LED street light fixture development focus

0 Preface

Energy ranks first in the world's most concerned issues in the 21st century, and lighting energy efficiency is ranked as the top priority. Road lighting is controlled by the government. In response to the trend of energy conservation and environmental protection, the business opportunities of D-lamps are constantly heating up. From the point of view of lighting, the road is divided into four categories: automobile-specific, general street, shopping street, and sidewalk. Generally speaking, road lighting refers to car-specific road lighting. The main function is to ensure that pedestrians and vehicle drivers at night roads can pass safely. In particular, drivers should be provided to see the shape of the road, the direction of travel and the surrounding environment to correctly identify Any obstacles that predict the direction of travel. Road lighting plays a very important role in reducing accidents and crime.

Due to the importance of road lighting, the design considerations include sufficient illumination, uniform brightness, pleasant light color, low glare, induced driving, energy saving and convenient maintenance. Therefore, street lamps should have high efficiency, good lighting utilization and proper light distribution. Long life, waterproof and dustproof, etc., to achieve safety, reliability, energy saving, visual mode, low light damage and low maintenance rate.

LED is regarded as the light of the 21st century. LED lighting is like vacuum tube to transistor, and CRT to FPD. It is a revolutionary breakthrough. After 2007, LED luminous efficiency is continuously improved. 1W cold white light product light efficiency is 110 lm/W, Nichia. The laboratory level of the small crystal ratio (20mA) is even 249 lm/W, and a large amount of investment has brought about an amazing growth rate. The 2008 Beijing Olympic Games also declared the coming of the LED lighting era. Before replacing indoor lighting, the most popular aspect of cutting people is the design of LED light source applied to street lighting modules. With a long-lasting life of over 5000Oh, it can significantly reduce future maintenance costs and reduce power consumption by 30%-50%. In addition, the excellent controllability of the LED street lamp can be fully utilized, such as the initial illumination adjustment, the time-controlled illumination setting, and the automatic dimming, etc., which can add another 10% to the road energy-saving benefit. At present, the system efficiency of white LEDs for lighting under normal lighting working conditions has exceeded 401m/W, and the manufacturing cost of LEDs has dropped drastically. Therefore, white LEDs for lighting have commercial conditions applied to some lighting fields. However, the current industrial groups that manufacture lighting LEDs often do not understand the basic knowledge and requirements of lighting appliances, and the traditional lighting lighting production groups often lack deep understanding of the LED characteristics of lighting, and do not fully utilize LED features in design and operation. To carry out the strengths and avoid weaknesses, resulting in many unreasonable designs.

1 About LED street light

1.1 Road lighting regulations and standards

Road lighting requirements are strict, road lighting setting specifications and street lamp performance standards are fairly complete and clear in the world; standards generally include illuminance and uniformity (minimum illuminance / average illuminance), CIE115 proposes reference specifications, dividing roads into four categories, pavement benchmarks It is 15, 20, 30, 501x with a uniformity of 0.4. Based on the influence of environmental and cultural factors, there are still gaps in national norms. The safety, performance and energy efficiency characteristics of road lighting fixtures are important aspects for evaluating the quality of road lighting fixtures. The relevant product standards include product safety standards, product performance standards and photometric test method standards. Segmented Product Safety Standard: GB 7000. 1 -2007 "Lighting Part 1: General Requirements and Tests" (idt IEC60598 -1 : 2002) and GB7000.5 -2005 "Safety Requirements for Road and Street Lighting" (idt IEC60598 - 2 -3:2003) Performance Standards for Segmented Products: GB/T24827-2009 "Performance Requirements for Road and Street Lighting", Photometric Test Method Standard: GB/T9468-2008 "General Requirements for Luminous Distribution Photometric Measurement", the standard The test method is consistent with CIE121. Lamps that use LED light sources in sections, in addition to the above two standards, shall also comply with the safety of GB7247.1-2001 laser products Part 1: Equipment classification, requirements and user guides (idt IEC 60825 -1: 1993). Regarding the detection of D-lights, IECEE has OSM/CTL resolution. The principle is that for four-D lamps, the type should first be judged according to IEC60825-1. If it is a Class 1 or 1M laser-safe LED, it is not required to IEC60825-1 test, otherwise required. The luminosity of the luminaire is specified by the CIE (International Commission on Illumination) publication, covering the glare and uniformity of CIE 31 -1978 road lighting, depreciation and maintenance of CIE33 -1977 road lighting, CIE 34 -1977 road lighting and installation data: luminosity Determination, classification and performance, CIE 47 -1977 Road lighting in humid environments, CIE 66 -1984 Road surface and materials, CIE 140 -2000 road lighting calculation, CIE 180 -2007 Road transport lighting in developing countries.

