Re: kablooey

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

John Larkin wrote:

One of my better customers just called. Seems six of my VME arbitrary
waveform generators failed, in the same VME crate, simultaneously.
We're talking over $30K of damage here.

I had them put a scope on the +5 supply and switch power on. It ramped
up in maybe 100 ms, peaked at 5.6 for a few ms, then settled down to
5.1. That's above the 5.5 abs max for my CPU and FPGA chips, but not
really that bad.

So I asked them to switch power off for various times, then back on. A
2 second delay gave 7.5 volts peak. An estimated 0.5 sec delay, as
fast as they could work the switch, ramped up to 8.8. I'm guessing an
optimum brownout might well hit 10 volts.

Dumb switching power supply design... stupid loop dynamics and no
crowbar! I mean, one could easily load a VME crate with $100K worth of
boards. They're ragging the crate vendor next. Don't know who is going
to pay to fix the modules.

I could add a transzorb to my boards, I guess, but the overshoot might
just blow a hole in the board. Transzorb+fuse would work, but that
would be an ugly kluge. 8.8 volts is sort of past my responsibility,
I'm thinking.

John

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.

John

.

Relevant Pages

  • Re: kablooey
    ... waveform generators failed, in the same VME crate, simultaneously. ... So I asked them to switch power off for various times, ... optimum brownout might well hit 10 volts. ... described a rash of component failures that ...
    (sci.electronics.design)
  • Re: kablooey
    ... waveform generators failed, in the same VME crate, simultaneously. ... So I asked them to switch power off for various times, ... optimum brownout might well hit 10 volts. ... Sounds to me like a rather ignorant PSU manufacturer. ...
    (sci.electronics.design)
  • kablooey
    ... waveform generators failed, in the same VME crate, simultaneously. ... So I asked them to switch power off for various times, ... second delay gave 7.5 volts peak. ... optimum brownout might well hit 10 volts. ...
    (sci.electronics.design)
  • Re: kablooey
    ... Jim Thompson wrote: ... waveform generators failed, in the same VME crate, simultaneously. ... So I asked them to switch power off for various times, ... Sounds to me like a rather ignorant PSU manufacturer. ...
    (sci.electronics.design)
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