The invention relates to a switched-mode power supply system comprising two buffer capacitors connected in series and connected between the two input terminals of a DC input voltage source, two switches connected in series, a primary inductive assembly connected in series with the two switches, and a secondary winding magnetically coupled to the primary inductive assembly in order to deliver a DC output voltage. A current injection module injects current at a mid-point of the two buffer capacitors in order to generate a current imbalance at this mid-point. A balancing circuit for balancing the leakage currents of the capacitors, one end of said balancing circuit being connected to the mid-point of the buffer capacitors, maintains this imbalance at a predetermined value. The invention also relates to a speed variator using such a power supply system.