EP0307325A1 - Ignition control circuit - Google Patents

Abstract

The invention relates to a control circuit comprising: a double pole power switch (4) connected in series to the primary winding of an ignition coil (2) and a detection resistor (R1) connected to a voltage divider (R2, R3) supplying a voltage VD; a controlled amplifier- comparator (9), the first input of which receives the detected voltage and the second input of which receives a reference voltage (8) and the output of which is connected to the base of the switch (4), this amplifier-comparator acting to limit the base current when the detected voltage nears the reference voltage; a series resistor (R10) between the output of the amplifier-comparator (9) and the base of the switch (4); a differential amplifier (11), the inputs of which are connected to the terminals of the series resistor and the output of which is connected to the first input of the amplifier-comparator. <IMAGE>

Description

The present invention relates to an ignition control circuit and in particular a so-called electronic control circuit for motor vehicles in which the spark of a spark plug is obtained at the secondary of a voltage-raising coil at the moment when the current passing through the primary of this coil.

In recent years, it has become possible to use an electronic switch as a switch. FIG. 1 represents an example of such a conventional electronic ignition system comprising an energy storage battery 1 (the automobile battery) and a lifting coil 2, the secondary of which is connected to a spark gap 3 and the primary of which is in series with an electronic switch 4, such as an assembly of Darlington transistors, making it possible to let the current pass or interrupt the current in the coil. A control member 5 makes it possible to control, as a function of information received from the motor 6, the closing then the opening of the switch 4, this control member acting on the base of the main switch which, in the example shown, is bipolar. Thus, when the switch 4 a is closed, current begins to flow in the primary of the coil and gradually increase. When switch 4 opens, a spark occurs in the spark plugs 3.

For proper operation of this device, it is necessary that when the power switch 4 is opened, the current reached in the primary of the coil 2 is sufficient to cause the spark in the secondary. Ideally, provision should be made for the power switch 4 closing time so that this current value is exactly reached. In fact, since the motors operate at variable speed, it is difficult to predict this duration very precisely and it is generally necessary to close the power switch 4 for a period longer than necessary. If no action was taken, it would result in the passage of too much current through the primary of the coil 2 and the power switch 4.

To avoid these excessive currents, it has conventionally been necessary to provide means for limiting the current, an example of which appears in FIG. 1.

Thus, there is a detection circuit 7 in series with the power switch 4. This detection circuit comprises a resistor R1 of low value in parallel with a divider circuit comprising resistors R2 and R3, the voltage detected at the common point between the resistors R2 and R3 being proportional to the current passing through the resistor R1. This detected voltage V _D is compared to a reference voltage 8 in a controlled comparator amplifier 9 whose output supplies a basic current to the power switch 4. Thus, when the power switch 4 is closed, when the current is low in the resistor R1, the base current is maximum, and when the detected voltage approaches the reference voltage 8, adjusted to correspond to the limit current, the base current is reduced. In this phase where the base current of the power switch 4 is reduced, it no longer behaves like a simple switch but in fact like a linear current amplifier.

In this system, all of the elements 4, 7, 8 and 9 constitute a control loop. In practice, this loop may not be stable. The amplifier 9, produced in an integrated circuit, is a high gain amplifier, for example with three stages. Current technologies mean that this gain can vary for each stage by a factor of 3, for example between 100 and 300. Similarly, the gain of the power switch 4, which is for example a Darlington assembly with several stages, can vary considerably , for example by a factor of 30. Under these conditions, it is very difficult in practice to ensure a predetermined value for the loop gain of the servo and consequently to ensure the stability of the servo in current.

Thus, in practical systems, we are obliged to subject the Darlingtons to severe sorting, which limits the possibility of choosing suppliers and adds to costs, and we have also had to use an external capacitor in the control loop. stabilization which increases the number of terminals of the integrated circuit to be provided and is also a factor of cost increase.

An object of the present invention is to provide an ignition control circuit whose control in the current limitation phase is stable thanks to a predictable loop gain.

Another object of the present invention is to provide that this control circuit which is constituted in practice by a specialized integrated circuit can be used in connection with different bipolar Darlington transistors or.

More particularly, an object of the invention is to make the contribution to the loop gain of the amplifier contained in the control member reproducible when the latter is produced in the form of an integrated circuit.

Another object of the present invention is to make this contribution externally adjustable so as to be able to compensate for the differences in gain provided by the different types of power members.

