The secondary side is a very different story. Low voltage caps that can supply a reasonable amount of ripple current have electrolytes and oxides that have to endure much higher field strengths without breaking down. It's an art to make these caps and they are subsequently usually the most expensive component after the transformer and non-BOM costs. That is why you almost always see crappy secondary caps and good primaries.
(...)
A Rubycon 680 uF / 25 V / 105°C, low ESR capacitor costs EUR 0.58 at Digikey, in single quantities. This power supply costs EUR 64 (at Conrad). Even if you calculate times four the BOM costs to get the retail price, and add a bit for the development costs and profit, it should be possible to use this output capacitor instead of the Samxon. Maybe they think if you can get the medical certification, then you can just double the price, it doesn't matter, the target group will pay it.
Why is there a green LED on the PCB?
I'm not sure if I said this before on the forum, I remember saying this on the internet once... The reason they are using Samxon (crap garbage bin caps) on the secondary and name brand on the primary is that the secondary caps are much more expensive, simple as that. High voltage aluminum electrolytics are peanuts to build, you can even literally do this yourself and get great stability with some oil paper and aluminum foil. The difference between a name brand and cheap chinese brand cap is less than a cent for these manufacturers, so the choice is easy.
The secondary side is a very different story. Low voltage caps that can supply a reasonable amount of ripple current have electrolytes and oxides that have to endure much higher field strengths without breaking down. It's an art to make these caps and they are subsequently usually the most expensive component after the transformer and non-BOM costs. That is why you almost always see crappy secondary caps and good primaries.
As for the general construction: Dave, you are giving them way too much credit. This is a crappy power supply. It's not $4 QA reject crappy, certainly, but it's not even near high quality in any respect. It's a phenolic, wave soldered, one-tier-up-from-the-shenzhen-market-brand-components, just-meets-requirements design. I don't necessarily blame them, but I've seen medical supplies from Edac that have proper (4 layer even) FR-4 boards, slot isolated transformers (not this 3M foil garbage), name brand controllers, 125C caps, great output filters, etc..
If you ever get a hold of an EEG power supply, I highly recommend tearing those down. They are the absolute best PSUs in the world.
I believe that the winding above the secondary is not a feedback coil. Just a winding to power the controller.
Alexander.
Why is there a green LED on the PCB?
There appears to be a plastic window on the front of the device near where the cable exits. You can see it near the start of the video.