Electric system of a road vehicle provided with a DC-DC electronic power converter

1. An electric system ( 17 ) of a road vehicle ( 1 ); the electric system ( 17 ) comprises:

a high-voltage electric circuit ( 18 ) provided with a first storage system ( 14 ) and with at least one electric machine ( 8 ) designed to generate electrical energy;

a low-voltage electric circuit ( 19 ) provided with a plurality of electrical loads, wherein a selected one of the low-voltage and high-voltage electric circuits is constituted by an electric starter motor ( 22 ) of an internal combustion heat engine ( 5 ) of the vehicle ( 1 ); and

a DC-DC electronic power converter ( 25 ), which connects the low-voltage electric circuit ( 19 ) and the high-voltage electric circuit ( 18 ) to one another in order to transfer electrical energy from the high-voltage electric circuit ( 18 ) to the low-voltage electric circuit ( 19 );

wherein the low-voltage electric circuit ( 19 ) is completely free from any storage system arranged on the outside of the DC-DC electronic power converter ( 25 );

wherein the DC-DC electronic power converter ( 25 ) comprises a high-voltage input ( 26 ), a low-voltage output ( 28 ), a conversion device ( 27 ), which is capable of varying the DC electrical voltage and is interposed between the high-voltage input ( 26 ) and the low-voltage output ( 28 ), and a control unit ( 31 ), which controls the conversion device ( 27 );

wherein the DC-DC electronic power converter ( 25 ) comprises a second storage system ( 29 ), which is connected to the low-voltage output ( 28 ) and is integrated in the DC-DC electronic power converter ( 25 );

wherein the control unit ( 31 ), when the vehicle ignition is on, is configured to continuously activate the conversion device ( 27 ) so as to have the conversion device ( 27 ) continuously deliver the electric power needed to supply the electrical loads of the low-voltage electric circuit ( 19 ) and needed to keep the second storage system ( 29 ) completely charged;

wherein the control unit ( 31 ), only when the vehicle ignition is off, is configured to cyclically detect a state of charge of the second storage system ( 29 ) and to activate the conversion device ( 27 ) only when the state of charge of the second storage system ( 29 ) is below a charge threshold and only for an amount of time needed to charge the second storage system ( 29 );

wherein the second storage system ( 29 ) has a nominal performance that is smaller than 10% of a nominal performance of the electric starter motor ( 22 ) such that the second storage system ( 29 ) is not capable of supplying the operating power needed by the electric starter motor ( 22 ) by itself;

wherein the DC-DC electronic power converter ( 25 ) comprises at least two conversion devices ( 27 ), which are identical to one another and are connected to one another in parallel both in the high-voltage input ( 26 ) and in the low-voltage output ( 28 );

wherein the control unit ( 31 ) is configured, when the vehicle ignition is on, to estimate an electric power requested by the low-voltage output ( 28 ) and decide how many converter devices ( 27 ) to operate together depending on the electric power requested by the low-voltage output ( 28 );

wherein only the cooperation of the at least two conversion devices ( 27 ) allows the DC-DC electronic power converter ( 25 ) to deliver a total power sufficient to power the electric starter motor ( 22 ); and

wherein a fraction of electric current powering the electric starter motor ( 22 ) also comes from the second storage system ( 29 ) and is negligible.

2. An electric system ( 17 ) according to claim 1 , wherein the control unit ( 31 ) is configured, only when the vehicle ignition is off, to turn off the conversion device ( 27 ) and to normally keep it turned off, so that the electrical loads of the low-voltage electric circuit ( 19 ) are exclusively supplied with power by the second storage system ( 29 ).

3. An electric system ( 17 ) according to claim 1 , wherein the control unit ( 31 ) is configured, only when the vehicle ignition is off, to activate one single conversion device ( 27 ) in order to charge the second storage system ( 29 ) when the state of charge of the second storage system ( 29 ) is below the charge threshold.

4. An electric system ( 17 ) according to claim 1 , wherein the control unit ( 31 ) is configured to keep track of the actual use time of each converter device ( 27 ) and to cyclically switch the actually active converter devices ( 27 ) so as to balance the actual use times of the converter devices ( 27 ).

5. An electric system ( 17 ) according to claim 1 , wherein the electrical loads of the low-voltage electric circuit ( 19 ) comprise:

at least one continuous electrical load, which must constantly be supplied with power regardless of the use of the vehicle ( 1 ); and

at least one occasional electrical load, which must be supplied with power only when the vehicle ( 1 ) is used.

6. An electric system ( 17 ) according to claim 5 , wherein:

a permanent electric line ( 33 ), to which the continuous electrical load is connected, originates from the DC-DC electronic power converter ( 25 );

an interruptible electric line ( 34 ), to which the occasional electrical load is connected, originates from the DC-DC electronic power converter ( 25 );

the permanent electric line ( 33 ) is directly connected to the conversion device ( 27 ) without the interposition of any switch;

the DC-DC electronic power converter ( 25 ) comprises a switch ( 35 ), which is interposed between the conversion device ( 27 ) and the interruptible electric line ( 34 ); and

the control unit ( 31 ) closes the switch ( 35 ) and keeps it closed, when the vehicle ignition is on, and opens the switch ( 35 ) and keeps it open, when the vehicle ignition is off.

7. An electric system ( 17 ) according to claim 6 , wherein two interruptible electric lines ( 34 ), to which different occasional electrical loads are connected, originate from the DC-DC electronic power converter ( 25 ).

8. An electric system ( 17 ) according to claim 1 , wherein:

the electrical loads of the low-voltage electric circuit ( 19 ) comprise at least one continuous electrical load, which must constantly be supplied with power regardless of the use of the vehicle ( 1 ); and

the second storage system ( 29 ) is sized so as to be capable of autonomously supplying with power the sole continuous electrical load.

9. An electric system ( 17 ) according to claim 1 , wherein the second storage system ( 29 ) has a nominal power below 30 Watt and a storable electric charge below 6 Ah.

10. An electric system ( 17 ) according to claim 1 , wherein the second storage system ( 29 ) only comprises supercapacitors.

11. An electric system ( 17 ) according to claim 1 , wherein the DC-DC electronic power converter ( 25 ) comprises one single outer protection casing ( 36 ), which houses, on the inside, the conversion device ( 27 ), the control unit ( 31 ) and the second storage system ( 29 ).

12. An electric system ( 17 ) according to claim 1 , wherein:

the DC-DC electronic power converter ( 25 ) comprises exactly three conversion devices ( 27 ), which are connected to one another in parallel both in the high-voltage input ( 26 ) and in the low-voltage output ( 28 ); and

all the three conversion devices ( 27 ) are used together only to power the electric starter motor ( 22 ) in case the internal combustion heat engine ( 5 ) has to be started.

13. An electric system ( 17 ) according to claim 12 , wherein:

the three conversion devices ( 27 ) are identical to one another and each have a nominal power of approximately 3500-4000 Watts;

the second storage system ( 29 ) has a nominal power below 30 Watt.

14. An electric system ( 17 ) according to claim 13 , wherein a fraction not greater than 0.2-0.3% of the total of electric current powering the electric starter motor ( 22 ) also comes from the second storage system ( 29 ).

15. An electric system ( 17 ) according to claim 1 , wherein a fraction not greater than 0.2-0.3% of the total of electric current powering the electric starter motor ( 22 ) also comes from the second storage system ( 29 ).

16. An electric system ( 17 ) according to claim 1 , wherein the nominal performance of the second storage system ( 29 ) is smaller than 1% of the nominal performance of the electric starter motor ( 22 ).