Voltage limiter integrated electronic circuit

Abstract: A voltage limiting circuit having a first and a second terminal between which is applied a voltage (Vs) to be limited of the type comprising at least one transistor (T2) having a first terminal connected to the first terminal of the circuit 1. This transistor T2 also comprises a second terminal kept at a reference voltage (E) and a control terminal coupled to the second terminal of the circuit 1 through at least one first equivalent resistive element (R'). The value of the equivalent resistive element (R') depends on the value of the voltage (Vs) to be limited so that the first transistor (T2) will have a temperature-stable breakdown voltage (Vcer).

Description: The present invention relates to an integrated electronic voltage limiting circuit of suitable voltage and especially to protect against overvoltage a switch placed in series with an electrical load connected to a power supply line. The field of application of the present invention concerns the automobile industry and specifically the field of electronic ignition with discrete components. As known, in many cases bipolar transistors are used as switches for supply electrical loads in switching with even very high currents. Particularly when the electrical load is of the inductive type, opening of the switch, i.e. the sudden change from a conduction state to an interdiction state of the transistor causes a transient overvoltage condition at its ends. This overvoltage can even be several times greater than the supply voltage and can reach values and durations such as to exceed the safety limits set by the structural characteristics of the transistor and even destroy it. A first technical solution of the prior art to obviate this shortcoming calls for the use of a voltage limiting circuit comprising a type NPN Darlington couple (T1,T2) whose drive transistor is indicated by T1 and at whose ends is present a voltage Vs to be limited as shown in FIG. 1. This voltage limiting circuit also comprises a Zener diode Z1 having an anode terminal connected to a base terminal of the transistor T1 and a cathode terminal connected to an intermediate node A of a resistive divider (R1,R2) at whose ends is present the voltage Vs to be limited. In this case the voltage Vs is limited to a maximum value equal to: Vs= [V(Z1)+2Vbe] * (R1+R2)/R2 where V(Z1) is the breakdown voltage of the diode Z1 and Vbe are the voltages present between the base and emitter terminals respectively of the transistors T1 and T2 incorporated in the Darlington couple (T1,T2). A second known technical solution, shown in FIG. 2, calls for the use of a voltage limiting circuit comprising a transistor T3 with open base placed between the transistors T1 and T2 of the Darlington couple (T1,T2). In this second case the voltage Vs is limited to a maximum value equal to: Vs=Vceo(T3)+Vbe(T2) where Vceo is the breakdown voltage between the collector and emitter terminals of the transistor T3 with open base and Vbe is the voltage present between the base and emitter terminals of the transistor T2. A last known technical solution of the prior art, shown in FIG. 3, requires that in substitution of the transistor T3 the voltage limiting circuit includes a Zener diode chain. Although advantageous from various viewpoints the above mentioned voltage limiting circuits are of poor precision because their breakdown voltage varies severely with temperature. The technical problem underlying the present invention is to provide an integrated electronic voltage limiting circuit which would be very accurate as regards its breakdown voltage and independent of temperature. The technical problem is solved by a circuit of the above mentioned type as defined in the characterizing part of the annexed claims. The characteristics and advantages of the circuit in accordance with the present invention are set forth in the description of an embodiment thereof given below by way of non-limiting example with reference to the annexed drawings. In the drawings: FIG. 1 shows a diagram of an integrated electronic voltage limiting circuit provided in accordance with the prior art, FIG. 2 shows a second diagram of a circuit provided in accordance with the prior art, FIG. 3 shows another diagram of a circuit provided in accordance with the prior art, FIG. 4 shows a basic diagram of an integrated electronic voltage limiting circuit in accordance with the present invention. FIG. 5 shows a possible embodiment of the diagram shown in FIG. 4, FIG. 6 shows a possible embodiment of a detail of the circuit diagram shown in FIG. 4, FIG. 7 shows a circuit diagram of a possible application of the circuit in accordance with the present invention, and FIG. 8 shows the relationship between two variable magnitudes present in the circuit in accordance with the present invention. Specifically, with reference to FIG. 4, the voltage limiting circuit comprises at least one first and one second terminal between which is present a voltage Vs to be limited. The circuit also comprises at least one first bipolar NPN transistor T2 having a first terminal connected to the first terminal of the circuit and