Dave Platt wrote:
Dave Platt wrote:

In this particular application - pass transistors in a linear
regulator - I don't see that MOSFETs would have any particular
advantage.
. . .


Someone mentioned the 2 to 4 amps of base current you'd have to supply
for bipolar pass transistors.


Good point. The base current wouldn't be wasted (it'd flow into the
load), but the need for a fairly high-current driver would certainly
complicate the circuitry and require the use of a higher-voltage
transformer and unregulated supply, compared to a MOSFET solution.


You wouldn't need it if you used FETs. I'd
look seriously at P-channel VMOS for pass devices, myself.


Might be even more advantageous to use N-channel VMOS, and regulate on
the negative side of the load. N-channel power MOSFETs are
significantly less expensive than the closest P-channel equivalents,
and there seems to be quite a bit more choice in terms of size and
rating. If I recall properly, the charge-carrier mobility in an N
channel (electrons) is a good deal higher than in a P channel (holes),
which means that N-channel MOSFETs can have smaller channel areas than
equivalent P-channel devices, hence more dice per wafer, hence lower
prices.


I built a cheap ps using a bunch of 2n3055's as pass transistors. (think
I had 5 or 6
of them in parallel). Each had a 0.1 ohm resistor in the emitter lead
to balance
out the current (or one might try to hog it all if the transistors
aren't well
matched). They well darlington connected to another 2n3055 which acted
as a driver (and helps increase the gain at high collector current).
Yet another
transistor was used to drive the driver (TO-5 si, probably a 2n3053 or
equal)
from the regulator IC. A large heat sink was used and I mounted surplus
computer power supply fans on it to add cooling.

I used a heavy transformer and the input voltage to the pass regulator was
about 15-16 volts for 13.6 out. Key here is that with a large
transformer and
enough filter C the voltage won't droop on high current (guess that means
it's well regulated without the regulator!) , and we don't ask the
transistors to disapate too much power.

Course' such a supply is HEAVY, a switcher is much lighter, but a good
switcher with low noise is hard to come by! (Not to mention problems
of rf feed back!).