Theory of Operation—2246 1Y and 2246 Mod A Service
restart after about half a second, but will shut down
again if the overvoltage condition continues (this is
the "c h irp ” mode).
PREREGULATOR OUTPUT CONTROL.
The voltage
across the Inverter current source transistor, FET
Q2214, is monitored by Q2208 (from the collector
voltage of either Q2209 or Q2210). That voltage has
to be maintained at the proper level to provide
enough regulation room for the secondary supply
voltages and still not dissipate more power than
necessary in Q2214. If the voltage across Q2214 is
too high, Q2209 is biased on harder and draws more
current from the input summing node (pin 2 of
U2201) of the voltage error amplifier in U2201, the
Preregulator Control 1C. The output of the error
amplifier at pin 3 of U2201 then rises, and the width
of the switching pulse to the Preregulator Switching
circuit narrows to decrease the +44 V output.
The Inverter Control circuit (Q2212 and Q2213)
senses the decreased voltage across the primary of
the
Inverter
power
transform er
(T2204)
and
responds by driving Q2214, the Inverter current-
source transistor, harder; thereby decreasing the
voltage across it.
Control response time in the feedback loop just
described is long; but it does not need a fast
response time, since the circuit only determines the
power dissipation in Q2214. Compensation of the
circuit to prevent oscillation is done by a low-pass
filter (10 Hz cutoff)
formed
by C2238, R2205, and
R2246.
Preregulator Switching Circuit
The Preregulator Switching circuit provides the
energy required to keep C2203 charged up to
+44 V. Switching FET Q2201 is driven by the pulse-
width modulated output of the Preregulator 1C
(U2201)
via
a
buffer
amplifier
circuit.
The
Preregulator 1C controls the on-tim e to maintain the
voltage across C2203 at +44 V.
For the following discussion of the switching circuit,
assume that Q2201 is off, C2201 is charged to the
rectified line voltage (160 V from the Primary Power
Rectifier), and the +44 V supply is up and driving a
circuit load.
When the Preregulator 1C turns on Q2201, the drain
of Q2201 is immediately clamped to 44 V. This
forces 116 V (160 V - 44 V) across pins 6 and 7 of
T2203. Current begins increasing linearly in that coil
as Q2201 supplies current to the +44 V supply. With
the one end of C2201 clamped to +44 V, and C2201
being charged to +160 V, the other end of C2201 is
pushed down with the anode of CR2201 going to
-116 V (44 V - 160 V). This places 116 V (0 -
116 V) across pins 1 and 2 of T2203 and current
begins increasing linearly in that coil, also flowing
through Q2201 to the +44 V supply. After a time
determined by Preregulator 1C U2201, the drive
signal to Q2201 is switched low, and the switching
FET is turned off.
The current flowing in both coils of T2203 must con
tinue as the magnetic field collapses, but it cannot
flow through Q2201. The only available path is
through CR2201 (previously biased o ff). The polarity
reversal of the voltage across T2203 that occurs for
ward biases CR2201, and permits the energy in the
magnetic field to be released to the +44 V supply.
When CR2201 is forward biased its cathode is
clamped to the +44 V supply level. With C2201 still
charged to +160 V (the supply voltage), its positive
end is pushed up to 204 V (44 V + 160 V ). Now there
is -4 4 V (160 V - 204 V) across the coil of T2203
from pin 7 to pin 6 and -4 4 V ( 0 - 4 4 V) from pin 2
to pin 1 (see Figure 3-1 0). Since C2201 is in parallel
with C2202 for dc voltages (coils are shorts to d c ),
the dc voltage across C2201 can change very little.
The capacitance of C2201 is large enough that the
charging and discharging currents do not have
enough tim e to change the voltage across C2201 in
normal operation.
The two coils of T2203 need not be coupled mag
netically for the circuit to operate. Both coils are
wound on the same core for convenience. Trans
form er action is minimal because the waveforms
impressed across both coils are nearly identical.
After a time controlled by the Preregulator 1C (the
dead tim e), the on-tim e cycle for Q2201 repeats.
On time depends on the line voltage level; a higher
3-56
Summary of Contents for 2246 1Y
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