Patent ID: 7254045

Claim:
A power supply circuit comprising: a direct-current power source from which a pulsating current is obtained; a choke coil connected to one terminal of said direct-current power source; a converter transformer having a primary coil, a secondary coil and a first auxiliary coil; a switching element which performs switching of an electric current of said converter transformer; a pulse width modulation control circuit which performs on-off control of said switching element; a first connection means for connecting the other end of said choke coil to one end of said first auxiliary coil of said converter transformer and for connecting the other end of said first auxiliary coil to the other terminal of said direct-current power source through a series circuit of a first diode and a first capacitor; a second connection means for connecting a connection midpoint between said first diode and first capacitor to one end of the primary coil of said converter transformer and for connecting the other end of said primary coil to the other terminal of said direct-current power source through the switching element; a third connection means for connecting the secondary coil of said converter transformer to a direct-current voltage output terminal through a rectifying circuit; and a fourth connection means for connecting said direct-current voltage output terminal to the input side of said pulse width modulation control circuit and for connecting an output terminal of the pulse width modulation control circuit to a control electrode of said switching element, wherein a connection point of said primary coil and switching element is connected to the connection point of said first diode and first capacitor through a series circuit of a second capacitor and a second diode, and a connection point of said second capacitor and second diode is connected to a connection point of said switching element and the other terminal of said direct-current power source through a series circuit of a second auxiliary coil having a winding of the same phase as said choke coil and a third diode.