Power supply unit for a self-powered intelligent electronic device

An apparatus comprising:

• an Intelligent Electronic Device (IED) for a LV or MV electric power applications, the IED including a power supply unit having:
• a power transformer stage, which is operatively coupled to a feeding conductor to harvest electric power from said feeder conductor, said transformer stage being adapted to provide a first voltage (V1);
• a first storage stage, which is electrically connected to said power transformer stage to receive said first voltage, said first storage stage being adapted to store electric energy and provide a first supply voltage (VS1), said power transformer stage being adapted to provide a first charging current (I1) to charge said first storage stage, when a current flows along said feeding conductor;
• a first step-down conversion stage including a DC/DC switching converter, said DC/DC switching converter structured to receive said first supply voltage (VS1) from a switching stage and provide a third voltage (V3) having a lower amplitude than said first supply voltage when said DC/DC switching converter is electrically connected with said first storage stage by way of the switching stage;
• a second storage stage, which is electrically connected to said first step-down conversion stage, said second storage stage being adapted to store electric energy and provide a second supply voltage (VS2), said first step-down conversion stage being adapted to provide a second charging current (I2) to charge said second storage stage, when said first step-down conversion stage is electrically connected to said first storage stage,
• wherein the switching stage includes a switching device structured to electrically connect/disconnect said DC/DC switching converter with/from said first storage stage and a driving circuit adapted to receive control signals (C) from a control unit and enable/disable the switching operation of said switching device in response to control signals (C) received in input, the switching stage being structured and operable so that if the control signals (C) are not available during a start-up event when the control signals (C) are unavailable from the control unit and the driving circuit does not receive the control signals (C) from the control unit, the driving circuit naturally turns the switching device to an ON to connect said DC/DC switching converter with said first storage stage, and
• wherein the control unit operates the switching device using a first duty cycle of less than 10% while the control unit operates the DC/DC switching converter using a second duty cycle greater than the first duty cycle.