In one embodiment, a source driving circuit configured for a switching power circuit, can include: (i) a source transistor coupled between a source of a main power transistor and ground, where the source transistor can be controlled by a PWM control signal; (ii) the main power transistor being on when the source transistor is on and a gate-source voltage of the main power transistor exceeds a conduction threshold voltage; (iii) a source diode having an anode coupled to the main power transistor source, and a cathode coupled to a delay circuit and a power supply capacitor; and (iv) the delay circuit controlling the main power transistor to turn off a delay time after the source transistor is turned off, where the delay time allows charging of the power supply capacitor such that a voltage across the power supply capacitor is at least a level of a reference voltage.