Key points in designing road lamps with LED light sources

introduction

The application of LED in lighting is a hot topic in the lighting industry in my country and even in the world. Through the continuous efforts of practitioners, the system efficiency (including driving circuit) of white light LED for lighting under normal lighting working conditions has reached 40lm/w. In addition, the manufacturing cost of LED for lighting has dropped significantly, so white light LED for lighting has met the commercial conditions for application in some lighting fields.

However, due to the situation caused by the industrial structure distribution in my country for many years since the planned economy era, the current industrial groups manufacturing lighting LEDs often lack the understanding of the basic knowledge and requirements of lighting appliances, and the traditional lighting fixture production groups also often lack a deep understanding of the characteristics of lighting LEDs. In the design and application, they do not fully utilize the characteristics of LEDs to make the best use of their strengths and avoid their weaknesses. As a result, many unreasonable and wrong designs are generated when lighting LEDs are applied to lighting devices. This article puts forward the following insights on some of the main problems of the current application of lighting LEDs in road lamps.

Features of LED lighting in road lighting

a The current status and defects of ordinary road lighting fixtures

At present, the light sources commonly used in road lamps are high-pressure sodium lamps or metal-toothed lamps. The biggest feature of these two light sources is that the arc tubes are small in size, and the small arc tubes produce a large light output and have a high light efficiency. The former, including the supporting electrical appliances, can reach 110 lm/w, and the latter, including the supporting electrical appliances, can reach 80 lm/w. However, when this type of light source is used in road lamps, only about 40% of the light directly reaches the road surface through the glass cover, and the rest of the light is projected out of the lamp through the lamp reflector. At present, traditional lamps that generally use this type of light source basically have two shortcomings. First, the illumination in the direction of direct illumination of the lamp is very high, which can reach more than 50 lx on secondary roads. This area is obviously over-illuminated, and the illumination at the intersection of the two lamps is only 40% to 20% of the illumination under the lamp. Second, the efficiency of the reflector of this type of lamp is generally only 50% to 60%, so about 60% of the light output is in the lamp, which is projected onto the road surface after losing 40% to 30%. The overall efficiency of such lamps is generally around 70% (only such road lamps of international famous brands can reach 80% efficiency), and the excessive illumination in the over-illuminated area is also a waste. Comprehensive analysis of the above traditional road lamps shows that their comprehensive effective lighting efficiency (excluding the over-illuminated part) is only 50%. Because of the above reasons, the lighting effect (road surface illumination and illumination uniformity) of my country's secondary roads currently basically does not meet the requirements of CJJ45-2006, CIE31 and CIE115 standards.

b Current lighting status of road lamps using LED light sources

At present, there are two main types of LEDs used in road lamps. One type uses the traditional road lamp housing, but installs a matrix LED inside the lamp on an almost flat mounting surface (also a reflective surface). This design method is impossible to obtain good light distribution of the lamp. The other type is to integrate multiple LEDs in a circular area (the diameter of the area 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 reflector for light distribution. However, the light distribution of the lamp in this design method will not be better than that of traditional road lamps, and because a high-density LED is integrated in a very small area, the heat dissipation of the LED is obviously poor, which not only affects the luminous efficiency of the LED, but also often affects the service life of the LED.

c Features and applications of LED lighting

The biggest feature of LED for lighting at present is that it has 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 reflector itself is already included in the light efficiency test of LED. Road lamps using LEDs should make full use of the characteristics of directional light emission of LEDs, so that each LED in the road lamps 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 road lamps. 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 lamps should contain the function of three-time light distribution 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 late September this year, the author participated in the acceptance of the test road section of Weihai City's secondary trunk roads using LED light source road lamps organized by the Shanghai Illumination Society. The lamps and layout of this test road are designed using the above principle. From the actual measurement results on site, this test section fully utilizes the appropriate directional light-emitting function of LED and adopts a three-way light distribution method. Not only does it make the illumination of the secondary trunk road and the illumination uniformity of the road surface meet the requirements of national standards, but it also prevents local excessive lighting in this area in the main lighting direction of the lamp. By making full use of the characteristics of LED and using a reasonable light distribution method to fully utilize the light output of LED, the lack of LED's own light efficiency compared to HID light sources is compensated, so that the energy consumption per unit area of ​​the entire road is only about 70% of the energy consumption of the original lighting design using high-pressure sodium lamp road lamps, while meeting the standard requirements, achieving significant energy-saving effects. China Lighting Network Technical Paper • LED Technology

