Jeff Liebermann wrote:
On Thu, 26 Jul 2012 22:57:25 -0400, "Tom Del Rosso"
wrote:

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?

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.


http://en.wikipedia.org/wiki/Power_Good_Signal

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?


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