A primary-side-control quasi-resonant flyback converter with tight output voltage regulation and self-calibrated valley switching

This paper presents methods and circuits to achieve tight output voltage regulation and self-calibrated valley switching for primary-side-control quasi-resonant (QR) flyback converters. By tracking the slope of auxiliary winding voltage, the controller retrieves the output voltage at a fixed diode current with a simple analog method, and it thus suppresses the load and line effect on the output voltage. The proposed self-calibrated valley switching circuit is based on the perturb and observe (P&O) method to automatically detect the QR valley, and no extra off-chip component is required. A primary-side flyback controller that integrates the two proposed circuits has been designed and simulated using the SPICE model for VIS 0.5-μm 5-V/40-V high-voltage CMOS process. Simulation results of a 6.2-V/5-W QR flyback converter exhibit a 0.8% output voltage variation with the load varying from 10% to full and the input voltage from 127 V to 375 V, and the validity of the proposed valley switching technique is also verified.