CIRCUIT FOR CONTROLLING THE POWER SUPPLY OF AN ELECTRICAL LOAD, HAVING A DEVICE FOR DETECTING A SHORT-CIRCUIT OF THE LOAD

Abstract: The detection device comprises a comparator (6) which supplies a signal representative of a short-circuit state of the load (2) when the voltage at the terminals of that load, after filtering, is below a reference vol­ tage Vref. The device comprises means (7, 8) for rapidly pre­ charging the capacitor (C) of the filter (3), when the load is switched on. The initial time interval during which the output signal of the comparator is not representative of the presence or absence of a short­ circuit is thus reduced. Application to the control of actuators in motor vehicle electronics.

Description: The present invention relates to a circuit for controlling the power supply of an electrical load provided with a device for detecting a short-circuit of the load. More particularly, the invention relates to such a circuit designed for controlling the power supply of an inductive load whose detecting device monitors the possible short-circuiting of the load in order that a corrective action may be triggered in response intended to prevent, for example, damage to the circuit by this short-circuit of the load or defective functioning of a unit fitted with this load. In motor vehicle electronics, numerous actuators comprise such inductive loads. There can be mentioned the winding of a fuel injector or that of an electrovalve varying the pressure of a braking fluid in an anti-locking device for the wheels of a vehicle. It is furthermore possible to mention the coil of an inductive primary ignition circuit. The circuits controlling such inductive loads in a motor vehicle environment are often fitted with diagnostic means enabling the detection, for example, of a disconnection or an accidental short-circuit of the inductive load rendering the latter inoperative, this detection being made for reasons of safety for example. These means comprise electronic circuits which monitor the voltage existing at the terminals of the load in order to detect, for example, the possible appearance of a short-circuit and to trigger alarm actions or appropriate corrective actions. Considering the high interference environment of a motor vehicle, the voltage sampled at the terminals of the load must be filtered in order that it may be used, the filtered signal being compared with a reference voltage. When the filtered signal is below this reference voltage, the means of comparison used trigger the supply of a signal representative of a short-circuited state of the load. Figure 1 of the accompanying drawing shows a circuit for controlling the power supply of an actuator having an inductive load, of the prior art. The circuit comprises a power transistor 1 controlled by its base 5 in order to initiate the power supply of an inductive load 2 connected in series with the emitter-collector circuit of the power transistor, between the collector of the latter and ground. In the circuit shown, the power supply voltage +Vbat is supplied by the battery of a motor vehicle. As has been seen above, a filter 3, for example a low pass capacitive filter, takes the voltage from the terminal which is common to the inductive load 2 and the collector of the transistor 1 in order to filter out the interference from this signal before applying it to an input E of the control unit 4 which is designed to use the signal received from the filter 3. The unit 4 also controls the conduction of the power transistor via a terminal S connected to the base of this transistor, through an actuator control sub-circuit 5. When the control unit triggers this conduction, it is clear that the capacitor C of the low pass filter 3 begins to charge with a certain time constant. In Figure 3 of the accompanying drawing, there is shown at A the control signal Vs of the transistor 1 output at S by the control unit, and at B, the voltage V E at the terminals of the capacitor. Up to the instant T where V E reaches a value Vref of a reference voltage generated by the control unit 4 in order to derive, from a comparison of the voltage at the terminals of the load 2 after filtering with this voltage Vref, the presence or absence of a short-circuit of this load, it is clear that this comparison is not significant since the voltage at the terminals of the load is constantly lower than Vref, even in the absence of a short-circuit. It is therefore necessary to provide means for preventing the control unit from taking account of any short-circuit information up to the instant T. In order to do this it is possible to consider generating a delay of the output signal of the filter by analog or digital means. When the control unit conventionally comprises a microprocessor, the delay is obtained by means of a parallel loaded counter initialized by the microprocessor. Incidentally, using this assumption, the voltage signal at the terminals of the load could also be filtered by digital means. The masking logic necessary for obtaining a digital delay has the disadvantage of being expensive. In fact, the delay to be generated must in practice be able to reach values in the order of 200 µs which, for a