At present, governments at all levels are vigorously promoting energy conservation and emission reduction. In indoor lighting, the limit of lighting power density (LPD) has been enforced. The technical standard for road lighting energy conservation certification in my country, "Technical Requirements for Energy Conservation Certification of Road Lighting Fixtures", will be completed at the beginning of next year. The technical requirements put forward the assessment requirements for road lighting power density (LPD). The core part is to reduce the lighting power density (unit: W/m2) as much as possible under the premise of meeting the illumination and illumination uniformity of the road surface, so as to achieve the purpose of energy conservation.

The design concept of the above-mentioned road lamps using LEDs meets the assessment requirements of the Technical Requirements for Energy-Saving Certification of Road Lighting Lamps. Currently, the Canadian government has promoted its road lighting energy-saving work according to the requirements of road lighting power density (LPD). It is estimated that more countries will promote their road lighting energy-saving work according to the requirements of LPD in the near future.

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 direction of each LED. When the fixture is used at different heights and irradiation widths, the spherical universal joint can be adjusted to achieve satisfactory results for the irradiation 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.

Other Problems and Solutions for Lighting Design Using LEDs

a LED heat dissipation conditions and IP protection

The working condition and heat dissipation of LED are not only directly related to the luminous efficiency of LED during actual operation, but also to the service life of LED. 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 LED. Solving these two contradictory but necessary 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 LED is applied to road lamps in China. The unqualified and unreasonable situations that have occurred in domestic use are basically:

(1) A heat sink is used for the LED, but the design of the LED wiring terminals and heat sink cannot reach IP45 or above, and cannot meet the requirements of the GB7000.5/IEC6598-2-3 standard.

(2) Using ordinary road lamp housings and matrix LEDs on the light-emitting surface of the lamp. Although this design can meet the IP test, the lack of ventilation in the lamp will cause the temperature inside the lamp cavity to rise to 50°C~80°C during operation. Under such high working conditions, the luminous efficiency of the LED cannot be high, and the service life of the LED will also be greatly reduced.

(3) An instrument fan is used inside 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-facing 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. (One company adopted the above design idea, and its product also passed the VDE IP65 test) 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 the use occasions of most road lamps in my country, the amount of flying dust in the air is large, sometimes reaching a large amount (such as sandstorms). After this type of lamp is used under normal conditions for a period of time (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 both the heat dissipation and IP protection of LEDs in road lamps, the more reasonable design guiding ideology is (1) to use heat conducting plates at key heat dissipation locations. The heat conducting plate is a metal plate with thin tubes for the flow of refrigerant evenly distributed inside it, 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 quickly and the heat will be quickly transferred. The thermal conductivity coefficient of a good heat conducting 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 locations, it will have a multiplier effect on the heat dissipation of LEDs. (2) Design the lamp housing to be a radiator. At present, most road lamp housings are made of aluminum. Directly using the outside of the lamp housing as a radiator can not only meet the IP protection level requirements, but also obtain a large heat dissipation area. In addition, the radiator composed of the lamp housing can be washed by natural wind and rain when there is dust, thereby ensuring the continued effectiveness of the radiator.

b Efficiency and output characteristics of LED driver circuit

LED requires the driver circuit to have 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 in my country, it is very necessary for the driver circuit of road lamps LED to have constant current output characteristics, which can ensure constant light output and prevent the LED from over-power operation.

