Patent Publication Number: US-10787136-B2

Title: Electric power system for controlling pre-charge of vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Japanese Application Serial No. 2017-166755, filed on Aug. 31, 2017. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND 
     Technical Field 
     The disclosure relates to an electric power system of a vehicle. 
     Related Art 
     An electric vehicle such as a hybrid car or an electric car has an electric power system mounted on, and runs by using electric power supplied from the electric power system to drive a motor. The electric power system is divided into a high-voltage system which has a high-voltage battery and mainly supplies the electric power to the motor, and a low-voltage system which has a low-voltage battery with an output voltage lower than the high-voltage battery and mainly supplies the electric power to a control device or auxiliary machines of the vehicle. 
     In Japanese Laid-open No. 2006-298240 (Patent Literature 1), a load which needs constant electric power supply (for example, an electric brake system which controls an electric foot brake or an electric parking brake) is defined as a backup object load, and an electric power system capable of supplying the electric power from both a low-voltage system and a high-voltage system to the backup object load is disclosed. 
     In the electric power system of Patent Literature 1, a power supply path which connects the high-voltage system to the backup object load, and a power supply path which connects the low-voltage system to the backup object load are connected via a diode. In this electric power system, usually the electric power is supplied from the low-voltage system to the backup object load, when the voltage of the low-voltage system is, due to some reason, below an output voltage of a DCDC converter which reduces the voltage of the high-voltage system and supplies the voltage to the backup object load, the electric power of the high-voltage system is supplied to the backup object load. As a result, the electric power is constantly supplied to the backup object load from the low-voltage system or the high-voltage system. 
     Meanwhile, a plurality of high-capacity smoothing condensers is arranged on the high-voltage system to stabilize the output electric power of the high-voltage system. For this reason, during the start of the vehicle, before the electric power is supplied to the motor and the vehicle starts to run, the high-voltage battery and the smoothing condenser are connected and a so-called pre-charge which charges the smoothing condenser is performed. 
     However, in the electric power system capable of supplying the electric power from the high-voltage system to the backup object load in the abovementioned way, during the pre-charge of the smoothing condenser, the electric power is supplied from the high-voltage battery to the backup object load, there is concern that the pre-charge is hindered, or it takes a long time to finish the pre-charge. 
     SUMMARY 
     An electric power system (for example, an electric power system  1  described below) of a vehicle (for example, a vehicle V described below) includes: a high-voltage circuit (for example, a high-voltage circuit  2  described below), which includes a main circuit (for example, a main circuit  29  described below) and a high-voltage battery (for example, a high-voltage battery  21  described below) connected to the main circuit; a low-voltage battery (for example, a low-voltage battery  31  described below), of which an output voltage is lower than that of the high-voltage battery; a system control part (for example, a system electronic control unit (ECU)  8  described below), which operates by the electric power supplied from the high-voltage circuit or the low-voltage battery and controls the main circuit; a first system control power line (for example, a first system control power line  32  described below), which connects the low-voltage battery and the system control part; a second system control power line (for example, a second system control power line  53  described below), which connects the high-voltage circuit and the system control part; a switch (for example, a second switch  59  described below), which is arranged on the second system control power line; and a switch control part (for example, a backup power supply ECU  57  described below), which opens the switch when the pre-charge of more than one condenser (for example, smoothing condensers C 1 , C 2  described below) which are arranged on the main circuit is performed by supplying electric power to the high-voltage battery. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a structure of an electric vehicle on which an electric power system of an embodiment of the disclosure is mounted. 
         FIG. 2  is a diagram schematically showing an operation example of the electric power system in a normal running of the vehicle. 
         FIG. 3  is a diagram schematically showing an operation example of the electric power system when the vehicle in the normal running collides with an object and a connection of a low-voltage battery with a backup power supply ECU and a system ECU disappears. 
         FIG. 4  is a time chart for describing procedures of a start treatment of the electric power system. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The disclosure provides an electric power system which uses a high-voltage circuit as a backup power supply of a low-voltage circuit, and which can quickly finishes a pre-charge of a condenser arranged in the high-voltage circuit. 
     In the disclosure, a voltage-reducing device (for example, a voltage-reducing device  55  described below) for reducing a voltage of the electric power from the high-voltage circuit is also included; one end of the second system control power line is connected to an output side of the voltage-reducing device, while the other end is connected to the first system control power line; on a side of the first system control power line that is closer to a low-voltage battery than a connecting point of the second system control power line, a first diode (for example, a first diode  33  described below) which permits a current from the low-voltage battery to the system control part is arranged, and on the second system control power line, a second diode (for example, a second diode  54  described below) which permits a current from the voltage-reducing device to the system control part is arranged; the voltage-reducing device preferably reduces the voltage of the electric power from the high-voltage circuit to a voltage that is within an operation voltage scope of the system control part and is lower than an output voltage in a normal state of the low-voltage battery. 
     In the disclosure, the system control part which controls the main circuit is connected to the low-voltage battery by the first system control power line, and the system control part is connected to the high-voltage circuit which includes the high-voltage battery by the second system control power line. In an ordinary vehicle, considering the maintainability by an operator, the system control part or the low-voltage battery which supplies electric power to vehicle auxiliary machines is arranged on a front side of the vehicle inside an engine room in most cases. For this reason, in a collision of the vehicle, a connection between the low-voltage battery and the system control part may disappear, and electric power cannot be supplied from the low-voltage battery to the system control part. In addition, even if the low-voltage battery is arranged inside the vehicle interior, in the collision of the vehicle, low-voltage system may cause layer short, and electric power cannot be supplied from the low-voltage battery to the system control part either. Besides, when the output voltage of the low-voltage battery decreases due to excessive degeneration of the low-voltage battery, there are also cases when the electric power cannot be supplied from the low-voltage battery to the system control part. In contrast, in the disclosure, when electric power cannot be supplied from the low-voltage battery to the system control part because the connection between the low-voltage battery and the system control part disappears, or layer short occurs in the low-voltage system, or the low-voltage battery degenerates excessively, the high-voltage circuit becomes a backup power supply of the low-voltage battery, and necessary electric power can be supplied from the high-voltage circuit to the system control part via the second system control power line. 