To achieve these and other objects, the present invention provides a control circuit comprising a bipolar power switch in series with the primary of an ignition coil and a detection resistor; a voltage divider in parallel on the detection resistor providing a detected voltage proportional to the current in the resistance; a controlled comparator amplifier, the first input of which receives the detected voltage and the second input of which receives a reference voltage, the output of which is connected to the base of the power switch and the control input of which can receive a general inhibition order, this amplifier-comparator acting to limit the base current when the detected voltage approaches the reference voltage; a series resistor between the output of the comparator amplifier and the base of the power switch; and a differential amplifier whose inputs are connected to the terminals of the series resistor and whose output is connected to the first input of the comparator amplifier.

Preferably, this circuit further comprises means for inhibiting the action of the differential amplifier when the detected voltage is less than a selected threshold at most equal to the reference voltage.

Preferably, the differential amplifier comprises multi-collector transistors whose emitters are connected to the terminals of the series resistor, whose first collectors are interconnected and whose second collectors are connected to active loads consisting of transistors connected in current mirror .

Preferably, the resistors of the voltage divider are adjustable so that their ratio is adjusted to obtain a limitation of the current in the power switch to a predetermined value and that the absolute value of the resistance in series with the input of the amplifier-comparator is chosen to determine the gain of the servo circuit.

• la figure 1 est un schéma partiellement sous forme de blocs illustrant un circuit de commande d'allumage selon l'art antérieur ;
• la figure 2 est un schéma partiellement sous forme de blocs illustrant la présente invention ; et
• la figure 3 représente de façon détaillée des parties du circuit selon la présente invention.

These objects, characteristics and advantages as well as others of the present invention will be explained in more detail in the following description of particular embodiments made in relation to the attached figures, among which:

• Figure 1 is a diagram partially in the form of blocks illustrating an ignition control circuit according to the prior art;
• FIG. 2 is a diagram partially in the form of blocks illustrating the present invention; and
• Figure 3 shows in detail parts of the circuit according to the present invention.

In the schematic view of FIG. 2, we find the same elements as in FIG. 1 designated by the same references, namely the power transistor 4, the detector 7, the reference voltage source 8 and the amplifier-comparator. This high-gain amplifier-comparator 9 receives, as in the case of FIG. 1, on its second input a voltage reference 8 and on its first input a voltage V _D proportional to the current in the resistor R1, the coefficient of proportionality being fixed by the ratio of the values of the resistances R2 and R3.

The output of this amplifier 9 acts on the control terminal of the power switch 4 via a resistor R10 in series. This resistor R10 constitutes a means of measuring the input current into the control terminal, commonly the base, of the bipolar power switch 4.

In addition, a differential amplifier 11 of transconductance y _D (ratio between the variation of output current and the variation of input voltage) has its inputs connected to the terminals of the resistor R10. The output of this differential amplifier 11 is connected to the first input of the amplifier-comparator 9 in a direction such that the loop formed by this amplifier-comparator, the resistor R10 and the differential amplifier 11 performs a feedback.

In the absence of the feedback loop caused by the differential amplifier 11, the voltage on the resistor R1 is first divided in the ratio of the resistors R3 / R2 + R3 then amplified by the amplifier-comparator 9 with a gain in overall tension equal to:
[R3 / (R2 + R3)]. G (9),
where G (9) is the gain of amplifier-comparator 9 (a high gain that is not very reproducible). On the other hand, when the differential amplifier 11 introduces a feedback, and if G (9) is high, the overall gain is such that the error signal between the inputs of amplifier-comparator A is always weak; this implies that any variation of the current coming from the connection point between the resistors R2 and R3 is absorbed by the amplifier 11. We deduce the slope or transconductance between the top of the resistance R1 and the control terminal of the power switch 4 :
dI / dV = 1 / R2.R10.y _D.

