Method and corresponding circuit for detecting an open load

Abstract: The invention relates to a method for detecting an open load by means of a driver having at least one main power transistor (M10) connected to the load (L) and one auxiliary transistor (M11) connected in parallel with the main transistor (M10) between a first power supply voltage reference (Vs) and a second voltage reference (GND), the method providing a comparison between a first voltage (V IN1 ) present on a terminal (S10) connected to the load of the main transistor (M10) and a second voltage (V IN2 ) present on a terminal (S11) of the auxiliary transistor (M11). The invention also relates to a circuit for detecting an open load (L), in which the said method is implemented.

Description: Scope This invention relates to a method for detecting an open load. More particularly, the invention relates to a method for detecting an open load by means of a driver having at least one main power transistor connected to the load and one auxiliary transistor connected in parallel with the main transistor between a first power supply voltage reference and a second voltage reference. The invention also relates to a circuit for detecting an open load, of the type comprising a first main transistor connected to the load and a second auxiliary transistor connected in parallel with the main transistor between a first power supply voltage reference and a second voltage reference, together with a control circuit connected between the terminal of the said main transistor connected to the load and a gate terminal of the said auxiliary transistor, and connected to the first power supply voltage reference. A detection circuit of this type may be used in the field of programmable process controllers, which are used, for example, in automated production lines of industrial systems. An automated production line essentially comprising a processing unit, or processor, which exchanges information with peripheral units located along the production line. These peripheral units may be, for example, solenoid valves, lamps, motors, etc., and the connections to the processor are made by connecting wires up to 400 metres in length, terminating at an input/output interface. A good diagnostic system associated with the processor is able to minimize the time required for tracing faults, and to reduce the cost of identifying and rectifying them. An example of such a cost is that associated with the loss of production which occurs immediately in an assembly line as a result of the interruption of a line or a break in the winding of a solenoid valve. In this context, it is particularly useful to be able to measure the absence of current in an output stage of the processor. The absence of current indicates the interruption of a connecting wire or a break in a winding of an actuator present in a peripheral. The invention relates particularly, but not exclusively, to a detection circuit for an output stage of an industrial process controller made with MOS power transistors, and the following description refers to this field of application with the sole purpose of simplifying the explanation of the invention. Prior art As is well known, the output stage 1 of an industrial processor is normally made with MOS power transistors in a "high side driver" configuration, as illustrated in the attached Figure 1. This output stage 1 comprises a power switch, in particular a MOS power transistor M1, having a drain terminal D1 connected to a power supply voltage reference Vs, a source terminal S1 connected to a second voltage reference, for example an earth GND, through a load L, and a gate terminal G1 connected to a sensor 2. More particularly, the power transistor M1 comprises a number N1 of elementary MOS cells. The sensor 2 is connected in turn to the power supply voltage reference Vs and to the second voltage reference GND through a reference voltage generator GN1. The sensor 2 is also connected to a first non-inverting (+) input terminal I1 of a comparator 3. A second, inverting (-) input terminal I2 of the comparator 3 is connected to the source terminal S1 of the power transistor M1. In particular, the sensor 2 comprises a MOS transistor M2, having a drain terminal D2 connected to the power supply voltage reference Vs, a gate terminal G2 connected to the gate terminal G1 of the power transistor M1, and a source terminal S2 connected to the reference voltage generator GN1. The transistor M2 comprises a number N2 of elementary MOS cells. A conventional method of diagnosis of the output stage 1 provides for the detection of the value of a saturation voltage V SAT of the power transistor M1, in other words the value of the voltage drop across the saturation resistance of the power transistor M1. This method, however, cannot be used to detect variations of a few mA when the power transistor, at low saturation resistance, is fully on. In fact, according to the circuit configuration shown in Figure 1, a current I 1 is present on one