Patent Publication Number: US-2017361712-A1

Title: Electric vehicle

Description:
TECHNICAL FIELD 
     The disclosure herein relates to an electric vehicle. The electric vehicle herein referred to generally means an automobile that has a motor configured to drive a wheel. The electric vehicle includes, but is not particularly limited to: a rechargeable electric vehicle recharged with external electric power; a fuel cell vehicle that has a fuel cell; a solar cell vehicle that has a solar cell; a hybrid vehicle that further has an engine; and an automobile that has two or more of these features. 
     BACKGROUND 
     The electric vehicle has been known. The electric vehicle has a motor that drives a wheel. In a power supply circuit that supplies electric power to the motor, a smoothing capacitor can be provided in addition to a DC-DC converter or an inverter, for example. The smoothing capacitor stores electrical charges to thereby restrain fluctuations in voltage within the power supply circuit. While the electric vehicle is used, electrical charges are stored in the smoothing capacitor at a high voltage. Accordingly, when the electric vehicle crashes, the smoothing capacitor is required to be quickly discharged. 
     To discharge the smoothing capacitor, the electric vehicle can further include a processor that performs a discharge process. The discharge process is a process of, when the electric vehicle crashes, discharging the smoothing capacitor by controlling the power supply circuit. For example, the processor can discharge the smoothing capacitor through the motor by controlling an inverter circuit. In this case, the processor can adjust a current that flows in the motor such that an output torque of the motor becomes zero. Such control is referred to as zero torque control. One example of the art described above is described in Japanese Patent Application Publication No. 2006-141158. 
     SUMMARY 
     The electric vehicle may further include a power source and a relay circuit. The power source may be an accessory battery, for example, and is electrically connected to each of a plurality of electric loads including the processor via a corresponding fuse. The relay circuit is electrically connected between the power source and the processor, and is driven to electrically connect between the power source and the processor in response to a relay drive signal outputted from the processor. According to such a configuration, when the processor quits its operation, for example, the processor can quit outputting the relay drive signal, to thereby electrically disconnect between itself and the power source. 
     When the electric vehicle crashes, a conductive path (e.g., a wire harness) that connects the power source and any of the electric loads, or the electric load itself may be damaged, which may cause a short circuit in the power source. In this case, a corresponding fuse is blown to thereby quickly resolve the short circuit in the power source, and electric power supply to the other electric loads is resumed. However, an output voltage of the power source temporarily decreases during a period from the occurrence of a short circuit to the blowout of the fuse, and hence there may be a case where the processor quits its operation. If the processor quits its operation, the output of the relay drive signal by the processor is also quitted, and the driving of the relay circuit is also quitted. Consequently, the power source and the processor are electrically disconnected. In this case, even if the output voltage of the power source is subsequently recovered, there may be a case where the processor cannot be activated again and cannot discharge the smoothing capacitor. 
     The present disclosure provides a technique capable of activating the processor again when the output voltage of the power source temporarily decreases and the processor quits its operation. 
     An electric vehicle disclosed herein may comprise: a motor configured to drive a wheel; a smoothing capacitor provided within a power supply circuit that supplies electric power to the motor; a processor configured to perform a discharge process when the electric vehicle crashes, the discharge process discharging the smoothing capacitor by controlling the power supply circuit; a power source connected to each of a plurality of each of electric loads including the processor via a corresponding fuse; a relay circuit electrically connected between the power source and the processor and configured to be driven to electrically connect between the power source and the processor in response to a relay drive signal outputted from the processor; and a holding circuit configured to temporarily hold the relay circuit in a driven state when the processor quits outputting the relay drive signal. 
     In this electric vehicle as well, when the above-mentioned short circuit in the power source occurs, there may be a ease where the processor quits its operation owing to a temporary decrease in the output voltage of the power source. When the processor quits its operation, the output of the relay drive signal from the processor is also quitted. However, even if the processor quits outputting the relay drive signal, the holding circuit temporarily holds the relay circuit in a driven state. Meanwhile, if the output voltage of the power source is recovered, the processor can be activated again and resume outputting the relay drive signal. Then the processor can discharge the smoothing capacitor by performing the discharge process. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram that schematically shows a configuration of a hybrid vehicle  10 ; 
         FIG. 2  schematically shows an internal configuration of a power supply circuit  32 ; 
         FIG. 3  schematically shows an internal configuration of a motor control unit  44 ; 
         FIG. 4  shows one example of a time chart according to a discharge process by a processor  62 ; 
         FIG. 5  shows one example of a short circuit that occurs in an accessory battery  34 ; 
         FIG. 6  shows one example of a time chart according to the discharge process by the processor  62  in a case where the accessory battery  34  is short-circuited; 
         FIG. 7  schematically shows an internal configuration of a motor control unit  144  in a variation; 
         FIG. 8  shows one example of a time chart according to the discharge process by the processor  62  in the variation. In  FIGS. 4, 6, and 8 , the same signs indicate the same or corresponding indices; and 
         FIG. 9  schematically shows an internal configuration of a motor control unit  244  in another variation. 
     
    
    
     DETAILED DESCRIPTION 
     Representative, non-limiting examples of the present invention will now be described in further detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Furthermore, each of the additional features and teachings disclosed below may be utilized separately or in conjunction with other features and teachings to provide improved electric vehicles, as well as methods for using and manufacturing the same. 
