Re: kablooey

On Sun, 12 Mar 2006 04:59:06 +1300, Terry Given <my_name@xxxxxxxx>
wrote:

Jon Elson wrote:


John Larkin wrote:

On Fri, 10 Mar 2006 16:03:48 GMT, Michael <em.pea.sea@xxxxxxx> wrote:

Just two nights ago a friend who works in the RAID industry, in Failure
Analysis, described a rash of component failures (capacitor
punch-through) that
he diagnosed as caused by SMPS spikes at turn-on. 10vdc caps on a
5vdc line
were being killed by 12vdc. Switchers save iron and weight, yes, but
poorly
designed ones can be a basket of headache.



Yikes! 12 is even worse than 8.8.

Now I'm beginning to wonder... how many unexplained, seemingly random,
logic failures are caused by smps's and random-duration brownouts?

Many switchers have huge open-loop voltage margins, especially 85..260
volt universal-input types run at 240 volts input. A 5-volt supply,
running wide-open with 240 in, could easily make 15, maybe even 20,
volts out.


Recent posts described a very common startup circuit that has a negative
aspect
related to static power dissipation in the high-value res from the
rectified line
to the control supply. (I put a FET in there to shut off the R when the
supply was
running on mine, to reduce that heat.) Also, if the supply fails to
start, that R
can get real hot.

Anyway, another type uses a very flaky "teaser" circuit to blip on of
the main
power FETs on every once in a while. If these supplies fail to start
for any
reason, they just sit there squeaking and chirping. The idea, when
everything
is running right is that the FET is just triggered one time for a few
microseconds
every couple of seconds, and just one "tease" is enough to charge up the
control
supply and start the whole supply up. This design usually uses fewer
parts, and
no big resistor. But, it is a mighty UN-controlled way to start the
supply!
I'll almost bet that this is how the supply that started this thread works.
If the variation of the components is too large, or the breakover device
that
triggers the FET is flaky and "sputters", it can result in an
overvoltage condition.
Even still, the crowbar should be a totally independent function, just a
Zener
and an SCR. Not having a crowbar on a multi-thousand $ VME crate that is
guaranteed to have even more thousands of $ of boards plugged into it is
totally
unbelievable. Our VME crates run about $7000, and have probably twice that
value of boards in them.

Of course, this may be a case of two points of failure. You'll never
know if your
crowbar is working unless you actually test it. So, the crowbar might
just have
been non-functional, and nobody noticed until the power supply
regulation went
haywire. I can only see one way a momentary power dip would cause an
overvolt.
The power supply probably has a slow turn-on arrangement that normally
keeps
it stable. The dip was enough that the switching transistors went to
100% (or
full) duty-cycle, thus causing the regulation to go open-loop. Integral
windup
occurs, then the mains power comes back on, and the loop can't react
fast enough
to rising main supply voltage. Whew, that takes some pretty crude
design of
the control loop! If it didn't have slow-up, it would do this EVERY
time it was
turned on. Also, a load dump would liekly trigger the same situation.
The maker
certainly should test to a worse load-dump than any user could ever
cause. Does
this crate allow hot-plugging? If so, then the load dump of a
hot-unplugging
needs to be covered.

Jon

any buck-derived converter can over-voltage its output, if poorly
designed. large L and small C means lots of overshoot when a large load
(or worse still, overload) is removed, and there is not a damn thing the
controller can do about it. At best the controller can add no energy to
the output inductor, so all the energy stored there gets dumped into
Cout. the cure is lots of C and not much L.

likewise a real slow controller and not much capacitance can also give
lots of overshoot (undershoot, too), but that is easily fixed by
choosing a higher closed-loop bandwidth.

Cheers
Terry

Or a shunt regulator for such short-term events. And a crow-bar
system... I did one years ago that had a circulating current of over
100A... fuse went like a flashbulb ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

It's what you learn, after you know it all, that counts.
.

Relevant Pages

  • Re: kablooey
    ... Just two nights ago a friend who works in the RAID industry, in Failure ... to the control supply. ... power FETs on every once in a while. ... Even still, the crowbar should be a totally independent function, just a Zener ...
    (sci.electronics.design)
  • Re: kablooey
    ... Just two nights ago a friend who works in the RAID industry, in Failure ... power FETs on every once in a while. ... is running right is that the FET is just triggered one time for a few microseconds ... Even still, the crowbar should be a totally independent function, just a Zener ...
    (sci.electronics.design)
  • Re: secret codes embedded in your printer output...
    ... >In every society you have those who exercise power and they generally ... introduce any law they wish with few constraints. ... and where there is no solid constitution and BoR to supervise them. ... >But I think there is an agenda to exert more and more control over ...
    (uk.politics.misc)
  • Iranian Power Grab Debacle (a primer)
    ... positions of power over who and what lives or, more likely, dies. ... be unleashed upon Iran. ... control over, ... The USA gathers petroleum from around ...
    (alt.politics)
  • Iranian Power Grab Debacle (a primer)
    ... positions of power over who and what lives or, more likely, dies. ... be unleashed upon Iran. ... control over, ... The USA gathers petroleum from around ...
    (alt.politics.bush)