Thus, this structure brings the following advantages:
- Despite all the variations which may affect the gain of the high gain amplifier-comparator 9, the transconductance between the detection resistor R1 and the control terminal can be maintained well defined provided that R2, R10 and y _D are defined . In fact, it is known that, while the voltage gain of an amplifier produced in the form of an integrated circuit is subject to large variations, which was the case with amplifier 9 in the use which was made of it, the transconductance of a differential amplifier is reproducible data. Thus, only the current gain of the power switch 4 is a cause of dispersion in the overall gain of the current control loop and the stability is easier to control.
- In addition, as the gain of this loop depends on the value of the resistor R2, it will be possible by choosing an adjustable resistor to modify this value to adapt the same integrated control circuit to power members of various types and characteristics. In manufacturing, it will therefore be possible to use a batch of power members whose gain would be particularly high by compensating for it by an increase in the resistance R2 which will be produced in the form of a discrete resistance external to the integrated control circuit.
- The oscillations which appeared in classical systems are eliminated. Indeed, the coil with its parasitic capacities constitutes an oscillating circuit which is excited for example ple when the power switch 4 switches from the closed switch mode (when a maximum base current is supplied) to the current regulator mode (when we begin to reduce, quite abruptly, the base current of the power switch), this is ie when the voltage across the coil drops suddenly from LdI / dt to a zero value (constant I). The large voltage variation at this time triggers damped oscillations which can disturb other organs connected to the circuit, for example, in automotive applications, the tachometer. To dampen these oscillations as best as possible, it is important that the impedance at the collector of the power switch remains low. For this, the gain of the control loop which tends to transform it into a source of high impedance must not be excessive. According to the invention, this gain can be controlled by adjusting the value of the resistance R2.

Thus the adjustment of the potentiometer R2, R3 has a double function:
by adjusting R2, the gain of the control loop is fixed,
by adjusting the ratio R2 / R3, the desired level of current limitation is fixed in the power switch.

A drawback of the circuit according to the present invention as described above could be that, from the initial phase of closing the power switch 4, the differential amplifier 11 begins to act to reduce the base current in this power switch and therefore to slow the rate of current increase in the coil primary. To overcome this drawback, it is provided according to the present invention to control the operation of the differential amplifier 11 by a circuit comprising a comparator 12 receiving on its first input the detected signal V _D at the connection point of the resistors R2 and R3 (as on the first input of the comparator amplifier 9) and on its second input a second source of reference voltage 13 with a value at most equal and preferably slightly lower than that of the reference source 8. Thus, the output of comparator 12 inhibits the operation of the differential amplifier 11 as long as the detected voltage, ie ie the current in the resistor R1, has not reached a certain threshold. It is only from this threshold that the feedback begins to act.

FIG. 3 represents a detailed embodiment of certain elements of the circuit shown very schematically in FIG. 2.

The amplifier-comparator 9 comprises an amplification chain made up of PNP T57 and NPN T59 and T62 transistors. At its input, there are two NPN transistors T51 and T55 mounted as a comparator, the emitter of transistor T51 being connected to an input terminal E9 which receives the signal V _D coming from the connection point of the resistors R2 and R3 mentioned above . The emitter of transistor T55 is connected to a resistor R8 which defines, in relation to a current source I8, the reference voltage represented by the voltage source 8 in FIGS. 1 and 2.

More specifically, the input comparator includes the following connections. The transistor T51 is connected by its collector to a current source I51 whose other terminal is connected to the supply voltage VCC. The emitter of transistor T51 is connected to terminal E9 as indicated above, the base of transistor T51 is connected to its collector and to the base of transistor T55. The collector of transistor T55 is connected to the supply voltage VCC by a current source I8 and its emitter is connected to ground via a resistor R8.

The signal at the output terminal 23 of the comparator is connected to the input transistor T57 of the amplifier circuit comprising the transistors T57, T59 and T62.

More specifically, the amplification part includes the following connections. The PNP transistor T57 whose base is connected to the output terminal 23 of the comparator has its transmitter connected to a current source I57 itself connected to the supply voltage VCC. The collector of transistor T57 is connected to ground by a transistor T45 maintained in the on state. In addition, the emitter and the collector of transistor T57 are connected to ground via NPN transistors T56 and T60, the role of which will be explained below. The emitter of transistor T57 is also connected to the base of the second NPN-type transistor T59 whose collector is connected to terminal VCC by a current limiting resistor R59 and whose emitter is connected to ground via a transistor T61 whose role will be explained below. The emitter of transistor T59 is also connected to the base of transistor T62 whose collector is connected to terminal VCC by a current limiting resistor R62 and whose emitter is connected to ground via a resistor R61 and to the output terminal S9 of circuit 9.

The three transistors T56, T60 and T61 contained in a block 30 constitute the inhibition control circuit of the amplifier 9. This circuit receives a signal from a control input terminal EC9 connected to a processing circuit signal receiving information from the motor 6. It will be seen that the circuit 30 also serves to disable the differential amplifier 11.

