Different types of common mode chokes for power filtering - Page 1
Forum section: electronics / projects

author: dkonigs
author_rank: Regular Contributor
timestamp: August 13, 2024, 05:42:59 am
content: So when I look at common mode chokes for power supply filtering, I've noticed that they come in a variety of core types.So for example, take these two choices:The particular CMCs in question are the Schaffner RN116-0.5-02-47M and the EPCOS TDK B82731M2501A030. Both have the same inductance rating, and very similar (though obviously not identical) specifications. Both also appear to be intended for similar use cases.Except that the Schaffner is a toroidal core (that's also potted), while the EPCOS is a D-shaped core.What reason would one have to prefer one construction style over the other?(beyond the question of potted vs non-potted)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 13, 2024, 08:57:52 am
content: Orientation, aspect ratio, profile, capacitance and other behavior into cutoff frequency band, etc.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: August 15, 2024, 02:11:30 am
content: The inductance specification of a common-mode choke can be deceiving, because it's usually taken at some reference frequency (e.g. 1KHz, 10KHz, or 100KHz), while for EMI, usually we care about how these parts look at 150KHz to 30MHz+.So for example, you (theoretically) could have a choke which measures a very high inductance at 10KHz, but if its self resonant frequency is 50KHz, it is practically useless in the typical 150KHz+ EMI band. This probably wouldn't be a very popular choke.For the examples you give, the attenuation and self-resonant frequencies are similar.The multi-slot bobbin types often have higher self-resonant frequency, but toroids are usually more compact. (Consider that they don't use a bobbin at all.) You may also be interested in differences between leakage inductance if you need additional differential-mode filtering.

author: Circlotron
author_rank: Super Contributor
timestamp: August 15, 2024, 02:41:09 am
content: Quote from: TimNJ on August 15, 2024, 02:11:30 amIndeed. When I was in the power supply game we made CM toroids both with the windings separated to each side of the core, and bifilar.

author: TimNJ
author_rank: Super Contributor
timestamp: August 15, 2024, 04:05:58 pm
content: There's also some interesting differences regarding core shape and frequency response. Distributed capacitances and so on...The square core common-mode chokes have gained popularity in recent years. Comparing to a toroid of similar size, inductance, and current rating, the square core chokes offer much broader band attenuation. But, the peak attenuation of a toroid choke is typically higher. It depends on the nature of the noise source. If you have a very strong 3rd or 5th harmonic of a switching frequency, you may indeed prefer higher peak attenuation at a small range of frequencies.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 15, 2024, 04:40:09 pm
content: You might find this page from Coilcraft of interest.www.coilcraft.com/en-us/edu/series/a-guide-to-understanding-common-mode-chokes/?srsltid=AfmBOopHyLvb4Yrzj0N09Rcn75PhBnuwC9uFiqGbA4kTuUFxJKZgl6x8

author: dkonigs
author_rank: Regular Contributor
timestamp: August 22, 2024, 09:28:56 pm
content: Quote from: TimNJ on August 15, 2024, 02:11:30 amYeah, that's the thing. On nearly all the specs, these two parts are basically equivalent. And in my initial conducted emissions bench tests, they also appear to perform the same.The reason I'm heading down this rabbit hole is because the AC/DC converter this is going on the front of has a recommendation for a 45mH 500mA CMC on its input. The manufacturer is planning to release such a part (its not a common thing in the usual catalogs), but won't have it out for a while yet. So I'm looking into off-the-shelf alternatives. Seems like basically anything reasonable will get the job done.From the preliminary datasheet screenshots the manufacturer has shared with me, the part they plan to release looks quite similar to the TDK EPCOS part I mentioned above. (Except it is a little bit smaller.) But unless their part actually does show up in time for my needs, I'll likely go with the Schaffner because its cheaper and because I'd prefer fully encapsulated parts in this area.