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SECTION 7
LOW VOLTAGE POWER SUPPLY (A1)
7-1.
General. The low voltage power supply converts either an AC line input or a DC supply input to the DC
operating voltages required by the system. Appropriate protection circuits are incorporated within the supply
to protect both the supply and the system in vhe event of certain common malfunctions. A block diagram of the
Low Voltage Supply module is shown in figure 7-1 with its schematic shown in figure 7-2.
7-2.
Input Power Control.
Whenever AC power is connected to the unit the DC BUS within the supply is
supplied by the AC rectifier and filter circuitry. The AC sense circuit provides a control voltage whenever AC is
present that isolates the DC input from the DC bus and drives the front panel AC indicator.
7-3.
With power on the DC buss the power supply control circuitry determines the operating mode of the
power supply. With the unit "OFF", the battery charger control circuit is turned on and the frequency standard
control and chopper generator circuits are turned off. When the unit is in"ST AND BY", the chopper generator is
off and the frequency standard supply and battery charger are enabled. Finally with the unit "ON", the
frequency standard supply and the chopper generator are enabled and the battery charger is off. Thus the
battery is charged in off and standby modes, and the frequency standard operates in standby and on modes.
7-4.
The voltage for the battery charger is boosted above the nominal DC bus voltage to 32 volts by the AC
boost winding. This increase in voltage is necessary for proper charge operation.
7-5.
For operation from a DC input, the AC power must be removed from the unit disabling the AC sense
voltage. With the AC power removed and the unit off, no power is present on the DC bus. When the unit is
switched to the standby mode, the DC relay closes, connecting the DC input to the DC bus and the supply
voltage to the frequency standard is enabled. Then with the unit turned on the chopper generator is enabled
and normal operation occurs.
7-6.
DC Output Control.
Regulation of the DC output voltages is accomplished by regulating only the +5V
output. The transformer winding ratios determing the other output voltages with respect to the +5 volt output.
The +5V output is compared with a stable reference voltage and the resultant control voltage is used to
determine the on time of the pulse-width modulator, thus regulating the input voltage to the chopper circuits.
7-7.
The chopper generator provides the 7.9 volt reference voltage, a 20 KHz square wave chopper drive
signal, and a 20 KHz triangle waveform output for pulse-width modulator control. The pulse-width control
comparator compares the triangle waveform with the control voltage. If the control voltage is equal to the mean
DC voltage of the triangle wave the pulse modulator has a 50% duty cycle. For control voltages above and
below the mean value the duty cycle is proportionally increased or decreased.
7-8.
The filtered DC output from the pulse-width modulator is chopped through the primary of the output
transformer at the 20KHz rate. The DC output is alternately switched between the upper half and the lower half
of the primary. winding. The current through the primary center tap is detected by a current transformer and its
output used for overcurrent protection.
7-9.
Protection Circuitry.
The power supply is protected against shorted outputs, high internal
temperatures, and low or high DC buss voltages. In each case the protection circuit pulls the control voltage
line to ground to open up the pulse-width modulator and shut down the supply.
7-1
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