• 1 - quand un signal d'inhibition est envoyé sur la borne EC9, les transistors T56, T60 et T61 sont conducteurs et donc au­cun signal n'est appliqué sur les bases des transistors amplifica­teurs T59 et T62. La tension de sortie de l'amplificateur-­comparateur 9 sur la borne S9 est donc à bas niveau et aucun courant n'est fourni à la résistance R10 reliée à cette borne S9.
• 2 - Dans le cas où un signal de validation est appliqué à la borne EC9, les transistors T56, T60, T61 sont bloqués et l'étage amplificateur peut fonctionner.
  • 2.1 - Initialement, la tension sur la borne d'entrée E9 croît progressivement en partant de zéro. La tension sur les bases des transistors T51 et T55 croît également en restant supérieure à la tension d'entrée d'une tension base/émetteur de transistor. En première approximation, le potentiel d'émetteur du transistor T55 reste égal à celui du transistor T51 (E9) ; on en déduit le cou­rant de collecteur du transistor T55, égal à V_E9/R8 ; tant que ce courant reste inférieur à celui de la source de courant I8, le transistor PNP T52 reste bloqué. Le courant de la source de cou­rant I57 est donc totalement injecté dans la base du transistor T59 qui injecte lui-même un courant important dans la base du transistor T62 et une tension maximale apparaît sur la borne de sortie S9.
  • 2.2 - Dès que le courant V_E9/R8 devient égal à I8, la tension sur l'émetteur du transistor T55 est égale à I8.R8 et le reste. Alors, le transistor T57 commence à conduire et dérive une partie du courant de base du transistor T59 en provenance de la source de courant I57. Le courant des transistors T59 et T62 est donc limité pour réduire le potentiel à la borne de sortie S9.

The operation of the amplifier-comparator 9 in association with the circuit 30 is as follows:

• 1 - when an inhibition signal is sent to the EC9 terminal, the transistors T56, T60 and T61 are conductive and therefore no signal is applied to the bases of the amplifier transistors T59 and T62. The output voltage of the amplifier-comparator 9 on the terminal S9 is therefore at low level and no current is supplied to the resistor R10 connected to this terminal S9.
• 2 - In the case where a validation signal is applied to the EC9 terminal, the transistors T56, T60, T61 are blocked and the amplifier stage can operate.
  • 2.1 - Initially, the voltage on the input terminal E9 gradually increases starting from zero. The voltage on the bases of transistors T51 and T55 also increases while remaining higher than the input voltage of a base / emitter voltage of transistor. As a first approximation, the emitter potential of transistor T55 remains equal to that of transistor T51 (E9); the collector current of transistor T55, equal to V _E9 / R8, is deduced therefrom; as long as this current remains lower than that of the current source I8, the PNP transistor T52 remains blocked. The current from the current source I57 is therefore completely injected into the base of the transistor T59 which itself injects a large current into the base of the transistor T62 and a maximum voltage appears on the output terminal S9.
  • 2.2 - As soon as the current V _E9 / R8 becomes equal to I8, the voltage on the emitter of transistor T55 is equal to I8.R8 and the rest. Then, the transistor T57 starts to conduct and derives a part of the basic current of the transistor T59 coming from the current source I57. The current of transistors T59 and T62 is therefore limited to reduce the potential at the output terminal S9.

The differential amplifier 11 has its input terminals connected to the terminals of the resistor R10 and its output terminal A connected to the terminal A indicated in block 9 at the emitter of transistor T51, that is to say tell at terminal E9 where the detected voltage V _D is applied. This differential amplifier 11 includes two input resistors R11 and R12 connected to the emitters of PNP transistors with two collectors T63 and T70 mounted in differential, that is to say connected by their bases, and has a constant gain thanks to an interconnection of the second collectors. The first collectors are connected to ground by a load made up of two NPN transistors T76 and T77 connected in current mirror. The collectors of transistors T76 and T77 are respectively connected to the collectors of transistors T63 and T70, their emitters being grounded, their bases being interconnected and the collector and the base of transistor T77 being connected. The signal on the collector of transistor T76, which is the amplified image of the current in the resistor R10, is supplied to the output terminal A, via a transistor T78 mounted as a diode, to provide a feedback. .

