A DC/DC converter comprising - a first conversion branch stretching between a first and a second DC terminal (18, 20), - a first controllable voltage source (Ub1p) in a first half of the first conversion branch, - a second controllable voltage source (Ub1n) in a second half of the first conversion branch, - a conversion unit converting between AC and DC and at one end connected to a third and a fourth DC terminal (22, 24), and - a transformer with - a primary winding connected in a first interconnecting branch stretching between a first junction at which the first and second controllable voltage sources are connected to each other and a first potential that lies in the middle between the potentials of the first and second DC terminal, and - a secondary winding connected to another end of the conversion unit, wherein - the first controllable voltage source is made up of a first group of series-connected cells (CCA; CCB) and the second voltage source is made up of a second group of series-connected cells, where each cell is controllable to provide a voltage contribution in the conversion branch, and - the first and second voltage sources are controllable to provide a common-mode DC voltage to the first and second DC terminals and a differential mode AC voltage to said junction. - a first conversion branch stretching between a first and a second DC terminal (18, 20), - a first controllable voltage source (Ub1p) in a first half of the first conversion branch, - a second controllable voltage source (Ub1n) in a second half of the first conversion branch, - a conversion unit converting between AC and DC and at one end connected to a third and a fourth DC terminal (22, 24), and - a transformer with - a primary winding connected in a first interconnecting branch stretching between a first junction at which the first and second controllable voltage sources are connected to each other and a first potential that lies in the middle between the potentials of the first and second DC terminal, and - a secondary winding connected to another end of the conversion unit, wherein - the first controllable voltage source is made up of a first group of series-connected cells (CCA; CCB) and the second voltage source is made up of a second group of series-connected cells, where each cell is controllable to provide a voltage contribution in the conversion branch, and - the first and second voltage sources are controllable to provide a common-mode DC voltage to the first and second DC terminals and a differential mode AC voltage to said junction. - a primary winding connected in a first interconnecting branch stretching between a first junction at which the first and second controllable voltage sources are connected to each other and a first potential that lies in the middle between the potentials of the first and second DC terminal, and - a secondary winding connected to another end of the conversion unit, - the first controllable voltage source is made up of a first group of series-connected cells (CCA; CCB) and the second voltage source is made up of a second group of series-connected cells, where each cell is controllable to provide a voltage contribution in the conversion branch, and - the first and second voltage sources are controllable to provide a common-mode DC voltage to the first and second DC terminals and a differential mode AC voltage to said junction.