Electrical load driving apparatus

The electrical load driving apparatus includes means for alternately lowering the gate voltages of two current supply transistors connected in parallel to each other at regular time intervals, a current being supplied to an electrical load through drain-source paths of both the current supply transistors, and means for detecting wire breakage in two current supply wires in which the current supply transistors are interposed respectively at portions near the electrical load with respect to the current supply transistors based on the drain-source voltages of the current supply transistors.

This application claims priority to Japanese Patent Application No. 2010-290903 filed on Dec. 27, 2010, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrical load driving apparatus for supplying a current to an electrical load through a plurality of transistors.

2. Description of Related Art

Japanese Patent Application Laid-open number 2003-158447 discloses such an electrical load driving apparatus having a structure in which a plurality of power supply wires are connected in parallel, one connection node of the power supply wires being connected to one end of an electrical load which is connected to one of the high voltage side and the low voltage side of a power source, the other node being connected to the other of the high voltage side and the low voltage side of the power source, and each of the power supply wires including a current supply transistor interposed therein. Accordingly, in this structure, a plurality of the current supply transistors are connected in parallel with one another, and connected in series to the electrical load.

In this apparatus, the current supply transistors are turned on during a current supply period in which the electrical load should be continuously supplied with a current.

According to the above structure, it is possible to maintain supply of a current to the electrical load even if one of the current supply wires is broken.

Further, the above electrical load driving apparatus is configured to generate sole-on periods within the current supply period, only a corresponding one of the current supply transistors being turned on during each sole-on period, and to monitor the voltage at the output terminals of the current supply transistors connected in parallel with one another during the sole-on period to determine whether or not the current supply transistor to be turned on within the sole-on period has an open failure in which the transistor is fixed to the off state.

This configuration makes it possible to detect an open failure of each current supply transistor, and also to detect a wire breakage of each current supply wire as an open failure of the current supply transistor regardless whether the wire breakage occurs at a portion on the near side of the current supply transistor, or at a portion on the far side of the current supply transistor with respect to the electrical load.

However, the above conventional electrical load driving apparatus has a disadvantage in reliability, because the sole-on period is generated within the current supply period.

More specifically, in a case where the electrical load is not a load that should be driven only a short period in response to a certain event, but a load that should be drive continuously (for example a relay for relaying power to an ignition system of a vehicle), since the failure detection is performed by turning on one of the current supply transistors during each sole-on period all the while the vehicle is running, the current supply transistors may degrade quickly.

In addition, if one of the current supply transistors has an open failure or if one of the current supply wires is broken, current supply to the electrical load is interrupted during each corresponding sole-on period.

SUMMARY

An exemplary embodiment provides an electrical load driving apparatus comprising:

a plurality of current supply wires connected in parallel with one another, each of the current supply wires being connected to one of a high voltage terminal and a low voltage terminal of a power source at one end thereof, and being connected to one end of an electrical load at the other end thereof, the other end of the electrical load being electrically connected with the other of the high voltage terminal and the low voltage terminal of the power source;

a plurality of current supply transistors having one control terminal and two output terminals, each of the current supply transistors being interposed in a corresponding one of the current supply wires at the two output terminals thereof to supply a current to the electrical load when being turned on during a current supply period in which the electrical load should be supplied with the current continuously;

a check period generating section that successively generates a first check period within the current supply period, a specific one of the current supply transistors being turned on in a completely turn-on state where a voltage difference between the two output terminals thereof is lower than or equal to a predetermined value so that the specific current supply transistor is turned on completely, the current supply transistors other than the specific current supply transistor being turned on in a low current supply performance state where a voltage difference between the two output terminals thereof is higher than the predetermined value so that the current supply transistors other than the specific current supply transistor are turned on incompletely, the specific current supply transistor being selected from among all of the current supply transistors in sequence within the current supply period; and

a wire breakage determining means configured to determine, upon detecting that a voltage variation in a voltage of one of the two output terminals which is located on the side near the electrical load with respect to the current supply transistors between any one of the current supply transistors and any other one of the current supply transistors exceeds a predetermined threshold, that a wire breakage is present in a portion of one of the current supply wires on the side near the electrical load with respect to the current supply transistors.

According to the exemplary embodiment, there is provided an electrical load driving apparatus including a plurality of current supply transistors connected in parallel with one another to supply a current to an electrical load, and capable of detecting wire breakage in a plurality of current supply wires in each of which a corresponding one of the current supply transistors is interposed without placing a large current burden on the current supply transistors and without interrupting current supply to the electrical load.

Other advantages and features of the invention will become apparent from the following description including the drawings and claims.

PREFERRED EMBODIMENTS OF THE INVENTION

First Embodiment

FIG. 1is a diagram showing the structure of an ECU11as an electrical load driving apparatus according to a first embodiment of the invention.

