A power supply unit for supplying electrical energy to an on-board network of a vehicle, including: • at least two direct current/direct current converters which are interlaced and reversible between a step-down voltage operating mode and a step-up voltage operating mode, the converters being intended to be connected to an electrical energy storage device (ST2) and able to provide a current to the on-board network; • a switch (K) enabling a source of electrical energy (ST1) to supply the on-board network when the switch (K) is in a first state, and enabling the electrical energy storage device (ST2) to supply the on-board network when the switch (K) is in a second state, characterized in that the converters are variable frequency converters, and in that the supply unit further includes a synchronisation device configured to synchronise the operation of the converters operating at variable frequencies and the current generation of the converters, said synchronisation device including: • reception means for receiving a switching signal generated by each of the converters; • means for detecting a transition type of the received switching signals; • means for generating a synchronisation signal when a transition is detected; • means (227, 2S1, 2S2) for providing the synchronisation signal to one of the converters , said means (227, 2S1, 2S2) being configured to provide the synchronisation signal to a different converter and in a successive order each time a transition is detected; and • synchronisation start-up means able to provide a synchronisation signal to a predetermined converter including means for receiving a value of a current passing through an inductance of one converter among the converters , and means for generating a synchronisation start-up signal (START SYNCHRO) when the value of said current reaches a predetermined value. • at least two direct current/direct current converters which are interlaced and reversible between a step-down voltage operating mode and a step-up voltage operating mode, the converters being intended to be connected to an electrical energy storage device (ST2) and able to provide a current to the on-board network; • a switch (K) enabling a source of electrical energy (ST1) to supply the on-board network when the switch (K) is in a first state, and enabling the electrical energy storage device (ST2) to supply the on-board network when the switch (K) is in a second state, • reception means for receiving a switching signal generated by each of the converters; • means for detecting a transition type of the received switching signals; • means for generating a synchronisation signal when a transition is detected; • means (227, 2S1, 2S2) for providing the synchronisation signal to one of the converters , said means (227, 2S1, 2S2) being configured to provide the synchronisation signal to a different converter and in a successive order each time a transition is detected; and • synchronisation start-up means able to provide a synchronisation signal to a predetermined converter including means for receiving a value of a current passing through an inductance of one converter among the converters , and means for generating a synchronisation start-up signal (START SYNCHRO) when the value of said current reaches a predetermined value. <EOS>