Switching power supply

A switching power supply, comprising:

• a choke coil being in series at one of a pair of source supply lines to which a DC input voltage is supplied;
• a switching element connected between said pair of source supply lines, said switching element intermittently cutting off said DC input voltage applied to said choke coil;
• and
• a rectifier/smoothing circuit being input with a voltage achieved by superimposing a flyback voltage onto said DC input voltage, said flyback voltage being generated at said choke coil while said switching element is set in an OFF state, said rectifier/smoothing circuit including:
• a synchronous rectifying device constituted of an NPN transistor;
• a smoothing capacitor having a first end and a second end, said smoothing capacitor outputting a stepped up DC voltage at said first end and said second end, said first end being connected to said synchronous rectifying device;
• and
• a base drive circuit including:
• an auxiliary winding magnetically coupled with said choke coil;
• a DC blocking capacitor provided between said auxiliary winding and a base of said NPN transistor;
• and
• a reset circuit connected between said base of said NPN transistor and any one of said first end and said second end of said smoothing capacitor.