     Besides, in the electric power system which uses the high-voltage circuit as the backup power supply of the low-voltage battery in this way, as mentioned above, there is concern that the pre-charge of the condenser arranged on the main circuit is hindered. Therefore, in the disclosure, a switch is arranged on the second system control power line, and when the pre-charge of the more than one condenser arranged on the main circuit is performed, the switch is opened by the switch control part. As a result, during the pre-charge of the condenser, the electric connection between the condenser and the system control part is broken, and the electric power from the high-voltage battery is not supplied to the system control part, so that the pre-charge of the condenser can be finished quickly. 
     In the disclosure, the output side of the voltage-reducing device which reduces the voltage of the electric power from the high-voltage circuit is connected to the first system control power line by the second system control power line, the first diode is arranged on the side of the first system control power line that is closer to the low-voltage battery than the connecting point of the second system control power line, and the second diode is arranged on the second system control power line. Besides, in the disclosure, the electric power from the high-voltage circuit is reduced by the voltage-reducing device to a voltage which is within the operation voltage scope of the system control part and is lower than the output voltage in the state when the low-voltage battery is normal. Therefore, according to the disclosure, during the normal state when the connection between the low-voltage battery and the system control part does not disappear and the output voltage of the low-voltage battery does not decrease because of excessive degeneration, the electric power is supplied from the low-voltage battery with a higher potential to the system control part. Moreover, when malfunction occurs, for example, when the connection between the low-voltage battery and the system control part disappears due to the collision or when the output voltage of the low-voltage battery decreases due to excessive degeneration, the electric power from the high-voltage circuit can be supplied to the system control part via the second system control power line and the voltage-reducing device, and a control by the system control part can be continued. Besides, in the disclosure, by arranging diodes on the first system control power line and the second system control power line separately, the potential on the second system control power line side can be kept by the voltage-reducing device with the operation voltage scope of the system control part, and the electric power from the low-voltage battery can be supplied to the system control part via the first system control power line. Accordingly, when the vehicle collides, power supply of the system control part can be directly switched from the low-voltage battery to the voltage-reducing device side instead of detecting the collision and performing special control accordingly. Therefore, in the disclosure, the electric power can be continuously supplied to the system control part before and after the collision of the vehicle without interruption. Similarly, when the output voltage of the low-voltage battery is lower than the output voltage of the voltage-reducing device due to excessive degeneration, power supply of the system control part can also be switched from the low-voltage battery to the voltage-reducing device side. In addition, when the electric power from the high-voltage circuit may be constantly supplied to the system control part in this way, there is concern that the pre-charge of the condenser arranged on the main circuit is hindered. In contrast, in the disclosure, during the pre-charge of the condenser, the switch arranged on the second system control power line is opened as mentioned above, so that there is no concern of the pre-charge being hindered either. 
     In the following part, an embodiment of the disclosure is described with reference to the drawings. 
       FIG. 1  is a diagram showing a structure of an electric vehicle V (referred to as “vehicle” only hereinafter) on which an electric power system  1  of the embodiment is mounted. In addition, in the embodiment, a so-called hybrid vehicle including an engine E, an electric motor M and a power generator G is described as an example of the vehicle V, but the disclosure is not limited to this. The electric power system of the disclosure can be applied to, without being limited to the hybrid vehicle, any vehicle as long as the vehicle is a vehicle which uses electric power stored in a battery to run, such as an electric car or a fuel cell car. 
     The vehicle V includes the electric power system  1 , the engine E, the electric motor M, the power generator G, and a drive wheel W. The electric motor M mainly generates a power with which the vehicle V runs. The electric motor M is connected to the drive wheel W. A torque generated in the electric motor M by supplying electric power from the electric power system  1  to the electric motor M is transmitted to the drive wheel W via a power transmission mechanism that is not shown, rotates the drive wheel W and makes the vehicle V run. Besides, the electric motor M functions as a power generator during a regenerative deceleration of the vehicle V. The electric power generated by the electric motor M is charged to a high-voltage battery  21  described below which is included in the electric power system  1 . 
     The engine E is connected to the power generator G. The power generator G is driven by the power of the engine E and generates electric power. The electric power generated by the power generator G is charged to the high-voltage battery  21 . In addition, the engine E is connected to the drive wheel W via a power transmission mechanism that is not shown, and the power of the engine E may be used to drive the drive wheel W. 
     The electric power system  1  includes a high-voltage circuit  2  arranged on the high-voltage battery  21 , a low-voltage circuit arranged on a low-voltage battery  31 , a backup power supply unit  5 , and a system ECU  8 . 
     The high-voltage circuit  2  includes: the high-voltage battery  21 ; a high-voltage DCDC converter  22 ; first power lines  26   p,    26   n,  which connect a positive electrode and a negative electrode of the high-voltage battery  21  to a low-voltage side positive electrode terminal  221  and a low-voltage side negative electrode terminal  222  of the high-voltage DCDC converter  22 ; a first inverter  23 ; a second inverter  24 ; second power lines  27   p,    27   n,  which connect a high-voltage side positive electrode terminal  223  and a high-voltage side negative electrode terminal  224  of the high-voltage DCDC converter  22  to a direct-current input-output side of each inverter  23 ,  24 ; a low-voltage DCDC converter  25 , which is connected to the first power lines  26   p,    26   n;  a drive circuit  25   d  of the low-voltage DCDC converter  25 ; and a second smoothing condenser C 2 , which is connected to the second power lines  27   p,    27   n.    
