PSU design planning, advice & play along! - Page 1
Forum section: electronics / projects

author: bsodmike
author_rank: Regular Contributor
timestamp: September 07, 2020, 12:07:59 pm
content: Hi all,For a good while now, I've been following many of Dave's teardown videos, and find stuff like this really exciting and interesting.At the moment I've got a simple 240VAC to 12VAC/3VA isolation transformer and I want to use this to build a basic "input stage" PCB to use for future projects.https://export.farnell.com/carel/tra12un100/transformer-240vac-in-12vac-out/dp/645400Ultimately this will become the first stage of a DC-DC switch-mode PSU. I want to repurpose the 'DaveCAD' version as the EMI filtering input stage.Looking at the mains input circuitry (attached), I've figured out a couple parts:(0) HRC fuses, can use something sensible.(1) TDK B32922, B32923, B32924, B32926 SeriesRadial Leaded, Metallized Polypropylene, Film Capacitorshttps://www.digikey.com/catalog/en/partgroup/b32922-b32923-b32924-b32926-series/74367The image shows a "u33" 300VAC jobbie, so I assume this is a 0.33uF?(2) Common-mode Ferrite choke with 2-separate windings. Can you recommend a part for this?(3) Grey caps - no idea?(4) 4x inductors with Yellow cores - ideas?(5) MOVs for overload protection- any ideas?Thanks!

author: bsodmike
author_rank: Regular Contributor
timestamp: September 07, 2020, 02:34:43 pm
content: Just to make it clear, I'm looking for suggestions on potential parts and values that can be used. Thanks!

author: WattsThat
author_rank: Frequent Contributor
timestamp: September 07, 2020, 04:22:35 pm
content: Why are you using a 1200 watt power supply as a reference design and then say you’ve got a 3 VA transformer for the “input stage”.Do you know what the 3 VA rating actually means, in terms of power?

author: bsodmike
author_rank: Regular Contributor
timestamp: September 08, 2020, 07:38:24 am
content: Quote from: WattsThat on September 07, 2020, 04:22:35 pmWell that's apparent power and can deliver around 3VA/12V = ~250mW. For larger current draw, I can shop for a transformer with a higher VA rating.I was looking at the input EMI filtering stage, and I don't see the harm in that. Anyways, I found two further excellent resources to aid me on this:(1)This helps to explain sections of the previous input stage circuitry.(2) An EMI filter kit (1-6A) off eBay:(Schematic attached)Best aspect, we get some component values.- 2x 470v MOVs- 3x X2 100nf self-healing film caps.- 1x Common mode-choke (no value specified in uH).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 08, 2020, 07:12:25 pm
content: So, is thisjustfor a 250mA (not mW?) power supply, or is this just the aux supply for a much bigger switching supply? (A common design approach, but no one uses iron transformers for this: they would have to be switched to run in different domains. A universal input SMPS is used instead.)'Cuz iron transformers and rectifiers need very little if any EMI filtering. The solution is trivial: 0dB attenuation is required, therefore adequate filtering is obtained from two pieces of wire.Otherwise, this very much smacks of looking at a shiny and wanting to build it, without understanding why it exists. Which is fine, electronic components are probably an underutilized medium and I'd love to see more. But for electrical purposes -- function dominates over form. What is your justification for it?Tim

author: bsodmike
author_rank: Regular Contributor
timestamp: September 09, 2020, 07:59:23 am
content: Quote from: T3sl4co1l on September 08, 2020, 07:12:25 pmI want to structure the PSU like this:(1) Input end, basic EMI filtering, could be as simple as MOV + Film cap.(2) I looked at the common-mode & differential filtering (as a learning exercise); I now understand the concepts there.(3) I found another resource to help me better understand SMPS design and I came across this:https://eevblog.com/files/SPS9400_ServiceManual.pdfThis uses an MC3426 and a similar modern alternative is this NCP1608 IC by On Semiconductor.http://www.farnell.com/datasheets/2355053.pdfAt the end this Datasheet shows an Application Note example using the NCP1608 as a pre-converter design. It seems the IC drives one side of a common-mode choke, it's other end is "series-wired" to the rectified 200VDC line. I suppose, this inductor is "switched" at high-frequency to provide the required current/voltage at the output (basic principle of Vout = L*di/dt)?

author: MosherIV
author_rank: Super Contributor
timestamp: September 09, 2020, 12:22:37 pm
content: QuoteThe transformer that you have is most likely a 50/60Hz step down transformer. These are not suitable for use as the isolation transformer in a switch mode power supply. The switching frequency is greater than 50KHz in most mains smps. The 50/60Hz transformer that you have will not work (very well) at those frequencies, it is just not designed to work at high frequencies.The isolation transformer used in most mains based smps are custom made for the smps application. They are just not easily available off the shelf from regular electronics suppliers.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 09, 2020, 03:00:57 pm
content: Quote from: bsodmike on September 09, 2020, 07:59:23 amHeh no, did you notice the dots -- that's differential mode actually, but that's still not a good description of it. It's asymmetrical. Check the turns ratio calculation on page 22, and notice the resistors connected to the secondary, they aren't exactly low values. (They don't give a calculation for R1 as it happens, but it'll be on the order of 10 or 100 ohms I suspect). It's stepping down, and it's not really transforming power, just sensing the inductor voltage, and tapping off a little power to run itself. This makes these inductors a bit hard to find, unless someone happens to stock the exact part advertised in the appnote.Which, don't forget to check out the appnote:https://www.onsemi.com/pub/Collateral/AND8396-D.PDFTim