The ECU11is mounted on a vehicle to turn on and off a relay13mounted on the vehicle by passing a current to a coil13aof the relay13. When the relay13is turned on, a battery voltage VB of a battery15is supplied to various vehicle-mounted units and devices related to the ignition system of the vehicle as a power supply voltage. More specifically, the ECU11turns on the relay13when a vehicle driver operates the ignition switch of the vehicle to supply the battery voltage VB to the various vehicle-mounted units and devices, and turns off the relay13when the vehicle driver turns off the ignition switch on condition that a predetermined power supply stop condition is satisfied.

In this embodiment, the relay coil13aof the relay13is high side driven by the ECU11. Accordingly, one end of the relay coil13ais connected to the negative terminal of the battery15through a ground line of the vehicle outside the ECU11.

The ECU11includes a terminal17connected to the other end of the relay coil13aopposite to the ground line, a terminal19connected to the positive terminal (high voltage terminal) of the battery15, and two current supply transistors Tr1and Tr2each of which passes a current to the relay coil13awhen turned on. In this embodiment, the transistors Tr1and Tr2are N-channel MOSFETs.

The two current supply transistors Tr1and Tr2are connected in parallel to each other, and connected in series to the relay coil13a.

More specifically, the ECU11includes current supply wires L1and L2connected in parallel to each other, each of which is connected to the terminal17at one end thereof and connected to the terminal19at the other end thereof. The current supply wire L1is interposed by the current supply transistor Tr1such that the drain terminal is on the upstream side (on the terminal19side) and the source terminal is on the downstream side. The current supply wire L2is interposed by the current supply transistor Tr2such that the drain terminal is on the upstream side (on the terminal19side) and the source terminal is on the downstream side.

Hence, the current supply wire L1includes a load side portion LS1located on the near side of the relay coil13aas an electrical load and connecting the source terminal of the current supply transistor Tr1to the terminal17of the ECU11, and an opposite load side portion LD1located on the far side of the relay coil13aand connecting the drain terminal of the current supply transistor Tr1to the terminal19of the ECU11. Likewise, the current supply wire L2includes a load side portion LS2located on the near side of the relay coil13aand connecting the source terminal of the current supply transistor Tr2to the terminal17of the ECU11, and an opposite load side portion LD2located on the far side of the relay coil13aand connecting the drain terminal of the current supply transistor Tr2to the terminal19of the ECU11.

The ECU11includes a microcomputer20which controls current supply to the relay coil13a, a drive circuit21which turns on and off the current supply transistor Tr1in accordance with an on/off command signal SD1outputted from the microcomputer20, and a drive circuit22which turns on and off the current supply transistor Tr2in accordance with an on/off command signal SD2outputted from the microcomputer20. The ECU11further includes a voltage variation control section31and a failure detecting section32for detecting wire breakage of the current supply wires L1and L2.

The drive circuit21turns on the current supply transistor Tr1by applying a drive voltage to the gate terminal as a control terminal of the current supply transistor Tr1when the on/off command signal SD1is at the active level (high level in this embodiment). The drive circuit22turns on the current supply transistor Tr2by applying a drive voltage to the gate terminal as a control terminal of the current supply transistor Tr2when the on/off command signal SD2is at the active level (high level).

Upon detecting that the vehicle driver turns on the ignition switch, the microcomputer20brings the on/off command signals SD1and SD2respectively supplied to the drive circuits21and22to the high level at the same time as shown inFIG. 3. Thereafter, upon detecting that the vehicle driver turns off the ignition switch, the microcomputer20brings the on/off command signals SD1and SD2to the low level on condition that a predetermined current supply stop condition is satisfied.

As explained above, the microcomputer20sets both the on/off command signals SD1and SD2at the high level to turn on both the current supply transistors Tr1and Tr2during a current supply period in which the relay coil13ashould be energized continuously, so that a current is supplied to the relay coil13athrough the two current supply transistors Tr1and Tr2.

Although not shown inFIG. 1, the microcomputer20is inputted with a signal indicating that the vehicle driver turns on or off the ignition switch to detect whether the ignition switch is on or off. The current supply stop condition may be that a current supply stop permission signal transmitted on a signal line (not shown) is received from a vehicle-mounted unit supplied with electric power through the relay13.

Next, the operations of the drive circuits21and22are explained in detail. Here, since the drive circuits21and22have the same structure, only the operation of the drive circuit21is explained.

As shown inFIG. 2, the drive circuit21includes a voltage step-up circuit25for stepping up the battery voltage VB (which is 12 to 14 V in this embodiment) being supplied to the ECU11through the terminal19, a diode26whose anode is applied with the battery voltage VB, a switching element27as an analog switch connected between the voltage output terminal of the voltage step-up circuit25and the gate terminal of the current supply transistor Tr1, and a switching element28as an analog switch connected between the cathode of the diode26and the gate terminal of the current supply transistor Tr1.

