JPH0281515A - Load driving circuit - Google Patents

Abstract

PURPOSE:To prevent heating of a load drive transistor(TR) and to attain a small capacity for the TR by providing a control circuit operating a 2nd TR so as to turn off a 1st TR in response to an overcurrent flowing to the 1st TR. CONSTITUTION:If a coating of a driving wire 14 between the load driving TR Q12 and a load 13 is damaged and brought into directly contact with earthing, since the potential of a control wire 14 is zero, if an output terminal P of a controller 11 remains at a high impedance, a voltage across resistors R14, R11 is lost. As a result, a TR Q11 is turned off, a base current of the load driving TR Q12 is lost and the TR Q12 is turned off. Since no current flows to the load driving TR Q12, the power consumption in the TR Q12 is quickly eliminated, the heat of the TR is prevented in advance and the capacity of the TR is made small.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a load driving circuit having an output short-circuit protection function.

[Prior Art] The computerization of various control systems in recent automobiles is remarkable.

Conventionally, as an electronic unit used in such an automobile, there is a load drive circuit as shown in FIG. 6, which drives loads such as relays, valves, or motors.

In this device, a control device 1 such as a microcomputer that is normally driven at 5V controls a drive switch circuit 3 via a voltage shift circuit 2, and a load 4 is driven via this drive switch circuit 3. . In this case, the power line 5 is connected to the drive switch circuit 3 side, and the ground 6 is connected to the load 4 side.

By the way, in general, in a car, the power line 5 is connected to the positive terminal of the battery, and the ground 6 is connected to the chassis. Therefore, if the coating of the load drive line 7 between the drive switch circuit 3 and the load 4 wired inside the vehicle is damaged and comes into direct contact with the chassis, if the drive switch circuit 3 is in the on state, the drive switch circuit An excessive current would flow through the drive switch circuit 3, destroying the switch elements of the drive switch circuit 3, making the electronic unit unusable, and possibly causing a fire.

Therefore, conventionally, in order to limit the excessive current flowing through the drive switch circuit 3, a circuit having a current limiting function as shown in FIGS. 7 and 8 has been considered. In the case of FIG. 7, when an rHJ output is generated at the output terminal P of the control device 1, the transistor Ql is turned on by the voltage divided between the resistors R1 and R2, and the base current flows through the drive transistor Q2. Q2 is turned on to drive the load 4, and a constant current drive output is obtained by connecting a resistor R to the emitter of the drive transistor Q2. Also, the one in Figure 8 is the same as the one in Figure 7.
The drive transistor Q2 is turned on to drive the load 4 by the same operation as shown in the figure, but here, by connecting the current limiting resistor R to the base of the drive transistor Q2, the output current is changed to the base current hf' I am trying to limit it to e times.

By the way, if the battery voltage is 12V, then 2.4W
A drive current of at least 200 mA is required to drive a load of (in reality, a little more drive current is required to fully saturate the drive transistor).

Under such conditions, in normal times, almost no voltage is applied to the drive transistor, and power consumption is very small. By the way, the saturation voltage of the drive transistor is set to 0.
When it is 2V, it is about 0.2V x 200mA-40mW.

However, as mentioned above, if the load drive line 7 comes into contact with the ground 6 (chassis), the 12V x 200mA-
The output power is 2.4W, which means that it consumes quite a large amount of power. This is based on the assumption that the load drive line 7 comes into contact with the chassis, which is unlikely to happen, and the drive transistor Q
Second, it is necessary to design the circuit of the electronic unit using devices with high allowable power consumption.Increasing the power consumption of transistors requires heat sinks, etc., which causes problems not only in terms of price but also in This was disadvantageous in terms of space, etc. Furthermore, the temperature rise inside the electronic unit limited high-density packaging on the unit board, which also became an obstacle to miniaturization.

