Power source faults can cause failures in a power supply. Conditions
such as loss of phase, under and over voltage, reverse polarity (DC input),
and incorrect phase sequence should be identified in the power supply
specification and addressed during the design phase.
faults can damage the power distribution system. When one of the power
supply components fails, fault currents may flow through distribution wiring
and input filter components. Input wiring should be properly protected
with a coordinated set of protection devices. More than one set of
current interrupt devices may be required between the power source and system
because current carrying capacity drops as the power is distributed.
This is especially true of higher power equipment. Components, wiring
and circuit traces within the power supply that may have to conduct fault
currents should also be designed to survive. Additionally, voltage
transients, generated when the circuit protective devices interrupt, should be
considered in the design.
Components in the power supply control
circuits can fail creating over-voltage or incorrectly sequenced output
voltages. These could damage the load. Also, failures in interface
circuits may allow undesirable or dangerous system operating modes. The
design should be reviewed for single point failures or sneak paths.
Redundancy and fail-safe operation in these circuits should be
Since switching power supplies present a constant power load
to the power source, the source current will increase with a reduced source
voltage. The worst case current should be considered as part of the
overall protection approach.