From: firstname.lastname@example.org (carltons)
Subject: Re: hi-capacity voltage controlled capacitor?
References: <email@example.com> <firstname.lastname@example.org> <3DA72D96.9F9514DE@NOSPAMboeing.com>
User-Agent: NewsWatcher-X 2.2.3b2
NNTP-Posting-Date: Sun, 13 Oct 2002 16:58:54 GMT
Date: Sun, 13 Oct 2002 16:58:54 GMT
In article , John Woodgate
> I read in sci.electronics.design that carltons
> wrote (in om>) about 'hi-capacity voltage controlled capacitor?', on Sun, 13 Oct
> > I
> >gather that there is a problem with ESR in some caps. If this has not
> >been pointed out before now, there is a reason why the parameter is called
> Yes, it's because it attempts to model ALL the losses as if they were
> due to a single series resistor. A better model has a series resistor
> and a shunt resistor, and you can carry on making better and better
> models by adding more resistors.
> >Current, as in circuit current, does not flow through a cap. Is
> >this magic or a violation of Kirchoff's laws? No. Charge is exchanged
> >from one plate to the other of the cap, this current is called
> >displacement current. Since this is a real world there is a limit.
> Displacement current behaves in every way as far as the outside world is
> concerned as if it DID flow through the capacitor (put an ammeter in
> series to see that!). It seems unnecessary to stress that that isn't
> actually true. There isn't any feature of displacement current itself
> that gives rise to ESR.
> > It is
> >called ESR and is an apparent, mesureable and different parameter from the
> >circuit resistance outside the component.
> Part of it is a REAL series resistance. Part of it is due to a REAL
> shunt resistance, but you could say (if it helps, which I doubt) that
> the part of ESR which is actually due to shunt losses is not 'real'.
> > Rds, as in FET Rds, is real and
> >is part of the normal thinking of circuits and circuit current.
> So is ESR, although it, like many other simply-modelled loss
> resistances, varies with frequency.
I guess what you're saying is that there is no easy answer, right? What
then is the solution? I would say that you should build the circuit as
best you can with the simulation tools that you have, based on the best
models that you can obtain, and see if it works. If it falls short,
breadboard, test and then parallel some more caps and keep going. The
schedule won't stop because of physical laws and explanations. I would
recommend, if you care about the future, that you continue to find better
modelling techniques etc., but if time is not allowing this to happen
right now, then do it when you can before the next design bites you. I
hate the idea of throwing parts into the solution, but use as much science
and engineering as you can and then get the job done no matter how painful
the solution. The CEO won't know the difference one way or the other. :-)
P.S. John, note my last name. I didn't get it from Ireland. ;-)