A switching power supply circuit achieves improvement of the power
conversion efficiency while a high power factor is maintained, in which
the power factor of a current resonance converter is improved by feeding
back a switching output to a rectification current path to selectively
interrupt the rectification current, thereby to expand the conduction
angle of ac input current. This allows elimination of a power choke coil
to achieve improvement of the power conversion efficiency. Further, the
magnetic flux density of an insulating converter transformer is set lower
than a predetermined level, so that secondary side rectification current
normally maintains a continuous mode irrespective of a variation of the
load or the input voltage. This decreases the power loss that appears
when the secondary side rectification current is placed into a
discontinuous mode and thereby achieves further improvement of the power
conversion efficiency.