Lighting requirements for roads/street lights (from CJJ45-2006 standard), road lighting regulations for CJJ45-2006 in Table 1, and luminosity indicators for different roads from road lighting requirements, if roads are mainly for motor vehicles The road surface should be evaluated by brightness. For example, the illuminance should be used as the main service object for pedestrians and non-motor vehicles. Since the pedestrian cares about the face of the other person and the situation on the ground, it is necessary to determine the illuminance of the vertical surface or the illuminance on the road surface (horizontal illumination).

Table 1 Regulations for road lighting on the CJJ45-2006 standard

Under the premise of energy saving, Japan sets the reference standard for street lighting fixtures. The energy efficiency part is bounded by 200w lighting fixtures. The luminous efficiency below 200W is 50lm/W, and the rest is higher than 60lm/W. Power density (LPD, W/m2) is included in the regulations, and this policy will accelerate the popularity of high-efficiency lamps and is also an opportunity for LED street lamps.

Taiwan actively promoted the development of LED lighting regulations, and officially announced the CNS15233 "Light-emitting diode road lighting" standard on December 4, 2008. Emphasis on energy saving, durability, safety, light distribution performance, etc., three levels of energy efficiency, minimum luminous efficiency of 451m / W, followed by continuous testing 360h under 50 °. Safety is focused on avoiding lightning damage and wind strength. The distribution of light intensity is mainly regulated on the light distribution, and the product must ensure the uniformity of the road illumination to ensure the safety of passers-by. The specification mainly includes basic performance (power factor, input power, current harmonic distortion and voltage variation), safety (insulation withstand voltage, IP, wind pressure), electromagnetic interference, optical characteristics (light color allowable color shift range, light distribution) ), luminous efficiency, durability reliability (temperature cycle and switch 8000 times), durability, etc. In addition, based on visual moderation, no LED is allowed to light or flicker. The most demanding standard is that after the aging test is required for 100Oh, in the 3000h test, the luminous flux should not be less than 92% of the original luminous flux, and the luminous efficiency is 90% of the original standard.

DOE's requirements for road lighting in the ENERGY STAR certification of SSL products, DOE announced the first batch of SSL product ENERGY STAR certification requirements in August 2007 (a total of 7 products, 3 outdoor and 4 indoors) In August 2008, the requirements for supplementary products including road/street lighting technical requirements were announced (a total of 10 products, 5 outdoor and 5 indoors). The technical requirements for road/street lighting are as follows (including technical requirements common to the first batch of products):

(1) Requirements for chromaticity and color change, the color specifications are shown in Table 2. The SSL product meets the requirements of the chromaticity coordinates required by ENERGY STAR. There is no limit to the color temperature of road/street luminaires in the DOE SSL specification.

Table 2 Rules for chromaticity, color change and color

Figure 1 The representation of the color temperature position in the chromaticity diagram

Figure 2 LED light decay and junction temperature curve

(2) Life (light fading) requires two grades of 2500Oh and 3500Oh. The detection method is to illuminate the SSL lamp after 6000 hours, and the lamp does not need to be aging. The measurement is performed every 1000h. The light decay to 600Oh is less than the initial 94.1%, the life is more than 35000h, and the light decay is 91.8%. The life expectancy is 25000h.

(3) Electrical requirements, Table 3 is the electrical requirements for SSL lamps.

Table 3 Electrical requirements for SSL lamps

(4) Energy efficiency requirements: The energy efficiency of the lamp is ≥50lm/W.

