There is provided a switch mode power supply circuit including at least one inductive component coupled to an associated switching device for cyclically connecting the inductive component to a source of power. The circuit includes a signal output representative of a voltage at a junction of the at least one inductive component to the switching device. The circuit further comprises a hard switching amplitude detector for deriving a measure of hard switching amplitude occurring in operation in the switching device the detector including a signal processing path for receiving the signal output and generating the measure of hard switching amplitude therefrom. The signal path includes: a signal differentiator for imperfectly differentiating the signal output to generate a corresponding imperfectly differentiated signal; and a signal integrator for integrating the imperfectly differentiated signal in a temporally-gated manner for generating the measure of hard switching.