1 Introduction

Primary-side regulation (PSR) is an observer-based approach to estimate the output voltage of a flyback converter using the reflected voltage on a primary-referenced winding. Previous Analog Design Journal articles^{[1, 2]} have described the operation and switching behavior of an auxless PSR flyback DC/DC converter for automotive and industrial applications, where appropriately timed sensing of the primary switch voltage at its resonant knee position provides a suitable proxy for the output voltage. This magnetically-sensed regulation technique supports a sub-1% output-voltage accuracy across load, line and temperature ranges.^[1] Moreover, it avoids the secondary-side optocoupler and error amplifier normally used for isolated feedback, thereby eliminating a component crossing the isolation barrier and enabling a cost-effective, high-reliability design with low component count.

Within this context, this article reviews the modes of operation and salient characteristics of a PSR flyback DC/DC converter and specifically examines output capacitor selection to meet system specifications for output voltage ripple and small-signal stability.