[sci.electronics] Power Supply question

gt7200a@prism.gatech.EDU (CHENG,ROBERT CHUANLUN) (02/27/91)

I'm trying to build a 12 volt DC power supply with high current
capability (10 amps +), but I want to keep the circuit simple
by using regulators.  Here's the question:  Can regulators be
wired in parallel to increase current handling if identical
regulators are used?

Thanks in advance,

tomb@hplsla.HP.COM (Tom Bruhns) (03/01/91)

RE: Wiring regulators in parallel

Because each regulator would have a slightly different voltage it was
trying to regulate to, this is generally not such a hot (or maybe make
that too hot) idea.  If you look in a National Semiconductor linear
applications book or voltage regulator book, you will find some
suggested ways to increase the current output.  One way, for example,
is to put a resistance in series with the input to the regulator IC.
Assuming a positive regulator, you then connect a PNP transistor
emitter to power source, base to resistor-regulator junction, and
collector to regulator output.  Select the resistor so that the
regulator is drawing, say, 3/4 of it's rated current to provide a
drop across the resistor that will get the PNP turned on.  This is a
pretty simple circuit, and deserves a bit more consideration for
protection of the parts, but this should give you the idea of what's
going on.

Simplicity has its price:  for every volt you drop in the regulator,
at 10 amps you will dissipate 10 watts.  Are you prepared to get rid of
50 watts?  You better be!  An alternative is a switching regulator...
more complicated, but you are getting into the power range where it's
worth considering.  

BTW, if you use a linear regulator IC, you may well want to go with a
fairly high-current one, like an LM350K (3 amp), LM338K (5 amp), or
LM396K (10 amp) (but at $31 in DigiKey's catalog).

myers@hpfcdj.HP.COM (Bob Myers) (03/02/91)

>by using regulators.  Here's the question:  Can regulators be
>wired in parallel to increase current handling if identical
>regulators are used?

A much better way to acheive higher current capacity is to use a single
regulator to set the base voltage for a pass transistor which can handle the
required current.  See my previous posting on the Zener-diode-based regulator
for a brief description of how a pass transistor is used; the regulator 
IC basically takes the place of the Zener, and so must be set up so that
its output voltage is greater than the desired output by the BE drop (about
0.6 to 0.7V; twice this if you're using a Darlington configuration for the
pass transistor).  Therfore, you'll want an adjustable, instead of a "fixed,"
regulator.  (And note that now you can use a regulator with a much lower
current rating!)

A good source of information on application of regulator ICs - this included -
is National's _Voltage_Regulator_Handbook_.  Try the library of the local
engineering school.


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers@fc.hp.com                          |  sentient life-form on this planet.