Patent Publication Number: US-2022216791-A1

Title: In-vehicle power supply system

Description:
TECHNICAL FIELD 
     The present invention relates to an in-vehicle power supply system for use in various types of vehicles. 
     BACKGROUND ART 
       FIG. 3  is a circuit block diagram of conventional in-vehicle power supply device  1 . In-vehicle power supply device  1  includes converter  2 . Input section  3  of in-vehicle power supply device  1  is connected to battery  4  while output section  5  is connected to battery  6 . In-vehicle power supply device  1  includes controller  7 . Controller  7  detects currents and voltages of input section  3  and output section  5  and controls an operation of converter  2  in response to the detected values. 
     When detecting an abnormal value of the current or the voltage at input section  3  or output section  5 , controller  7  determines that in-vehicle power supply device  1  has failure, turns off switch  8  between output section  5  and battery  6 , and outputs a warning signal to the outside of in-vehicle power supply device  1 . 
     A conventional in-vehicle power supply device similar to in-vehicle power supply device  1  is disclosed in, for example, PTL 1. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Patent Laid-Open Publication No. 2007-202290 
       
    
     SUMMARY 
     An in-vehicle power supply system includes a high-voltage direct-current (DC) power supply, a low-voltage storage battery, a DC-DC converter, and a controller. The DC-DC converter includes a conversion circuit including a high-voltage terminal and a low-voltage terminal, an input switch connected between the high-voltage DC power supply and the high-voltage terminal, and an output switch connected between the low-voltage terminal and the low-voltage storage battery. The controller is configured to, after detecting that a current flowing through the conversion circuit exceeds a predetermined current threshold or that a charge voltage of the low-voltage storage battery exceeds a predetermined voltage threshold, execute a failure determination of the DC-DC converter after instructing the conversion circuit to stop a voltage conversion operation, causing the input switch to switch to the disconnected state, and instructing the output switch to switch to the disconnected state. The controller is configured to, in the failure determination execute a first determination operation including an output short-circuit failure determination, an output open-circuit failure determination, and an input short-circuit failure determination. If determining, in the first determination operation, that the output switch does not have failure, controller is configured to execute a second determination operation of determining whether or not the conversion circuit has failure. If determining, in the second determination operation, that the conversion circuit does not have failure, the controller is configured to execute a third determination operation of determining whether or not the input switch has failure. 
     The in-vehicle power supply device determines the location of failure easily. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit block diagram of an in-vehicle power supply system in accordance with an exemplary embodiment. 
         FIG. 2  is another circuit block diagram of the in-vehicle power supply system in accordance with the embodiment. 
         FIG. 3  is a circuit block diagram of a conventional in-vehicle power supply system. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is a circuit block diagram of in-vehicle power supply system  11  in accordance with an exemplary embodiment. In-vehicle power supply system  11  includes high-voltage DC power supply  12 , low-voltage storage battery  13 , DC-DC converter  14 , and controller  15 . 
     DC-DC converter  14  is connected to high-voltage DC power supply  12  and low-voltage storage battery  13  between high-voltage DC power supply  12  and low-voltage storage battery  13 . DC-DC converter  14  includes input switch  16 , conversion circuit  17  configured to perform a bidirectional operation, and output switch  18  which are connected in series from high-voltage DC power supply  12  to low-voltage storage battery  13  in this order. DC-DC converter  14  further includes current detector  19 . High-voltage terminal  17 A of conversion circuit  17  is connected to input switch  16 . Low-voltage terminal  17 B of conversion circuit  17  is connected to output switch  18 . 
     Input switch  16  includes one terminal connected to high-voltage DC power supply  12  and another terminal. Input switch  16  is configured to selectively switch to a connected state of connecting the one terminal thereof to another terminal thereof and to a disconnected state of disconnecting the one terminal thereof from another terminal thereof. Converter circuit  17  includes high-voltage terminal  17 A and low-voltage terminal  17 B. High-voltage terminal  17 A is connected to another terminal of input switch  16 . Converter circuit  17  is configured to perform a bidirectional operation of performing a boosting-up operation of boosting up a voltage of low-voltage terminal  17 B to obtain a voltage of high-voltage terminal  17 A, and a stepping-down operation of stepping down the voltage of high-voltage terminal  17 A to obtain the voltage of the low-voltage terminal  17 B. Output switch  18  includes one terminal connected to low-voltage terminal  17 B of conversion circuit  17  and another terminal connected to low-voltage storage battery  13 . Output switch  18  is configured to selectively switch to a connected state of connecting the one terminal thereof and to another terminal thereof and to a disconnected state of disconnecting the one terminal thereof from another terminal thereof. 
