> A friend of mine has asked me, in desperation, if the list
> has any comments about his circuit.
>
> http://home.clear.net.nz/pages/joecolquitt/12vmotor.html
>
> The problem he's got is that this circuit tested OK on the bench,
> and 2/3rds of the finished units seem to be alright in use, but
> the others fry all 3 FETs short circuit, causing the motors to run
> full tilt, causing a scramble to disconnect the battery. It even
> happened twice this week, on separate occasions, with units
> that were seemingly turned off, then spontaneously burst into
> life a few minutes after being used. I've suggested a varistor
> across the FETs, but that doesn't explain why the units start
> on their own. Any clues ? (personally I'd have made the circuit
> more fail-safe, but he's very cost-conscious. Perhaps to the
> extent of false economy it seems)

First - what may be happening.
Second - what to do.

The Taiwanese call this failure 'turbo mode" :-).

The present arrangement appears to be attempting to prevent the FETs being
over voltaged by turning them on if the drain voltage exceeds the 27v zener
ratings.
If this happens the gates can be pulled up hard as high as you want to pull
them until the FET turns on and clamps the spike. Which gets there first
depends on parameters which need to be examined in more detail.

The gates have no direct protection at the gate but only behind the 68 ohm
drive resistors. There is very significant capacitive coupling between gate
and drain during switching and transient oscillatory modes can occur which
generate large short term signals on the gate. Having protection right at
the gate (electrically AND physically) helps.

Zener gate protection is often OK but for negative going oscillatory
transients (which do happen in the real world and which can be quickly
fatal) a reverse biased schottky at the gate is even better. A zener can
still be used to limit the positive excursions. This adds cost and
complexity so if this is to be avoided then starting with zeners alone at
the gate would be worthwhile.
.

SO

I would try zeners slightly more than 12v at each FETS gate OR if he must
save money then tying all 3 FETS gates together and having a common zener to
ground from the gates. (I realise that this removes the spreading affect of
the 3 x 68r base drive resistors but as the FETs are being driven at well
above gate threshold voltage this loss should be more than made up for by
the gain in protection.

RShorting out the 3 x 68r and seeing how it affects operation and
reliability may be instructive.

If the 3 x gate to drain zeners are felt to be providing valuable
protection, consider placing a resistor in  series with each one so that
rapid positive dv/dt transients cannot pull  the gate above the FET's
maximum Vgs rating.

I question why these zeners are needed here.
IF the motor flywheel Schottky is operating correctly then the FET drains
should see minimal effects from the motors inductive reactance when the FETs
are off as all motor current will flywheel through the Schottky.

QUESTION - After blowup - is the motor Schottky still OK?
If this ever fails open the FET is indeed subject to motor current and
inductive kick.
If it fails short the FET is switching a hard DC supply.
Either case is liable to be fatal.




regards


        Russell McMahon

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