     Moreover, combinations of features and steps disclosed in the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Furthermore, various features of the above-described and below-described representative examples, as well as the various independent and dependent claims, may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings. 
     All features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter, independent of the compositions of the features in the embodiments and/or the claims. In addition, all value ranges or indications of groups of entities are intended to disclose every possible intermediate value or intermediate entity for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter. 
     A hybrid vehicle  10  in one embodiment will be described with reference to the drawings. The hybrid vehicle  10  is one example of an electric vehicle disclosed herein. The configuration of the hybrid vehicle  10  described below can also be applied to other types of electric vehicles. As shown in  FIG. 1 , the hybrid vehicle  10  in the present embodiment includes a vehicle body  12 , and four wheels  14  and  16  supported rotatably relative to the vehicle body  12 . The four wheels  14  and  16  include a pair of driving wheels  14  and a pair of driven wheels  16 . The pair of driving wheels  14  is connected to an output shaft  20  via a differential gear  18 . The output shaft  20  is supported rotatably relative to the vehicle body  12 . As one example, the pair of driving wheels  14  is rear wheels positioned at a rear portion of the vehicle body  12 , while the pair of driven wheels  16  is front wheels positioned at a front portion of the vehicle body  12 . The pair of driving wheels  14  is disposed coaxially with each other, and the pair of driven wheels  16  is also disposed coaxially with each other. 
     The hybrid vehicle  10  further includes an engine  22 , a first motor generator  24  ( 1 MG in the drawing), and a second motor generator  26  ( 2 MG in the drawing). The engine  22  combusts fuel such as gasoline, and outputs power. Each of the first and second motor generators  24  and  26  is a three-phase motor generator that has a U phase, a V phase and a W phase. In the following, the first motor generator  24  is simply referred to as the first motor  24 , and the second motor generator  26  is simply referred to as the second motor  26 . The engine  22  is connected to the output shaft  20  and the first motor  24  via a power distribution mechanism  28 . The power distribution mechanism  28  distributes the power outputted by the engine  22 , to the output shaft  20  and the first motor  24 . As one example, the power distribution mechanism  28  in the present embodiment has a planetary gear mechanism. The second motor  26  is connected to the output shaft  20 . With such a configuration, the first motor  24  functions as a generator driven by the engine  22 . Moreover, the first motor  24  also functions as a starter motor for starting the engine  22 . On the other hand, the second motor  26  primarily functions as a motor that drives the pair of driving wheels  14 . Moreover, the second motor  26  also functions as a generator when the hybrid vehicle  10  carries out regenerative braking. 
     The hybrid vehicle  10  further includes a main battery  30  and a power supply circuit  32 . The main battery  30  is electrically connected to the first and second motors  24  and  26  via the power supply circuit  32 . The main battery  30  is a rechargeable battery, and although no particular limitation is imposed on the main battery  30 , it has a plurality of lithium-ion cells. The power supply circuit  32  supplies electric power from the main battery  30  to each of the first and second motors  24  and  26 . Moreover, the power supply circuit  32  supplies electric power generated at the first motor  24  or the second motor  26  to the main battery  30 . As one example, the main battery  30  in the present embodiment has a rated voltage of approximately 200 volts, and each of the first and second motors  24  and  26  has a rated voltage of approximately 600 volts. In other words, the main battery  30  has a rated voltage lower than that of each of the first and second motors  24  and  26 . It should be noted that no particular limitation is imposed on specific values of rated voltages of the main battery  30 , the first motor  24 , and the second motor  26 , or on a relation in magnitude among the rated voltages. 
     As shown in  FIG. 2 , the power supply circuit  32  includes a DC-DC converter  50 , a first inverter  52 , and a second inverter  54 . The DC-DC converter  50  is a DC-DC converter that enables step-up and step-down of voltage. As one example, the DC-DC converter  50  includes an inductor L 1 , an upper arm switching element Q 13 , a lower arm switching element Q 14 , an upper arm diode D 13 , and a lower arm diode D 14 . The DC-DC converter intermittently turns on the lower arm switching element Q 14 , to thereby function as a step-up converter. Moreover, the DC-DC converter intermittently turns on the upper arm switching element Q 13 , to thereby function as a step-down converter. 
     The first inverter  52  has a plurality of switching elements Q 1  to Q 6 , and a plurality of diodes D 1  to D 6 . Each of the plurality of diodes D 1  to D 6  is connected in parallel with corresponding one of the plurality of switching elements Q 1  to Q 6 . The first inverter  52  selectively turns on and off the plurality of switching elements Q 1  to Q 6 , to thereby convert the DC electric power from the DC-DC converter  50  into AC electric power. Similarly, the second inverter  54  has a plurality of switching elements Q 7  to Q 12 , and a plurality of diodes D 7  to D 12 . Each of the plurality of diodes D 7  to D 12  is connected in parallel with corresponding one of the plurality of switching elements Q 7  to Q 12 . The second inverter  54  selectively turns on and off the plurality of switching elements Q 7  to Q 12 , to thereby convert the DC electric power from the DC-DC converter  50  into AC electric power. 