     Controller  15  monitors a current flowing through conversion circuit  17  of DC-DC converter  14  and detected by current detector  19  and a he charge voltage of low-voltage storage battery  13 . Controller  15  controls an operation of DC-DC converter  14  based on the values of the current flowing through conversion circuit  17  and the charge voltage of low-voltage storage battery  13 . 
     If controller  15  detects that the current flowing through conversion circuit  17  exceeds a predetermined current threshold or that the charge voltage of low-voltage storage battery  13  exceeds a predetermined voltage threshold, controller  15  instructs conversion circuit  17  to stop a power conversion operation, and instructs input switch  16  and output switch  18  to be in the disconnected state. After that, controller  15  executes a failure determination of DC-DC converter  14 . 
     The operations of the failure determination include a first determination operation, a second determination operation, and a third determination operation, which will be described below. 
     At first, the first determination operation is executed. In the first determination operation, while controller  15  instructs output switch  18  to be in the disconnected state, controller  15  detects low-voltage value VL 1  of low-voltage terminal  17 B of conversion circuit  17 . Based on low voltage value VL 1 , controller  15  performs an output short-circuit failure determination of determining whether output switch  18  is continuously short-circuited thus having failure due to short-circuit thereof, or is in a normal state in which output switch  18  is disconnectable. 
     Further, controller  15  instructs output switch  18  to be in the connected state. After that, controller  15  detects low voltage value VL 2  of low-voltage terminal  17 B of conversion circuit  17 . Based on low voltage value VL 2 , controller  15  performs an output open-circuit failure determination of determining whether output switch  18  is continuously in the disconnected state, i.e., continuously opens, thus having failure due to open-circuit, or output switch  18  is in a normal state in which output switch  18  is connectable. 
     Further, while controller  15  instructs input switch  16  to be in the disconnected state, controller  15  detects high voltage value VH 1  of high-voltage terminal  17 A of conversion circuit  17 . High voltage value VH 1  may be obtained by detecting the voltage of the high potential side of input capacitor  20 . Based on high voltage value VH 1 , controller  15  performs a short-circuit failure determination of determining whether input switch  16  is continuously short-circuited, thus having failure due to short-circuit, or is in a normal state in which input switch is disconnectable. 
     At this moment, since conversion circuit  17  is stopped, electric power is not supplied from conversion circuit  17  to high-voltage terminal  17 A of conversion circuit  17 . If input switch  16  is properly disconnectable, the voltage from high-voltage DC power supply  12  through input switch  16  is not applied to high-voltage terminal  17 A of conversion circuit  17  while input switch  16  is in the disconnected state. Therefore, when high voltage value VH 1  at high-voltage terminal  17 A of conversion circuit  17  is higher than high-voltage threshold VTH 1 , controller  15  determines that input switch  16  has failure due to short-circuit thereof. When high voltage value VH 1  is lower than high-voltage threshold VTH 1 , controller  15  determines that input switch  16  dos not have failure and is normal. For example, high-voltage threshold VTH 1  is set to be a value approximately within a range of one-tenth to one-half of a reference voltage value of high-voltage DC power supply  12  such that, when input switch  16  has failure due to short-circuit, the failure is detected properly even if a voltage drop occurs due to a contact resistance of input switch  16 . As described above, controller  15  obtains the voltage of high-voltage terminal  17 A by detecting the voltage of input capacitor  20 , which is connected to ground GND and a node at which conversion circuit  17  is connected to input switch  16 . 
     In the first determination operation, the controller  15  preferably may initially execute the input short-circuit failure determination of determining a short-circuit failure of input switch  16 , after that, execute the output short-circuit failure determination for determining a short-circuit failure of output switch  18 , and, after that, execute the output open-circuit failure determination of determining an open-circuit failure of output switch  18 . Alternatively, the controller  15  may initially execute the determination of a short-circuit failure of output switch  18 , after that, execute the determination of a short-circuit failure of input switch  16 , and, after that, execute the determination of an open-circuit failure of output switch  18 . 
