A power converter comprising: a DC/DC converting circuit configured to provide a variable electric power, which comprises a first output terminal electrically connectable to one terminal of an external load and a second output terminal; a first energy storage element comprising: a first terminal electrically connected to the second output terminal of the DC/DC converting circuit; and a second terminal, wherein the first energy storage element is a rechargeable battery or a super capacitor; and a switch comprising: a first connecting terminal electrically connected to the second output terminal of the DC/DC converting circuit; a second connecting terminal electrically connected to the second terminal of the first energy storage element; and a third connecting terminal electrically connected to the other terminal of the external load, wherein the power converter is adapted to provide a two-stage continuous power supply to the external load by the switching operation of the switch, wherein when the third connecting terminal and the first connecting terminal of the switch are electrically connected, the power converter is adapted to provide the power supply to the external load by use of the variable electric power; and when the third connecting terminal and the second connecting terminal of the switch are electrically connected, the power converter provides the power supply for the external load according to the DC/DC converting circuit and the first energy storage element, and wherein the power converter comprises a controller, and the controller comprising: a detection terminal electrically connected to the first output terminal of the DC/DC converting circuit, and configured to detect the voltage across the external load; and a control terminal electrically connected to the switch, wherein when the voltage across the external load is equal to or greater than a predetermined voltage, the control terminal is configured to output a control signal to the switch, thereby allowing the second connecting terminal and the third connecting terminal of the switch to be electrically connected; and when the voltage across the external load is less than the predetermined voltage, the control terminal is configured to output another control signal to the switch, thereby allowing the first connecting terminal and the third connecting terminal of the switch to be electrically connected. a DC/DC converting circuit configured to provide a variable electric power, which comprises a first output terminal electrically connectable to one terminal of an external load and a second output terminal; a first energy storage element comprising: a first terminal electrically connected to the second output terminal of the DC/DC converting circuit; and a second terminal, wherein the first energy storage element is a rechargeable battery or a super capacitor; and a first terminal electrically connected to the second output terminal of the DC/DC converting circuit; and a second terminal, wherein the first energy storage element is a rechargeable battery or a super capacitor; and a switch comprising: a first connecting terminal electrically connected to the second output terminal of the DC/DC converting circuit; a second connecting terminal electrically connected to the second terminal of the first energy storage element; and a third connecting terminal electrically connected to the other terminal of the external load, a first connecting terminal electrically connected to the second output terminal of the DC/DC converting circuit; a second connecting terminal electrically connected to the second terminal of the first energy storage element; and a third connecting terminal electrically connected to the other terminal of the external load, wherein the power converter is adapted to provide a two-stage continuous power supply to the external load by the switching operation of the switch, wherein when the third connecting terminal and the first connecting terminal of the switch are electrically connected, the power converter is adapted to provide the power supply to the external load by use of the variable electric power; and when the third connecting terminal and the second connecting terminal of the switch are electrically connected, the power converter provides the power supply for the external load according to the DC/DC converting circuit and the first energy storage element, and wherein the power converter comprises a controller, and the controller comprising: a detection terminal electrically connected to the first output terminal of the DC/DC converting circuit, and configured to detect the voltage across the external load; and a control terminal electrically connected to the switch, wherein when the voltage across the external load is equal to or greater than a predetermined voltage, the control terminal is configured to output a control signal to the switch, thereby allowing the second connecting terminal and the third connecting terminal of the switch to be electrically connected; and when the voltage across the external load is less than the predetermined voltage, the control terminal is configured to output another control signal to the switch, thereby allowing the first connecting terminal and the third connecting terminal of the switch to be electrically connected.