Driving Small FETs

Driving a small FET directly from a micro port is a fairly common requirement, however minor errors here can lead apparently inexplicable device failures.

FET No gate resistor

What could be wrong with that?  A FET is a voltage driven device so the micro port will have no problems.  Well there are several problems here.  During reset the micro ports are set to high impedance, at which point the voltage level on the FET gate is undefined, a word that should strike fear into the heart of any designer.  Well who cares you might think, the micro is not is reset for long, that is unless the under voltage reset is active.  If the micro has a brown out detector or an external under voltage reset then when the supply is low it will be held in reset for a long time.  If the gate can float about the FET might enter the linear region and expire from over-dissipation.  Small FETs can sink several amps when saturated but may only be able to dissipate a few hundred milliwatts.

Talking of undefined, when driving a filiament lamp the turn on current can be 10 or more times the running current.  Make sure that the FET does not fail due to this transient dissipation. If the track from the gate to the micro is long then the FET might decide that it wants to oscillate, perhaps just on the transistions, but if you are unlucky it might scream away all the time that the load is on.  This can lead to the device failing and generates a depressing amount of noise. Bear in mind that the turn on voltage for some FETs may be lower than the guaranteed "0" volts from the micro port. There is no protection against transients on the power rail, or those that might be caused by the fast switching of the FET and the inductance of the track to the lamp.

Notice that when the FET is off there is a nice capacitive coupling between the power rail and the gate.  Transients there are rather generously coupled straight into the micro port.

It doesn't take much to fix all these problems.

FET with gate snubber

The series gate resistor spoils the Q on any inductance present on the gate preventing oscillation and limiting transient currents into the micro port.  Put it right at the FET gate.  The parallel resistor defines the FET as off when the micro port is high impedance and attenuates the drive from the micro so that a "0" means the FET is really off.  Finally our old standby the zener lops off any nasty transients that might zap the FET.  Bear in mind that the input resistors will slow down the rise and fall times of the gate as Mr Miller and his capacitance to come into play.  Make sure that you do not over-dissipate the FET by making this too slow.  Finally, ensure that the FET is large enough to stand the surge current of switching on the filiament lamp, yes FETs have a safe operating area as well as bipolars.