The switching elements27and28turn on when the on/off command signal SD1outputted from the microcomputer20is at the high level to connect the voltage output terminal of the voltage step-up circuit25and the cathode of the diode26to the gate terminal of the current supply transistor Tr1, and turn off when the on/off command signal SD1outputted from the microcomputer20is at the low level to disconnect the voltage output terminal of the voltage step-up circuit25and the cathode of the diode26from the gate terminal of the current supply transistor Tr1.

The voltage step-up circuit25includes a voltage generating section25awhich generates a voltage higher than the battery voltage VB by stepping up the battery voltage VB and outputs the generated voltage from the voltage output terminal of the voltage step-up circuit25, and a voltage control section25bwhich adjusts the voltage generated by the voltage generating section25a.

The voltage control section25badjusts the voltage generated by the voltage generating section25ato a voltage (20 V, for example) high enough to turn on the current supply transistor Tr1completely in its saturation state by being applied to the gate terminal of the current supply transistor Tr1, when a later explained voltage variation command signal SC1outputted from the voltage variation control section31is at the non-active level (low level in this embodiment). This voltage is referred to as “completely turning-on voltage” hereinafter.

On the other hand, the voltage control section25badjusts the voltage generated by the voltage generating section25ato a specific voltage higher than the battery voltage VB and lower than the completely turning-on voltage. This specific voltage is a voltage for turning on the current supply transistor Tr1by being applied to the gate terminal of the current supply transistor Tr1in a state where the current supply performance of the current supply transistor is lower than that when being applied with the completely turning-on voltage. This specific voltage (15 V, for example) makes the drain-source voltage of the current supply transistor Tr1at a certain voltage higher than that when the current supply transistor Tr1is in the completely-on state. This specific voltage is referred to as “current supply performance lowering voltage” hereinafter.

Hence, as shown inFIG. 3, when the on/off command signal SD1outputted from the microcomputer20is at the high level, and the voltage variation command signal SC1is at the low level, the gate voltage VG1of the current supply transistor Tr1is at the completely turning-on voltage (20 V) generated by the voltage step-up circuit25, and as a result, the drain-source voltage of the current supply transistor Tr1become 0.5 V, for example. Accordingly, when the on/off command signal SD1outputted from the microcomputer20is at the high level, and the voltage variation command signal SC1is at the low level, the source voltage VS1as the output voltage of the current supply transistor TR1becomes 13.5 V which is lower than the drain voltage VD1equal to the battery voltage VB (14 V) by 0.5 V.

On the other hand, when the on/off command signal SD1outputted from the microcomputer20is at the high level, and the voltage variation command signal SC1is at the high level, the gate voltage VG1of the current supply transistor Tr1is at the current supply performance lowering voltage (15 V), and as a result, the current supply transistor Tr1turns on in the low current supply performance state where the drain-source voltage is at a value lower than that when the current supply transistor Tr1is in the completely on state. Hence, as shown inFIG. 3, when both the on/off command signal SD1and the voltage variation command signal SC1are at the high level, the source voltage VS1of the current supply transistor becomes 13.0 V which is lower than the drain voltage VD1equal to the battery voltage VB (14 V) by 1.0 V.

The above descriptions of the structure and operation of the drive circuit21can be applied to the drive circuit22except that the on/off command signal SD1outputted from the microcomputer20is replaced by the on/off command signal SD2, the voltage variation command signal SC1outputted from the voltage variation control section31is replaced by the voltage variation command signal SC2, and the current supply transistor Tr1is replaced by the current supply transistor Tr2.

Although not shown in the drawings, each of the current supply transistors Tr1and Tr2is provided with a malfunction-preventing resistor connected between the gate terminal and the source terminal. Accordingly, when the on/off command signals SD1or SD2is changed to the low level to turn off the switching element27or28of the drive circuit21or22, since the gate-source voltage of the current supply transistor Tr1or Tr2becomes 0 V, the current supply transistor Tr1or TR2completely turns off.

Further, in the unlikely case where the output voltage of the voltage step-up circuit25decreases below the current supply performance lowering voltage (decreases to 0 V, for example) when the on/off command signal SD1or SD2is at the high level, since the gate terminal of the current supply transistor Tr1or TR2is applied with a voltage equal to the battery voltage VB minus the forward voltage Vf (about 0.6 V) of the diode26, it is possible to turn on the current supply transistor Tr1or Tr2in a state where a necessary minimum current is passed to the relay coil13aalthough the current supply performance in this state is lower than that in the low current supply performance state. That is, the diode26and the switching element28are provided to enable the fail-safe operation for each of the drive circuits21and22.

