EP0272946A1 - Circuit to regulate current in an inductive load, and its application to the ignition coil control of an internal-combustion engine - Google Patents

Abstract

The circuit according to the invention comprises a hysteresis comparator 9 connected to a transistor 2 for controlling the current flowing in an inductive load 1. The voltage drawn across a measurement resistor 3 is compared to a reference voltage established by a source 6. According to the invention, a feedback loop 10 establishes a proportional action of the transistor 2 on the current in the load 1, when the voltage at S exceeds a predetermined threshold. An oscillation of the current is thus obtained between two values framing a nominal value, these two values being linked to the thresholds of the hysteresis comparator. Application to the production of an integrated circuit for controlling the ignition coil of an internal combustion engine.

Description

The present invention relates to a current regulation circuit in an inductive load, designed for an integrated circuit embodiment. The invention also relates to such a circuit which finds particular application in the electronic control of an ignition coil of an internal combustion engine.

The regulation of the current in an inductive load intervenes in many fields of the technique such as, for example, the feeding of electric motors, and, more particularly in the field of the propulsion of the motor vehicles by internal combustion engines, in controlling the supply of the ignition coils or the solenoid valves of the fuel injectors.

This often poses the problem of limiting the power dissipated in an inductive winding during a permanent power supply, to avoid, for example, excessive heating or excessive electrical energy consumption likely to quickly exhaust a source of energy. on-board energy, such as the battery of a motor vehicle.

To limit the current in an inductive load, a circuit such as that shown in FIG. 1 of the accompanying drawing, in which the inductive load 1 is mounted in series with an electronic control member 2 such as a transistor, a measurement resistor 3, and a source of electrical energy 4 supplying this circuit. One of the inputs of a differential amplifier 5 is supplied by a reference voltage source 6, the other input of the amplifier being supplied by the voltage drawn across the measurement resistor, voltage whose amplitude is representative of the intensity of the current flowing in the inductive load 1. The output of the differential amplifier 5 supplies the base of the transistor 2 to control this transistor so that the current flowing in the load and in the emitter-collector circuit of the transistor either regulated to the value which corresponds to the chosen reference voltage. The current in the inductive load can be cut by closing a switch 8 which connects the base of transistor 2, of the NPN type, to ground.

The circuit thus includes a feedback loop which controls the current in the load to a chosen value. To ensure the stability of this loop, a corrector sub-circuit 7 is generally provided, installed between the output of the differential amplifier 5 and the input of this amplifier which is supplied by the measurement signal. This corrector is conventionally constituted by an arrangement of resistors and capacitors chosen from various configurations well known in the art of servo-control. In certain applications, and this is particularly the case for the ignition control circuits of internal combustion engines, these resistors and these capacities having the disadvantage of being too large to be able to be incorporated into an integrated circuit in a reasonable cost.

The object of the present invention is therefore to produce a circuit for regulating the current flowing in an inductive load, which can be easily integrated by current techniques for manufacturing integrated circuits by photochemical etching and diffusion of impurities, for example.

The present invention also aims to produce such an integrated circuit, more particularly intended for controlling the current flowing in a coil of an electronic ignition control device for an internal combustion engine.

These aims of the invention are achieved with a circuit for regulating the current flowing in an inductive load. powered by a voltage source, comprising an electronic control member placed in series with this load and a comparator powered by a first signal representative of a reference current intensity and by a second signal representative of the instantaneous current intensity flowing in the load and in the electronic control member, a control electrode of which is connected to the output of the comparator, this circuit being characterized in that it comprises a feedback loop between the terminal common to the load and the electronic control member, and the control electrode thereof, suitable for controlling a conduction with proportional action of the member when the voltage on said common terminal reaches a predetermined threshold, this conduction replacing the conduction in saturation of the electronic control unit established by the comparator, when the voltage at the common terminal is less than the predetermined threshold, so as to cause oscillation ler the intensity of the current in the load around a nominal value.