output terminal O1 of the comparator 3, as determined by the following equation: I 1 = (N1/N2) * I REF - Voff/Ronp where I REF is the reference current supplied by the generator GN1, Voff is the offset voltage of the comparator 3, and Ronp is the drain-source resistance of the power transistor M1 when it is on. The term Voff/Ronp limits the minimum current detectable with this configuration when the power transistor is on and, consequently, the saturation voltage V SAT determined by the equation V SAT = R SAT * I I where R SAT is the saturation resistance of the power transistor M1. In the case of an output stage 1 of an industrial process controller it may be necessary to measure variations of output current I 1 less than 3 mA for a power transistor M1 with a saturation resistance R SAT of 70 mΩ, in other words a saturation voltage V SAT of approximately 210 µV, which is therefore too small to be detected by the technique illustrated above. A first solution to enable small variations of output current to be measured has been described in the article "Driver intelligenti per controllori industriali" (Intelligent drivers for industrial controllers) by Herbert Sax and Michael Barou, published in the journal "Elettronica Oggi" on 30 June 1992. The method proposed in this article can be applied with a circuit architecture 4 illustrated in Figure 2. The architecture 4 comprises a MOS power transistor M3 connected between a first power supply voltage reference Vs and a circuit node X1 connected to a first inverting (-) input terminal I3 of a comparator 5. This comparator 5 has a second, non-inverting (+) input terminal I4 connected to the power supply voltage reference Vs through a constant voltage generator GN2. A load L is connected between the circuit node X1 and a second voltage reference, for example an earth GND. The power transistor M3 has a gate terminal G3 connected to a gate terminal G4 of an auxiliary MOS transistor M4 connected between the said circuit node X1 and the power supply voltage reference Vs and connected to this reference through a detection resistance R1. The auxiliary transistor M4 comprises a number N4 of elementary MOS cells which is a submultiple of the number N3 of cells contained in the power transistor M3 (N4 = N3/n, where n is any whole number). Part of a load current I L runs through the transistor M3. The transistors M3 and M4 have their source terminals, S3 and S4 respectively, connected to the load L. They also have their gate terminals D3 and D4 in common and connected to a control circuit 6 connected between the second voltage reference GND and a third control voltage reference V CP . The comparator 5 has an output terminal 02 connected to the said control circuit 6. The auxiliary transistor M4 has its drain terminal D4 connected to a first inverting (-) input terminal I5 of a further comparator 7. The comparator 7 has a second non-inverting (+) input terminal I6 connected to the power supply voltage reference Vs through a voltage generator GN3 and an output terminal 03, which signals when the load L is open. In particular, the control circuit 6 comprises a further MOS transistor M5 having a source terminal S5 connected to the second earth reference GND, a gate terminal G5 connected to the output terminal 02 of the comparator 5 and a drain terminal D5 connected to the gate terminal G4 of the auxiliary transistor M4 and to a switch SW1, which in turn is connected to the third control voltage reference V CP through a further current generator GN4. The switch SW1 is controlled by an ON signal external to the architecture 4. We shall now examine the method of detection according to the architecture 4, which is based on the measurement of the current passing through the resistance R1. To ensure that the voltage drop across the transistors M3 and M4 does not approach zero for very small load currents, or for an open load L, while maintaining the voltage drop across the resistance R1 at measurable levels, the architecture 4 provides for the control circuit 6 to be connected in a negative feedback loop which connects the circuit node X1 to the gate terminal G4 of the transistor M4 through the comparator 5. This feedback loop prevents the voltage drop between the source terminal S3 and the drain terminal D3 of the transistor M3 from falling below a voltage V R supplied by the voltage generator GN2. In other words, with small load currents the two MOS transistors M3 and M4 operate in saturation conditions, working in analog mode and no longer as switches. Although certain aspects of this first solution are advantageous, it has a number of disadvantages, as follows: the transistors M3 and M4 have their drain terminals D3 and D4 separate: they cannot therefore be made in a single pocket, and this diminishes the precision of the device, since the coupling between the structures becomes less effective. Moreover, in the technologies for construction of integrated circuits in which the drain terminal is made to coincide with the substrate