JP3632385B2 - Inductive load drive circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an inductive load driving circuit for driving an inductive load upon receiving power supply from a DC power source.
[0002]
[Prior art and problems to be solved by the invention]
Conventionally, a drive circuit that drives an inductive load such as a solenoid valve is normally energized to an inductive load separately from a switching element that conducts or cuts off a power supply path from a DC power source to the inductive load. Clamp circuit that clamps the voltage applied to the switching element from the inductive load to a predetermined clamp voltage that is smaller than the withstand voltage of the switching element in order to protect the switching element from the back electromotive force generated in the inductive load when shut off Is provided.
[0003]
As the clamp circuit, a CR circuit including a power zener diode or a parallel circuit of a capacitor and a resistor is provided between a connection point between the switching element and the inductive load and a ground (or power supply) line. When a back electromotive force is generated, a current is passed through a power zener diode or CR circuit, and a high voltage applied to the switching element from an inductive load is clamped to a predetermined clamp voltage or less. When a zener diode is provided between the connection point with the inductive load and the control input terminal of the switching element (base if the switching element is a bipolar transistor, gate if the switching element is FET), and the back electromotive force is generated Clamps the voltage at both ends to a predetermined clamping voltage with a Zener diode At the same time, to operate the switching element by a current flowing through the Zener diode, which the energy stored in the inductive load and configured to emit via the switching element, etc. have been known.
[0004]
Of these, the latter clamp circuit using a Zener diode and a switching element allows a current to flow through the switching element during voltage clamping, so that the current capacity of the Zener diode used as the clamp circuit can be reduced and is relatively inexpensive. Can be realized.
[0005]
By the way, in a drive circuit equipped with this type of clamp circuit, if the clamp voltage is set large, the current flowing through the switching element after the occurrence of the back electromotive force can be quickly attenuated, and the responsiveness when driving an inductive load is improved. Although it could be improved, there was a problem that radio noise was generated due to a steep current change.
[0006]
Hereinafter, this problem will be described using a specific circuit.
First, FIG. 3 shows an example of a conventional driving circuit 100.
As shown in FIG. 3, the drive circuit 100 includes an NPN-type bipolar transistor (hereinafter simply referred to as a transistor) 3 as a switching element, and a drive signal generation circuit 5 is connected to a base B 1 that is a control input terminal of the transistor 3. The inductive load 7 is connected to the collector C1 of the transistor 3, the emitter E1 of the transistor 3 is grounded to the ground line, and the positive power supply voltage + B is applied to the opposite side of the inductive load 7 from the transistor 3. By connecting the power supply lines, the transistor 3 is turned on when a high level drive signal is output from the drive signal generation circuit 5, and a current flows through the inductive load 7. A Zener diode 101 is provided between the base B1 and the collector C1 of the transistor 3 with the base B1 side as an anode.
[0007]
In the drive circuit 100 configured in this way, as shown in FIG. 4A, when the transistor 3 is in the on state and the inductive load 7 is energized, the drive signal from the drive signal generation circuit 5 Is stopped (output is open), the transistor 3 is turned off, the energization to the inductive load 7 is cut off, and a back electromotive force is generated in the inductive load 7, so that the voltage at the connection point with the transistor 3 (Collector voltage Vc) rises steeply. However, since the Zener diode 101 is provided between the collector C1 and the base B1 of the transistor 3, the collector voltage Vc is clamped by the breakdown voltage of the Zener diode 101. Therefore, when the breakdown voltage of the Zener diode 101 is set to a voltage value lower than the breakdown voltage of the transistor 3 such that the breakdown voltage of the Zener diode 101 is 70 V when the breakdown voltage of the transistor 3 is 100 V, an inductive load is obtained. Thus, the transistor 3 can be protected from the counter electromotive force generated when the power supply 7 is turned off.
[0008]
Further, when the collector voltage Vc is clamped by the zener diode 101 in this way, a current flows from the inductive load 7 to the base B1 of the transistor 3 via the zener diode 101. A corresponding collector current Ic flows, and the energy stored in the inductive load 7 is quickly released. When the collector voltage Vc becomes lower than the clamp voltage due to this energy release, no current flows through the Zener diode 101 (and hence the base of the transistor 3), and the collector current Ic flowing from the inductive load 7 side is also cut off. Is done.