• - la figure 1 représente schématiquement un circuit de régulation du courant dans une charge inductive, de la technique antérieure,
• - la figure 2 est une représentation schématique du circuit de régulation suivant la présente invention,
• - la figure 3 est un graphe qui représente la caractéristique de transfert d'un comparateur à hystérésis qui fait partie du circuit de la figure 2,
• - les figures 4A et 4B sont des graphes qui représentent l'évolution du courant dans la charge inductive et celle de la tension prélevée au point S du circuit suivant l'invention, respectivement, et
• - la figure 5 est une représentation schématique d'un deuxième mode de réalisation du circuit suivant la présente invention.

In the attached drawing, given only by way of example:

• FIG. 1 diagrammatically represents a circuit for regulating the current in an inductive load, of the prior art,
• FIG. 2 is a schematic representation of the regulation circuit according to the present invention,
• FIG. 3 is a graph which represents the transfer characteristic of a hysteresis comparator which is part of the circuit of FIG. 2,
• FIGS. 4A and 4B are graphs which represent the evolution of the current in the inductive load and that of the voltage taken at point S of the circuit according to the invention, respectively, and
• - Figure 5 is a schematic representation of a second embodiment of the circuit according to the present invention.

Before undertaking a detailed description of the circuit according to the invention, it is useful to recall some characteristics of a particular, but not limiting, application of the invention, namely the regulation of the current in a coil of an electronic ignition device of an internal combustion engine. In this application, a current must be passed through the coil for a sufficient time interval to charge the latter with a predetermined electromagnetic energy which is then suddenly released by opening the coil supply circuit, which causes the emission of an ignition spark in a spark plug forming part of a secondary circuit coupled to the coil by mutual inductance. The emission of this spark is triggered at a determined instant of ignition advance calculated as a function of certain engine operating parameters (rotation speed, intake pressure, etc.). Of course, it is necessary that at this instant, the ignition coil is sufficiently charged by the current having passed through the coil before this instant and after the emission of the last spark. At the closing of the supply circuit, this current first increases to a predetermined value and then must be stabilized until the coil discharges, to prevent it from being subjected to excessive heating, and d 'elsewhere useless, from the moment when the energy stored in the coil is sufficient to allow the triggering of a suitable energy ignition spark. It is the stabilization of the current in the time interval called "regulation time" which precedes the ignition instant that the present invention is intended for, in its application to the ignition control of an engine with internal combustion.

We now refer to Figure 2 of the drawing where the reference numerals 1,2,3,4 and 8 correspond to corresponding components or bodies of the circuit according to the invention which are identical to those of the circuit of Figure 1, commented in the preamble to this description, a circuit in which the electronic control member takes the form of a transistor 2 of the NPN type.

In this figure, it appears that the electrode of control of this member, the base of transistor 2 in this case, is controlled by the output of a hysteresis comparator 9, the transfer characteristic of which is shown in Figure 3. The positive input of the comparator is connected to the reference voltage source 6, of value Uo, while the negative input is supplied by the voltage taken from the measurement resistor 3. According to the invention, an additional feedback loop 10 causes the collector voltage of transistor 2 to react on the basis of this transistor, to restore the conduction thereof when this voltage exceeds a predetermined threshold.

We now refer to Figs. 3 and 4 to explain the operation of the regulation circuit according to the invention.

