Abstract:
A circuit comprising a switch series connected to a load; a first and second current recirculating branch alternately connectable parallel to the load, for reducing the current in the same; and a logic control unit for opening and closing the switch and recirculating branches, so that the load is supplied with a current rising to a peak value and then falling rapidly to and oscillating about a lower hold value; a transistor being provided for reducing the voltage supplied to the load by the first recirculating branch at the end of the fast fall phase, so as to eliminate uncontrollable operating zones and prevent the load current from falling below the hold value.

Description:
This is a continuation of application Ser. No. 07/810,950 filed Dec. 19, 1991, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an inductive load drive circuit, particularly for fuel injectors. 
     On automotive electronic injection systems, fuel supply is enabled by means of an electronically controlled valve, operation of which is controlled by the magnetic field produced by an electromagnet roughly describable as an inductor wound about a core and through which a control current is supplied. 
     To reduce dissipation, control is effected in two phases: a first phase requiring a strong magnetic field for opening the valve (peak phase); and a second phase in which the valve is kept open (hold phase), and in which a lower magnetic field and, consequently, a lower control current are required. 
     FIG. 2 shows a rough half line graph of the control current I L  of an injector. As can be seen, the peak phase extends up to instant t 1 , with current I L  increasing up to a maximum value I p . This is followed by phase t 1  -t 2  in which the current falls sharply, depending on application requirements; an uncontrollable phase t 2  -t 3  ; and, from t 3  onwards, the actual hold phase, chopped to prevent active elements in a linear zone resulting in dissipation. 
     The passage from peak current I p  to the hold current (ranging between a maximum I HMAX  and a minimum I HMIN ) must be effected rapidly, for which purpose provision is made for recirculating high voltage current (freewheeling zone), i.e. to reduce the current in the inductor, this is supplied with a high voltage for forcing a fall in current. 
     The high speed required in passing from the peak to the hold current results in magnetic problems, the effect of which is to create a zone (interval t 2  -t 3  in FIG. 2) that is hard to control. 
     As the presence of uncontrollable zones may result, in some cases, in malfunctioning or at any rate in impaired reliability of the circuit, such zones must perforce be eliminated. One known method of doing this is to maintain the high speed recirculating or free-wheeling phase until the current in the load drops to a so-called &#34;undershoot&#34; value, lower than that of the hold current, and to only subsequently commence the hold phase. The corresponding current pattern in the inductor is as shown in FIG. 3, wherein the recirculating phase is maintained up to instant t 4 , at which point the recirculating current reaches the undershoot value I UND , marking the start of the hold phase in which the current in the inductor oscillates between I HMAX  and I HMIN  as in FIG. 2. 
     The above solution, however, is also unsatisfactory, in that it requires a highly accurate I UND  value at which to arrest the high speed recirculating phase, to prevent too low a current value, and consequently too low a magnetic field, from closing the valve. The attainment of a sufficiently high degree of accuracy inevitably results in difficulties (again affecting the reliability of the circuit) or at any rate in complex design and high manufacturing cost of the circuit. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a drive circuit of the type described above, designed to eliminate the presence of uncontrollable zones without reducing the control current below the hold value, thus safeguarding against undesired closure of the valve, and providing for maximum reliability, straightforward design and low-cost manufacture of the circuit. 
     According to the present invention, there is provided an inductive load drive circuit, particularly for fuel injectors, as claimed in claim 1. 
     The present invention is based on maintaining the high speed recirculating phase until a value close to the hold value range is reached, thus preventing it from falling below the minimum hold value, and in subsequently forcing a slower reduction to the minimum hold value. The follow-up current reduction phase is effected by the same branch as the high speed recirculating phase, and is controlled accurately so that the high speed recirculating branch applies a predetermined recirculating voltage, lower than that of the high speed recirculating phase. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: 
     FIG. 1 shows an operating block diagram of the circuit according to the present invention; 
     FIG. 2 shows a current graph relative to a known circuit; 
     FIG. 3 shows a current graph relative to a further known circuit; 
     FIG. 4 shows a current graph relative to the FIG. 1 circuit according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, the electromagnet controlling the valve on the injector is shown schematically by inductor L, which also represents the load of control circuit 1 according to the present invention. 
