Common Mode Chokes - ugh

techydude 2015-04-24 05:51:49 UTC #1

Chris, you knew this post was coming...
(delayed a week assisting my boss who had to fly over to your side of the planet, to assisting him debug EMI-related IrDA issues at one of our test sites - just when I thought I'd have my most productive week ever with him gone... not so much )

so I need a common-mode choke, 85-265Vac rated, <1.0Amps operational current, but UL/CSA/VDE/whateverthefluff rated, Because Standards (which have not been formally stated requirements for the product at this stage, but we know it'll come down the track).

so I can see @ DK there's all sorts. my height restriction is about 27-28mm, but there's plenty of options, some half that height, at various PCB footprints. but unfortunately DK doesn't give agency ratings as a searchable criteria sigh

hope over here. um, where do I start? tried transformers, 1:1 ratio, saw a delicious-looking Approvals search criteria, but clearly I wasn't in the right place. did a search for a part# that was on my maybe-short list, just to get an idea where CM-chokes were categorised here - ah! inductors! duh!

so, Inductors > Fixed Inductors > Inductor Type: CM Choke; while furiously ignoring the criteria "Connector Type" that was left with "Card Edge Connector" as the only available type, & same with "Resistor Type: Fixed Resistor" -weird! But then 'Package Height' ranged only from 0.3 to 10.0mm, hmmm, something wrong here...

Undeterred, I tick 'filter choke' & 'common mode choke' in the 'Inductor Application' criteria (the kind of thing I'd never dream of ticking, it sounds like the list of 'Applications' on the front page of a datasheet - oh damn, they don't have my application listed, so I can't use this part?? you know the joke...

but the result of narrowing my search by ticking these two 'Applications' left me with results whose Package Height now ranged from 31.2 to 35.6mm, up from 0.3 to 10.0mm blank stare blink blank stare please, don't even try to make a Venn Diagram of this situation, you'll do yourself an injury.

so, The Truth Is In There, somewhere, but seemingly not searchable in a manner that makes sense to me

tomacorp 2015-04-30 07:00:51 UTC #2

This is an old problem - shopping for magnetics has been difficult for a long time. The manufacturer's data sheet authors (or the translators) don't always clearly convey the concepts or terminology of magnetics. It's tough to get the database right when the data sheets are spotty.

Safety approvals in component databases are even worse. I never have enough faith in any database to rely on them for safety information. So if you can't rely on it, why bother having it? That's my excuse, anyway.

Unlike most power transformers, common mode chokes are often designed to have a relatively large, controlled leakage inductance. This enables them to be used as common-mode suppressors and differential mode filters at the same time. Also unlike most power transformers, you really need to know the common-mode inductance value to design the filter.

Like most difficult categories, this is how I find them:
1. Find a family that has some stock at a distributor that you like.
In this example I chose PLY10 from Murata.
2. Look at the data sheet to find approvals, specifications, etc.
Looks like maybe the Safety ones start with PLY10AS.
Note the other weird terminology in the data sheet. Normal mode inductance? I know what they mean but I haven't heard it said that way before.
3. Put PLY10AS into Findchips and see what you can find.
In this case Mouser has stock.

It does look like there is a problem with this search in parts.io. I searched for PLY10AS and got 40 results. When I check the 'In stock' button, there are only 2 results. There should be many more results, based on what I see at Findchips.

This used to be in my sig:
"Inductors designed while-u-wait. Transformers take longer."

techydude 2015-05-01 04:40:34 UTC #3

thanks Tomacorp,
i think you've hit a few nails - the inconsistency of datasheet data & terminology, but also parts.io's search for this category "having issues"
I saw Murata's PLA10/PLA10AS & PLY10/PLY10AS range, with the S variants having "Safety approval", EN60065. I was all set to use it, but I'm quite a noob when it comes to modern standards for stuff like this, so I emailed Murata to ask if they had any mains-voltage CM chokes that were UL-listed. Answer was no, which seemed odd.

ChrisGammell 2015-05-01 05:05:21 UTC #4

Hey Tom and Anthony,

I definitely agree that certain passives like this can be a hairy search.

OK, so first off: the data on these components obviously has some outlier data that needs to be cleaned up. There is zero reason there should be a "card edge connector" type. This is likely a single record or two with this erroneous column, but even if a single part has it, it will show up in the filters. We'll get working on those oddball criteria.

Next, I also did a search for common mode and yes, there is a bit of confusion there. So here's the steps I did, after knowing some of your pitfalls to see if I found anything different: http://parts.io/search/term-common%20mode%20choke/Class-Inductors/Category-Fixed%20Inductors?infer=partial

I did notice this field, which perhaps is what you're actually looking for?

Like perhaps because it's a toroid the diameter of the coil is actually the spec?

techydude 2015-05-01 05:31:17 UTC #5

I wasn't specifically looking for toroidal, so I hadn't selected Construction: Circular, but maybe that was skewing my perceptions when looking at the various dimension ranges? In fact I furiously avoided that Construction criteria, because it was clearly filled with a cornucopia of not-mututally-exclusive criteria, and - a recurrence of an old gripe - I can't select multiple options .