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PC PSU shutdown condition with bad mobo caps

T

Tom Del Rosso

When a mobo has bad caps the PSU shuts down, but what triggers it? What
does the PSU see from its point of view? Peak current spikes?
 
T

Tom Del Rosso

Michael said:
Excessive ripple keeps the 'Power good signal' set to false.

But whether or not that signal is involved in the chain of events (normally
it drives reset and maybe not all designs necessarily use it that way) what
is sensed on the PSU outputs is what I'm thinking about. Would it be
voltage ripple and not just current all the way back in the PSU even if the
PSU caps are good?

And are PSU caps likely to be damaged too after use with a bad mobo?
 
J

Jamie

Tom said:
When a mobo has bad caps the PSU shuts down, but what triggers it? What
does the PSU see from its point of view? Peak current spikes?
PG wire?

Jamie
 
T

Tom Del Rosso

Jeff said:
The purpose of the caps that are close to the CPU is to reduce the
ripple on the processor power line to tolerable levels. The processor
can go from zero to 40 amps at anywhere from zero (ground bounce) to
GHz rates. The capacitors have to smooth all that out. Basically,
they form an energy storage system to deliver power during the high
current spikes to the power hungry CPU.

When the ESR (equivalent series resistance) of the caps increases due
to electrolyte loss caused by overheating, less energy is available to
the CPU during high current peaks. The result is lots of ripple and
noise on the power supply line. Eventually, this gets the attention
of the "power good" line to the power supply, which shuts down to
protect the MB and CPU.

That's the point of my question. So it sees increased ripple voltage and
not just ripple current. That means the PSU caps are overwhelmed and might
suffer some damage too.


I've known the power-good signal since the XT, but it used to be just for
releasing RESET after the voltages came up. It isn't a very sensitive
detector of excessive ripple current since the latter has to be worse than
the load regulation spec to trigger it. In old designs it probably wouldn't
have detected ripple at all since it had its own filtering to create a delay
after power came up.

Note that better MB's use polymer capacitors instead of electrolytics.

But you can't substitute them for electrolytics, can you?
 
M

Mike Tomlinson

Tom Del Rosso said:
That means the PSU caps are overwhelmed and might
suffer some damage too.

Yes. All motherboards I've seen with failed caps in the processor
voltage regulator cct have also had failing caps in the PSU.

Replacing the PSU as well as recapping the board is what makes repair
uneconomic.
 
T

Tom Del Rosso

Jeff said:
Sorta. If any part of the noise or ripple on any of the power supply
lines goes below the defined threshold, the power good line will drop.
<http://www.formfactors.org/developer\specs\ATX12V_PSDG_2_2_public_br2.pdf>
See Fig 7 on Pg 25 and 3.3.1 on Pg 26.

I don't see any mention of current so it isn't 'sorta' triggered by ripple
voltage.

Nope. There's quite a bit of DC resistance between the power supply
and the ripple source, which is the CPU.

That would matter if the supply had sense lines, but PC supplies don't. The
voltage is sensed at the supply's output.

But I understand you to mean that although the caps in the PSU are working
harder, the discharge is small enough that they won't be heated unless they
were defective as well.
 
J

JW

The purpose of the caps that are close to the CPU is to reduce the
ripple on the processor power line to tolerable levels. The processor
can go from zero to 40 amps at anywhere from zero (ground bounce) to
GHz rates. The capacitors have to smooth all that out. Basically,
they form an energy storage system to deliver power during the high
current spikes to the power hungry CPU.

40A? Try 110A! Take a look at page 82.
http://www.intel.com/content/dam/ww...eets/3rd-gen-core-desktop-vol-1-datasheet.pdf

For the high end 130W Xeon over 200A:
http://www.intel.com/content/dam/ww...7-8800-4800-2800-families-vol-1-datasheet.pdf
Page 22.
 
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