Patent ID: 8582328

Claim:
A power supply circuit for providing a power voltage to a back-stage circuit, the power supply circuit comprising: a switching power circuit, controlling the power voltage; a closed loop feedback control that, in a normal mode, feeds back the power voltage to the switching power circuit in a closed loop; an open loop feedback control that, in a power saving mode, feeds back the power voltage to the switching power circuit in an open loop; and a transformer, having a primary side coupled to the switching power circuit, and a secondary side coupled to the back-stage circuit and the closed loop feedback control, wherein when the switching power circuit continuously detects the open loop and after the switching power circuit substantially enters in a zero output status for a delay period, the switching power circuit enters into a power down mode, the delay period related to voltage delay and voltage comparison, wherein the transformer couples an output voltage of the switching power circuit to the secondary side to generate the power voltage, and wherein the switching power circuit comprises a signal generating circuit coupled to the transformer, and a control circuit coupled to the signal generating circuit, for controlling the signal generating circuit to enter into the power down mode, the control circuit comprising a first comparer coupled to the closed loop feedback control, the first comparer comparing a first reference voltage and a first voltage fed back by the closed loop feedback control, and outputting a first comparing signal, a counter, counting the first comparing signal to generate a counting signal, and a second comparer, comparing the counting signal with a reference count number to output a control signal to the signal generating circuit, the control signal controlling whether the signal generating circuit enters into the power down mode.