resolution of 2 µs, involves the use of a parallel loaded 7-bit counter in order to be programmable, a requirement which additionally demands the presence of a 7-bit register and the associated logic. The assembly requires the integration of about 200 MOS transistors per counter resulting in an expensive production. Such a solution does not however enable a reduction in the length of the time interval during which a short-circuit of the load cannot be detected. As seen above, the generated delay must in practice be in the order of 200 µs, whereas experience shows that a short-circuit of the load for more than 100 µs not taken into account can affect the safety of the control circuit of the power supply of this load. It is therefore an object of the present invention to provide a circuit for controlling the power supply of an electrical load, comprising a device for detecting a short-circuit of the load, this circuit being provided with means enabling the duration during which a possible short-circuit of the load cannot be observed by the device to be reduced to the minimum. Another object of the present invention is to provide such an integrated circuit which is less expensive than that of the digital masking means mentioned above. Another object of the present invention is to provide such a device in which the characteristics of the device for detecting a short-circuit of the load are independent of those of the filter used for removing interference from the voltage sampled at the terminals of the load. Such a circuit is thus easily adapted to a modification of this filter. These objects of the invention, as well as others which will appear below, are achieved with a circuit for controlling the power supply of an electrical load, having a device for detecting a short-circuit of the load by comparison of a reference voltage with a voltage sampled at the terminals of the load and filtered in a capacitive filter, this circuit being characterized in that it comprises means, actuated by the switching on of the electrical load, for pre-charging the capacitor of the filter in order to initially establish on the output of this capacitor a voltage substantially equal to the reference voltage, the time constant of the charging of the capacitor by these means being low with respect to that which corresponds to the charging of this capacitor by the supply voltage of the load alone. These pre-charging means comprise a connecting transistor for connecting the capacitor of the filter to a pre-charging voltage source and a logic sub-circuit for triggering the conduction of the transistor when the load is switched on. In one embodiment of the circuit according to the invention, the comparison of the reference voltage with the voltage sampled at the terminals of the load is performed by a comparator whose inputs are respectively supplied by these voltages. The logic sub-circuit then comprises a logic AND gate having two inputs respectively connected to the output of the comparator and to a source of a validation signal of predetermined duration, the output of the AND gate supplying a control pin of the connecting transistor. The invention will now be described by way of example with reference to the accompanying drawing in which: Figure 1 is a diagrammatic representation of a circuit for controlling the power supply of an electrical load, provided with a device for detecting a short-circuit of the load, of the prior art, Figure 2 is a diagrammatic representation of a circuit for controlling the power supply of an electrical load, provided with a device for detecting a short-circuit of the load according to the present invention , and, Figure 3 shows graphs of the variations in time of voltages sampled in the circuits of Figures 1 and 2, these graphs being used for understanding the operation of the circuit according to the present invention. Reference is made to Figure 2 of the drawing in which the circuit shown again uses various elements of the circuit in Figure 1 which is part of the prior art, as explained in the preamble of the present description. In Figures 1 and 2 identical numbers denote identical or similar devices. In Figure 2, a power transistor 1 controls the electrical power supply of a load 2. The transistor 1 can be a transistor of the MOS technology type, or of the bipolar technology type as shown in the drawing in which it takes the form of a PNP type transistor connected by its emitter to the + pole of an electrical power supply source. In motor vehicle electronics, this power supply source is the battery of the vehicle which therefore supplies a voltage + Vbat. The load 2, typically an inductive winding in motor vehicle electronics, is then in this case part of an actuator, for example, an electrovalve which varies the pressure or the flow rate of a fluid. Such an electrovalve can thus initiate a supply of liquid fuel at a given time, or cut off this supply at a given time. Safety reasons lead to the monitoring of the correct operation of such an actuator and, for this purpose, the detecting of a possible disconnection or short-circuiting of its winding. For short-circuit detection, there is conventionally provided, as seen above, a device