(5) Minimum light output required for light distribution: 23001m (initial); lumen density: 0°~90° is 100%, luminous flux is 80°~90°, maximum 10% luminous flux; beam uniformity requires maximum light intensity At 55 ° - 65 ° and so on.

(6) Other content is the exception that the standby power consumption requirement does not consume electricity when not working, but it is used together with the monitoring equipment that detects the presence of personnel; it is guaranteed to replace the replacement light source or driver for free for three years from the date of purchase. The date of guarantee shall be calculated from the date of transportation; the luminaire manufacturing shall be accompanied by the description of the thermal management of the luminaire, the certificate file and the test procedure; the minimum required lumen of the LED light source to decay to 70% of the initial value is 35000h; attached to the building SSL devices with a power greater than 13W must be equipped with a light control device that can be automatically turned on and off, and can be manually or automatically controlled within 24 hours.

1. 2 LED street light status

(1) Compared with traditional road lighting fixtures, LED street lamps are commonly used as high-voltage uranium lamps or metal halide lamps. The biggest feature is that the arc-emitting arc tube is small in size, has a large light output, and has high light. Efficiency, system efficiency (including supporting electrical appliances) can reach 80 -110 lm / W, but this kind of light source is used in road lamps, only about 40% of the light directly passes through the glass cover to reach the road surface, and other light passes through the lamp reflector and then emits the lamp . Traditional street lamps have obvious deficiencies. First, the illumination in the direction of direct illumination of the lamps is very high, and the over-illumination is obvious. The secondary roads can reach 50 lx or more. Second, the efficiency of the reflectors is 50%-60%, and the light output of about 60% is In the luminaire, after losing 30% -40%, the technology is shot on the road. The overall efficiency of such luminaires is 60-70% (very few international brands have efficiencies of up to 80%). The comprehensive effective lighting efficiency is only 50%. At present, the lighting effect (illuminance and uniformity) of the secondary trunk road can hardly meet the requirements of CJJ45-2006 and CIE31, CIE115 standards. LED as a street light source, it has many advantages compared with the traditional street light source, long life, good seismic performance, white LED light color is better than high-pressure lamp, can achieve a perfect dimming function (LED is frequently switched on it is not too big The effect can be dimmed by changing the current method, or the pulse width adjustment method can be used for dimming, and the LED luminous intensity can be effectively adjusted by adjusting the voltage duty ratio and the operating frequency), and the luminous flux loss of the LED light source in the optical system of the lamp is the smallest, LED The light source does not contain harmful metal records.

(2) Current status of road light fixtures with LED light sources. At present, there are two main types of LEDs used in street lamps: one uses traditional road lamp housings, but only the matrix LEDs mounted on the flat panel mounting surface; The class integrates multiple LEDs on a cylinder, the light output density is close to that of a high-intensity gas discharge lamp, and then the illuminator reflector is used for light distribution, but the design of the luminaire distribution is not superior to the traditional road lamps, and in one The integration of high-density LEDs in a small area makes the LED heat dissipation poor, which not only affects the luminous efficiency of the LED, but also affects the service life of the LED. The LED street lamp modified by the traditional street lamp head adopts the aluminum profile as the radiator, and the whole light source and the driving circuit are incorporated into the aluminum alloy lamp shell. Although the protection level of the lamp is solved, the whole light source is in the sealed lamp housing. The heat element method generated in the work of the lamp is emitted into the air, which causes the LED module and the drive circuit to work in a very harsh environment, the working temperature rises sharply, and the LED has a dead light and a light decay as the temperature rises, the driver It is also burnt due to high temperature, which greatly reduces the reliability and service life of the luminaire (see Figure 2). Therefore, this program is not mature. To make LED street lights, it is not feasible to simply install LED light sources with traditional lamp heads, as shown in Figure 3.

