Techniques for avoiding false negative sense (NSN) detection in a flyback AC-DC converter are described herein. In an example embodiment, a secondary side controller of the AC-DC converter comprises a frequency detector, a negative sense detector, and control logic. The frequency detector is configured to determine a frequency of an input signal from the drain node of a synchronous rectifier (SR) circuit on the secondary side of the AC-DC converter. The negative sense detector is configured to determine a negative voltage of the input signal. The control logic is configured to: enable the negative sense detector, when the frequency of the input signal rises above a frequency threshold value; and turn on the SR circuit to transfer power to the secondary side of the AC-DC converter, when the negative voltage of the input signal falls below a voltage threshold value.