BCCs - mainly for low Vcesat "trick" at end.

> > One down-side to this approach is that you won't get all the way to 5V
> > and 0V - the best you can do is one diode drop away from each rail.

Fine enough for almost all FETs.
As long as Vgsth >> 1 Vbe.

You can use push pull drive with the emitters on the rails if you want
true rail to rail response (less a much smaller Vce_sat*) , with more
effort then required to prevent shoot through.

> True. I guess that's good enough for FET gates. It does drive up the
> dissipation, though.

Dissipation is very largely only when load takes the current offered.
I use BC807-40 and BC817-40 with superb  Beta.

> On FET drivers, I'm not seeing any ratings on
> continuous output current. I'm mostly seeing peak current ratings.

FET drivers are rated that way because that's what matters for a FE
gate (but, you know that).
Most will probably carry near their max rating the rest of the time as well=
..
Sensible thermal rating is probably the key.
It  may be that some use special drive techniques to hit the gate hard
for a limited time only but I'd expect most to be relatively dumb.

An issue with any such driver is shoot through.
I use this with an MV34063 (again) at not much over 100 kHz and
transition seems to be fast enough to have minimal shoot through
issues.
The MC34063 has single sided (high side only) drive and I use a 1K
pull down at about 3-4V drive and it works well [tm].

In that case the MOSFET has suitable low Vgsth to allow full on drive
but not so low that the Vbe to ground matters at turnoff. [A beautiful
little Taiwanese CES2310 SOT23 "Golidlock"s FET :-) ].




        Russell McMahon
       Applied Technology ltd
       New Zealand


* Interest only:

If you want very very very low Vcesat for some switching purpose and
efficiency is not important then unusually high forced betas can
decrease Vcesat to much lower than usual levels.
You might use this for eg switching a voltage reference )(that was not
too too crucial).

I recall one design (but can't as yet recall what it was
for)(Taiwanese exercise bike?)  that I used a forced beta of about 0.1
:-) !!!! (ie Ib =3D 10 x Ic!!!) to get a superbly low Vcesat where his
mattered.
--=20
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