Voltage/current characteristics control circuit particularly for protecting power transistors

Abstract: Voltage/current characteristics control circuit particularly for protecting power transistors, which comprises at least one power transistor; the emitter terminal of a second transistor (5) is directly connected to the output of the power transistor; the emitter terminal of a first transistor (4) is connected to the output of the power transistor by means of a first resistor (6). The collector terminal and the base terminal of the second transistor (5) are connected to a current source (3). The base terminal of the first transistor (4) is connected to the base terminal of the second transistor (5), and the circuit furthermore comprises a protection circuitry (2). The circuitry is connected to the collector terminal of the first transistor (4) through a differential stage which comprises a third transistor (9) and a fourth transistor (10); the third transistor (9) and fourth transistor (10) have a respective second resistor (12) and third resistor (13) arranged in series. Divider means (14) are furthermore provided and are interposed between the input terminal of the power transistor and the base terminals of the third transistor (9) and fourth transistor (10).

Description: The present invention relates to a voltage/current characteristics control circuit particularly for protecting power transistors. In power integrated circuits, the need is increasingly felt to adequately protect power transistors, whether in MOS technology or in bipolar technology, from overloads. Power transistors, like all transistors, in fact have, in the voltage/current plane, as more clearly illustrated in figure 1, an operating area usually termed SOA, which is the acronym of Safe Operating Area. The working point of the device must always be within the SOA, under penalty of the destruction of the device itself; such an event obviously entails rapid and drastic maintenance interventions, thus increasing the management costs of the machine or apparatus in which this event occurs. The boundaries of the SOA are usually defined by three limits: the maximum current I MAX which can be withstood by the output device, its maximum voltage V MAX and the maximum power which can be dissipated by the device. In the case of bipolar devices, the situation is further limited by the well-known phenomenon of II breakdown, indicated in figure 1 by the broken line, which is ultimately the main restriction in the manufacture of bipolar power devices for high voltages, i.e. with a supply voltage V cc higher than 30 volts. Furthermore, in particular conditions, such as an amplifier with a partially inductive load, the SOA can rather often be abandoned by the locus of the points of the voltage/current plane which represent the work point region, as more clearly illustrated in figure 2. This boundary crossing unavoidably entails the final destruction of the device. The technical problem therefore consists in manufacturing protection circuits which detect when the power transistor is going to cross the boundaries of the SOA and switch it off in this case. The situation is even worse when the difference between the boundary of the SOA and the work point is very small; the protection circuits must therefore have such a resolution as to follow the SOA as closely as possible, so as to avoid switching the transistor off when instead it is still within the SOA, i.e. in normal operating conditions. Finally, the manufacture of said protection circuits and most of all their operating stability with respect to variations of the production processes for their manufacture is one of the main problems of protection circuits. The aim of the present invention is to eliminate or substantially reduce the disadvantages described above in known types of known control circuits by providing a voltage/current characteristics control circuit particularly for protecting power transistors which faithfully follows the boundaries of the SOA in order to switch the power transistor off when the work point exceeds said boundaries. Within the scope of this aim, an object of the present invention is to provide a control circuit which is stable in operation even if the manufacturing process varies. Not least object of the present invention is to provide a voltage/current characteristics control circuit particularly for protecting power transistors which is relatively easy to manufacture and at competitive costs. This aim, these objects and others which will become apparent hereinafter are achieved by a voltage/current characteristics control circuit for protecting at least one power transistor according to the invention, comprising: a first transistor having an emitter terminal connected to the output of said power transistor by means of a first resistor, a second transistor having an emitter terminal directly connected to the output of said power transistor, the collector terminal and the base terminal of said second transistor being connected to a current source, the base terminal of said first transistor being connected to the base terminal of said second transistor, and a protection circuitry, characterized in that said protection circuitry is connected to the collector terminal of said first transistor through a differential stage which comprises a third transistor and a fourth transistor, said third and fourth transistors having respective second and third resistors arranged in series therewith, an in that divider means are provided between the input terminal of said power transistor and the base terminals of said third transistor and fourth transistor to provide the differential stage with a difference voltage which is a function of the voltage at the input terminal of said power transistor. Further features and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment of a voltage/current characteristics control circuit particularly for protecting power transistors according to the invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is a voltage/current plot of the Safe Operating Area (SOA); figure 2 is a voltage/current plot of the SOA with a region of working points; figure 3 is a diagram of a known protection circuit; figure 4 is a diagram of another known protection circuit; figure 5 is a voltage/current plot of the SOA of the circuit of figure 4; figure 6 is a diagram of a protection circuit according to the invention; figure 7 is a voltage/voltage plot of the behavior of a differential voltage of the circuit according to the invention; figure 8 is a voltage/current diagram of the SOA of the circuit of figure 7; figure 9 is a diagram of a nonlinear divider comprised within the circuit of figure 6; figure 10 is a voltage/voltage plot of the behavior of a differential voltage in the divider of figure 9; and figure 11 is a voltage/current plot of the SOA of the circuit of figure 6 completed by the divider of figure 9. With reference to figure 3, said figure schematically illustrates the basic circuit for detecting the current which flows in a power device 1. Said circuit comprises a protection circuitry 2 and a current source 3, which are respectively connected to respective collector terminals of a first NPN transistor 4 and of a second NPN transistor 5 which are current-mirror connected. In other words, the base terminals of the two transistors 4 and 5 are connected to one another, to the collector terminal of the second transistor 5 and to the current source 3. The emitter terminal of the first transistor 4 is connected to a terminal of a first resistor 6 and is connected to the ground, whereas the emitter terminal of the second transistor 5 is connected to an output terminal of the power device 1 and to the other terminal of the first resistor 6. If the current in output from the device 1, termed I₀, is zero, the voltage drop across the first resistor 6 is zero and the current I₂ which is mirrored by the transistors 4 and 5 is equal to the current I₁ generated by the source 3. If a non-zero current I₀ flows in the device 1, the voltage drop across the first resistor 6 causes the current I₂ to increase until it reaches the threshold value I ₂, whose exceeding causes the intervention of the protection circuit 2. In this circuit, the protection 2 is thus sensitive only to the output current and is therefore not suitable for high-voltage circuits. The circuit illustrated in figure 4 is an improvement with respect to the above described circuit. In this known type of circuit, without changing the references previously provided, a series constituted by a resistor 7 connected to the anode of a Zener diode 8 is connected to the base terminal, to the collector terminal of the second transistor 5 and to the current source 3. The cathode of the Zener diode 8 is connected to the input terminal of the power device 1. In this last circuit, so long as the voltage V₀ across the power device 1 is lower than the Zener voltage V Z , the circuit limits the current I₀ to the value I 0MAX as in the preceding circuit; if the voltage V₀ exceeds the Zener voltage V Z , the current which flows in the first transistor 4 also increases, and a smaller voltage drop across the first resistor 6 is accordingly necessary in order to reach the current value I ₂ which activates the protection circuitry 2. The protection curve of the circuit of figure 4 is shown in figure 5. With reference to figure 6, a voltage/current characteristics control circuit particularly for protecting power transistors comprises the above components, whose reference numerals are maintained in figure 6 for the sake of clarity. The protection circuitry 2 is connected to the collector terminal of the first transistor 4 through a differential stage which comprises a third transistor 9 and a fourth transistor 10. Said third and fourth transistors have a respective second resistor 12 and third resistor 13 connected in series thereto. Divider means 14, described hereinafter, are furthermore provided and interposed between the input terminal of the power transistor or power device 1 and the base terminals of the third transistor 9 and of the fourth transistor 10. The third transistor 9 has its collector terminal connected to a supply voltage 15 which is equal in value to V cc and its emitter terminal connected, by means of the second resistor 12, to the collector terminal of the first transistor 4 and to the emitter terminal of the fourth transistor 10 by means of the third resistor 13. The collector terminal of the fourth transistor 10 is connected to the protection circuitry 2. The current I₂ is a function of the current I₀ according to the following rule: I₂ = I₁ (A₄/A₅) exp (I₀R₆/V T ) where R₆ is the resistance value of the resistor 6, V T is the thermal voltage, which is usually equal to 26 millivolts, and (A₄/A₅) is the area ratio between the first transistor 4 and the second transistor 5. In order to also make the circuit sensitive to the voltage V₀ of the power device 1 to be protected, the current I₂ is divided by the differential stage constituted by the third transistor 9 and by the fourth transistor 10. The differential stage is furthermore intentionally degenerated by means of the presence of the second resistor 12 and of the third resistor 13. The described differential stage is driven by the divider means 14, which are better explained hereinafter and provide it both with bias and with the difference voltage V D . The voltage V D is a function of the voltage V₀ and has a behavior which is qualitatively illustrated in figure 7. For high voltages V₀, the voltage V D is positive and all the current I₂ flows through the fourth transistor 10. When the current I₂ reaches the threshold value I ₂, the protection intervenes and switches off the device 1. In this region of the SOA, as illustrated in figure 8, the current I₀ is limited to the value I₀₂ for V₀ > V₀₂. With lower voltages V₀, V D is initially annulled and then becomes increasingly negative. The current I₂ thus flows partly in the third transistor 9 and partly in the fourth transistor 10. A higher current I₀ is therefore necessary in order to reach the threshold current I ₂ and to make the protection