GB2170931A

GB2170931A - Driver circuit for solenoids

Info

Publication number: GB2170931A
Application number: GB08529458A
Authority: GB
Inventors: Kenneth Ian Forster
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1985-02-02
Filing date: 1985-11-29
Publication date: 1986-08-13
Also published as: DE3602826A1; GB8529458D0; GB2170931B; GB8502705D0

Abstract

The invention relates to a circuit for driving current through the winding of a solenoid 10, e.g. in an engine fuel injection system. The circuit has a current output stage comprising a first transistor 14 having a current switching path connected in a series path containing the winding 10 and a heat dissipating resistor 16, and a second transistor 12 having a switching path connected directly in series with the winding 10 and in parallel with the series combination of the switching path of the first transistor 14 and the resistor 16. Diodes 26,28 ensure that second transistor 12 is only conductive until the current through the winding 10 reaches a preset value. Both transistors are controlled by negative feedback derived from a common emitter resistor 20 coupled to a comparator amplifier 34. The resistor 16 prevents excessive power dissipation in the transistor 14 but is bypassed initially by the transistor 12 to avoid limitation of the current rise time. <IMAGE>

Description

SPECIFICATION Driver circuit for solenoids The present invention relates to a current driver circuit for applying a controlled current to the winding of a solenoid and is intended primarily for driving a control current through a fuel injector of an engine to control the quantity of fuel delivered.

In a motor vehicle, it is already known to provide signals from a computer to control the opening of fuel injectors associated with the different cylinders. The signals for each injector comprise a first signal, herein termed the enable signal, which permits the injector to open and the absence of which inhibits the flow of current through the injector. A second signal from the computer, herein termed the peak signal, sets the level of the current flowing through the injector. Initially, to open the injector, a short pulse of high magnitude causes a high current flow but the peak signal then drops to a lower value sufficient to keep the injector open but not to cause it to open.

The current which must flow through the solenoid is fairly high being typically 2 amps and the winding may typically have a resistance of 1 ohm. As he vehicle supply is typically 12 volts, this results in the driver circuit having at its output a 10 volt drop giving rise to a power dissipation of some 20 watts.

Such high power dissipation within the driver circuit gives rise to severe problems, especially when the driver circuit is part of an integrated circuit housed in the hot and hostile environment of the engine compartment of a motor vehicle. It is possible to reduce the power dissipation within the driver circuit by means of an external series resistor but this has the disadvantage of limiting the rate of increase of the current through the solenoid winding.

It has already been proposed, in US-A-4,314,305, to supply current to the winding of a solenoid through a drive circuit having two output transistors in parallel paths of high and low resistance, respectively. A comparator circuit compares a voltage related to the current through the winding and switches on one or other of the two transistors.

In this earlier proposal, the current in the winding is not regulated by the feedback and instead switching between the two current paths is relied upon to maintain the current at the desired level. In particular, no negative current dependent feedback regulates the current supply when the low resistance path is connected to the winding. As a result the current regulation in the prior art is not fully satisfactory, and the operation of the injectors is dependent upon the supply voltage.

Other proposals are to be found in the prior art, in which two current paths of differing series resistance are used to supply current to the winding of a solenoid, but all the known circuits have in common that the switching element in the low resistance path is conductive is either on or off and is not regulated in dependence upon the current through the winding.

The present invention seeks to provide a circuit capable of driving current through the winding of an injector which enables the current through the winding of the solenoid to be more fully regulated.

According to the present invention, there is provided a current driver circuit for driving current through the winding of a solenoid having a current output stage comprising a first switching element having a current switching path connected in a series path containing the winding of the solenoid and a heat dissipating resistor and a second current switching element having a switching path connected directly in series with the winding of the solenoid and bypassing the resistor, characterised by means for monitoring the sum of the currents flowing through the two current switching elements and providing a negative feedback control signal dependent upon the sum of the two currents to both the switching elements whereby to regulate the current flowing through the winding of the solenoid, the second switching element serving to conduct current to the winding only when the current through the winding is below a predetermined level.

As a result of the above construction, the heat dissipation occurs mainly in the series resistor of the first switching element which may be mounted externally of the integrated circuit, thereby reducing the problem of heat dissipation within the integrated circuit. The series resistor however does not affect the rise time of the current as the parallel connected second switching element permits the resistor to be by-passed until a current sufficiently high to open the injector is flowing through the solenoid winding.

In the present invention, by contrast with the prior art proposals mentioned above, both the current paths to the winding are regulated by negative feedback dependent upon the total current through the winding, with the result that current through the winding is regulated at all times in the operating cycle.

It is preferred that the switching elements should be transistors.

Advantageously, the emitters of the two transistors are connected to ground by way of a common resistor serving to provide a feedback voltage dependent upon the current flowing through the solenoid winding.

The feedback signal is conveniently supplied to one input of a differential amplifier which receives a second input representative of the desired current flow through the solenoid winding, the output of the differential amplifier being applied to the bases of the transistors constituting the switching elements.

In order to ensure that the transitor should cease to conduct when the current through the solenoid winding reaches a predetermined value, a plurality of forward diodes may be arranged in series with the base emitter path of the second transistor. In this way, the second transistor is only conductive when the difference between the current dependent feedback voltage and the output of the differential amplifier exceeds the voltage drop across the forward biassed diodes.

