An 1326 Application Note

Over the past two decades plenty of resonant and quasi-resonant converters have been developed and proposed as an answer to the difficulties raised by square-wave converters, especially those related to their parasitic elements. The basic idea is to put these parasitics in use. The core of both classes of converters is a tank circuit. Unlike resonant converters, where it takes part actively in the power conversion process, quasi-resonant converters use the tank circuit only to create either a zerovoltage or a zero-current condition for the power switch, to turn on or turn off respectively. The existing types of quasi-resonant converters can be then classified as either 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. The superposition of the resonant voltage across the tank circuit (and the switch, while it is in off state) on 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. The superposition of the resonant current flowing through the tank circuit (and the switch, while it is in on state) on the normal current flow generates the zero-current condition for the switch to turn off. An interesting property of quasi-resonant converters is that they will be turned back into a normal square-wave converter if the tank circuit is removed. Vice versa, a quasi-resonant converter can be obtained starting from a normal square-wave topology.