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 cannot be high under the premise of basically meeting the constant current output. The correct design scheme is to use an active electronic switching circuit or a high-frequency current to drive the LED. The above two schemes can make the drive circuit have a high conversion efficiency while maintaining a good constant current output characteristic (for an introduction to improving the efficiency of LED drive circuits, please refer to "China Lighting Appliances" 2007 Issue 6).

c Lightning protection for electronic drive circuits in outdoor lighting fixtures

At present, most of the road lamps in my country adopt the mode of HID light source with trigger and inductive ballast. Although this mode has the problems of low energy efficiency and flicker, its reliability is very high. When LED lamps with electronic drive circuits are used in outdoor lighting occasions, an important aspect threatening their reliability is the problem of lightning induction.

As we all know, lightning in the sky emits a broad spectrum of radio waves, and overhead road lamp power supply lines are good receivers 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 down 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.

In addition, since my country's power supply lines are three-phase four-wire polarity power supplies with a grounded neutral line, at the moment when the radio waves of lightning are sensed in each section of the two overhead power lines, a differential-mode interference voltage is generated between the two power lines due to the different instantaneous impedances of the two power lines to the ground. This instantaneous differential-mode interference voltage can also reach hundreds of volts to more than 3,000 volts. This voltage often breaks down the power rectifier diode of the drive circuit and the electrical gap between the electrodes of different polarities on the printed circuit board, which will also damage the drive circuit.

To solve this problem, a fast-response varistor must be connected to the input end of the LED drive circuit to ensure the discharge of differential mode interference. Since the inductive interference of lightning is repeated many times, when the interference voltage is high, the varistor may discharge a large current when it is instantaneously turned on. Therefore, the varistor used should not only have a fast response capability, but also have the ability to discharge tens of amperes instantaneously without damage. In addition to using a varistor, the input end of the LED drive circuit should also be combined with the protection of conducted interference (EMI) and designed with a composite LC network, so that these LC networks can not only prevent the internal EMI from leaking to the power grid, but also have a significant inhibitory effect on the interference signal of lightning.

In addition, the electrical clearance between each point of the LED drive circuit and the ground should be kept above 7mm. The grounding capacitor of EMI protection and the insulation strength of the drive circuit to the ground should meet the requirements of reinforced insulation (4V+2750V). This will enable the LED drive circuit to have good resistance to differential mode and common mode lightning induction.

Conclusion

As a novel solid light source, LED has many unique advantages, such as almost no pollution in the production process and products, no fear of vibration, 0~100% continuous dimming, operation under ultra-low voltage, continuous operation in switching and flashing working states, and directional output light. In recent years, its obvious progress in light efficiency and light color has enabled it to enter commercial applications. However, in designing and using, we must pay full attention to the characteristics of LED and make full use of its strengths and avoid its weaknesses in order to achieve the expected good results.

At present, LEDs used in road lamps should give full play to their directional light output function, use the specific beam output angle and different light intensity output of each single LED, as well as the reasonable design of the installation direction and the role of the lamp reflector to achieve three-dimensional light distribution, maximize the use of LED direct light and directionality to maintain the full utilization of LED emitted light, maintain the uniformity of road lighting, thereby preventing excessive lighting in local areas, making up for the lack of its own luminous efficiency, and significantly improving the efficiency of the system. On the premise that the illumination meets the standard requirements, the lighting energy consumption per unit area can be achieved by about 30% energy saving compared with lamps using high-pressure sodium lamps on secondary roads. However, in terms of the current energy efficiency level and output light intensity level of LED, there is still a lot of gap for commercial applications of LEDs as main lighting on trunk roads.

In addition, when designing road lamps that use LED as light source, the heat dissipation should be reliable and effective while meeting IP protection requirements. The LED driving circuit should have high efficiency and perfect EMI and EMS protection characteristics. LED road lamps with these characteristics not only have good lighting power density (LPD) indicators, but also have the advantage of output light without flicker, which is very beneficial for lighting moving objects.