[sdiy] SSM2164 protection

[David G. Dixon]

> > OK, where's the flaw?
> 
> If you must ask, there are several. :-)

I did, or I wouldn't have.  I don't do this for a living, you know, nor did
I study it in school.  I'm just a weekend warrior with a soldering iron and
a Rick Wakeman fixation!

> Count how much capacitance is downstream from the switch.  Now consider
> that
> all capacitors are discharging through the single module that decides to
> latch up first... at least it'd probably protect the others from failing,
> but Murphys law will make it the module you can spare the least.  A
> latchup
> happens very fast, so by the time your protection switch knows that
> something is wrong the modules may have well given up the ghost already.

I hadn't thought about capacitance.  However, I just simulated my little
solution again with 100uF of capacitance (ten 10uF caps in parallel -- I use
10uF caps on each rail on each module, and 10 modules per cabinet is about
my upper limit) to ground between the FET and the (assumed) 100 ohm load.
It took 40 or 50 ms for the voltage to disappear.  However, the current was
cut off instantly (of course).

There would be about 1.5mC of charge stored on all those capacitors (100uF x
15V).  That's the equivalent of 1.5mA of current flowing for 1s.  I can't
imagine that tiny bit of charge frying anything (but, again, IMBSC).

> Also that switch doesn't protect against reverse voltage at all, the body
> diode will open and if you prevent this, there's still the path to the
> unprotected ground rail.  Failsafe power distribution is hard to design
> and
> costly to implement due to the multitude of possible failure modes, even
> if
> you consider only a single mode for each failure event (you usually don't
> have that luxury, either).

Well, it's not meant to protect against everything.  For reverse current,
polyfuses and rectifiers to ground (or even just rectifiers in line) work
exceedingly well.  It's only meant to protect against the loss of the
negative rail, and only for the benefit of the 2164s (nothing else really
cares, as far as I can tell).