A little help describing a switch mode PSU - Page 1
Forum section: electronics / beginners

author: windmill john
author_rank: Contributor
timestamp: January 05, 2023, 11:12:02 am
content: Thanks to all re my thread: "Isolation transformer wanted for oscilloscope", a bit like wood turning, learn all the safety stuff first.Anyway, picture attached of an smps from a PC at work.Still trying to get my head around it, but there appears to be three transformers? A, B and C.I get one will be for the input voltage and one, I've read for the digital circuit (little vague on that) what is the third for? I may be wrong, time for Benta to take the mick.Yes Benta, I am aware of the 420 volt capacitor and I'm still here!!!! Amazing!!

author: TimNJ
author_rank: Super Contributor
timestamp: January 05, 2023, 02:17:27 pm
content: A is the main transformer for the presumably 12V, high power output.B is the auxiliary transformer to provide the other miscellaneous rail voltages and standby supply required by the computer. (Historically at least, 3.3V, 5VSB -12V.) There are various reasons to implement this separately. One reason is that the standby supply is always required to be on, even if the computer is “off”. Theoretically, you could derive all voltages (including the main output) from a single transformer but this usually leads to high no-load losses, circulating losses and so on (depending on the topology). On top of that, it’s hard to make a jack of all trades transformer which is high efficiency/good regulation for the main output, whilst also providing all the other rail voltages.C is the PFC inductor.

author: windmill john
author_rank: Contributor
timestamp: January 05, 2023, 06:30:02 pm
content: Thanks Tim. I’m new and hadn’t heard of a pfc inductor. More reading, thanks.Purely out of interest, this is from a Dell 3030 AIO PC. We’ve had many psus fail; power clicks on and off.So, we got replacement psus from eBay. As I’m trying to improve my knowledge of electronics, I decided to take a look. As psus are now £90, my boss has said bin the PCs as they are at least five to seven years old.Had an electrician in for other reasons and we had a play. It looks like the relay, marked R is the reason. £5 to replace, might have a play.

author: tautech
author_rank: Super Contributor
timestamp: January 06, 2023, 06:06:10 am
content: Quote from: windmill john on January 05, 2023, 11:12:02 amAnd you won't just looking at the top side of the PCB without some experience.First to have any clue of how it might work is to identify the SMPS controller IC and get its datasheet and/or App note.Here's a very common SMPS controller IC datasheet and typically they control switching frequency/duration of rectified mains across a step down transformer of one or more secondary windings and sometimes a primary side flyback winding that supplies therunpower to the controller IC.https://www.ti.com/lit/an/slua143/slua143.pdf?ts=1672983602501&ref_url=https%253A%252F%252Fwww.google.com%252FWhile it's useful to understand the internal construction of the IC its basic application diagram is really the key to how it's used and shows the core components that have it operate. See Fig 7However CIRCUIT EXAMPLES is where we should really focus as so often they are copied exactly in commercial applications. See Fig 31Here we can plainly see pin 7 is Vcc and unless the IC's UVLO is exceeded the IC will not run and there are 2 paths for the Vcc current source, namely for start up via R2 and from the Aux/Flyback primary winding rectified by diodes and smoothed by caps.3 devices types have just been mentioned and any of these are known failure points for SMPS.One last word, aproperlydesigned and inproperworking order a SMPS should continue to operate after mains power is removed until the rectified mains input bulk capacitor is near depleted. < Ignore this at your peril.