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7-9.
PROTECTION CIRCUIT. This power supply is protected from shorted outputs, high internal
temperatures, and high or low DC bus voltage. In each case, the protection circuit pulls the control voltage line
low, disabling the pulse width modulator and shutting down the power supply outputs.
7-10.
Short circuit protection is provided by monitoring the current in the primary winding of the output
transformer T2. When an output is shorted, the primary winding current will increase significantly. This causes
the overcurrent detector to pull the control voltage line low, disabling the pulse width modulator and shutting
down the output. With the output shut down, there is no primary winding current, causing the control voltage
line to be released. When the control voltage line is released, the pulse width modulator is again enabled and
the power supply outputs are available again. If the short circuit is still present, the shutdown sequence will be
repeated. A delay is provided in the overcurrent detector causing the shutdown sequence to cycle at an
approximately 0.5 second rate.
7-11.
Over temperature protection is provided by a thermal switch. When the temperature of the power
supply exceeds the setting of the thermal switch, the switch closes, pulling the control voltage line low,
disabling the pulse width modulator and shutting down the power supply outputs. Normal power supply
operation will resume when the temperature returns to a safe operating level.
7-12.
Protection against high or low DC or AC line inputs is provided by monitoring the DC bus voltage. Wheh
the DC bus voltage exceeds 20 volts, or falls below 10 volts, the high/low shutdown circuit pulls the control
voltage line low, disabling the pulse width modulator and shutting down the power supply outputs. When the
DC bus voltage returns to normal, the power supply will automatically resume normal operation.
7-13.
HIGH VOLTAGE CONTROL. The HV BIAS V line and the HV SOURCE V line provide the high voltage
power supply A10 with bias voltage and primary power, respectively.
7-14. SWITCHER MODULE A1A1. The switcher module (figure 7-2) contains the pulse width modulator
and chopper circuits. The input PWM DRIVE signal, from the control module, switches the chopping circuit on
and off. This produces a rectangular wave output which is filtered and applied to transformer choppers A and
B. In effect, this action regulates the voltage which is applied to transformer T201 on the output module. The
PWM OUT signal is a secondary input to the voltage regulator comparator on the control module.
7-15.
Transformer choppers
A
and Bare driven by CHOPPER DR A and CHOPPER DR B signals from the
control module. Output signals XFMR DR A and XFMR DR Bare 180-degrees out-of-phase and XFMR DR A'
and XFMR DR B' are 180-degrees out-of-phase. An output, HV SOURCE V, from the chopping circuit is the
primary power source for the high voltage power supply.
7-16.
OVP (Overvoltage Protection).
The OVP zener is connected to the +5V output from the output module
and limits the maximum +5 volt level to +6.3 volts.
7-17.
OUTPUT MODULE A1A2. The output module (figure7-3) provides the regulated output voltages and
the current sense voltage for the overcurrent protection circuit. Input power is provided by signals XMFR DR A,
A', B, and
8'.
These signals are 20kHz squarewaves and drive the primary windings of transformerT201. After
full wave rectification and filtering, the nominal output voltages are available as shown in figure 7-3.
7-18.
The primary current of transformer T201 is monitored by transformer T202. The voltage developed
across T202 is full wave rectified and applied to the current limit circuit on the control module by the
CURRENT LIMIT SENSE signal. An increase in the primary current of T201 produces a corresponding
increase in the voltage developed across T202. This increase is applied to the current limit circuit and
overcurrent protection is initiated.
7-2
7-19.
Regulation of the output voltage is accomplished by the +5-volt feedback. When the +5-volt output is
regulated, the remaining output voltages will be regulated because of the turns ratio of the windings between
the outputs. When the +5-volt output is held to one percent regulation, the other outputs will be held to live
percent regulation.
7-20.
The OVP (Overvoltage Protection) output is applied to
a
6.2-volt zener diode mounted on the chopper
assembly. This provides overvoltage protection to the +5-volt output.
7-21.
CONTROL MODULE A1A3. The control module (figure 7-3) provides pulse width modulation control
and contains the protection circuits. Pulse width modulation control is accomplished by comparing a 7.9V
reference voltage to the +5-volt feedback from the output module. The resultant integrated control voltage is
applied to the pulse width control. This voltage is compared to the 20kHz triangle voltage to determine the duty
cycle of the pulse width modulator. The chopper drive outputs are squarewaves and are 180-degrees out-of-
phase with each other.
7-22.
When the DC BUS voltage is over 20 or under 10 volts DC, the over/under voltage protection circuit
pulls the control voltage signal to the pulsewidth control circuit low. This action shuts down the pulsewidth
modulator.
7-23. The soft start circuit slows the rise lime of the control signal to the pulsewidth control circuits. When the
signal reaches the operating level the soft start circuit is switched out of the control loop.
7-24.
The overcurrent detector compares a signal that is proportioned to the current in the current
transformer, to a reference. When the current is too high, the control signal is pulled low, shutting down the
output module. After a delay, the output module operates again, if the malfunction causing the overcurrent is
still present, the module will shutdown again. This sequence will cycle at a 0.5 second rate until the
malfunction is corrected.
7-25.
When the internal temperature of the power supply rises above 85" C, the overtemp shutdown circuit
causes the control signal to go low, shutting down the pulsewidth control circuit. The control logic functions
are shown in table 7-1.
Table 7-1.
Control Logic Functions
Output Signals
Input Signals
Batt Chr
HV Bias
DC Led
Pwr5n
AC Sense
Enable
Supply V
AC Led
Anode
Relay Enable •
Low
Low
High
On
Off
On
Low
Low
High
Low
On
On
Off
High
High
Low
Low
Off
Off.
Off
Low
High
High
High
Off
On
Off
High
• Note that RELAY ENABLE low, does not imply that the relay is closed. The PWR OFF signal on the relay
module must also be high to close the relay.
7-3
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