Design requirements for LED street lights
1. Directional Light Emission
LEDs used in street lighting offer the advantage of directional light emission, enhanced by integrated reflectors that are more efficient than traditional lamp reflectors. The efficiency of these reflectors is included in the LED’s light effect measurement. Road luminaires should fully utilize LEDs’ directional emission, ensuring each LED targets specific areas of the road. The luminaires’ reflectors can then assist in achieving comprehensive light distribution, meeting the illuminance and uniformity standards of CJJ45-2006, CIE31, and CIE115.
Each LED’s installation position and emission direction should be designed based on the streetlight’s height and the road’s width to achieve optimal secondary light distribution. Reflectors in these luminaires serve as auxiliary elements to ensure uniform road illumination. Adjustable spherical universal joints can be used to set the lighting direction of each LED, allowing for adjustments according to varying heights and illumination widths.
Using the inverse square law E(lx)=I(cd)D(m)2E(lx) = \frac{I(cd)}{D(m)^2}, the power and beam output angle for each LED can be determined. Adjusting the power output of each LED via the drive circuit can achieve the desired light output, reducing lighting power density while meeting illuminance and uniformity requirements and promoting energy efficiency.
2. Power System Requirements
The power system for LED street lights differs from traditional light sources. LEDs require constant current drive power for normal operation, as simple switching power supplies can damage LED devices. Ensuring tightly packed LEDs with a constant current drive circuit is crucial. The low junction voltage of LEDs in the forward direction necessitates a constant drive current to maintain constant output power, which is essential given the unstable power supply voltage in some regions.
The drive circuit must have high internal impedance to ensure constant current characteristics. A step-down, rectification, and filtering followed by a DC constant current source circuit or a switching power supply with a resistance circuit might consume excessive power, reducing efficiency. An active electronic switching circuit or high-frequency current drive can maintain high conversion efficiency and good constant current output characteristics.
Traditional road lamps using HID light sources and inductive ballasts suffer from low energy efficiency and stroboscopic issues. For LED lamps with electronic drive circuits, lightning induction poses a significant threat.
3. Lightning Induction Protection
Lightning emits broad-spectrum radio waves, which overhead power supply lines for road lamps can easily receive, causing common mode interference in the drive circuit. This interference can reach high voltages, potentially damaging the drive circuit.
Given the three-phase four-wire neutral line grounded power supply, lightning-induced radio waves can cause differential mode interference voltage between power lines, which may break down power rectifier diodes and circuit boards. To mitigate this, fast-response varistors should be connected to the LED drive circuit input to discharge differential mode interference. These varistors must respond quickly and handle high instantaneous conduction and discharge currents.
Additionally, EMI protection with a composite LC network should be designed to prevent internal EMI from leaking and inhibit lightning interference signals. Electrical clearance between circuit points and the ground should be above 7mm. The grounding capacitance of EMI protection and the drive circuit’s ground insulation strength should meet reinforced insulation requirements (4V+2750V) to enhance resistance to differential and common mode lightning induction.