on which the circuit is formed, for example in VIP technology, the circuit architecture 4 cannot be produced; the voltage drop across the detection resistance R1 alters the drain-source voltage of the auxiliary transistor M4, thus changing the value of the current mirrored by it and further reducing the precision of the circuit. This last problem is highly disadvantageous in cases in which the open load current to be detected is very small; in these conditions, the transistors M3 and M4 operate below the threshold, in other words with drain voltages highly dependent on the drain-source voltage altered by the voltage drop across the resistance R1. A second known solution, based on a completely different approach to the circuit from that described immediately above, is described in the article "A New Open Load Circuit for Very Low Current Level Detection" by Jean-Luis Siaudeau, published in "Power Conversion June 1994 Proceedings". This method of detection is used in the device 8 illustrated in Figure 3. The device 8 comprises a MOS power transistor M6 connected between a first power supply voltage reference Vs and an output terminal 03. The device 8 comprises a further auxiliary MOS power transistor M7 connected between the power supply voltage reference Vs and the output terminal 03, which in turn is connected to a second voltage reference, for example an earth GND, through the load L. The transistors M6 and M7 have their drain terminals D6 and D7 and their source terminals S6 and S7 connected to each other, and also to the power supply voltage reference Vs and to the output terminal 03 respectively. The transistor M6 receives a main control signal ON1 at its gate terminal G6, while the auxiliary transistor M7 has its gate terminal G7 connected to the gate terminals G8 and G9 of two further MOS power transistors M8 and M9 respectively, connected in parallel with each other, between the power supply voltage reference Vs and the second voltage reference GND. The transistors M8 and M9 have their drain terminals D8 and D9 connected to each other and to the power supply voltage reference Vs and receive a common auxiliary control signal ON2 at their gate terminals G8 and G9. The transistor M8 has its source terminal S8 connected to the second voltage reference GND through a current generator GN5 and to a first input terminal I7 of a control logic circuit 9 which in turn is connected between the output terminal 03 and a second output terminal 04. The transistor M9 has its source terminal S9 connected to the second voltage reference GND through a further current generator GN6 and to a second input terminal I8 of the control logic circuit 9. The power transistors M6 and M7 of the device 8 have their drain terminals D6 and D7 in common. This solution may therefore be implemented in technologies in which the drains of the power transistors coincide with the substrate of the integrated circuit, such as VIP technology. The device 8 also uses an auxiliary power transistor M7 with a saturation resistance R SAT2 which is higher than the saturation resistance R SAT1 of the power transistor M6, known as the main transistor. In normal operating conditions, the two transistors M6 and M7 operate in parallel, and the saturation resistance R SAT of the device 8, which is the result of the parallel connection of the saturation resistances R SAT1 and R SAT2 of the transistors M6 and M7 is practically identical to the saturation resistance R SAT1 of the main transistor M6. However, if the load L becomes open, the main power transistor M6 goes off when the current passing through it reaches a first threshold I R1 , equal to the current supplied from the current generator GN5 multiplied by the W/L ratio of the areas of the transistors M6 and M7. When the transistor M6 is turned off, all the load current flows through the auxiliary transistor M7. It is therefore possible to make a more accurate measurement of the load current from the saturation resistance R SAT2 of the auxiliary transistor M7, which has a higher value than the saturation resistance R SAT of the circuit in normal conditions. Although it meets the requirements, this solution also has certain problems, such as the following: the main power transistor M6 and auxiliary power transistor M7 of the device 8 have their gate terminals G6 and G7 separate: these therefore require separate drivers, thus considerably increasing the complexity of the device's circuit; when the load current decreases, there is a switch from the main transistor M6 to the auxiliary transistor M7: this causes a peak in the output voltage. A further prior art solution is disclosed in the EP application No. 0 594 517 relating to an open load detection circuit for a power MOS transistor, which operates in a switching mode. However, that circuit cannot be used to measure variations of low output currents since, in case of a saturation resistance of the main power transistor having a low value, the saturation voltage would be too small to be detected