a second terminal kept at a reference voltage E. This first transistor T2 also comprises a control terminal coupled with the second terminal of the circuit through at least one first equivalent resistive element R'. In FIG. 5 it is seen that the first equivalent resistive element R' incorporates at least one second bipolar PNP transistor T1 having a first terminal connected to the control terminal of the first transistor T2 and a second terminal connected to the second terminal of the circuit. In addition, the second transistor T1 is equipped with a control terminal driven through a resistive divider (R1,R2) incorporating a second R1 and a third R2 resistive element connected in series together and inserted between the first and second terminals of the circuit. With reference to FIG. 6 it is seen that the reference voltage E is generated by a plurality of one-way current flow circuit elements (Z1,D1,...Dn) connected together in series. These circuit elements (Z1,D1,...Dn) comprise at least one Zener diode Z1 with positive temperature coefficient and at least some junction diodes (D1,...Dn) with negative temperature coefficient so as to make the reference voltage E constant with the temperature. FIG. 7 shows a circuit diagram of a possible application of the circuit. Specifically the circuit is connected by its first terminal to a first terminal of an inductive electrical load ZL having a second terminal connected to a power supply line AL. The second terminal of the circuit is connected to a control terminal of a switch T3 placed in series with the electrical load ZL. The above mentioned control terminal is also connected to drive circuit means P not shown in FIG. 7 because conventional. It is seen that when the drive circuit means P open the switch T3 provided e.g. through a third bipolar NPN transistor the circuit limits the overvoltage created at the ends of the above mentioned switch. There is now described operation of the voltage limiting circuit in accordance with the present invention. It is noted that on the basis of the collector-emitter breakdown characteristic of a bipolar NPN transistor having a resistance between base and emitter the first transistor T2 has a breakdown voltage Vcer greater than the breakdown voltage Vceo present between the first and second terminals of the transistor T2 when the control terminal is open. This breakdown voltage Vcer is also smaller than the breakdown voltage Vces present between the first and second terminals of the same transistor T2 when the control terminal is short circuited. In addition, the breakdown voltage Vcer depends on the value of the first equivalent resistive element R'. Now referring to FIG. 5 it is seen that the reference voltage E is equal to: E=VR2+VbeT1-VbeT2 where VR2 is the voltage present at the ends of the third resistive element R2, VbeT1 is the voltage present between the first terminal and the control terminal of the second transistor T1 and VbeT2 is the voltage present between the second terminal and the control terminal of the first transistor T2. When the first transistor T2 is in conduction state we have: Vs=VR2*(R1+R2)/R2 If in a first approximation it is assumed that: E=VR2 from the formula (1) it is inferred that the reference voltage E and the resistive divider (R1,R2) are temperature constant, the voltage Vs will also be temperature constant. In this application it is assumed that the first transistor T2 has a voltage Vceo=350V and a voltage Vces=800V. It is also assumed that the reference voltage E is 20V and the second R1 and third R2 resistive elements are 95kΩ and 5kΩ respectively. In addition, to the circuit is applied a voltage Vs of increasing value. Now referring to FIGS. 4 and 5 it is seen that the first equivalent resistive element R' is equal to: R' = E*R1*R2/{(h fe *(R1*R2)*[E-Vs*R2/(R1+R2)]} where h fe is the current gain of the second transistor T1. From the above formula it is inferred that for low values of voltage Vs the first resistive element R' has a very low value as shown in the chart represented in FIG. 8. In this case the first transistor T2 is in a condition near breakdown voltage Vces. When approaching the condition: E=Vs*R2/(R1+R2) by substituting this value in the formula (2) it is found that the value of the first resistive element R' tends to infinite. In this case the first transistor T2 is in a condition near the breakdown voltage Vceo. Consequently, being placed in the condition Vs>Vceo the first transistor T2 tends to go into breakdown, reducing the voltage at its ends and hence reducing the voltage Vs. Summing up, the voltage Vs modulates the value of the first resistive element R' in accordance with the formula (2) and the chart shown in FIG. 8 keeping the first transistor T2 in breakdown at a voltage Vs which is taken from the formula (3) and is equal to: Vs (Vcer) = E*(R1+R2)/R2 As may be seen, this breakdown voltage is temperature independent as the voltage E is temperature independent. In conclusion the circuit in accordance with the present invention has a breakdown voltage which is temperature-stable and also of predetermined value between the values of the voltages Vceo and Vces.

