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PSU design planning, advice & play along!

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bsodmike:
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/645400

Ultimately 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 Series
Radial Leaded, Metallized Polypropylene, Film Capacitors
https://www.digikey.com/catalog/en/partgroup/b32922-b32923-b32924-b32926-series/74367

The 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!

bsodmike:
Just to make it clear, I'm looking for suggestions on potential parts and values that can be used.  Thanks!

WattsThat:
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?

bsodmike:

--- Quote from: WattsThat on September 07, 2020, 04:22:35 pm ---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?

--- End quote ---

Well 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).

T3sl4co1l:
So, is this just for 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

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