This document introduces resonant and quasi-resonant converters which have been developed and proposed as a solution to the difficulties of square-wave converters. Quasi-resonant converters use a tank circuit to create a zero-voltage or zero-current condition for the power switch to turn on or off. Quasi-resonant converters can be classified as Zero-Voltage Switching turn-on (ZVS) or Zero-Current Switching turn-off (ZCS) converters. In ZVS converters, the power switch is dynamically connected in parallel to the tank circuit, and the superposition of the resonant voltage and the DC input voltage generates the zero-voltage condition for the switch to turn on. Conversely, in ZCS converters, the power switch is connected in series to the tank circuit, and the superposition of the resonant current and the normal current flow generates the zero-current condition for the switch to turn off.
The document describes the principles of quasi-resonant zero-voltage switching (ZVS) flyback converters, highlighting their advantages over standard square-wave designs, particularly regarding conducted EMI emissions. It details the circuit topology involving leakage inductance and drain capacitance to create zero-voltage conditions for the power switch.
The device uses a dedicated ZCD pin to detect the negative-going portion of the drain voltage, requiring just one resistor.