Voltage detection method and circuit and associated switching power supply

A voltage detection circuit, comprising:

• a) a first current generating circuit configured to generate a first current according to a first voltage and a converting resistor, wherein said first voltage comprises an excitation voltage on an inductor of a switching power supply during a first time period of a switching cycle of said switching power supply, and wherein said first time period comprises a rising portion of a current flowing through said inductor;
• b) a second current source configured to output a second current, wherein said first and second currents flow in opposite directions;
• c) a first switch coupled between said first current generating circuit and a detection capacitor;
• d) a second switch coupled between said second current source and said detection capacitor, wherein said first switch is on and said second switch is off such that said first current charges said detection capacitor via said first switch during said first time period, and wherein said first switch is off and said second switch is on such that said second current discharges said detection capacitor via said second switch during a second time period of said switching cycle;
• and
• e) wherein a voltage across said detection capacitor is detected to obtain a detection voltage at an end time of said second time period, said detection voltage being used to determine a state of a present output voltage of said switching power supply, wherein said second time period comprises a decreasing portion of said inductor current, and wherein said converting resistor comprises a proportion of a predetermined output voltage of said switching power supply and said second current.