     When executing the determination for short-circuit failure before the determination for the open-circuit failure, the controller may prevent an overcurrent from flowing into a switch for which a failure is to be detected, which is caused by controller  15  instructing the switch to enable connection in a later failure determination operation. This prevents the occurrence of a secondary failure associated with the failure determination. 
     Next, a second determination operation is performed. The second determination operation is performed subsequent to the first determination operation if controller  15  determines, in the first determination operation, that output switch  18  is neither in short-circuit failure nor in open-circuit failure and also input switch  16  is not in short-circuit failure so that output switch  18  is properly controllable by controller  15 . In the second determination operation, first, controller  15  instructs output switch  18  to be in the connected state. Further, controller  15  controls conversion circuit  17  to boost up the voltage of low-voltage terminal  17 B of conversion circuit  17 . In that condition, controller  15  detects high voltage value VH 2  of high-voltage terminal  17 A of conversion circuit  17 . Based on high voltage value VH 2 , controller  15  determines whether conversion circuit  17  is capable of properly performing a boosting-up operation, that is, whether or not conversion circuit  17  has failure. 
     Next, a third determination operation is performed. The third determination operation is performed subsequent to the second determination operation if controller  15  determines, in the previously-performed second determination operation, that conversion circuit  17  has no failure to perform the boosting-up operation. In the third determination operation, first, controller  15  controls conversion circuit  17  to cause conversion circuit  17  not to perform the boosting-up operation. In that condition, controller  15  instructs input switch  16  to be in the connected state. Further, controller  15  detects high voltage value VH 3  of high-voltage terminal  17 A of conversion circuit  17 . Based on high voltage value VH 3 , controller  15  performs an input open-circuit failure determination of determining whether input switch  16  has failure due to open-circuit thereof, or input switch  16  is connectable in a normal state. 
     With the configuration and operation as described above, it is possible to easily and accurately identify a portion of DC-DC converter  14  causing the failure. 
     Conventional in-vehicle power supply device  1  shown in  FIG. 3  is able to identify the occurrence of a failure in in-vehicle power supply device  1 , but unable to identify the portion of converter  2  causing the failure. 
     In-vehicle power supply system  11  preferably initially perform the short-circuit failure determination, then perform a failure determination for input switch  16  which is on the high voltage side, and after that, perform a failure determination for the low voltage side successively. In other words, the second determination operation may be executed in the middle of the first determination operation. This means that, even when a short-circuit failure occurs in conversion circuit  17 , no voltage is applied to conversion circuit  17  or output switch  18  during the failure determination that proceeds from the high voltage side to the low voltage side. In other words, when an open-circuit failure determination is performed at the low voltage side, it has already been determined that conversion circuit  17  is not in short-circuit failure and that input switch  16  is normal and not in failure. Thereby, it is possible that, especially when determining a failure of output switch  18 , no electric power is supplied from conversion circuit  17  or input switch  16 . As a result, it is possible to prevent a secondary failure from occurring. Therefore, it is unnecessary to increase the withstand voltage of output switch  18  in consideration of the failure determination. 
     The configuration and operation of in-vehicle power supply system  11  will be detailed below.  FIG. 2  is another circuit block diagram of in-vehicle power supply system  11  in accordance with the embodiment. In-vehicle power supply system  11  is mounted to vehicle body  22  of vehicle  21 . 
     High-voltage DC power supply  12  may be implemented only by storage battery  12 A, such as a lithium-ion battery with, e.g. a DC voltage of 48 V, or may include storage battery  12 A and power generator  23  connected in parallel with storage battery  12 A. Power generator  23  may include, for example, an alternator and a rectifier. Low voltage storage battery  13  is, for example, a lead-acid storage battery with a DC voltage of 12 V, which is lower than the voltage of high-voltage DC power supply  12 . During driving, such as normal traveling of vehicle  21 , low voltage storage battery  13  is charged by the electric power of high-voltage DC power supply  12 . 
     As mentioned above, DC-DC converter  14  is connected between high-voltage DC power supply  12  and low voltage storage battery  13 . 
     DC-DC converter  14  includes input switch  16 , conversion circuit  17  capable of a bidirectional operation, output switch  18 , and current detector  19  that are connected in series from high-voltage DC power supply  12  to low voltage storage battery  13  in this order. 
