Single-stage switching power supply

A single-stage switching power supply for generating an output voltage to a system circuit, said single-stage switching power supply comprising:

• a transformer having a first primary winding assembly, a second primary winding assembly and a secondary winding assembly;
• a voltage level generation circuit connected to the first primary winding assembly or said second primary winding assembly for generating an auxiliary voltage;
• a first switching circuit connected to said first primary winding assembly, said second primary winding assembly, an input-side common terminal and said voltage level generation circuit, and including a first switch element and a second switch element;
• a second switching circuit connected to said first primary winding assembly or said second primary winding assembly and said input-side common terminal, and including a third switch element;
• a rectifying and filtering circuit connected to said secondary winding assembly and said system circuit for rectifying and filtering and generating said output voltage;
• a feedback circuit connected to said rectifying, filtering circuit and generating a feedback signal in response to said output voltage;
• and
• a control circuit connected to said first switching circuit, said second switching circuit, said feedback circuit and said system circuit, wherein in response to said feedback signal and an operating-status signal issued by said system circuit, said first switching circuit and said second switching circuit are alternately enabled under control of said control circuit such that electric energy of a first DC voltage is transmitted from said first primary winding assembly or said second primary winding assembly to said secondary winding assembly, wherein said first switching circuit is disabled but said second switching circuit is enabled under control of said control circuit when said operating-status signal is at a standby operating status.