This application discloses a converter and a power adapter, to reduce an energy loss of the power adapter. The converter includes: a DC power supply, a primary power transistor, an auxiliary power transistor, a first capacitor, a transformer, and a control circuit. The first capacitor is connected in series to the transformer to form a series circuit, the series circuit is connected in parallel to a first terminal and a second terminal of the auxiliary power transistor. The control circuit is configured to: when an excitation current in the transformer is in a continuous state, regulate a target voltage to a preset voltage threshold, and control the primary power transistor to be turned on when first dead time ends, where the target voltage is a voltage between the first terminal of the primary power transistor and the ground.