Small-signal modeling for design of robust variable-frequency flyback battery chargers for portable device applications

A novel methodology to derive small-signal models for a peak-current controlled variable-frequency flyback converter operating in boundary mode is presented. The loop gain of a universal input 5-W battery charger for mobile phones is analyzed and compared to the predictions. The results obtained show a good correlation between predictions and measurements validating the modeling. The implications (i.e. RHP and ESR zeros as well as opto-coupler in the feedback loop) for controller design is discussed.