     Each of input switch  16  and output switch  18  may be implemented by a semiconductor switch, such as a field effect transistor (FET) or an insulated gate bipolar transistor (IGBT), or a relay switch having mechanical contacts. Converter circuit  17  is a bidirectional conversion circuit that is capable of both a stepping-down operation of stepping down a high voltage of high-voltage DC power supply  12  to a low voltage of low voltage storage battery  13  and a boosting-up operation of boosting up a low voltage of low voltage storage battery  13  to a high voltage of high-voltage DC power supply  12 . 
     Converter circuit  17  includes high-side switch  24 , choke coil  25 , low-side switch  26 , and smoothing capacitor  27 . High-side switch  24  and choke coil  25  are connected in series in this order from high-voltage terminal  17 A to low voltage terminal  17 B of conversion circuit  17 . Low-side switch  26  is connected to ground GND and node  17 P at which high-side switch  24  is connected to choke coil  25 . Smoothing capacitor  27  is connected to low voltage terminal  17 B and ground GND. 
     High-side switch  24  thus includes one terminal and another terminal. One terminal of high-side switch  24  is connected to another terminal of input switch  16 . Choke coil  25  has one terminal connected to another terminal of output switch  18 , and another terminal connected to another terminal of high-side switch  24  at node  17 P. Low-side switch  26  includes one terminal connected to node  17 P and another terminal connected to ground GND. 
     High-side switch  24  and low-side switch  26  are controlled by controller  15  so as to perform a synchronously-rectifying operation so as to perform the boosting-up operation and the stepping-down operation. 
     Resistor  28  having a high resistance value is connected to low voltage terminal  17 B of conversion circuit  17  and ground GND. In other words, resistor  28  has one terminal connected to low voltage terminal  17 B of conversion circuit  17  and another terminal connected to ground GND. Resistor  28  is mainly used for detecting the voltage of low voltage terminal  17 B. 
     Current detector  19  detects a he current flowing through conversion circuit  17 . Controller  15  monitors the current detected by current detector  19  and the charge voltage for charging low voltage storage battery  13  which is the voltage across both ends of low voltage storage battery  13 . Further, controller  15  controls DC-DC converter  14  based on the current flowing through conversion circuit  17  and the charge voltage of low voltage storage battery  13 . This operation of controller  15  is an operation during driving, such as normally traveling of vehicle  21 . 
     In  FIG. 1 , current detector  19  is provided between input switch  16  and high-side switch  24  to detect the current flowing through high-side switch  24 . However, current detector  19  may be provided to detect a he current flowing through low-side switch  26 . Alternatively, current detector  19  may be provided at the one terminal or another terminal of choke coil  25  to detect a he current flowing through choke coil  25 . Further, current detector  19  may be provided at plural locations to detect currents flowing through plural components. Current detector  19  detects the current of DC-DC converter  14 . 
     However, current detector  19  may detect the current flowing through low voltage storage battery  13 . 
     When controller  15  detects that the current flowing through conversion circuit  17  or the charge voltage of low voltage storage battery  13  exceeds a predetermined threshold, controller  15  instructs input switch  16 , output switch  18 , high-side switch  24 , and low-side switch  26  to be continuously in the disconnected state. This configuration ensures safety from abnormality due to short-circuit. After that, controller  15  executes a failure determination operation of determining a failure of DC-DC converter  14 . The failure determination operation may be executed either during traveling of vehicle  21  or immediately after vehicle  21  has been started with a starter switch. 
     In the first determination operation performed initially in the failure determination, the controller executes the following three determinations: a short-circuit failure determination for output switch  18 , an open-circuit failure determination for output switch  18 , and a short-circuit failure determination for input switch  16 . Controller  15  detects low voltage value VL 1  of low voltage terminal  17 B of conversion circuit  17 . Based on low voltage value VL 1 , controller  15  performs an output short-circuit failure determination of determining whether output switch  18  has failure due to short-circuit thereof in which output switch  18  is continuously short-circuited, or output switch  18  is in an normal state, i.e., disconnectable. 