by the technique disclosed in the same document. The technical problem underlying the present invention is that of devising a method and a corresponding circuit for detecting the load current when the load becomes open; this method and circuit must be capable of overcoming the limitations and/or problems which have hitherto limited the solutions in the known art. Summary of the invention The concept on which the present invention is based is that of forcing the operation of the power transistors in the region of near-saturation, even at very low load currents. On the basis of this concept, the technical problem is resolved by a method for generating an open load detection voltage, of the type indicated previously and defined by the characterizing clause of Claim 1. The technical problem is also resolved by a detection circuit of the type indicated previously and defined by the characterizing clause of Claim 6. The characteristics and the advantages of the method according to the invention will be made clear by the following description of one of its embodiments, provided purely for guidance and without restriction, with reference to the attached drawings. Brief description of the drawings In these drawings, Figure 1 is a schematic illustration of an output stage of an industrial process controller according to the known art; Figure 2 is an illustration of a circuit architecture for detecting an open load according to the known art; Figure 3 shows a further known technical solution for a circuit architecture for detecting an open load according to the known art; Figure 4 is an illustration of a detection circuit according to the invention. Detailed description With particular reference to Figure 4, the number 10 indicates in a general way a detection circuit according to the invention. The circuit 10 comprises a MOS power transistor M10, which will be called the main transistor, connected between a first power supply voltage reference Vs and a first output terminal 05. There is an auxiliary MOS transistor M11 in parallel with the main transistor M10. In particular, the transistors M10 and M11 have their drain terminals D10 and D11 in common and connected to the power supply voltage reference Vs. The main transistor M10 has its source terminal S10 connected to the first output terminal 05, while the auxiliary transistor M11 has its source terminal S11 connected to a comparator with hysteresis 11. Advantageously, according to the invention, the auxiliary transistor M11 may comprise a number of cells N6 which is a submultiple of the number of cells N5 contained in the main transistor M10 (N6 = N5/n, where n is any whole number equal to the ratio of the areas of the transistors). The first output terminal 05 is connected to a second voltage reference, for example an earth GND, through a load L, and to a first input I9 of the comparator with hysteresis 11. A second input terminal I10 of the comparator with hysteresis 11, namely that connected to the source S11, is connected to the second voltage reference GND through a current generator GN7. The comparator 11 also has an output terminal 06. The output terminal 05 of the detection circuit 10 is also connected to a first input terminal I11 of a second comparator 12 having a second input terminal I12 connected to the first power supply voltage reference Vs through a further constant voltage generator GN8. The comparator 12 has an output terminal O7 connected to the gate terminal of a MOS transistor M12, connected between the second voltage reference GND and a third control voltage reference V CP . In particular, the transistor M12 has its drain terminal D12 connected to the third control voltage reference V CP through an interface circuit 14 comprising a switch SW, controlled by a control signal ON, in series with a current generator GN9. The said drain terminal D12 is connected to the gate terminals G10 and G11 of the main transistor M10 and auxiliary transistor M11 respectively. The comparator 12 and the MOS transistor M12 form a control circuit 13 which provides a negative feedback between the output terminal 05 and the gate terminals G10 and G11 of the main transistor M10 and auxiliary transistor M11. To make it possible to measure an appreciable voltage between the source and drain terminals of the main transistor M10 and the auxiliary transistor M11 even at very low currents, both these transistors are maintained in the near-saturation operating region. For this purpose, the first input I9 of the comparator 11, which also coincides with the source terminal S10 of the main transistor M10 is maintained at a predetermined voltage V R by the control circuit 13. The current I L is detected by comparing the voltage of the source terminal S11 of the auxiliary transistor M11, polarized by the reference current generated by the current generator GN7 at a predetermined voltage V R , with the voltage of the source terminal S10 of the power transistor M10 polarized by the current I L flowing in the load L. This