A converter includes an input capacitor, a primary-side switch circuit, a magnetic element circuit, a secondary-side switch circuit, and an output capacitor. The magnetic element circuit includes a transformer and an inductor. The input capacitor is configured to receive an input voltage. The primary-side switch circuit is coupled to the input capacitor. The magnetic element circuit is coupled to the primary-side switch circuit. The inductor is a leakage inductor of the transformer or an external inductor coupled between the transformer and the primary-side switch circuit. The secondary-side switch circuit is coupled to the magnetic element circuit. The output capacitor is coupled to the secondary-side switch circuit. The input capacitor and the inductor generate an oscillating current. The primary-side switch circuit is switched at an adjacent region of a wave trough of the oscillating current.