A synchronous rectifier comprises at least one rectification switch (102, 103), and a control circuit (104) for controlling the at least one rectification switch to allow unidirectional current flow only. The control circuit comprises at least one current sensor (105, 106) for sensing an alternating component of current of the at least one rectification switch, and at least one driver circuit (107, 108) for controlling the at least one rectification switch at least partly on the basis of the direction of the sensed alternating component. Using the alternating component for controlling the rectification switch removes a need to compare the current to any non-zero constant or adjustable threshold value, and thus challenges related to defining the threshold value can be avoided. The synchronous rectifier can be for example a part of a resonant converter.