Solutions to electromagnetic compatibility of switching circuits
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, reduce the disturbance signal generated by the disturbance source; second, cut off the propagation path of the disturbance signal; third, Enhance the ability of the harassed body to resist harassment. When solving the internal compatibility of switching power supplies, the above three methods can be comprehensively utilized, based on the cost-effectiveness ratio and the ease of implementation. Therefore, external disturbances generated by switching power supplies, such as harmonic currents in power lines, conducted disturbances in power lines, and electromagnetic field radiation disturbances, can only be solved by reducing the source of disturbance. On the one hand, it can enhance the design of the input/output filter circuit, improve the performance of the APFC circuit, reduce the voltage and current change rate of the switching tube and the rectifier and freewheeling diode, and adopt various soft switching circuit topologies and control methods; on the other hand, In terms of aspects, strengthen the shielding effect of the casing, improve the gap leakage of the casing, and conduct good grounding treatment. The ability to resist external disturbances (such as surges and lightning strikes) should be optimized by optimizing the lightning protection capabilities of the AC input and DC output ports. Usually, for the combined lightning strike waveform of 1.2/50μs open circuit voltage and 8/20μs short circuit current, due to the small energy, a combination method of zinc oxide varistor and gas square tube 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 ports and control ports, and the electrical distance between the small signal circuit and the chassis is increased, or devices with anti-static harassment are selected. Fast transient signals contain a wide spectrum and are easily transmitted into the control circuit in the form of common mode. The same method as anti-static is used to reduce the distributed capacitance of the common mode inductor and strengthen the common mode signal filtering of the input circuit (add Common mode capacitors or insertion loss ferrite magnetic rings, etc.) to improve the anti-interference performance of the system.
To reduce the internal disturbance of the switching power supply, realize its own electromagnetic compatibility, and improve the stability and reliability of the switching power supply, we should start from the following aspects: ① Pay attention to the correct zoning of digital circuits and module circuit PCB wiring; ② Digital circuits and Decoupling of analog circuit power supply; ③ Single-point grounding of digital circuits and analog circuits, single-point grounding of large current circuits and small currents, especially current and voltage sampling circuits, to reduce common resistance disturbance and the influence of ground loops. Pay attention to this when wiring. The spacing between adjacent lines and signal properties should be used to avoid crosstalk, reduce the area surrounded by the output rectifier circuit, freewheeling diode circuit and tributary filter circuit, reduce the leakage of the transformer and the distributed capacitance of the filter inductor, and use high resonant frequency Filter capacitors, etc.
