A converter includes a switching circuit, a resonant circuit, a rectifying circuit, and a transformer including a primary winding, and a secondary winding. The switching circuit is configured to convert a DC input voltage to a switching signal. The resonant circuit is electrically coupled to the switching circuit and configured to receive the switching signal to provide a primary current. The primary winding is coupled to the resonant circuit. The rectifying circuit is coupled to the secondary winding and configured to rectify a secondary current outputted by the secondary winding so as to provide an output voltage. The resonant circuit includes a variable inductor to adjust the characteristic curve of the converter.