[0009]
By the way, in such a drive circuit 101, when the clamp voltage (that is, the breakdown voltage of the Zener diode 101) is set low, the current change flowing from the inductive load 7 to the transistor 3 becomes gentle and accumulates in the inductive load 7. The transistor 3 is completely discharged because the current continues to flow through the transistor 3 even though the generated energy is sufficiently discharged to a region not affected by the operation of the inductive load (for example, opening / closing of the solenoid valve). There is a problem that it takes time to turn off.
[0010]
Therefore, in this type of drive circuit 101 (particularly, a drive circuit for an inductive load such as a spill valve (solenoid valve) of a fuel injection pump that injects fuel to a diesel engine), a clamp voltage is required. Is set to a voltage value as large as possible within a voltage range lower than the withstand voltage of the transistor 3, the current flowing in the transistor 3 after the inductive load 7 is cut off is attenuated in a shorter time, and the transistor 3 is completely Therefore, the time until the operation is stopped is shortened to ensure the response of the drive circuit.
[0011]
However, if the clamp voltage is set high in order to improve the response of the drive circuit in this way, the collector current Ic suddenly changes in a decreasing direction immediately after the counter electromotive force generated in the inductive load reaches the clamp voltage. For this reason, there is a problem that high-frequency radio noise is generated due to the current change. When radio noise is generated, it affects the operation of surrounding electronic devices. For example, as a driving circuit for an automobile equipped with various electronic devices such as a driving circuit for driving a spill valve of a fuel injection pump. When used, other measures such as shielding the drive circuit are necessary so that the radio noise does not affect other electronic devices.
[0012]
On the other hand, as a technique for reducing radio noise while ensuring the responsiveness of the drive circuit, a capacitor 103 is provided in parallel with the Zener diode 101 for voltage clamping as shown by a dotted line in FIG. When back electromotive force is generated due to the interruption of energization, the capacitor 103 is charged to the breakdown voltage of the Zener diode 101 with the counter electromotive force, so that the collector current Ic immediately after the energization of the inductive load 7 is interrupted. It is conceivable to suppress significant changes.
[0013]
However, although such a measure can reduce radio noise, as shown in FIG. 4B, a delay time occurs until the collector current Ic starts to decay after the inductive load 7 is turned off. For this reason, there is a problem in that good drive characteristics cannot be obtained as a drive circuit for an inductive load that requires high responsiveness.
[0014]
In this circuit, since the capacitor 103 is directly connected between the collector C1 and the base B1 of the transistor, there is a possibility of oscillation depending on the combination of the transistor 3 and the capacitor 103.
The present invention has been made in view of these problems, and includes a clamp circuit that discharges a high voltage generated when the energization to the inductive load is interrupted via a switching element that conducts and interrupts the energization path of the inductive load. In the drive circuit of the inductive load, the response of the drive circuit is ensured and the occurrence of radio noise is suppressed.
[0015]
[Means for Solving the Problems and Effects of the Invention]
The inductive load drive circuit according to claim 1, which is made to achieve the above object, includes a voltage clamp provided between a connection point between the switching element and the inductive load and a control input terminal of the switching element. The means is constituted by a series circuit of a plurality of Zener diodes, and a capacitor is connected in parallel to a part of the plurality of Zener diodes.
[0016]
For this reason, in the drive circuit according to the present invention, the switching element changes from the on state to the off state in accordance with the change of the drive signal input to the control input terminal, and the energization to the inductive load is cut off and the inductive load is changed. When the back electromotive force is generated, the connection point voltage between the switching element and the inductive load first rises sharply to a predetermined voltage determined by the sum of the breakdown voltages of the Zener diodes not connected in parallel with the capacitor, and thereafter, As the capacitors connected to the remaining Zener diodes are charged, they gradually increase to a predetermined clamp voltage that is the sum of the breakdown voltages of all Zener diodes.
[0017]
As a result, according to the present invention, after the current supply to the inductive load is cut off, the current change of the switching element is suppressed until the capacitor is completely charged and the connection point voltage reaches the clamp voltage. Although the current decay is delayed as compared with a drive circuit that is not provided, the delay time is shorter than when a capacitor is connected in parallel to the entire voltage clamp means.
[0018]
If the capacitance of the capacitor is constant, this delay time changes according to the breakdown voltage of the Zener diode to which the capacitor is connected in parallel, and becomes shorter as the breakdown voltage becomes smaller. By changing the ratio of the sum of the breakdown voltage of the Zener diode and the sum of the breakdown voltage of the Zener diode that is not connected in parallel with the capacitor, the delay time can be shortened and the response of the drive circuit can be emphasized. The delay time can be lengthened so that the radio noise can be reduced.