     The high-voltage battery  21  is a secondary battery capable of a discharge which converts chemical energy to electric energy and a charge which converts electric energy to chemical energy. In the following part, a situation in which a so-called lithium ion battery which performs charge and discharge by a movement of the lithium ion between electrodes is used as the high-voltage battery  21  is described, but the disclosure is not limited to this. 
     A positive electrode contactor  28   p  and a negative electrode contactor  28   n  are arranged separately on the first power lines  26   p,    26   n.  The contactors  28   p,    28   n  are normally open contactors which open to break a conduction between two electrodes of the high-voltage battery  21  and the first power lines  26   p,    26   n  in a state when a command signal from outside is not input, and close to connect the high-voltage battery  21  and the first power lines  26   p,    26   n  in a state when the command signal is input. In accordance with the command signal transmitted from a battery ECU  36 , the contactors  28   p,    28   n  use the electric power supplied from the low-voltage battery  31  to open and close. In addition, the positive electrode contactor  28   p  becomes a pre-charge contactor which has a pre-charge resistance for relaxing an inrush current to a plurality of smoothing condensers arranged on the high-voltage circuit  2 . 
     The high-voltage DCDC converter  22  is arranged between the first power lines  26   p,    26   n  and the second power lines  27   p,    27   n.  The low-voltage side positive electrode terminal  221  and the low-voltage side negative electrode terminal  222  of the high-voltage DCDC converter  22  are connected to the high-voltage battery  21  via the first power lines  26   p,    26   n  separately as described above. The high-voltage side positive electrode terminal  223  and the high-voltage side negative electrode terminal  224  of the high-voltage DCDC converter  22  are connected to the first inverter  23  and the second inverter  24  via the second power lines  27   p,    27   n  separately as described above. 
     The high-voltage DCDC converter  22  is a bidirectional DCDC converter which is formed by a combination of a reactor L, a first smoothing condenser C 1 , a high-arm element  225 H, a low-arm element  225 L, and a negative bus  227 . 
     The negative bus  227  is a wiring which connects the low-voltage side negative electrode terminal  222  and the high-voltage side negative electrode terminal  224 . One end side of the first smoothing condenser C 1  is connected to the low-voltage side positive electrode terminal  221 , and the other end side is connected to the negative bus  227 . One end side of the reactor L is connected to the low-voltage side positive electrode terminal  221 , and the other end side is connected to a connection node of the high-arm element  225 H and the low-arm element  225 L. 
     The high-arm element  225 H includes an existing power switching element such as an IGBT or a MOSFET, and a diode connected in parallel to the power switching element. The low-arm element  225 L includes an existing power switching element such as an IGBT or a MOSFET, and a diode connected in parallel to the power switching element. The high-arm element  225 H and the low-arm element  225 L are connected in series in this order between the high-voltage side positive electrode terminal  223  and the negative bus  227 . 
     A collector of the power switching element of the high-arm element  225 H is connected to the high-voltage side positive electrode terminal  223 , and an emitter of the power switching element of the high-arm element  225 H is connected to a collector of the low-arm element  225 L. An emitter of the power switching element of the low-arm element  225 L is connected to the negative bus  227 . A forward direction of the diode arranged on the high-arm element  225 H is a direction heading from the reactor L toward the high-voltage side positive electrode terminal  223 . Besides, a forward direction of the diode arranged on the low-arm element  225 L is a direction heading from the negative bus  227  toward the reactor L. 
     The high-arm element  225 H and the low-arm element  225 L are separately turned on or off by a gate drive signal generated by a gate drive circuit (not shown) included in the system ECU  8 . 
     The high-voltage DCDC converter  22  fulfills a voltage-raising function and a voltage-reducing function by driving the element  225 H,  225 L ON/OFF according to the gate drive signal generated from the gate drive circuit of the system ECU  8  in a predetermined timing. The voltage-raising function is a function which raises a voltage applied to the terminals  221 ,  222  on the low-voltage side and outputs the voltage to the terminals  223 ,  224  on the high-voltage side, by which a current flows from the first power lines  26   p,    26   n  to the second power lines  27   p,    27   n.  Besides, the voltage-reducing function is a function which reduces a voltage applied to the terminals  223 ,  224  on the high-voltage side and outputs the voltage to the terminals  221 ,  222  on the low-voltage side, by which a current flows from the second power lines  27   p,    27   n  to the first power lines  26   p,    26   n.  In addition, in the following part, a potential difference between the first power lines  26   p  and  26   n  is referred as V 1 , and a potential difference between the second power lines  27   p  and  27   n  is referred to as V 2 . 
     The first inverter  23  and the second inverter  24  are, for example, pulse width modulation PWM inverters including a bridge circuit formed by bridge connecting a plurality of switching elements (for example, IGBT), and have a function of converting a direct-current electric power and an alternating-current electric power. The first inverter  23  is connected to the second power lines  27   p,    27   n  on the direct-current input-output side, and is connected to a U-phase coil, a V-phase coil and a W-phase coil of the electric motor M on the alternating-current input-output side. The second inverter  24  is connected to the second power lines  27   p,    27   n  on the direct-current input-output side, is connected to a U-phase coil, a V-phase coil and a W-phase coil of the power generator G on the alternating-current input-output side. 
