> This is why there are dedicated chips just
> to drive FETs. =A0They take logic levels in and put out the gate voltage.
> They are optimized for a short but high burst of current when switching t=
o
> overcome the effective gate capacitance of the FET. =A0High end FET drive=
rs
> are rated for over 1A during switching.

A FET gate driver good enough for many applications where voltage is
OK but current is lacking can be made with 2 jelly-bean bipolars and
little  else. (Any 'glue' parts needed would be needed regardless of
driver used)

A better driver that also allows whatever voltage drive is required
can be made with 2 bipolars and a little extra glue and a better again
version with 3 transistors plus glue.
When you get to this sort of drive requirement a bipolar will probably
also need a driver if using a uP but a different circuit.

Circuits available if wanted - information access and time both
unavailable at this moment.

Simplest FET current driver.

PNP =3D P  Pe =3D emitter etc
NPN =3D N

Pc - V+
Pe - Ne
Pb - Nb
PIC drive to Pb + Nb
Drive output from Pe + Ne.
Rseries series R from drive output to FET gate to limit current.
Reverse Zener -  FET gate to ground to limit Miller effect gate
spiking. Not usually optional*.
Optional reverse biased Schottky FET gate to ground mounted as close
to FET as possible to kill parasitic gate ringing.

Commoned-emitter, commoned-base arrangement (rare) tends to stop shoot
through which appears likely at 1st glance.

BC327 / BC337 or SMD equivalent pair good for 1A gate drive provided
adequate Beta jellybeans used.
Using highest Beta BC337 (BC337-400) of 250 guaranteed needs ~4 mA
base drive for 1A FET drive.
Rseries adjusted to suit. Can be SC / 0R but provides super fast gate
drive and more EMI and switching losses.
Actual drive power low as gate current on;y drawn to charge add
discharge FET gate capacitance.
Power ~~~~~=3D 2 x 0.5 x C_gate_FET x Vgate^2 x frequency =3D low
(0.5 CV^2 with two edges per cycle).

This stunningly simple circuit works very well. There is drive voltage
loss due to emitter follower drive so only FETs with Vgs-drive well
inside Vcc can be driven.

Adding 1 more bipolar will allow any desired gate drive voltage.
Anon.



           Russell

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