A method and apparatus for providing improved electromagnetic compatibility between a power source (20) and a switching element (22) in a switched mode power supply includes a device (24) for measuring an electrical variable, for example current, that fluctuates as the switching element (22) switches power received from the power source (20). A signal injector (28) is arranged to generate an electrical signal, for example current, of substantially the same amplitude, but opposite polarity, as the measured current, and to inject it between the power source (20) and the switching element (22) so as to tend to minimize fluctuations in the electrical variable.