On Sat, 28 Jul 2012 01:18:15 -0400, "Tom Del Rosso"
wrote:
So it sees increased ripple voltage and
not just ripple current.
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%5Cspecs%5CATX12V_PSDG_2_2_public_br2.pdf
See Fig 7 on Pg 25 and 3.3.1 on Pg 26.
That means the PSU caps are overwhelmed and might
suffer some damage too.
Nope. There's quite a bit of DC resistance between the power supply
and the ripple source, which is the CPU. The ripple is there not
because the power supply is producing the ripple or noise, but because
the CPU is NOT a constant DC load. The power consumption of the CPU
varies radically what's being computed, power saving features, data
transfer speeds, etc. The filter caps on the motherboard are actually
protecting the power supply from too much ripple produced by the CPU.
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.
Things have changed a bit. See the timing specs in the ATX12V Power
Supply Design Guide in the first URL.
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.
If it meets the ATX design specs, *ANY* voltage excursion below the
allowable design tolerances on any of the voltage lines, should
initiate a shutdown. In general, the voltage tolerances at +/-5%. See
3.2.1.
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.
Yep. The old designs also didn't sense excursions of the 117VAC power
supply input.
Note that better MB's use polymer capacitors instead of electrolytics.
But you can't substitute them for electrolytics, can you?
Yes, you can, but only if the ESR is low enough. Many motherboards
have a mix of solid polymer and electrolytic caps with the polymer
handling the high ripple current and high temperature filtering, and
the electrolytics handling the less critical filtering. The issue is
primarily cost. Solid polymer caps are about 5-10 times as expensive
as electrolytic.
http://capacitorlab.com/capacitor-types-polymer/
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Jeff Liebermann
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