[0019]
That is, according to the present invention, according to the response required for driving the inductive load and the surrounding environment, the ratio of the breakdown voltage between the Zener diode to which the capacitor is connected in parallel and the other Zener diode is appropriately set. Thus, the responsiveness of the drive circuit and the radio noise generated from the drive circuit can be set optimally.
[0020]
In particular, when driving an inductive load that needs to be switched between energization and non-energization at high speed, the responsiveness of the drive circuit is required. Therefore, the capacitors are not connected in parallel as described in claim 2. What is necessary is just to set the breakdown voltage of each Zener diode so that the sum of the breakdown voltage of a Zener diode may become larger than the sum of the breakdown voltage of the Zener diode with which the capacitor | condenser was connected in parallel.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is an electric circuit diagram showing a configuration of a drive circuit 1 according to an embodiment to which the present invention is applied. The drive circuit 1 of this embodiment is for driving a spill valve (solenoid valve) used to determine the fuel injection end timing in a fuel injection pump that supplies fuel to a diesel engine. As with the conventional drive circuit 100 shown in FIG. 3, an NPN-type bipolar transistor 3 provided in the energization path of the inductive load 7 that is a solenoid of the spill valve is provided. The collector C1 of the transistor 3 is connected to the power supply line (+ B) via the inductive load 7, the emitter E1 is grounded, and the base B1 is connected to the drive signal generating circuit 5.
[0022]
3 is different from the conventional driving circuit 100 shown in FIG. 3 in that the two are connected in series between the collector C1 and the base B1 of the transistor 3 with the anode on the base B1 side and the cathode on the collector C1 side. Two Zener diodes 9 and 11 are provided, and a capacitor 13 is paralleled to one Zener diode 11.
[0023]
In the present embodiment, the breakdown voltage of the Zener diodes 9 and 11 is set to the same voltage “35 V” with respect to the withstand voltage “100 V” of the transistor 3. The clamp voltage between the base B1 and the collector C1 to be clamped is set to “70V”.
[0024]
In the drive circuit 1 of the present embodiment configured as described above, as shown in FIG. 2, when the transistor 3 is in the ON state and the inductive load 7 is energized, the drive from the drive signal generation circuit 5 When the signal output is stopped (output open) and the inductive load 7 is de-energized, back electromotive force is generated in the inductive load 7 and the collector voltage Vc rises sharply. Since the capacitor 13 is connected in parallel to one of the two Zener diodes 9 and 11 provided between B1 and the collector C1, the collector voltage Vc at that time is that of the Zener diode 9 to which the capacitor 13 is not connected. The breakdown voltage Vt (35 V) is obtained. Thereafter, the collector voltage Vc gradually increases as the capacitor 13 is charged, and is finally clamped to a voltage (70 V) that is the sum of the breakdown voltages of the two Zener diodes 9 and 11.
[0025]
For this reason, in the drive circuit 1 of the present embodiment, after the energization of the inductive load 7 is interrupted, the charging of the capacitor 13 is completed and the collector current Vc reaches the clamp voltage (70V). Since Ic is not greatly attenuated, the current decay is delayed as compared with the drive circuit 100 shown in FIG. 3 in which the capacitor 13 is not provided, and the responsiveness of the drive circuit 1 is lowered, but is caused by a steep current change. Radio noise can be reduced. Conversely, the delay time of the current change is shorter than that in the case where a Zener diode and a capacitor are connected between the base B1 and the collector C1 of the transistor 3 in order to reduce radio noise. In contrast, the responsiveness can be improved. Therefore, according to the drive circuit 1 of the present embodiment, radio noise can be reduced while ensuring the responsiveness of the drive circuit.
[0026]
Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and it goes without saying that the present invention may be implemented in various modes.
For example, in the drive circuit 1 of the above embodiment, the description has been made assuming that the breakdown voltages of the Zener diode 11 to which the capacitor 13 is connected in parallel and the Zener diode 9 that is not connected in parallel are made the same, but the ratio of these breakdown voltages is changed. Thus, for example, if the breakdown voltage of the Zener diode 11 to which the capacitor 13 is connected in parallel is made smaller than the breakdown voltage of the other Zener diodes 9, it is possible to obtain a circuit configuration that emphasizes the high responsiveness of the drive circuit, Conversely, if the breakdown voltage of the Zener diode 11 to which the capacitor 13 is connected in parallel is made larger than the breakdown voltage of the other Zener diodes 9, it is possible to achieve a circuit configuration that emphasizes the prevention of radio noise generation. What is necessary is just to set suitably about the breakdown voltage of the diodes 9 and 11 according to use conditions.
