A power supply apparatus which includes a transformer having a primary winding and a secondary winding, whereby magnetic flux generated by a varying primary voltage applied to the primary winding induces a varying secondary voltage on the secondary winding, a torroidal transformer core over which said primary winding and secondary winding are applied, and at least one magnetic shunt arranged to provide a diversion path for magnetic flux generated by the primary winding which diverts magnetic flux from the secondary winding.