The invention relates to a synchronous rectifier (16) for integration into a power source (10) in order to provide a direct current, in particular in a cube or ashlar-shaped unit of a heavy-current transformer (12), comprising circuit elements (24), an actuation circuit (17) for actuating the circuit elements (24) and a supply circuit (48), wherein a printed circuit board (35) with conductor tracks and connection surfaces is provided for receiving electronic components. For reduction of losses and improvement of efficiency, the circuit elements (24), the actuation circuit (17) and the supply circuit (48) are arranged on the circuit board (35) for the autonomous operation thereof, wherein several openings (37), which are arranged in parallel and in series, are provided on the circuit board (35) for receiving protrusions (36) of a contact plate (29), above which openings (37) the circuit elements (24) are arranged and are connected or soldered and can be contacted with the protrusions (36) of the contact plate (29).