JP7616399B2 - Load Driver - Google Patents

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on Patent Application No. 2021-137349, filed on August 25, 2021, the contents of which are incorporated herein by reference.

The present disclosure relates to a load driving device used to supply power to a load.

Vehicle control systems are required to diagnose abnormalities in the path that supplies power to exhaust gas treatment-related sensor devices. In general, the power source and sensor device are connected via a load drive device equipped with a switching circuit, and when an abnormality such as a short circuit or break in the power supply path is detected, an abnormality detection signal is output from the load drive device to the outside. In addition, consideration is being given to adding a function to protect the internal circuitry of the load drive device when an abnormality occurs.

For example, Patent Document 1 discloses a circuit configuration for detecting ground faults in a switching circuit for an in-vehicle device, in which a microcomputer interface circuit consisting of two resistors is connected in parallel with a load to the output side of a transistor, and a microcomputer is connected to this, so that the output voltage of the transistor can be determined by the microcomputer. In this configuration, when the microcomputer detects a drop in output voltage due to a ground fault, it turns off the transistor, thereby preventing a large current from continuing to flow and protecting the transistor.

Japanese Patent Application Publication No. 8-317545

For example, an in-vehicle sensor device uses a load drive device to control the power supply to a heater section, which serves as a load. A switching circuit opens and closes the power supply path to the heater section by driving a switching element on and off in response to a drive command from a control section, thereby maintaining the sensor device at a predetermined temperature. The control section can be configured to monitor, for example, the voltage or current of the power supply path, diagnose the presence or absence of a ground fault or other anomaly, and stop the power supply operation when an anomaly is detected.

When a ground fault occurs, overheating due to an overcurrent can easily occur, damaging circuit elements and causing malfunctions, making it desirable to stop the power supply as quickly as possible. Furthermore, it has been found that in a load drive device that uses an on-board battery as its power source, if the wiring between the on-board battery is long due to restrictions on placement within the vehicle, a high-frequency surge is more likely to occur in the event of a ground fault, which could lead to damage to the internal circuitry. This is due to inductance and capacitance components in the ground fault path, and it is believed that if the surge current flowing around from the ground side becomes large, it could exceed the set tolerance of the circuits used for ground fault detection and load drive.

The objective of this disclosure is to provide a load driving device that can quickly stop power supply operation in the event of a ground fault and protect the circuit from high-frequency surges that occur in the event of a ground fault.

One aspect of the present disclosure is
A load driving device including a switching circuit that opens and closes a power supply path from a power source to a load, a drive control unit that controls driving of the switching circuit, and a circuit protection mechanism that protects the switching circuit when a power supply abnormality occurs,
The switching circuit is
A voltage-driven switching element;
an input terminal unit to which a drive instruction signal is input from the drive control unit;
an output terminal portion to which the load is connected via a power supply line that serves as the power supply path;
The circuit protection mechanism is as follows:
a protection diode having an anode terminal connected to the input terminal section and a cathode terminal connected to the output terminal section;
a power supply abnormality detection unit that detects a ground fault abnormality in the power supply path based on a voltage detection signal at a detection point provided between the input terminal unit and the protection diode;
The load driving device has a ground fault protection element connected between the input terminal portion and the protection diode, and which limits the lower limit level of the drive instruction signal input to the switching circuit in the event of a ground fault abnormality.
Another aspect of the present disclosure is
A load driving device including a switching circuit that opens and closes a power supply path from a power source to a load, a drive control unit that controls driving of the switching circuit, and a circuit protection mechanism that protects the switching circuit when a power supply abnormality occurs,
The switching circuit is
A voltage-driven switching element;
an input terminal unit to which a drive instruction signal is input from the drive control unit;
an output terminal portion to which the load is connected via a power supply line that serves as the power supply path;
The circuit protection mechanism is as follows:
a protection diode having an anode terminal connected to the input terminal section and a cathode terminal connected to the output terminal section;
a power supply abnormality detection unit that detects a ground fault abnormality in the power supply path based on a voltage detection signal at a detection point provided between the input terminal unit and the protection diode;
a ground fault protection element that limits a lower limit level of the drive instruction signal input to the switching circuit when a ground fault occurs ,
The earth fault protection element in the load driving device is a capacitor element having one terminal connected between the input terminal portion and the protection diode and the other terminal grounded .

In the load driving device of the above configuration, the switching circuit drives the switching element to open and close based on a drive instruction signal input from the input terminal section, and supplies power to the load via a power supply line connected to the output terminal section. When a ground fault occurs in the power supply path on the output side of the switching circuit, current flows through a protection diode whose cathode terminal is connected to the output terminal section, and the voltage level of the drive instruction signal input to the input terminal section whose anode terminal is connected drops. This immediately turns off the switching element, making it possible to protect the switching element from overheating due to overcurrent. In addition, the occurrence of a ground fault can be detected by monitoring this drop in voltage level with the power supply abnormality detection section. Under normal conditions, the protection diode protects the input terminal section so that the voltage on the power supply side is not applied.

Furthermore, in the case of a high-frequency surge caused by a ground fault, the ground fault protection element connected between the input terminal and the protection diode operates to limit the lower limit level of the drive instruction signal, protecting the switching circuit from surge currents that sneak in from the ground side. In this way, the circuit protection mechanism includes a protection diode, a power supply anomaly detection unit using the protection diode, and a ground fault protection element, so that the internal circuitry and switching elements of the switching circuit can be quickly protected while the occurrence of a ground fault can be reliably detected.

As described above, according to the above aspect, it is possible to provide a load driving device that can quickly stop power supply operation in the event of a ground fault and protect the circuit from high-frequency surges that occur in the event of a ground fault.