Figure 3 Traditional street light converted into LED street light

At present, LED street lights cannot be used for trunk roads. The main road is a trunk road connecting the main sections of the city, and is separated by a motor vehicle and a non-motor vehicle. The speed of the motor vehicle on the main road is faster than that of the secondary road, and the lighting level is higher than that of the secondary road. The illuminance requirement of the main road is up to 30 lx, the illuminance uniformity is 0.4, the average brightness requirement is 2.0 cd/m2, the total brightness uniformity is 0.4, the brightness longitudinal uniformity is 0.7, and the threshold increment is ≤10. Street lamps that meet the energy-saving level should have the lighting quality above the above requirements, and the lighting power density should be less than the national standard. When the number of lanes is ≥6, the lighting power density is 1.05W/m2, and the number of lanes is <6. The illumination power density is 1.25 W/m2. Most of the main roads use 250W or 400W high-pressure uranium lamp street lamps. The luminous flux of 250W or 400W high-pressure nano lamps reaches 28000lm (250 W) and 48000 lm (400 W). The luminaire efficiency of such high-power street lamps is generally between 75% and 85%. Therefore, the luminous flux output of the lamps is also at least 2100 Olm. Taking a 250W high-voltage lamp street lamp as an example, the lamp efficiency is 83.5%, and the light distribution curve is as shown in Fig. 4(a). The maximum light intensity is 10° C angle and 70.0° γ angle. The shape of the light distribution curve in which c is 0°, 180°, and 10° is good. This street light can be used on the main road of four to six lanes. At present, the small luminous flux is the main reason why LED street lights cannot be applied to the main road. Luminaires with different luminous flux grades should be applied to different levels of roads, otherwise energy saving effects will not be achieved.

Figure 4 LED light distribution

(3) LED light source road lamp driving status, LED drive circuit requirements are to ensure the characteristics of constant current output, because the relative change region of the junction voltage is small when the LED is working forward, so the LED drive current is guaranteed to be basically guaranteed. The LED output power is constant. Due to the unstable supply voltage supply in China, it is necessary to have a constant current output characteristic of the driving circuit of the road lighting LED to ensure the light output while avoiding the super power operation of the LED. At present, a large part of the first type of driving circuit is composed of buck, rectification and filtering, DC constant current traveling, and circuit; the other type is composed of a general-purpose switching power supply plus a resistance circuit, which consumes a large amount of active power. Therefore, the two types of driving circuits are not efficient under the premise of basically satisfying the characteristics of constant current output. In addition, a large part does not have lightning protection.

(4) LED light source road lamp heat dissipation and IP protection status, LED working conditions and heat dissipation are not only directly related to the luminous efficiency of the LED actually working, but also related to the service life, and good IP protection will also hinder the heat dissipation of the LED. . At present, a large part of the LED street lamp is partially modified in the traditional lamp. The heat of the lamp is concentrated in the optical cavity of the lamp. On the one hand, the characteristics of the LED light source are violated, the light distribution is not up to the requirement, and the heat dissipation effect is accompanied by another fatal defect. Not good, seriously affecting light decay and longevity. Because the traditional lamp is partially modified, even if the IP protection is good, it affects the heat dissipation of the LED, and some lamps fail to reach the IP65 level, which directly causes the short circuit of the light source and the power supply to fail.