     At this moment, since high-side switch  24  and low-side switch  26  are continuously in the disconnected state and conversion circuit  17  is stopped, electric power is not supplied from conversion circuit  17  to output switch  18 . Then, if output switch  18  is in the disconnected state properly in response to the instruction from controller  15 , no voltage is applied to resistor  28  from low voltage storage battery  13  via output switch  18 . Therefore, when low voltage value VL 1  that occurs at the high potential side of resistor  28  or smoothing capacitor  27  is higher than low voltage threshold VTL 1 , controller  15  determines that output switch  18  has failure due to short-circuit in which output switch  18  is continuously short-circuited and thus has failure due to the short-circuit. If low voltage value VL 1  is lower than low voltage threshold VTL 1 , controller  15  determines that output switch  18  is normal. For example, low voltage threshold VTL 1  is set to be approximately within a range of one-tenth to one-half of the reference voltage value of low voltage storage battery  13  such that, at the time of a short-circuit failure of output switch  18 , the failure can be detected properly even if a voltage drop occurs due to a contact resistance of output switch  18 . 
     Alternatively, controller  15  may detect the voltage across both terminals of output switch  18 . Controller  15  may determine that output switch  18  has failure due to short-circuit thereof if the voltage across both terminals of output switch  18  is less than a threshold. In this case, controller  15  determines that output switch  18  is normal and has no failure if the voltage is greater than the threshold. For example, when output switch  18  has failure due to short-circuit thereof, the resistance between both terminals of output switch  18  is almost zero, so that the voltage is approximately zero. If output switch  18  is in the disconnected state properly in response to the instruction from controller  15 , a voltage appears across both terminals of output switch  18 , i.e., between low voltage storage battery  13  and grounded resistor  28 . Accordingly, it is possible that the threshold here may range from about 0.1 V to about 1.0 V. 
     In the first determination operation, controller  15  instructs output switch  18  to be in the connected state. In this condition, controller  15  detects low voltage value VL 2  of low voltage terminal  17 B of conversion circuit  17 . Based on low voltage value VL 2 , controller  15  performs an output open-circuit failure determination of determining whether output switch  18  is continuously in the disconnected state, i.e., continuously opens to have failure due to open-circuit, or output switch  18  is properly connectable. 
     At this moment, since conversion circuit  17  is stopped, electric power is not supplied from conversion circuit  17  to output switch  18 . If output switch  18  is properly connectable in response to the instruction from controller  15 , output switch  18  is switched to the connected state so that a voltage is applied to resistor  28  from low voltage storage battery  13  via output switch  18 . Therefore, if low voltage value VL 2  at the high potential side of smoothing capacitor  27  is lower than low voltage threshold VTL 2 , controller  15  determines that output switch  18  has failure due to open-circuit thereof. If low voltage value VL 2  is higher than low voltage threshold VTL 2 , controller  15  determines that output switch  18  is normal, having no failure. When output switch  18  is switched to the connected state properly, the voltage drop due to output switch  18  is a very small value. Therefore, the target of low voltage threshold VTL 2  may be set to a lower limit value within a normal fluctuation range of low voltage storage battery  13 , such as a value that is about 1 V lower than the reference value of low voltage storage battery  13  or a value that is about 10% lower than the reference value of low voltage storage battery  13 . 
     Alternatively, if controller  15  detects the voltage across both terminals of output switch  18  and the voltage between across both terminals of output switch  18  is greater than a threshold, controller  15  may determine that output switch  18  has failure due to open-circuit thereof. If the voltage is less than the threshold, controller  15  may determine that output switch  18  is normal, having no failure. For example, if output switch  18  has failure due to open-circuit, a voltage appears across both terminals of output switch  18 , between low voltage storage battery  13  and grounded resistor  28 . On the other hand, if output switch  18  is in a normal, connectable state, the resistance between both terminals of output switch  18  is almost zero, almost no voltage appears the voltage is about zero. Accordingly, the threshold may be set within a range from about 0.5 V to about 1.0 V. 
     In the first determination operation, controller  15  further detects high voltage value VH 1  of high-voltage terminal  17 A of conversion circuit  17 . Based on high voltage value VH 1 , controller  15  performs an input short-circuit failure determination of determining whether input switch  16  is continuously in the connected state, i.e., continuously short-circuited to have failure due to the short-circuit, or input switch  16  is in a normal state, disconnectable. At this moment, since conversion circuit  17  is stopped, electric power is not supplied from conversion circuit  17  to input switch  16 . If input switch  16  is properly in the disconnected state in response to the instruction from controller  15 , no voltage is applied to high-voltage terminal  17 A from high-voltage DC power supply  12  via input switch  16 . 
