Patent ID: 8659915

Claim:
A valley-detection method for quasi-resonance switching, which respectively uses a first capacitor and a second capacitor to connect with a comparator, and said comparator connects with an N-channel metal oxide semiconductor field effect transistor (NMOSFET), and a transformer has a primary winding and a secondary winding, and said primary winding connects with a drain of said NMOSFET and a high voltage terminal, and a source of said NMOSFET connects with a low voltage terminal, and said first capacitor and said second capacitor connects with said transformer; said method comprising steps of: Step (A): turning on said NMOSFET; Step (B): energy from said high voltage terminal is stored in said primary winding; Step (C): turning off said NMOSFET whereby said energy is discharged from said secondary winding and a resonant signal across said source and said drain is generated, and providing a first constant current to charge said first capacitor at a start time point of said resonant signal until a voltage of said resonant signal first reaches to a crossing voltage, which equals to a high voltage of said high voltage terminal plus a low voltage of said low voltage terminal, wherein the voltage of said resonant signal oscillates around a mid-level, and is hereby referred to as said crossing voltage; Step (D): providing a second constant current to charge said second capacitor when said voltage of said resonant signal equals to said crossing voltage while said voltage of said resonant signal varies from high to low, and a product of said second constant current and a capacitance of said first capacitor equals to a product of said first constant current and a capacitance of said second capacitor; and Step (E): said comparator generating a driving signal to turn on said NMOSFET when a voltage of said second capacitor equals to a voltage of said first capacitor.