DC-to-DC converter having - a first partial converter which has at least a first converter phase, wherein the first converter phase of the first partial converter comprises a first phase branch and a second phase branch, wherein - the first phase branch of the first partial converter extends between a first primary-side DC voltage pole and a first AC voltage terminal of the first partial converter, and - the second phase branch of the first partial converter extends between the first AC voltage terminal of the first partial converter and a first secondary-side DC voltage pole, and - a second partial converter which has at least a first converter phase, wherein the first converter phase of the second partial converter comprises a first phase branch and a second phase branch, wherein - the first phase branch of the second partial converter extends between the first secondary-side DC voltage pole and a first AC voltage terminal of the second partial converter, and - the second phase branch of the second partial converter extends between the first AC voltage terminal of the second partial converter and a second secondary-side DC voltage pole, wherein the two partial converters (2, 3) are connected in series with one another and form at least part of a converter series circuit which extends between the first and a second primary-side DC voltage pole (5, 6), wherein the phase branches (4a, 9a) of the partial converters (2, 3) each have a series circuit of two-pole submodules which each comprise at least two power semiconductor switches (102, 104) and an energy store, and the first AC voltage terminal of the first partial converter and the first AC voltage terminal of the second partial converter are connected to one another via a coupling device, with the result that an exchange of electric power between the first and the second partial converter is enabled, wherein the partial converters (2, 3) are designed to provide a constant primary-side output voltage in the case of a variable secondary-side output voltage, wherein the number of submodules in the first and second phase branch (14, 15) of the first partial converter and the number of submodules in the first and second phase branch (20, 21) of the second partial converter are such that the constant primary-side output voltage is able to be provided independently of the secondary-side output voltage, characterized in that the number of submodules of the first and second partial converter (2, 3) is able to be varied by means of a suitable step-down switching unit in dependence on the secondary output voltage. - a first partial converter which has at least a first converter phase, wherein the first converter phase of the first partial converter comprises a first phase branch and a second phase branch, wherein - the first phase branch of the first partial converter extends between a first primary-side DC voltage pole and a first AC voltage terminal of the first partial converter, and - the second phase branch of the first partial converter extends between the first AC voltage terminal of the first partial converter and a first secondary-side DC voltage pole, and - the first phase branch of the first partial converter extends between a first primary-side DC voltage pole and a first AC voltage terminal of the first partial converter, and - the second phase branch of the first partial converter extends between the first AC voltage terminal of the first partial converter and a first secondary-side DC voltage pole, and - a second partial converter which has at least a first converter phase, wherein the first converter phase of the second partial converter comprises a first phase branch and a second phase branch, wherein - the first phase branch of the second partial converter extends between the first secondary-side DC voltage pole and a first AC voltage terminal of the second partial converter, and - the second phase branch of the second partial converter extends between the first AC voltage terminal of the second partial converter and a second secondary-side DC voltage pole, wherein - the first phase branch of the second partial converter extends between the first secondary-side DC voltage pole and a first AC voltage terminal of the second partial converter, and - the second phase branch of the second partial converter extends between the first AC voltage terminal of the second partial converter and a second secondary-side DC voltage pole, wherein