Choosing transformer for ac-dc converter - Page 1
Forum section: electronics / projects

author: MattHollands
author_rank: Frequent Contributor
timestamp: August 08, 2015, 12:10:25 am
content: Hi! I'm trying to design an ac-dc converter that will work in both US and UK (115Vac & 230Vac), producing 12V at ~1.5A.This is my first SMPS design so I'm having a bit of trouble. I've pretty much settled on the controller chip etc but I really can't figure out what transformer to use. I want to operate somewhere between 40-80kHz.I'm using this configurationhttps://upload.wikimedia.org/wikipedia/commons/thumb/8/86/Flyback_conventions.svg/250px-Flyback_conventions.svg.pngThe best match I've found so far is the ECO2225SEO-D01H016 (http://media.digikey.com/pdf/Data%20Sheets/TDK%20PDFs/ECO_Series.pdfpage 70-72). But I have some reservations.The transformer is operated at 67kHz, has a turns ratio of ~6.33 and has a primary inductance of 567uH. Therefore it has a secondary inductance of 14.2uH.If it is suppling 18W to the load, then it must be supplying 269uJ per cycle. Therefore the peak current in the secondary coil is 6.16A (E = 0.5*L*I^2). Therefore the peak current in the primary coil is 0.97A.Assuming my calculation is correct then this means that the current in the secondary coil exceeds the maximum 2.4A current, although not always.So my first question is, considering that the current in the secondary coil ramps up, can I get away with having higher peak currents? Otherwise I can't find any feasible transformers that can take 6A in their secondary coils.Can anyone suggest how to choose a transformer for an ac-dc converter?Sorry for the long winded question and I hope it makes sense.

author: MattHollands
author_rank: Frequent Contributor
timestamp: August 08, 2015, 12:53:13 am
content: Ok cool, that's what I thought. So I've calculated the wave forms etc for this setup, and the RMS current in the secondary coil is ~2.5A. Is there any way of knowing if that will be fine?Another way of putting it is that the current in the secondary coil will only exceed 2.4A for about 3 microseconds of a 14.9 microsecond cycle, ie 20% of the time. Sure, it's a lot less than 3 seconds, but its repeated every 14.9 microseconds.Thanks

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 08, 2015, 01:13:58 am
content: Is there any reason you can't use an off the shelf solution?

author: MattHollands
author_rank: Frequent Contributor
timestamp: August 08, 2015, 12:14:12 pm
content: @NiHaoMikeI'm messing with the idea of having a fair number of these manufactured so it wouldn't really be feasible to do so unless there was a very cheap one I could findFair suggestion though.@AcHmed99They look like two really useful tools! Thanks. The waveform drawings are very convenient. I'm still slightly confused as to what current rating I should look for in the secondary coil. (From my best experience) I'm going to struggle to find a transformer that takes 3A rms at 12V. Sorry for being a bit dumb, I've not had much experience with transformers before other than simple toroidal step downs. Can you recommend anywhere for buying transformers?

author: MattHollands
author_rank: Frequent Contributor
timestamp: August 08, 2015, 04:09:50 pm
content: Ok thank you for the help, I understand better now.I'm in the UK at the moment but moving to US in a month. I am definitely doing it as a learning exercise. In a way I'm doing the whole project as an exercise, from designing to selling a few. The testing/approvals has been a thing I've been worried about.Perhaps I'll look more deeply into buying a pre-built solution, but ideally the product would be as small as possible. But I understand it may be worthwhile buying something pre-made.I still want to try building an SMPS anyway