The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a circuit diagram showing an overall configuration of a load driving device according to a first embodiment; FIG. 2 is a circuit diagram showing a current path in the load driving device in the event of a ground fault in the first embodiment; FIG. 3 is a time chart for explaining a ground fault detection operation by a circuit protection mechanism of the load driving device in the first embodiment; FIG. 4 is a time chart for explaining a basic operation of a circuit protection mechanism of a load driving device in the first embodiment; FIG. 5 is a voltage waveform diagram at section A for explaining the effect of a circuit protection mechanism of the load driving device according to the first embodiment; FIG. 6 is an overall configuration diagram and an equivalent circuit diagram of a load driving device according to the first embodiment; FIG. 7 is a voltage waveform diagram of part A when a ground fault occurs in the load driving device according to the first embodiment; FIG. 8 is a diagram showing a relationship between a power harness length and a peak voltage of a load driving device according to the first embodiment; FIG. 9 is an equivalent circuit diagram of a load driving device according to the first embodiment; FIG. 10 is a voltage waveform diagram at section A for explaining the effect of a circuit protection mechanism of the load driving device according to the first embodiment; FIG. 11 is a diagram showing a comparison of the relationship between the power harness length and the peak voltage depending on whether or not a resistive element is added to the load driving device in the first embodiment; FIG. 12 is a diagram showing a relationship between an added resistance value to a load driving device and an A section voltage in the first embodiment; FIG. 13 is a circuit diagram showing an overall configuration of a load driving device according to a second embodiment; FIG. 14 is an equivalent circuit diagram for explaining a configuration of a load driving device according to a second embodiment; FIG. 15 is a time chart showing the configuration of the load driving device according to the first and second embodiments and the first embodiment, and comparing the changes in the A section voltage and the B section voltage when a ground fault occurs; FIG. 16 is a diagram showing a comparison of the relationship between the power harness length and the peak voltage depending on whether or not a capacitor element is added to a load driving device in the second embodiment; FIG. 17 is a diagram showing the relationship between the capacitance value of a capacitor added to the load driving device and the peak voltage in the second embodiment.

(Embodiment 1)
A first embodiment of a load driving device will be described with reference to FIGS. 1 to 12. FIG.
1, the load driving device 1 of this embodiment includes a switching circuit 2 that opens and closes a power supply path L1 from a power source B to a load L, a drive control unit 3 that controls the driving of the switching circuit 2, and a circuit protection mechanism 4 that protects the switching circuit 2 in the event of a power supply abnormality. The power source B is, for example, an on-vehicle battery, and the load driving device 1 is used to supply battery power to various electrical devices such as a sensor S mounted on the vehicle. The load L is, for example, a heater unit built into the sensor S.

The switching circuit 2 has a voltage-driven switching element 21, an input terminal T1 to which a drive instruction signal S1 is input from the drive control unit 3, and an output terminal T2 to which a load L is connected via a power supply line L11 that serves as a power supply path L1. Preferably, the switching circuit 2 further includes a power supply terminal Tb and a gate driver unit (hereinafter, appropriately abbreviated as the driver unit) 22. Here, "connection" means electrical connection, and the same applies hereinafter.

In the switching circuit 2, the power supply terminal Tb is connected to the power supply B via a power supply harness H which serves as the power supply path L1. The switching element 21 is connected in series between the power supply terminal Tb and the output terminal T2 to open and close the power supply path L1. The driver unit 22 is connected between the input terminal T1 and the gate terminal of the switching element 21 to drive the switching element 21 based on a drive instruction signal S1.

The drive control unit 3 is connected to the input terminal unit T1 via the drive signal line L2, and can be configured to output a drive instruction signal S1, which is a binary voltage signal, to the switching circuit 2. The circuit protection mechanism 4 includes a protection diode D1, a power supply abnormality detection unit 41 using the protection diode D1, and a ground fault protection element 40.

The protective diode D1 has an anode terminal connected to the input terminal T1 side of the switching circuit 2 and a cathode terminal connected to the output terminal T2 side, and under normal conditions protects the input terminal T1 from being applied with a voltage from the power supply side. In the event of a ground fault, a ground fault current path is formed through the protective diode D1, and the signal level of the drive instruction signal S1 is lowered, thereby immediately turning off the switching element 21.

The power supply abnormality detection unit 41 provides a detection point P1 between the input terminal unit T1 and the protection diode D1, and detects a ground fault abnormality in the power supply path L1 based on a voltage detection signal S2 at the detection point P1. Preferably, the power supply abnormality detection unit 41 is connected to the detection point P1 via a detection signal line L3, and periodically monitors the potential of the detection point P1, which is input as a voltage detection signal S2, thereby making it possible to detect a ground fault abnormality by utilizing a voltage drop in the ground fault current path caused by the protection diode D1.

The circuit protection mechanism 4 further includes a ground fault protection element 40 to protect the circuit from high-frequency surges due to a ground fault. The ground fault protection element 40 is connected between the input terminal T1 and the protection diode D1, and limits the lower limit level of the drive instruction signal S1 input to the switching circuit 2 in the event of a ground fault. Specifically, the ground fault protection element 40 operates so that the minimum value of the voltage signal input to the input terminal T1 in the event of a ground fault, i.e., the lower limit signal level of the drive instruction signal S1, does not fall below the set withstand voltage of the switching circuit 2.

A resistor element R1 connected in series between the input terminal T1 and the protection diode D1 can be preferably used as the ground fault protection element 40. The resistor element R1 is disposed in the current path in the event of a ground fault and consumes the ground fault surge current that occurs, thereby suppressing the negative fluctuation of the drive instruction signal S1 and quickly protecting the switching circuit 2.

