Within the wide family of Switch Mode Power Supplies (SMPS), the Flyback converters represent the structure of choice for use in small and medium power applications. For
compact designs and radio?frequency sensitive applications, e.g. TV sets or set?top boxes, Quasi? Resonant power supplies start to take a significant market share over
the traditional fixed frequency topology. However, if the feedback loop control is well understood with this latter, for instance via a comprehensive literature and SPICE models,
the situation differs for self?oscillating variable switching frequency structures where no model still exists. This article will show how a simple large?signal averaged SPICE model
can be derived and used to ease the design work during stability analysis.