Theory of Operation-2445A/2455A Service
Opto-isolator U1040
and
resistor
R1044
form a control
network that allows a voltage
sensed
at the feedback
input
(FB)
to slightly alter the voltage-sense reference
applied
to
pin
2 of
U2.
The
FB
signal
is
generated
by
the
+5
V Inverter
Feedback
amplifier (U1371, diagram
10)
and
is
directly related to the
level
of the
+5VD
supply
line.
Base drive to the shunt transistor
(in
opto-isolator U1040)
is increased should the
FB
signal go below its nominal
value.
Additional current
is
shunted around
R1045
(via
R1044)
and
raises the voltage-sense reference
level
to
error-amplifier
U2.
This increases the voltage applied to
the primary winding of the output transformer,
since
U2
sensing depends
on
a balanced condition. Higher currents
are
induced
in
the secondary windings,
and
the secondary
voltages
begin
to return to their nominal values. As the
+5VD
line returns
to
its nominal level, base drive to the
shunt transistor
will
be
reduced
and
the voltage
in
the pri-
mary winding
will
follow. Should the
FB
signal
level
tend
too
high,
opposite control responses occur. Further infor-
mation about the
FB
signal
is
given
in
the
+5
V Inverter
Feedback
description.
Error amplifier
U4
and
the voltage divider composed of
R1035
and
R1031
provide a backup sensing circuit. Its
operation
is
similar to that of error amplifier
U2
, just
described, but it senses at a slightly higher
level.
As
long
as
U2
is
operating properly,
U4
will
be
inactive. However,
should a failure occur
in
the
U2
sensing circuitry,
the
volt-
age
on
the primary winding of T1060
will
rise
to the
sens
-
ing
level
at
pin
15
of
U4.
Sense
amplifier
U4
will
then
take
over, preventing a damaging over-voltage condition.
Inverter Drive
The Inverter Drive circuit performs the necessary
switching to drive the inverter output transformer. Like the
Preregulator Control
IC,
the Inverter Drive circuit requires
an
independent power supply, since it must
be
operational
before
any
of the secondary supply voltages
can
be
gen-
erated.
INVERTER DRIVE POWER SUPPLY. This power supply
consists of 01062, VR1062,
and
their associated
components.
As
power
is
first applied, the initial charging
current through T1050 induces a current
in
the
transformer secondary winding (pins 8
and
9)
. The
alternating current
is
rectified
by
the diode bridge
composed of CR1062, CR1063, CR1064,
and
CR1065
and
stored
in
C1066, providing power for the Inverter Drive
circuitry.
When
the Preregulator Control
IC
turns switching
transistor 01050
on
for the first time,
the
charge stored
on
C1066 during the initial charging period
is
sufficient to
properly tum
on
one
of the current-switching transistors
(either 01060 or 01070) for the first cycle. After that, the
alternating drive signals continue to induce current into the
3-42
secondary winding of T1050 to provide operating power
as
long
as
the instrument
is
turned on.
The current rectified
by
the diode bridge
and
stored
on
capacitor C1066
is
regulated down to the required voltage
level
by
R1061, VR1062,
and
01062. Zener diode
VR
1062 references emitter-follower 01062
and
holds the
supply output at approximately + 11.4 V.
INVERTER DRIVE GENERATOR.
The
Inverter Drive
generator consists of U1062, U1064, U1066, switching
transistors 01060, 01070
and
their associated
components.
The
circuitry alternately switches current
through
each
leg
of the output transformer (T1060)
primary winding
and
produces the
ac
current required for
transformer action.
Out-of-phase input signals to comparator U1062C
come
from two resistive voltage dividers place
in
either
leg
of
one secondary winding of T1050. The comparator detects
the phase changes (crossover points) of the secondary
current
caused
as
01050 switches
on
and
off. Every com-
plete on-off cycle of 01050 produces a positive clock at
pin
14 of U1062C that toggles flip-flop U1064B.
The
tog-
gling alternately turns switching transistors 01060
and
01070
on
,
each
with
an
approximate 50% duty cycle.
Comparators U1062A
and
U1062B,
at
the Q
and
Q
output of the flip-flop, detect the precise crossing point of
the toggling drive signals
and
ensure that only one switch-
ing
transistor will
be
on
at
any
one
time. These mutually-
exclusive drive signals are buffered
by
inverters U 1 066A
and
U1066B
and
applied to switching transistors 01060
and
01070 to alternately turn
them
on
and
off at one-half
the switching rate of 01050. By alternately switching
opposite ends of the primary winding to ground, the
current flowing through switching transistor 01050 will
flow alternately
in
each
half of the primary winding. This
produces
ac
voltages at the secondary windings that
are
then rectified, providing the various unregulated
de
supply
voltages.
Current
Limit
The Current Limit circuit, composed of transistor 01040
and
the associated components, limits the maximum
current flow
in
the output transformer to about 1
ampere.
Resistor
R1040
(connected to the Preregulator Control
IC
+ 15 V supply) forward biases germanium diode CR1040
and
applies approximately
+0.3
V across the base-to-
emitter junction of 01040. Current flowing to the output
transformer develops a voltage drop across R 1 050 that
adds to the bias developed
by
CR1040.
As
the current to
the transformer increases, the voltage drop across
R1050
also increases until, at around 1 A, the combined voltage
drop across R1050
and
CR1040 forward biases transistor
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