Source-Load Interactions In Multi-Unit Power Systems
Switching regulators have negative input impedance, at least at low
frequency. This is evidenced by the fact that as the input voltage
increases, the input current decreases, and vice-versa. Before switching
regulators came into such wide-spread use, they were mainly stand-alone
units operating from a low source impedance and driving a passive load.
Oscillation was rarely a problem. Now, as switching regulators become lower
in cost and more wide-spread in application, there are many instances where
switching regulators serve as both source and load. This is happening both
at low power, as in distributed power systems, and at high power, in utility
systems with DC links. The space station power system is a good example of
switching regulators driving other switching regulators. If a source has a
resonant output characteristic, connecting a negative resistance to that
source can allow the resonant output to oscillate. This characteristic is
shared by both L-C input filters and switching power supply outputs. This
paper discusses various criteria that have been developed recently to
assure stability under these source-load conditions and gives practical
suggestions of design and testing criteria to assure stability under all
operating conditions.
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