Re: A prospective MOSFET driver

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

Original poster: "David Sharpe by way of Terry Fritz <twftesla-at-qwest-dot-net>" <sccr4us-at-erols-dot-com>

Hi Malcolm, Ken, Ed et al

Might be an interesting experiment, measure knee voltage at
some reverse current and the forward bias at say 50% of rated
current at 100% power rating.  Play this game through
10, 50 100 500 1k current reversals and see if there is a knee
voltage shift, or sensitivity to temperature.

When I first started in industry (1974) fresh out of school
the first thing the "dogs" threw me on was helping a factory rep
perform factory field mods on a room full (12 or 14 if memory
serves me correct) SCR based second generation variable
frequency drives.  Auxiliary commutation circuits, switching
reactors, 2000A 250VDC bus bars (SC of ~35kA) rubber mats
everywhere, the works.  A sink, swim or die (literally!!)
introduction to Power Electronics.  One of the first things
the field tech did is replace ALL of the zener references in
the drives, voltage shifts with temperature could cause all kinds
of excitement
(like DC shoot through faults...rrrrrrrrrrrrrKABAAAMM......
%%##$%% !!!... its dam... DARK in here; where is the
flashlight???   :^C  )
That's where I remember the comment about forward biasing
affecting the zener diode knee voltage.  These were not chump
drives either, the smallest one was 50kVA, and there
were two master/slave independent units of 200kVA ea.,
the rest were 100kVA units.

Processing is definitely a lot better now then 28 years ago, maybe
not an issue but worth a relatively simple experiment outlined above
to see.  Generally with my designs, I don't use a zener in the gate
drive circuits at all, except as a bi-directional clamp to protect
GS from exceeding +/- Vgs ratings.  Designers / Engineers
preference.

Regards
Dave Sharpe, TCBOR
Chesterfield, VA. USA

Tesla list wrote:

> Original poster: "Malcolm Watts by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>
>
> Hi Dave,
>
> On 6 Aug 2002, at 22:04, Tesla list wrote:
>
> > Original poster: "David Sharpe by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <sccr4us-at-erols-dot-com>
> >
> > Ken
> > I see what you're trying to accomplish.  My only comment is
> > when you are biasing Q2 on via the pulse transformer via D1
> > with a positive polarity (and of course biasing Q1 off at the
> > same time).
> >
> > I've always been told to NEVER forward bias a zener diode,
> > you will hose its knee characteristics.  Comments from the
> > list???
>
> Interesting. I haven't heard that before (gulp!). I haven't seen
> degradation in zeners I've forward biased but the current was pretty
> modest and zeners are not noted for their current handling
> capability. In fact, I think you've hit on a severe drawback to the
> circuit right there! I never thought about it. The peak gate current
> could be in the Amps range.
>
> Regards,
> malcolm