A charging circuit includes a main power circuit, a DC-to-DC converting circuit, a detection circuit and a controller. The main power circuit is electrically connected to a power source for converting an input voltage from the power source into a first voltage. The DC-to-DC converting circuit is electrically connected to the main power circuit for converting the first voltage into a second voltage to charge the energy storage element. The detection circuit is electrically connected to the main power circuit and the DC-to-DC converting circuit for detecting a terminal voltage of the energy storage element and the first voltage from the main power circuit, thereby generating a feedback signal. The controller is electrically connected to the detection circuit and the main power circuit for controlling operations of the first switching element in response to the feedback signal, so that the first voltage is adjustable according to the second voltage.