Design standards and lighting requirements of LED street lights

　　The difference between LED street lights and conventional street lights is that the LED light source adopts low-voltage DC power supply and is a high-efficiency white light synthesized by GaN-based power blue LED and yellow. It has high efficiency, safety, energy saving, environmental protection, long life, fast response speed, high color rendering index, etc. Unique advantages, can be widely used on roads. High temperature resistance up  135 degrees, low temperature resistance up  -45 degrees. The LED street light design has the following requirements:

　　1. The biggest feature of lighting LEDs is the function of directional light emission, because almost all power LEDs are equipped with reflecrs, and the efficiency of such reflecrs is significantly higher than that of lamps.

　　In addition, the efficiency of the self-reflecr has been included in the detection of the light efficiency of the LED. Road luminaires using LEDs should make full use of the directional emission characteristics of LEDs as much as possible, so that each LED in the road luminaires directly shoot the light  each area of ??the illuminated road, and then use the auxiliary light distribution of the luminaire reflecr  achieve Very reasonable comprehensive light distribution of road lamps. It should be said that road lamps must truly meet the illuminance and uniformity requirements of CJJ45-2006 and CIE31 and CIE115 standards, and the three-time light distribution function in the luminaire can be better realized. The LED with a reflecr and a reasonable beam output angle itself has a good primary light distribution function. In the luminaire, the installation position and emission direction of each LED can be designed according  the height of the road luminaire and the width of the road surface  achieve a good secondary light distribution function. The reflecr in this kind of lamps is only used as an auxiliary three-time light distribution method  ensure a better uniformity of road illumination.

　　In the design of actual road lighting fixtures, each LED can be fixed on the fixture with a spherical universal joint under the premise of basically setting the lighting direction of each LED. When the fixture is used in different heights and illumination widths At the same time, the spherical universal joint can be adjusted  make the illumination direction of each LED reach a satisfacry result. When determining the power and beam output angle of each LED, according  E(lx)=I(cd)/D(m)2 (light intensity and illuminance distance inverse square law), the basic selection of each LED can be calculated 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 different power output  each LED by the LED drive circuit. These adjustment methods are peculiar  road lamps using LED light sources. Making full use of these features can reduce the lighting power density under the premise of meeting the road surface illuminance and uniformity of illuminance, and achieve the purpose of energy saving.

　　2. The power supply system of LED street lights is also different from traditional light sources. The constant current drive power required by LEDs is a cornersne  ensure its normal operation. Simple switching power supply solutions often bring damage  LED devices.

　　How  make a group of LEDs tightly packed gether is also an indicar for investigating LED street lights. The requirement of the LED for the drive circuit is  ensure the characteristics of constant current output. Because the junction voltage is relatively small when the LED is working in the forward direction, the constant LED drive current is guaranteed  basically ensure the constant output power of the LED. For the current situation of unstable power supply voltage in my country, it is very necessary for the driving circuit of the road lamp LED  have a constant current output characteristic, which can ensure the constant light output and prevent the LED from overpowering. In order  make the LED drive circuit exhibit constant current characteristics, looking inward from the output end of the drive circuit, its output internal impedance must be high. When working, the load current also passes through this output internal impedance. If the drive circuit is composed of a step-down, rectification and filtering plus a DC constant current source circuit or a general switching power supply plus a resistance circuit, it must also consume a lot of active power. Therefore, the efficiency of these two types of drive circuits can not be high under the premise of basically satisfying the constant current output.

　　The correct design scheme is  use an active electronic switch circuit or a high-frequency current  drive the LED. Using the above two schemes can make the drive circuit have a high conversion efficiency while maintaining a good constant current output characteristic.

　　The road lamps in our country basically adopt the mode of HID light source plus trigger and inductive ballast, although this mode has the problems of low energy efficiency and stroboscopic. When LED lamps with electronic drive circuits are used in outdoor lighting situations, an important aspect that threatens their plasticity is the lightning induction problem. As we all know, lightning in the sky emits a broad-spectrum radio wave, while the power supply lines of overhead road lamps are well-received wireless. 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 of volts  thousands of volts  the ground, and it is easy  break down in the drive circuit. EMC grounding capacitance or a small electrical gap  the ground ( the shell) can cause damage  the drive circuit.

　　In addition, since China’s power supply line is a three-phase four-wire neutral line grounded polar power supply, in each section of the two overhead power supply lines, at the moment when the radio wave of lightning is induced, the two power supply lines are connected  the ground. The instantaneous impedance is different and a differential mode interference voltage is generated between the two power supply lines. This instantaneous differential mode interference voltage can also reach hundreds of volts  more than 3000 volts. This voltage will often break down the power rectifier diode and printed circuit  control the electrical gap between the electrodes of different polarities on the circuit board, the LED controller will also damage the drive circuit.

　　solve this problem, a fast-response varisr must be connected  the input end of the LED drive circuit  ensure the discharge of differential mode interference. Since the inductive interference of lightning is repeated many times, when the interference voltage is high, the instantaneous conduction and discharge current of the varisr may be large. Therefore, the varisr used should not only have a fast response capability, but also have an instantaneous conductivity. The discharge capacity of dozens of amperes is not damaged.

　　In addition  the use of varisrs, the input end of the LED drive circuit should also be combined with conducted interference (EMI) protection, and a composite LC network should be designed so that these LC networks can not only prevent internal EMI from leaking  the grid, but also The interference signal of lightning has an obvious inhibiry effect. In addition, the electrical clearance between each point of the LED drive circuit and the ground should be kept above 7mm. The grounding capacitance of EMI protection and the ground insulation strength of the drive circuit should meet the requirements of reinforced insulation (4V+2750V), which can make the LED The drive circuit has good resistance  differential mode and common mode lightning induction.