     The first inverter  23  is framed by bridge connecting, in each phase, a high-side U-phase switching element and a low-side U-phase switching element which are connected to the U-phase of the electric motor M, a high-side V-phase switching element and a low-side V-phase switching element which are connected to the V-phase of the electric motor M, and a high-side W-phase switching element and a low-side W-phase switching element which are connected to the W-phase of the electric motor M. 
     By driving the switching elements of each phase ON/OFF according to the gate drive signal generated from the gate drive circuit of the system ECU  8  in a predetermined timing, the first inverter  23  converts the direct-current electric power supplied from the high-voltage DCDC converter  22  to an alternating-current electric power and supplies to the electric motor M, or converts the alternating-current electric power supplied from the electric motor M to a direct-current electric power and supplies to the high-voltage DCDC converter  22 . 
     The second inverter  24  is formed by bridge connecting, in each phase, a high-side U-phase switching element and a low-side U-phase switching element which are connected to the U-phase of the power generator G, a high-side V-phase switching element and a low-side V-phase switching element which are connected to the V-phase of the power generator G, and a high-side W-phase switching element and a low-side W-phase switching element which are connected to the W-phase of the power generator G. 
     By driving the switching elements of each phase ON/OFF according to the gate drive signal generated from the gate drive circuit of the system ECU  8  in a predetermined timing, the second inverter  24  converts the direct-current electric power supplied from the high-voltage DCDC converter  22  to an alternating-current electric power and supplies to the electric motor G, or converts the alternating-current electric power supplied from the electric motor G to a direct-current electric power and supplies to the high-voltage DCDC converter  22 . 
     The low-voltage DCDC converter  25  is connected in parallel with the high-voltage DCDC converter  22  to the first power lines  26   p,    26   n.  By using the electric power supplied from the backup power supply unit  5  to drive the switching element of the low-voltage DCDC converter  25  ON/OFF, the drive circuit  25   d  reduces the voltage V 1  between the first power lines  26   p,    26   n  and supplies the voltage to the low-voltage battery  31  to charge the low-voltage battery  31 . 
     The system ECU  8  is a microcomputer which is responsible for a running control of the vehicle V, more specifically, a control of the main circuit  29  including the high-voltage DCDC converter  22 , the first inverter  23 , the second inverter  24 , and the second smoothing condenser C 2  in the high-voltage circuit  2 . The system ECU  8  uses the electric power supplied from the low-voltage battery  31  or the backup power supply unit  5  to operate. When a start switch (not shown) is turned on by a driver to start the electric power system  1 , the system ECU  8  starts under the electric power supplied from the low-voltage battery  31 , and then uses the electric power supplied from the low-voltage battery  31  or the backup power supply unit  5  to operate. 
     The low-voltage circuit includes the low-voltage battery  31 , a first system control power line  32 , a first diode  33 , a collision detection part  35  and a battery ECU  36 . 
     The low-voltage battery  31  is a secondary battery capable of a discharge which converts chemical energy to electric energy and a charge which converts electric energy to chemical energy. In the embodiment, a situation is described in which a lead battery using lead in an electrode is used as the battery  3 , but the disclosure is not limited to this. Besides, in the following part, a situation is described in which the output voltage of the low-voltage battery  31  is lower than the output voltage of the high-voltage battery  21 . In addition, in the following part, a situation is described in which the low-voltage battery  31  is arranged, with consideration of the maintainability by the operator, on a vehicle front side in an un-illustrated engine room of the vehicle V, but the disclosure is not limited to this. 
     The first system control power line  32  is a power-feeding line which connects the low-voltage battery  31  with a backup power supply ECU  57  and the system ECU  8 , and supplies electric power from the low-voltage battery  31  to the backup power supply ECU  57  and the system ECU  8 . In addition, in the following part, a voltage of the first system control power line  32 , namely the output voltage of the low-voltage battery  31  is referred to as VB. 
     The first diode  33  is arranged on the first system control power line  32 . A forward direction of the first diode  33  is a direction heading from the low-voltage battery  31  toward the backup power supply ECU  57  and the system ECU  8 , and permits a current from the low-voltage battery  31  to the backup power supply ECU  57  and the system ECU  8 . 
     The collision detection part  35  determines whether the vehicle V collides or overturns by using a detection signal of an acceleration sensor (not shown), and transmits a collision detection signal to the battery ECU  36  when a collision or overturning is determined. The collision detection part  35  uses the electric power supplied from the low-voltage battery  31  to operate. 
     The battery ECU  36  is a microcomputer which is responsible for a control related to monitoring of ON/OFF of the contactors  28   p,    28   n  or the state of the high-voltage battery  21  and the low-voltage battery  31 . The battery ECU  36  uses the electric power supplied from the low-voltage battery  31  to operate. 
     When the start switch is turned on by the driver, the battery ECU  36  starts under the electric power supplied from the low-voltage battery  31 , and starts the pre-charge of a plurality of smoothing condensers C 1 -C 2  arranged on the high-voltage circuit  2 . More specifically, the battery ECU  36  performs the pre-charge of the smoothing condensers C 1 -C 2  by turning on the contactors  28   p,    28   n  and connecting the high-voltage battery  21  to the first power lines  26   p,    26   n.  In addition, when performing the pre-charge of the smoothing condensers C 1 -C 2 , the battery ECU  36  turns on the negative electrode contactor  28   n,  and turns a contactor with a pre-charge resistance in the positive electrode contactor  28   p.  Besides, after the pre-charge of the smoothing condensers C 1 -C 2  is finished, the battery ECU  36  turns on a contactor without a pre-charge resistance in the positive electrode contactor  28   p.  Accordingly, an inrush current to the smoothing condensers C 1 -C 2  during the implementation of the pre-charge can be relaxed. 
