An isolated bidirectional DC-DC converter, comprising: a high voltage side first DC power source, a first switching circuit connected between said first DC power source and a primary winding of a transformer for converting power mutually between direct current and alternating current, and a second switching circuit (22; 24) connected between a secondary winding of said transformer and a low voltage side second DC power source for converting power mutually between direct current and alternating current, for transferring power from said first to said second DC power source by a buck operation and from said second to said first DC power source by a boost operation, Wherein the second switching circuit (22,24) comprises two main switching devices (22;24) respectively connected between a respective AC terminal of said transformer and a respective terminal of said second DC power source, wherein the control unit is adapted to switching on both main switching devices (22;24) in certain time periods ((a),(c)), characterised in that between the AC terminals of said second switching circuit and one terminal of said second DC power source, a voltage clamp circuit, is connected, the voltage clamp circuit comprising, connected to each AC terminal of said second switching circuit, a series unit of a respective switching device (21; 23) with a reverse parallel diode (31; 33) and a capacitor, and further comprising a control unit adapted for, at voltage boost operation, turning on said switching devices (21, 23) of said voltage clamp circuit during an off period of a switching device of said second switching circuit (22;24) that is connected with the switching device of the voltage clamp circuit, and further adapted for at voltage buck operation and at the timing of short-circuiting both ends of the primary winding by the first switching circuit after that the primary winding of the transformer is supplied with a voltage from the first switching circuit, turning off once the switching device of said second switching circuit (22; 24) set in the continuity state for synchronous rectification and turning on and then turning off said switching device (21, 23) of said voltage clamp circuit that is connected with the other switching device of said second switching circuit (22;24)such that a current flowing through the primary winding is reduced. a high voltage side first DC power source, a first switching circuit connected between said first DC power source and a primary winding of a transformer for converting power mutually between direct current and alternating current, and a second switching circuit (22; 24) connected between a secondary winding of said transformer and a low voltage side second DC power source for converting power mutually between direct current and alternating current, for transferring power from said first to said second DC power source by a buck operation and from said second to said first DC power source by a boost operation, Wherein the second switching circuit (22,24) comprises two main switching devices (22;24) respectively connected between a respective AC terminal of said transformer and a respective terminal of said second DC power source, wherein the control unit is adapted to switching on both main switching devices (22;24) in certain time periods ((a),(c)), characterised in that between the AC terminals of said second switching circuit and one terminal of said second DC power source, a voltage clamp circuit, is connected, the voltage clamp circuit comprising, connected to each AC terminal of said second switching circuit, a series unit of a respective switching device (21; 23) with a reverse parallel diode (31; 33) and a capacitor, and further comprising a control unit adapted for, at voltage boost operation, turning on said switching devices (21, 23) of said voltage clamp circuit during an off period of a switching device of said second switching circuit (22;24) that is connected with the switching device of the voltage clamp circuit, and further adapted for at voltage buck operation and at the timing of short-circuiting both ends of the primary winding by the first switching circuit after that the primary winding of the transformer is supplied with a voltage from the first switching circuit, turning off once the switching device of said second switching circuit (22; 24) set in the continuity state for synchronous rectification and turning on and then turning off said switching device (21, 23) of said voltage clamp circuit that is connected with the other switching device of said second switching circuit (22;24)such that a current flowing through the primary winding is reduced.