Next, the operation of the voltage variation control section31is explained. The voltage variation control section31is configured to vary the source voltages VS1and VS2of the current supply transistors TR1and Tr2by varying their gate voltages VG1and VG2during the current supply period in which both the on/off command signals SD1and SD2outputted from the microcomputer20are at the high level. More specifically, the voltage variation control section31is configured to alternately bring the voltage variation command signals SC1and SC2respectively supplied to the drive circuits21and22to the high level at intervals of a predetermined time Ta for a predetermined time Tb shorter than the time Ta, as shown inFIG. 3. In other words, the voltage variation control section31sets each of the voltage variation command signal SC1and SC2at the high level during the time Tb at intervals of the time of 2×Ta with a phase difference of the time Ta between these signals.

Accordingly, the period of the time Tb in which one of the current supply transistors Tr1and Tr2is in the completely on state, and the other is in the low current supply performance state occurs at intervals of the time Ta within the current supply period of the relay coil13a, and the current supply transistors Tr1and Tr2are set to the high level alternately at intervals of the time Ta. In this embodiment, the period of the time Tb is a check period for detecting wire breakage of the load side portion LS1or LS2of the current supply wire L1or L2.

As shown inFIG. 3, in this embodiment, assuming that the battery voltage VB is 14 V, when the voltage variation command signal SC1is changed to the high level as a result of which the gate voltage VG1of the current supply transistor Tr1decreases from the completely turn-on voltage (20 V) to the current supply performance lowering voltage (15 V), since the current supply transistor Tr1turns on in the low current supply performance state, the drain-source voltage of the current supply transistor Tr1increases from 0.5 V to 1.0 V, and the source voltage of the current supply transistor Tr1decreases from 13.5 V to 13.0 V.

Likewise, when the voltage variation command signal SC2is changed to the high level as a result of which the gate voltage VG2of the current supply transistor Tr2decreases from the completely turn-on voltage (20 V) to the current supply performance lowering voltage (15 V), since the current supply transistor Tr2turns on in the low current supply performance state, the drain-source voltage of the current supply transistor Tr2increases from 0.5 V to 1.0 V, and the source voltage of the current supply transistor Tr2decreases from 13.5 V to 13.0 V.

At this time, since the source terminals of the current supply transistors Tr1and Tr2are connected to the terminal117of the ECU11, the load voltage (the voltage applied to the relay coil13a) which is equal to the voltage of the terminal17decreases from 13.5 V to 13.0 V as shown inFIG. 3.

The voltage variation control section31is allowed to operate to set the voltage variation command signal SC1or SC1at the high level by the microcomputer20. Next, the operation of the failure detecting section32is explained.

The failure detecting section32is connected to the output terminals (the drain and source terminals) of each of the current supply transistors TR1and Tr2, and performs a load side wire breakage detection process shown inFIG. 4to detect wire breakage in the load side portions LS1and LS2of the current supply wires L1and L2.

As shown inFIG. 4, this process begins in step S110to determine whether or not a difference between the source voltage VS1and VS2of the current supply transistors Tr1and Tr2exceeds a predetermined threshold value. If the determination result in step S110is affirmative, the process proceeds to step S120to output a load side wire breakage detection signal to the microcomputer20assuming that a wire breakage has occurred in the load side portion LS1or LS2.

Incidentally, although the current supply transistors and the current supply wires are two in number in this embodiment, they may be three or more in number. In short, step S120determines that there is a wire breakage in one of the current supply wires when a difference in the source voltage between any one of the current supply transistors and any other one of the current supply transistors exceeds the threshold value. The failure detecting section32may be configured to perform the load side wire breakage detection process constantly, or only within the current supply period of the relay coil13a, or only during the check period within the current supply period. Upon receiving the load side wire breakage detection signal from the failure detecting section32, the microcomputer20stores a failure code indicating occurrence of a failure identified by the load side wire breakage detection signal in a nonvolatile memory, and performs a warning process to inform the vehicle driver of the occurrence of the failure by turning on a warning light or showing a message corresponding to the detected failure on a display.

The principle of detecting wire breakage in the load side portion LS1or LS2is explained in the following. In the normal state where there is no wire breakage, since the source terminals as the load side output terminals of the current supply transistors Tr1and Tr2are connected to each other through the load side portions LS1and LS2, the source voltages VS1and VS2of the source terminals are always the same with each other.

If a wire breakage occurs in any one of the load side portions LS1and LS2, the current supply transistor TRx (x being 1 or 2) connected to the load side portion LSx having the wire breakage is disconnected from the source terminal of the other current supply transistor and the relay coil13a.