     Besides, the battery ECU  36  and the backup power supply ECU  57  described below can perform a CAN communication via a CAN bus that is not shown. Accordingly, during the pre-charge of the smoothing condensers C 1 -C 2  performed by procedures as mentioned above, the battery ECU  36  transmits a CAN signal showing that the pre-charge of the smoothing condensers C 1 -C 2  is being implemented to the backup power supply ECU  57  so that that the pre-charge being performed is not hindered. 
     Besides, after the contactors  28   p,    28   n  are turned on in the abovementioned way, when the start switch is turned off by the driver to stop the electric power system  1 , or when a collision detection signal is received from the collision detection part  35 , the battery ECU  36  turns off the contactors  28   p,    28   n  and disconnects the high-voltage battery  21  from the first power lines  26   p,    26   n.    
     The backup power supply unit  5  includes a third power line  51 , a second system control power line  53 , a second diode  54 , a voltage-reducing device  55 , a power supply IC  56 , the backup power supply ECU  57 , a first switch  58 , and a second switch  59 . 
     The third power line  51  is a power-feeding line which connects the second power line  27   p  of the high-voltage circuit  2  and the drive circuit  25   d  of the low-voltage DCDC converter  25 , and supplies electric power from the second power line  27   p  to the drive circuit  25   d.    
     The voltage-reducing device  55  is arranged on the third power line  51 . The voltage-reducing device  55  is an insulating DCDC converter which includes: a transformer, of which a primary side is connected to the second power line  27   p  side, and a secondary side is connected to the drive circuit  25   d  side; and a switching element, which interrupts a current flowing to the primary side of the transformer. After the start switch is turned on and the contactors  28   p,    28   n  are turned on in the abovementioned way, by using the electric power supplied from the first power line  26   p  (or the second power line  27   p ) to drive the switching element of the voltage-reducing device  55  ON/OFF, the power supply IC  56  reduces a voltage of the electric power supplied from the second power line  27   p  and outputs the electric power to the drive circuit  25   d  side. 
     The first switch  58  is arranged on the side of the third power line  51  that is closer to the drive circuit  25   d  than the voltage-reducing device  55 , and interrupts the connection between the voltage-reducing device  55  and the drive circuit  25   d.  The first switch  58  is a normally open switch which opens to break the conduction between the voltage-reducing device  55  and the drive circuit  25   d  in a state when a command signal from outside is not input, and closes to connect the voltage-reducing device  55  and the drive circuit  25   d  in a state when the command signal is input. The first switch  58  opens and closes in accordance with the command signal transmitted from the backup power supply ECU  57 . 
     The second system control power line  53  is a power-feeding line which connects a position in the third power line  51  and between the first switch  58  and the voltage-reducing device  55  with the side of the first system control power line  32  that is closer to a backup power supply ECU  57  and system ECU  8  than the first diode  33 , and supplies electric power from the voltage-reducing device  55  to the backup power supply ECU  57  and the system ECU  8 . In addition, in the following part, a voltage of the second system control power line  53 , namely the output voltage of the voltage-reducing device  55  is referred to as Vcc. In addition, in the following part, a situation is described in which the second system control power line  53  is connected to the first system control power line  32 , but the disclosure is not limited to this. The second system control power line  53  may also be directly connected to the backup power supply ECU  57  and the system ECU  8  without using the first system control power line  32 . 
     The second switch  59  is arranged on the second system control power line  53 , and interrupts the connection of the voltage-reducing device  55  with the backup power supply ECU  57  and the system ECU  8 . The second switch  59  is a normally open switch which opens to break the conduction of the voltage-reducing device  55  with the backup power supply ECU  57  and the system ECU  8  in a state when a command signal from outside is not input, and closes to connect the voltage-reducing device  55  with the backup power supply ECU  57  and the system ECU  8  in a state when the command signal is input. The second switch  59  opens and closes in accordance with the command signal transmitted from the backup power supply ECU  57 . 
     The second diode  54  is arranged on the side of the second system control power line  53  that is closer to the first system control power line  32  than the second switch  59 . A forward direction of the second diode  54  is a direction heading from the voltage-reducing device  55  toward the backup power supply ECU  57  and the system ECU  8 , and permits a current from the voltage-reducing device  55  to the backup power supply ECU  57  and the system ECU  8 . 
     Here, a setting of the output voltage Vcc of the voltage-reducing device  55  is described. As shown in  FIG. 1 , the low-voltage battery  31  and the voltage-reducing device  55 , which are electric power supply source, are connected separately via the diodes  33 ,  54  to the system ECU  8  which is responsible for the running control of the vehicle V. For this reason, in the system ECU  8 , electric power may be selectively supplied from one of the two electric power supply sources with a higher potential. In the embodiment, the low-voltage battery  31  is set as a main power supply of the system ECU  8 , and the voltage-reducing device  55  is used as a backup power supply when a failure occurs in the low-voltage battery  31  (more specifically, when the connection between the low-voltage battery  31  and the system ECU  8  disappears due to the collision of the vehicle V, or when the low-voltage battery  31  is not in a normal state), so that the output voltage Vcc of the voltage-reducing device  55  is set so as to be lower than the output voltage VB which is within the operation voltage scope of the system ECU  8  and is a voltage in the normal state of the low-voltage battery  31 . Here, the abnormal state of the low-voltage battery  31  means, for example, a state in which the output voltage of the low-voltage battery  31  is much lower, due to an excessive degeneration of the low-voltage battery  31 , than the voltage of the time when it is newly made. 