which samples the voltage at the terminals of the load 2 and which compares this voltage with a reference voltage Vref. When the sampled voltage is less than Vref it is derived from this that the load is short-circuited. The device then generates a signal processed by a control unit 4 for triggering corrective actions. The control unit typically comprises one or more microprocessors, memories and interfaces by which are received signals from sensors monitoring values associated with the functioning of the vehicle: engine intake pressure, speed of rotation of the engine, temperature of the air or of a cooling liquid etc... These signals are processed by the unit 4 which generates control signals for actuators which enable the adjustment of certain operating parameters for example. Thus the control unit 4 generates at its output S a signal for triggering the conduction or non-conduction of the transistor 1 via a control sub-circuit 5 of the actuator of which the inductive load 2 is a part. In motor vehicle electronics, in particular, the environment is highly disturbed. The short-circuit device detection must therefore be supplied by a signal from which the interference has been filtered out and which represents the voltage at the terminals of the load. For this purpose there is conventionally used a low pass capacitive filter 3 which comprises, for example, a capacitor C and a resistor R2 connected in parallel between the ground and one terminal of a resistor R1 whose other terminal is connected to the point common to the load 2 and the collector of the power transistor 1. At the point E′, which is the point common to the capacitor C and to the resistor R1, there is therefore available a voltage signal which is free of interference and which is compared with the voltage Vref in a comparator 6 in order to detect the presence of the possible short-circuit of the load. When the control unit triggers, by means of the actuator control 5 and the transistor 1, the power supply of the load, this power supply will also cause the charging of the capacitor C, through the resistor R1, with a time constant determined by the values of the resistors R1 and R2 and of the capacitor C. If what then occurs in the circuit of the prior art shown in Figure 1 is examined in conjunction with graph B in Figure 3 which shows the variation of the voltage at the output E of the filter of this circuit starting from the switching on at t = 0 of the load 2, triggered by the signal Vs applied by the control unit 4 at its output S, it can be seen that the voltage at E increases until it reaches, at the instant T, the value Vref at which the control unit 4 of the circuit in Figure 1 compares the filtered voltage in order to derive, in the case of the filtered voltage exceeding this value, the absence of any short-circuit of the load 2. Up to the instant T, with the filtered voltage remaining below Vref, the detection device could confuse this situation with that resulting from a genuine short-circuit, even in the absence of such a short-circuit of the load. In order to avoid such an erroneous detection, a masking of the information provided by the detection device is provided up to the instant T. In order to do this, it is possible to use an analog delay or digital masking means such as described in the preamble of the present description. The latter are expensive and do not solve the problem of the detection device not supplying significant information up to the instant T. Now if the load is already short-circuited at the instant t = 0, or if a short-circuit occurs between t = 0 and t = T, it is clear that the detection device of the circuit in Figure 1 is not capable of indicating this short-circuit before the instant T because of the masking. In motor vehicle electronics a value in the order of 200 µs is usually observed for the duration of the interval [0, T], taking into account the time constant of the filter necessary for a good removal of interference from the voltage signal sampled at the load. Now it has been observed that a short-circuit which remains uncorrected for more than about 100 µs could inflict damage on the circuit. According to the present invention such a situation is prevented by providing the detection device of the circuit with means, shown in Figure 2, which enable the time interval during which a short-circuit of the load cannot be indicated by the detection device to be reduced to a few microseconds only. These means comprise a voltage source V DD connected by means of a connection transistor 7 to the output terminal of the filter 3. The transistor 7 can, for example, be of the N channel MOS type, the source of the transistor being connected to the capacitor C, at the output of the filter, while the drain is connected to the voltage source V DD . The gate of the transistor is controlled by the output of a pre-charged control logic sub-circuit 8, this sub-circuit comprising two inputs 9 and 10 respectively connected to the control unit 4 and to the output of the comparator 6. The transistor 7, the sub-circuit 8 and the comparator 6 can be integrated with the control unit in order to form a single integrated circuit 11 controlling via its output