A Self-Oscillating Flyback Converter with Primary Feedback Control

Due to its simplicity and low component count, a self-oscillating flyback converter is an ideal topology for cost sensitive applications, such as mobile phone chargers and auxiliary power supplies. To regulate the output voltage, the conventional self-oscillating flyback converter employs a secondary feedback current mode control scheme whose major components include a TL431 error amplifier and optocoupler. These two components nonetheless contribute significantly to a total cost of the self-oscillating flyback converter. A further cost reduction can be achieved, if the error amplifier and optocoupler can be eliminated altogether. This paper proposes a self-oscillating flyback converter with primary feedback control. In the proposed converter, the error amplifier and optocoupler are replaced by a Zener diode, and the output voltage from the auxiliary winding is used as a feedback variable, instead of the actual output voltage from the secondary winding. In the paper, the operation of the proposed converter is described. The prototype converter operating from a 220 V AC main and producing an output of 24 V/0.5 A is fabricated and tested. Experimental results are presented to verify the converter performance as well as to validate the principle of operation.