Accordingly, during the check period of the length of the time Tb in which the current supply transistor other than the current supply transistor Trx is turned on in the low current supply performance state, the drain-source voltage of the current supply transistor Trx is at 0.5 V as is expected in the completely on period, while the drain-source voltage of the other current supply transistor is at 1.0 V. Accordingly, the source voltage of the current supply transistor Trx becomes the battery voltage VB minus 0.5 V, while the source voltage of the other current supply transistor becomes the battery voltage VB minus 1.0 V. Therefore, there occurs a difference larger than a certain threshold value between the source voltage VSx of the current supply transistor VSx and the source voltage of the other current supply transistor.

Accordingly, in this embodiment, the threshold value used in step S110is set to a value (0.4 V, for example) which is equal to or slightly smaller than the difference between 1.0 V and 0.5 V, and it is determined that there is a wire breakage in one of the load side portions LS1and LS2if a difference between the source voltages VS1and VS2is detected to be larger than the threshold value.

Accordingly, as shown inFIG. 3, if a wire breakage occurs in the load side portion LS2of the current supply wire L2at time t1, and the battery voltage VB is 14.0 V, when there comes the check period of the length of the time Tb in which the voltage variation command signal SC1is set at the high level, the voltage variation command signal SC2is set at the low level so that the current supply transistor TR2turns on in the completely on state, and the current supply transistor Tr1turns on in the low current supply performance state, the source voltage VS1of the current supply transistor Tr1becomes 13.0 V, while the source voltage VS2of the current supply transistor Tr2becomes 13.5 V. As a result, since the difference between the source voltage VS1and VS2exceeds the threshold value, it is determined that one of the load side portions LS1and LS2is broken.

Incidentally, when the load side portion LS2of the current supply wire L2is broken, the current supply transistor Tr1supplies a current to the relay coil13during the check period in which the voltage variation command signal CS1is at the low level and the voltage variation command signal CS2is at the high level even if the gate voltage VG1is not decreased. Accordingly, in this case, although the current supply transistor Tr2is not connected to the relay coil13a, since the gate voltage VG2decreases to 15 V, the source voltage VS2is lowered compared to the normal state. Hence, since the difference between the source voltages VS1and VS2does not exceed the threshold value, no wire breakage is detected.

The failure detecting section32includes an on-state monitoring section32a. The on-state monitoring section32ais activated when both the on/off command signals SD1and SD2outputted from the microcomputer20are at the high level to monitor the drain-source voltages of the current supply transistors Tr1and Tr2. Upon detecting that any one of the drain-source voltages of the current supply transistors Tr1and Tr2exceeds a specific value Vdso set for detecting abnormality, the on-state monitoring section32aoutputs a circuit abnormality signal to the microcomputer20.

The specific value Vdso is set to a value (4 V, for example) higher than the drain-source voltage (1 V, for example) when the current supply transistors Tr1or Tr2is turned on in the low current supply performance state. The voltage variation control section31is configured to set a current-supply-period signal outputted to the failure detecting section32to the high level during the current supply period in which both the on/off command signals SD1and SD2outputted from the microcomputer20are at the high level. The failure detecting section32operates while the current-supply-period signal is at the high level. The failure detecting section32may be inputted with the on/off command signals SD1and SD2outputted from the microcomputer20instead of the current-supply-period signal.

Next, a voltage variation control inhibition process which the microcomputer20performs in connection with the wire breakage detection for the current supply wires L1and L2is explained with reference toFIG. 5. The microcomputer20performs this voltage variation control inhibition process at regular time intervals during the current supply period in which the microcomputer20sets both the on/off command signals SD1and SD2at the high level. The microcomputer20allows the voltage variation control section31to operate immediately after being powered on.

As shown inFIG. 5, the voltage variation control inhibition process begins in step S210to determine whether the drain-source voltage of any one of the current supply transistors Tr1and Tr2exceeds the specified value Vdso based on whether or not the circuit abnormality signal has been outputted from the failure detecting section32.

If the determination result in step S210is negative, the process proceeds to step S220. In step S220, it is determined whether or not the failure detecting section32has detected a wire breakage of the current supply wire L1or L2. More specifically, it is determined whether or not the load side wire breakage detection signal has been outputted from the failure detecting section32. If the determination result in step S220is negative, the process proceeds to step S240.

On the other hand, if the determination result in step S210or step S220is affirmative, the process proceeds to step S230. In step S230, the voltage variation control section31is inhibited from operating to prevent the voltage variation command signals SC1and SC2from being set to the high level, and then this process is terminated.

The inhibition to the voltage variation control section31continues after the present current supply period of the relay coil13aends and the next current supply period comes. The microcomputer20releases the inhibition to the voltage variation control section31to allow the voltage variation control section31to operate when the microcomputer20restarts thereafter, or when the microcomputer20receives an inhibition release signal transmitted from outside of the ECU11.

In step S240, the battery voltage VB received through the terminal19is A/D-converted, and it is determined whether or not the battery voltage VB has decreased below a predetermined low voltage detection threshold Vb1. If the determination result in step S240is negative, the process proceeds to step S250. The low voltage detection threshold Vb1is set to a value (10 V, for example) lower than the normal range of the value of the battery voltage VB.