pin S the switching on of the load 2, and receiving at its input pin E′ the filtered voltage signal sampled at the terminals of this load. This integrated circuit can of course comprise other input and/or output terminals in order to receive or to supply signals processed in the control unit for monitoring purposes or for the control of other functions. The circuit according to the invention therefore operates as follows. When the control unit supplies on its output pin S a signal commanding the switching on the load 2, this unit simultaneously sends to the input pin 9 of the pre-charge sub-circuit 8 a validation signal of predetermined duration To (see Figure 3 Graph D). This signal is received by a logic AND gate 12 which has a second input connected at 10 to the output of the comparator 6. As soon as the load 2 is switched on, the comparator 6 supplies a "high" level logic signal indicating that the voltage at the terminals of the load 2 is then less than Vref because of the starting of the charging of the capacitor C of the filter 3. The AND gate then immediately triggers the conduction of the transistor 7 which then causes the voltage V DD to be applied to the capacitor C which then charges very rapidly. In fact, the resistance R DSON of the transistor 7 when it is conducting is very low with respect to Rl and R2 and the time constant R DSON x C of the charging of the capacitor by the voltage source V DD is also low. Because of this low time constant, the capacitor C charges very rapidly as shown in full line in Figure 3, Graph C, which shows the variation of the voltage V E on the input pin E′ of the circuit 11. The voltage Vref is reached at the instant Toc. If the charge 2 is not then short-circuited, the voltage V E will continue to increase, which causes the output of the comparator 6 to switch over to the low state. The AND gate 12 cuts off and the transistor 7 stops conducting. The charging of the capacitor C of the filter 3 continues under the effect of the voltage +Vbat alone with the time constant of the filter 3 as shown in full line in graph C. The output of the comparator 6 is connected to an input pin 13 of the control unit. Thus from the instant Toc the control unit receives a "low" signal from the comparator representative of the absence of a short-circuit of the load 2. If the case of a load in a short-circuited state when this load is switched on is now considered, the voltage at E′ at the instant Toc will not continue to increase as shown in full line in the graph C but will decrease until it drops below the value Vref again, because the voltage +Vbat is then short-circuited at the input of the filter 3. This decrease causes the output of the comparator to switch over to the high state and therefore puts the transistor 7 back into conduction which recharges the capacitor C to the voltage Vref, causing another switchover of the comparator etc... In Figure 3, Graph E shows the switchovers of the comparator between the instants Toc and To, these switchovers producing a regulation of the voltage V E about the value Vref between these two instants, because of the loopback of the circuit of the gate 12 and the transistor 7 by the comparator 6. However, at the end of the validation pulse of duration To applied to the input 9 of the gate 12, this gate cuts off the transistor 7. The voltage V E then decreases because of the discharging of the capacitor C which is no longer charged neither by the source V DD nor by the source +Vbat because of the short-circuiting of the load 2. The output of the comparator then returns in a stable way to the high state to indicate the short-circuit of the load to the control unit 4. According to one embodiment of the invention given by way of example, the value of the duration To of the validation pulse is set at 2 µs. Thus by means of the invention, the control unit is informed of the short-circuit of the load at the end of 2 µs, compared with a duration of T in the order of 200 µs of the corresponding delay for the circuit of the prior art shown in Figure 1. An important advantageous characteristic of the circuit according to the invention depends on the production by the control unit of the validation pulse of duration To (Figure 3,D). The latter delays the production by the comparator of a signal representative of a state of short-circuit of the load by To. The value of To is chosen such that it is longer than Toc (Figure 3, C), the time required for the charging of the capacitor C by the source V DD to reach the value Vref. Now Toc is a function of the capacitor C of the filter 3 external to the circuit 11. This filter 3 must be able to be adapted to modifications of the load of the actuator to be monitored, without this resulting in a modification of the circuit 11. Such modifications can result, for example, from a technological development of the actuator incorporating this load or from a reduction in the sensitivity of the short-circuit detection device to induced interference, such modifications normally resulting from the development of products in time through the efforts of their designers. By suitably choosing To in such a way that To remains longer than Toc, whatever the