     The backup power supply ECU  57  is a microcomputer responsible for a control related to the ON/OFF of the first switch  58  and the second switch  59 . The backup power supply ECU  57  uses the electric power supplied from the low-voltage battery  31  or the backup power supply unit  5  to operate. 
     When the start switch is turned on by the driver, similar to the system ECU  8 , the backup power supply ECU  57  starts under the electric power supplied from the low-voltage battery  31 , and then uses the electric power supplied from the low-voltage battery  31  or the backup power supply unit  5  to operate. Besides, after starting under the electric power supplied from the low-voltage battery  31 , the backup power supply ECU  57  turns on both the first switch  58  and the second switch  59 , and becomes a state capable of supplying electric power from the voltage-reducing device  55  to the drive circuit  25   d,  the backup power supply ECU  57 , and the system ECU  8 . 
     On the other hand, when the start switch is turned on as mentioned above, the contactors  28   p,    28   n  are turned on by the battery ECU  36 , and the pre-charge of the smoothing condensers C 1 -C 2  is performed. Accordingly, while a CAN signal showing that the pre-charge of the smoothing condensers C 1 -C 2  is being implemented is received from the battery ECU  36 , the backup power supply ECU  57  turns on at least the second switch  59  in the first switch  58  and the second switch  59  in a way that the pre-charge of the smoothing condensers C 1 -C 2  is not hindered. After that, when the pre-charge of the smoothing condensers C 1 -C 2  is finished, and a CAN signal showing that the pre-charge is finished is received from the battery ECU  36 , the backup power supply ECU  57  turns on the switch which is in an off state till then. Besides, by starting the pre-charge of the smoothing condensers C 1 -C 2  as mentioned above, if the voltage V 1  or V 2  exceeds a predetermined threshold, the power supply IC  56  starts the voltage-reducing device  55 , and uses the output electric power of the voltage-reducing device  55  to start the drive of the low-voltage DCDC converter  25  by the drive circuit  25   d.    
     Next, an operation example of the electric power system  1  is described with reference to  FIG. 2  and  FIG. 3 . 
       FIG. 2  is a diagram schematically showing an operation example of the electric power system  1  in the normal running of the vehicle V. Here, in the normal running means, more specifically, after the pre-charge of the smoothing condensers C 1 -C 2  is finished. 
     As mentioned above, the backup power supply ECU  57  turns on both the first switch  58  and the second switch  59  in the normal running, and the voltage-reducing device  55  reduces the voltage of the electric power supplied from the second power line  27   p  and outputs the electric power. The drive circuit  25   d  consumes the output electric power of the voltage-reducing device  55  to drive the low-voltage DCDC converter  25 . As a result, the low-voltage DCDC converter  25  reduces a voltage of the electric power supplied from the first power line  26   p  and charges the low-voltage battery  31 . In addition, at this moment, the output voltage Vcc of the voltage-reducing device  55  is set so as to be lower than the output voltage VB of the low-voltage battery  31  in this way. For this reason, the electric power from the low-voltage battery  31  with a higher potential is supplied to the backup power supply ECU  57  and the system ECU  8 . The system ECU  8  operates by the electric power supplied from the low-voltage battery  31  and controls the main circuit  29 . 
       FIG. 3  is a diagram schematically showing an operation example of the electric power system  1  when the vehicle V in the normal running collides with an object and a connection of the low-voltage battery  31  with the backup power supply ECU  57  and the system ECU  8  disappears. In addition, for example, when the voltage of the low-voltage battery  31  decreases due to an occurrence of layer short in the low-voltage system to which the electric power from the low-voltage battery  31  is supplied, the operation is also the same. 
     When the vehicle V collides, the battery ECU  36  turns the contactors  28   p,    28   n  off and disconnects the high-voltage battery  21  from the first power lines  26   p,    26   n  as mentioned above. 
     In addition, immediately after the high-voltage battery  21  is disconnected from the first power lines  26   p,    26   n,  a large amount of electric charge remains in the smoothing condensers C 1 -C 2  arranged on the high-voltage circuit  2 , therefore it is necessary to discharge quickly and quickly decrease the potential of the high-voltage circuit  2 . Accordingly, the voltage-reducing device  55  reduces the voltage of the electric power supplied from the smoothing condensers C 1 , C 2  via the second power line  27   p  and supplies the electric power to the drive circuit  25   d.  The drive circuit  25   d  uses the electric power supplied from the voltage-reducing device  55  to drive the switching element of the low-voltage DCDC converter  25  ON/OFF, thereby consuming the electric power remaining in the smoothing condensers C 1 -C 2 . 
     Besides, when the connection between the low-voltage battery  31  and the system ECU  8  disappears due to the collision of the vehicle V, the output voltage Vcc of the voltage-reducing device  55  is higher than the output voltage VB of the low-voltage battery  31 , and a state is achieved in which the electric power can be supplied from the voltage-reducing device  55  to the backup power supply ECU  57  and the system ECU  8 . Accordingly, the voltage-reducing device  55  reduces the voltage of the electric power supplied from the smoothing condensers C 1 , C 2  via the second power line  27   p,  and as mentioned above, supplies the electric power to the drive circuit  25   d  as well as the backup power supply ECU  57  and the system ECU  8 , thereby consuming the electric power remaining in the smoothing condensers C 1 -C 2  by the backup power supply ECU  57  and the system ECU  8 . 
     Besides, after the collision of the vehicle V, the system ECU  8  uses the electric power supplied from the backup power supply unit  5  to operate as mentioned above, and performs, for example, a control which stops an idling of the electric motor  3 , or a discharge control which actively consumes the electric power stored in the smoothing condensers C 1 -C 2 . 