2 LED street light design concerns and solutions

2.1 LED light distribution design

The LED street light is different from the optical design using conventional light source street lights. Conventional light source street lamps use a reflector to evenly distribute the luminous flux of a light source onto an illuminated road surface. The light source of the LED street lamp is composed of a plurality of LEDs, and the illumination direction of the illuminated road surface is obtained by designing the projection direction of each LED. The biggest feature of LED is the function of directional emission of light. The power type LED is equipped with a transmitter, and its efficiency is obviously higher than that of the luminaire reflector (primary light distribution). The LED road luminaire should utilize the directional emission characteristics of the LED as much as possible. The LEDs are directly directed to the illuminated road surface (secondary light distribution), and then the auxiliary light distribution (three times light distribution) of the lamp reflector is used to realize reasonable road light integrated light distribution to meet CJJ45-2006 and CIE31, CIE115 standard illumination and uniformity requirements. Due to the semi-space illumination of the LED light source, the LED light source is used in the luminaire, and the luminous flux loss of the light source is less than that of the high pressure uranium lamp or the metal halide lamp. For the time being, the luminous efficacy of the LED light source does not reach the level of the high-pressure nano lamp or the metal halide lamp, that is, the light source of the same power, and the luminous flux is not as high as the high-intensity gas discharge lamp. However, due to the half-space illumination of the LED light source, the luminous flux output of the LED luminaire can be close to that of the high-intensity discharge lamp. When the street light illuminates the road, there is only half the space in the direction of the light. The luminous space of the high-intensity discharge lamp occupies the entire space, see Figure 4(a). If there is no luminaire, the light in the upper half of the light source is completely wasted. Using a luminaire, the reflector can reflect the light in the upper half space into the lower half space, but the absorption of the light by the reflector, and the occlusion of the light source itself is converted into thermal energy, or the reflector reflects the light twice. Reabsorption causes a portion of the light from the source to be lost. Luminaire efficiency is a parameter that reflects the extent of light loss from the source. From the luminosity performance of small power high-pressure uranium lamp street lamps (70 W -150 W), the luminaire efficiency is generally between 65% and 75%. Taking a 100W high-pressure uranium lamp as an example, the luminous flux of the light source reaches 8000 1m, and the luminous output of the street lamp is 52001m - 6000 1m. LED light sources are advantageous in this respect. LED illumination is only present in half of the space, see Figure 4(b), which can also project 100% of the light onto the road without the use of a luminaire. The need to design the luminaire is to change the direction of light projection over a small range of angles, during which the loss of light is minimal. At present, the luminous efficacy of LED street lamps has reached 40-50 lm/W. Taking 100W LED street lamps as an example, the luminous flux output of lamps can reach 4000-5000 1m. It is believed that by continuously strengthening the design power, the light distribution of the street lamps will be more optimized and finally perfect.

Governments at all levels are vigorously promoting energy conservation and emission reduction. In indoor lighting, the limits of lighting power density (LPD) have been enforced. The technical standard for road lighting energy-saving certification in China has been proposed for road lighting. The core part of the assessment of power density (LPD) is to reduce the lighting power density as much as possible under the premise of meeting the road surface illumination and uniformity (unit: W/m2, thus achieving energy saving purposes. The above design ideas just meet the assessment requirements.

2.2 LED street light cooling

LEDs are used in embedded lighting fixtures. Due to the large amount of heat generated by LED chips, it is difficult to make high-power lamps. One of the issues that companies are currently working on is "heat dissipation." Although the luminous efficiency is getting higher and higher, the heat generation of the LED chip is still large. If the heat dissipation measures are not taken, the temperature of the LED chip will be too high, resulting in deterioration of the performance of the chip itself and the encapsulating resin, thereby eventually causing a decrease in luminous efficiency and a shortened life. The most important performance of LEDs is "long life". In order not to affect this indicator, it is necessary to try to dissipate the heat generated on the chip. Improving the heating of LEDs is the key to capturing high-power lighting. At present, the more practical solution is to design the aluminum casing of the lamp as a radiator. After the inner surface is flat, the light source aluminum substrate is installed, and the aluminum substrate and the inner surface are connected by a thermal silica gel to ensure the conduction heat dissipation effect, and the outer surface is made of 鳝. The heat dissipation structure can ensure the continuous heat dissipation work even if the outer casing is dusted. In addition, the heat-dissipating plate can be used for the key heat-dissipating position. The heat-conducting plate is inside the metal plate, and is provided with a thin pipe for the refrigerant to flow. When a certain part of the heat-conducting plate is heated, the refrigerant in the thin pipe will flow quickly to make the heat rapidly Conduction, good thermal conductivity plate heat transfer coefficient is 8-12 times the same thickness of copper plate.

In addition, the heat dissipating fins are simple and effective heat dissipating units for small heat generating devices, but it is unscientific for a high-power heating device such as a street lamp to simply amplify the heat dissipating fins. According to the most basic thermal principle, a large area of heat-dissipating cymbal has a serious heat island effect. In order to obtain the required heat-dissipating effect, the size of the heat sink is increased as the power is increased, and the size of the heat sink is additionally increased. The size and weight of the heat sink are greatly increased, resulting in a large weight of the lamp.