     In the first determination operation, the controller preferably executes the input short-circuit failure determination of input switch  16  first among the output short-circuit failure determination of output switch  18 , the output open-circuit failure determination of output switch  18 , and the input short-circuit failure determination of input switch  16 , and then, executes the output short-circuit failure determination of output switch  18  subsequently. After that, the controller executes the output open-circuit failure determination of output switch  18 . Alternatively, the controller may execute the output short-circuit failure determination of output switch  18  first, then, execute the input short-circuit failure determination of input switch  16 , and after that, execute the output open-circuit failure determination of output switch  18 . 
     The determination of short-circuit failure is executed before the determination of open-circuit failure. This configuration prevents an overcurrent from flowing into a switch for which a failure is to be detected when controller  15  instructs the switch to switch be in a connected state in subsequent failure determination operations. This operation prevents a secondary failure associated with the failure determination. The secondary failure is more effectively prevented especially when the input short-circuit failure determination of input switch  16  is initially executed before the output short-circuit failure determination of output switch  18 . 
     In the first determination operation, controller  15  executes the input short-circuit failure determination, the output short-circuit failure determination, and the output open-circuit failure. If determining that both input switch  16  and output switch  18  are normal and do not have failure in the input short-circuit failure determination, the output short-circuit failure determination, and the output open-circuit failure determination, controller  15  executes the second determination operation described later. If determining that any of input switch  16  and output switch  18  is in failure in these determinations, it is determined that DC-DC converter  14  has failure, and a switch with failure is identified. Controller  15  continuously stops DC-DC converter  14 . Further, controller  15  transmits a warning signal reporting the failure to a vehicle control unit mounted to vehicle body  22 . 
     Furthermore, after determining that output switch  18  is able to properly switch to both the connected state and the disconnected state and has no failure in the first determination operation, the controller  15  may determine whether or not low-side switch  26  has failure, as a preliminary determination operation. Herein, first, controller  15  instructs low-side switch  26  to be in the disconnected state. Then, in a limited short length of period PT 1 , output switch  18  that has been determined to be normal in the first determination operation is switched to the connected state. Then, after period PT 1  has elapsed, output switch  18  is switched to the disconnected state. 
     Here, if low-side switch  26  is properly in the disconnected state in response to an instruction from controller  15 , the voltage charged in smoothing capacitor  27  during period PT 1  is maintained after completion of period PT 1  and even when period PT 2  has elapsed after the disconnecting of output switch  18 . On the other hand, when low-side switch  26  is unable to be in the disconnected state properly in response to the instruction from controller  15  and has failure due to short-circuit thereof, the voltage charged in smoothing capacitor  27  during period PT 1  is discharged through the short-circuited low-side switch  26  to ground GND, so that the voltage becomes almost zero after period PT 2  has elapsed from the time when low-side switch  26  is instructed to be in the disconnected state after completion of period PT 1 . 
     In other words, after charging smoothing capacitor  27  in period PT 1  by connecting output switch  18 , controller  15  causes output switch  18  to be in the disconnected state. Controller  15  detects, based on the voltage across both terminals of resistor  28 , the state of charge of smoothing capacitor  27  after the lapse of period PT 2  from the disconnecting of output switch  18 . If this voltage is lower than a predetermined voltage, controller  15  determines that low-side switch  26  has failure due to short-circuit thereof. If this voltage is equal to or higher than the predetermined voltage, controller  15  determines that low-side switch  26  has no failure doe to short-circuit. 
     The second determination operation is performed subsequent to the first determination operation. If controller  15  determines that output switch  18  has neither failure due to short-circuit nor open-circuit so that controller  15  can control output switch  18  properly and determines that input switch  16  has no failure due to short-circuit, controller  15  executes the second determination operation subsequent to the first determination operation. 
     In the second determination operation, first, controller  15  instructs output switch  18  to be in the connected state. Further, controller  15  instructs conversion circuit  17  to perform the boosting-up operation of boosting up the voltage of low voltage terminal  17 B of conversion circuit  17 . Then, controller  15  detects high voltage value VH 2  at high-voltage terminal  17 A of conversion circuit  17 , which is obtained by the boosting-up operation. High voltage value VH 2  may be obtained by detecting the voltage of the high potential side of input capacitor  20 . Based on high voltage value VH 2 , controller  15  determines whether conversion circuit  17  may properly perform the boosting-up operation and whether or not conversion circuit  17  has failure. 
