Electromagnetic interference EMI noise source and propagation process analysis

There are two types of electromagnetic interference (EMI Electromagnetic Interference): conducted interference and radiated interference. Conducted interference is mainly caused by interference signals generated by electronic devices transmitted through wires or common power lines, causing interference with each other. Further subdivision, conducted interference is divided into common mode interference and differential mode interference. Here we talk about the propagation process of EMI.


This is to say the propagation process of EMI, the interference source-interference path-receiver. The source of interference can be understood as the interference discovered by your device. After the transmission route, for the power supply, you can only start from two aspects, reduce the interference source, or cut off the interference path. The last one is generally not needed. Let's take a look at a diagram, first FFT the several different waveforms.
See how his higher harmonics are distributed.

Let’s look at the two slopes, what is the study?

There are many engineers who only know that the slope is different. The fast EMI will be worse, but I don't know why it is worse. You will see this picture clearly. You can use the anti-true FFT to see. A comparison of triangular waves and other waveforms will be described later.

Let’s take a look at the coupling mechanism.

Now let's talk about the principle of electric field interference: look at the picture first.

Let's continue:

We continue, the analysis of the noise source is summarized: dv/dt generates an electric field, and di/dt generates a magnetic field. The magnetic field is related to the differential mode current. The electric field is related to the common mode current. In the near-field region, the impedance of the radiation source determines whether the electric field dominates or the magnetic field dominates. The magnetic field is typically generated by a current loop. These loops can be analyzed in differential mode due to the opposite current directions of the signal lines and the return lines. The generation of the electric field requires a high-impedance source. Therefore, the noise current of each wire can be regarded as the same direction, which can be analyzed as a common mode.