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​LED street light design requirements


2. The power system of LED street light is 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 tightly compressed together 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.

China's road lamps 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. While LED lamps using electronic drive circuits are used in field lighting applications, an important aspect that threatens their plasticity 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.

In addition, since the power supply line in China is a three-phase four-wire neutral line grounded polar power supply, in the segments of the two overhead power supply lines, at the moment when the radio waves of the lightning are sensed, the two power supply lines are grounded. The instantaneous impedance is different, which causes a differential mode interference voltage between the two power supply lines. The instantaneous differential mode interference voltage can also reach hundreds of volts to more than 3,000 volts. This voltage often breaks down the power supply rectifier diode and the printed circuit of the driver circuit. The electrical gap between the electrodes of different polarity on the board, the LED controller will also damage the drive circuit.

To solve this problem, it is necessary to connect the fast-responding varistor to the input terminal of the LED driving circuit to ensure the leakage of differential mode interference. Since the inductive interference of lightning is repeated many times, when the interference voltage is high, the current of the varistor transient conduction bleed may be large, so the varistor should not only have a fast response capability, but also have a transient guide. The venting capacity of ten amps is not damaged. In addition to the use of varistor, the input of the LED driver circuit should also be combined with conducted interference (EMI) protection, designed with a composite LC network, so that these LC networks can not only hinder internal EMI leakage to the grid, but also The interference signal of lightning has a significant inhibitory effect.

In addition, the electrical clearance of each point-to-ground of the LED driver circuit should be kept above 7mm. The grounding capacitance of the EMI protection and the insulation strength of the grounding circuit of the driving circuit should meet the requirements of reinforced insulation (4V+2750V), which can make the LED The drive circuit has good resistance to differential mode and common mode lightning induction.