PREREGULATION.
Once
the initial charging
at
power-
up
is
accomplished,
as
just described, the voltage-sensing
circuitry begins controlling the Inverter switching action.
The
actual voltage sensing is done
by
error amplifier
U2.
The
level
at the center tap of output transformer
T1
060 is
applied to
pin
1
and
is
compared to the reference
established
by
R1045
and
R1046
at
pin
2.
If
the
sensed
level
at
pin
1 is lower than the reference level
(as
it will
always
be
for the first few switching cycles), the of error-
amplifier
U2
will
be
LO.
The
LO,
applied to the inverting
input of
U3,
results
in
a long-duty-eycle drive signal to
transistor 01050
(via
CR1030).
Since
the Inverter Drive
stage will alternately turn either 01060 or 01070
on,
relatively large current pulses will result
in
the primary
winding of inverter output transformer
T1060.
1 0 .
1V
FB
Theory of
Operation-2445A/2455A
Service
These
large current pulses, over the period of a few
cycles, will increase the charge
on
the storage capacitors
on
the secondary side of the transformer
and
will reduce
the current demand
on
the inverter output transformer.
As
the
demand
increases, the voltage across the primary
winding will increase until it reaches the point where the
two inputs of
U2
are
at the same potential. At this point,
the output of
U2
(to
U3)
will settle to a
level
approximately
equal to the midpoint of the triangular waveform applied to
the other input of
U3
.
The
resulting drive signal
has
an
approximate 50% duty cycle
and
will respond to changes
in
either the
ac
line
voltage or supply load conditions.
Depending
on
the output levels sensed, the duty cycle of
the drive signal
will
change (sensed
level
rises or falls with
respect to the triangular waveform) to
hold
the secondary
supplies at their proper levels.
8
I
01060
SWITCH
SIGNALS
___
11060
Figure 3-11. Simplified schematic of control
3-41
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