The signal applied to the input of the differential amplifier to represent the desired value of the current through the solenoid winding is conveniently provided by a voltage divider, one part of the divider being connected in parallel with a further switching element controlled by the peak signal from the computer.

The invention will now be described further, by way of example, with reference to the accompanying drawing, which is a diagram of a driver circuit in accordance with the invention.

The winding 10 of the solenoid operated injector is connected to the positive battery voltage terminal of the vehicle 12 volt supply and is connected to earth through two control paths. The first path comprises a power dissipating resistor 16, the collector-emitter path of a transistor 14 and a feedback emitter resistor 20. The second control path consists of the collector-emitter path of a second transistor 12 and the feedback resistor 20 which is common to both control paths and has the same current passing through it as the winding 10. A zener diode 18 also connected to the winding 10 acts as a free-wheeling diode to prevent overloading pf the transistors 12 and 14 when the switch off.

The voltage developed across the resistor 20, which is proportional to the current flowing through the winding 10 is fed back to the input of an operational amplifier 34 by way of a resistor 32. This voltage is compared to a signal representing the desired current by the operational amplifier 34 the output of which is connected to the base of transistor 14 by way of a resistor 24 and to the base of the transistor 12 by way of two forward biassed diodes 26, 28 and a resistor 30.

When the output voltage of the operational amplifier 34 is high, the base of transistor 14 receives a signal from a voltage divider comprising the resistor 24 and a resistor 22 which is connected to ground. This causes the transistor 14 to turn on and causes current to flow through the winding 10 and the resistor 16. The full output voltage of the operational amplifier is also applied to the base of the transistor 12 through the series connected diodes across which there is developed a voltage drop. The voltage applied to the base of the transistor 12 is reduced by the latter voltage drop and when the voltage developed across the resistor 20 by the winding current reaches the reduced voltage applied to the base of the transistor 12, the latter ceases to conduct and the winding current is maintained by the flow through the first transistor 14 and the resistor 16.As a result the rise time of the current through the winding is the same as in the absence of the resistor 16 but in the steady state the power dissipation occurs outside the integrated circuit comprising the transistors 12, 14.

The previously mentioned ENABLE signal is applied by way of an input terminal of the driver circuit and a resistor 50 to the base of a transistor 48 acting as a buffer amplifier.

When the enable signal is high, the transistor 48 is switched off and the 5 volt reference signal is applied by way of a resistor 46 and a diode 60 to the inverting input pf the operational amplifier 34, thereby maintaining the output of the amplifier permanently low and preventing operation of the injector. When the transistor 48 is turned on, however, the low collector voltage of the transistor 48 is isolated from the operational amplifier 34 by the diode 60 which now ceases to conduct.

The PEAK signal previously mentioned is applied from a second input terminal of the driver circuit by way of a resistor 36 to a transistor 38 which varies the voltage applied to the non-inverting input of the operational amplifier 34 between two fixed levels. When the transistor 38 is off the voltage at the noninverting input of the amplifier 34 is determined by the voltage divider comprising tworresistors 40, 44. This sets the low reference voltage for the sustaining current of the solenoid winding. On the other hand, when the transistor 38 is rendered conductive by the PEAK signal, the resistor 42 is connected in parallel with the resistor 40 thereby reducing the resistance of the upper half of the divider and raising the reference voltage to correspond to the peak current value required to open the injector.

The purpose of the remaining elements shown in the drawing will be self evident to the person skilled in the art but it is mentioned for completeness that the various capacitors 52, 54, 56 & 58 provide decoupling and serve to suppress high frequency interference.

Claims

1. A current driver circuit for driving current through the winding of a solenoid (10) having a current output stage comprising a first switching element (14) having a current switching path connected in a series path containing the winding of the solenoid (10) and a heat dissipating resistor (16) and a second current switching element (12) having a switching path connected directly in series with the winding of the solenoid (10) and bypassing the resistor (16), characterised by means (20) for monitoring the sum of the cur rents flowing through the two current switching elements (12, 14) and providing a negative feedback control signal dependent upon the sum of the two currents to both the switching elements (12, 14) whereby to regulate the current flowing through the winding of the solenoid (10), the second switching element (12) serving to conduct current to the winding only when the latter current through the winding is below a predetermined level.

2. A circuit as claimed in claim 1, wherein both the switching elements (12, 14) are transistors.

3. A circuit as claimed in claim 2, wherein the emitters of the two transistors (12, 14) are connected to ground by way of a common resistor (20) serving to provide a feedback voltage dependent upon the current flowing through the solenoid winding (10).

4. A circuit as claimed in claim 3, wherein the feedback signal is supplied to one input of a differential amplifier (34) which receives a second input representative of the desired current flow through the solenoid winding (10), the output of the differential amplifier (34) being applied to the bases of the transistors (12, 14) constituting the switching elements.

5. A circuit as claimed in claim 4, wherein a plurality of forward biassed diodes (26, 28) are be arranged in series with the base emitter path of the second transistor.

6. A circuit as claimed in claim 4 or 5, wherein the signal applied to the input of the differential amplifier (34) to represent the desired value of the current through the solenoid winding (10) is provided by means of a voltage divider (40, 44), one part (40) of the divider being connected in parallel with a further switching element (38) controlled by an input signal to the curcuit.