main switching device a control circuit varies the off period during which the main switching device and the auxiliary switching device are off simultaneously according to power consumption by a load on an apparatus in which the switching power supply unit is installed application_number US 96169401 A description the present invention will now be described in detail with reference to the accompanying drawings showing a preferred embodiment thereof fig1 is a schematic circuit diagram showing the arrangement of a switching power supply unit according to an embodiment of the present invention the switching power supply unit according to the present embodiment has a switching power supply unit comprised of a partial resonance type forward converter in fig1 a resonance inductor lr a primary winding lp of an insulating transformer t and a main switching device q 1 n channel type mos fet in this embodiment are connected to a direct current power supply vin that is comprised of a waveform shaping smoothing circuit connected to an alternating current power supply a diode d 1 and a capacitor c 1 are equivalently connected to the main switching device q 1 specifically the diode d 1 is included in the main switching device q 1 and is connected in anti parallel between the.