Claims: Voltage limiting circuit having a first and a second terminal between which is applied a voltage (Vs) to be limited of the type comprising at least one first transistor (T2) having a first terminal connected to the first terminal of said voltage limiting circuit , a second terminal, and a control terminal coupled with the second terminal of said voltage limiting circuit through at least one first equivalent resistive element (R'), a reference voltage (E) being applied between said second terminal of said first transistor (T2) and said second terminal of said voltage limiting circuit, said reference voltage (E) being temperature independent and said equivalent resistive element (R') having a value depending on the value of the voltage (Vs) to be limited so that the first transistor (T2) will have a temperature-stable breakdown voltage (Vcer). Circuit in accordance with claim 1 and characterized in that the first equivalent resistive element (R') comprises at least one second transistor (T1) having a first terminal connected to the control terminal of the first transistor (T2) and a second terminal connected to the second terminal of said voltage limiting circuit . Circuit in accordance with claim 2 and characterized in that the second transistor (T1) is equipped with a control terminal driven through a resistive divider (R1,R2) inserted between the first and second terminals of said voltage limiting circuit . Circuit in accordance with claim 3 and characterized in that said resistive divider (R1,R2) comprises at least one second (R1) and one third (R2) resistive element connected in series together. Circuit in accordance with claim 1 and characterized in that the first transistor (T2) is a bipolar transistor of the NPN type. Circuit in accordance with claim 2 and characterized in that the second transistor (T1) is a bipolar transistor of the PNP type. Circuit in accordance with claim 1 and characterized in that the reference voltage (E) is generated by at least a plurality of circuit elements with one-way current flow (Z1,D1,...,Dn) connected in series together. Circuit in accordance with claim 7 and characterized in that said circuit elements with one-way current flow (Z1,D1,...,Dn) comprise at least one Zener diode with positive temperature coefficient and at least one of the junction diodes with negative temperature coefficient so that the reference voltage (E) is temperature constant. Electronic ignition system for an internal combustion motor of the type comprising: an electrical load (ZL) of the inductive type having a first terminal connected to a power supply line (AL) and a second terminal, and a switch (T3) inserted with a first and a second terminal between the second terminal of the electrical load (ZL) and a reference potential (GND) and having a control terminal connected to drive circuit means (P); at least one voltage limiting circuit connected between the second terminal of the electrical load (ZL) and the control terminal of the switch (T3) to limit an overvoltage present between the first and the second terminals of the switch (T3); characterized in that said circuit comprises a first and a second terminals between which is applied a voltage (Vs) to be limited, said first and second terminals being respectively connected to said second terminal of the electrical load (ZL) and to said control terminal of said switch (T3) and at least one first transistor (T2) having a first terminal connected to the first terminal of the circuit, a second terminal, and a control terminal coupled to the second terminal of the switch (T3) through at least a first equivalent resistive element (R'), a reference voltage (E) being applied between said second terminal of said first transistor (T2) and said second terminal of said voltage limiting circuit, said reference voltage (E) being temperature independent and said equivalent resistive element (R') having a value depending on the value of the voltage (Vs) to le limited so that the first transistor (T2) has a temperature-stable breakdown voltage (Vcer).

Inventors: PALARA SERGIO, PALARA, SERGIO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1995
Main classification: H03K17
IPC codes: F02P3, H03K17