Quasi-resonant controlled switching power supply with an adaptive upper limit of valley number

A switching power supply, comprising:

• a power switch;
• an energy storage element coupled to the power switch, during an OFF period of the power switch, a winding voltage of the energy storage element oscillates to provide an oscillation signal having one or more valleys;
• and
• a controller, comprising:
• a target valley number generator configured to provide a target locked valley number corresponding to a valley switching of the power switch, wherein the target locked valley number will not exceed an upper limit stored in a register;
• a first comparison circuit configured to compare a voltage sensing signal representative of the oscillation signal with a first threshold voltage during the OFF period of the power switch, to provide a first square wave signal;
• a second comparison circuit configured to compare the voltage sensing signal with a second threshold voltage during the OFF period of the power switch, to provide a second square wave signal, wherein the second threshold voltage is less than the first threshold voltage;
• and
• an upper limit updating circuit configured to determine if to update the upper limit based on the first square wave signal and the second square wave signal.

A control method used in a switching power supply having a power switch and an energy storage element coupled to the power switch, a winding voltage of the energy storage element oscillates to provide an oscillation signal having one or more valleys during an OFF period of the power switch, the control method comprising:

• providing a target locked valley number corresponding to a valley switching of the power switch, wherein the target locked valley number will not exceed an upper limit stored in a register;
• comparing a voltage sensing signal representative of the oscillation signal with a first threshold voltage during the OFF period of the power switch, to provide a first square wave signal;
• comparing the voltage sensing signal with a second threshold voltage during the OFF period of the power switch, to provide a second square wave signal, wherein the second threshold voltage is less than the first threshold voltage;
• and
• determining if to update the upper limit based on the first square wave signal and the second square wave signal.