[Problems to be Solved by the Invention] As described above, in the conventional device, assuming that the load drive line comes into contact with the chassis, one or more drive transistors (with large capacity and power consumption) are used. Because of this, a heat sink is required, which is disadvantageous not only in terms of price but also in terms of installation space.Furthermore, the temperature rise inside the electronic unit limits high-density mounting on the unit board. However, there was a problem that became an obstacle to miniaturization of electronic units.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to develop a load drive circuit that can prevent heat generation in load drive transistors, make it possible to reduce the capacity of transistors, and contribute greatly to the miniaturization and cost reduction of electronic units. It is to provide a protection circuit.

[Means for Solving the Problems] The present invention provides a load that is connected in series to a load to which power is supplied,
The first transistor that controls the on/off state of the load is controlled on and off by a second transistor that responds to the load on/off control command, and the second transistor responds to the overcurrent flowing through the first transistor. A control circuit operates to turn off the first transistor.

[Function] According to the present invention, when an overcurrent flows through the load driving transistor connected in series to the load due to a short circuit accident between the source of γ and the ground, this transistor can be immediately turned off. Since heat generation in the transistor can be prevented, the capacity of the transistor can be reduced.

[Example] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the circuit configuration of the same embodiment. In the figure, 11 is a control device, and this control device 11 is +5
It consists of a microcomputer driven by a separate V power supply. This control device 11 has rHJ at the output terminal P.
In addition to level and rLJ level outputs, a high-impedance state can be set.It always generates an rLJ level output, and when driving a load, it immediately enters a high-impedance state after generating a pulsed rHJ level output. I'm trying to make it happen. Such an operation can be easily handled by a normal microcomputer control circuit.

The output terminal P of the control device 11 is connected to ground via a resistor R11 and to the base of a transistor Qll via a resistor R13. The resistor R13 prevents excessive current from flowing from the output terminal P of the control device 11 to the transistor Qll.

This resistor RH may be connected between a connection point between resistors R11 and R14, which will be described later, and an output terminal P of the control device 11.

The emitter of the transistor Qll is connected to ground, and the collector is connected to the load driving transistor Q through a resistor R12.
Connected to 12 bases. The load driving transistor Q12 has an emitter connected to the power supply line 12 and a collector connected to ground via the load 13. In addition, the load driving transistor Q12 has its collector connected to a resistor R
14, connected to ground through R11, and these resistors R1
4. The connection point of R11 is connected to the base of the transistor Qll via a resistor R13. Resistance R1 here
1 is not necessarily required.

Here, the power line 12 is connected to the positive terminal of the automobile battery, and the ground is connected to the chassis of the automobile.

Next, the operation of the embodiment configured as described above will be explained.

If the load is not driven now, the output terminal P of the control device 11 generates an "L" level output as shown in period A in FIG. 2(a). In this state, transistor Qll remains off, and load driving transistor Q12
is also off, and the load 13 is in an off state as shown in period A in FIG. 2(b).

If you want to drive the load from this state, see Figure 2(a).
As shown in period B, the output terminal P of the control device 11 immediately shifts to a high impedance state after generating a pulsed rHJ level output. Then, the pulsed rHJ level output turns on the transistor Qll, and the base current flows through the load driving transistor Q12.
The same transistor Q12 is also turned on. This allows power line 1
2 to the load 13 via the load driving transistor Q12.
7fi current is supplied to the load 13, and the load 13 is turned on as shown in period B of FIG. 2(b). In this state, load 13
is applied across resistors R14 and R11, and the voltage at the connection point of these resistors R14 and R11 is applied to the base of transistor Qll via resistor RI3. Therefore, in this state, even if the output terminal P of the control device 11 becomes a high impedance state, the transistor Qll is maintained in the on state, and the base current continues to flow through the load driving transistor Q12, so that the load 13 is kept in the on state. maintained.

Thereafter, when the load drive is stopped, the output terminal P of the control device 11 generates an rLJ level output as shown in period C in FIG. 2(a). Then, the transistor Qll is turned off and the base current of the load driving transistor Q12 disappears, so the transistor Q12 is also turned off, and as shown in FIG.
), the load 13 returns to the off state.

