Re: Solid-state status

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "R.E.Burnett by way of Terry Fritz <twftesla-at-uswest-dot-net>" <R.E.Burnett-at-newcastle.ac.uk>


Hi Ken,

There are a couple of things that you can do to minimise the
"Miller" effect (caused by MOSFET Gate-Drain capacitance.)

1. Reduce the impedance of the gate drive circuit.  This involves 
minimising stray resistance and inductance in the entire gate-source loop.
You could use a twisted pair if it is not practical to locate the driver
stage close to the MOSFET gate terminals.  Keep PCB traces like 1/2" or
shorter in length,  and only add enough resistance to damp ringing.

2. Apply a small negative bias to the gate with respect to the source of
the device that is intended to remain off.  If the gate is biased to -10V
when off then the Miller effect must cause almost 13volts rise in the
gate potential in order to risk spurious turn-on.

( The negative gate bias method is a very effective "Miller killer" but it
has one disadvantage.  Turn-on delay is increased because the gate now
has to be charged to the threshold voltage from -10volts or so ! )

I am not sure of your circuit topology (it is not published anywhere ?)
so I cannot really suggest any other tricks.  But,  I hope this info is of
some help.
							Cheers,

							-Richie,

> Original poster: "Kennan C Herrick by way of Terry Fritz
<twftesla-at-uswest-dot-net>"
> <kcha1-at-juno-dot-com>
> 
> Holiday madness being upon us, my T.c. work has slowed.  But some may
> have interest in the current problem:  Turns out my gate-drive lines are
> too long.  Some ringing but the more serious problem is Miller current at
> MOSFET turn-off, causing turning on again of those that are supposed to
> be going off, by virtue of the voltage drop in the too-long lines.  Such
> turn-on is a major no-no, of course, so I am having to re-do my
> gate-driving scheme.  It will take some semi-major scrambling-around of
> circuits, cutting of p.c.-board traces, adding jumpers, etc.  Dog-work,
> like much of the previous toil leading up to the present design.
> 
> But basically, my "current-loop" scheme works and--as soon as we clear
> this season's frenetic activities, I'll get to that dog work & advise.
> No offense to dogs.
> 
> KCH
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