The present invention relates to a method which operates a power supply device having a power factor correction (PFC) stage and an LLC converter coupled to the PFC stage. The method comprises the steps of: implementing a current control loop which has an alternating current (AC) input voltage measurement value as inputs to operate a PFC stage from a normal state mode to a continuous-conduction mode (CCM) of a power supply device by using a current mode controller; implementing a voltage control loop which does not have the AC input voltage measurement value as the inputs to operate the PFC stage in a discontinuous-conduction mode (DCM) during at least a part of a burst mode of the power supply device by using a voltage mode controller; and modifying the voltage control loop when the output of the voltage mode controller in the burst mode proportional to power in average current mode control is out of a predetermined range.
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