     As mentioned above, immediately after the collision of the vehicle V, the electric power remaining in the smoothing condensers C 1 -C 2  is supplied to the drive circuit  25   d,  the backup power supply ECU  57 , and the system ECU  8  via the backup power supply unit  5  and consumed by these parts, therefore the potential of the high-voltage circuit  2  can be quickly reduced. Here, as shown in  FIG. 3 , the electric power remaining in the first smoothing condenser C 1  is supplied to the voltage-reducing device  55  via the reactor L, the diode of the high-arm element  225 H, the second power line  27   p  and the third power line  51 . Besides, as shown in  FIG. 3 , the electric power remaining in the second smoothing condenser C 2  is supplied to the voltage-reducing device  55  via the second power line  27   p  and the third power line  51 . 
     Here, an advantage of connecting the third power line  51  to the second power line  27   p  of the high-voltage circuit  2  is described by a comparison with a situation in which the third power line is connected to the first power line  26   p  of the high-voltage circuit  2 . The second smoothing condenser C 2  is connected in parallel with respect to a serial connection body of the high-arm element  225 H and the low-arm element  225 L of the high-voltage DCDC converter  22 . For this reason, when the third power line is connected to the first power line  26   p,  the electric charge remaining in the second smoothing condenser C 2  cannot be supplied to the voltage-reducing device  55  unless the high-arm element  225 H is turned on. On the contrary, in the electric power system  1 , by connecting the third power line  51  to the second power line  27   p,  when the vehicle V collides, the electric charge remaining in the two smoothing condensers C 1 -C 2  is quickly supplied to the voltage-reducing device  55  via the third power line  51  and can be consumed by the drive circuit  25   d.    
     Next, a start treatment of the electric power system  1  is described with reference to  FIG. 4 .  FIG. 4  is a time chart for describing procedures of the start treatment of the electric power system  1 . In  FIG. 4 , the output voltage VB of the low-voltage battery  31 , the backup power supply ECU  57 , the first switch  58 , the voltage V 1  between the first power lines  26   p,    26   n  of the high-voltage circuit  2 , the voltage V 2  between the second power lines  27   p,    27   n  of the high-voltage circuit  2 , the output voltage Vcc of the voltage-reducing device  55 , and the second switch  59  are sequentially shown from the top. 
     At the beginning, at time t 1 , the start switch is turned on in order to start the electric power system  1 , the output voltage VB of the low-voltage battery  31  rises accordingly, and the electric power from the low-voltage battery  31  is started to be supplied to the backup power supply ECU  57 , the system ECU  8 , and the battery ECU  36 . After that, at time t 2 , the backup power supply ECU  57  rises. At time t 3 , the backup power supply ECU  57  turns the first switch  58  and the second switch  59  on and connects the voltage-reducing device  55  to the drive circuit  25   d  of the low-voltage DCDC converter  25 . 
     After that, at time t 4 , the battery ECU  36  turns on the contactors  28   p,    28   n  to connect the high-voltage battery  21  to the smoothing condensers C 1 -C 2 , and starts the pre-charge of the smoothing condensers C 1 -C 2 . As a result, after time t 4 , the voltage V 1 , V 2  of the high-voltage circuit  2  starts to increase. Moreover, at time t 4 , the backup power supply ECU  57  receives, from the battery ECU  36 , a CAN signal showing that the pre-charge of the smoothing condensers C 1 -C 2  is being implemented; accordingly, a portion of the electric power necessary for the pre-charge is supplied to the system ECU  8  or the backup power supply ECU  57 , and the second switch  59  is turned off so that the pre-charge is not hindered. 
     After that, at time t 5 , the voltage V 1  between the first power lines  26   p,    26   n  (or the voltage V 2  between the second power lines  27   p,    27   n ) exceeds a predetermined start voltage; accordingly, the power supply IC  56  starts and the output voltage Vcc of the voltage-reducing device  55  rises. However, at this time, the backup power supply ECU  57  turns the second switch  59  off, so that the output electric power of the voltage-reducing device  55  is not supplied to the backup power supply ECU  57  or the system ECU  8 . 
     After that, at time t 6 , the voltage V 1 , V 2  of the high-voltage circuit  2  increase up to the predetermined threshold; accordingly, the battery ECU  36  determines that the pre-charge of the smoothing condensers C 1 -C 2  is finished, and transmits a CAN signal expressing this meaning to the backup power supply ECU  57 . The backup power supply ECU  57  turns on the second switch  59  which is off till then according to the receiving of the CAN signal. As a result, the conduction of the voltage-reducing device  55  with the backup power supply ECU  57  and the system ECU  8  is achieved, and after time t 6 , the electric power can be supplied from the voltage-reducing device  55  to the ECU  57  and the ECU  8 . 
     According to the electric power system  1  of the embodiment, the following effects can be achieved. 
     (1) In the electric power system  1 , the system ECU  8  controlling the main circuit  29  is connected to the low-voltage battery  31  by the first system control power line  32 , and the system ECU  8  is connected to the high-voltage circuit  2  including the high-voltage battery  21  via the second system control power line  53  and the voltage-reducing device  55 . In the electric power system  1 , when the electric power cannot be supplied from the low-voltage battery  31  to the system ECU  8  because the connection of the low-voltage battery  31  and the system ECU  8  disappears due to a collision, or because layer short occurs in the low-voltage system, or because the low-voltage battery  31  degenerates excessively, the backup power supply unit  5  sets the high-voltage circuit  2  as the backup power supply of the low-voltage battery  31 , and is able to supply necessary electric power from the high-voltage circuit  2  to the system ECU  8  via the second system control power line  53  and the voltage-reducing device  55 . 
