Abstract: A series-parallel resonant power converter comprises a primary-side start-up controller and a secondary-side controller, wherein the primary-side start-up controller sends power to the secondary-side controller when power (voltage) is first applied to the series-parallel resonant power converter. The start-up controller starts up the series-parallel resonant power converter using an open-loop start-up technique wherein the secondary-side closed-loop controller takes over control of the series-parallel resonant power converter once it becomes powered and activated. During light-load or no load conditions, the secondary-side controller sends an off resonance higher frequency or a standby code inhibit (disable) command to the start-up controller.