The invention relates to a switched mode power converter and a method of operating such a converter A switched mode power converter according to the invention includes a transformer (2) having a primary winding (2a) and at least one secondary winding (2b) and a secondary side rectifier circuit including an output filter (6, 10) coupled to the at least one secondary winding (2b), and a secondary side active switch device (S3) coupled between the at least one secondary winding and the output filter. The converter further includes primary side and secondary side control means (12, 16, 18) for regulating the switching of the primary side and secondary side switches, respectively, and configured so as to reduce the duty cycle of the primary side switch device (S1) during a lower power mode of operation of the converter, the reduction of the duty cycle of the primary side switch being determined with reference to the duty cycle of the secondary side switch (S3). This leads to a substantial increase in the efficiency of operation in the low power mode.