(located
on
the instrument rear panel). Power Switch S350
applies the selected line voltage to power supply rectifier
CR1011.
With the selector switch
in
the
115
V position, the
rectifier
and
storage capacitors
C1021
and
C1022 operate
as
a full-wave voltage doubler.
When
operating
in
this
configuration,
each
capacitor
is
charged
on
opposite half
cycles of the ac input,
and
the voltages across the two
capacitors
in
series will approximate the peak-to-peak
value of the source voltage. For 230 V operation, switch
S90 connects the rectifier as a conventional bridge
rectifier. Both capacitors charge
on
both input half cycles,
and
the voltage across
C1021
and
C1022
in
series will
approximate the
peak
value of the rectified source voltage.
For either configuration, the
de
voltage supplied to the
power supply inverter is the
same.
Thermistors
RT101
0
and
RT1016 limit the surge
current when the power supply
is
first turned
on.
As
current flow warms the thermistors, their resistances
decrease
and
have little effect
on
circuit operation.
Spark-gap electrodes
E1001
and E1002 are surge-voltage
protectors. If excessive source voltage
is
applied to the
instrument, the spark-gaps conduct,
and
the extra current
flow quickly exceeds the rating of fuse
F90.
The fuse then
opens to protect the instrument's power supply.
The
EMI
(electromagnetic interference) filter, inductors L
1011
and
L1012, capacitors C1016
and
C1018,
and
resistors R1011,
R1012, R1016
and
R1018 form a line-filter circuit. This
filter, along with common mode rejection transformer
T1020, prevents power-line interference from entering the
instrument
and
prevents power supply switching signals
from entering the supply
line.
Preregulator Control
The Preregulator Control circuit monitors the drive volt-
age
applied to inverter output transformer T1060
and
holds it at the
level
that produces proper supply voltages
at the secondary windings.
The Preregulator Control circuit consists primarily of
control
IC
U1030, its switching buffers,
and
its power sup-
ply components.
The
control
IC
senses voltage
on
the pri-
mary winding of T2060
and
varies the "on time" of a
series-switching transistor, depending
on
whether the
sensed voltage was too
high
or too low.
The
switching
transistor 01050, rectifier CR1050, choke T1050,
and
capacitor C1050 form a buck-switching regulator circuit.
The
output voltage at W1060 is proportional to the pro-
duct of the rectified line voltage
on
C1020-C1022
and
the
duty cycle of 01050.
In
normal operation, 01050 is
on
about one-half the time.
When
01050
is
off, current flows
to
W1060
and
T1060 through CR1050.
Theory of Operation-2445A/2455A Service
PREREGULATOR CONTROL POWER SUPPLY. Since
the Preregulator Control network controls supply startup
and
preregulates the secondary supplies,
an
independent
power source must
be
established for it before
any
of the
other power supplies will operate.
The
independent power
supply for the control circuitry
is
composed of 01021,
01022,
and
associated components.
Initially, when instrument power
is
applied, the positive
plate of capacitor C1025 is charged toward the positive
rectified line voltage through R1020.
The
voltage at the
base of 01022 follows at a
level
determined
by
the voltage
divider composed of R1022, R1024, CR1023,
and
the load
within U1030.
When
the voltage across C1025 reaches
about +21
V,
the
base
voltage
of
01022 reaches
+6.8
V
and
01022 turns
on,
saturating 01021. The +21 v
on
the emitter of 01021 appears at its collector
and
estab-
lishes the positive voltage supply for the Preregulator
IC
.
With 01021
on,
R1024
is placed
in
parallel with R1022,
and
both 01022
and
01021 remain saturated.
The
+
21
V
level
begins
to
drain down as the control
IC
draws current from C1025. If the Preregulator Control
IC
doesn't start the switching supply
(and
thus recharge
C1025
and
C1023
via
CR1022)
by
the time the voltage
across C1025 reaches about
+8
V,
01021 will turn off.
Resistor R1024 pulls the base of 01022 low
and
turns
that transistor off also. (Capacitor C1025 would only
discharge low enough to turn off the transistors under a
fault condition.)
In
this event, C1025 would then charge
again to +
21
V,
and
the start sequence would repeat.
Normally, the control
IC
will start Inverter action before the
+ 8 V
level
is
reached,
and
current
is
drawn through
T1050
via
01050. This induces a current
in
the secondary
winding of T1050
via
01050. This induces a current
in
the
secondary winding of T1050 and charges
C1
025
positive
via
diode CR1022.
The
turns ratio of T1050 sets the
secondary voltage at approximately +
15
V;
and,
as
long
as the supply
is
being
properly regulated, C1025 will be
charged up to that
level
and
held
there.
PREREGULATOR START-UP. As the supply for the
Preregulator Control
IC
is
established,
an
internal
switching oscillator begins to
run.
The
oscillator generates
a repetitive triangular wave (as shown
in
Figure 3-10) at a
frequency determined primarily by R1032
and
C1032.
The
simplified schematic of Figure
3-11
illustrates the voltage
control functions of U1030.
As the Preregulator power supply turns
on,
capacitor
C1034 charges from the + 5 V reference
level
toward
ground potential through R1034
and
R1037. As it does,
the voltage at pin 4 (one input of Dead-Time Comparator
U1)
will pass through the positive-peak value of the tri-
angular waveform
on
the other input of the Dead-Time
3-39
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