The invention relates to a power supply unit (3) for supplying power to an on-board electrical network of a vehicle, including: at least two DC-to-DC converters (9A, 9B) which are interleaved and reversible between an opera-ting mode for lowering voltage and an operating mode for raising voltage, the converters (9A, 9B) being intended for being connected to a power storage device (ST2) and being capable of supplying current to the on-board network; and a switch (K) enabling a power source (STI) to supply power to the on-board network when the switch (K) is in a first state, and enabling the power storage device (ST2) to supply power to the on-board network when the switch (K) is in a second state. The unit is characterized in that the converters (9A, 9B) are variable-frequency converters, and in that the power supply unit (3) also includes a synchronization unit (200) configured such as to synchronize the operation of the converters (9A, 9B) operating at variable frequencies and the current generation of the converters.