PC PSU shutdown condition with bad mobo caps

[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?


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[Michael A. Terrell]

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?

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

 

[Tom Del Rosso]

Michael A. Terrell wrote:

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?


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?


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[Michael A. Terrell]

Tom Del Rosso wrote:

Michael A. Terrell wrote:
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?


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?

No. If they are good, the PS will just shut down. The CPU PS
generates its own control signals. They no longer run from 3.3V. The
voltage is lower, and at a lot higher currents. That's why the quality
of the electrolytic caps is so critical. I see people complain about
the caps being too close to the CPU, but 1.8V @ 30A doesn't allow for
long traces.

 

[Jeff Liebermann]

On Thu, 26 Jul 2012 22:57:25 -0400, "Tom Del Rosso"
<td_03@verizon.net.invalid> 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.

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

One irritating problem is the dominoes effect on multiple MB filter
capacitors failing. When the ripple voltage is low across the
capacitors, there is little ripple current through the capacitors.
Capacitors only conduct during the time the voltage changes across
them. When the voltage is pure DC, no current is drawn. They also
only get warm when there's ripple current through the capacitor.

When one capacitor (usually nearest the CPU) fails, the ripple current
trhough the others increases slightly. That means that the internal
heat generated in the other caps increases. When the next capacitor
fails, the ripple current and internal heating of the remaining
capacitors again increases, result in even more heating. Eventually,
the few remaining caps can't handle the self-heating, overheat, bulge,
and die. This is why you want to replace ALL the capacitors in a
parallel string when one of them fails. Measuring the ESR of such a
parallel string is impossible.

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


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

 

[Jeff Liebermann]

On Fri, 27 Jul 2012 09:48:48 -0700 (PDT), spamtrap1888
<spamtrap1888@gmail.com> wrote:

On Jul 27, 9:22 am, Jeff Liebermann <je...@cruzio.com> wrote:
On Thu, 26 Jul 2012 22:57:25 -0400, "Tom Del Rosso"

td...@verizon.net.invalid> 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 here: 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 jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[spamtrap1888]

On Jul 27, 9:22 am, Jeff Liebermann <je...@cruzio.com> wrote:

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

td...@verizon.net.invalid> 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 here: are there not also small value
capacitors in parallel, to take care of the GHz spikes?

 

[Jamie]

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?


PG wire?

Jamie

 

[Tom Del Rosso]

Jeff Liebermann wrote:

On Thu, 26 Jul 2012 22:57:25 -0400, "Tom Del Rosso"
td_03@verizon.net.invalid> 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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[Michael A. Terrell]

Tom Del Rosso wrote:

Jeff Liebermann wrote:
On Thu, 26 Jul 2012 22:57:25 -0400, "Tom Del Rosso"
td_03@verizon.net.invalid> 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.

The computer's power supply is on the other side of the CPU supply,
but a ittle ripple can get into the 3.3V or 5 V rails and cause erratic
operation. If the power supply is failing, it's usually just a
coincidence unless it was a really crappy design.

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?

They are also called solid polymer electrolytics. Substitution will
depend on the max & min ESR allowed in the circuit.

http://thecapking.com/plg1c821mdo1.html is one example.

http://www.thecapking.com/whatwestock.html is a list of common low ESR
caps used on motherboards.

 

[Jeff Liebermann]

On Sat, 28 Jul 2012 01:18:15 -0400, "Tom Del Rosso"
<td_03@verizon.net.invalid> 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/>





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

 

On Thursday, July 26, 2012 11:12:57 PM UTC-7, 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?

I don't know if the Power_Good signal from the PSU is just a power-on reset signal for the motherboard or if the mobo is designed to shut down if that signal goes false. OTOH the mobo doesn't drive the Power_Good signal; the only thing it does drive is the Power_On signal on the green wire.

Motherboards have several voltage regulators -- CPU, AGP or PCI-E, and memory, and I'm guessing they're designed to shut down the power in case of overload because it could be too dangerous not to (tons of amps for creating arcs and fires); I can't say for sure because I haven't looked for this protection on many mobos, but my ancient PC-2007 had a "U" shaped bar to sense CPU current.

