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3-76
SPECIAL OPERATING
CONSIDERATIONS
3-77
Pulse Loading
3-78
The power supply automatically crosses over from
constant-voltage to constant-current operation, or the
reverse, in response to an increase beyond the preset volt¬
age or current limit. Although the preset limit may be set
higher than the average output current or voltage, high
peak currents or voltages may exceed the preset limit and
cause crossover to occur. If this current or voltage limiting
is not desired, set the current or voltage controls somewhat
above the peak rather than the average requirement.
3-79
Output Capacitance
3-80
An internal capacitor (A3C3) connected across
the output terminals of the supply helps to supply high-
current pulses of short duration during constant voltage
operation. Adding capacitance externally would improve
the pulse current capability of the supply but would
decrease the load protection provided by the constant-
current circuit. A high-current output pulse could damage
load components before the average output current is large
enough to cause the constant-current circuit to operate.
Another drawback to adding additional capacitance is the
possibility of causing the supply to become unstable and
oscillate.
3-81
During constant-current operation, additional
capacitance connected across the output of the supply
would have the following disadvantages:
a. With additional capacitance connected, the output
impedance of the supply decreases with increasing
frequency.
b. With additional capacitance connected, the output
current takes longer to recover from the effects of a change
in the load resistance.
c. With additional capacitance connected, a rapid
reduction in load resistance can produce a larger than
normal surge current that could cause a high power
dissipation in the load.
3-82
Reverse Voltage Protection
3-83
One or two internal diodes (A4CR106, or
A4CR106 and A4CR107) connected with reverse polarity
across the output terminals of the supply protect the
output electrolytic capacitors and series transistors from
the effects of a reverse voltage applied across the supply's
output. Such a reverse voltage might inadvertently be
applied when operating supplies in series if one of them
were to be turned on or off before the other. The current
rating of the output diodes is equal to the rated output
current of the supply.
3-84
The series regulator transistors are also protected
against reverse voltage by diode A4CR105. This diode
shunts the series regulators if the supply is connected in
parallel with another supply but is deenergized.
3-85
Reverse Current Loading
3-86
An active load connected to the power supply may
actually deliver a reverse current to the supply during a
portion of its operating cycle. If an external source is
allowed to pump current into the supply, it will cause a
loss of regulation and might possibly damage the output
capacitor. To avoid these effects, it is necessary to pre¬
load the supply with a dummy load resistor so that it
delivers current through the entire operating cycle of the
load device.
3-87
Battery Charging
3-88
The automatic crossover between constant-voltage
and constant-current exhibited by these supplies makes
them ideal for battery charging applications. Using this
EXT
CROWBAR
TRIGGER
^ Al
A2
A3
A4 A5
A6
A7 A8 +-S -S
A9
Figure 3-15. Auto-Tracking Operation of Three Units
3-12
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