V G wrote:
> Then in which case would you suggest a FET over a BJT?

For power applications, the two main things to consider are power
dissipation and how the part needs to be driven.  FETs look more like a
resistor when on, and bipolars more like a voltage source.  A FET with
really low Rdson can often have a lower voltage drop accross it, and
therefore lower power dissipation, at the design current.  This becomes les=
s
of a advantage as the current goes up.

Drive issues are a consideration too.  FETs are driven by a voltage, but fo=
r
large FETs it needs to swing 12 to 15 volts between on and off, and that
quickly changing voltage will see some capacitance.  This is not something =
a
microcontroller can do directly.  This is why there are dedicated chips jus=
t
to drive FETs.  They take logic levels in and put out the gate voltage.
They are optimized for a short but high burst of current when switching to
overcome the effective gate capacitance of the FET.  High end FET drivers
are rated for over 1A during switching.  "Logic level" FETs are designed so
that they turn on pretty well with only 3.3V or 5V base drive.  The
IRLML2502 and IRLML0030 are in this catagory.  However, these FETs are
limited to relatively low voltages.  The two I mentioned above are rated fo=
r
20V and 30V, respectively.  These are desirable characteristics, so also
cost a premium.

Bipolars are driven with current, not voltage.  That means that a micro
running at 5V or 3.3V supply can drive a bipolar gate, up to some current
limit.  You can switch 100V from a micro easily with just a NPN transistor
and a base resistor.  Drawbacks here are that the base current needs to be
on continuously to keep the transistor on, and that the output current is
limited by the gain.  Let's say the 100V NPN can be counted on to have a
gain of 50.  That means with 10mA base drive is can only switch 500mA.  If
that's all you need then that's not bad.  If you need 5A, then it will take
extra circuitry to provide the larger base current.

Bipolar transistors are also generally cheaper than than FETs with similar
voltage and current capabilities.  Sometimes that's a important
consideration.  They also tend to be a little more robust against the kinds
of transients and the like things sometimes get exposed to despite best
intentions.


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