage to be generated between the two power supply lines. This instantaneous differential mode interference voltage can also reach hundreds of volts to more than 3,000 volts. This voltage will often breakdown the driver. The electrical gap between the circuit's power rectifier diode and the electrodes of different polarities on the printed circuit board will also damage the drive circuit.
To solve this problem, a varistor must be installed at the input end of the LED drive circuit and respond quickly to ensure the discharge of differential mode interference.
Since the induced interference of lightning is repeated many times, when the interference voltage is high, the instantaneous conduction and discharge current of the varistor may be very large, so the varistor used should not only have a fast response. It should also have the ability to instantly conduct tens of amps without damage. In addition to using varistor, the input end of the LED drive circuit should also be combined with conductive interference (EMI) protection and designed with a composite LC network, so that these LC networks can not only prevent internal EMI from leaking to the power grid, but also prevent internal EMI from leaking to the power grid. The interference signal of lightning plays a significant inhibitory role.
Also, the electrical gap between each circuit of the LED drive circuit and the ground should be maintained at more than 7 mm. The ground capacitor for EMI protection and the ground insulation strength of the drive circuit should meet the requirements of reinforced insulation (4V 2750V). Requirements, so that the LED drive circuit has good resistance to differential mode and maximum mode lightning induction.
Conclusion
As a novel solid light source, LED has almost no pollution in the production process and products, is not afraid of vibration, can achieve continuous dimming from 0 to 100, and can be dimmed at extremely low It works under voltage, can continuously work in switching and flashing states, and has many unique advantages such as directional output light. In recent years, significant progress in its light efficiency and light color has enabled it to enter commercial applications. However, when designing and using LEDs, you must fully understand the characteristics of LEDs and exploit their strengths and avoid weaknesses to achieve the expected application results.
The current application of LED in road lighting fixtures should give full play to its function of directional light output, using the specific beam output angle and different light intensity output of each single LED as well as the reasonable design and installation direction. The role of lamps is to achieve three-dimensional light distribution, maximize the use of direct light and directionality of LEDs to maintain full utilization of LED emitted light, maintain the uniformity of road lighting, prevent over-illumination of local areas, and make up for the lack of its own luminous efficiency. The efficiency of the system has been significantly improved. On the premise that the illumination meets the standard requirements, the lighting energy consumption per unit area on the secondary arterial road can be about 30 times more energy-saving than that of lamps using high-pressure sodium lamps. However, there are still many gaps in LED's current energy efficiency level and output light intensity level when used as main lighting on trunk roads.
In addition, road lighting fixtures that use LED as the light source should be designed to ensure reliable and continuous effective heat dissipation on the premise of meeting IP protection. The LED drive circuit should have high efficiency and complete EMI and Protective properties of EMS. LED road lighting fixtures with these characteristics not only have good lighting power density (LPD) indicators, but also have the advantage of output light without flickering, which is very beneficial for the lighting of moving objects.
(Excerpted from "China Lighting" 2008.No.7)