Star Tech Journal

STAR*TECH Journal
•
January 1987 (13)
Power Supplies:
Theory of Operation, Part 2
By
output voltage would j1..1np up to about the
unregulated input voltage. At this point
the crowbar circuit would kick in and do
its thing.
James Beck
Greater Southern Distributing Co.
Atlanta, Georgia
If Ql2 opened up, it would shut off the
output because it v.uuld no longer be
pulling the base potential of Qll down. If
it shorted, it v.uuld have just the
opposite effect, and once again the
crowbar circuit takes over. An open or
shorted Qll would have the same effect as
an open or shorted Ql2.
I.AST ISSUE: The components and their
descriptive actions in the conventional +5
volt OC power supply circuit were
discussed.
PCWERING-UP OUR SUPPLY:
Well, let's take our power supply and give
it some juice. Referring to the schematic
shown below, let's assume it is in
operation with a load of 1 amp and an
unregulated input of 11.5 volts.
Let's j1..1np real c:uick to Dll and Rll. Dll
has about a 3 volt drop across it and Rll
is left to drop the other 2 volts. Using
Ohms Law (I=E/R), the 2 volts dropped
across Rll gives us an LED current or
approximately 11 milliamps.
Note: Keep in mind that resistors and
semiconductors vary so any voltages given
are approximations.
011
2N58'79
+11,5V DC
+5V DC
H.S.31
R12
100
1W
R13
12
2W
+
C12
C13
100UFI 0.1UFI
5V
1sv J_ sov.J_
-
-
~ C14
o.22uF
35V
TANT
U11 K
TL431 ___
R -----+------<11>----e
T0-92 A
Ull supplies about 2-1/2 volts to the base
of Ql2, which turns it on, and due to the
signal inversion that takes place at the
collector it "pulls down" the voltage on
the base of Qll. Since Qll is a PNP type,
the lower the potential on the base in
respect to the emitter -the harder it is
turned on (Qll's base voltage referenced
to ground works out to be approx. 10.7V).
Since a 1 amp load is small compared to
the board-set this supply was intended to
power, the series-pass transistor is
barely turned on.
.
012
1N4734
SCR
MCR 68-1
If Ull shorted, it would effectively
ground the base of Ql2, turning it off.
Which in turn would turn off Qll shutting
down the supply. On the other hand if it
opened up, Ql2 would get turned on hard
which v.uuld turn on Qll hard and the
I ~~a
+
RH
1
R11
180
1/4W
1 1011
MV5752
Now, we're going to take a look at the
overvoltage crowbar circuit that keeps
your boards from becoming a French Fry in
the event of regulator failure!
Let's now assume Qll has developed an
emitter-collector short and the
unregulated voltage is now going to the
output. Dl2 is a 5 volt zener so it is
going to drop 5 volts of the 11.5. This
leaves 6.5 volts to be dropped by Rl8.
This voltage is going to be applied
through Rl7 to the gate of the SCR. Rl7
and Cl4 form a RC timer that keeps the SCR
from firing during brief power spikes.
Since we are assuming a dead short in Qll,
the RC network tirres out the SCR fires and
poof the fuse blows saving the boards from
alrrost certain destruction.
I hope this has helped a few of you to
understand the basics of power supply
operation. I've been doing my best to stay
away from any heavy duty semiconductor
theory and still get the point across, so
if I've left something out and you have a
question, feel free to call. The nurrber is
404/352-3040.