DC-DC converter, comprising - an inverter having four primary semiconductor switches (Q1 to Q4) connected together to form an inverter full-bridge for converting an input DC voltage (U in) into an AC voltage, - a transformer having a primary winding arranged in the bridge branch of the inverter full-bridge and a secondary winding, - a rectifier which is connected to the secondary winding for rectifying a secondary voltage of the transformer, - a clamping assembly which comprises an additional semiconductor switch and a clamping capacitor connected in series with the additional semiconductor switch and to which the rectified secondary voltage (U 2) of the transformer is applied, - and a control unit which is designed to actuate the primary semiconductor switches (Q1 to Q4) and the additional semiconductor switch, characterised in that - the transformer is an air gap transformer, and - the control unit is designed to actuate the primary semiconductor switches (Q1 to Q4) and the additional semiconductor switch in such a way that - an AC voltage is generated by the inverter and alternatingly assumes an upper voltage level during a first active phase and a lower voltage level during a second active phase, - between two successive active phases (T1, T2), the transformer is operated in a freewheeling phase in which the primary winding is short circuited, - and the clamping assembly is switched off by the additional semiconductor switch after the start of each freewheeling phase and is switched on after the start of each active phase (T1, T2), - wherein a switching-off delay duration is used as a manipulated variable for the switching off of the clamping assembly after the start of a freewheeling phase for regulating a primary current (I TR,1) which flows during the freewheeling phase as a primary freewheeling current through the primary winding of the transformer and which is regulated by means of the switching-off delay duration to a target value, wherein the target value of the primary freewheeling current is determined dependent upon the input DC voltage (U in) and/or upon an output current (I out) of the DC-DC converter. - an inverter having four primary semiconductor switches (Q1 to Q4) connected together to form an inverter full-bridge for converting an input DC voltage (U in) into an AC voltage, - a transformer having a primary winding arranged in the bridge branch of the inverter full-bridge and a secondary winding, - a rectifier which is connected to the secondary winding for rectifying a secondary voltage of the transformer, - a clamping assembly which comprises an additional semiconductor switch and a clamping capacitor connected in series with the additional semiconductor switch and to which the rectified secondary voltage (U 2) of the transformer is applied, - and a control unit which is designed to actuate the primary semiconductor switches (Q1 to Q4) and the additional semiconductor switch, characterised in that - the transformer is an air gap transformer, and - the control unit is designed to actuate the primary semiconductor switches (Q1 to Q4) and the additional semiconductor switch in such a way that - an AC voltage is generated by the inverter and alternatingly assumes an upper voltage level during a first active phase and a lower voltage level during a second active phase, - between two successive active phases (T1, T2), the transformer is operated in a freewheeling phase in which the primary winding is short circuited, - and the clamping assembly is switched off by the additional semiconductor switch after the start of each freewheeling phase and is switched on after the start of each active phase (T1, T2), - wherein a switching-off delay duration is used as a manipulated variable for the switching off of the clamping assembly after the start of a freewheeling phase for regulating a primary current (I TR,1) which flows during the freewheeling phase as a primary freewheeling current through the primary winding of the transformer and which is regulated by means of the switching-off delay duration to a target value, wherein the target value of the primary freewheeling current is determined dependent upon the input DC voltage (U in) and/or upon an output current (I out) of the DC-DC converter.