     The main battery  30  is connected to the first motor  24  via the DC-DC converter  50  and the first inverter  52 . If the first motor  24  functions as a motor, the DC electric power from the main battery  30  is stepped up in voltage in the DC-DC converter  50 , subsequently converted into AC electric power in the first inverter  52 , and then supplied to the first motor  24 . On the other hand, if the first motor  24  functions as a generator, the AC electric power from the first motor  24  is converted into DC electric power in the first inverter  52 , next, stepped down in voltage in the DC-DC converter  50 , and then supplied to the main battery  30 . 
     Similarly, the main battery  30  is connected to the second motor  26  via the DC-DC converter  50  and the second inverter  54 . If the second motor  26  functions as a motor, the DC electric power from the main battery  30  is stepped up in voltage in the DC-DC converter  50 , next, converted into AC electric power in the second inverter  54 , and then supplied to the second motor  26 . On the other hand, if the second motor  26  functions as a generator, the AC electric power from the second motor  26  is converted into DC electric power in the second inverter  54 , next, stepped down in voltage in the DC-DC converter  50 , and then supplied to the main battery  30 . It should be noted that the configuration of the power supply circuit  32  in the present embodiment is one example, and can be changed as appropriate in accordance with the configuration of each of the main battery  30 , the first motor  24 , and the second motor  26 . For example, if the main battery  30  has the same rated voltage as that of each of the first and second motors  24  and  26 , the DC-DC converter  50  is not necessarily required. 
     The power supply circuit  32  further includes a first smoothing capacitor C 1  and a second smoothing capacitor C 2 . The first smoothing capacitor C 1  is positioned between the main battery  30  and the DC-DC converter  50 , and the second smoothing capacitor C 2  is positioned between the DC-DC converter  50  and the first inverter  52 , and between the DC-DC converter  50  and the second inverter  54 . Each of the first and second smoothing capacitors C 1  and C 2  stores electrical charges, to thereby restrain fluctuations in voltage within the power supply circuit  32 . For example, the first smoothing capacitor C 1  restrains fluctuations in the DC voltage outputted from the DC-DC converter  50  to the main battery  30 . Moreover, the second smoothing capacitor C 2  restrains fluctuations in the DC voltage outputted from the DC-DC converter  50  to the first and second inverters  52  and  54 . It should be noted that the power supply circuit  32  may include only one of the first and second smoothing capacitors C 1  and C 2 , or may further include another smoothing capacitor. The number and positions of the smoothing capacitors can be changed as appropriate in accordance with the configuration of the power supply circuit  32 . 
     Returning to  FIG. 1 , the hybrid vehicle  10  further includes a hybrid control unit  40  (HV-ECU in the drawing), an engine control unit  42  (ENG-ECU in the drawing), a motor control unit  44  (MG-ECU in the drawing), and an air bag control unit  46  (AB-ECU in the drawing). The engine control unit  42  is communicatively connected to the engine  22 , and controls an operation of the engine  22 . The motor control unit  44  is communicatively connected to the power supply circuit  32 , and controls an operation of the power supply circuit  32 . Specifically, the motor control unit  44  controls the switching elements Q 1  to Q 14  in the power supply circuit  32 , to thereby control an operation of each of the first and second motors  24  and  26 . The hybrid control unit  40  can communicate with a plurality of control units including the engine control unit  42 , the motor control unit  44 , and the air bag control unit  46 , via a communication path  48 , and gives them an operation command to thereby control the entire operation of the hybrid vehicle  10 . 
     The air bag control unit  46  controls an operation of one or more air bags (not shown) provided in the hybrid vehicle  10 . The air bag control unit  46 , in particular, has an acceleration sensor, for example, and can detect a crash of the hybrid vehicle  10 . When detecting a crash of the hybrid vehicle  10 , the air bag control unit  46  operates the air bag(s). Moreover, when detecting a crash of the hybrid vehicle  10 , the air bag control unit  46  transmits a prescribed crash signal to the plurality of control units including the hybrid control unit  40  and the motor control unit  44 . As one example, the crash signal may be a train of pulse signals with prescribed periodicity. Notably, the hybrid vehicle  10  may include another crash detection device that detects a crash of the hybrid vehicle  10 , in place of, or in addition to the air bag control unit  46 . 
     As shown in  FIGS. 1 and 2 , the hybrid vehicle  10  further includes an accessory battery  34  and a charging circuit  36 . The accessory battery  34  is electrically connected to the main battery  30  via the charging circuit  36 . The accessory battery  34  is a power source that supplies electric power to the plurality of electric loads mounted on the hybrid vehicle  10 , including the motor control unit  44 , for example. As one example, the accessory battery  34  has a rated voltage of 12 volts. The accessory battery  34  is a rechargeable battery, and charged with electric power supplied from the main battery  30 . The charging circuit  36  has a step-down-type DC-DC converter, and steps down the DC voltage from the main battery  30  to a DC voltage suitable for charging of the accessory battery  34 , to thereby charge the accessory battery  34 . 
     As shown in  FIG. 3 , the accessory battery  34  is electrically connected to the plurality of electric loads including the motor control unit  44  via the corresponding fuses  104 . It should be noted that the plurality of electric loads also include the air bag control unit  46  and other electric loads  58 . It should be noted that other electric loads  58  shown in  FIG. 3  include the hybrid control unit  40  and the engine control unit  42 , which have been mentioned above, for example. The air bag control unit  46  is provided with a first back-up power source  47 . The first back-up power source  47  has a rechargeable power storage element (e.g., a capacitor or a secondary battery), and is charged by the accessory battery  34 . When the electric power supply from the accessory battery  34  to the air bag control unit  46  is stopped, the first back-up power source  47  substitutes for the accessory battery  34  and supplies electric power to the air bag control unit  46 . This enables the air bag control unit  46  to continue its operation for a prescribed time even when the corresponding fuse  104  between the accessory battery  34  and the air bag control unit  46  is blown, for example. 