In step S250, it is determined whether or not at least one of specific electrical loads (the starter or defogger, for example) which may cause the battery voltage VB to drop below the low voltage detection threshold Vb1has been powered on or supplied with a current, based on information showing control states of the specific electrical loads received from vehicle-mounted units for controlling theses electrical loads. If the determination result in step S250is negative, the process proceeds to step S260to release inhibition made in the later described step S270, and then this process is terminated.

If the determination result in step S240or step S250is affirmative, the process proceeds to step S270assuming that the battery voltage VB is lower than the low voltage detection threshold.

In step S270, the voltage variation control section31is inhibited from operating, and then this process is terminated. The inhibition to the voltage variation control section31, which is released in step S260, is for inhibiting the voltage variation control section31from operating when the determination result in step S240or S250is affirmative.

According to the above described process performed by the ECU11makes it possible to detect wire breakage in the load side portions LS1and LS2of the current supply wires L1and L2respectively connected with the current supply transistors Tr1and Tr2. Further, since the relay coil13ais supplied with a current through all the current supply transistors Tr1and Tr2during the current supply period of the relay coil13a, it can be prevented that the current supply transistors Tr1and Tr2are overloaded, and that current supply to the relay coil13ais interrupted when one of the current supply wires L1and L2is broken.

Incidentally, the check period of the length of the time Tb in which one of the voltage variation command signals SC1and SC2is at the high level may be generated continuously and successively. However, generating the check period intermittently at regular time intervals of the time Ta is advantageous in view of reduction of power loss in the current supply transistors Tr1and Tr2.

In this embodiment, when the drain-source voltage of any one of the current supply transistors Tr1and Tr2exceeds the specific value Vdso during the current supply period of the relay coil13a, it is detected in steps S210and S230shown inFIG. 5, and as a result the voltage variation control section31is inhibited from operating thereafter.

Accordingly, if there occurs a circuit abnormality in which the output voltage of the voltage step-up circuit25decreases below the current supply performance lowering voltage when the voltage variation command signal SC1is set to the high level, due to abnormality in the voltage control section25bof the voltage step-up circuit25of one or both of the drive circuits21and22(it is assumed that the abnormality is in only the drive circuit21in the following description), and as a result, the drain-source voltage of the current supply transistor Tr1exceeds the specified value Vdso, the voltage variation command signals SC1and SC2are inhibited from being set to the high level thereafter. Accordingly, according to the above embodiment, it is possible to prevent the power loss of the current supply transistor Tr1from becoming excessively large, and to prevent the current supplied to the relay coil13afrom being reduced. The above explanation applies also to the drive circuit22and the current supply transistor Tr2.

The on-state monitoring section32aof the failure detecting section32may be configured to monitor the drain-source voltage of only one of the current supply transistors Tr1and Tr2. However, in this embodiment, the on-state monitoring section32aof the failure detecting section32is configured to monitor the drain-source voltages of both the current supply transistors Tr1and Tr2to increase the reliability of the failure detection. The on-state monitoring section32amay be configured to operate to monitor the drain-source voltages of the current supply transistors Tr1and Tr2only during the check period in which one of the voltage variation command signals SC1and SC2is set to the high level within the current supply period of the relay coil13a.

According to this embodiment, since the voltage variation control section32is inhibited from operating after any one of the current supply wires L1and L2is detected to be broken by the failure detecting section32, it is possible to prevent the current supplied to the relay coil13afrom being reduced because the current supply transistor connected to the normal one of the current supply wires is not set to the low current supply performance state.

Further, according to this embodiment, it is possible to turn on the current supply transistors Tr1and Tr2in a state enabling to supply a current to the coil13anecessary to turn on the relay13, even if the drive voltage applied to the gate terminals of the current supply transistors Tr1and Tr2is decreased below a value minimally necessary to turn on the current supply transistors Tr1and Tr2due to failure of the voltage step-up circuit25or the switching element27of the drive circuit21or22, because of the provision the diode26and the switching element28.

In addition, according to this embodiment, it is possible to prevent the current supplied to the relay coil13afrom being excessively reduced when the battery voltage VB is low, causing the voltage variation command signals SC1and SC2to be at the high level, because step S270inhibits the voltage variation control section31from operating when the battery voltage VB is detected to be lower than the low voltage detection threshold Vb1in step S240or S250. One of steps S240and S250may be omitted.

In this embodiment, the relay coil13ais an object to be supplied with a current, however, the relay13itself can be assumed as an object to be driven.