foreseeable development of the actuator and of the associated filter may be, the characteristics of this circuit 11 are fixed and this circuit can subsequently be adapted to the foreseeable modifications of the filter. The characteristics of the short-circuit detection device of the control circuit according to the invention are thus rendered independent of subsequent developments of the filter. The means of pre-charging the capacitor C built into the circuit can easily be integrated at low cost because they require the integration of only one logic gate and one MOS transistor. Because the time period To to be masked is extremely short with the circuit according to the invention, it is possible to almost completely dispense with the masking logic described in the preamble to the present invention, which logic can contain several hundred MOS transistors. The saving in area for the integrated circuit is therefore considerable. The safety and reliability of the control circuit according to the invention are greatly improved with respect to those of the circuit in Figure 1 (of the prior art), because the response time of the short-circuit detection device is reduced to very small values, much less than the critical value of 100 µs mentioned above. By way of example, for an implementation of the invention in a control circuit for an electrovalve to cut off the supply of the fuel, the following values have been selected for certain parameters of the circuit:

Claims: Circuit for controlling the power supply of an electrical load (2), having a device for detecting a short-circuit of the load by comparison of a reference voltage with a voltage sampled at the terminals of the load (2) and filtered in a capacitive filter (3), this circuit being characterized in that it comprises means (7, 8), actuated by the switching on of the electrical load, for pre-charging the capacitor (C) of the filter (3) in order to initially establish on the output of this capacitor a voltage substantially equal to the reference voltage, the time constant of the charging of the capacitor by these means being low with respect to that which corresponds to the charging of this capacitor by the supply voltage of the load (2) alone. Circuit according to Claim 1, characterized in that the said means comprise a connecting transistor (7) for connecting the capacitor (C) of the filter (3) to a pre-charging voltage source (V DD ) and a logic sub-circuit (8) for triggering the conduction of the transistor (7) when the load is switched on. Circuit according to Claim 2, in which the inputs of a comparator (6) are respectively supplied by a source of reference voltage Vref and by the output voltage (V E ) of the filter, characterized in that the logic sub-circuit (8) comprises a logic AND gate (12) having two inputs (10, 9) respectively connected to the output of the comparator (6) and to a source (4) of a validation signal of predetermined duration (To), the output of the AND gate being connected to a pin controlling the conduction of the connection transistor (7), the output of the comparator supplying a signal representative of a state of short-circuit of the load (2) when the output voltage of the filter remains below Vref at the end of the validation signal (To). Circuit according to Claim 3, characterized in that it comprises a control unit (4) connected to a control pin of a transistor (1) controlling the flow of a current between the positive terminal of a power supply source (+Vbat) of the load (2) and that load, this control unit (4) also being connected to the AND gate (12) of the logic sub-circuit, in order to supply to the control pin of the control transistor (1) a signal for putting that transistor into conduction and, simultaneously, a validation signal to the AND gate (12) of the logic sub-circuit (8). Circuit according to Claim 4, characterized in that the control unit (4), the comparator (6), the connection transistor (7) and the logic sub-circuit (8) controlling this transistor constitute a single integrated circuit (11), the capacitive filter (3) being external to this integrated circuit. Circuit according to any of Claims 3 to 5, characterized in that the duration To of the validation signal, starting from the switching on of the load (2) is longer than the duration Toc of the charging of the capacitor (C) to the reference voltage Vref, when this capacitor is connected by the connection transistor (7) to the pre-charging voltage source (V DD ) Circuit according to any of the preceding Claims, characterized in that the capacitive filter is a low pass RC filter comprising a capacitor (C) and a resistor (R2) in parallel between the ground of the circuit and a terminal of a second resistor (R1) whose other terminal is connected to the load (2), at the end at which it is supplied, the other terminal of the load (2) being connected to ground. Circuit according to any of the preceding Claims, characterized in that the load is an inductive load forming part of an actuator.

Inventors: BAYLAC, BRUNO YVES HENRI, CASTEL, MICHEL
Applicants: SIEMENS AKTIENGESELLSCHAFT
Application year: 1989
Main classification: H02H3
IPC codes: H02H3, H03K17, F02D41, H02H1, F02D45, G05F1, H02M3