Patent ID: 8456867

Claim:
An isolated switched mode power supply (SMPS), comprising: 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; a transformer having a primary winding; 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 having: a first switching element; 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; and 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; wherein 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 at an output of the isolated SMPS; and generating the start-up switching control signals, comprising a first voltage pulse of duration P 1 to temporarily switch ON the second switching element, and a subsequent second voltage pulse of duration P 2 to temporarily switch ON the first switching element, wherein P 1 <P 2 and P 2 <DT, where T is a switching period of the isolated SMPS, thereby allowing the boot-strap capacitor to be charged before the first switching element is turned ON by the second voltage pulse.