2011/5/26 Ruben J=F6nsson <ruben@pp.sbbs.se>

> > > Drive issues are a consideration too.  FETs are driven by a voltage,
> but
> > > for
> > > large FETs it needs to swing 12 to 15 volts between on and off, and
> that
> > > quickly changing voltage will see some capacitance.
> >
> >
> > What effect will the capacitance have?
>
> This matters when the FET is used in a switching application, where you
> want
> the FET to be either completely on or completely off as much of the time =
as
> possible. This is because when the FET is half on, much of the load volta=
ge
> appears across the FET instead of over the load, which causes a lot of
> power
> dissipation in the FET.
>
> So to prevent this you need to switch it on or off as fast as possible.
> This
> becomes harder to do if the gate driver (a PIC output pin for example) se=
es
> a
> lot of capacitance. You actually need to charge and discharge this
> capacitance
> each time the FET is switched on or of. Since the driver has limitations =
in
> how
> much current it can deliver (it has an internal resistance which is more
> than 0
> ohms) the switching on and off takes some time and during this time the F=
ET
> is
> somwhere inbetween fully on and fully of and it is dissipating power (its
> Drain
> Source resistance is changing from infinitely high to very low). The bigg=
er
> the
> capacitance is, the more current is needed to switch the FET on or off in
> the
> same time.
>
> This really only matters when the FET is coninously switched on and of at=
 a
> high frequency such as in PWM applications or switching power supplies.
> When
> switching relays on and of at a low frequency, the gate capacitance is of
> less
> importance.
>

Very well written, explains a lot! Thanks!
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.