     As shown in  FIG. 3 , the motor control unit  44  includes a power source circuit  60  and a processor  62 . The processor  62  is electrically connected to the accessory battery  34  via the power source circuit  60 , and operates by the electric power supplied from the accessory battery  34 . A corresponding fuse  104  and a relay circuit  80 , which will be mentioned below, are electrically intervened between the power source circuit  60  and the accessory battery  34 . The power source circuit  60  adjusts the voltage inputted from the accessory battery  34  to a voltage corresponding to the rated voltage of the processor  62 . As one example, the power source circuit  60  in the present embodiment adjusts a voltage of 12 volts inputted from the accessory battery  34  to 5 volts, and outputs the adjusted voltage. The processor  62  has a CPU and a memory, and can use a plurality of programs and a plurality of parameters stored in the memory to perform a plurality of processes. As schematically shown in  FIG. 3 , the plurality of processes includes a relay driving process  64 , an abnormal quitting detection process  66 , a crash determination process  68 , and a discharge process  70 . Moreover, although not shown, the processor  62  can perform a process of controlling an operation of the power supply circuit  32 , based on an operation command by the hybrid control unit  40  (e.g., a target torque of each of the first and second motors  24  and  26 ). For this purpose, the motor control unit  44  may further include at least one processor in addition to the processor  62  shown in  FIG. 3 . 
     The crash determination process  68  is a process of determining whether or not the hybrid vehicle  10  has crashed based on the crash signal outputted from the air bag control unit  46 . To the processor  62 , the crash signal outputted from the air bag control unit  46  is inputted via an interface circuit  102 . The discharge process  70  is a process of; when the crash determination process  68  determines that the hybrid vehicle  10  has crashed, discharging the first and second smoothing capacitors C 1  and C 2  by controlling the power supply circuit  32 . As one example, in this discharge process  70 , it is possible to discharge the first and second smoothing capacitors C 1  and C 2  through the second motor  26  by controlling the DC-DC converter  50  and the second inverter  54 . In this case, the current that flows in the second motor  26  may be preferably adjusted such that the output torque of the second motor  26  becomes zero. In other words, the zero torque control on the second motor  26  is preferably performed. Notably, in other embodiments, if the power supply circuit  32  has another circuit structure that can discharge the first and second smoothing capacitors C 1  and C 2 , that circuit structure may be utilized in the discharge process  70 . Notably, when the discharge process  70  is performed, the main battery  30  is electrically disconnected from the power supply circuit  32  by a switch or a relay, not shown. The relay driving process  64  and the abnormal quitting detection process  66  will be described later. 
     By performing the crash determination process  68  and the discharge process  70 , the processor  62  can discharge the first and second smoothing capacitors C 1  and C 2  in the power supply circuit  32  when the hybrid vehicle  10  crashes. As shown in  FIG. 4 , assume that the hybrid vehicle  10  crashes at a time point t 1 , for example. In this case, at a time point t 2 , the air bag control unit  46  starts outputting the crash signal (see A 1  in the drawing). A time T 1  from the time point t 1  to the time point t 2  represents a processing time necessary for the air bag control unit  46  to detect the crash. When the air bag control unit  46  starts outputting the crash signal, the processor  62  starts the discharge process  70  at a time point t 3  (see  42  in the drawing). A time T 2  from the time point t 2  to the time point t 3  is a time necessary for the processor  62  to perform the crash determination process  68 . To avoid erroneous determination caused by a noise signal, the processor  62  determines that the hybrid vehicle  10  has crashed when the processor  62  keeps receiving the crash signal for the time T 2 . As one example, in the present embodiment, a design value of the time T 1  is 50 milliseconds, and a design value of the time T 2  is 180 milliseconds. 
     Returning to  FIG. 3 , the motor control unit  44  further includes the relay circuit  80 . The relay circuit  80  is electrically connected between the accessory battery  34  and the power source circuit  60 . The relay circuit  80  is driven to electrically connect between the accessory battery  34  and the power source circuit  60  in response to a relay drive signal outputted from the processor  62 . In other words, while the processor  62  is outputting the relay drive signal, the accessory battery  34  and the processor  62  are electrically connected, and electric power is supplied from the accessory battery  34  to the processor  62 . On the other hand, when the processor  62  quits its operation, the processor  62  quits outputting the relay drive signal, and interrupts, by itself, the electric power supply from the accessory battery  34 . The relay drive signal in the present embodiment is a signal having a prescribed DC voltage (e.g., 3 to 5 volts). The motor control unit  44  may further include a diode  98  for circuit protection, and a capacitor  96  for noise prevention. 
