On Fri, 27 Jul 2012 09:48:48 -0700 (PDT), spamtrap1888
wrote:

On Jul 27, 9:22*am, 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.

I'm going to expose my ignorance he are there not also small value
capacitors in parallel, to take care of the GHz spikes?

Yes. There are ceramic chip caps found on the bottom of the PCB,
directly under the CPU. Short lead length required for high
frequencies. None of the external CPU pins actually see GHz range
signals as they are divided down before they hit the various buses.
However, the rise times are well into the GHz (picosecond) ranges,
which can have components in the GHz range, and must be bypassed lest
they radiate RF and attract the attention of the FCC.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558