comparison is made by the comparator with hysteresis 11 and the result V OUT is present on the output terminal 06 of this comparator 11. Advantageously, according to the invention, the main transistor M10 and the auxiliary transistor M11 have their gate terminals G10 and G11 connected to each other, and are therefore controlled by the same circuit 13 through the interface circuit 14. The operation of the detection circuit 10 in different conditions of operation of the load L will now be examined. For high load currents, both transistors M10 and M11 are completely on, in other words they operate as switches, and the feedback loop formed by the control circuit 13 is open. When the load current decreases, the voltage drop across the saturation resistance of the power transistor M10 decreases. When this drop reaches the value of the reference voltage V R , the feedback loop is closed, bringing the transistors M10 and M11 into the near-saturation operating region. The control circuit 13 therefore permits very precise measurement of the voltage between the drain and source terminals of the power transistor M10, by determining its value for low output currents, in other words when the load L is open. The current is detected by means of the comparator 11, by comparing the voltage of the source terminal S11 of the auxiliary transistor M11, polarized by the current I B supplied by the generator GN7, with the voltage of the source terminal S10 of the main power transistor M10, polarized by the load current I L . Advantageously, according to the invention, the polarization current I B is equal to I S /n where I S is a threshold current for the detection of the open condition of the load L and n is the ratio of the areas of the power transistors, in other words the ratio between the number of cells N5 of the main power transistor M10 and the number of cells N6 of the auxiliary transistor M11. The output voltage of the comparator 11 therefore changes state when the load current I L reaches the threshold current value I S . In particular, for load currents I L greater than the threshold value I S , the voltage of the source terminal S11 of the auxiliary transistor M11 is greater than the voltage of the output terminal 05; in these conditions, the voltage present on the output terminal 05 of the comparator 11 is at a low level V0. Conversely, if the load current I L goes below the threshold I S , the voltage of the source terminal S11 of the auxiliary transistor M11 goes below the voltage of the output terminal 05, causing this terminal 05 to switch to a high value V1, thus signalling the open condition of the load L. For this purpose, the circuit 10 in Figure 4 uses a polarization current I B n times smaller than the source current I S , where n is the ratio between the number of cells contained in the main and auxiliary transistors. It should be noted that the load current I C at which the negative feedback 13 comes into effect is greater than the open load detection threshold current I S . The load voltage present on the output terminal 05 decreases in a linear way with the decrease of the load current I L to the threshold voltage V R generated by the voltage generator GN8: it is at this point that the negative feedback comes into play. The principal advantages of this solution are as follows: the main and auxiliary transistors M10 and M11 have their gate terminals G10 and G11 in common and therefore require only one driver; this results in a simplicity of circuit design which decreases the cost of the device; the transistors M10 and M11 also have their drain terminals D10 and D11 in common; they may therefore be made in a single epitaxial pocket and with any technology, with a gain in terms of precision of the circuit due to the better coupling of the structures; the drain-source voltage of the auxiliary transistor M11 is not altered by the presence of a detection resistance and, when the load current I L is near the threshold value I S , is equal to the drain-source voltage of the main transistor M10; this further improves the precision of measurement; there is no noise component in the output voltage, since there is no switch between the main transistor M10 and the auxiliary transistor M11. The applicant has implemented the circuit solution shown in Figure 4 in BCD technology, using a main power transistor M10 of the vertical DMOS type comprising N5 = 18000 cells and an auxiliary transistor made in the same pocket with a cell ratio n = 20. Since the saturation resistance of the power transistor used is 70 mΩ and, at the nominal current of 2.5 A, the voltage drop on the output terminal is equal to 175 mV, a reference voltage V R of 60 mV was chosen, with a threshold current I S of 3 mA, with a consequent polarization current I B of 150 µA. The results obtained by simulation, in the worst case, show variations of less than 20% in the typical value. Modifications and variations may be made to the invention while remaining within the scope of the following claims.

