Among the circuits, the issues of EMC and EMI are topics that people always care about. In LED circuits, the requirements for EMC and EMI are also very high. This article will introduce you to the solution to solve EMI and EMC in LED product design. Interested friends can take a look. To solve the electromagnetic interference problem of the LED driving power supply, the following aspects can be started from the hardware: Reduce the interference of the switching power supply itself Soft-switching technology adds inductance and capacitance components to the original hard-switching circuit, and uses the resonance of the inductor and capacitor to reduce the du/dt and di/dt during the switching process, so that the voltage drop before the switching device is turned on before the current rises. , or the drop in current before the turn-off precedes the rise in voltage to eliminate the overlap of voltage and current. The switching frequency modulation technique disperses the energy concentrated on fc and its harmonics 2fc, 3fc... into the frequency bands around them by modulating the switching frequency fc to reduce the EMI amplitude at each frequency point. The choice of components, select components that are not easy to produce noise, difficult to conduct and radiate noise. Of particular note is the selection of winding components such as diodes and transformers. The fast recovery diode with low reverse recovery current and short recovery time is an ideal device for the high-frequency rectification part of the switching power supply. Reasonable use of electromagnetic interference filter, one of the main purposes of EMI filter, grid noise is a kind of electromagnetic interference, it belongs to radio frequency interference (RFI), its conducted noise spectrum is roughly 10KHz ~ 30MHz, up to 150MHz. Under normal circumstances, the differential mode interference amplitude is small, the frequency is low, and the interference caused by the difference is small; the common mode interference amplitude is large, the frequency is high, and the radiation can be generated by the wire, and the interference caused by the interference is large. The most effective way to reduce conducted interference is to install an electromagnetic interference filter in the switching power supply input and output circuits. LED power supplies generally use a simple single-stage EMI filter, which mainly includes common mode chokes and filter capacitors. The EMI filter effectively suppresses electromagnetic interference from the switching power supply adapter. Reduce electromagnetic interference by cutting off the propagation path of interfering signals In the first case, power line interference can be filtered out using the power line filter. A reasonable and effective switching power supply EMI filter should have strong suppression of differential mode and common mode interference on the power line. Improving Electromagnetic Compatibility of PCB Boards Design PCBs are the support for circuit components and devices in LED power systems that provide electrical connections between circuit components and devices. With the rapid development of electronic technology, the density of PCBs is getting higher and higher. The quality of the PCB design has a great impact on the electromagnetic compatibility of the LED power system. Practice has confirmed that even if the schematic design of the circuit is correct and the printed circuit board is not properly designed, it will adversely affect the reliability of the LED power system. The PCB anti-interference design mainly includes PCB layout, wiring and grounding, and its purpose is to reduce the electromagnetic interference between the PCB and the crosstalk between the circuits on the PCB. In addition, the frequency of the ac vibration caused by the electromagnetic interference of the general transformer is generally about 50HZ, and the humming sound caused by the improper wiring of the grounding wire, because the frequency multiplication frequency of the rectifying circuit is about 100HZ, careful distinction can still be detected. Therefore, when designing a printed circuit board, you should pay attention to the correct method, follow the general principles of PCB design, and should meet the anti-interference design requirements. Actively enhance the anti-interference ability of the interfered body In the LED power system, the input/output is also the transmission line of the interference source, and the detection source for receiving the RF interference signal. We generally take effective measures when designing: use the necessary common mode/differential mode suppression circuit, and also Take certain filtering and anti-electromagnetic shielding measures to reduce the interference. Whenever possible, take various isolation measures (such as optical isolation or magnetoelectric isolation) to block the spread of interference. Lightning protection measures, outdoor use of the LED power system or the introduction of indoor power lines, signal lines from the outside, to consider the system's lightning protection. Commonly used lightning protection devices are: gas discharge tubes, TVS (Transient Voltage Suppression) and the like. The gas discharge tube is a gas breakdown discharge when the voltage of the power source is greater than a certain value, usually tens of V or hundreds of V, and the strong shock pulse on the power line is guided into the earth. The TVS can be viewed as two parallel and opposite-direction Zener diodes that conduct when the voltage across the terminals is above a certain value. It is characterized by the ability to transiently pass hundreds of thousands of currents. Through this paper, we can summarize the main control technologies for LED power supply EMC/EMI: circuit measures, EMI filtering, component selection, shielding and anti-interference design of printed circuit boards. If these problems can be solved correctly and reasonably, the 3C certification will be passed smoothly.
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