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I have yet to undestrand exactly how to use common mode chokes inductors, according to the datasheet they have 2 operation modes like in the picture common mode and differential mode, however im afraid i dont understand the datasheet itself

enter image description here

enter image description here

As you can see, the inductor has 4 terminals, each pair its made of isolated wiring, and theres is a ferrite inside the windings to make sure it complies with its datasheet inductance however

1) what is the intended use for common mode? (what kind of aplications)

2) what is the intended use for differential mode?

Dmitry Grigoryev
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GoatZero
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    The datasheet's drawing of common and differential mode is flat out wrong. Common mode involves two signals. One would go through pins 1 and 3, the other through 2 and 4. What they a have shown is differential mode, with the impedance lowered by paralleling both coils. – Matt Young May 12 '15 at 14:19
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    I have the feeling that there are some useful questions for everyone struggling to get out, but by throwing multiple questions at once out, the format of this site isn't served well. I suggest that you split it up into multiple questions (possibly the next asked after you understood the previous one) so all can build up on another. Probably like asking for the difference between 2 and 3 or so. – PlasmaHH May 12 '15 at 14:21
  • @PlasmaHH you are right, i just edited the question to understand the modes of choke inductors first – GoatZero May 12 '15 at 14:27
  • @MattYoung Now im a bit scared this is the first time i hear about a datasheet being wrong , i just added the "original" pin out from the datasheet – GoatZero May 12 '15 at 14:28
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    @GoatZero As you get more experience and look at more datasheets, you'll see stuff like that all over the place. – Matt Young May 12 '15 at 14:45
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    The datasheet drawing is a description of how they wired things up to produce those two graphs, not a suggestion of how to use the filter in real life. –  Oct 13 '15 at 18:16
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    The schematic is not wrong. It shows the test setup on how the performance was measured. Common mode means you can have two independent lines with the same signal imposed on both of them. –  Oct 26 '15 at 21:14

1 Answers1

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Chokes are used to suppress noise, i.e. to prevent noise and other EMI both from entering and going out of some piece of equipment.

What is usually referred to as common mode noise or as differential mode noise are simply two modes in which noise can be coupled conductively (i.e. through wires) into the piece of equipment.

This document explains in more detail the issue.

Essentially common mode (CM) noise is an unwanted signal which couples into both line conductors in the same direction, whereas differential noise is coupled into a single conductor. For simplicity I'm talking about noise coupled into mains line here, where common mode chokes are frequently used, but the same problem arises whenever some wire comes out of an apparatus, for example the leads of a multimeter, an oscilloscope probe or even the USB cable connecting an external HD to the PC.

Common mode chokes usually have two separate windings which are each put in series with each line conductor. These two windings are wound on the same ferromagnetic core in a way that exploits the different path that power line current and CMN currents take in the circuit. Therefore the choke presents very little impedance to the power line current, whereas common mode noise currents see each winding as much higher impedances, and this attenuates the amplitude of the noise.

Another interesting document is this application note about line filters in switching power supplies.

LorenzoDonati4Ukraine-OnStrike
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  • The link doesn't work, could you update it please (the one on the application note) – gfmoore Jun 04 '23 at 08:32
  • @gfmoore Sadly that document doesn't seem to be available any longer. I will leave the link as is so that anyone willing to perform a more in-depth search using the link data could have a chance to do it. – LorenzoDonati4Ukraine-OnStrike Jun 04 '23 at 09:03