A DC-DC converter (200) comprising a first (201) and a second (202) variable voltage source, a capacitor (203), an alternating current filter (204), and controlling means (205), is provided. A first DC voltage (UDC1) is provided over a series-connection of the first and the second voltage source, and a second DC voltage (UDC2), being lower in magnitude than (UDC1), is provided over the second voltage source. The conversion between (UDC1) and (UDC2) is effected by circulating an alternating current within a circuit comprising the two voltage sources and the capacitor, thereby exchanging power between the two voltage sources. The alternating current is driven by AC voltage components provided by the first and the second voltage source. The controlling means is arranged for controlling the first and the second voltage source so as to maintain a phase difference between the AC components to be close to π.