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Theory of Operation
3–116
1780R-Series Service Manual
Pin 1 of U460 is the Error Amplifier output. Pin 2 is the minus input to the Error
Amplifier, the positive terminal is internally connected to +2.5 volts. Pin 3 is the
current sense input, and pin 6 is the controller output, driving the base drive
circuit. The Current Mode Controller has an internal oscillator and the frequency
of operation is determined by a resistor/capacitor (RC) circuit (R560 and C461)
connected to pin 4. Pin 8 is the +5 volt reference output.
The signal from the Opto-Isolator drives R462; R461 is the feedback resistor.
Because R461 and R462 are equal, the gain of the signal (at the output of the
Error Amplifier) is equal to one.
Starter. Q471 and Q470 form the starter circuit. Because CR540 cannot supply
base drive power when the instrument is first turned on, an alternative start is
needed. Prior to power supply start, Q470 is off because U460 cannot power its
base. Q471 is on, with R574 supplying its base current.
The collector resistor for Q471 (R575) limits current to a small value, which is
just large enough to charge C540 in a fraction of a second. Once C540 charges to
16 volts, U460 turns on and provides base drive. Once the supply is operating,
the power for the base drive circuit is supplied by CR540.
When the supply starts, Q470 turns on, lights DS470, and turns off Q471. If for
some reason the power supply does not start (e.g., the power switch is off), C540
is quickly depleted and when its charge reaches 10 volts, U460 shuts down. This
cycle repeats at a periodic rate of a few cycles per second.
Under Voltage Disabler. Q560 turns on when the base drive power supply drops
below about 13 volts. When it turns on, it pulls up the current sense line to U460
to shut down the supply and ensure that the base drive signal is shut off before it
gets too weak.
Low Line Lockout. Q472 is normally held in the off condition by the voltage
divider on its base (R570, R571, and R572). If the line voltage drops below
about 80 volts AC, Q472 turns on and pulls the base of Q460 high to pull pin 2
of U460 high and turn it off.
Current Limit Time Latch. The Comp input of U460 rises above 4 volts when it is
in current limit. If U360 senses this condition, its output goes low and charges
C360. After about 40 milliseconds, C360 charges enough to turn on Q360. Once
it is turned on, it will not turn off again until the power source decays. Q360
pulls pin 3 of U460 high to shut down base drive power.
When the power supply load is too great, and it is in current limit, it typically
runs in short bursts of 40 milliseconds or so, followed by a half second of rest
time.