A power adapter to receive at least one AC input power and transform to DC primary output power includes a power factor correction circuit to receive the AC input power and modulate to become a modulated power, an isolation voltage step-down circuit connecting to the power factor correction circuit to modulate the modulated power to a modulated lower voltage power, a switch voltage regulation circuit connecting to the isolation voltage step-down circuit to receive the modulated lower voltage power, and a voltage stabilization circuit connecting to the switch voltage regulation circuit. The switch voltage regulation circuit sets a determined output level and regulates the modulated lower voltage power to become a determined power at the determined output level. The voltage stabilization circuit modulates the determined power to become the primary output power and supplies the primary output power to a primary output end.