     Accordingly, if conversion circuit  17  increases high voltage value VH 2 , which is obtained by detecting the voltage of the high potential side of input capacitor  20 , to be higher than high-voltage threshold VTH 2 , the controller determines that conversion circuit  17  is able to perform the boosting-up operation properly and conversion circuit  17  has no failure. High-voltage threshold VTH 2  may be set to a value that is about 1 V higher than the reference value of high-voltage DC power supply  12  or a value that is about 10% higher than the reference value of high-voltage DC power supply  12 . 
     If controller  15  determines that conversion circuit  17  has no failure in the second determination operation, controller  15  executes the following determination operation. When a failure is determined in the second determination operation, controller  15  determines that conversion circuit  17  has failure and identifies that a portion with failure is conversion circuit  17 . Furthermore, controller  15  transmits a warning signal indicating the failure to the vehicle control unit mounted to vehicle body  22 . 
     At the beginning of the second determination operation, controller  15  causes output switch  18  to switch to the connected state. Here, in the case where high-side switch  24  of conversion circuit  17  is implemented by a MOSFET having a parasitic diode that allows current to constantly flow in a direction from output switch  18  toward input switch  16 , or in the case where high-side diode  24 A that allows a current to constantly flow in the direction from output switch  18  toward input switch  16  is provided in parallel with high-side switch  24 , a large current flows within a short time from low voltage storage battery  13  to input capacitor  20  when output switch  18  is switched to the connected state. Therefore, before controller  15  causes output switch  18  to continuously be in the connected state in the second determination operation, controller  15  may cause output switch  18  to operate as a switching element of a stepping-down converter so as to charge input capacitor  20  to a predetermined voltage. 
     The operation for charging input capacitor  20  will be detailed below. In conversion circuit  17 , high-side switch  24  connected to input switch  16  is connected in series to choke coil  25  connected to output switch  18 . Low-side switch  26  is connected to the ground and to node  17 P at which high-side switch  24  is connected to choke coil  25 . High-side diode  24 A having a cathode connected to input switch  16  is connected in parallel to high-side switch  24 . The cathode of converter diode  29  is connected low voltage terminal  17 B of conversion circuit  17 . The anode of the diode is connected to ground GND. Input capacitor  20  is connected to ground GND and high-voltage terminal  17 A of conversion circuit  17 . 
     If controller  15  determines that conversion circuit  17  has no failure in the first determination operation, controller  15  executes the second determination operation. In the second determination operation, controller  15  periodically turns on and off output switch  18  by driving output switch  18  with, e.g. a PWM signal to control switching of output switch  18 , thereby operating choke coil  25 , converter diode  29 , and output switch  18  as a step-down converter. This converter operation produces a voltage lower than the voltage of low voltage terminal  17 B at node  17 P, and controller  15  charges input capacitor  20  through high-side diode  24 A until both terminals of input capacitor  20  reaches a predetermined value. After that, controller  15  quits this converter operation, to complete charging of input capacitor  20 . 
     After that, controller  15  instructs output switch  18  to continuously be in the connected state, and further, instructs conversion circuit  17  to perform the boosting-up operation of boosting up the voltage of low voltage terminal  17 B of conversion circuit  17 . After that, controller  15  detects high voltage value VH 2  of high-voltage terminal  17 A of conversion circuit  17 , and determines, based on high voltage value VH 2 , whether or not conversion circuit  17  has failure. 
     The failure determination of conversion circuit  17  based on high voltage value VH 2  is similar to the second determination operation, and will not be described further. In the case that output switch  18  is implemented by a semiconductor switch, such as a field effect transistor (FET) or an insulated gate bipolar transistor (IGBT), controller  15  can perform control on the switching operations. 
     Controller  15  performs the third determination operation after the second determination operation. If controller  15  determines that conversion circuit  17  is normal and conversion circuit  17  is able to perform the boosting-up operation and has no failure in the second determination operation, controller  15  executes the third determination operation subsequent to the second determination operation. 