     Besides, as mentioned above, in the electric power system  1  which uses the high-voltage circuit  2  as the backup power supply of the low-voltage battery  31  in this way, there is concern that the pre-charge of the smoothing condensers C 1 -C 2  arranged on the main circuit  29  is hindered. Accordingly, in the electric power system  1 , the second switch  59  is arranged on the second system control power line  53 , and when performing the pre-charge of the smoothing condensers C 1 -C 2  arranged on the main circuit  29 , the second switch  59  is turned off by the backup power supply ECU  57 . As a result, during the pre-charge of the smoothing condensers C 1 -C 2 , the electric connection between the smoothing condensers C 1 -C 2  and the system ECU  8  is blocked, and the electric power from the high-voltage battery  21  is not supplied to the system ECU  8 , so that the pre-charge of the smoothing condensers C 1 -C 2  can be quickly finished. 
     (2) In the electric power system  1 , the output side of the voltage-reducing device  55  is connected to the first system control power line  32  by the second system control power line  53 , the first diode  33  is arranged on the side of the first system control power line  32  that is closer to the low-voltage battery  31  than the connecting point of the second system control power line  53 , and the second diode  54  is arranged on the second system control power line  53 . Besides, in the electric power system  1 , the voltage of the electric power from the high-voltage circuit  2  is reduced by the voltage-reducing device  55  to a voltage which is within the operation voltage scope of the system ECU  8  and is lower than the output voltage in the normal state of the low-voltage battery  31 . Therefore, according to the electric power system  1 , in the normal state when the connection between the low-voltage battery  31  and the system ECU  8  does not disappear and the output voltage of the low-voltage battery  31  does not decrease due to excessive degeneration, electric power is supplied from the low-voltage battery  31  with a higher potential to the system ECU  8 . Besides, when malfunction occurs, for example, when the connection between the low-voltage battery  31  and the system ECU  8  disappears due to collision or when the output voltage of the low-voltage battery  31  decreases due to excessive degeneration, the electric power from the high-voltage circuit  2  can be supplied to the system ECU  8  via the second system control power line  53  and the voltage-reducing device  55 , and the control by the system ECU  8  can be continued. In addition, in the electric power system  1 , the diodes  33 ,  54  are arranged separately on the system control power lines  32 ,  53 , by which the potential on the second system control power line  53  side can be maintained within the operation voltage scope of the system ECU  8  by the voltage-reducing device  55 , and the electric power from the low-voltage battery  31  can be supplied to the system ECU  8  via the first system control power line  32 . As a result, when the vehicle V collides, the power supply of the system ECU  8  can be directly switched from the low-voltage battery  31  to the voltage-reducing device  55  side instead of detecting the collision and performing special control accordingly. Therefore, in the electric power system  1 , electric power can be continuously supplied to the system ECU  8  before and after the collision of the vehicle V without interruption. Similarly, when the output voltage VB of the low-voltage battery  31  is lower than the output voltage Vcc of the voltage-reducing device  55  due to excessive degeneration, the power supply of the system ECU  8  can also be switched from the low-voltage battery  31  to the voltage-reducing device  55  side. In addition, when the electric power from the high-voltage circuit  2  can be constantly supplied to the system ECU  8  in this way, there is concern that the pre-charge of the smoothing condensers C 1 -C 2  arranged on the main circuit  29  is hindered. In contrast, in the electric power system  1 , as mentioned above, the second switch  59  arranged on the second system control power line  53  is turned off during the pre-charge of the smoothing condensers C 1 -C 2 , so that there is no concern that the pre-charge is hindered. 
     In the above, one embodiment of the disclosure is described, but the disclosure is not limited to this. Detailed structure may be properly altered within the scope of the spirit of the disclosure. For example, detailed circuit structures of the high-voltage DCDC converter  22 , the first inverter  23 , and the second inverter  24  are not limited to the structures shown in  FIG. 1 - FIG. 3 . In particular, other existing circuits besides the circuits shown in  FIG. 1 - FIG. 3 , such as a multilevel chopper circuit, an interleaving circuit, and a circuit with magnetic coupling, may be used as the high-voltage DCDC converter  22 . For example, the multilevel chopper circuit includes a condenser which is independent of the first smoothing condenser C 1  and is arranged in parallel to a serial connection body of the switching element or the diode. During the collision of the vehicle V, similar to the second smoothing condenser C 2 , the condenser can quickly supply the remaining electric charge to the voltage-reducing device  55  via the third power line  51  and consume the electric charge by the drive circuit  25   d.  For this reason, the multilevel chopper circuit has many advantages when applied to the disclosure. 
     Besides, in the abovementioned embodiment, the situation is described in which the backup power supply ECU  57  turns the first switch  58  on and turns the second switch  59  off while the pre-charge of the smoothing condensers C 1 -C 2  is performed by the battery ECU  36 , but the disclosure is not limited to this. According to the abovementioned embodiment, by turning the first switch  58  on even when the pre-charge is performed, a state can be achieved in which the low-voltage DCDC converter  25  can be quickly utilized because the voltage-reducing device  55  is connected to the drive circuit  25   d  of the low-voltage DCDC converter  25 . However, when the consumed electric power of the drive circuit  25   d  increases, there is also concern that the pre-charge is hindered if the first switch  58  is continued to be turned on even when the pre-charge is performed. Therefore, in such a case, the backup power supply ECU  57  may turn off both the second switch  59  and the first switch  58  while the pre-charge of the smoothing condensers C 1 -C 2  is performed by the battery ECU  36 .