Next, various modifications of the above described embodiment are described. The above embodiment may be modified such that the voltage variation command signals SC1and SC2are inputted also to the failure detecting section32to enable the failure detecting section32to determine that it is during the check period at present, and which of the current supply transistors is in the completely on state during this check period. This is because, if any one of the voltage variation signals SC1and SC2is at the high level, it can be determined that it is during the check period at present, and the current supply transistor corresponding to one of the voltage variation signals SC1and SC2which is at the low level is turned on completely, and the other current supply transistor is in the low current supply performance state.

In this modification, the failure detecting section32performs the load side wire breakage detecting process shown inFIG. 4only during the check period. Upon detecting that the difference between the source voltages VS1and VS2exceeds the threshold value, the failure detecting section32determines that the load side portion of the current supply wire to which the current supply transistor set in the completely on state is connected is broken, and outputs a load side wire breakage detection signal added with information to identify the broken current supply wire to the microcomputer20.

The above modification makes it possible that the microcomputer20identifies which of the load side portions of the current supply wires L1and L2is broken. For example, in the case shown inFIG. 3, if the load side portion LS2is broken at time t1, there is determined, during the check period thereafter in which the voltage variation command signal SC1is set to the high level and the voltage variation command signal SC2is set to the low level, that the load side portion LS2of the current supply wire L2is broken, and the load side wire breakage detection signal indicating that effect is outputted from the failure detecting section32to the microcomputer20.

Further, the above embodiment may be modified such that, with the decrease of the battery voltage VB, the degree of lowering of the current supply performance of the current supply transistors Tr1and Tr2during the check period is decreased, the check period of the length of time Tb is shortened, or the generation interval of the time Ta of the check period is lengthened, in order to prevent the current supplied to the relay coil13afrom being reduced.

Further, the above embodiment may be modified such that the circuit abnormality signal and the load side wire breakage detection signal outputted from the failure detecting section32are inputted to the voltage variation control section31, and the failure detecting section32stops operation when any one of the circuit abnormality signal and the load side wire breakage detection signal is received. In this modification, steps S210to S230in the process shown inFIG. 5may be omitted.

Further, steps S240to S270may be performed by a circuit other than the microcomputer20.

Second Embodiment

Next, a second embodiment of the invention is described.

The ECU11of the second embodiment is different from that of the first embodiment in the following points (1) and (2).

(1) The voltage variation control section31sets the voltage variation command signals SC1and SC2to the high or low level not only in the pattern shown inFIG. 3, but also in the pattern shown inFIG. 6.

In the second embodiment, after a first check period in which the voltage variation command signals SC1and SC2are set to the high level alternately during the predetermined time Tb at the time intervals of the time Ta as shown inFIG. 3is generated2N times (N being an integer larger than or equal to 1), that is after each of the voltage variation command signals SC1and SC2is changed to the high level N times, a second check period in which both the voltage variation command signals SC1and SC2are set to the high level at the same time during the predetermined time Tb at regular time intervals of the time Ta is generated M times (M being an integer larger than or equal to 1). The 2×N times generations of the first check period and the M times generations of the second check period are repeated during the current supply period.

FIG. 6shows the case in which only the second check period is generated. However, it is possible to change the voltage variation command signals SC1and SC2between the high level and the low level independently in the pattern shown inFIG. 7, for example.FIG. 7shows an example where the N and M are1. Accordingly, in the example shown inFIG. 7, a check period in which only the voltage variation command signal SC1is set to the high level, a check period in which only the voltage variation command signal SC2is set to the high level, and a check period in which both the voltage variation command signal SC1and SC2are set to the high level are included in each cycle of the length of 3×Ta.

In the second embodiment, the check period in which both the voltage variation command signal SC1and SC2are set to the high level is a period for detecting wire breakage in the opposite load side portions LD1and LD2, and is different in kind from the check period in which one of the voltage variation command signals SC1and SC2is set to the high level. Hereinafter, this period is referred to as “different kind period”.

(2) The voltage variation command signals SC1and SC2outputted from the voltage variation control section31are inputted also to the failure detecting section32. The failure detecting section32performs an opposite load side wire breakage detection process shown inFIG. 8to detect wire breakage of the opposite load side portions LD1and LD2of the current supply wires L1and L2during the different kind check period in which both the voltage variation command signals SC1and SC2are set to the high level.

As shown inFIG. 8, the opposite load side wire breakage detection process begins in step S310to determine whether or not each of the drain-source voltages of the current supply transistors Tr1and Tr2is lower than a predetermined threshold value. If the determination result in step S310is affirmative, the process proceeds to step S320to output an opposite load side wire breakage detection signal indicating that one of the opposite load side portions LD1and LD2, which is connected with the current supply transistor whose drain-voltage source is detected to be lower than the threshold value, to the microcomputer20.