     No particular limitation is imposed on the specific configuration of the relay circuit  80 . As one example, the relay circuit  80  in the present embodiment has a p channel-type field-effect transistor  82  (hereinafter p-FET  82 ) and an n channel-type field-effect transistor  88  (hereinafter n-FET  88 ). A source of the p-FET  82  is electrically connected to the accessory battery  34 , and a drain of the p-FET  82  is electrically connected to the power source circuit  60 . The p-FET  82  can thereby be electrically connected and disconnected between the accessory battery  34  and the power source circuit  60 , A gate and the source of the p-FET  82  are electrically connected via a resistor  84 . The gate of the p-FET  82  is electrically connected to a drain of the n-FET  88  via a resistor  86 . A source of the n-FET  88  is electrically grounded, and a gate and the source of the n-FET  88  are electrically connected via a resistor  90 . The relay drive signal is then inputted to the gate of the n-FET  88 . With such a configuration, when the processor  62  outputs the relay drive signal, the n-FET  88  and the p-PET  82  are turned on, causing the accessory battery  34  and the processor  62  to be electrically connected. In other words, the relay drive signal has a DC voltage higher than a threshold voltage of the n-FET  88 . When the processor  62  then quits outputting the relay drive signal, the n-FET  88  and the p-FET  82  are turned off, causing the accessory battery  34  and the processor  62  to be electrically disconnected. 
     The relay drive signal outputted by the processor  62  is inputted to the relay circuit  80  through a signal path  76 . Here, the signal path  76  is provided with an OR circuit  74  and a resistor  78 . To the OR circuit  74 , a relay activation signal outputted from one of other electric loads  58  (e.g., the hybrid control unit  40 ) is inputted via an interface circuit  100 , in addition to the relay drive signal. Usually, when the processor  62  is to be activated, the relay circuit  80  is driven by the relay activation signal outputted from one of other electric loads  58 . This starts the electric power supply from the accessory battery  34  to the processor  62 , causing the processor  62  to be activated. After the processor  62  is activated, the processor  62  starts outputting the relay drive signal, and the relay circuit  80  is maintained in a driven state. Here, no particular limitation is imposed on the configuration of the OR circuit  74 , and the OR circuit  74  may be configured with use of an integrated circuit, or may be a discrete circuit that has one or more semiconductor elements. Notably, in other embodiments, a second path for supplying electric power from the accessory battery  34  to the processor  62  may separately be provided. In this case, a second relay circuit may be provided on the second path, and a relay activation signal outputted from one of other electric loads  58  (e.g., the hybrid control unit  40 ) may be configured to be inputted to the second relay circuit. According to such a configuration, when the processor  62  is to be activated, electric power is supplied from the accessory battery  34  to the processor  62  via the second path. Accordingly, the OR circuit  74  is not required. 
     The motor control unit  44  further includes a holding circuit  92 . The holding circuit  92  is connected to the signal path  76 . The holding circuit  92  is configured to temporarily hold the relay circuit  80  in a driven state when the processor  62  quits outputting the relay drive signal. 
     The holding circuit  92  in the present embodiment has a power storage element  94 . This power storage element  94  is a capacitor, but the power storage element  94  may be a secondary battery or another power storage element. The power storage element  94  has one end electrically connected to the signal path  76 , and the other end electrically grounded. The processor  62  is also electrically grounded, and hence the processor  62  and the power storage element  94  are connected in parallel with each other with respect to the relay circuit  80 . More specifically, the processor  62  and the power storage element  94  are connected in parallel with each other with respect to an input portion of the relay circuit  80  to which the relay drive signal is inputted. 
     As mentioned above, the relay drive signal outputted by the processor  62  is a signal having a prescribed DC voltage. Accordingly, while the processor  62  is outputting the relay drive signal, the power storage element  94  is charged by the relay drive signal. Even if the processor  62  quits outputting the relay drive signal, the power storage element  94  thus charged inputs a voltage equivalent to or corresponding to the relay drive signal to the relay circuit  80 . This enables the relay circuit  80  to be temporarily held in a driven state even after the processor  62  quits outputting the relay drive signal. The resistor  90  in the relay circuit  80  is connected in parallel with the power storage element  94 . Accordingly, the power storage element  94  is gradually discharged via the resistor  90 , causing the relay circuit  80  to be turned off eventually. The time during which the power storage element  94  holds the relay circuit  80  in a driven state can be adjusted by means of a capacity of the power storage element  94  and a resistance value of the resistor  90 . 
     As mentioned above, in the hybrid vehicle  10  in the present embodiment, when the hybrid vehicle  10  crashes, the first and second smoothing capacitors C 1  and C 2  in the power supply circuit  32  can be discharged. However, when the hybrid vehicle  10  crashes, there may be a case where the vehicle body  12  is significantly deformed, for example, to thereby cause a short circuit in the accessory battery  34 . As shown in  FIG. 5 , for example, assume that a wire harness X 1  that electrically connects the accessory battery  34  and one electric load  58   a  is damaged and brought into contact with the vehicle body  12 , to thereby be electrically grounded. In this case, the accessory battery  34  is short-circuited, to thereby generate a large short circuit current SC. It should be noted that, owing to a blowout of the fuse  104   a,  the short circuit in the accessory battery  34  is quickly resolved, and electric power supply to the other electric loads including the motor control unit  44  is resumed. 
