A method for operating a series-parallel resonant power converter, said method comprising the steps of: applying a first DC voltage to a primary-side start-up controller ; during start-up, turning on and off through a gate driver at least one power switch (222; 224), preferably a MOSFET, coupled with a series-parallel resonant circuit that includes a primary winding of a transformer, whereby an AC voltage is produced on a secondary winding of the transformer; rectifying the AC voltage from the secondary winding of the transformer with second rectifiers, preferably synchronous rectifiers, to provide a second DC voltage for powering a secondary-side controller and a load; and after start-up, transferring control of the at least one power switch from the primary-side start-up controller to the secondary-side controller when the second DC voltage reaches a predetermined value; characterized in that during start-up, the start-up controller performs start-up control by turning on and off of the at least one power switch (222; 224) at a frequency higher than a resonant frequency of the series-parallel resonant circuit , and by slowly reducing the on and off frequency of the at least one power switch (222; 224) toward the resonant frequency of the series-parallel resonant circuit, wherein a rate of reducing the frequency is preferably determined by a resistor coupled with the primary-side start-up controller ; and wherein the secondary-side controller comprises a power operating mode, wherein the secondary-side controller provides drive waveforms to the gate driver that control said at least one power switch to regulate the second DC voltage based on a sensed output voltage of the power converter and a low standby power mode, wherein the secondary-side controller monitors the second DC voltage and depending on the monitored output voltage level sends a control signal (Dout) to the primary-side start-up controller to repeat said start-up control or to stop turning on and off the at least one power switch (222; 224). applying a first DC voltage to a primary-side start-up controller ; during start-up, turning on and off through a gate driver at least one power switch (222; 224), preferably a MOSFET, coupled with a series-parallel resonant circuit that includes a primary winding of a transformer, whereby an AC voltage is produced on a secondary winding of the transformer; rectifying the AC voltage from the secondary winding of the transformer with second rectifiers, preferably synchronous rectifiers, to provide a second DC voltage for powering a secondary-side controller and a load; and after start-up, transferring control of the at least one power switch from the primary-side start-up controller to the secondary-side controller when the second DC voltage reaches a predetermined value; characterized in that during start-up, the start-up controller performs start-up control by turning on and off of the at least one power switch (222; 224) at a frequency higher than a resonant frequency of the series-parallel resonant circuit , and by slowly reducing the on and off frequency of the at least one power switch (222; 224) toward the resonant frequency of the series-parallel resonant circuit, wherein a rate of reducing the frequency is preferably determined by a resistor coupled with the primary-side start-up controller ; and wherein the secondary-side controller comprises a power operating mode, wherein the secondary-side controller provides drive waveforms to the gate driver that control said at least one power switch to regulate the second DC voltage based on a sensed output voltage of the power converter and a low standby power mode, wherein the secondary-side controller monitors the second DC voltage and depending on the monitored output voltage level sends a control signal (Dout) to the primary-side start-up controller to repeat said start-up control or to stop turning on and off the at least one power switch (222; 224). <EOS>