In this embodiment, the following will be described as an example in which a resistor element R1 for earth fault protection is used in the circuit protection mechanism 4. The resistor element R1 is preferably configured as, for example, a chip resistor, and is arranged in the current path during an earth fault closer to the input terminal portion T1. It is preferably arranged closer to the input terminal portion T1 than the detection point P1, and can be inserted in series, for example, between the connection point P2 between the protection diode D1 and the drive signal line L2 and the input terminal portion T1.

The drive control unit 3 and the power supply abnormality detection unit 41 can be configured, for example, as part of a control device 5 that controls the overall operation of the sensor S, including the driving of the heater unit by the load drive device 1. The control device 5 is configured with a microcomputer (hereinafter, appropriately referred to as a microcomputer) that has a memory for storing programs and the like and a CPU and executes arithmetic processing based on the programs.

2, if a ground fault occurs in the power supply line L11 to the load L, the current due to the high frequency surge may flow to the ground side without passing through the load L and may sneak into the input side of the switching circuit 2 from the ground terminal Tg. In this case, as shown by the solid arrow in Fig. 2, a current path is formed connecting the input side and output side of the switching circuit 2 via the protective diode D1, causing a drop in the voltage signal level input to the input terminal T1.

3, when a ground fault occurs in response to the drive instruction signal S1, the voltage detection signal S2 at the detection point P1 drops to a level equivalent to the forward voltage _Vf1 of the protection diode D1. Therefore, if this voltage _Vf1 corresponds to a low-level signal of the drive instruction signal S1 and is set so as to be recognized in the driver section 22 as a signal level for turning off the switching element 21, the switching element 21 can be immediately turned off due to the drop in the voltage signal level.

Moreover, by monitoring this drop in the voltage signal level with the power supply abnormality detection unit 41, it becomes possible to detect a ground fault. Specifically, the occurrence of a ground fault can be detected by periodically capturing the potential of the detection point P1 and comparing it with a preset ground fault detection threshold TH.

Figure 4 shows the relationship between the ground fault detection operation in the power supply abnormality detection unit 41 and the operation of each unit, with the voltage at the input terminal T1 of the switching circuit 2 being the A voltage and the voltage at the output terminal T2 being the B voltage. When a ground fault occurs while the load drive device 1 is operating normally, the A voltage (i.e., the signal level of the drive instruction signal S1) drops from high level as the B voltage drops, and when it falls below the ground fault detection threshold TH, a ground fault is detected. That is, when the voltage detection signal S2 is received by the power supply abnormality detection unit 41, the recognized voltage in the control device 5 (i.e., the microcomputer recognized voltage in Figure 4) switches from high level to low level.

As a result, when the drive control unit 3 stops the operation of outputting the drive instruction signal S1, the input of the drive instruction signal S1 to the driver unit 22 of the switching circuit 2 is stopped. However, depending on the timing of the ground fault detection and the calculation processing in the control device 5, a delay may occur before the drive instruction operation by the drive control unit 3 is stopped. Even in such a case, since the switching element 21 can be turned off when the voltage of section A drops to a low level regardless of the drive instruction operation, the switching operation can be stopped more quickly.

Furthermore, as shown in Figure 5, by inserting resistive element R1 into the current path during a ground fault, fluctuations in the voltage signal level can be suppressed for high-frequency surges that occur during a ground fault (e.g., as shown by the solid line in Figure 5). This significantly reduces the negative surge (hereinafter, appropriately referred to as a negative surge) compared to when resistive element R1 is not provided (e.g., as shown by the dotted line in Figure 5), and prevents problems caused by the negative surge leaking into the input side of the switching circuit 2. In addition, the signal level input to the switching circuit 2 is stabilized, enabling rapid ground fault detection.

In this way, the load driving device 1 of this embodiment is equipped with a circuit protection mechanism 4, and is able to detect a ground fault anomaly in the power supply path L1 to which the load L is connected, and quickly stop driving the switching circuit 2, thereby protecting the circuit from overcurrents and high-frequency surges caused by ground faults.

The load driving device 1 of this embodiment will be described in detail below. In Fig. 1, an on-vehicle sensor S to which the load driving device 1 is applied is, for example, a gas sensor such as a NOx sensor constituting an exhaust gas treatment device, and a sensor element having a built-in heater section (for example, resistor R _H ) serving as a load L is installed in an exhaust gas passage of the vehicle. The load driving device 1 supplies power via a power supply path L1 to cause the heater section to generate heat, thereby heating the sensor element to a temperature suitable for detecting NOx.

The power supply path L1 is a path for supplying a DC power supply voltage VD from a power source B to a load L, and the load drive device 1 is equipped with a control device 5 having a switching circuit 2 for opening and closing the power supply path L1 and a drive control unit 3 for controlling the switching circuit 2. The control device 5 uses the drive control unit 3 to control the power supply to the load L, and also controls the overall operation of the sensor S, including a NOx detection unit (not shown) that is built into the sensor element together with a heater unit.

The switching circuit 2 is configured as a high-side switch IC and includes a switching element 21, a driver unit 22, and various terminal units. A power source B is connected to the power source terminal unit Tb of the switching circuit 2 via a power source harness H (e.g., inductance Lv, resistance Rv), and a load L is connected to the output terminal unit T2 of the switching circuit 2 via a power supply line L11. At this time, the path from the power source harness H following the power source B to the load L via the inside of the switching circuit 2 and the power supply line L11 becomes the power supply path L1.

The switching element 21 is connected in series to the power supply path L1 between the power supply terminal Tb and the output terminal T2, and is turned on and off by a driver 22 connected to the input terminal T1 to make the power supply path L1 conductive or cut off. For example, a gate voltage-driven MOSFET (i.e., a field effect transistor) is used as the switching element 21, and the drain terminal of the MOSFET is connected to the power supply terminal Tb, and the source terminal of the MOSFET is connected to the output terminal T2. The driver 22 is connected to the gate terminal of the MOSFET.

