Overvoltage sensor with hysteresis

Abstract: First and second resistors are connected in series with a Zener diode between first and second points of operating potential. The base-to-emitter of an NPN transistor is connected across the first resistor to sense the current through the series path. The collector-to-emitter of a PNP transistor is connected across the second transistor, whereby when the PNP transistor is turned-on hard and into saturation, the voltage drop across the second transistor decreases. The collector of the NPN transistor is connected to the base of the PNP transistor, whereby when an overvoltage condition exists and the Zener diode breaks down, the two transistors are driven regeneratively and form a latch and the operating point of the circuit is shifted.

Description: BACKGROUND OF THE INVENTION This invention relates to circuitry for sensing when the operating voltage applied to the circuitry exceeds a predetermined level and for producing a control signal in response to an overvoltage condition. In many applications, such as, for example, automotive systems, the supply voltage may vary over a wide range. Circuits powered by the supply voltage may be damaged when the supply voltage exceeds a certain overvoltage level (VOV). To prevent the circuits from being damaged, the overvoltage condition must be sensed and power must be removed from the circuits or the circuits must be deactivated. A known circuit for sensing an overvoltage condition is shown in FIGURE 1. The circuit of FIGURE 1 includes a PNP transistor, Q1, connected as a diode which is used to prevent current flow between the positive supply line (Vs) and ground when the supply and ground connections are interchanged. A Zener diode, Z1, used to sense the overvoltage condition is connected in series with Q1 and resistors R1 and R2 between Vs and ground. Resistor R1 is used to limit the current which flows through Q1 and Z1 and the value of resistor R2 is selected to ensure the voltage across R2 will be less than 0.5 or 0.6 volts when Z1 is not conducting. An NPN transistor, Q2, whose base-to-emitter junction is connected across R2, is used to control the load circuitry 7 when Z1 breaks down and causes Q2 to conduct. The operation of the circuit of FIGURE 1 may be briefly described as follows: Assume that Z1 has a breakdown voltage Vz and that Q1 has a forward voltage of Vf. For values of supply voltage (Vs) less than Vz + Vf, there is only leakage current flowing through Q1, Z1, R1 and R2. When Vs exceeds Vz + Vf, a current, Ix, flows through Q1, Z1, R1 and R2. VOV is the value of Vs at which Vs exceeds Vz + Vf and produces a current Ix which causes Q2 to conduct. Transistor Q2 conducts when a voltage drop equal to VBE2 is developed between its base and emitter terminals. The VBE2 drop is produced when the current Ix flowing through Q1, Z1, R1 and R2 reaches a level such that [Ix . R2] exceeds the VBE of Q2. For values of Vs much less than VOV, the current through Z1 is small (leakage) generating a voltage much less than VBE2 across R2. As Vs increases and approaches VOV, Z1 breaks down and the current through Z1 increases causing the voltage across R2 to rise. When Vs equals VOV, the voltage developed across R2 equals VBE2, current flows into the base of Q2 and the collector current of Q2 is sufficient to turn off (or otherwise deactivate) the load circuitry 7 connected to the collector of Q2. The circuit of FIGURE 1 performs a useful function but suffers from the following disadvantages: 1. When Vs rises to a voltage level where the Zener diode, Z1, just begins to conduct, noise signals may be generated which cause the collector current of Q2 to vary widely. This results in an oscillatory signal being applied to control the load circuitry 7 connected to the collector of Q2. 2. The voltage developed across R2 and the resulting conduction level of Q2 changes as the supply voltage is varied in the vicinity of VOV. If Vs changes gradually, the load circuitry 7 connected to the collector of Q4 will be turned off or on gradually over a range of several millivolts. In this range, noise signals can cause erratic operation of the circuit under control. US-A-4 868 703 discloses a solid state switching device allowing independent control of a latching and a holding current. Each transistor device has a resistance between its base and emitter and has its base connected to the collector of the other transistor device. SUMMARY OF THE INVENTION It is an object of the present invention to overcome or at least reduce one or more of the above disadvantages. Accordingly, the present invention provides an overvoltage sensing circuit