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区 JALLINCKRODT
The potentiometer R80 (R27 on boards 800906 Rev G and earlier)
is preset at the factory to generate a 100kHz frequency. The PWM
output, U16 pin 14, controls FET Q4.
This FET acts as the switch
in the flyback regulator circuit shown in Figure 3-4. T1 transfers
energy from input to output.
During the FET "on" time (i.e. gate
voltage = 10V), energy builds up in the core.
During the FET "off?
© time (i.e. gate voltage = OV), the total stored energy is transferred
to the secondary winding and current is delivered to the load.
Flyback converters require a clamp to protect the FET switch from
overvoltage spikes when it opens. These spikes are caused by
leakage inductance in the transformer.
D7, R23 and C12 form a
voltage clamp which limits the voltage spikes from exceeding the
BVDSS rating of 4,
Capacitor C11 smooths out the ripple to provide a DC voltage to
the motor. The motor voltage is routed through relay K1 which
controls the direction of the motor movement.
When the relay is
off, the motor will expel; when the relay is on, the motor will fill.
Stopping
The motor can be braked by either shorting the motor leads to-
gether, shorting the output of the flyback transformer, T1, or by
setting the STOP pin on U16 pin 10 high.
When the SAFE RELAY K2 is off, any voltage impressed on the
motor will turn on triac Q1 which shorts the motor leads together.
In order for the motor to move, therefore, it is necessary to turn the
SAFE RELAY on. To turn on the SAFE RELAY it is necessary to
set the SAFE RELAY CTL line (U6 pin 9) high and to toggle the
watchdog fail-safe (U17 pin 11) at least once every 170 ms. This
sets the fail-safe line (U6 pin 10) high.
The output of the flyback transformer is shorted by the brake circuit
consisting of Q2, Q3, R20-R22.
If U15 pin 16 is low the gate of Q2
is low, so it is "off". If U15 pin 16 is high the gate of Q2 is high, Q2
turns on and the output of the servo transformer is shorted.
Motor
current flowing through Q2 generates a voltage across R21.
If
excessive current flows through Q2 the voltage across R21 will rise
high enough to turn on Q3.
When Q3 turns on, Q2 is turned off
thus preventing excessive current from damaging Q2. When the
DEAD STOP signal from the CPU is high and if either SIGN or
INJECT are high (but not both), U15 pin 16 is low (i.e. brake is off).
800961-C Nov. 2000
3-11
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