Solutions to electromagnetic compatibility of switching power supplies
From the perspective of the three elements of electromagnetic compatibility, to solve the electromagnetic compatibility problem of switching power supplies, we can start from three aspects: first, reducing the interference signals generated by the interference source; Secondly, cut off the transmission channels of harassing signals; Thirdly, enhance the anti-interference ability of the harassed body. When solving the internal compatibility of switch mode power supplies, the above three methods can be comprehensively utilized, based on cost-effectiveness and implementation difficulty. Therefore, the external disturbances generated by switching power supplies, such as harmonic currents in power lines, conducted disturbances in power lines, and electromagnetic radiation disturbances, can only be solved by reducing the sources of disturbances. On the one hand, it can enhance the design of input/output filtering circuits, improve the performance of APFC circuits, reduce the voltage and current change rates of switching tubes, rectifiers, and freewheeling diodes, and adopt various soft switching circuit topologies and control methods; On the other hand, strengthen the shielding effect of the chassis, improve the gap leakage of the chassis, and carry out good grounding treatment. And for external anti-interference capabilities (such as surges and lightning strikes), the lightning protection capabilities of AC input and DC output ports should be optimized. Usually, for the combined lightning waveform of 1.2/50 μ s open circuit voltage and 8/20 μ s short circuit current, due to the small energy, a combination of zinc oxide varistors and gas square tubes is usually used to solve the problem. For electrostatic discharge, TVS tubes and corresponding grounding protection are usually used in the small signal circuits of communication and control ports, and the electrical distance between the small signal circuit and the casing is increased to solve or select devices with anti-static interference. Fast transient signals contain a wide frequency spectrum and are easily transmitted into the control circuit in a common mode manner. The same method as anti-static is used to reduce the distributed capacitance of common mode inductance and strengthen the common mode signal filtering of the input circuit (by adding common mode capacitors or insertion loss type ferrite magnetic rings, etc.) to improve the system's anti-interference performance.
To reduce the internal interference of switching power supplies, achieve their own electromagnetic compatibility, and improve the stability and reliability of switching power supplies, the following aspects should be taken into consideration: ① Pay attention to the correct zoning of digital circuits and module circuit PCB wiring; ② Decoupling of digital and analog circuit power supplies; ③ Single point grounding for digital circuits and analog circuits, as well as single point grounding for high current circuits and low current, especially current and voltage sampling circuits, is used to reduce common resistance interference and minimize the impact of ground loop grounding. When wiring, attention should be paid to the spacing and signal properties between adjacent lines to avoid crosstalk, reduce the area enclosed by the output rectifier circuit, freewheeling diode circuit, and tributary filter circuit, reduce transformer leakage and distributed capacitance of filter inductance, and use high resonant frequency filter capacitors.