     Inductor L is connected between a supply line V CC , constituting a first reference potential line, and point A, which is grounded (constituting a second reference potential line) via a controlled power switch 2 consisting, in this case, of a DMOS transistor, and a sensing resistor 3. The mid point S between transistor 2 and resistor 3 is connected to a first input of four comparators 4, 5, 6, 7 forming part of a logic control unit 14. That is, point S is connected to the inverting inputs of comparators 4 and 7, and to the non-inverting inputs of comparators 5 and 6, while the non-inverting inputs of comparators 4 and 7 are connected respectively to reference voltage sources 8 and 11, and the inverting inputs of comparators 5 and 6 to respective sources 9 and 10. Source 9 supplies a voltage V 1  equal to that at the terminals of resistor 3 when supplied with current I p  ; sources 8 and 10 supply a voltage V 2  corresponding to current I HMAX  ; and source 11 supplies a voltage V 3  corresponding to current I HMIN . 
     The output of comparator 4 is connected to a MOS control transistor 16, the source terminal of which is grounded, and the drain terminal of which is connected to the base of a PNP transistor 17, the emitter of which is connected to point A, and the collector of which is connected to the mid point of a series of Zener diodes 18 1 , 18 2 , . . . , 18 i , 18 i+1 , . . . , 18 n . Diodes 18 are connected in the same direction, with the cathode of diode 18 n  connected to point A, and the anode of diode 18 1  series connected to the emitter of a PNP transistor 20. Transistor 20 presents its base connected to supply line V CC , and its collector connected to a point P connected directly to the control terminal of switch 2, and grounded via resistor 21. Point P is also connected to the drain terminal of a MOS P channel transistor 22, the source terminal of which is connected to supply line V CC , and the gate terminal of which is connected to an output of logic control unit 14 and, via resistor 23, to the supply line. 
     Logic unit 14 presents a further output connected to the base terminal of a recirculating PNP transistor 26, the collector of which is connected to the supply line, and the emitter to point A. 
     In addition to comparators 4-7, logic unit 14 also comprises an input comparator 30 having its non-inverting input connected to input terminal 31 of circuit 1 and receiving injection control signal IN; and its inverting input connected to a source 32 supplying a reference voltage V 4 . The output of comparator 30 drives a MOS control transistor 33 having its source terminal grounded, and its drain terminal connected to the gate of transistor 22. The output of comparator 30 is also connected to the drain terminal of a further MOS transistor 34, the source terminal of which is grounded, and the gate terminal of which is connected to output Q of a storage element or flip-flop 35. Flip-flop 35 presents an input S connected to the output of an OR circuit 36 having two inputs connected respectively to the outputs of comparators 5 and 6. 
     The output of comparator 7 is connected to the set input S of a second flip-flop 38 and to the drain terminal of a MOS transistor 39, the source terminal of which is grounded, the gate terminal of which is connected, via inverter 40, to output Q of a further flip-flop 50. Flip-flop 50 presents its set input S connected to the output of comparator 5, and its reset input R connected to the output of an OR circuit 51, one input of which is connected to the output of comparator 7, and the other input of which receives the inverted value of injection control signal IN. Reset input R of flip-flop 38 is connected to the inverted value of injection control signal IN, while output Q is connected to the gate terminal of a MOS transistor 42, the source terminal of which is grounded, and the drain terminal of which is connected to the base of recirculating transistor 26. The output of flip-flop 38 is also connected, via inverter 44, to the gate terminal of a MOS transistor 45, the source terminal of which is grounded, and the drain terminal of which is connected to the output of comparator 6. 
     Output Q of flip-flop 38 is also connected to a first input of an AND circuit 46, the other input of which is connected to the output Q of flip-flop 35. Via a delay element or timer 47, e.g. capacitive type, the output of circuit 46 is connected to one input of an OR circuit 48 having a second input receiving the inverted value of injection control signal IN, and a third input connected to the output of comparator 7. Finally, the output of OR circuit 48 is connected to the reset input of flip-flop 35. 
     Operation of circuit 1 will be described with reference to FIG. 4. At the start, when signal IN is low, flip-flop 38 and, via circuit 48, flip-flop 35 are reset, so that output Q is also low. Similarly, flip-flop 50 is reset via circuit 51, so that its output is low, thus switching on transistor 39, which maintains a low output of comparator 7. Also, the output of comparator 30 is low, switch 2 is open, and no current flows through L. 
     As soon as signal IN switches to high (instant t O ), comparator 30 switches, so as to switch on transistor 33, and consequently transistor 22, and close switch 2. Inductor L is thus connected between supply V CC  and ground, and begins conducting an increasing current. Initially (as long as the voltage drop of resistor 3 is less than V 2 ) comparator 4 supplies a high output signal, but the voltage drop at the base-emitter junction of transistor 17 is such that the transistor remains off. Moreover, the output of comparator 6 is kept low by activated transistor 45. 
     When the current in the inductor reaches the peak value I p  (instant t 1 ), comparator 5 switches to high, thus switching flip-flop 35, which in turn turns on transistor 34, turns off transistors 33 and 22, and opens switch 2. Consequently, voltage V L  at the terminals of inductor L rises rapidly to a value V CL  equal to: 
     
