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output voltage of the supply with the drop across the
VOLTAGE controls. If these voltages are not equal, the
comparator produces an error signal that is amplified and
fed back to the series regulator with the correct phase and
amplitude to make them equal. In this manner, the
constant-voltage comparator holds the output voltage at
the level established by the VOLTAGE controls.
4-7
During constant-current operation, the constant-
current comparator detects any difference between the
voltage drop developed by load current flowing through
the current sampling resistor and the voltage across the
CURRENT controls. If the two inputs to the comparator
are momentarily unequal, an error signal is generated
that alters the conduction of the series regulator by the
amount necessary to reduce the error voltage at the
comparator's input to zero. Hence, the I R drop across the
current sampling resistor, and therefore the output current,
is held at a constant value.
4-8
Since the constant-voltage comparator causes the
output impedance to be low and allows the output current
to change whenever the load resistance changes, while
the constant-current comparator causes the output
impedance to be high and allows the output voltage to
change in response to a load resistance change, it is obvious
that the two comparison amplifiers cannot control the
output simultaneously. For any given value of load resis¬
tance, the power supply must act either as a constant-
voltage source or as a constant-current source. To enable
one comparator or the other to take control, the outputs
of both comparators are connected to the input of the mixer
amplifier through an OR gate.
4-9
Figure 4 2 shows the output characteristic of a
constant-voltage/constant-current power supply. With no
load connected, the output current (IquT^ is zer0 3nd the
output voltage (EqUT) equals the front panel voltage con¬
trol setting (Eg). When a load resistance is connected to the
output terminals of the supply, the output current increases
while the output voltage remains constant. Point D thus
represents a typical constant-voltage operating point.
Further decreases in load resistance are accompanied by
further increases in IqijT w't*1 no c^ange 'n t^ie outPut
voltage until the output current reaches Ig, a value equal
to the front panel current control setting. At this point
the supply automatically changes its mode of operation and
becomes a constant-current source. Still further decreases
in the value of load resistance are accompanied by a drop
in the supply's output voltage with no accompanying
change in its output current. With a short circuit across
the load terminals, IquT ~'s and ^OUT ~
4-10
The "crossover" value of load resistance can be
defined as Rq = Eg/Ig. Adjustment of the front panel
voltage and current controls permit this "crossover"
resistance Rc to be set to any desired value from 0 to
If RL is greater than
Rq,
the supply is in constant-voltage
operation. If
Rq
is less than
Rq,
the supply is in constant-
current operation.
4-11
The short-circuit protection circuit (see Figure 4-1)
protects the series regulator if the supply's output is shorted
while its controls are set to a high output voltage and current.
The protection circuit monitors the voltage drop across the
series regulator. If this voltage rises above a preset level,
the protection circuit limits the current through the series
regulator until the preregulator can reduce the voltage
across the series regulator. Once this voltage returns to
normal, the short-circuit protection circuit is turned off and
has no further effect on the operation of the supply.
4-12
The overvoltage protection crowbar monitors the
output of the supply and fires an SCR which short circuits
the supply if its output exceeds a preset threshold. The
crowbar also sends a turn-down signal to the preregulator
control circuit.
4-13
The overvoltage limit circuit protects the main
rectifier diodes and filter capacitors from damage if the
series regulator transistors short or the voltage programming
control opens. The circuit monitors the output voltage of the
supply and sends a turn-down signal to the preregulator
control circuit if the voltage exceeds approximately 120%
of its rated maximum.
OPEN
Es = FRONT PANEL VOLTAGE CONTROL SETTING
IS =FRONT PANEL CURRENT CONTROL SETTING-
-Li.'CRmCAL" OR "CROSSOVER"
VALUE
C ° ls ' OF LOAD RESISTOR
Figure 4-2. Operating Locus of a CV/CC Power Supply
4-2
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