A controller and a method for controlling a power converter includes a sample block coupled to generate a first, second, and third sample by sampling an input sense signal that is representative of an input voltage of the power converter. An enable signal is asserted when a first difference between the first sample and the second sample exceeds a first threshold. An edge signal is asserted when both the enable signal is asserted and a second difference between the first sample and the third sample exceeds a second threshold. A drive circuit is coupled to output a drive signal in response to the edge signal. The drive signal is for controlling a switch coupled to regulate an output of the power converter.