     However, during a period from the occurrence of the short circuit to the blowout of the fuse  104   a,  the output voltage of the accessory battery  34  temporarily decreases. Consequently, there may be a case where the supply voltage to the processor  62  also decreases, and the processor  62  quits its operation. When the processor  62  quits its operation, the output of the relay drive signal by the processor  62  is also quitted. At this time, if the motor control unit  44  does not include the holding circuit  92 , the driving of the relay circuit  80  is disadvantageously stopped unless a relay activation signal is provided by the interface circuit  100 . In this case, even if the output voltage of the accessory battery  34  is subsequently recovered, the processor  62  cannot receive electric power supply from the accessory battery  34 . The processor  62  can neither be activated again nor perform the discharge process  70 . 
     In contrast to the above, the motor control unit  44  in the present embodiment includes the holding circuit  92 , and even if the processor  62  quits outputting the relay drive signal, the holding circuit  92  temporarily holds the relay circuit  80  in a driven state. Meanwhile, if the output voltage of the accessory battery  34  is recovered, the accessory battery  34  is electrically connected to the processor  62 , enabling the processor  62  to be activated again and resume outputting the relay drive signal. Then the processor  62  can discharge the first and second smoothing capacitors C 1  and C 2  by performing the crash determination process  68  and the discharge process  70 . As such, according to the hybrid vehicle  10  in the present embodiment, When the hybrid vehicle  10  crashes, the first and second smoothing capacitors C 1  and C 2  can more reliably be discharged. 
     With reference to  FIG. 6 , a specific example of a series of the flows described above will be described. Similarly to the example in  FIG. 4 , when the hybrid vehicle  10  crashes at the time point t 1 , the air bag control unit  46  starts outputting the crash signal at the time point t 2  (see A 1  in the drawing). Assume that a short circuit occurs one or more times in the accessory battery  34 , mentioned above, after the time point  11 , and the output voltage of the accessory ,  battery  34  decreases to approximately zero volt for a time T 3  from a time point t 4  to a time point t 5  (see A 3 ). In this case, at the time point t 4 , the output voltage of the power source circuit  60  also decreases to approximately zero volt (see A 4 ), thereby causing the processor  62  to quit its operation (see A 5 ). Accordingly, the output of the relay drive signal is quitted (see A 6 ). At this stage, however, the power storage element  94  in the holding circuit  92  is charged, and hence by the output voltage of the holding circuit  92  (see A 7 ), the relay circuit  80  is maintained in a driven state even after the time point t 4  (see A 8 ). 
     Afterwards, when the output voltage of the accessory battery  34  is recovered to 12 volts at the time point t 5 , the output voltage of the power source circuit  60  is also recovered to 5 volts at a time point t 6 , and the processor  62  is activated again. In other words, even at the time point t 6 , the holding circuit  92  holds the relay circuit  80  in a driven state, and electric power supply from the accessory battery  34  to the processor  62  is resumed. A time T 4  from the time point t 5  to the time point t 6  is a time necessary for the output voltage of the power source circuit  60  to reach 5 volts, which is a target voltage, by feedback control within the power source circuit  60 . 
     When the processor  62  is activated again at the time point t 6 , the processor  62  carries out a prescribed initialization process, and subsequently performs the abnormal quitting detection process  66  (see  FIG. 3 ). The abnormal quitting detection process  66  is a process of detecting whether or not the last quitting of operation of the processor  62  is abnormal. The abnormal quitting of operation herein referred to includes quitting of operation due to a loss of power source electric power, as occurring at the time point t 4 . The memory of the processor  62  records an operation history of the processor  62 , and in the abnormal quitting detection process  66 , the operation history is referenced. For example, if no normal quitting of operation is recorded at the last of the operation history stored in the memory, the last quitting of operation of the processor  62  is determined to be abnormal. 
     If the last quitting of operation of the processor  62  is abnormal, the processor  62  performs the relay driving process  64  (see  FIG. 3 ), and starts outputting the relay drive signal at a time point t 7 . Notably, if the last quitting of operation of the processor  62  is normal, the processor  62 , before performing the relay driving process  64 , performs some other processes necessary for the control on the power supply circuit  32 . In other words, if the last quitting of operation of the processor  62  is abnormal, the processor  62  skips some processes to be performed at normal times, and starts outputting the relay drive signal early. A time T 5  from the time point t 6  to the time point t 7  is a time necessary for the processor  62  to perform the initialization operation mentioned above, the abnormal quitting detection process  66 , and the relay driving process  64 . Afterwards, the processor  62  performs the crash determination process  68 , and subsequently performs the discharge process  70  at a tithe point t 8 . The time T 2  from the time point t 7  to the time point t 8  is a time necessary for the processor  62  to perform the crash determination process  68 , as mentioned above. 
     As described above, during a period from the time point t 4  at which the processor  62  quits outputting the relay drive signal to the time point t 7  at which the processor  62  resumes outputting the relay drive signal, the holding circuit  92  maintains the relay circuit  80  in a driven state. In other words, the holding circuit  92  can hold the relay circuit  80  in a driven state at least for a time equal to the total of the times T 3 , T 4 , and T 5 . When the output voltage of the accessory battery  34  is recovered, electric power supply from the accessory battery  34  to the processor  62  can thereby be resumed without the need of the relay drive signal provided by the processor  62 . As one example, in the present embodiment, the maximum values of the times T 3 , T 4 , and T 5  are assumed to be 300 milliseconds, 80 milliseconds, and 120 milliseconds, respectively. Accordingly, the holding circuit  92  in the present embodiment is designed to be able to hold the relay circuit  80  in a driven state at least for 500 milliseconds or more after the processor  62  quits outputting the relay drive signal. 
