Design-oriented Small-signal Modeling of Primary-Side Regulated Flyback Converters

Primary-Side Regulation (PSR) is a popular technique used today in many low-power high-volume applications, such as travel adapters and chargers for portable equipment, to reduce their size and cost. At these power levels the flyback converter is the topology of choice and several PSR methodologies to control both its output voltage (CV-PSR, Constant Voltage PSR) and its output current (CC-PSR, Constant Current PSR) are available. As to a flyback converter with CV-PSR, the implementation of the S&H circuit, a fundamental building block of the CV-PSR loop, plays a key role in the output-to-control transfer function. This paper, after briefly reviewing the concepts related to CV-PSR, describes a typical, silicon efficient implementation of an S&H circuit specific for CV-PSR usage. Then, it illustrates how its model in the frequency domain is determined with a system identification approach in the time domain. The model is verified and validated by means of PSIM simulations.