Solutions for electromagnetic compatibility (EMC)
In terms of the three elements of electromagnetic compatibility, to solve the electromagnetic compatibility problem of switching power supply, we can start from three aspects: firstly, to reduce the nuisance signals generated by the nuisance source; secondly, to cut off the propagation path of the nuisance signals; and thirdly, to enhance the anti-harassment capability of the nuisance body. In solving the internal compatibility of switching power supplies, the above three methods can be utilised in a comprehensive manner, subject to the cost-benefit ratio and the ease of implementation. Consequently, external nuisances generated by switching power supplies, such as power line harmonic currents, power line conduction nuisances, and electromagnetic field radiation nuisances, can only be solved by reducing the nuisance sources. On the one hand, you can enhance the design of the input/output filter circuit, improve the performance of the APFC circuit, reduce the switching tube and rectifier, current diode voltage, current rate of change, the use of a variety of soft-switching circuit topology and control, etc.; on the other hand, to strengthen the shielding effect of the chassis, to improve the chassis leakage of the gaps, and a good grounding process. And the external nuisance resistance (such as surge and lightning strike) should be optimised for the lightning protection of AC input and DC output ports. Usually, for the combination of 1.2/50μs open-circuit voltage and 8/20μs short-circuit current lightning strike waveforms, a combination of zinc oxide varistors and gas square electrodes, etc. is usually used to solve the problem due to the small energy. For electrostatic discharge, usually in the communication port and the control port of the small signal circuit, the use of TVS tubes and the corresponding grounding protection, increase the electrical distance between the small signal circuit and the chassis and so on to solve the problem or the use of anti-static nuisance devices. Fast transient signals contain a wide frequency spectrum, it is easy to common mode way into the control circuit, using the same method with the anti-static and reduce the distribution capacitance of the common-mode inductor, strengthen the common mode signal filtering of the input circuit (plus common-mode capacitance or insertion loss type ferrite ring, etc.) to improve the system's anti-interference performance.
Reduce the internal harassment of the switching power supply, to achieve its own electromagnetic compatibility, improve the stability and reliability of the switching power supply, should start from the following aspects:
① Pay attention to the correct partitioning of PCB wiring for digital and modular circuits;
② Decoupling of digital circuits and analogue circuit power supplies;
③ digital circuits and analog circuits, single-point grounding, high-current circuits and small-current, especially current and voltage sampling circuits, single-point grounding in order to reduce the common resistance harassment, reduce the impact of the ground loop, wiring, pay attention to the spacing between adjacent wires and the nature of the signal, to avoid crosstalk, reduce the output rectifier circuit and the diode circuit and tributary filter circuits surrounded by an area of reduced leakage of transformers, the filtering capacitance of the inductors, and the distribution of capacitance. The use of high resonant frequency filter capacitors and so on.
