Need feedback on Mains to 12/5/3.3V Supply - Page 1
Forum section: electronics / projects

author: mkalte
author_rank: Newbie
timestamp: November 19, 2020, 12:23:04 am
content: Heya,i have been working on a mains to 12V/3.3V/5V supply.This is mostly as an exercise in working with mains voltage and dealing with dc/dc converters.I have already prototyped the PCB once, to check for (and successfully find and fix!) issues in my schematic, so now it appears to be working and im seeing the voltages. I am now wondering if there is anything i should be taking care of, what else to look out for etc etc.Im using an isolating (at least according to the datasheet) dc/dc converter for mains to 12V. After that i use two MC33063A to get down to 3.3V and 5.0V. I want to be able to draw about 0.5A on each.I also added a thyristor crowbar in case the mains converter decides to fail anyway.Component values are calculated (or copied) from the datasheet. I decided against pours for the power nets, for easier soldering, and because it is low current anyway.I know that at least L1 and L3 could be smd, but my brain farted and now i have 10 of them, so i kept them tht.What did i miss? What do i need to look out for?Any feedback would be really appreciated.Thanks a lot!Project files:https://github.com/mkalte666/supplyboxAnd images:

author: dkonigs
author_rank: Regular Contributor
timestamp: November 19, 2020, 04:09:39 am
content: No feedback on the design, per se, but I'm interested in seeing what advice others have to offer. A project I'm working on right now has need for the same 3 voltages (12V to drive relays, 3.3V for most of the board, 5V for some external ports). My approach so far has been to use an AC/DC converter (PSK-6B-S12) and a pair of mostly-integrated Recom switching regulators (R-78E3.3-0.5 and R-78E5.0-0.5). I'm pretty sure my component count is lower, and my circuit simpler, but I wouldn't be surprised if my total BOM cost is higher. (But I really wouldn't know, without trying to price out everything.)

author: mariush
author_rank: Super Contributor
timestamp: November 19, 2020, 08:14:22 am
content: I agree that MC34063 and the likes have no place on a board, low frequency makes it require big inductors and resistors/capacitors, few protections, etc. Lots of switching regulators that are cheap and better.As for 3.3v, unless you're aiming for efficiency, I'd just go with a good LDO (linear regulator) with less than ~1.5v drop, so pretty much any.With the right package, for example a to252 / dpak , you can do 5v -> 3.3v at 0.5A just fine, and you'll only dissipate 1.7v x 0.5 = < 1w, so with a bit of pcb around the chip to act as heatsink you're fine.You could also just add a diode in front of the LDO to more or less cause a voltage drop of around 0.7v...1v and therefore reduce the voltage drop on the regulator (and now the heat is spread across two components)I'd say forget about the module, slap a barrel jack connector and now you can power it with a 16..20v laptop adapter brick.Actually, no, you can get cheap wallwart adapters for less than 10$, for example here's a 18v 13w one for 7$ :https://www.digikey.com/en/products/detail/tri-mag-llc/L6R12-180/7681522- 13w is enough for 12v @ 0.5A and 5v @ 1A (assuming you'll use a ldo to do 0.5A on 3.3v, leaving 0.5A on 5v)Use a switching regulator for 12v and 5v , for example TPS54386 or TPS54383 (same chip, but runs at 300kHz switching freq.)Here's datasheet for these two, see page :https://www.ti.com/lit/ds/symlink/tps54386.pdfPage 37 shows schematic for 22...26v to 12v @ 2A and 5v @ 2A .. but you can easily modify it for 16v...20v input and 12v/5v output

author: mkalte
author_rank: Newbie
timestamp: November 19, 2020, 09:28:15 am
content: Thanks for your replies.I failed to make it clear above, but this is not part of a bigger design.While it will be nice to have around to power the breadboard (after shoving it in a no touchy 3d printed box with some banana jacks and a proper AC connector), im mostly going for the learning experience here.So nothing wrong with redoing everything 5 times. And as it appears, this is coming up^^Quote from: blueskull on November 19, 2020, 05:29:41 amJup, thats why im hereNever really gone beyond linear or fully integrated buck converters before, mostly powered from battery or usb.QuoteYou make a good point here, and i wonder what was going on inside my head that i was coming to the conclusion that this was a good idea.I'm probably going with rolling my own, as this is more interesting. Thanks!QuoteOk.QuoteI see, i have fallen into the trap of sorting by cheapest part and reading into the topic from there. I'll have to look into this further.Thanks a lot, time to figure it out from here.Quote from: mariush on November 19, 2020, 08:14:22 amI will keep this in mind if im building an "real" design in this power region, but id like to keep using a buck converter for this one.QuoteI am doing this to get a better hang of building a supply, not for a "real" design. Again, ill be keeping this in mind for my next project with a bigger use, but for this exercise ill keep juggling AC.Though as said above, i am ditching the tamura module.Thanks for your feedback, this helps a lot.

author: tggzzz
author_rank: Super Contributor
timestamp: November 19, 2020, 10:21:22 am
content: You have been thinking about how it works. Youmustconsider how itfails.Start by understanding why class X and Y capacitors exist.