How to use common mode inductors?

sky999

How to use common mode inductors? [Copy link]

 

How to use common mode inductors? I don't have any application, but I've always been curious about this component.

Let’s not talk about using it in communication, because I rarely come into contact with communication. After studying it for a few days, I only understood what common-mode interference and differential-mode interference roughly mean.

But I can't seem to find any information on how to choose the model. It only says that you can choose based on current and impedance. So how do you know the impedance of the circuit board?

In what environment will there be common mode interference? Thank you

maychang

[What kind of environment will cause common mode interference? ]

Common-mode interference exists in any environment, unless it is a wireless receiver that uses only one antenna and one ground wire. Even if it is a wireless receiver that uses only one antenna and one ground wire, if it uses AC mains, common-mode interference may be transmitted from the AC mains wire.

sky999

maychang published on 2022-11-30 15:36 [What kind of environment will have common mode interference? ] Common mode interference exists in any environment, unless it is a wireless receiver that only uses one antenna and one ground wire. Even if...

If the circuit board is powered by a switching power supply, which usually contains a common-mode inductor, is the common-mode interference suppressed on the switching power supply side?

maychang

sky999 posted on 2022-11-30 16:07 If the circuit board is powered by a switching power supply, there is usually a common-mode inductor in the switching power supply. Then, is the common-mode interference suppressed on the switching power supply side?

[If the circuit board is powered by a switching power supply, which generally contains a common-mode inductor, is the common-mode interference suppressed on the switching power supply side?]

The common-mode inductor in the switching power supply is usually placed at the power supply input line, such as the AC mains input line. Its purpose is not to prevent common-mode interference from entering the switching power supply, but to prevent the interference generated by the switching power supply from entering the AC mains. The common-mode inductor in the switching power supply is not set to prevent interference from entering the load (your circuit board).

sky999

maychang published on 2022-11-30 16:22

Oh, I made a mistake. If the board does not use AC power but only uses DC power, will there be common mode interference? How should I choose the model?

chunyang

Impedance refers to the impedance of the common-mode inductor under a specific frequency spectrum, not the impedance of the circuit board or load.

As for under what circumstances common-mode interference occurs, there are too many to list. What we should know is under what circumstances common-mode inductors are needed.

The application of common-mode inductors can be roughly divided into two categories: one is to deal with external common-mode noise, and the other is to deal with internal common-mode noise. In actual application, specific analysis must be made based on specific circumstances.

maychang

sky999 posted on 2022-11-30 16:42 Oh, I made a mistake. If the board does not use AC power but only uses DC power, will there be common mode interference? How to choose the model?

[If the board does not use AC power and only uses DC power, will there be common mode interference? ]

Circuit boards do not use AC power, but only DC power, and they may also be affected by common-mode interference. A board usually has more than two power lines, and most likely has signal input and output lines. Since there are signal input lines, they may be affected by common-mode interference. Of course, in this case, the interference is usually weak.

chunyang

sky999 posted on 2022-11-30 16:42 Oh, I made a mistake. If the board does not use AC power but only uses DC power, will there be common mode interference? How to choose the model?

You may want to observe the power adapters of digital devices such as laptops and monitors. Those power adapters (especially digital products sold in the European market with stricter requirements) often have a cylindrical bump near the output plug, which is the common-mode inductor.

How to select the model is also determined by the current carrying capacity and noise spectrum.

sky999

chunyang published on 2022-11-30 17:32

Can you give an example of how to select a common mode inductor? For example, 485 and CAN, why can there be a fixed common mode inductor model?

sky999

maychang published on 2022-11-30 17:30

If there are a lot of input signals on the board, will the interference be stronger?

chunyang

sky999 posted on 2022-11-30 17:57 Can you give an example of how to choose the model? For example, 485 and CAN, why can there be a fixed model of common mode inductor?

485 and CAN are common-mode inductors used in signal circuits. This is different from the power circuit. Because the current is very small, the specifications of common-mode inductors for signal circuits are relatively few, and of course they share the same model. The application environments of 485 and CAN are similar, so sharing the same specifications is also convenient for production management.

chunyang

sky999 posted on 2022-11-30 17:57 Can you give an example of how to choose the model? For example, 485 and CAN, why can there be a fixed model of common mode inductor?

However, it should be noted that communication interfaces such as 485 and CAN, which already have strong common-mode suppression capabilities, rarely use common-mode inductors because they are unnecessary.

chunyang

sky999 posted on 2022-11-30 17:58 If there are a lot of input signals on the board, will the interference be stronger?

The number of signal pathways has nothing to do with the intensity of interference. It can only be said that a greater number of signal pathways makes it more susceptible to external interference. Pay attention to cause and effect.

maychang

sky999 posted on 2022-11-30 17:58 If there are a lot of input signals on the board, will the interference be stronger?

[If there are a lot of input signals on the board, will the interference be stronger?]

That's not the case. The intensity of interference has nothing to do with the number of channels, but is mainly related to the surrounding environment.

sky999

chunyang posted on 2022-11-30 18:07 The number of signal pathways has nothing to do with the intensity of interference. It can only be said that a larger number of signal pathways is more susceptible to external interference. Pay attention to cause and effect.

I think I understand it a little bit, but I still don’t know how to choose

sky999

maychang posted on 2022-11-30 18:11 [If there are a lot of input signals on the board, will the interference be stronger?] That's not the case. The intensity of interference has nothing to do with the number of channels...

Which of the following situations is more susceptible to common mode interference?

1. An electrical box surrounded by three-phase electricity, with a single circuit board in the middle.

2. There is no AC around the circuit board, but there are many electric towers outside.

3. There is a relay on the circuit board, and the contacts of the relay are connected to alternating current.

maychang

sky999 published on 2022-11-30 19:21

[Which of the following situations is more susceptible to common mode interference? ...]

It’s hard to say. I haven’t seen the specific situation, so I can’t say.

Chunyang on the 12th floor has already given an example: 485 communication interfaces rarely use common-mode inductors, because the 485 interface itself has strong resistance to common-mode interference. So this sentence can be said in reverse: if there is strong interference in the surrounding environment (electromagnetic environment), if your circuit has strong anti-interference performance, it will not affect the circuit operation.

chunyang

sky999 posted on 2022-11-30 19:18 I have a rough idea of it, but I still don’t know how to choose it

Don't you know that the selection is based on current and impedance? In most cases, these two parameters are enough. The current should not be difficult to understand, just pay attention to leaving a margin. As mentioned earlier, the impedance is selected based on the noise spectrum, that is, first find out the range and intensity of the noise frequency that affects the sensitive circuit, and then calculate the attenuation rate based on the upper limit of the noise that the sensitive circuit can tolerate, and then convert it into impedance based on the attenuation rate. However, it is not necessary to be so troublesome in engineering, because the impedance converted from the attenuation rate is the lower limit, and the value in actual use should be higher than the lower limit, and then comprehensively consider factors such as cost and volume, so you see relatively few varieties of common-mode inductors, and the difference is mainly in the current parameter.

chunyang

sky999 published on 2022-11-30 19:21

It’s hard to say, the conditions are too crude and all kinds of possibilities exist.

sky999

Well, thank you both, I can only say that this component is not yet ready, I am still confused