A DC-DC converter system having a DC-DC converter including a power transistor and a control unit is provided, which control unit provides control signals for triggering the power transistor. The DC-DC converter is operable in two operating modes: in a first operating mode, the DC-DC converter operates at high efficiency; and in a second operating mode, the DC-DC converter operates at low efficiency. The high power loss released in the second operating mode is used in the warm-up phase after the start of a motor vehicle engine as an additional heating system.