2
\$\begingroup\$

I understand there are both coupled and un-coupled versions of differential mode chokes as shown below.

Which one is better? I see most of the designs having un-coupled version. What is the reason?

Coupled differential mode inductors:

enter image description here

Un-Coupled differential mode inductors: enter image description here

Picture source

\$\endgroup\$
6
  • 3
    \$\begingroup\$ @winny it looks like OP is only asking about DM noise, and is not interested in CM noise (unless thats relevant to the answer of why a coupled DM choke is different to an uncoupled one performance-wise) \$\endgroup\$
    – BeB00
    May 2 at 8:55
  • 1
    \$\begingroup\$ @Winny: Agree with BeB00 \$\endgroup\$
    – Divya K.S
    May 2 at 9:16
  • 1
    \$\begingroup\$ @BeB00 Ah! So the question is why one would use two separate ones or one with two windings on the same core? \$\endgroup\$
    – winny
    May 2 at 9:21
  • 1
    \$\begingroup\$ Yes. Thats the question \$\endgroup\$
    – Divya K.S
    May 2 at 9:30
  • 1
    \$\begingroup\$ If you have more height than PCB surface available, two inductors on the same core would waste less PCB space. For high DC bias, I don't think it's a very practical solution though. \$\endgroup\$
    – winny
    May 2 at 10:10

1 Answer 1

4
\$\begingroup\$

Coupled common mode choke real model includes stray inductances, and these inductances work as differential mode chokes and thus they help to tame the differential mode noise.

If these stray inductances are not enough the designer may want to put discrete inductors for better differential mode noise filtering (2nd image is a good example).

As a direct comparison, coupled ones generally have higher inductance in a smaller volume.

\$\endgroup\$
3
  • \$\begingroup\$ If coupled ones provide better inductance in a smaller volume, shouldn't that be the popular choice? \$\endgroup\$
    – Divya K.S
    May 2 at 9:16
  • 2
    \$\begingroup\$ @DivyaK.S It's not that simple. And also popularity is subjective. You can't exactly know that there'll be differential noise only, or common mode noise only. If you are working with live voltages then there'll always be a common mode noise due to the existence of earth, so a CMC is needed. In case of DM noise existence, as I stated in my answer, the leakage of a CMC is used as to filter the differential mode noise. If the DM noise occupies a wide frequency range and the leakage is not enough then you might want to think about using external inductors considering cost, Q factor, SRF, etc. \$\endgroup\$ May 2 at 11:59
  • 1
    \$\begingroup\$ @DivyaK.S That would also imply that every designer has the same knowledge. They most definitely don't. And it would also imply that the market is optimal, i.e. that part prices capture all efficiencies of design. The truth is, part prices aren't so ideal, and parts that might cost less to make don't have lower price, and vice versa. \$\endgroup\$ May 2 at 12:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.