How does an ATX power supply work?

[MrOmnos]

I was looking to build a bench power supply. So, I broke into an old ATX power supply to salvage parts. I found 3 power transistors [KSE13009L], and 3 schottky barrier rectifiers [S30D40C] but I didn't find any voltage regulators which I was hoping to find. I did find a packaged Bridge Rectifier though which will come in handy. And a KA7500B smps controller chip. I don't understand how the power supply manages to give out 5, 12 and 3.3 volts without any voltage regulators ics like LM7805 ?

 

[Martaine2005]

@MrOmnos
I think most ATX psu's use components to achieve the different voltages rather than dedicated regulators.
The KA7500B (switched mode power supply controller) is in fact a 5 volt regulator (or 5 volt reference) Or 5 volt standby for the motherboard.
This 5 volts is converted to 3.3 volts using components (not a regulator).
Once the psu's POST has completed it will (allow) the 12 volt to be present by components (not a regualator).
I think that if REGULATORS were used, the transformer would be quite hefty.
So the three transistors are probabaly for the 3.3v, 5v and 12v..
I am not saying how they work!!
Just my assumption.
Martin

 

[ramussons]

You may not have to breakup a ATX PSU to make a bench supply.
Google and you may get an idea that suits your requirement.

 

[BobK]

It is a switch mode power supply, so you will not find any linear regulators in it. Typically, they use a transformer with multiple windings to get the various output voltages. One voltage is regulated, and the others are simply based on the ratio of turns in their windings.

Bob

 

[MrOmnos]

You may not have to breakup a ATX PSU to make a bench supply.
Google and you may get an idea that suits your requirement.

I was just trying to salvage usable parts. But now I have learned alot about SMPS and may be I'll be build one in coming future.