A drive instruction signal S1 is input to the driver unit 22 via a drive signal line L2 connected to the input terminal T1 of the switching circuit 2. The drive instruction signal S1 is a binary voltage signal of high level (H) or low level (L) generated in the drive control unit 3 of the control device 5 (see, for example, FIG. 3), and the driver unit 22 outputs a gate signal for turning the switching element 21 on and off based on the signal level of the input drive instruction signal S1. The drive control unit 3 generates the drive instruction signal S1 based on, for example, the temperature detection result of the load L, and controls the on/off operation of the switching element 21 so that the sensor S is maintained at a predetermined temperature.

In the switching circuit 2, the driver section 22 and the protective diode D2 are connected in parallel between the input terminal section T1 and the ground terminal Tg, and the reverse connection protection resistor R2 is connected in series between the driver section 22 and the ground terminal Tg. The protective diode D2 has a cathode terminal connected to the signal line L21 connecting the input terminal section T1 and the driver section 22, and an anode terminal connected to the ground line L4 between the driver section 22 and the reverse connection protection resistor R2. The protective diode D2 is set to ensure a predetermined surge tolerance against, for example, electrostatic surges input from the input terminal section T1. The reverse connection protection resistor R2 is set to protect the circuit by reducing the voltage when, for example, a reverse connection of the power supply B occurs.

In this way, the high-side switch IC that constitutes the switching circuit 2 is usually configured to have a built-in protective diode D2 and reverse connection protection resistor R2 to protect the internal circuit. However, the settings of a typical high-side switch IC are not necessarily sufficient to protect the switching circuit 2 when a ground fault occurs in the power supply path L1 that connects the heater part of the sensor S to the load L.

Therefore, in this embodiment, a circuit protection mechanism 4 is provided outside the switching circuit 2 to reliably protect the switching circuit 2 from ground faults. The circuit protection mechanism 4 includes a protection diode D1 and a power supply abnormality detection unit 41 using the protection diode D1, and detects a ground fault in the power supply path L1 and quickly stops the switching element 21. The circuit protection mechanism 4 further includes a resistive element R1 as a ground fault protection element 40, and reliably protects the switching circuit 2 from a surge current that occurs in the event of a ground fault.

The protective diode D1 has an anode terminal connected to the drive signal line L2 leading to the input terminal T1 of the switching circuit 2, and a cathode terminal connected to the power supply line L11 leading to the output terminal T2 of the switching circuit 2. At this time, the protective diode D1 is connected between the input terminal T1 and the output terminal T2 of the switching circuit 2, with the direction from the drive signal line L2 to the power supply line L11 being the forward direction. This prevents the voltage of the power supply line L11 (i.e., ≒ power supply voltage VD) from being applied to the input terminal T1 under normal conditions, preventing damage to the internal circuitry due to overvoltage.

2, when a high-frequency surge occurs due to a ground fault, a current path is formed that passes through a ground fault location with lower resistance (i.e., resistor Rs) without passing through the load L (i.e., resistor R _H ) and returns from the ground terminal Tg to the input terminal T1 side of the switching circuit 2. This current path forms a closed circuit that runs from the drive signal line L2, via the protective diode D1, to the power supply line L11 on the output terminal T2 side of the switching circuit 2.

At this time, the power supply abnormality detection unit 41 detects a ground fault based on the potential of a detection point P1 provided on the anode terminal side of the protection diode D1 in the ground fault current path. The detection point P1 is provided between the protection diode D1 and the drive signal line L2, and is connected to the power supply abnormality detection unit 41 via the detection signal line L3. The power supply abnormality detection unit 41 is provided with a ground fault detection circuit 42 that determines whether or not a ground fault exists, and a drive prohibition unit 43 that prohibits the output of the drive instruction signal S1 when a ground fault exists.

3, in response to a drive instruction signal S1 output from the drive control unit 3 to the drive signal line L2, a voltage detection signal S2 at a detection point P is input from the detection signal line L3 to the ground fault detection circuit 42 at a predetermined timing. Here, at time T0 in FIG. 3, when the drive instruction signal S1 is at a high level and a ground fault abnormality occurs in the power supply path L1, the voltage level of the voltage detection signal S2 input to the ground fault detection circuit 42 drops to the forward voltage _Vf1 of the protection diode D1.

Therefore, the ground fault detection circuit 42 can be configured to set a detection threshold value TH for detecting a ground fault in advance, and to determine that a ground fault has occurred when the voltage detection signal S2 has a voltage level lower than the detection threshold value TH using a comparison circuit or the like. The detection threshold value TH in the ground fault detection circuit 42 is set to be between the high-level voltage (e.g., about 5 V) of the drive instruction signal S1 and the forward voltage _Vf1 of the protection diode D1.

When a ground fault is detected, the ground fault detection circuit 42 can output a ground fault signal S0 to an external vehicle control device (not shown) to warn of an abnormality in the sensor S. The drive inhibition unit 43 can also be activated to output a drive inhibition signal S3 to the drive control unit 3. The drive control unit 3 is configured to stop outputting the drive instruction signal S1 (i.e., low level) when the drive inhibition signal S3 is input while the drive instruction signal S1 is being output (i.e., high level), or to output the drive instruction signal S1 only when the drive inhibition signal S3 is not input.

