Modding a server power supply for higher voltage... - Page 1
Forum section: electronics / projects

author: Hiemal
author_rank: Regular Contributor
timestamp: March 17, 2019, 11:33:40 pm
content: Hi ya'll.Some number of weeks ago I bought an HP 460 watt server PSU and noticed that it has no OVP protection on the outputs. Ialsonoticed that you can change the output voltage consequently, to whatever value you want (I got it to go as high as 30v!)The control circuitry has a separate rail that stays steady at 12v.Assuming I change out the output caps to properly rated ones, would it be problematic to use it as a fully adjustable power supply? Or would bad things happen? I know there's potential issues with snubbers and whatnot but otherwise... what would one expect?

author: coromonadalix
author_rank: Super Contributor
timestamp: March 18, 2019, 12:03:13 am
content: If the psu has no ovp as you say, i'm surprised if there is no ovp ??Do some tests to be sure of it, and know how it will react under light and heavy loads ... even a short

author: jackbob
author_rank: Regular Contributor
timestamp: March 18, 2019, 02:48:56 am
content: If the supply does not monitor the input or switching current at the primary, this could be problematic. Surely it monitors the output current for overcurrent conditions but if the voltage is more than double on the output, and it allows the same current on the output, the power will essentially be more than double. This will cause problems with the transformer saturating and the switching fets will most likely fail.

author: magic
author_rank: Super Contributor
timestamp: March 18, 2019, 07:11:44 am
content: A flyback transformer may look like magic which turns arbitrary voltage into 12V, but there is actually nothing magical about it.It's still a transformer and it still obeys Vp=n×Vs at all time.Increase peak secondary voltage and you increase peak primary voltage. The latter is boosted on top of rectified mains when the primary switch turns off and the switch has to withstand all of that.Some headroom was likely left in the designand you get additional headroom from running a 230V compatible supply on 110V, but it's going to blow if you push the output voltage too far, even with no load.Wait, what a nonsense. You don't actually get any extra headroom from using lower mains voltage if the PSU has active PFC. But you would with a simple switcher.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 18, 2019, 10:24:03 am
content: you may loose load and line stability if you mod the output voltage, the psu was designed in a way, by having a higher output you change a few things, the behaviour can change too ...

author: Hiemal
author_rank: Regular Contributor
timestamp: March 18, 2019, 03:02:00 pm
content: I'll try doing some more tests when I get the chance to characterize it under different loads and voltages, and see if it survives or not. I'm pretty sure it monitors primary side current draw, since theres some hefty sized shunt resistors on that side of the PCB.And I understand it definitely would not be able to supply the same current at higher voltages. I'd try to keep it within it's original power rating if this does somehow work out properly.As to the switch ratings... Well. Is there a rule of thumb or something I could follow in regards to output voltage vs. what the main switching transistors should be rated at?

author: iWei
author_rank: Contributor
timestamp: March 18, 2019, 04:07:31 pm
content: Is it one of those 511777 power supplies? How did you manage to modify the voltage so much?