In accordance with an embodiment, a method of operating a switched-mode power converter includes defining at least one of a minimum switching frequency threshold and a maximum switching frequency threshold; detecting valleys occurring in a voltage over the switching element when the switching element is in an off-state; and either in a quasi-resonant mode, switching the switching element on when an nth consecutive valley occurs, n being an integer equal to or greater than one, so that the actual switching frequency is at least one of: below the maximum switching frequency threshold and above the minimum switching frequency threshold, or in a forced switching frequency mode, switching the switching element on so that the actual switching frequency is the maximum or the minimum switching frequency.