However, as mentioned above, if a delay occurs in the detection operation by the ground fault detection circuit 42 and the halt of the drive instruction signal S1 is delayed, there is a risk of malfunction such as overheating of the switching element 21. In addition, if the high-frequency surge generated with the occurrence of a ground fault becomes large at time T0 in FIG. 3, it may lead to damage to the internal circuit of the switching circuit 2. For this reason, a protection diode D1 and a resistance element R1 are arranged in the current path in the event of a ground fault, and as shown in FIG. 4 and FIG. 5, the drive of the switching element 21 is quickly stopped and the input of the high-frequency surge generated in the event of a ground fault is limited. Thereafter, the operation of the switching circuit 2 can be reliably stopped by the output of the drive prohibition signal S3 from the drive prohibition unit 43.

Next, the mechanism of earth fault detection and circuit destruction due to earth fault will be described with reference to the first embodiment shown in Figures 6 to 8. As shown in the left diagram of Figure 6, the first embodiment shows a basic circuit configuration of this embodiment, and is the same as the load driving device 1 of Figure 1 except that the circuit protection mechanism 4 does not include the resistive element R1, and includes a power supply abnormality detection unit 41 using a protective diode D1. The right diagram of Figure 6 shows a simplified circuit excluding the switching circuit 2 and the control device 5. In response to the application of the power supply voltage VD, a current Iv flows from the power harness H to the load L, and in the event of an earth fault, an earth fault current I _short flows from the ground terminal Tg side through the resistor Rs at the earth fault location in the forward direction of the protective diode D1. In Figure 6, _Vf2 is the forward voltage of the protective diode D2, and _RGRD is the resistance value of the reverse connection protection resistor R2.

Here, when focusing on the input side of the switching circuit 2, the voltage (i.e., the A section voltage V _A ) is expressed by the following equation (1) using the output side voltage (i.e., the B section voltage V _B ) and the forward voltage _Vf1 of the protection diode D1.
V _A = V _B + V _f1 ...Formula (1)
In the event of a ground fault, V _B ≈0 V, so the voltage V _A at part A in the event of a ground fault is expressed by the following equation (2) and is approximately equal to V _f1 .
V _A ≒ V _f1 ...Formula (2)

As shown in Fig. 7, the A section voltage _VA is normally at a signal level equivalent to the drive instruction signal S1, and when a ground fault occurs, it drops to _Vf1 . That is, this voltage drop can be recognized by the power supply abnormality detection unit 41 and detected as a ground fault abnormality. To this end, the power supply abnormality detection unit 41 captures the A section voltage _VA as a voltage detection signal S2 at the detection point P1 and compares it with a ground fault detection threshold TH (see Fig. 3, for example). The ground fault detection threshold TH corresponds to a signal level lower than the signal level recognized as a high level and higher than _Vf1 (i.e., H level>TH> _Vf1 >0).

However, if the high-frequency surge generated during a ground fault exceeds the range of the circuit withstand voltage indicated by the dashed line in Figure 7, there is a risk of circuit destruction. Here, the upper or lower limit of the withstand voltage of the switching circuit 2 (i.e., the destruction limit) is generally set based on the high or low level signal of the drive instruction signal S1, and the withstand voltage setting for negative side surges is lower than that for the positive side. Therefore, if the negative surge becomes large due to the wiring length of the power supply path L1, it may fall below the lower limit of the circuit withstand voltage, damaging the circuit components.

On the other hand, the minimum value of the A part voltage V _A when a ground fault occurs, i.e., the negative peak value V _A-peak caused by a high-frequency surge, is expressed by the following equation (3) using the wiring length X of the power harness H (hereinafter, appropriately referred to as the power harness length).
V _A-peak = {a/(bX+c)}+d×eX...Formula (3)
In formula (3), a, b, c, d, and e are constants determined by the circuit configuration. Formula (3) is calculated based on the following formula (31) for calculating the B section voltage _VB , assuming that the power harness length X is proportional to the resistance Rv and inductance Lv of the power harness H (i.e., X∝Rv∝Lv).
V _B = V _B-DC + V _B-AC ...Formula (31)
In equation (31), V _B-DC is the DC component of V _B , and V _B-AC is the AC component of V _B.
Details of this will be provided later.

8 shows the relationship between the power harness length X (unit: mm) and the peak value V _A-peak (unit: V) based on the above formula (3), and as the power harness length X becomes longer from 0 mm, the peak value V _A-peak decreases and the risk of reaching the voltage breakdown region increases. This tendency continues until the power harness length X exceeds 10,000 mm (i.e., region X1), and then converges to a substantially constant range (i.e., region X2). For this reason, measures are needed to reliably prevent circuit breakdown due to ground faults in large vehicles and commercial vehicles in which the power harness length X is relatively long.

Therefore, in this embodiment, in addition to the power supply abnormality detection unit 41 of the circuit protection mechanism 4, a resistive element R1 is arranged in the current path in the event of a ground fault to reduce the effect of the high-frequency surge on the A part voltage _V'A . Fig. 9 is an equivalent circuit diagram showing the configuration of this embodiment with the addition of a resistive element R1 (e.g., resistance value R), and the A part voltage _V'A is expressed by the following equation (41).
V' _A = V _B + V _f1 + R×I _short ...Formula (41)
Here, from the above formula (1), V _A =V _B +V _f1 , and therefore the above formula (41) can be expressed by the following formula (42).
V' _A = V _A + R×I _short ...Formula (42)
Therefore, when the above formula (3) is taken into consideration, the peak value _V'A-peak of the A-section voltage _V'A is expressed by the following formula (4).
V' _A-peak = V _A-peak +R×I _short
= {a/(bX+c)}+d×eX+R×I _short ...Formula (4)

10, it is possible to raise the entire A section voltage _V'A , including the peak value _V'A-peak , by an amount equivalent to R× _Ishort , and bring it within the range of the circuit withstand voltage. In other words, the addition of resistance element R1 has the effect of offsetting the A section voltage _V'A in the positive direction with respect to the A section voltage _V'A before the addition, making it possible to set the peak value _V'A-peak so that it does not fall below the lower limit of the circuit withstand voltage.

