Electromagnetic compatibility issues that need to be paid attention to about LED street lights

1. Applicable standards

When applying for voluntary certification in China, the electromagnetic compatibility of LEDs needs to meet the requirements for surge immunity in the standards GB17743-2007 Limits and Methods of Measurement of Radio Disturbance Characteristics of Electrical Lighting and Similar Equipment GB17625.1-2012 Limits for Harmonic Current Emission and Electromagnetic Compatibility (Input Current per Phase of Equipment ≤ 16A), and GB/T18595-2014 Requirements for Electromagnetic Compatibility Immunity of General Lighting Equipment. Disturbance voltage, radiated field disturbance, and surge immunity are the most problematic items in the three standards.

2. Disturbance voltage

The LED controller circuit did not consider electromagnetic compatibility issues in the design process of street lamps, and the design of the power filter circuit was not ideal, resulting in low frequency radiation exceeding limits. The product disturbance voltage items did not meet the requirements of GB17743-2007 "Limits and Measurement Methods for Radio Disturbance Characteristics of Electrical Lighting and Similar Equipment". Unqualified projects can easily lead to distorted voltage and current waveforms in the power grid, contaminating the power grid, and affecting the normal operation of other electrical equipment, especially highly sensitive equipment.

Disturbance voltage disturbances mainly include differential mode disturbances and common mode disturbances. For conducted disturbances in different frequency bands that exceed the standard, common corrective measures include:

(1) Below 5MHz, there is mainly a mixture of differential mode and common mode. You can adjust the mode disturbance exceeding X capacitance, add a differential mode inductor, and adjust the differential mode inductance (as shown in Figure 1); If the common mode interference exceeds the standard, you can add a common mode inductor, select a reasonable inductor to suppress or increase Y, and adjust the parameters and position of the capacitance Y appropriately (as shown in Figure 2).

(2) 20~30MHZ disturbance: it is possible to adjust the commonly used corrective measures Y capacitance or change the position of Y capacitance; Or the transformer is wrapped with copper foil; Or connect a small common mode inductor to the front of the output line and have a double wire.

(3) 5M~20MHz is mainly based on co touch disturbance, and the method of suppressing co touch is adopted. Using a magnetic loop around the housing ground wire 2-3 turns will align it for 10 turns of MHz. The above interference has a significant attenuation effect. Or adjust reasonable filter parameters by connecting RC in parallel at both ends of the output rectifier tube (as shown in Figure 3).

3. Radiation field disturbance

The main reason is that there are defects in the overall design of LED street lights, such as unreasonable design of constant current modules outside the source for lighting or power shielding effects, unreasonable grounding wiring, unreasonable internal wiring of lighting, and unreasonable application of sensing elements, which lead to unreasonable interference. GB Limit Requirements 17743-2007 Limits and Measurement Methods for Radio Disturbance Characteristics of Electric Lighting and Similar Equipment.

As a measure to suppress radiation disturbances, special attention should be paid to the layout and wiring of switching power supplies. As a remedial measure, appropriate measures (such as shielding and filtering) can also be used for the structure and equipment adoption of switching power supplies. Common suppression measures are as follows:

1) Use shielding to suppress radiated noise

The principle of the shielding effect is to use a shield to reflect or absorb noise. The metal shield mainly adopts a complete shielding structure, requiring the spacing between the opening size and fasteners to be as small as possible, and avoiding insulation in certain areas due to coating.

2) Common rectification methods for disturbance sources are as follows:

Adjust the absorption circuit parameters of the output rectifier tube, or string the output rectifier tube and MOS magnetic beads on the pin, or connect a small absorption circuit in parallel with the D-S pin of the MOSFET on the pin.

3) Design of Printed Circuit Board

Circuit design cannot achieve optimal wiring, but the most important parts should be considered. From the perspective of electromagnetic interference emissions, the most important signals are high current and voltage rate of change signals, such as clock lines, data lines, high-power square wave oscillators, and so on. From a sensitivity perspective, the most important signals are trigger input circuits, clock systems, small signal analog amplifiers, and so on. The switch adjustment circuit and the rectifier output circuit are the most important electromagnetic interference emission lines; Feedback control is the most important sensitive line.

4. Surge (impact) immunity

The main reason is that the surge circuit design of LED street lamps and LED control devices is not reasonable, or the selection of varistors, transient suppression diodes, or gas discharge tubes is not reasonable, resulting in inconsistency with the limit requirements of the GB/T17626.5-2008 Electromagnetic Compatibility Testing and Measurement Techniques Surge (Impact) Immunity Test. If the project is not qualified, the lightning protection effect of LED street lamps is poor, and the equipment cannot be well protected from normal operation in thunderstorm weather, and even the lamp failure quickly occurs, affecting the safety of driving on normal roads at night.

According to the electromagnetic compatibility surge test principles and product design principles, the main corrective measures are to increase the design of surge protection devices and grounding treatment.