comprising: first and second nodes at which first and second voltages are provided; a current flow path formed of a first resistor, a second resistor and a voltage reference circuit element coupled in series between said first and second nodes; a first bipolar transistor having a base, an emitter and a collector, the base and emitter of said first transistor being connected across said first resistor, said first transistor being operative to sense current flow through said current flow path in response to current flow through said first resistor forward biasing the base-emitter of said first transistor and causing current flow in the collector of said first transistor; and a second bipolar transistor having a base, an emitter and a first collector, the first collector (C01) and emitter of said second transistor being connected across said second resistor and the base of said second transistor being connected to the collector of said first transistor, so that current flow in the collector of said first transistor is operative to turn on said second transistor and reduce the voltage drop across said second resistor, the first and second bipolar transistors being of complementary polarity types such that when the first and second transistors are rendered conducting, regenerative feedback causes them to go into a latch condition; which circuit is characterised in that the first collector of said second transistor is connected to the base of the first transistor via said voltage reference element; the second transistor has a second collector; and a controllable switching element is arranged to be coupled in with a load and has a control input coupled to the second collector of said second transistor. Overvoltage sensing circuits embodying the invention include positive feedback means for causing the overvoltage sensing circuit to go into a latch condition and produce a definite overvoltage indication upon the occurrence of an overvoltage condition. Circuits embodying the invention also include hysteresis for causing the circuit to latch up for one value of supply voltage and to drop out of the latch condition for another value of supply voltage. BRIEF DESCRIPTION OF THE DRAWING In the accompanying drawing, like reference characterisitics denote like components; and FIGURE 1 is a schematic diagram of a prior art circuit; and FIGURE 2 is a schematic diagram of a circuit with hysteresis embodying the invention. DETAILED DESCRIPTION OF THE INVENTION The circuit of FIGURE 2 includes a first power terminal 20 to which is applied ground potential and a second power terminal 22 to which is applied the supply voltage, Vs. A PNP transistor, Q1, is connected at its emitter to terminal 22 and at its base and collector to Node 24. Q1 functions to block reverse current when the positive supply and ground connections are interchanged. A resistor R1 is connected between nodes 24 and 26. A Zener diode, Z1, is connected at its cathode to node 26 and at its anode to node 28. A resistor R2 is connected between nodes 28 and 30. An NPN transistor, Q2, is connected at its base to node 28, at its emitter to node 30 and at its collector to a node 23 to which is connected the base of a PNP transistor, Q3. Q2 functions to sense the current level through R2 and draws collector current when the voltage across its base and emitter exceeds a voltage defined as VBE2. A resistor, R3, connected between node 30 and ground terminal 20, functions to limit the current that can flow between Vs and ground via Q1, R1, Z1, R2 and Q2. The emitter of PNP transistor, Q3, is connected to node 24, one of its collectors (CO1) is connected to node 26 and its other collector (CO2) is connected to the base of NPN transistor, Q4. The connection of CO1 of Q3, via Z1, to the base of Q2 and the connection of the collector of Q2 to the base of Q3 forms a latch circuit which functions like a silicon controlled rectifier (SCR) when Q2 goes into conduction. A resistor R4 is connected between the emitter and the base of Q3 to ensure that Q3 is turned off in the presence of leakage current through Q2 and/or Q3. The emitter of Q4 is returned to ground potential. A resistor, R5, connected between the base and emitter of Q4, ensures that Q4 remains cut off in the presence of leakage current through Q2 and Q3. Q4 functions to amplify the control signal produced by Q3 at CO2 and couples the amplified signal to the load circuitry 7A connected to its collector. The load circuitry may take many different forms. For purpose of illustration, three types of loads are shown connected to the collector of Q4. These loads may in fact comprise many other elements or portions of integrated