Help with switch mode power supply - Page 1
Forum section: electronics / projects

author: lanmi93
author_rank: Newbie
timestamp: June 26, 2017, 02:05:31 pm
content: Hey. I'm new at this, so please bee kind and don't be mean.I need help with design of switch mode power supply. Problem with design is that I'm very limited with space and it needs to be able to deliver 3A on 5 volts. Found some reference design online and started from that. It would bee really bad if it blew up because of design flaw. Can you guys please look at it and point to any flaws before i send it for manufacturing, I would really appreciate it.Current version:27.06.2017 V1.01(+ Included file from PI Expert Online for transformer) (got some extra traces at AC filtering part, it is fixed)Dismissed:26.06.2017 V1.00(Feedback network was messed up / Transformer badly defined)

author: Leon23
author_rank: Contributor
timestamp: June 26, 2017, 02:19:20 pm
content: Design of the schematic or layout? Where is the schematics? What is the input voltage?

author: lanmi93
author_rank: Newbie
timestamp: June 26, 2017, 02:23:32 pm
content: Quote from: Leon23 on June 26, 2017, 02:19:20 pmSchematic is last, in PDF, and input voltage is 230V AC.

author: fourtytwo42
author_rank: Super Contributor
timestamp: June 26, 2017, 03:41:34 pm
content: Hello and welcome. It would help to have some transformer info, phasing not marked on schematic, is it gapped, what core, winding data please.

author: lanmi93
author_rank: Newbie
timestamp: June 26, 2017, 04:02:52 pm
content: Quote from: fourtytwo42 on June 26, 2017, 03:41:34 pmStill didn't figured that out. I know its going to bee EFD-25, Ferrite core. Primary and bias winding on one side, secondary on other. Like i said, new to all this, first time designing SMPS so i would appreciate any info of what should i put... I usedhttps://www.power.com/sites/default/files/PDFFiles/der114.pdfschematic, and modified it a bit. Any suggestion about transformer are welcome, but it needs to fit this footprint... Tnx.Edit:Ill probably use same one...

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 26, 2017, 05:33:03 pm
content: Hmmm... the "core" (ahem) of SMPS design is the transformer (the magnetic components in general) so saying that's "all" you need help with is basically saying "design it for me".That said, the phasing dots traditionally indicate the start of a winding and in this case should be assigned to pins 3, 5 and 10 of your schematic (alternatively, pins 1, 4 and 7 could be the dot ends, but this puts the swinging node of all the windings closest to the outside which increases noise emissions).The EFD25 core size is a bit big for a 15W/>100kHz power supply but its always possible to use a bigger core, whereas the converse is not true. Regardless of which size you choose I would strongly suggest getting a ferrite core set from one of the major electronics parts distributors (e.g. - DigiKey) which has been pre-gapped to a specific ALvalue (which gives the inductance in nH per turn squared; e.g. - a 100 turn winding on a core with an ALof 250 results in 2.5mH inductance). Note that the ALvalue for a pre-gapped set assumes the use of one ungapped and one gapped core half. For this frequency range EPCOS/TDK N87 material is ideal and is widely available.The TNY280 runs at 132kHz and has a 700V switch so for a max 265VAC input a Np:Ns turns ratio in the range of 30:1 to 40:1 would be good for a 5V output - turns ratio is not terribly critical in a flyback, btw. The primary inductance should be somewhere between 2mH to 2.5mH.As per usual with SMPS noobs, I highly recommend tinkering around with the calculator for a flyback converter on this site:http://schmidt-walter-schaltnetzteile.de/smps_e/spw_smps_e.html.At that site, plug in 280V and 360V for the min and max Vin, respectively, and manually set L1 to 2.4E-3 (2.4mH) and N1/N2 (aka "Np:Ns") to 30. Click on calculate, then on transformer data and scroll through the selections. Note that you can add cores to the table manually and it will remember them for your browser (using cookies, I guess?) for quite some time. For example, plugging in the data for an EFD25 core with an ALvalue of 250 (one of the stock gap values available) results in a primary with 98 turns and a secondary with 4 turns.Don't forget you need 3 layers of mylar or kapton tape between primary and secondary. I would use a higher clamping voltage for DZ1 - it needs to be higher than Vo * Ns:Np by a good margin for the flyback converter to even work - or else skip it completely and rely on the RCD clamp alone.On a related note, you want C10 to bridge pins 1 and 7, rather than 1 and 10, and note that this is a safety-critical component (needs to be "Y2" rated).Your feedback network is messed up, but I'm done for now.

author: lanmi93
author_rank: Newbie
timestamp: June 26, 2017, 09:15:46 pm
content: Wow, wow. I'm new at this SMPS stuff so be patient with me. I don't want you to design SMPS for me, where is fun in that, i just need you to point me in right direction. Like, i didn't know that I need to pay more attention to Transformer part. Ill research it much more now. Also English is not my main language so its not going that fast but i'm getting there. Also, this is not only project that i'm currently working on, so there is that. I had to start from scratch because i didn't know much about SMPS, used only linear till now. I saw where mistakes are and after fixing it ill get back with it. Thanks a lot for pointing them out. Also thanks for calculator, great stuff, i wonder how i didn't saw it before.

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 26, 2017, 09:31:19 pm
content: Quote from: lanmi93 on June 26, 2017, 09:15:46 pmHey, you said don't be mean; you didn't say anything about also being patient...Quote from: lanmi93 on June 26, 2017, 09:15:46 pmYeah, that calculator is a real gem... if for no other reason than it makes responding to posts like yours a lot easier for me!More seriously, Power Integrations has a web-based application for designing with their parts at:https://ac-dc.power.com/design-support/pi-expert/I downloaded and tried out their "PI Expert" standalone application a while ago and it did a surprisingly good job so if the web application is similar then I would trust it. And while I applaud the desire to do things on your own, there is no shame in borrowing heavily from the examples in the datasheet or the PI Expert software for your first offline SMPS.edit: grammar

author: lanmi93
author_rank: Newbie
timestamp: June 26, 2017, 09:38:33 pm
content: Wow, this looks great. Ill definitely try it out tomorrow. Meh, if screw up i could use that Transformer from example.... "Bobbin: EEL-22, 10 Pin Bobbin (Ying Chin YC-2207/ Pin Shine P-2204 or equivalent)". I think it could fit.

author: lanmi93
author_rank: Newbie
timestamp: June 27, 2017, 12:19:10 pm
content: Updated design. Tried but could not keep signal lines on TNY290 from nosy ones. Also, current PSU dimensions are 45mm x 100mm.

author: fourtytwo42
author_rank: Super Contributor
timestamp: June 27, 2017, 12:46:01 pm
content: Quote from: MagicSmoker on June 26, 2017, 05:33:03 pmYou also need 2mm margin tape at the ends of all windings to provide the required 4mm creepage distance within the transformer. If you are not experienced at manufacturing transformers for the degree of safety required it might be better to buy one. I believe the chip manufacturers often arrange sources that are published in there apps data, better safe than sorry

author: lanmi93
author_rank: Newbie
timestamp: June 27, 2017, 01:06:48 pm
content: Quote from: fourtytwo42 on June 27, 2017, 12:46:01 pmYes. I was planning to order transformer, as you said better safe than sorry. For that purpose i put EFD25 type footprint when EEL-22 would be enough... Just in case.

author: lanmi93
author_rank: Newbie
timestamp: June 28, 2017, 09:43:08 am
content: Some more suggestion guys....about spacing....gaps...traces width?