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This article shows you the principle of electromagnetic compatibility interference and what the PCB EMC design criteria are, the content is concise and easy to understand, it can definitely brighten your eyes. I hope you can get something through the detailed introduction of this article.

PCB electromagnetic interference principle

The external interference caused by the veneer PCB of electronic products may be either differential mode interference or common mode interference. The reason for the interference is the corresponding common mode (CM) interference current and differential (DM) interference current on the veneer PCB. The interference generated on the veneer is transmitted by means of conduction or radiation, which leads to the EMC problem of the product.

Common mode interference current and differential mode interference current are shown in the following two figures:

For common-mode interference (Common-mode) current, it means that the amplitude of the interference current on the two wires is equal and the direction is the same. As shown in the following figure, when a common-mode loop is formed with the ground reference plane through the distribution parameters, the common-mode current flows through the loop.

For differential mode interference (Differential-mode) current, it refers to the interference current on two wires with equal amplitude and opposite direction. As shown in the following figure, the current between the differential signal pair and the signal and its ground reflux belong to the differential mode current.

According to the theory of electromagnetic field, the differential interference current, the signal and the reflux current are all opposite to each other, and the phase difference of the differential current is 180 degrees, so the magnetic field of the differential mode current is mainly concentrated in the loop area composed of the differential mode current. the magnetic lines outside the loop area will cancel each other out. The direction of the common-mode current is the same. Outside the loop area, the magnetic field produced by the common-mode current is in the same direction, but the magnetic field intensity is enhanced. Usually, the external interference of the product is mainly common mode.

It is difficult to calculate the external radiation of PCB accurately. The industry usually calculates the field strength of external interference, that is, the RF energy radiated from PCB, according to the approximate radiation antenna model of differential mode current and common mode current on PCB.

There are many signal loops on the PCB veneer, such as the loop between the signal and the signal ground, and the loop between the power supply and its corresponding.

It's a differential mode current loop. When calculating the differential mode radiation intensity, it can be approximately equivalent to the loop antenna model.

The radiation intensity of the ring antenna model is calculated by the following formula:

EBay 263mm 10-16f2AI/r

Among them

E: electric field intensity (MHz); f: current frequency (MHz); A: current loop area (cm2); I: current intensity (mA)

R: distance from the test point to the current loop (m)

There is also a common mode signal loop on the PCB veneer, such as the loop between the external cable and the earth, which is the common mode current loop. When calculating the common-mode radiation intensity, it can be approximately equivalent to a single-stage antenna model, also known as a rod antenna model.

The radiation intensity of the monopole antenna model is calculated by the following formula: eject 12.6 × 10-7fIL/r.

Among them

E: electric field strength (MHz); f: current frequency (MHz); L: cable length (m) I: current intensity (mA)

R: distance from the test point to the current loop (m)

Because of differential mode interference and common mode interference on PCB veneer, it is necessary to reduce the area of differential mode signal loop and common mode current according to the above interference principle in PCB layout and wiring, so as to reduce the external interference of veneer PCB.

Design principles of PCB EMC

According to the electromagnetic compatibility requirements, the product needs to be designed through PCB to reduce the external electromagnetic interference of the veneer and suppress the external electromagnetic interference at the same time. Therefore, we need to pay attention to the following major principles when designing PCB:

The main results are as follows: 1. It is necessary to reduce the alignment length and differential mode loop area of veneer, because the larger the differential mode loop area is, the greater the external interference is, and the weaker the anti-jamming ability of veneer is.

2. pay attention to the ground design of the veneer, it is necessary to reduce the ground impedance of the veneer, reduce the amplitude of the common mode voltage of the veneer, reduce the external radiation interference, and at the same time reduce the common mode voltage difference formed by external interference on the veneer.

3. Pay attention to the distance between the strong interference module and other digital and analog circuits, because the farther the distance is, the smaller the interference coupling is.

4. Pay attention to the layout and wiring design of components, because the veneer PCB wiring is the antenna in the case of high-frequency electromagnetic interference, which can easily lead to external radiation and coupling external interference.

PCB EMC Design process and steps

According to the characteristics of the veneer, the PCB design is carried out in the following order:

1. First of all, understand the function realization principle on PCB, and divide the modules according to different circuit functions.

2. Understand the connection relationship between the functional circuits on PCB, and consider the smoothness of wiring.

3. Determine the location of the main devices in each functional module on the PCB.

4. Determine the land partition on the PCB and the connection between the land.

5. Layout of important functional devices and filter devices

6. carry on the wiring design of the veneer.

The above content is the principle of electromagnetic compatibility interference and what are the PCB EMC design criteria? have you learned the knowledge or skills? If you want to learn more skills or enrich your knowledge reserve, you are welcome to follow the industry information channel.

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