When the switch 8 opens, the comparator sends a "high" signal on the base of transistor 2 (Fig. 3) which switches the transistor to its conductive state. The current I in the load 1, the transistor 2 and the measurement resistance then begins to increase (part a of the graph I = f (t) of Fig. 4A). The voltage drop across the measurement resistor then increases until it exceeds a value Uo + H / 2 corresponding to the crossing of the "low" switching threshold - H / 2 of comparator 9 (Fig. 3). Indeed, at the time of passage, the potential difference between the positive and negative inputs of the comparator is:
V⁺ - V⁻ (Uo - (Uo + H / 2) = - H / 2
At this time, the current in the inductive load reaches an IN + value (Fig. 4A). Switching the comparator output O to the "low" state then blocks transistor 2. This immediately results in an overvoltage at S, on the collector of the transistor. According to the present invention, this overvoltage then establishes a proportional action conduction of the transistor 2, by means of the feedback loop 10 which comprises a member 11 sensitive to this overvoltage. In a first embodiment of the invention, this member 11 is constituted by a Zener diode 12 mounted in reverse and a resistor 13 serial. When the overvoltage at S is such that the breakdown voltage of the diode is exceeded, a voltage is transmitted on the base of transistor 2 which then conducts again.

In a second embodiment, the member 11 is constituted by a resistor which conducts an unblocking current for the transistor 2 when the voltage at S reaches a predetermined threshold.

The conduction of transistor 2 decreases the voltage at S and the current I in the inductive load 1 (phase b on the graph I = f (t) of Fig. 4A). When I becomes lower than the value IN- which corresponds to a voltage drop Uo - H / 2 in the measurement resistance, there is again switching of the hysteresis comparator 9, the output of which returns to the "high" state to saturate the transistor (2) and thus cause a rise in the current in the inductive load 1.

In stabilized operation, the current in the load (1) and in the transistor (2) therefore oscillates between the values IN + and IN- while the voltage Vs in S oscillates between a controlled overvoltage close to the voltage E supplied by the source 4 and a value very close to zero (Fig. 4B). One can choose a hysteresis comparator 9, in which the thresholds - H / 2 and + H / 2 are very close, which provides a quasi-stabilization of the current in the load.

Of course, it is necessary to ensure the stability of the reaction loop 10 which makes it possible to achieve this result. According to a particularly advantageous characteristic of the present invention, the capacities and resistances of the correction network (not shown) used for this purpose are of much lower values than those of the corresponding components necessary in the circuit of the prior art shown in FIG. 1. Indeed, the critical frequency of loop 10 is much higher than that of the loop of FIG. 1. For example, in an application of the invention to regulating a current in an ignition coil of a combustion engine internal, a critical frequency of the order of 2 MHz has been noted for loop 10, to be compared with a critical frequency of approximately 30 kHz for the loop of a circuit of the type of that in FIG. 1. The time constants of the correction network associated with loop 10 are therefore lower, as are the values and the dimensions of the resistive and capacitive components used in this network. Therefore, they can easily be incorporated into an integrated circuit embodiment of the circuit according to the invention, which is an essential objective of the present invention. In some applications, the use of a correction network may even be completely eliminated. This will be the case, for example, when the overvoltage in phase b (Fig. 4) does not need to be precisely controlled. The gain of loop 10 can then be low and not require a correction network.

We can, at the limit, obtain a non-oscillating current in the inductive load. For example, in the case of current control in an internal combustion engine ignition coil, it is possible to ensure that the duration of phase b (Fig. 4) is longer than the desired regulation time of the current. This can be obtained with a loop 10 such that the threshold V for re-conduction of the transistor 2 is sufficiently close to the value E - Rc.I or Rc is the resistance of the inductive load 1 and E the supply voltage, l equality leading to a phase b of infinite duration.

de cette bascule étant connectée à la base du transitor 2. Ainsi, le transistor fonctionne en limiteur de tension à action proportionnelle, à chaque fois que le courant dépasse le courant nominal, pendant un intervalle de temps To. A l'expiration de cet intervalle, le transistor 2 est saturé de nouveau alors que le courant I repasse en dessous du courant nominal.According to another embodiment of the invention, shown in FIG. 5, the hysteresis comparator 9 is replaced by a conventional comparator 9ʹ, the output of which supplies a monostable flip-flop 14 of period To, the output

of this flip-flop being connected to the base of the transitor 2. Thus, the transistor operates as a proportional action voltage limiter, each time the current exceeds the nominal current, during a time interval To. At the expiration of this interval , transistor 2 is saturated again while current I goes back below nominal current.