In this embodiment, since the drain-source voltages of the current supply transistors Tr1and Tr2are higher than 1 V when the voltage variation command signals SC1and SC2are at the high level, the threshold value is set lower than 1 V. Upon receiving the opposite load side wire breakage detection signal from the failure detecting section32, the microcomputer20stores a failure code indicating occurrence of a failure identified by the opposite load side wire breakage detection signal in the nonvolatile memory, and performs a warning process to inform the vehicle driver of the occurrence of the failure by turning on a warning light or showing a message corresponding to the detected failure on a display.

Next, the principle of detecting wire breakage of the opposite load side portions LD1and LD2is explained. In the normal state where there is no wire breakage (before time t2inFIG. 6), the drain-source voltages of the current supply transistors Tr1and Tr2are at 1 V during the different kind check period in which both the voltage variation signals SC1and SC2are set to the high level. InFIG. 6, the description “VS1−VD1” means the source voltage VS1minus the drain voltage VD1of the current supply transistor Tr1, which is the sign-inverted version of the drain-source voltage of the current supply transistor Tr1. Likewise, the description “VS1−VD1” means the sign-inverted version of the drain-source voltage of the current supply transistor Tr2.

If any one of the opposite load side portions LD1and LD2is broken, since no current flows through the current supply transistor Trx (x being 1 or 2) connected to the broken opposite load side portion LDx, the relay coil13ais supplied with a current only through the other current supply transistor. As a result, the drain-source voltage of the current supply transistor connected to the opposite load side portion which is not broken becomes higher than or equal to 1 V, while the drain-source voltage of the current supply transistor Trx connected to the opposite load side portion LDx which is broken becomes lower than 1 V reliably, because no current flows through the current supply transistor Trx.

Accordingly, in this embodiment, the threshold value used in step S310shown inFIG. 8is set lower than 1 V (0.7 V, for example) to determine that the opposite load side portion of the current supply wire connected with the current supply transistor whose drain-source voltage is lower than the threshold value during the different kind check period is broken.

For example, if the opposite load side portion LD2of the current supply wire L2is broken at time t2, when there comes the different kind check period of the length of the time Tb in which both the voltage variation command signals SC1and SC2are set at the high level, the drain-source voltage of the current supply transistor Tr1becomes 1 V, while the drain-source voltage of the current supply transistor Tr2becomes 0.5 V lower than the threshold value. Accordingly, the opposite load side portion LD2connected with the current supply transistor Tr2is determined to be broken.

According to the second embodiment, also wire breakage of the opposite load side portions LD1and LD2of the current supply wires L1and L2can be detected. In the second embodiment, since the different kind check period is generated intermittently, the wire breakage detection can be performed for the opposite load side portion LD1and the opposite load side portion LD2intermittently and repeatedly.

In this embodiment, it is determined whether or not the opposite load side wire breakage signal has been outputted from the failure detecting section32in step S220of the voltage variation control inhibition process shown inFIG. 5, and if the determination result in step S220is affirmative, the process proceeds to step S230to inhibit the voltage variation control section31from operating thereafter. Accordingly, according to this embodiment, it is possible to prevent the current supplied to the relay coil13afrom being reduced also in a case where any one of the opposite load side portions LD1and LD2of the current supply wires L1and L2is broken, because the current supply transistor connected to the current supply wire with no wire breakage is not set to the low current supply performance state in this case. It is a matter of course that various modifications can be made to the above embodiments as described below.

The microcomputer20may be configured to output the on/off command signals SD1and SD2from the same terminal instead of from the separate two terminals. In this modification, the common on/off command signal is inputted to the drive circuits21and22, and the voltage variation control section31.

The current supply wires and the current supply transistors may be three or more in number. The current supply transistors may be P-channel MOSFETs. The current supply transistors may be IGBTs or bipolar transistors. In a case where bipolar transistors, which are current-driven type transistors, are used as the current supply transistors, the low current supply performance state can be achieved by reducing a current supplied to the base terminal as a control terminal of each current supply transistor.

The relay coil13amay be a low side driven relay coil. In this case, one end of the relay coil13ais connected to the positive terminal of the battery15, the other end of the relay coil13ais connected to the terminal19of the ECU11, and the terminal17of the ECU11is connected to the ground line. Accordingly, the portions designated by LD1and LD2of the current supply wires L1and L2inFIG. 1correspond to the load side portions, and the portions designated by LS1and LS2correspond to the opposite load side portions.

In the above embodiments, the relay coil13aof the relay13is the electrical load as an object to be supplied with a current. However, the electrical load as an object to be supplied with a current is not limited to the relay coil13a. For example, it may be an actuator such as an electrical motor whose output power depends on a current supplied. In this case, in addition to, or instead of steps S240and S250, a step for determining whether or not a current supplied to the actuator should be increased as much as possible is provided in the process shown inFIG. 5, and if the determination result in this step is affirmative, the process proceeds to step S270.