An isolated switched mode power supply comprises a transformer, with primary and secondary windings, and a rectification network connected to the secondary winding. The rectification network and the transformer are arranged such that, during a free-wheeling period of operation of the switched mode power supply, a magnetic flux from a first portion of a secondary winding of the transformer substantially cancels a magnetic flux from a second portion of the secondary winding between the first and second portions of the secondary winding. Additionally, a secondary side circuit connected to the secondary winding comprises a switching device, which is connected to a centre-tap, provided between the first and second portions of the secondary winding, and an output of the rectification network so as to conduct at least a part of a free-wheeling current flowing in the secondary side circuit during the free-wheeling period.