Next, if the cover of the drive line 14 between the load drive transistor Q12 and the load 13 is damaged and comes into direct contact with the ground, if the load drive transistor Q12 is in the on state, the load drive transistor Q12 Since a voltage is applied between the battery and the chassis and a large current flows for driving the load, the power consumption of the load driving transistor QI2 becomes extremely large. However, in this case, since the potential of the control line 14 is 0, the output terminal P of the control device 11
remains in a high impedance state, the resistors R14 and R
The voltage across the transistor Q11 disappears, and as a result, the voltage across the transistor Q
ll is turned off, the base current of the load driving transistor Q12 disappears, and the same transistor Q12 is also turned off. As a result, no current flows through the load driving transistor Q12, so that power consumption in the load driving transistor Q12 can be quickly eliminated.

By the way, in such a circuit configuration, once the load driving transistor Q12 is turned off, the circuit will not return to the on state if the load driving transistor Q12 is turned off.

Therefore, in order to restore the circuit in such a case, the rHJ level output for time TI as shown in Fig. 3(a) is intermittently generated twice at each time T2, and a high impedance state is set during this time. The output terminal P of the control device 11 is made to generate such an output. The time T2 here is an allowable time since the load 13 is in the off state when it should be on, and is sufficiently larger than the time TI.

When the first pulse-like rHJ level output is applied, the transistor Qll turns on, causing the base current to flow through the load driving transistor Q12.
12 turns on the load 13. However, if the contact state of the drive line 14 to the ground has not been recovered at this point, the transistor QIL load driving transistor Q12 is turned off again, and the current flowing through the load driving transistor Q12 is cut off. Become. Then, after a predetermined time T2, the pulse-like rHJ level output is applied again, the transistor Qll is turned on, the base current flows to the load driving transistor QI2, the transistor Q12 is turned on, and the load 13 is turned on again. state. If the circuit has recovered normally at this point, the load 13 is maintained in the on state.

In this case, for the generation time T1 of the first pulsed rHJ output, the transistor Q11 and the load driving transistor Q1
2 is turned on, a large current flows through the load driving transistor Q12 during this time, but this time is TI/T2 of the entire time, and power consumption can be suppressed to an extremely small amount.

Next, another embodiment of the invention will be described with reference to FIG.

FIG. 4 shows a case where the power line 22 is connected to the load 23 side. In this case, the control device 21 outputs the output terminal P
In addition to rHJ level and "L" level output, a high impedance state can be set, so r
When an HJ level output is generated and the load is driven, a high impedance state is set immediately after a pulsed rLJ level output is generated. Then, the output terminal P of the control device 21 is connected to the ground via the resistor R21, and the transistor Q is connected via the resistor R23.
Connect to the base of 21.

The emitter of the transistor Q21 is connected to ground, and the collector is connected to the base of the load driving transistor Q22. The load driving transistor Q22 has its emitter connected to ground, and its collector connected to the power supply line 22 via the load 23. In addition, the load driving transistor Q
22 connects the base to the power supply + via the pull-up resistor R21.
Connect to VCC and connect the collector to resistors R24, R
21 to ground, and these resistors R24, R2
1 is connected to the transistor Q2 via the resistor R23.
It is connected to the base of 1. The resistor R21 here is not necessarily required.

Here, the power line 22 is connected to the positive terminal of the automobile battery, and the ground is read directly to the chassis of the automobile.

According to such a configuration, when the load is not driven, an rHJ level output is generated at the output terminal P of the control device 11 as shown in period A in FIG. 5(a). Then, the transistor Q21 is turned on and no base current flows through the load driving transistor Q22, so the load driving transistor Q22 is also turned off, and the load 23 is turned off as shown in period A in FIG. 5(b). become a state.

