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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

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    \$\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
    Commented May 2, 2022 at 8:55
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    \$\begingroup\$ @Winny: Agree with BeB00 \$\endgroup\$ Commented May 2, 2022 at 9:16
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    \$\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
    Commented May 2, 2022 at 9:21
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    \$\begingroup\$ Yes. Thats the question \$\endgroup\$ Commented May 2, 2022 at 9:30
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    \$\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
    Commented May 2, 2022 at 10:10

3 Answers 3

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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.

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  • \$\begingroup\$ If coupled ones provide better inductance in a smaller volume, shouldn't that be the popular choice? \$\endgroup\$ Commented May 2, 2022 at 9:16
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    \$\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\$ Commented May 2, 2022 at 11:59
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    \$\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\$ Commented May 2, 2022 at 12:04
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Saturation may be a concern. In a common-mode choke the power supply outbound and return currents cancel out. In a differential mode choke they add together. If the ferrite starts to saturate its inductance will be greatly reduced.

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    \$\begingroup\$ Hi and welcome -- but a question: how would turns/saturation differ between single and multi-winding types? \$\endgroup\$ Commented May 8, 2023 at 14:46
  • \$\begingroup\$ @Tim In my experience, wire current ratings on multi-winding chokes are much higher than the saturation current, as they are designed with the cancellation effect in mind. This is why multi-winding chokes are a fraction of the weight and cost of single-winding chokes of the same inductance and wire current. \$\endgroup\$ Commented Jul 2, 2023 at 13:33
  • \$\begingroup\$ @personal_cloud That checks out for CMCs which typically have minuscule saturation current (I've measured as low as 10mA), but for differential (dual winding inductor) types it is a design parameter, and could be higher or lower than the total DC rating. \$\endgroup\$ Commented Jul 2, 2023 at 20:11
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I can think of several reasons:

  • Single winding inductors are cheap and plentiful
  • Dual windings invite high-frequency modes between windings, which might complicate or frustrate treatment of the common mode
  • Single inductors act independently, with fewer terminals/ports, so are easier to model / solve
  • If only additional DM attenuation is required, a single inductor (not a pair) can be used

The HF modes are probably a subtle point that bears more detail. As is the case with most anything in the topic of EMC...

In the same way that the CMC has self-resonant modes, it also has modes coupling between windings. A dual-winding DMC will have this as well.

Consider this curve fit of a T604050-R6161-X504:

T604050-R6161-X504 CM impedance model, curve fit

T604050-R6161-X504 CM impedance model, equivalent circuit

This was solved by superimposing the datasheet plot on the analysis output, and tweaking values until they match. I gave up towards the HF end as you can see; the data probably aren't too meaningful up there anyway.

Note that this is the common mode equivalent, so acts in parallel with the magnetizing inductance of the CMC (or rather, is the magnetizing impedance). Most of the inductance will be shared between the two windings, but some leakage and capacitive elements will couple across them.

Note that the legend says "1 phase"; sometimes the windings are wired in parallel for these measurements, but evidently they left the other open-circuit. This will cause it to act as a resonant trap, its terminal capacitance (approx. C1) resonating with leakage inductance (approx. L2 + L4), hence the notch and peak at (and a bit above) 10MHz.

There will be higher-order effects as you go up from 30MHz, of course; effects that depend ever more critically on what's nearby, and which radiate into space, not remaining confined to the measurement terminals. While we can measure components up there, the measurements become less and less meaningful as we go up. So this is a fine stopping point; and indeed, most datasheets end around 30MHz (conducted emissions limit), or even 10MHz.

Anyway, in the same way that the CM windings couple, giving a blip in the transfer function here -- so too, the DM windings will couple to each other. What would be CM effects, become DM effects, and vice versa, as we cross-connect the windings for the respective purposes. A highly reactive notch at 10MHz might have a substantial impact on the CM response of the filter otherwise. We can't simply stack these impedance plots together as if they were resistors; we must account for their reactance, and avoid creating notches in the overall filter response that lead to emissions (or susceptibility) problems.

In contrast, if we have single inductors, even if they have a response like the above -- that is, with ugly peaks and notches at some points -- we have a better chance of working with them, because we can for example dampen peaks by placing an R+C in parallel with it, and dampen valleys by putting an L||R in series with it. In general, where an inductive component becomes dominant capacitive (such as the above, >600kHz), we can add another inductor in series with it, and the added inductor resonates with the effective capacitance of the first; and we can ensure the series combination remains well-behaved by damping the resonance. And we can do all this without having to concern ourselves with more than two pins at a time, multiple modes between multiple pins.

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