Technical difficulties facing LED street light design

Output power and luminous flux, secondary optical design, thermal design and power system design are the technical difficulties faced by LED street lighting design.

1, output power and luminous flux

The increase of output power and luminous flux also needs to be further improved from the basic level of high-power white LED epitaxial technology and chip technology. In terms of secondary optical design, the radiation forms of LEDs are Lambertian, side-fire, batwing and concentrating. In the field of road lighting, according to the design experience, Lambertian and batwing are more suitable. Through the secondary optical design, the illumination range and luminosity curve of LED meet the requirements of road lighting.

At present, the most common form of LED used in road luminaires is mainly in the luminaire. On an almost flat mounting surface (also a reflective surface), a matrix LED is mounted. This design method is impossible to get good. Lights are equipped with light.

At present, the most important feature of LEDs for illumination is the function of directional emission of light, because currently power-type LEDs are almost equipped with reflectors, and the efficiency of such reflectors is significantly higher than the reflector efficiency of the lamps. In addition, the efficiency of the self-reflector has been included in the LED efficacy detection. Road luminaires using LEDs should utilize the characteristics of directional emitted light of LEDs as much as possible, so that each LED in the road luminaire directly directs the light to each area of the road surface to be illuminated, and then uses the auxiliary light distribution of the luminaire reflector to realize Very reasonable light distribution for road lamps.

It should be said that the road lamps should meet the requirements of illumination and illumination uniformity of CJJ45-2006 and CIE31 and CIE115 standards. The function of three light distributions in the lamps should be better. The LED with reflector and having a reasonable beam output angle has a good primary light distribution function. In the luminaire, a good secondary light distribution function can be realized by designing the mounting position of each LED and the direction of the emitted light according to the height of the road lamp and the width of the road surface. Reflectors in such luminaires are only used as an auxiliary three-time light distribution to ensure better uniformity of road illumination.

In the design of the actual road lighting fixtures, each LED can be fixed on the luminaire with the spherical universal joint under the premise of basically setting the direction of each LED, when the luminaire is used at different heights and illumination widths. At the same time, the spherical gimbal can be adjusted to achieve satisfactory results for each LED. When determining the power and beam output angle of each LED, the LEDs can be calculated according to E(lx)=I(cd)/D(m)2 (the inverse law of the intensity and illuminance distance). The power that the beam should have when outputting the angle, and the light output of each LED can be predicted by adjusting the power of each LED and the different power output from the LED driver circuit to each LED. These adjustment methods are unique to road lamps that use LED light sources. By fully utilizing these features, it is possible to reduce the illumination power density and achieve energy saving while satisfying the illumination and uniformity of road surface.

2, LED street light cooling

The heat dissipation of LED street lamps is one of the key problems to be solved. It is not only directly related to the luminous efficiency of the LEDs, but also because the LED street lamps have high brightness requirements and large heat generation, and the outdoor environment is 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 dustproof and waterproof function (IP), good IP protection will often hinder the heat dissipation of the LED. Solving this contradictory problem that has to be solved is an important aspect that should be paid attention to when designing road lamps. In this respect, it is also the most unqualified and unreasonable situation in the domestic application of LEDs in road lamps. There are basically unqualified and unreasonable situations in the current domestic use:

(1) The radiator is used for the LED, but the design of the terminal and the heat sink of the LED connection cannot reach the IP45 and above, and cannot meet the requirements of the GB7000.5/IEC6598-2-3 standard.

(2) Using ordinary road lamp housings, matrix LEDs are used in the illuminating surface of the luminaire. Although this design can meet the IP test, the temperature inside the luminaire will rise due to the non-ventilation in the luminaire. To 50 ° C ~ 80 ° C, under such high conditions, the luminous efficiency of LED is impossible to high, and the service life of LED will also be greatly reduced, in fact there are obvious unreasonable conditions.

(3) The instrument fan is used to dissipate the LED and the heat sink in the lamp. The air inlet is designed below the lamp to avoid the entry of rainwater. The air outlet is designed to be around the LED light source. This can also effectively prevent the entry of rainwater. In addition, the heat sink and the LED (light source cavity) are not in the same cavity. This design is good, and it can pass smoothly according to the IP test requirements of the luminaire. This solution not only solves the heat dissipation problem of the LED, but also meets the requirements of the IP level.

But this seemingly good design actually has obvious unreasonable circumstances. Because in the use of most road lamps in China, the amount of flying dust in the air is large, sometimes it will reach a large extent (such as sandstorms). After using this type of lamp for a period of time under normal conditions (about three months to In the half year), the gap inside the internal radiator will be filled with dust, which will greatly reduce the effect of the radiator. Finally, the LED will have a significantly shortened service life due to the high operating temperature. The shortcoming of this program is that it cannot be used well and well.

To balance the heat dissipation and IP protection of LEDs in road lamps, a more reasonable design guideline is

a. In the critical heat dissipation position, a heat conducting plate is used. The heat conducting plate is inside the metal plate, and is uniformly provided with a thin pipe for the refrigerant to flow, and the small pipe is filled with the refrigerant. When a certain part of the heat conducting plate is heated, the refrigerant in the thin pipe flows quickly to make the heat quickly. Ground conduction. The heat transfer 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 high, if it is used in a key part, the heat dissipation of the LED will have a multiplier effect.

b. Design the outer casing of the luminaire into a radiator. At present, most of the road lamp housings are made of aluminum. Directly using the outside of the lamp housing as a heat sink can not only ensure the IP protection level, but also obtain a large heat dissipation area. In addition, the radiator of the lamp housing is in the presence of dust. It can be flushed by natural wind and rain to ensure the continuous effectiveness of the radiator work.

3, LED street light power system

The power system of LED street light is also different from the traditional light source. The constant current driving power supply required by LED is a cornerstone to ensure its normal operation. The simple switching power supply scheme often brings damage to LED devices. How to make a group of LEDs that are tightly compressed together work safely and reliably is also an indicator of LED street lights.

The requirement of the LED for the driving circuit is to ensure the characteristics of the constant current output. Because the relative change region of the junction voltage is small when the LED is working in the forward direction, the constant driving current of the LED ensures that the output power of the LED is constant. For the current situation of unstable supply voltage supply in China, it is necessary to have a constant current output characteristic of the driving circuit of the road lamp LED, which can ensure the light output is constant and prevent the super 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 viewed from the output end of the drive circuit. During operation, the load current also passes through the output internal impedance. If the drive circuit consists of buck, rectification and filtering, plus DC constant current source circuit or general-purpose switching power supply plus resistance circuit, it must consume a lot of active power. Power, so the two types of drive circuits under the premise of basically meeting the constant current output, the efficiency is impossible. The correct design scheme is to use an active electronic switching circuit or to use high-frequency current to drive the LED. The above two schemes can make the driving circuit still have high conversion efficiency while maintaining good constant current output characteristics.

At present, road lamps in China basically adopt the mode of HID light source with trigger and magnetic ballast. Although this mode has the problems of low energy efficiency and stroboscopic, its reliability is very high. While LED lamps using electronic drive circuits are used in field lighting applications, an important aspect that threatens their reliability is the lightning induction problem.

It is well known that lightning in the air emits a broad spectrum of radio waves, while overhead road lighting power lines are well accepted for wireless. The radio waves emitted by the same lightning output received by the two power lines belong to the common mode interference signal for the driving circuit. The common mode interference can reach several volts to several thousand volts, which easily breaks through the driving circuit. The EMC grounding capacitor or a small clearance to the ground (to the outer casing) causes damage to the drive circuit.

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