     In the third determination operation, first, controller  15  instructs conversion circuit  17  not to perform the boosting-up operation. 
     Output switch  16  has been instructed to be in the disconnected state until then Controller  15  instructs output switch  16  to be in the connected state. After that, controller  15  detects high voltage value VH 3  of high-voltage terminal  17 A of conversion circuit  17 . High voltage value VH 3  may be obtained by detecting the voltage of the high potential side of input capacitor  20 . Based on high voltage value VH 3 , controller  15  performs an input open-circuit failure determination of determining whether input switch  16  continuously opens and has failure due to open-circuit thereof or input switch  16  is properly connectable. 
     At this moment, conversion circuit  17  is stopped, so that electric power is not supplied from conversion circuit  17  to input switch  16 . Then, if output input switch  16  is properly connectable in response to an instruction from controller  15 , a voltage is applied to high-voltage terminal  17 A from high-voltage DC power supply  12  via input switch  16  in the connected state. Therefore, if high voltage value VH 3  of high-voltage terminal  17 A of conversion circuit  17  is lower than high-voltage threshold VTH 3 , controller  15  determines that output switch  18  has failure due to open-circuit. If high voltage value VH 3  is higher than high-voltage threshold VTH 3 , controller  15  determines that output switch  18  has no failure and is normal. For example, the target of high-voltage threshold VTH 3  is set to a lower limit value within the normal fluctuation range of high-voltage DC power supply  12 , such as a value that is about 10% lower than the reference value of high-voltage threshold VTH 3 . Similarly to the short-circuit failure determination, the controller  15  may obtain the voltage of high-voltage terminal  17 A of conversion circuit  17  by detecting the voltage of input capacitor  20  connected to the high-voltage terminal  17 A of conversion circuit  17  and to ground GND. 
     In the third determination operation, controller  15  determines whether or not input switch  16  has failure due to open-circuit thereof. If determining that input switch  16  has no failure and is normal, all the determinations are ended. On the other hand, if is determining that input switch  16  has failure due to open-circuit, controller  15  determines that DC-DC converter  14  has failure. Controller  15  continuously stops DC-DC converter  14  and identifies a portion with failure as input switch  16 . Further, controller  15  transmits a warning signal indicating the failure to the vehicle control unit mounted to vehicle body  22 . 
     The configuration and operation as described above allows the controller to easily and accurately identify a portion of DC-DC converter  14  with failure. 
     Moreover, as described earlier, in-vehicle power supply system  11  starts to determine a failure from input switch  16 , which is on the high voltage side, and proceeds the determination successively to the low voltage side. This means that, when a switch in conversion circuit  17  has failure due to short-circuit, subsequent determination is not performed after that. Therefore, no voltage is applied to conversion circuit  17  or output switch  18  in the failure determination proceeding from the high voltage side to the low voltage side. In other words, when the determination of the low voltage side is performed, it has already been determined beforehand that conversion circuit  17  and input switch  16  are normal. Thereby, it is possible to set an appropriate voltage for conversion circuit  17  and output switch  18  especially when performing a failure determination operation of output switch  18 . This configuration prevents a secondary failure. Furthermore, it is unnecessary to increase the withstand voltage of output switch  18  in consideration of the failure determination. 
     In in-vehicle power supply system  11  described above, DC-DC converter  14  and controller  15  may be accommodated in, e.g. a single housing, or DC-DC converter  14  and controller  15  may be separately arranged in vehicle body  22 . 
     Particularly in the case that input switch  16  is implemented by a semiconductor switch, input switch  16  may preferably include two semiconductor switches connected in series such that the parasitic diodes are connected in opposite directions. 
     REFERENCE MARKS IN THE DRAWINGS 
     
         
           11  in-vehicle power supply system 
           12  high-voltage DC power supply 
           12 A storage battery 
           13  low voltage storage battery 
           14  DC-DC converter 
           15  controller 
           16  input switch 
           17  conversion circuit 
           17 A high-voltage terminal 
           17 B low voltage terminal 
           18  output switch 
           19  current detector 
           20  input capacitor 
           21  vehicle 
           22  vehicle body 
           23  power generator 
           24  high-side switch 
           24 A high-side diode 
           25  choke coil 
           26  low-side switch 
           27  smoothing capacitor 
           28  resistor 
           29  converter diode