Can common-mode inductors and differential-mode inductors be used for DC filtering?

oyhprince

Can common-mode inductors and differential-mode inductors be used for DC filtering? [Copy link]

 

RT! Is there something wrong with the circuit design?

okhxyyo

The title should be OK. But whether there is any problem with this circuit still needs an experienced expert to answer.

maychang

“Can common-mode inductors and differential-mode inductors be used for DC filtering?”

Yes. In some cases it is necessary.

maychang

There are some conceptual issues involved here.

What is “DC”? If it is “DC”, why does it need “filtering”?

maychang

In the picture in the first post, there is a GND on the left and another GND on the right. The two GNDs short-circuit the 1-2 pin windings of L5 and L3.

qwqwqw2088

When the battery is powered by a large current, in order to pass the EMC, adding a common mode inductor is easy to pass

oyhprince

maychang posted on 2020-4-15 07:07 This involves some conceptual issues. What is "DC"? If it is "DC", why do we need to "filter"...

There is an interference source nearby

oyhprince

maychang posted on 2020-4-15 07:13 In the first post, there is a GND on the left and a GND on the right. The two GNDs short-circuit the 1-2 pin windings of L5 and L3.

Wrong label, one should be the ground on the right, it should be Vss

oyhprince

The circuit is corrected as follows:

maychang

oyhprince posted on 2020-4-15 08:56 There is a source of interference nearby

Yes. It is precisely because of the interference that the filter is added here. So the input "DC" is actually a stable DC superimposed with the interference signal.

maychang

oyhprince posted on 2020-4-15 09:29 The circuit is corrected as follows:

This circuit is different from the first one, because it lacks an inductor L5. It is best not to delete this L5.

oyhprince

Plus L5

oyhprince

maychang posted on 2020-4-15 10:15 This circuit is different from the first post circuit, and the inductor L5 is missing. It is best not to delete this L5.

Added, follow-up

oyhprince

maychang posted on 2020-4-15 07:06 "Can common mode inductors and differential mode inductors be used for DC filtering?" Yes. In some cases, it is necessary.

In what cases, can you give examples or share your valuable experience?

maychang

oyhprince posted on 2020-4-15 11:11 In what situations, can you give examples or share your valuable experience

Example?

Generally speaking, the two power sources on the left may be batteries, industrial frequency AC rectifier power sources, or switching power sources. If they are switching power sources, and the device on the right is sensitive to noise, it is best to add common-mode inductor filtering. Even if the power sources on the left are "very clean", but the distance from the power source to the device is far (your USB and battery are not far away, so they are not included), strong interference may occur on the long-distance connection, so it is best to add common-mode inductor filtering.

chunyang

It is very common to use common-mode inductors in DC. Notice that many power adapters for digital products such as big-name LCD monitors and laptops often have a cylindrical object wrapped around the wire near the plug. That is the magnetic core, which forms a common-mode inductor with the wire to suppress noise. Of course, many similar power adapters omit this common-mode inductor to save costs. As for differential-mode inductors in DC circuits, there are even more. There are inductors in pie filters, and magnetic bead inductors are a very common application in the power circuits of digital products.

oyhprince

chunyang published on 2020-4-15 11:29 Common mode inductors are commonly used in DC. Note that the power adapters of many big-name LCD monitors, laptops and other digital products often have...

Thanks for your advice

oyhprince

maychang posted on 2020-4-15 11:17 For example? Generally speaking, the two power supplies on the left may be batteries, industrial frequency AC rectifier power supplies, or switching power supplies...&h ...

Thanks for your advice

慢跑蜗牛

maychang posted on 2020-4-15 10:15 This circuit is different from the first post circuit, and the inductor L5 is missing. It is best not to delete this L5.

Why can't L5 be deleted? Can someone please give me some advice?

maychang

Jogging Snail posted on 2020-9-15 16:54 Why can't L5 be deleted? Please give me some advice.

The common mode inductor is used to block the common mode interference signal, and L4 and L5 are used to block the differential mode interference signal. If L5 is removed, the differential mode interference will be transmitted directly from G0 to GND through the wire where L5 is located.