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regulating elements. Thus, current foldback is especially useful if the supply is operating in a remote location
and a long term short-circuit occurs. For switching regulated supplies, current foldback does not significantly
reduce dissipation within the supply. It does, however, provide superior load protection as mentioned
previously.
All Agilent supplies that employ current foldback are self restoring and are designed to withstand worst case
dissipation, which occurs under partial overload conditions when the supply is operating along the upper half of
the foldback locus.
Foldback current limiting is incorporated more often into fixed output voltage supplies because full rated output
current can be obtained only at the maximum rated output voltage. As indicated in Figure 20, reductions in
output voltage are accompanied by linear decreases in the available output current. Conventional current
limiting techniques, on the other hand, are used mostly in supplies that furnish continuously variable output
voltages. Note in Figure 19 that maximum rated current can be obtained at any rated output voltage.
Figure 20. Current Foldback Characteristic
PROTECTION CIRCUITS
Today, many power supply manufacturers are placing increasing emphasis on protection for both the power
supply and its load. A supply with extensive protection networks is of obvious benefit to the user, particularly
if used in a crucial application such as a system power source or in the testing of critical components.
Protection Circuits for Linear Type Supplies
The following paragraphs describe protection circuits that are commonly used in Agilent series regulated and
series/preregulated type supplies (Figure 21).
A. Varistor - A voltage-dependent resistor that protects preregulator triac against ac line spikes. Its resistance
decreases abruptly if line voltage exceeds a harmful level.