Power Supplies Specs Question

**rushnrockt** · 02-18-2005, 03:29 AM

So I am on my way to building my own power supply and right now have to figure out what the specifications are going to be for the whole unit.
I am aiming at ~250W total output.

I've been checking out various regular computer PSU's and their specs, and as most of you know, if you were to multiply each rail's voltage by each supposed normal current, and then add all numbers up, the power will total more than what the power supply is rated for.

Most of the time there is a limit to the total power that 3.3V and 5V rail can output but other rails are separate in that respect.

My question is if anyone can tell by what logic is the power being calculated for the power supplies. My understanding is that each rail can supply a certain amount of power but always at the expense of other rails. Exactly how and why I am not sure yet, so anyone with an insight on that, I will appreciate your input. Thank you for your time.

Disclaimer: yes, I am going to build it. It is my senior project and buying opus is not an option so please don't bother with suggestions "why don't you buy." This has nothing to do with me doing it "better" I just needed to make a project and thought that this would be a good one.

**eCar™** · 02-18-2005, 03:55 AM

http://www.formfactors.org/developer...20PSDG2.01.pdf

**rushnrockt** · 02-18-2005, 05:34 PM

Originally Posted by eCar™

http://www.formfactors.org/developer...20PSDG2.01.pdf

Thank you for that source, it is rather helpful, but I was not able to find out any a reason for why the 3.3V and 5V rails are put together in power calculations. I guess it has something to do with the design that is assumed to be used.

**andrewb** · 02-19-2005, 02:21 AM

That's probably for 110/220V AC designs. Will not apply in a DC-DC converter topology, unless you step down 3.3V from the 5V or share some sort of input for both rails.

-A

Originally Posted by rushnrockt

Thank you for that source, it is rather helpful, but I was not able to find out any a reason for why the 3.3V and 5V rails are put together in power calculations. I guess it has something to do with the design that is assumed to be used.

**rushnrockt** · 02-19-2005, 02:34 AM

Originally Posted by andrewb

That's probably for 110/220V AC designs. Will not apply in a DC-DC converter topology, unless you step down 3.3V from the 5V or share some sort of input for both rails.

-A

That's what I figured. From the ATX specifications I've been looking at, it seems like they consider only one type of topography, and switching regulators are in no way part of it. Already this project looks more difficult than it seemed.

Thank you for your input.