As explained in connection with FIG. 2, it is desired to be able to inhibit the operation of the differential amplifier 11 in the first phase of closing the main switch, that is to say when the voltage V _D is less than a certain threshold itself lower than the threshold defined by the reference voltage 8. The inhibition action is ensured by the transistor T79 whose collector is connected to the bases of the transistors T63 and T70 by means of a resistor R79, whose transmitter is connected to ground, and whose base is connected to a terminal B putting this conductor in conduction when one wants to inhibit the differential amplifier.

The control of the transistor T79 is ensured by a circuit comprising, between the voltage VCC and the ground, a resistor R16, a transistor T43 and a resistor R20. The base of transistor T43 is connected to the collector of transistor T45 and the emitter of transistor T43 is connected to the base of transistor T79. Thus, when the current in the transistor T57 and in the transistor T45 exceeds a certain threshold, the transistor T43 becomes conductive which turns on the transistor T79. By this means the amplifier 11 starts to operate very shortly before the moment when it is desired to limit the current in the power switch 4.

The output current of the differential amplifier 11 is available on the collectors of the transistors T63 and T76. The role of transistor T78, connected as a diode, is to allow transistor T76 to operate with a sufficient collector voltage; indeed the terminal E9 always remains at a potential close to that of the ground.

The advantages of this achievement are as follows:
- slope of the amplifier 11 which is easily calculable and sparsely dispersed (depends only on the bias current sent by the resistor R79);
- low phase rotation, because there is only one amplification stage (the transistor T55, the following transistors being followers which multiply the current);
- the system has no other power source than the voltage present on the control terminal of switch 4;
- the amplifier 11 is put into service or out of service very simply:
. if the transistor T79 is conductive, the resistor R79 supplies a basic current to the transistors T63 and T70 and the amplifier 11 operates;
. if the transistor T79 is blocked, the amplifier 11 does not work and has no influence.

The circuit is arranged so that the transistor T79 becomes conductive very shortly before the current in the coil reaches the set value. So :
- the gain of the servo remains very high (amplifier 11 off) for almost the entire time of current rise in the coil (period when it is important that the switch 4 receives the maximum base current);
- The gain of the control falls to a low value (amplifier 11 in operation) shortly before the set value is reached, thus providing the advantages which have been described.

Of course, the invention is in no way limited to the above embodiment which has only been described by way of example, and numerous variants and additions can be made by those skilled in the art. For example, as shown in FIG. 3, provision could be made to continuously pass through the transistor T51 a current substantially equal to I8. This is achieved by the transistor T50 associated with the transistor T46 and with the current source I46. Likewise, provision could be made for additional stabilization of the amplifier-comparator 9 by bringing a portion of the output voltage of this amplifier to the transistors T55 and T51 by means of a time constant.

Claims (5)

1. Ignition control circuit comprising:
- a bipolar power switch (4) in series with the primary of an ignition coil (2) and a detection resistor (R1),
- a voltage divider (R2, R3) in parallel on the detection resistance providing a detected voltage proportional to the current in this resistance (R1),
- a controlled comparator amplifier (9), the first input (E9) of which receives the detected voltage (V _D ) and the second input of which receives a reference voltage (8), the output of which is connected to the base of the power switch ( 4) and of which a control input (EC9) can receive an inhibition order signal, this amplifier-comparator acting to limit the base current when the detected voltage approaches the reference voltage,
characterized in that it further comprises:
a series resistor (R10) between the output of the comparator amplifier (9) and the base of the power switch (4), and
- a differential amplifier (11) whose inputs are connected to the terminals of the series resistor and whose output (A) is connected to the first input (E9) of the comparator amplifier (9). 2. Ignition control circuit according to claim 1, characterized in that it further comprises means (12; B, T79) for inhibiting the action of the differential amplifier (11) when the detected voltage is lower at a selected threshold at most equal to the reference voltage. 3. Ignition control circuit according to one of claims 1 or 2, characterized in that the differential amplifier (11) comprises transistors with two collectors (T63, T70) whose emitters are connected to the terminals of the series resistor (R10), whose first collectors are interconnected and whose second collectors are connected to active loads (T77, T78) made up of transistors (T77, T78) connected in current mirror . 4. Ignition control circuit according to any one of claims 1 to 3, characterized in that the resistors (R2, R3) of the voltage divider are adjustable so that their ratio is adjusted to adjust the value of the limit power switch current and that the absolute value of the resistance (R2) in series with the input of the comparator amplifier is chosen to determine the gain of the servo circuit. 5. Ignition control circuit according to claim 1, characterized in that it further comprises means (30) for starting or abruptly interrupting the action of the amplifier-comparator (9).

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