September 1996
selves with the current rating and
little else. We are powering logic
circuits here, so a few millivolts
of ripple one way or the other is
of little consequence. Likewise,
the load on the supply is rela-
tively stable so regulation differ-
ences rarely matter.
Heat can be a killer, so if you are
using most of a supply's capacity
it needs to be well ventilated or
fan cooled. Otherwise you need
to derate a supply before install-
ing. Any supply should have
more capacity than you need, but
using a twenty amp supply where
you need three amps is just over-
kill. Try putting an ammeter in
the SV line to some of your logic
boards and measure the current
they actually use.
BASICALLY THE SAME
My discussion centers on a hy-
pothetical switcher reminiscent
of the popular and easily repaired
Peter Chou type. Under the skin
they are all similar.
In a game, it is not necessary to
use isolated power for a switcher,
since the outputs are all isolated
by design. However, on your
bench you should use isolated
AC for testing. If you power the
switcher directly from the AC
mains you CAN NOT connect
your 'scope ground to anything
in the primary circuit.
The primary side of the circuit
directly rectifies the AC line to
make a high DC voltage. This
HV supply is then switched on
and off at a high rate (50,000
times a second or more, under
CD
STAR*TECH
control of the regulator circuitry)
through the transformer so as to
keep the secondaries fully pow-
ered. The secondaries are simple
half wave and full wave rectifiers
feeding into choke input filters.
Journal
Monitor
Proble111s?
Order This Tech Pub
These are not sine waves. The 5
volt output is sampled by the con-
troller for regulation. There is no
direct connection between the
primary and secondary circuits.
Any signal that must get from one
side to the other is done through
transformers or optical couplers.
J\,IC:,N/TORS:
SYMPTOMS/ SOLUTIONS
Includes all S* TJ monitor
articles from 1990, 1991,
1992, 1993,1994, 1995
& 1996
CIRCUIT DESCRIPTION
From the AC line input: Fuse, RF
filter to keep noise off the AC
line, LV supply transformer (T3),
current limiting resistor (RI) to
save the semiconductors from the
inrush current surge at turn on,
bridge rectifier and filter caps to
make HV supply, transformer
primary driven by switching tran-
s is tor, darn per diode across
switching transistor. The switch-
ing transistor usually gets its base
drive via transformer. I lump the
control and drive circuitry all to-
gether. I won't discuss them here,
they rarely fail, and the technical
description is beyond the scope
of this article.
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Circuit description and simpli-
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