The choice of heat-dissipating materials, in order to do LED street lights, we first choose the heat-dissipating materials. At present, the heat sinks are basically made of metal materials. This is mainly due to three considerations: good thermal conductivity, easy processing, and easy access. Table 4 shows the heat transfer coefficient of commonly used materials for heat sinks and common metal materials. Among the five different aluminum alloys listed in Table 4: AA6061 and AA6063 have good thermal conductivity and processability, suitable for extrusion forming processes, in heat sinks. It is widely used in processing. ADC12 is suitable for die-casting, but the heat transfer coefficient is low. Therefore, AA1070 aluminum alloy is usually used in heat sink processing. Unfortunately, the processing performance is not as good as that of ADC120AA1050. It is suitable for stamping process and is used for manufacturing thinner. Fins.

Table 4 Thermal Conductivity of Common Metal Materials

2.3 LED street light drive

To ensure the long-life characteristics of LEDs, the driving of LED street lights is very critical, and its conversion efficiency is very important. The use of active electronic switching circuits or high frequency currents to drive the LEDs allows the drive circuit to maintain good constant current output characteristics while providing high conversion efficiency. However, the LED luminaires of electronic drive circuits, when used outdoors, have the greatest threat to their reliability due to lightning induction. Common mode interference can reach hundreds of volts to several thousand volts to the ground. It is easy to break through the EMC grounding capacitance of the driver circuit or the small ground clearance, causing damage to the drive circuit. At the same time, the differential mode interference voltage can also be reached. From hundreds to more than 3,000 volts, this voltage can break through the gap between the power rectifier diode of the driver circuit and the electrodes of different polarity on the printed circuit board, damaging the driver circuit. The drive circuit should be equipped with varistor and conducted interference (EMI) protection. At the same time, the electrical clearance of the LED drive circuit should be kept above 7mm, and the insulation strength should be strengthened to (4U+2750V), so that the LED drive circuit has good performance. Anti-lightning capability.

2.4 IP protection of LED street lights

Not only the LED light source part is required to have IP65 protection capability, but also the driving part must meet the requirements of IP65 level or above, which not only ensures the safety of the LED and the driving work, but also ensures the light output maintenance rate. Generally, the structure waterproof design and the silicone sealing strip scheme are adopted. The silicon silicone rubber can only be used as an auxiliary waterproofing method, and the driving part can adopt the potting sealing rubber, which not only solves the waterproof purpose, but also can achieve the purpose of driving heat dissipation.

3 other points of attention for LED street lights

LED street light lamp structure overall consideration, integrated design; modular design of LED light source, easy to assemble maintenance and heat dissipation, as much as possible composed of multi-module light source to form a high-power light source; four D light source, light color choice: LED street light source cavity Separate design from power supply cavity; LED street light accessory material selection, such as lens material, heat dissipation material, etc.; power input terminal junction box design, consider convenient and safe disassembly and assembly, design power cord junction box at the same time pay attention to waterproof; LED street lamp anti-wind pressure requirements, Taking into account the total weight of the luminaire, the design of the windward area; the realization of the LED street light dimming control function, realizing energy saving; the perfect combination of LED street light and solar energy, effectively utilizing green energy; of course, as an outdoor product, we must also consider the design of the product, pay attention to the solution. Daytime landscape appreciation issues and more.

4 Summary

LED is applied in road lighting, giving full play to its directional light output characteristics. Reasonable secondary light distribution combined with lamp reflector three times of light distribution meets road lighting design requirements, system efficiency is continuously improved, and energy consumption per unit area is saved by 30% compared with high pressure uranium lamp. The purpose of the design is to ensure reliable and continuous heat dissipation under the premise of IP protection. The LED driver circuit should have high efficiency and perfect EMC protection characteristics, and should meet the lighting power density (LPD) index. With the advancement and development of LED technology, the application technology of LED street lamps has also progressed and developed. Most LED street lamp products meet or exceed national road lighting standards. The effect is obvious, the situation is gratifying and the outlook is optimistic. The strong support of the government is the source of the development of LED lighting technology. It should pay attention to the research of application technologies such as drive system, optical system, heat conduction and heat conduction system, protection level, processing technology and aesthetic design, and strive to improve the stability and reliability of LED lamps themselves. Sex. The application of LED street lamps should lead to the "practice" point of view, while strengthening the research and development of core technologies.

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