A method for starting up a power converter, said power converter comprising a start-up controller, a power switch, a transformer, a secondary-side rectifier, a secondary-side controller, a shunt resistor, an external capacitor, an isolation circuit and a switch and said method comprising the steps of: applying a first DC voltage to the start-up controller, wherein the start-up controller is configured to operate in a first and in a second mode, wherein in the first mode, during start-up of the power converter, the start-up controller operates as an open loop current regulator turning on and off the power switch, wherein the first DC voltage and the power switch are coupled to a primary winding of the transformer, whereby an AC voltage is produced on a secondary winding of the transformer; and rectifying the AC voltage from the secondary winding of the transformer with the secondary-side rectifier to provide a second DC voltage for powering the secondary-side controller and a load; characterized in that in the first mode, current through the primary winding of the transformer is measured by measuring a current sense voltage through the shunt resistor coupled with the power switch and the current sense voltage is provided to the start-up controller, wherein the current in the primary winding of the transformer is allowed to rise to a maximum current level monitored by the start-up controller; in the first mode, operation of the start-up controller is based upon regulation of a peak current through the power switch, wherein after turning ON the power switch, the current sense voltage is used to adjust an ON time of the power switch and wherein an OFF time of the power switch is set by the external capacitor, and wherein when the power switch is turned ON, the current sense voltage is initially ignored for a fixed blanking time, and then the current sense voltage is compared with a first reference voltage for only a predetermined brief time interval after the blanking time, the said power switch being turned off if the current sense voltage exceeds the first reference voltage and the current sense voltage is compared with a second reference voltage representing a maximum allowable current and control of the power switch is transferred from the start-up controller to the secondary-side controller when the second DC voltage is at a desired voltage value by operating the primary-side controller in the second mode, in which the start-up controller receives external PWM pulses through the isolation circuit from the secondary side controller and the switch is controlled to feed the PWM pulses to the power switch. applying a first DC voltage to the start-up controller, wherein the start-up controller is configured to operate in a first and in a second mode, wherein in the first mode, during start-up of the power converter, the start-up controller operates as an open loop current regulator turning on and off the power switch, wherein the first DC voltage and the power switch are coupled to a primary winding of the transformer, whereby an AC voltage is produced on a secondary winding of the transformer; and rectifying the AC voltage from the secondary winding of the transformer with the secondary-side rectifier to provide a second DC voltage for powering the secondary-side controller and a load; characterized in that in the first mode, current through the primary winding of the transformer is measured by measuring a current sense voltage through the shunt resistor coupled with the power switch and the current sense voltage is provided to the start-up controller, wherein the current in the primary winding of the transformer is allowed to rise to a maximum current level monitored by the start-up controller; in the first mode, operation of the start-up controller is based upon regulation of a peak current through the power switch, wherein after turning ON the power switch, the current sense voltage is used to adjust an ON time of the power switch and wherein an OFF time of the power switch is set by the external capacitor, and wherein when the power switch is turned ON, the current sense voltage is initially ignored for a fixed blanking time, and then the current sense voltage is compared with a first reference voltage for only a predetermined brief time interval after the blanking time, the said power switch being turned off if the current sense voltage exceeds the first reference voltage and the current sense voltage is compared with a second reference voltage representing a maximum allowable current and control of the power switch is transferred from the start-up controller to the secondary-side controller when the second DC voltage is at a desired voltage value by operating the primary-side controller in the second mode, in which the start-up controller receives external PWM pulses through the isolation circuit from the secondary side controller and the switch is controlled to feed the PWM pulses to the power switch.