[0027]
In the above embodiment, the drive circuit 1 in which the transistor 3 serving as a switching element is provided as a low-side switch on the lower potential side than the inductive load 7 has been described. However, the present invention is illustrated in FIG. As shown, the present invention can also be applied to a drive circuit 20 that uses a PNP-type bipolar transistor 23 as a switching element and is provided on the higher potential side than the inductive load 7 as a high-side switch.
[0028]
That is, in the drive circuit 20, the emitter E2 of the transistor 23 is connected to the power supply line (+ B), the collector C2 is grounded via the inductive load 7, and the base B2 is connected to the drive signal generation circuit 5 ′. Thus, when the transistor 23 is provided as a high-side switch for the inductive load 7, the collector C2 of the transistor 23 connected to the inductive load 7 and the base B2 that is the control input terminal of the transistor 23 The series circuit of the Zener diodes 9 and 11 is connected with the anode of each Zener diode 9 and 11 facing the collector C2, and the capacitor 13 is connected in parallel to one of the Zener diodes (the Zener diode 11 in the figure). Thus, the same effect as in the above embodiment can be obtained.
[0029]
In this drive circuit 20, since a PNP-type bipolar transistor 23 is used as a switching element, when the inductive load 7 is energized, a low level signal is output from the drive signal generation circuit 5 ', and the inductive load is output. When the power supply is cut off, the output is stopped (output open).
[0030]
In the drive circuits 1 and 20 shown in FIGS. 1A and 1B, bipolar transistors are used as switching elements. However, the switching elements are not limited to bipolar transistors. Any FET (electrolytic effect transistor) that has been conventionally used as a switching element for driving an inductive load can be used.
[0031]
In the drive circuits 1 and 20 shown in FIGS. 1A and 1B, a series circuit of two Zener diodes is used as the voltage clamping means. It is not necessary to configure with a Zener diode, and it may be configured with three or more Zener diodes.
[0032]
In the above embodiment, the spill valve of the fuel injection pump is driven. However, as the inductive load 7, an electromagnetic valve such as an spill valve such as an actuator that displaces an object by energizing the solenoid is used. Needless to say, the drive circuit of the present invention can be applied to other than the above.
[Brief description of the drawings]
FIG. 1A is a circuit diagram illustrating a drive circuit 1 according to an embodiment, and FIG. 1B is a circuit diagram illustrating a drive circuit 20 according to a modification.
FIG. 2 is a time chart showing changes in a collector voltage Vc and a collector current Ic when energization to an inductive load is interrupted in the drive circuit 1 of the embodiment.
FIG. 3 is a circuit diagram showing a conventional driving circuit 100. FIG.
FIG. 4 is a time chart showing changes in collector voltage Vc and collector current Ic when energization to an inductive load is interrupted in conventional drive circuit 100;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Drive circuit 3 ... Transistor 5 ... Drive signal generation circuit,
7 ... Inductive load 9 ... Zener diode 11 ... Zener diode 13 ... Capacitor 20 ... Drive circuit 23 ... Transistor

Claims (2)

A switching element that is provided in a power supply path from a DC power supply to the inductive load and that conducts and cuts off the power supply system path according to a drive signal input to a control input terminal;
The inductive load is provided between a connection point between the switching element and the inductive load and a control input terminal of the switching element, and when the switching element is changed from an on state to an off state by the drive signal. Voltage clamping means for causing a current to flow through the switching element due to the back electromotive force generated in the voltage and clamping a voltage generated between the connection point and the control input terminal to a predetermined clamping voltage smaller than the withstand voltage of the switching element. When,
In an inductive load drive circuit comprising:
An inductive load drive circuit, wherein the voltage clamping means is constituted by a series circuit of a plurality of Zener diodes, and a capacitor is connected in parallel to a part of the plurality of Zener diodes. The breakdown voltage of each Zener diode is set so that the sum of the breakdown voltages of the Zener diodes to which the capacitors are not connected in parallel is larger than the sum of the breakdown voltages of the Zener diodes to which the capacitors are connected in parallel. The inductive load drive circuit according to claim 1.

Images