Need a torroid for offline common mode choke and a surface mount holder for it. - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: September 26, 2017, 01:36:37 pm
content: Hello,We need a small common mode choke for our offline 230VAC, 40W, linear led driver which is not smps based but rather switches banks of leds in and out as the mains half cycles rise and fall.It must be SMD.There are none off the shelf.We need to but a torroid , wind it, then mount it in a surface mount holder.Do such pairs exist? (ie surface mount holders and torroids)?

author: capt bullshot
author_rank: Super Contributor
timestamp: September 26, 2017, 02:04:02 pm
content: Yes, they do exist. Usually a toroidal core of your choice and a plastic SMT mounting piece with either SMT or THT pins. The wire ends are soldered to the pins and the core gets potted inside the plastic piece. For SMT, the holder piece usally is rather small, e.g. 12mm x 12mm x 8mm, depending on your requirements (isolation and EMI suppression, the core might not fit.We (the company I work for) usually go to our favourite inductive components supplier and request a quote for a custom wound one if we can't find one off the shelf.These are companies like HARTU (http://www.hartu.de/en/), Voltis (http://www.voltis.de/en/) or a smaller local one.They have all the components (cores, mounting hardware for THT and SMT, ...) and if we ask nicely, we can get also samples of the components and wind our own inductors (only for evaluation purposes).What you're asking for looks like a quite common part as THT, but less common as SMT. It depends on the size of the toroid if your device is suitable for SMT, considering the size and weight it may be a better choice to go for THT.

author: Giaime
author_rank: Regular Contributor
timestamp: September 26, 2017, 03:11:34 pm
content: You could take a look to Wuerth WE-LF series:http://katalog.we-online.com/en/pbs/WE-LF-SMD?sid=4ba2f5e723

author: ocset
author_rank: Super Contributor
timestamp: September 27, 2017, 05:06:29 am
content: Thanks, we've looked into it now andwhat we need is 3 turns for each coil around this torroid core, and for them to mount it in a surface mount holder for us.Ferrite torroid datasheet:https://www.mag-inc.com/Media/Magnetics/Datasheets/0M40603TC.pdfIt must be Surface mount, there are no PTH components on our board and we cannot have any for cost reasons.Height of the finished assembly must be no more than 4.5mm and LxW no more than 8x8mm.Do you think this would be possible at this small size.?The great thing about this is that because its a common mode choke, even this tiny ferrite will not saturate, (not on diff mode current anyway, and common mode current is so small that it will never saturate it) when used in live and neutral. -Even the magnetising current of the coils cancels out.

author: capt bullshot
author_rank: Super Contributor
timestamp: September 27, 2017, 05:44:05 am
content: I can't speak for an inductive components manufaturer, my guess is: Yes, should be possible, but not easy.Check with a manufacturer of your choice if they have appropriate materials and encasings. Don't forget electrical safety regulations - these apply here for the required isolation characteristics of the wire, and clearance / creepage minimum distances must be obeyed.Double check the cost THT/SMT: Sometimes it turns out that introducing a THT component is less than the price difference of the SMT component to the THT component

author: jbb
author_rank: Super Contributor
timestamp: September 27, 2017, 05:59:09 am
content: Does it have to be a toroid?Other core shapes are available with flat SMT bobbins. They might be cheaper as well (because the bobbins are easier to wind).

author: ocset
author_rank: Super Contributor
timestamp: September 28, 2017, 05:10:41 am
content: Thanks, it doesnt have to be torroid, but must be smd as the boad is all smd, also, it should be ht<4.5mm and lxw <(8x8mm)

author: jbb
author_rank: Super Contributor
timestamp: September 28, 2017, 06:38:22 am
content: OK, I thought an EPCOS EFD core+bobbin might fit, but it seems they're a bit too big. Sorry.I second the suggestion of tryingWuerth. They have quite a range and relatively good pricing (for some reason Digikey whacks a massive mark up). And they actually have stuff in stockunlike others who sometimes offer a product that's never been made...