Fig. 11 shows the relationship between the power harness length X (unit: mm) and the peak value V _A-peak (unit: V) based on the above formula (4), in comparison with the relationship shown in Fig. 8 above. As shown enlarged in Fig. 11, the relationship between the power harness length X and the peak value V _A-peak shows a similar trend to Fig. 8, but is largely offset in the positive direction overall. Therefore, the magnitude of the peak value V _A-peak is greatly reduced overall, making it possible to prevent the breakdown limit region from being reached, regardless of the length of the power harness H.

12 shows an example of a simulation result in which a resistive element R1 is placed on the input side of the switching circuit 2 and its resistance value R (unit: kΩ) is changed. For a breakdown voltage (e.g., lower limit: −0.3 V) set in the driver unit 22 of the switching circuit 2, the resistance value R can be determined by solving the following equation (5).
V' _A-peak ≧V _break ...Formula (5)
In equation (4), V _break is the driver breakdown voltage, and V′ _A-peak is a function of the power harness length X and resistance value R expressed by equation (4) above.

From the results of Fig. 8 described above, in a circuit configuration that does not have resistive element R1, the peak value _VA-peak of the A part voltage _VA is minimum when the power harness length X is 30,000 mm. Fig. 12 shows the relationship between the A part voltage _VA and the resistance value R when the power harness length X in the above formula (4) is set to 30,000 mm. As shown in the figure, the A part voltage _VA increases as the resistance value R increases, and it can be seen that if the resistance value R is set to 30 kΩ or more, the above formula (5) is established.

The above formula (3) is calculated as follows. In the equivalent circuit of Fig. 9, the B section voltage _VB can be expressed as the sum of a DC component ( _VB-DC ) and an AC component ( _VB-AC ) as shown in the above formula (31). Of these, the DC component ( _VB-DC ) is expressed by the following formula (32) as a voltage divided by the resistance Rv of the power harness _H , the resistance RH of the parallel-connected load L, and the resistance Rs of the ground fault location (i.e., _RH // _Rs ).
V _B-DC = V _D ×{R _H・R _S / (R _H・R _S +R _V・R _S +R _V・R _H )}...Formula (32)
When this equation (32) is transformed using constants A, B, and C, it can be expressed as the following equation (33).
V _B-DC = A/(BR _V + C)...Formula (33)
A: V _D・R _H・R _S
B: R _H + R _S
C: R _H R _S

On the other hand, the AC component (V _B-AC ) is proportional to the electromotive force due to the inductance Lv of the power harness H, and is therefore expressed by the following equation (34).
V _B-AC ∝Lv×(dIv/dt)...Formula (34)
Here, the current Iv is expressed by the following equation (35), which can be substituted into the above equation (34) to transform it into the following equation (36).
Iv=V _D × {(R _H +R _S )/(R _V・R _H +R _V・R _S +R _H・R _S )}...Formula (35)
V _B-AC ∝Lv/R _V =D×(Lv/R _V )...Formula (36)
D: Proportional constant

The above formula (1) representing the A section voltage V _A and the above formulas (31), (33), and (36) representing the B section voltage V _B can be put together to obtain the following formula (37).
V _A = {A/(BR _V +C)}+V _f1 +D×(Lv/R _V )...Formula (37)
Of these, the first term on the right side is a DC fluctuation term, the second term is a constant term, and the third term is an AC fluctuation term. As described above, the power harness length X, the resistance R _V, and the inductance L _V are proportional to each other, so this equation (37) can be modified using X. In other words, if X ∝ R _V ∝ L _V , then we get the following, which can be expressed as the above-mentioned equation (3).
V _A-peak = {a/(bX+c)}+d×eX...Formula (3)

In this manner, according to the configuration of this embodiment, the circuit protection mechanism 4 includes the resistive element R1, so that the signal level input to the switching circuit 2 in the event of a ground fault can be maintained within the set voltage resistance range of the circuit. This makes it possible to quickly reduce the signal level input to the driver unit 22 and turn off the switching element 21 while preventing circuit destruction due to a high frequency surge. Therefore, by combining ground fault detection by the power supply abnormality detection unit 41 using the protective diode D1 and ground fault protection by the resistive element R1, it is possible to reliably protect the switching circuit 2.

(Embodiment 2)
A second embodiment of the load driving device will be described with reference to FIGS.
13, the load driving device 1 of this embodiment uses a capacitor element C1 instead of the resistor element R1 in the first embodiment as the ground fault protection element 40 connected between the input terminal T1 and the protection diode D1 in the circuit protection mechanism 4. The basic configuration of the load driving device 1 is the same as that of the first embodiment, and the following description will focus on the differences.
In addition, among the symbols used in the second and subsequent embodiments, the same symbols as those used in the previous embodiments represent the same components, etc. as those in the previous embodiments, unless otherwise specified.

In this embodiment, the load driving device 1 also includes a circuit protection mechanism 4 that protects the switching circuit 2 that opens and closes the power supply path L1 when the switching circuit 2 is driven by a drive command from the drive control unit 3 of the control device 5. The circuit protection mechanism 4 detects a ground fault using a protection diode D1 that forms a current path in the event of a ground fault and a power supply abnormality detection unit 41 that uses the protection diode D1, and protects the switching circuit 2 in the event of a ground fault abnormality using a capacitor element C1 as a ground fault protection element 40. The capacitor element C1 can be configured, for example, as a chip capacitor integrated to have a predetermined capacitor capacitance value C.

