Tektronix 7834

Under normal operating conditions, only the E Sense input
at pin 15 controls the hold-off time . However, various
fault
conditions can affect hold-off time or stop Inverter
opera-
tion altogether . The operation of each individual
function
of the Inverter Control stage is described in the following
discussion .
Pre-Regulator
The
Pre-Regulator operation of U1275, maintains constant
voltages at the outputs of the Low-Voltage Rectifiers . It also
provides
constant peak-to-peak voltage to the High-Voltage
supply .
Transformer T1235 provides Inverter phase information
and power to U 1275 . The phase information is connected
to pins 10 and 11 through C1275 and C1276 . Bridge
recti-
fier CR1280, CR1281, CR1282, and CR1283, provides
positive and negative operating voltages to U1275 . A shunt
regulator in U1275 maintains +7 .5 volts at pin 6 . The -2
volt (nominal) supply connected to pin 7 is unregulated .
Zener diode VR1279 is for protection against open circuit
condition (U1275 removed) and is normally notconducting .
Pin 15 is the voltage sensing (E Sense) point of the Pre-
Regulator circuit . Zero volts at pin 15 indicates proper
regulation . Zener diode VR1297 provides a stable reference
voltage for the sensing-divider resistors R1290, R1291,
R1293, R1294, and R1296 . Resistor R1293 in this divider
adjusts the ratio of the divider to adjust the output of the
+108 volt supply . Outputs of the other supplies are then set
by the turns ratio of T1310 .
Integrated circuit U1275 regulates the Inverter by varying
the hold-off time of the switching transistors, 01234 and
01241 . A variable pulse-width monostable multivibrator in
U1275 is triggered at pins 10 and 11 whenever the Inverter
current changes direction . The pulse width holds off the
Inverter by turning
on transistor 01252 through pin 9 of
U1275,
thus shorting out
the base drive to 01234 and
01241 . The pulse width
and therefore hold-off is controlled
by a ramp at pin 12 . If the voltage at the E Sense input,
pin 15, is too low, the ramp is not allowed to rise very high
and the pulse width and hold-off are short . As the E Sense
voltage rises, the ramp is allowed to rise to a higher voltage
level, increasing the hold-off time .
Fault
Protection
The fault-protection
portions of U1275 provide protection
for the power-supply
components from damage due to
short circuits,
turn-on surge currents, and other malfunc-
tions . When a fault is
detected at the Bal Sense input (pin
2)
or I Sense input (pin
13), a
current from
the Fault Holdoff
Time output (pin
1)
charges C1264 . If
the detected fault
lasts longer than about
10 milliseconds, C1264 will charge
positive enough
to initiate a positive output at pin 8 . This
output is applied through U1278B
and U1 278C to turn on
01254 and 01252 which
turns off the Inverter . The Invert-
Theory of Operation-7834
er will remain off while C1259 discharges
through R 1261
which
keeps 01254 and
01252
turned on
. The Inverter
restarts in roughly 500 milliseconds when the
current
through R1 261 is insufficient to keep 01254 and
01252
turned on . When the inverter restarts C1259 is recharged
through CR1259
and
R1259 . This
cycle repeats
until the
fault is corrected, with the Inverter
on
for about 10 milli-
seconds, and off for about 500 milliseconds .
Inverter Current Limiter
- The Inverter current limiter provides protection for the In-
verter components from damage due to excessive turn-on
current
or
short circuits . Operation of this
stage
is similar to
the Pre-Regulator (voltage regulation) . The Inverter current
limiter takes control of the Inverter hold-off time whenever
pin 13 starts to
go
negative . T1235 is a current step-down
transformer . The current is rectified and flows through
R1287, the current-sensing resistor . The voltage across
R1287 is negative and proportional to the Inverter current .
The I Sense input at pin 13 is normally held positive through
divider R1285 and R1286 . "1 - he Inverter current limiter
takes control of regulation when pin 13 approaches zero
volts . Peak Inverter current is limited to about 5 amperes .
If the voltage at pin 13 remains near zero for more than
about 10 milliseconds, pin 8 will go positive to turn off the
Inverter .
Balance
The Balance portion of U1275 provides overload protection
for the +5 volt light supply, +108 volt supply, +130 volt
supply,
+600
volt supply, focus supply and cathode supply,
as well as backup protection for several regulated supplies .
The
+5
volt, +108 volt and --17 volt supplies are applied to
the Bal Sense input at pin 2 of U1275 through resistive net-
work of R 1303, R 1304, and R 1305 . Other inputs are from
a network on the Low-Voltage Regulator board and current
sensing resistors on the Focus board . During normal opera-
tion, the voltage at the Bal Sense input remains near zero . If
one of the inputs changes sufficiently to cause this voltage
level
to vary about 200 millivolts (positive or negative) for
more than
about 10
milliseconds,
a positive output
is pro-
duced at pin 8 of U1275 to stop the Inverter .
Line Stop
The Line Stop portion of U1275 stops the Inverter when
the POWER switch is turned off . The Line Stop stage will
also stop the Inverter if the ac line voltage falls below a
minimum value .
The line-frequency signal from transformer T1208 is con-
nected to pin 4, the Line Stop Sense input of U1275 .
During normal operation, the line-frequency signal causes
the Line Stop Timer terminal (pin 3) to periodically dis-
charge to ground . When the line-frequency signal is inter-
rupted or falls below a minimum value, C1267 will charge
to approximately
+p .7
volt causing the Line Stop stage to
produce a positive output at pin 8 of U1275 to stop the
Inverter .
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