> > Drive issues are a consideration too.  FETs are driven by a voltage, bu=
t
> > for
> > large FETs it needs to swing 12 to 15 volts between on and off, and tha=
t
> > quickly changing voltage will see some capacitance.
>=20
>=20
> What effect will the capacitance have?

This matters when the FET is used in a switching application, where you wan=
t=20
the FET to be either completely on or completely off as much of the time as=
=20
possible. This is because when the FET is half on, much of the load voltage=
=20
appears across the FET instead of over the load, which causes a lot of powe=
r=20
dissipation in the FET.

So to prevent this you need to switch it on or off as fast as possible. Thi=
s=20
becomes harder to do if the gate driver (a PIC output pin for example) sees=
 a=20
lot of capacitance. You actually need to charge and discharge this capacita=
nce=20
each time the FET is switched on or of. Since the driver has limitations in=
 how=20
much current it can deliver (it has an internal resistance which is more th=
an 0=20
ohms) the switching on and off takes some time and during this time the FET=
 is=20
somwhere inbetween fully on and fully of and it is dissipating power (its D=
rain=20
Source resistance is changing from infinitely high to very low). The bigger=
 the=20
capacitance is, the more current is needed to switch the FET on or off in t=
he=20
same time.

This really only matters when the FET is coninously switched on and of at a=
=20
high frequency such as in PWM applications or switching power supplies. Whe=
n=20
switching relays on and of at a low frequency, the gate capacitance is of l=
ess=20
importance.

/Ruben



> --=20
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>=20

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
Ruben J=F6nsson
AB Liros Electronic
Box 9124, 200 39 Malm=F6, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D

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