One of the terminals of the capacitor element C1 is connected to the drive signal line L2 between the input terminal T1 and the drive control unit 3, and the other terminal is connected to the ground line L4 between the reverse connection protection resistor R2 and the ground terminal Tg. A protection diode D1 is connected to the connection point P2 between the capacitor element C1 and the drive signal line L2 via the detection point P1. That is, the capacitor element C1, which is the ground fault protection element 40, is connected in parallel to the ground fault current path from the ground terminal Tg through the input terminal T1 to the protection diode D1, making it possible to absorb the surge current generated in the event of a ground fault. Even with this configuration, it is possible to suppress the fluctuation of the drive instruction signal S1, limit the decrease in the signal level, and quickly protect the switching circuit 2 in the event of a ground fault abnormality.

In this way, the ground fault protection element 40 is connected between the input terminal T1 and the protection diode D1, and operates so that the lower limit level of the drive instruction signal S1 input to the input terminal T1 does not fall below the allowable limit of the switching circuit 2. By using the capacitor element C1, the signal level of the drive instruction signal S1 is prevented from changing significantly to the negative side, and the signal level can be gradually decreased toward the minimum value.

The equivalent circuit diagram shown in FIG. 14 has a configuration in which a capacitor element C1 (for example, a capacitor with a capacitance value C) is added to the basic circuit of this embodiment shown in FIG. 6 , and the A part voltage V″ _A is expressed by the following equation (6) with respect to the A part voltage V _A in FIG. 6 .
V" _A = V _A × E × (1-exp (-F/C)) + H...Formula (6)
In equation (6), E, F, and H are constants that are determined depending on the circuit configuration.
Therefore, taking into consideration the above formula (3), the peak value V _A-peak , which is the minimum value of the A-section voltage V _A, is expressed by the following formula (7).
V” _A-peak = V _A-peak ×E×(1-exp(-F/C))+H...Formula (7)
= [{a/(bX+c)}+d×eX]×E×(1-exp(-F/C))+H...Formula (7)
The above formula (6) will be described in detail later.

FIG. 15 shows the waveform of the A part voltage V″ _A in this embodiment when a ground fault occurs, in comparison with the waveforms in the above-mentioned Embodiment 1 and Reference Form 1. The top row of FIG. 15 shows a comparative embodiment that does not have the protection diode D1 and the ground fault protection element 40 (i.e., the resistance element R1 or the capacitor element C1). In that case, the A part voltage has an equivalent waveform to the B part voltage, and the B part voltage is applied to the switching circuit 2, which may exceed the upper breakdown limit. In addition, a high-frequency surge in the event of a ground fault may cause the lower breakdown limit to be exceeded. In contrast, in Reference Form 1 in which the protection diode D1 is added, the voltage applied to A is limited, and in Embodiment 1, the resistance element R1 is further added to consume the high-frequency surge, and the A part voltage is maintained within a range that does not exceed the breakdown limit.

Also, as shown in the bottom row of Figure 15, in this embodiment, the A part voltage is absorbed by the parallel connection of capacitor element C, which absorbs the surge flowing through the ground fault current path, suppressing voltage fluctuations on the input side of switching circuit 2. As a result, the A part voltage gradually drops from the signal level (high level voltage) at which switching element 21 is turned on to the signal level (low level voltage) at which switching element 21 is turned off. This keeps the A part voltage within a range that does not exceed the breakdown limit.

16 shows the relationship between the power harness length X (unit: mm) and the peak value V _A-peak (unit: V) based on the above formula (7), in comparison with the relationship shown in the above-mentioned FIG 8. In this case too, the peak value V _A-peak is largely offset overall in the positive direction by the addition of capacitor element C1. This prevents the peak value V _A-peak from decreasing, making it possible to prevent the breakdown limit region from being reached, regardless of the length of the power harness H.

17 shows an example of a simulation result in which a capacitor element C1 is connected in parallel between the input side of the switching circuit 2 and the ground terminal Tg, and the capacitor capacitance value C (unit: μF) is changed. For a breakdown voltage (e.g., lower limit: −0.3 V) of the driver unit 22 of the switching circuit 2, the capacitor capacitance value C can be determined by solving the following equation (5), as in the first embodiment.
V' _A-peak ≧V _break ...Formula (5)

As in the first embodiment, Fig. 17 shows the relationship between the A part voltage V _A and the capacitor capacitance value C when the power harness length X is set to 30,000 mm. As shown in the figure, the A part voltage V _A increases as the capacitor capacitance value C increases, and it can be seen that if the capacitor capacitance value C is set to 13 µF or more, the above formula (5) is established.

The above formula (6) is calculated as follows. The A section voltage V″ _A when the capacitor element C1 is added is expressed by the following formulas (61) and (62).
V” _A =-R _GRD ×Ig-V _f2 ...Formula (61)
V” _A =-(1/C)∫(Icdt)...Formula (62)
Moreover, the ground fault current I _short is expressed by the following equation (63).
I _short = I _G + I _C = V _B /Rs...Formula (63)
In equations (61) to (63), I _G is the current flowing through the reverse connection protection resistor R2, and I _C is the current flowing through the capacitor element C1.

