Re: gEDA-user: basic anti-EMI design q

["Phil Taylor" <phil@xxxxxxxxxxxxxxxxxxxxxxxx>]

DJ Delorie <dj@xxxxxxxxxxx> wrote:

> > A fet gate is only high impedance at DC. All that gate-drain capacitance
> > matters.
> 
> Oh crap, you're right.  Still, I was planning on a 100k or more
> resistor in series with the gates.  Sufficient?  Or plan on an opto?
> 
> Actually, the drive FETS have both a resistor and a zener in series,
> so that a floating I/O will drive neither FET.  It's a funny circuit,
> and I probably should work on that first and post it.  And the I/O
> line has a pair of inductors on it (LCL filter), probably a 12MHz low
> pass filter.

Zeners in gate circuits are not as foolproof at higher frequencies as they
seem to be at DC.  They have capacitance that makes them turn on slow (perhaps
not fast enough to prevent your fet gates from blowing up, if that's their
role in your circuit) and they're also very odd devices until you get up to
enough current to hit the zener voltage.  Are you sure you're circuit does
this.

As for the 100k, this value is kinda big in a gate circuit to my ears.    100k
seems like a far too high a drive impedance, as it is likely way bigger than
the zout of whatever is driving the gate.  100 kohm driving a nominal 15pf
input fet, ignoring miller effect and other internal capacitances, will roll
off above 130khz.

Then if there is much distance between your 100k and the gate you have an
antenna amplifying broadband in the middle of the switching transition (noise)
because the gate circuit is not driven hard by anything.  If you falsely
transistion the gate drive from low to high, say with a generator running
slower than your pulses on that line (and so it'll trigger),  there will be
noise (which gets amplified by your fet) during the transition.  You probably
won't need a motor running right next to it either to see it, and it may only
show up when you're not looking.

If you have fast logic following this fet, it will gladly switch as rapidly as
the noise falsely moves your fet above and below threshold during these noisy
transitions.

best, pt