A power supply system prevents the output voltage from dropping during the changeover
of the power source from a series regulator to a DC-DC converter. The power supply
system has two types of power sources, a DC-DC converter and a series regulator
that are connected parallel to each other. The first switching element of the DC-DC
converter transfers the input voltage to the load. The second switching element
connects the load to the ground. The control circuit turns ON and OFF the first
and second switching elements on receipt of a first selection signal so that the
number of turn-ONs of the second switching element gradually increases. This suppresses
the current from flowing backward through the second switching element SW2 to
the ground while the power source is changing over, preventing drop in the output voltage.
