Abstract: Control arrangement for interfacing a convertor (4) in a switched mode power supply with a mains power supply (1) comprising a switch (14) for controlling input current to the arrangement from the mains power supply to have the same waveform as the input voltage to the arrangement from the mains supply, wherein there is provided a controller (16, 17, 25) operable to control the duty ratio of the switching means in dependence on a predetermined relation between a predetermined time varying signal generated by the controller, and a signal I derived from the input current.