A power supply should never be like this

I'll say right away: this isn't a full review of the power supply, but more of a look inside, with a demonstration of how power supplies are sometimes assembled and why it's a good idea to open one up before using it.

So, I bought a set of LED panels, a controller, and a power supply for a friend.
The power supply was bought here, though it doesn't really matter, since you can run into the same kind of problem anywhere.

I had no complaints about the package itself. The seller packed the goods quite well, wrapping them in bubble wrap, with the PSU in a cardboard box.


The power supply was actually purchased as a separate lot, but I don't think that matters.
Inside the box, the PSU had extra protection in the form of an envelope made from more of the same bubble wrap.


It looks great, too. Who could tell from the photos that anything was wrong with the PSU?


The power supply uses what's known as an "ultra-slim" design. It was intended for a specialized installation, where this form factor is preferable.
But it's not just thinner; it's also smaller overall. For comparison, here's a standard 12 V, 240 W PSU next to it.



The PSU is rated for 200 W, with an output voltage of 5 V and, accordingly, a current of 40 A. It's marked S-200-5, i.e. 200 W, 5 V.


The terminals are standard screw terminals, but they're not covered, which is unsafe.


As usual, we grab a screwdriver and open up the power supply.
At first glance, it looks perfectly fine.


It even has a fuse, a thermistor, and an input filter. The measured capacitance of the input capacitor is 134 µF, which is far too little for a 200 W PSU.


The bridge rectifier is made up of individual 1N5408 diodes. I wouldn't call that bad, but I wouldn't call it good either. In my opinion, it's better to have some current headroom, since these diodes are only rated for 3 A.


And here's the first fail. Honestly, I was in shock when I saw that the power transistor wasn't even screwed to the heatsink. This PSU is a clear candidate for failure. I ran it at about 25% load before taking it apart; I'll show you the thermal camera photos later.
And the fact that this screw is covered with a sticker saying "Quality check OK" is just brutal sarcasm on the manufacturer's part.


But it's not just unscrewed; it's not even touching the heatsink. There's about 1 mm of space between the transistor and the heatsink.
Honestly, this is a complete disaster.


The feedback circuit, along with the optocoupler and trimmer resistor, is tucked between the heatsink and the transformer.


One thing I can say is that the manufacturer installed a whole bunch of proper Y1 capacitors. I guess they got spooked :)


No complaints about the transformer. It's also built in a "slim design," with a busbar winding rather than just wire or, as is more often the case, litz wire.


They installed no fewer than four output diode assemblies, but there are problems here, too. The retaining bars barely held them in place, and you could easily unscrew the screws with your fingers. Overheating is almost inevitable.



If you think that's the end of the problems, you're very much mistaken. One pair of diode assemblies is screwed down through an aluminum plate—and it's CROOKED!


The output inductor is wound with four wires and, in my opinion, is too small for this current and power. It makes an unpleasant buzzing sound under loads above 25%.


The output capacitors have a decent capacitance—3300 µF each, about 15000 µF measured in total—and are rated for 10 V, but they look very cheap. I don't think they'll last long.


And here's how hot the power transistor got at around 25% load: 80 degrees! At even 50% load, the PSU would have died very quickly.


So, this is one lousy PSU. One good thing is that it has all the components needed for a proper PSU on board. But some components are low quality, and the PSU itself has major assembly defects.

The bad
The input capacitor has too low a capacitance
The power transistor isn't screwed down, and there's a gap between its case and the heatsink
The output diode assemblies are different
One pair of diode assemblies has a bent plate
The output capacitors are low quality.

After inspecting the PSU, I remembered a good video on a similar topic.