A conversion circuit and an adapter that resolve a voltage drop problem of a power supply of a driver in an ACF circuit. The conversion circuit includes an active clamp flyback circuit, a drive circuit, and a replenishment power transistor. The active clamp flyback circuit is configured to perform power conversion. The drive circuit is configured to output a drive signal and a reference voltage. The drive signal is used to drive the active clamp flyback circuit. A first terminal of the replenishment power transistor is coupled to an input terminal of the active clamp flyback circuit, a second terminal of the replenishment power transistor is coupled to a power supply terminal of the drive circuit, and a gate of the replenishment power transistor is configured to receive the reference voltage.