From the above equations (61) to (63), the ground fault current I _short is expressed by the following equation (64) when _Vf2 is ignored.
I _short =-{(V” _A +V _f2 )/R _GRD }-C(dV” _A /dt)
=-(V" _A /R _GRD )-C(dV" _A /dt)...Formula (64)
Since the above equation (64) is a first-order differential equation, solving this equation gives the following equation (65).
V" _A = R _GRD × Ishort × [1-exp{-t/(R _GRD・C)}]...Formula (65)
When this equation (65) is transformed using the above equation (63), the following equation (66) is obtained.
V” _A = (R _GRD /Rs) × V _B × [1-exp{-t/(R _GRD・C)}]...Formula (66)

Furthermore, the following equation (67) is derived from the above equation (66) and the above equation (1).
V” _A = (R _GRD /Rs) × (V _A +V _f1 ) × [1-exp{-t/(R _GRD・C)}]
=V _A × (R _GRD /Rs) + [1-exp{-t/(R _GRD・C)}]-V _f1 × (R _GRD /Rs)×[1-exp{-t/(R _GRD・C)}]...Formula (67)
In this formula (67),
E = (R _GRD /Rs)
F={-t/(R _GRD・C)}]
H=V _f1 × (R _GRD /Rs) × [1-exp{-t/(R _GRD・C)}]
Then, the following is obtained, which can be expressed as the above-mentioned equation (6).
V" _A = V _A × E × (1-exp (-F/C)) + H...Formula (6)

With the configuration of this embodiment, the circuit protection mechanism 4 includes the capacitor element C1, and thus the same effect as in the first embodiment can be obtained. That is, the signal level input to the switching circuit 2 during a ground fault can be quickly reduced to turn off the switching element 21 while preventing circuit destruction due to a high-frequency surge, thereby ensuring protection of the switching circuit 2.

The present disclosure is not limited to the above-mentioned embodiments, and can be applied to various embodiments within the scope of the present disclosure. For example, the on-board sensor S to which the load driving device 1 is applied may be mounted in an exhaust gas treatment device for a vehicle to detect various components contained in the exhaust gas, and may be a gas sensor such as a NOx sensor, or a PM sensor for detecting particulate matter. It may also be applied to a load device that is not limited to a heater portion provided in a sensor for a vehicle. In addition, although a MOSFET is exemplified as a voltage-driven switching element, a semiconductor power element other than a MOSFET, such as an IGBT (insulated gate bipolar transistor), may also be used.

Although the present disclosure has been described with reference to an embodiment, it is understood that the present disclosure is not limited to the embodiment or structure. The present disclosure also encompasses various modifications and modifications within the scope of equivalents. In addition, various combinations and forms, as well as other combinations and forms including only one element, more than one element, or less than one element, are also within the scope and spirit of the present disclosure.

Claims (10)

A load driving device (1) including a switching circuit (2) that opens and closes a power supply path (L1) from a power source (B) to a load (L), a drive control unit (3) that controls the driving of the switching circuit, and a circuit protection mechanism (4) that protects the switching circuit in the event of a power supply abnormality,
The switching circuit is
A voltage-driven switching element (21);
an input terminal unit (T1) to which a drive instruction signal (S1) from the drive control unit is input;
an output terminal unit (T2) to which the load is connected via a power supply line (L11) that serves as the power supply path;
The circuit protection mechanism is as follows:
a protection diode (D1) having an anode terminal connected to the input terminal section and a cathode terminal connected to the output terminal section;
a power supply abnormality detection unit (41) for detecting a ground fault abnormality in the power supply path based on a voltage detection signal (S2) at a detection point (P1) provided between the input terminal unit and the protection diode;
a ground fault protection element (40) connected between the input terminal portion and the protection diode, for limiting a lower limit level of the drive instruction signal input to the switching circuit in the event of a ground fault abnormality. The load driving device according to claim 1, wherein the switching circuit is connected in series between a power supply terminal section (TB) connected to the power supply via a power supply harness (H) serving as the power supply path and the output terminal section, and a gate driver section (22) for driving the switching element is connected between the input terminal section and the gate terminal of the switching element. The load driving device according to claim 1 or 2, wherein the drive control unit is connected to the input terminal unit via a drive signal line (L2) and outputs the drive instruction signal, which is a binary voltage signal, to the switching circuit. The load driving device according to claim 3, wherein the ground fault protection element operates so that the minimum value of the voltage signal input to the input terminal section during a ground fault abnormality does not fall below the set withstand voltage of the switching circuit. The load driver according to any one of claims 1 to 3, wherein the ground fault protection element is a resistive element (R1) connected in series between the input terminal and the protection diode. The load driver according to claim 5, wherein the resistance value of the resistor element is set to 30 kΩ or more. A load driving device (1) including a switching circuit (2) that opens and closes a power supply path (L1) from a power source (B) to a load (L), a drive control unit (3) that controls the driving of the switching circuit, and a circuit protection mechanism (4) that protects the switching circuit in the event of a power supply abnormality,
The switching circuit is
A voltage-driven switching element (21);
an input terminal unit (T1) to which a drive instruction signal (S1) from the drive control unit is input;
an output terminal unit (T2) to which the load is connected via a power supply line (L11) that serves as the power supply path;
The circuit protection mechanism is as follows:
a protection diode (D1) having an anode terminal connected to the input terminal section and a cathode terminal connected to the output terminal section;
a power supply abnormality detection unit (41) for detecting a ground fault abnormality in the power supply path based on a voltage detection signal (S2) at a detection point (P1) provided between the input terminal unit and the protection diode;
a ground fault protection element (40) that limits a lower limit level of the drive instruction signal input to the switching circuit when a ground fault occurs;
The load driving device , wherein the earth fault protection element is a capacitor element (C1) having one terminal connected between the input terminal portion and the protection diode and the other terminal grounded. The load driver of claim 7, wherein the capacitance value of the capacitor element is set to 13 μF or more. The power supply abnormality detection unit includes a ground fault detection circuit (42) that compares the voltage detection signal with a ground fault detection threshold (TH) to determine the presence or absence of a ground fault abnormality, and a drive prohibition unit (43) that outputs a drive prohibition signal (S3) that prohibits the output of the drive instruction signal to the drive control unit when a ground fault abnormality occurs. The load drive device according to any one of claims 1 to 8. 10. The load driving device according to claim 1 , wherein the load is a heater section provided in an on-vehicle sensor (S).

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