Patent ID: 7310249

Claim:
A switching power supply circuit, comprising: switching means including a switching element for receiving a DC input voltage as an input thereto to perform switching operation; switching driving means for switching driving said switching element; a converter transformer formed by winding thereon at least a primary winding to which a switching output obtained by the switching operation of said switching means is supplied and a plurality of secondary windings in each of which an alternating voltage as the switching output obtained in said primary winding is excited; first DC output voltage production means for receiving the alternating voltage obtained in one of said plural secondary windings of said converter transformer as an input thereto to perform rectification operation to produce a first DC output voltage; second DC output voltage production means for receiving the alternating voltage obtained in a different one of said plural secondary windings as an input thereto to perform rectification operation to produce a second DC output voltage, said second DC output voltage production means supplying a load thereto with power lower than that supplied by said first DC output voltage production means; frequency controlled type constant voltage controlling means for controlling said switching driving means in response to the level of a first secondary side DC output voltage produced by said first DC output voltage production means to adjust the switching frequency of said switching means to perform constant voltage control for the first DC output voltage; and inductance controlled type constant voltage controlling means including a control transformer as a saturable reactor having a controlling winding and a controlled winding wound thereon, said controlled winding being inserted between the different secondary winding and said secondary DC output voltage production means, said inductance controlled type constant voltage controlling means adjusting the level of control current to be supplied to said controlling winding in response to the level of the second DC output voltage from said second DC output voltage production means to adjust the inductance of said controlled winding to perform constant voltage control for the second DC output voltage.