Electromagnetic interference is the most common problem in the design of electronic circuits. Designers are always looking for ways to completely eliminate or reduce electromagnetic interference (EMI). But to completely eliminate EMI interference, the first thing we need is to understand what EMI is and what its propagation process is. This article will give an overview of the EMI propagation process.

EMI is a general term for electromagnetic interference, but electromagnetic interference is actually divided into two types, one is interference to the interference and the other is radiation interference. Conducted interference is mainly the interference signal generated by electronic equipment is transmitted through the wire or the public power line, mutual interference. Further subdivided, conducted interference is divided into common mode interference and differential mode interference.

The EMI transmission process mainly passes through three parts, interference sources, interference channels, and receivers. For the switching power supply, the last part does not need to be considered, and the interference source can not be eliminated, because it is also the source of the switching power supply can work, but the interference of the interference source can be reduced through soft switching, buffering, etc. . Controlling the interference path is an important part of reducing the EMI of the switching power supply and is also the focus of this article.

The spectrum of the periodic signal is not even harmonics, and the positive and negative symmetric waveforms produce fewer frequency components, like a bridge circuit. High numbers are forgotten, and I'm interested in doing some FFT.

The larger the duty cycle, the greater the amplitude of the interference, which can be calculated from the FFT coefficients.

The slope of the waveform has a large effect on the high frequency part of the disturbance. The low frequency part has almost no effect. The low-frequency part is mainly determined by the amplitude of the waveform and the width of the high-level part, but the turning point where the high-frequency part is greatly reduced is 1/(3.14*tr), so the larger the tr, the lower the frequency of the turning point and the lower the high frequency. The bigger.

So we should think of measures to reduce the slope, buffer circuit.

· Both voltage and current waveforms are rich in frequency components

· When the amplitude is over 200M, the amplitude is very low, so the impact is very small

· The waveform affects the low-frequency part

· Rising and falling edges affect the high frequency section

· Duty cycle has a little effect on the spectral amplitude

It can be seen that the electromagnetic interference process is not simple, but it is not complicated and difficult to understand. Only after fully understanding the original strength of EMI can we effectively avoid and suppress EMI. I hope that after reading this article, we can have a better understanding of EMI.

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