     The power storage element  94  in the holding circuit  92  only has to store just enough electric power to temporarily hold the relay circuit  80  in a driven state. The electric power necessary to hold the relay circuit  80  in a driven state is smaller than the electric power necessary to maintain the operation of the processor  62 . For example, it is also contemplated that the processor  62  is provided with a back-up power source so as to prevent unintentional quitting of operation of the processor  62 . However, the back-up power source for the processor  62  needs to have a capability to store much electric power, resulting in an increase in physical size of the back-up power source. When compared with such a back-up power source, the power storage element  94  in the holding circuit  92  has a small size. Accordingly, the holding circuit  92  can be provided within the motor control unit  44  without increasing the size of the motor control unit  44 . 
     Next, with reference to  FIGS. 7 and 8 , a motor control unit  144  in a variation will be described. As shown in  FIG. 7 , the motor control unit  144  may further include a crash signal processing device  110  and a second back-up power source  112 . The crash signal processing device  110  receives the crash signal from the air bag control unit  46 , and outputs a second crash signal in accordance with the received crash signal to the processor  62 . As one example, the crash signal processing device  110  herein described counts the number of the received pulse signals, and when the count value of the pulse signals reaches a prescribed threshold value, outputs the second crash signal to the processor  62 . The crash signal processing device  110  is connected to the accessory battery  34  via a diode  114 , and operates by the electric power from the accessory battery  34 . 
     The second back-up power source  112  has a rechargeable power storage element (e.g., a capacitor or a secondary battery). The second back-up power source  112  is electrically connected to the accessory battery  34  via an electric power line  116  that has the diode  114 , and is charged with the electric power from the accessory battery  34 . When the electric power supply from the accessory battery  34  to the crash signal processing device  110  is stopped, the second back-up power source  112  substitutes for the accessory battery  34  and supplies electric power to the crash signal processing device  110 . This enables the crash signal processing device  110  to continue its operation even if the output voltage of the accessory battery  34  temporarily decreases, for example. 
     As shown in  FIG. 7 , assume that a wire harness X 2  that electrically connects the accessory battery  34  and the air bag control unit  46  is damaged and brought into contact with the vehicle body  12 , to thereby be electrically grounded. In this case, the fuse  104  between the accessory battery  34  and the air bag control unit  46  is blown, and electric power supply from the accessory battery  34  to the air bag control unit  46  is interrupted. The air bag control unit  46  is provided with the first backup power source  47 , and hence even after the electric power supply from the accessory battery  34  is interrupted, the air bag control unit  46  can temporarily continue its operation. Accordingly, as shown by A 1  in  FIG. 8 , the air bag control unit  46  can detect a crash and output a crash signal. However, the crash signal is outputted from. the air bag control unit  46  exclusively for a time T 6 , which is a limited time. Accordingly, if the crash signal from the air bag control unit  46  has already been interrupted when the processor  62  is activated again at the time point t 6  and the initialization process is completed at the time point t 7 , the processor  62  can no longer receive the crash signal from the air bag control unit  46 . 
     In view of the issues above, the motor control unit  144  shown in  FIG. 7  is provided with the crash signal processing device  110  and the second back-up power source  112 . As shown by A 10  in  FIG. 8 , the crash signal processing device  110  counts pulse signals in the crash signal, which is a train of pulse signals, and when the count value thereof reaches a prescribed threshold value X 10 , starts outputting the second crash signal to the processor  62 . Here, the crash signal processing device  110  can continue its operation by the electric power from the second back-up power source  112  even while the output voltage of the accessory battery  34  temporarily decreases (see A 9  in the drawing). When the initialization process is completed at the time point t 7 , the processor  62  can determine the presence or absence of a crash of the hybrid vehicle  10 , based on the second crash signal from the crash signal processing device  110 . In this case, the processor  62  only has to determine the presence or absence of the second crash signal in the crash determination process  68 , and the time necessary for the crash determination process  68  becomes extremely short. The processor  62  can thereby start the discharge process  70  early immediately after the time point t 7  (see A 2  in  FIG. 8 ). 
     As described above, according to the motor control unit  144  shown in  FIG. 7 , even if the crash signal from the air bag control unit  46  is interrupted, the processor  62  can perform the discharge process  70 . Moreover, the crash determination as to the hybrid vehicle  10  is made by the crash determination process  68  independent of the processor  62 , and hence the processor  62  can start and complete the discharge process  70  early. 
     The second back-up power source  112  only has to store just enough electric power to temporarily operate the crash signal processing device  110 . The electric power necessary for the crash signal processing device  110  to operate is smaller than the electric power necessary for the processor  62  to operate. Accordingly, when compared with the back-up power source for the processor  62 , mentioned above, the second back-up power source  112  is also decreased in size. Accordingly, the second back-up power source  112  can be provided within the motor control unit  144  without increasing the size of the motor control unit  144 . 
     The configuration of the crash signal processing device  110  is not limited to the above-mentioned examples, and can be changed in accordance with a crash detection signal, for example. The crash signal processing device  110  does not necessarily need to make a crash determination as to the hybrid vehicle  10 , and may also be configured to simply record the crash signal from the air bag control unit  46 . In this case, after being activated again, the processor  62  can reference the crash signal recorded in the crash signal processing device  110 . In other words, the crash signal processing device  110  outputs a part or a whole of the recorded crash signal to the processor  62  as the second crash signal in response to an instruction from the processor  62 , for example. The processor  62  can perform the crash determination process  68  and the discharge process  70  based on the second crash signal from the crash signal processing device  110 . 