Claims: Method 9 detecting an open load by means of a driver having at least one main power transistor (M10) having a source terminal (S10) connected to the load (L) and one auxiliary transistor (M11) having its gate and drain terminals respectively connected to the gate and drain terminals of the main transistor (M10), the drain-source paths of said main power transistor (M10) and auxiliary transistor (M11) being connected between a first power supply voltage reference (Vs) and a second voltage reference (GND), said method providing a comparison between a first voltage (VIN1) present on the terminal (S10) connected to the load of the main transistor (M10) and a second voltage (VIN2) present on the source terminal (S11) of the auxiliary transistor (M11), said comparison being made by means of a comparator (11) which receives at its inputs said first (VIN1) and second (VIN2) voltages respectively, and which has an open load detection voltage (VOUT) at its output (06), wherein a polarization current (IB)is applied to said source terminal (S11) of the auxiliary transistor (M11), characterized in that said first voltage (VIN1) is kept below a reference value (VS-VR) when the load current (IL) varies, by means of a negative feedback loop (13) connected between the terminal (S10) which is connected to the load of the main transistor (M10) and a gate terminal (G11) of the auxiliary transistor (M11) for load currents (IL) below a predetermined value (IC), such that when the negative feedback loop is closed, said main transistor (M10) and auxiliary transistor (M11) are kept in the near-saturation operating region by the said negative feedback loop (13). Method according to Claim 1, characterized in that said polarization current (IB) is equal to the ratio between the threshold value (IS) and the ratio of the areas (n) of the main and auxiliary transistors. Circuit for detecting an open load (L), of the type comprising: a first main transistor (M10) having a source terminal (S10) connected to the load (L); a second auxiliary transistor (M11)having its gate and drain terminals respectively connected in parallel to the gate and drain terminals of the main transistor (M10), the drain-source paths of said main power transistor (M10) and auxiliary transistor (M11) being connected between a first power supply voltage reference (Vs) and a second voltage reference (GND) a control circuit (13) connected between the terminal (S10) connected to the load of the said main transistor (M10), a gate terminal (G11) of said auxiliary transistor (M11), and connected to the first power supply voltage reference (Vs); a comparator (11) having a first input (I9) connected to the said terminal (S10) connected to the load of the said main transistor (M10) and a second input (I10) connected to the source terminal of the auxiliary transistor (M11); an open load detection voltage (VOUT) present on the output (06) of the comparator (11); and a polarization circuit (GN7) for the auxiliary transistor (M11) connected between said second input (I10) and the second voltage reference (GND), characterized in that the circuit comprises a negative feedback loop (13) connected between the terminal (S10) which is connected to the load of the main transistor (M10) and a gate terminal (G11) of the auxiliary transistor (M11) for load currents (IL) below a predetermined value (IC), for keeping said first voltage (VIN1) below a reference value (VS-VR) when the load current (IL) varies, such that when the negative feedback loop is closed, said main transistor (M10) and auxiliary transistor (M11) are kept in the near-saturation operating region by the said negative feedback loop (13). Circuit according to Claim 3, characterized in that the said main transistor (M10) comprises a number (N5) of elementary cells which is equal to a whole number (n) multiplied by the number (N6) of elementary cells contained in the auxiliary transistor (M11). Circuit according to Claim 3, characterized in that the said polarization circuit comprises a current generator (GN7) which supplies a polarization current (IB) equal to the ratio between a threshold value (IS) and the ratio of the areas (n) of the main transistor (M10) and auxiliary transistor (M11). Circuit according to Claim 3, characterized in that the said comparator (11) is a comparator with hysteresis. Circuit according to Claim 3, characterized in that the said main transistor (M10) and auxiliary transistor (M11) are connected to an interface circuit (14) comprising a third voltage reference (VCP) and a switch (SW) controlled by a control signal (ON). Circuit according to Claim 3, characterized in that the said main transistor (M10) and auxiliary transistor (M11) have their drain terminals (D10, D11) connected to the power supply voltage reference (Vs).

Inventors: GARIBOLDI ROBERTO, PULVIRENTI FRANCESCO, GARIBOLDI, ROBERTO, PULVIRENTI, FRANCESCO
Applicants: CONS RIC MICROELETTRONICA, ST MICROELECTRONICS SRL, CO.RI.M.ME., STMICROELECTRONICS S.R.L.
Application year: 1995
Main classification: G01R31
IPC codes: G05B19, G01R19, G01R31