Building An Average Model For Primary-Side Regulated Flyback Converters

by Yann Vaquette, ON Semiconductor, Toulouse, France, How2Power Today, Mar 15 2017

Focus:
Primary-side-regulated (PSR) controllers can reduce the size and cost of flyback-based ac-dc adapters by simplifying the feedback chain. These controllers are now found in a variety of applications and compete against existing optocoupler-based designs. This article describes the main differences between the classical flyback converter with optocoupler and the optocouplerless PSR flyback. Then it shows how to develop an average model for the PSR flyback—one that includes a sample-and-hold network—and simplify it without impacting the transfer function. The transfer function is evaluated and results obtained with Mathcad plots of the transfer function are compared with simulations of the converter. Finally, the loop compensation is plotted and calculations needed to adjust the phase margin are detailed. A PSR converter based on the NCP1365 controller is used to demonstrate the effectiveness of this approach to loop compensation.

What you’ll learn:

• How to build an average model for the (optocouplerless) primary-side regulated flyback converter that can be used in compensating the converter
• How to understand operation of the primary-side regulated flyback converter

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