An isolated switched mode power supply, SMPS, comprises a switching controller operable to generate start-up switching control signals during start-up of the isolated SMPS, and operational switching control signals of period T during subsequent operation of the isolated SMPS. The SMPS further comprises a transformer having a primary winding and a full-bridge drive circuit arranged to drive the primary winding of the transformer in response to the switching control signals. The full-bridge drive circuit has: a first switching element and a boot-strap driving circuit arranged to switch the first switching element in response to the switching control signals, the boot-strap driving circuit having a boot-strap capacitor. The full-bridge drive circuit further comprises a second switching element connected to the first switching element and to the boot-strap driving circuit, and arranged to conduct a current to charge the boot-strap capacitor when switched ON in response to the switching control signals. The switching controller is operable to start up the isolated SMPS by: determining a duty cycle D for the operational switching control signals based on a voltage an output of the isolated SMPS and generating the start-up switching control signals comprising a first voltage pulse of duration P1 to temporarily switch ON the second switching element, and a subsequent second voltage pulse of duration P2 to temporarily switch ON the first switching element, wherein P1