To drive the load from this state, the output terminal P of the control device 11 generates a pulsed rLJ level output, and then immediately sets the high impedance state, as shown in period B in FIG. 5(a). do. Then, the pulsed rL
The transistor Q21 is turned off by the J level output, and the load driving transistor Q2 is turned off via the pull-up resistor R22.
A base current flows through the transistor Q22, turning on the transistor Q22. Due to this? Current begins to flow from the Il source 1i122 through the load 23 and the load driving transistor Q22, and the load 23 is turned on as shown in period B in FIG. 5(b). In this state, load driving transistor Q
22 is applied across resistors R24 and R21, and the voltage at the connection point of these resistors R24 and R21 is applied to resistor R2.
However, at this time, the load driving transistor Q22 is in a saturated state, and the collector current of the load driving transistor Q22 is very small, so the base current is supplied to the transistor Q21. It remains off. As a result, the load driving transistor Q22 is maintained in the on state.
The load 23 is maintained in the on state.

Thereafter, when the load drive is stopped, the output terminal P of the control device 11 generates an rHJ level output as shown in period C in FIG. 5(a). Then, the transistor Q21 turns on, and the base current of the load driving transistor Q22 disappears, so the transistor Q22 also turns off, and as shown in FIG.
As shown in period C of b), the load 23 is turned off.

Next, if the coating of the drive line 24 between the load drive transistor Q22 and the load 23 is damaged and comes into direct contact with the power supply line 22, if the load drive transistor Q22 is in the on state, the load drive transistor Q22 A large current flows through the circuit, resulting in high power consumption.

However, in this case, the voltage at the collector of the load driving transistor Q22 increases and the output terminal P of the control device 21 is in a high impedance state, so the divided voltage of the resistors R24 and R21 is applied to the base of the transistor Q21, turns on. Then, the base current of the load driving transistor Q22 disappears, and the load driving transistor Q22 loses its base current.
22 is turned off. As a result, the load driving transistor Q
Since no current flows through Q22, power consumption in the load driving transistor Q22 can be quickly eliminated.

Note that even when an rLJ level output is intermittently generated at the output terminal P of the control device 11 in order to restore the circuit, a large current flows through the load driving transistor Q22 as in the above-described embodiment. Time Tl /T2
This can reduce power consumption to a small level.

[Effect of the invention] This invention provides a load that is connected in series to a load to which power is supplied,
The first transistor that controls the on/off state of the load is controlled on and off by a second transistor that responds to the load on/off control command, and the second transistor responds to the overcurrent flowing through the first transistor. A control circuit operates to turn off the first transistor. This results in
If an overcurrent flows into the first transistor for driving the load due to a short-circuit accident between the power supply and ground, etc., this transistor can be immediately turned off, which prevents heat generation in the transistor and reduces the transistor's small capacity. It is possible to aim for Compared to conventional systems, this technology does not require any heat sinks, making it significantly more advantageous in terms of cost and installation space.It also eliminates temperature rise inside the electronic unit, allowing for high-density packaging. This makes it possible to greatly contribute to the miniaturization and cost reduction of electronic units.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing one embodiment of this invention, FIGS. 2 and 3 are time charts for explaining the operation of the same embodiment, and FIG. 4 is a circuit diagram showing another embodiment of this invention. , 5th
The figure is a time chart for explaining the operation of this embodiment, and FIGS. 6 to 8 are circuit diagrams showing an example of a conventional load drive circuit. 11.21...Control device, 12.22...Power line,
13.23...Load, 14.24...Drive line, Ql
l, Q12, Q21, Q22...transistor, R1
1-Rl4, R21-R24...Resistance. Applicant's agent Patent attorney Takehiko Suzue

Claims (1)

[Claims] a power source, a load to which the power is supplied, a first transistor connected in series to the load to control the on/off state of the load, and an on/off state of the first transistor in response to an on/off control command for the load. and a control circuit that operates the second transistor to turn off the first transistor in response to an overcurrent flowing through the first transistor. Load drive circuit.