Whatever the embodiment chosen of the circuit according to the invention, the current in the inductive load 5 can be interrupted at any time to create an overvoltage at the terminals thereof, by closing the switch 8 connected between the ground of the circuit and the base of transistor NPN 2, under the control of a signal T for example. It is, of course, necessary that the input impedance of the feedback loop 10 of the circuit is suitably chosen so as not to prevent and support this overvoltage without damage.

Thus, in an application of the circuit according to the invention in automotive electronics, it is possible to cause an overvoltage, at a predetermined instant, on the inductive load (5). When the latter is coupled by mutual inductance to a secondary circuit comprising a spark plug forming part of an internal combustion engine, it results from the closing of the switch 8 the generation of an ignition spark in this candle.

The circuit according to the invention likewise makes it possible to obtain stabilization and then rapid cut-off of the current in the winding of an electromagnet, such as that conventionally found in a solenoid valve for a fuel injector for a internal combustion.

Of course, the applications of the invention mentioned above are not limiting. The invention finds application, on the contrary, wherever it is necessary to regulate the current flowing in an inductive load, as is also the case, for example, in the control of the windings of electric motors. The invention also extends to all applications where the regulated current of the inductive load must be able to be suddenly interrupted, for example what is practiced in the operation of certain electromagnets where rapid demagnetization conditions the obtaining short response times.

It is clear that the invention is not limited to a current regulation circuit where the electronic control member takes the form of a transistor made in bipolar technology. The use of MOS or CMOS technology, for example, is particularly suitable for integrating the circuit according to the invention.

Claims (10)

1. An integrated circuit for regulating the current flowing in an inductive load supplied by a voltage source, comprising an electronic control member placed in series with this load and a comparator supplied by a first signal representative of a reference current intensity and by a second signal representative of the instantaneous intensity of the current flowing in the load and in the electronic control member, of which a control electrode is connected to the output of the comparator, this circuit being characterized in that it comprises a loop feedback (10) integratable stability correcting network, between the common terminal to the load and to the electronic control member, and the control electrode thereof, suitable for controlling a conduction with proportional action of this organ when the voltage on said common terminal reaches a predetermined threshold, this conduction replacing the saturation conduction of the organ established by the comparator, when the voltage at the common terminal is lower than the predetermined threshold, so as to oscillate the intensity of the current in the load around a nominal value. 2. Circuit according to claim 1, characterized in that said feedback loop comprises a resistor. 3. Circuit according to claim 1, characterized in that said feedback loop comprises a Zener diode mounted in reverse and placed in series with a resistor. 4. Circuit according to any one of the preceding claims, characterized in that the comparator has a hysteretic transfer characteristic, including the thresholds - H / 2; + H / 2 define the passages of the electronic control member (2) in the blocked state or in the conductive state in saturation, so as to oscillate the current in the load between two values (IN +, IN-) close by excess or by default, respectively, to the nominal current to be established in this load. 5. Circuit according to any one of claims 1 to 3, characterized in that the output of the comparator feeds a monostable rocker, one output of which feeds the control electrode of the electronic control member, which then operates as a limiter. proportional action voltage each time the current in the load exceeds the value of the nominal current, during a time interval To equal to the period of the flip-flop, so as to bring this current back to its nominal value. 6. Circuit according to any one of claims 1 to 5, characterized in that it takes the form of an integrated circuit. 7. Circuit according to any one of the preceding claims, characterized in that it comprises a switch for selectively connecting the control electrode to a blocking voltage of the electronic control member. 8. Application of the circuit according to claim 7, to the generation of an ignition spark for an internal combustion engine, characterized in that said blocking voltage causes the formation of a spark in a secondary circuit cast by induction mutual inductive charge. 9. Application of the circuit according to claim 7, to breaking the current in the winding of an electromagnet. 10. Application according to claim 10, wherein said winding is part of a solenoid valve for injecting fuel into an internal combustion engine.

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