A resonant DC-DC converter includes an inverter circuit, a rectifier circuit and a transformer unit. The inverter circuit is connected to a first terminal of the transformer unit, and the rectifier circuit is connected to a second terminal of the transformer unit. The first and second terminals are galvanically isolated from one another by a first transformer. The transformer unit is configured to adapt a transformation ratio of the transformer unit. The transformer unit has an energy transmission path which includes an inverter and a second transformer, with an input of the energy transmission path being arranged parallel to a primary side of the first transformer and an output of the energy transmission path being arranged in series with the secondary side of the first transformer.