Switching power supply circuit

A switching power supply circuit, comprising:

• a first switching element with a fly-back transformer having a magnetizing coil and a drive coil at the primary side and an output coil at the secondary side and a first bipolar transistor applying a voltage from said drive coil for self-oscillation and then magnetizing said magnetizing coil,
• a second switching element with a second bipolar transistor to control the switching time of the first switching element,
• a timing condenser connected between the base and emitter of the second switching element, and supplying a current from said drive coil through an impedance circuit comprising a serial circuit of resistances and inductances in order to supply a direct current voltage rectified from AC voltage to said magnetizing coil and outputting said voltage from said output coil by switching, and
• a sensing coil being connected magnetically closely with said output coil and connected with a rectifying and smoothing circuit that in turn is connected with said timing condenser through a Zener diode facing in the inverse direction to the resistance, the difference between a detection voltage detected as a direct current voltage of the secondary side being rectified and smoothed from alternate current voltage output from said sensing coil and the basic voltage of said Zener diode being supplied to said timing condenser for changing the discharge and charge time and then changing the timing of the switching of said second switching element to control the switching time of said first switching element, and the discharge and charge property of said timing condenser being adjusted by resistances connected in series with said Zener diode in order to make the timing for said second switching element to turn on at the non-loading time shorter than that at the ordinary switching time.