intervene. The resulting curve is shown in figure 8. As is evident from the plot of figure 8, if the device 1 is to be protected below the voltage V₀ = V₀₁ at a constant current I₀ = I₀₁, it is necessary to fix the differential voltage V D at the output of the divider means 14 at a constant value equal to V D = V D1 . Therefore the transfer function of the divider means 14, i.e. V D = V D (V₀), must be calculated in order to follow the SOA curve as closely as possible. With reference to figure 9, the divider means 14 comprise a fourth resistor 16 which is connected to the supply voltage 15 and to the anode of a diode 17 whose cathode is connected to a fifth resistor 18 which is connected to the ground. The anode of the diode 17 is connected to the base terminal of the third transistor 9. The cathode of a Zener diode 19 is connected to the cathode of the diode 17; the anode of said Zener diode is connected, by means of a sixth resistor 20, to the input terminal of the power device 1 and is connected to the base terminal of the fourth transistor 10 by means of a seventh resistor 21. The divider means 14 are thus a non-linear divider with various slopes which allows to follow the SOA curve with good approximation, and the plot of the transfer function V D = V D (V₀) of the divider means 14 described above is shown in figure 10. The protection curve thus obtained is the one graphically illustrated in figure 11. The present invention conveniently allows to approximate the protection curve of the power device 1 very closely and in an easily reproducible manner even with a simple three-slope non-linear divider 14 such as the one illustrated by way of example in figure 9. The curve defined by the divider 14 is shown in figure 10, and the protection curve of the device 1 obtained by the circuit of figure 6 is the one illustrated in figure 11. With the previously described known circuit of figure 4, V₀ is in fact detected by increasing the reference current I₁ by means of the Zener diode 8 and of the resistor 7, and it is difficult to couple this additional current to the trigger current I₂, especially from the point of view of temperature variations and of variations in the Zener voltage V Z which are due to the production process. Advantageously, in the circuit according to the invention one acts by accurately dividing the current I₂ by means of the differential stage constituted by the transistors 9 and 10 and by the divider 14 which depends exclusively on resistance ratios, as illustrated in figure 6, obtaining protection curves which are more reproducible and temperature-stable. The invention thus conceived is susceptible to numerous modifications and variations within the scope of the inventive concept as defined in the appended claims. From the structural point of view, the transistors 4, 5, 9 and 10 are NPN bipolar transistors, but nothing conceptually forbids the execution of a circuit according to the invention with PNP bipolar transistors, with the obvious necessary circuital adaptations. Details may furthermore be replaced with other technically equivalent elements. In practice, the materials employed, as well as the dimensions, may vary according to the requirements. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims: Voltage/current characteristics control circuit for protecting at least one power transistor (1), comprising: a first transistor (4) having an emitter terminal connected to the output of said power transistor by means of a first resistor (6), a second transistor (5) having an emitter terminal directly connected to the output of said power transistor, the collector terminal and the base terminal of said second transistor (5) being connected to a current source (3), the base terminal of said first transistor (4) being connected to the base terminal of said second transistor (5), and a protection circuitry (2), characterized in that said protection circuitry (2) is connected to the collector terminal of said first transistor (4) through a differential stage which comprises a third transistor (9) and a fourth transistor (10), said third (9) and fourth (10) transistors having respective second (12) and third (13) resistors arranged in series therewith, and in that divider means (14) are provided between the input terminal of said power transistor (1) and the base terminals of said third transistor (9) and fourth transistor (10) to provide the differential stage with a difference voltage which is a function of the voltage at the input terminal of said power transistor. Control circuit according to claim 1, characterized in that said third transistor (9) has its collector terminal connected to a supply voltage and its emitter terminal connected, by means of said second resistor (12), to the collector terminal of said first transistor (4) and to the emitter terminal of said fourth transistor (10) by means of said third resistor (13). Control circuit according to any of the preceding claims, characterized in that said fourth transistor (10) has its collector terminal connected to said protection circuitry (2). Control circuit according to any of the preceding claims, characterized in that said divider means (14) comprise a fourth resistor (16) which is connected to said supply voltage (15) and to the anode of a diode (17) whose cathode is connected to a fifth resistor (18) which is connected to the ground. Control circuit according to claim 4, characterized in that the base terminal of said third transistor (9) is connected to said anode of said diode (17). Control circuit according to claim 5, characterized in that the cathode of a Zener diode (19) is connected to said cathode of said diode (17), the anode of said Zener diode (19) being connected, by means of a sixth resistor (20), to the input terminal of said power transistor and, by means of a seventh resistor (21), to the base terminal of said fourth transistor (10).

Inventors: BOTTI EDOARDO, BRAMBILLA DAVIDE, BOTTI, EDOARDO, BRAMBILLA, DAVIDE
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1991
Main classification: H02H7
IPC codes: G05F1, H03K17