Hi, 

I use the top261 to create a flyback power supply with an output voltage of 54V, for the purpose I have modified the feedback network which requires the use of the TL431, adding a 48V zener in series. For output current values ​​equal to 1-2A, the board works correctly and the transformer used by me is barely audible. For low values ​​of the output load, ie below 30mA, the transformer has a strong hum, which also appears to be irregular, as if the TOP were changing the switching frequency. Can you tell me something about it? Thank you.

Regards