STAR*TECH Journal
Inside Switching Regulators (Part 2)
By Jarres Beck
Greater Southern Dist. Co.
Atlanta, Georgia
OK, there it is. The schematic shows the
main power handling circuit of our
switching regulator. I didn't include a
fuse on one of the AC inputs, so we will
just assume there is an external fuse in
place. The two AC terminals represent the
115V AC wall voltage input. Cl, C2, C3,
C4, and Ll compromise a noise filter. The
pur:rose is not so much to keep noise out,
as it is to keep it in.
BRl and CS form a very simple unregulated
DC supply. BRl full wave rectifies the
115V (rrns), and CS filters it to 163V DC.
TRl and TR2 are the switching transistors.
'Ihey alternately turn on and off to pulse
the 163V DC through Tl. Dl and D2 are used
to protect their associated transistors
from damage caused by collapsing field
spikes. Tl is used to take the high
voltage, high frequency pulses induced in
its primary, by the switching transistors,
and convert it to a low voltage, high
frequency AC source for our output.
'Ihat just about covers what the parts are
suppose to do, what about when they fail?
April 1987 (10]
Failure lt>des
If Cl or C4 shorts out (this is a very
co:rranon occurance) it would effectively
connect the two AC lines together.
Needless to say this would cause the fuse
to blow.
Hare comes the good one, if C2 or C3
shorts, and you don't have the case
grounded, you could end up with a supply
with a "hot" case. A nice surprise for you
when the +5 needs to be adjusted!
If BRl were to short it would cause the
fuse to go. On the other hand if it
opened, there would be no voltage for CS
to filter, and no output voltage.
If CS shorted, once again, the fuse would
blow. This assumes that the surge rating
of the bridge is higher than the fuse
protecting it. If not the BRl may also
fail.
Let's suppose CS became "leaky". The AC
ripple on the unregulated supply would
increase. This would reduce the effective
DC value of the supply, which in turn
would cause the regulator (not yet shown)
to increase the switching rate of the
transistors. The increase in the switching
rate would cause an increase in the
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MAIM P~W£R.. 1-/AAJDLIN.6 C.K.1
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