A circuit for Flyback switching power supply includes a transformer having a primary winding, a secondary winding and an auxiliary winding, a power switch coupled to a dotted terminal of the primary winding, and a switch. A first terminal of the switch is connected to a non-dotted terminal of the auxiliary winding through a capacitor, a second terminal of the switch and a dotted terminal of the auxiliary winding are connected, respectively, to a ground. A common node of the capacitor and the auxiliary winding is configured to connect to a non-dotted terminal of the primary winding. A control circuit is configured to generate, based on a voltage at the common node of the capacitor and the auxiliary winding, a control signal to control the switch in order to achieve zero voltage switch (ZVS) of the power switch.