I don't know if PSU caps are damaged by bad caps on the mobo, but I doubt it's due to voltage but only current, and some reviewers have seen PSU caps and other PSU components fail when loaded to the PSU's full rated power with a testing machine -- see JonnyGuru.com , HardOCP.com , HardwareSecrets.com, which disect what they review, failed or not.

 

[Mike Tomlinson]

En el artículo <jutbnk$9dp$1@dont-email.me>, Tom Del Rosso
<td_03@verizon.net.invalid> escribió:

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

Yes.

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(='.'=)
(")_(")

 

[Mike Tomlinson]

En el artículo <juvsvt$afp$1@dont-email.me>, Tom Del Rosso
<td_03@verizon.net.invalid> escribió:

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.

--
(\_/)
(='.'=)
(")_(")

 

[Michael A. Terrell]

Mike Tomlinson wrote:

En el artículo <juvsvt$afp$1@dont-email.me>, Tom Del Rosso
td_03@verizon.net.invalid> escribió:

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.

I don't see that combination of failures in the PCs I rebuild. Power
supplies are usually easier to recap than a motherboard. Good 105C low
ESR caps are cheap in quantity. Jeff posted one link. Here are some
others that I've bought from:


http://www.thecapking.com/whatwestock.html

I buy some Panasonic FM series in bulk from www.digikey.com

 

[Jeff Liebermann]

On Sat, 28 Jul 2012 13:49:19 +0100, Mike Tomlinson
<mike@jasper.org.uk> wrote:

En el artículo <juvsvt$afp$1@dont-email.me>, Tom Del Rosso
td_03@verizon.net.invalid> escribió:

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.

That's not what I've been seeing. I don't do too much recapping, but
replacing the MB caps usually solves the problem. I have seen bad
power supplies, but MB cap failures seem more common. One common
characteristic I see in failing ATX power supplies is that they tend
to have only a few physically small capacitors in the DC output filter
section. The ATX supplies that are crammed full of big electrolytics
seem to survive nicely.

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

Ummm... it's easier to recap a power supply than a mother board. The
PS comes out easily, is easily disassembled, usually has a single
sided low density PCB, and is easy to test after recapping. The MB
requires removing lots of hardware, replacing many more small caps, is
a multi-layer PCB, and takes more time to test.

What makes MB recapping uneconomical is that rather large number of
caps that usually need replacement. Testing the caps on the MB is
problematic because many are in parallel. The only sure way is to
replace anything that might fail. That's usually about 12-15 caps on
a typical MB. I can frequently buy a used MB on eBay for less than
what I charge for my time. I still do recapping because I detest
creating eWaste, but I don't make much money on recapping.



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

 

[Tom Del Rosso]

Jeff Liebermann wrote:

On Sat, 28 Jul 2012 01:18:15 -0400, "Tom Del Rosso"
td_03@verizon.net.invalid> 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.

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

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.

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.


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On Saturday, July 28, 2012 5:49:19 AM UTC-7, Mike Tomlinson wrote:

En el artículo <juvsvt$afp$1@dont-email.me>, Tom Del Rosso <td_03@verizon.net.invalid> escribió:

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.

The OST brand caps in that voltage regulator in my ECS K7S5A Pro failed, as did the PC-Tur Elite caps (CEC International?) in that regulator in my ECS P4S5A, but the caps in their PSUs were fine, and all were Panasonic, Rubycon, or Nichicon.

 

[JW]

On Fri, 27 Jul 2012 09:22:19 -0700 Jeff Liebermann <jeffl@cruzio.com>
wrote in Message id: <4qe518dd0g3d0683tinu931qilvve6v1t3@4ax.com>:

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/www/public/us/en/documents/datasheets/3rd-gen-core-desktop-vol-1-datasheet.pdf

For the high end 130W Xeon over 200A:
http://www.intel.com/content/dam/www/public/us/en/documents/datasheets/xeon-e7-8800-4800-2800-families-vol-1-datasheet.pdf
Page 22.