     Next, with reference to  FIG. 9 , a motor control unit  244  in a variation will be described. In this variation as well, the motor control unit  244  includes the crash signal processing device  110  and the second back-up power source  112 . The motor control unit  244 , however, does not include the relay circuit  80 , and the processor  62  is always electrically connected to the accessory battery  34  and the charging circuit  36 . With such a configuration as well, when the output voltage of the accessory battery  34  decreases owing to a blowout of the fuse  104 , there may be a case where the processor  62  temporarily quits its operation. Furthermore, if the blowout of the fuse  104  occurs between the accessory battery  34  and the air bag control unit  46 , there may also be a case where the crash signal from the air bag control unit  46  has already been interrupted at the time point when the processor  62  completes the initialization process. However, after being activated again, the processor  62  can perform the crash determination process  68  and the discharge process  70  by referencing the crash signal recorded in the crash signal processing device  110 . As such, the configuration according to the crash signal processing device  110  and the second back-up power source  112  can effectively function regardless of the presence or absence of the relay circuit  80 . 
     Some specific examples have been described above in details. However, these are mere examples, and do not limit the scope of the claims. For example, the motor control units  44 ,  144 , and  244 , mentioned above, can be adopted not only in the hybrid vehicle  10 , but also in various electric vehicles such as a rechargeable electric vehicle, a fuel cell vehicle, and a solar cell vehicle, for example. Notably, the accessory battery  34  in the embodiment is one example of the power source described in the claims. The air bag control unit  46  in the embodiment is one example of the crash detection device described in the claims. The second back-up power source  112  in the embodiment is one example of the back-up power source described in the claims. 
     The technical issues understood from the disclosure of the present specification will hereinafter be listed. 
     An electric vehicle ( 10 ) disclosed herein comprises: a motor ( 26 ) configured to drive a wheel ( 14 ); a smoothing capacitor (C 1 , C 2 ) provided within a power supply circuit ( 32 ) that supplies electric power to the motor ( 26 ); a processor ( 62 ) configured to perform a discharge process ( 70 ) when the electric vehicle ( 10 ) crashes, the discharge process discharging the smoothing capacitor (C 1 , C 2 ) by controlling the power supply circuit ( 32 ); a power source ( 34 ) connected to each of a plurality of electric loads ( 44 ,  46 ,  58 ,  62 ) including the processor ( 62 ) via a corresponding fuse ( 104 ); a relay circuit ( 80 ) electrically connected between the power source ( 34 ) and the processor ( 62 ) and configured to be driven to electrically connect between the power source ( 34 ) and the processor ( 62 ) in response to a relay drive signal outputted from the processor ( 62 ); and a holding circuit ( 92 ) configured to temporarily hold the relay circuit ( 80 ) in a driven state when the processor ( 62 ) quits outputting the relay drive signal. According to this configuration, the smoothing capacitor (C 1 , C 2 ) within the power supply circuit ( 32 ) can be discharged reliably when the electric vehicle ( 10 ) crashes. 
     The holding circuit ( 92 ) may include a power storage element ( 94 ) configured to be charged by the relay drive signal outputted from the processor ( 62 ). According to such a configuration, the holding circuit ( 92 ) can drive the relay circuit ( 80 ) by electrical power charged within the power storage element ( 94 ). 
     If the relay drive signal has a prescribed DC voltage, the power storage element ( 94 ) in the holding circuit ( 92 ) may be connected in parallel with the processor ( 62 ) with respect to the relay circuit ( 80 ). According to such a configuration, the charged power storage element ( 94 ) can substitute for the processor ( 62 ) and output a signal equivalent to or corresponding to the relay drive signal. 
     At least one resistor ( 90 ) may be connected in parallel with the power storage element ( 94 ) in the holding circuit ( 92 ). According to such a configuration, after the output of the relay drive signal is quitted, the power storage element ( 94 ) is gradually discharged to thereby temporarily hold the relay circuit ( 80 ) in a driven state. 
     The electric vehicle ( 10 ) may further include: a crash detection device ( 46 ) configured to output a prescribed crash signal when the electric vehicle ( 10 ) crashes; a crash signal processing device ( 110 ) configured to receive the crash signal outputted from the crash detection device ( 46 ) and to output a second crash signal in accordance with the received crash signal to the processor ( 62 ); and a back-up power source ( 112 ) configured to supply electric power to the crash signal processing device ( 110 ) when electric power supply to the crash signal processing device ( 110 ) is interrupted. According to such a configuration, even when the crash signal from the crash detection device ( 46 ) is interrupted while the processor ( 62 ) temporarily quits its operation, the processor ( 62 ) can perform the discharge process ( 70 ) after being activated again, based on the second crash signal from the crash signal processing device ( 110 ). 
     The electric vehicle ( 10 ) may further include a main power source ( 30 ) configured to supply electric power to the motor ( 26 ) via the power supply circuit ( 32 ). The main power source ( 30 ) may be a rechargeable battery, a fuel cell, a solar cell, another electric power generating device, or a combination of at least two of them, for example.