         V.sub.L =V.sub.CL =V.sub.CC +n*V.sub.z +V.sub.BE20 
    
     where V BE20  is the base-emitter voltage drop of transistor 20; V z  is the breakdown voltage of each Zener diode; and n the number of Zener diodes 18. 
     Switching of comparator 5 also switches flip-flop 50, which receives a high signal at input S and, via inverter 40, turns off transistor 39, thus releasing the output of comparator 7, which nevertheless remains low. When voltage V CL  is reached, Zener diodes 18 and the base-emitter junction of transistor 20 are biased to such a value as to turn on transistor 20 in the active region and diodes 18 in the Zener zone. Transistor 20 therefore supplies the gate of transistor 2 with such a current as to again turn on (close) transistor 2. Resistor 21 in particular is so sized as to guarantee the bias current of Zener diodes 18 and transistor 20, while maintaining transistor 2 in the saturated zone and preventing a fall in voltage at point A, which would turn off Zener diodes 18 and, consequently, switch 2. The branch consisting of transistor 20 and diodes 18 locks the voltage at the terminals of inductor L to value V CL , so that current I L  falls steadily, as shown in FIG. 4 (interval t 1  -t 5 ). 
     When current I L  reaches the maximum hold value I HMAX  at instant t 5 , the output of comparator 4 switches to high, and turns on control transistor 16 and, consequently, transistor 17, which saturates. This therefore short-circuits diodes 18 i+1  -18 n  connected between the collector and the emitter, so as to reduce the voltage at the terminals of inductor L to a value for V CL&#39;  equal to: 
     
         V.sub.CL&#39; =V.sub.CC +V.sub.BE20 +i*V.sub.z +V.sub.CE17 
    
     where &#34;i&#34; is the number of the on Zener diodes; and V CE17  is the collector-emitter fall in voltage of transistor 17. 
     Inductor L therefore continues discharging, but less rapidly (and consequently less sharply). This phase lasts up to instant t 6 , at which point, comparator 7, detecting voltage V 3  on resistor 3, i.e. corresponding to current value I HMIN , switches to high, thus switching flip-flop 38. Output Q of flip-flop 38 therefore switches to high, which turns on transistor 42, thus enabling the recirculating circuit including PNP transistor 26, and turns off transistor 45, thus enabling the output of comparator 6, which nevertheless remains low. Via OR circuit 48, the high signal at the output of comparator 7 also resets flip-flop 35, the output Q of which switches to low, thus turning off transistor 34 and activating transistor 22 and switch 2, so that the current in inductor L rises. Finally, via circuit 51, the high signal at the output of comparator 7 resets flip-flop 50, which turns on transistor 39 for again maintaining a low output of comparator 7. 
     The current in the inductor therefore continues rising until it reaches value I HMAX  (instant t 7 ), at which point, the output of comparator 6 switches to high, thus switching output Q of flip-flop 35 once more to high, and turning off transistors 33, 22 and switch 2. The opening of switch 2 again causes an increase in the voltage at point A, which, in this case, rises high enough to start PNP transistor 26. The current therefore decreases through transistor 26, but, as the voltage is not sufficient for turn on the recirculating branch including transistor 20 and diodes 18, and therefore closing switch 2, the recirculating current does not flow through resistor 3. The end of this phase is determined by the switching of timer 47, which, enabled by circuit 46 receiving two high input signals, after a given time period (that required for lowering current I L  to roughly the I HMIN  value) resets flip-flop 35, thus turning off transistor 34 and closing switch 2 (instant t 8 ). 
     The current in the inductor therefore increases once more, as following instant t 6 , and the hold phase continues in this way, supplying the inductor with a hold current oscillating between I HMAX  and I HMIN , thus ensuring that the injector valve remains open. 
     The advantages of the FIG. 1 circuit according to the present invention will be clear from the foregoing description. By applying a predetermined recirculating voltage lower than that of the fast fall phase, immediately following the fast fall phase and commencing from a value higher than I HMIN , the circuit according to the present invention provides for reducing the current in perfectly controlled manner, thus eliminating the uncontrollable zone, which would otherwise impair the reliability of the injector-control circuit system. 
     Moreover, by eliminating the undershooting phase, the circuit according to the present invention, which is both easy to produce and readily integratable, ensures against undesired closure of the valve. 
     Finally, the circuit according to the present invention provides for troublefree variation of the voltage in the settling or slower recirculating phase as a function of the load, by varying the number of short-circuited Zener diodes. 
     To those skilled in the art it will be clear that changes may be made to the circuit as described and illustrated herein without, however, departing from the scope of the present invention. In particular, logic unit 14 may be employed differently, providing switch 2 and the recirculating branches are so controlled as to produce the FIG. 4 pattern.