In a complex resonance type switching power supply circuit including: a
series resonant circuit formed by a leakage inductance component having a
primary-side winding of an isolation converter transformer driven by
switching devices formed by half-bridge coupling and a primary-side series
resonant capacitor connected in series with the primary-side winding; and
a partial voltage resonance circuit connected in parallel with one of the
switching devices mentioned above, for effecting partial voltage resonance
in time with the turning off of the switching devices, an auxiliary
switching device of a series circuit is provided connected in parallel
with the primary-side series resonant capacitor, formed by the auxiliary
switching device and a capacitor, and is operated only during a period
when the switching device connected in parallel with a primary-side
partial resonance capacitor is on. During this period, the capacitor of
the series circuit is connected in parallel with the primary-side series
resonant capacitor. Thus, even when the gap of the isolation converter
transformer is zero, the switching devices are operated stably by zero
voltage switching and zero current switching.