circuits. A load, L1, is connected between terminal 22 and the collector of Q4. When Q4 is turned on, current can flow between VS and ground via load L1 and the collector-to-emitter path of Q4. When Q4 is turned off, current can not flow through L1 and load L1 floats at a potential equal to or close to the supply voltage. The collector of Q4 is also connected via a resistor R9 to the base of a PNP transistor, Q5, whose emitter is connected to terminal 22 with a resistor R8 being connected between the base and emitter of Q5 to ensure its nonconduction in the presence of leakage currents. A load L2 is connected between the collector of Q5 and ground potential. When Q4 is turned on, it causes the turn-on of Q5 which provides a current path between Vs and load L2. When Q4 is turned off, Q5 is also turned off and the current path between Vs and load L2 is removed. The collector of Q4 may also be connected to the base of an NPN transistor such as Q6 whereby when Q4 is turned-on, Q6 is turned-off and the load circuit L3 in the collector of Q6 is disconnected from ground and hence, deactivated. CIRCUIT OPERATION In the description to follow, the overvoltage condition, VOV, is defined as the voltage condition for which Q2 is rendered conductive. This occurs when the current through R2 results in a voltage which exceeds the VBE of Q2 and causes Q2 to conduct. When the supply voltage level, Vs, is much less than VOV, no substantial current, except for leakage, flows via Q1, R1, Z1, R2 and R3 to ground. The resistor, R2, is chosen such that normally expected values of leakage current through Z1 will not create a voltage across the base-emitter junction of Q2 which is large enough to cause Q2 to enter the forward active region of operation. Therefore, when Vs is less than VOV, Q2 is in the cutoff region. Likewise, the values of R4 and R5 are chosen to ensure that Q3 and Q4, respectively, are in the cutoff region under this condition. When the supply voltage level, Vs, is increased to a value which exceeds the sum of the Zener breakdown voltage, Vz, of Z1 and the forward voltage, Vf, of Q1, a current Ix flows via Q1, R1, Z1, R2 and R3 to ground. When Vs reaches VOV, the current Ix is of sufficient magnitude to cause the voltage drop across R2 to forward bias the base-emitter junction of Q2 sufficiently to place it in the forward active region. The resulting increase in the collector current of Q2 causes a voltage drop to be developed across R4 with a polarity which forward biases the base-emitter junction of Q3. When the voltage applied to Q3 exceeds VBE3, Q3 begins to conduct. It then supplies additional current via CO1 into node 26 which then flows through Z1 and into the parallel combination of R2 and the base of Q2. As the voltage drop across R2 increases, more current flows into the base of Q2, causing the conduction level of Q2 to increase. The increase in the collector current of Q2 causes an increase in the base current of Q3, causing Q3 to conduct more heavily and supplying more current into the base of Q2. Clearly, the current which flows from collector CO1 of Q3 which is connected to Z1, flows via Z1 into the base of Q2 providing positive feedback to make the loop formed by Z1, Q2 and Q3 regenerative. The positive feedback continues until Q2 and Q3 latch up similar to a silicon controlled rectifier(SCR). When regeneration occurs, the conduction level of Q3 increases quickly and dramatically. The collector current of Q3, which is supplied via collector CO2 to R5, causes an increase in the voltage across the base-emitter junction of Q4 which is sufficient to cause Q4 to enter the forward active region. The conduction level of Q4 changes rapidly when regeneration occurs going quickly from a fully-off to a fully-on condition. Even though the increase in Vs may be gradual, once the regenerative loop of Q2 and Q3 is energized, the turn-on of Q4 will be rapid and Q4 will switch the load circuitry 7A connected to its collector in an equally rapid fashion. In addition to functioning as part of a latch, Q3, as connected, also functions to provide hysteresis to the circuit. As Q3 conducts more and more, the collector CO1 of Q3 goes into saturation and the voltage drop across R1 is decreased causing an effective increase in the voltage across, and the currents drawn by, Z1, R2, R3 and Q2. Just prior to the onset of regeneration, when Vs is less than VOV, the voltage drop across R2 is just less than VBE2. Since the currents flowing in R2 and R1 are approximately equal (neglecting base current in Q2) the voltage drop across R1 is equal to VR1 ≈ [R1/R2](VBE2) When regeneration occurs, transistor Q3 is driven into saturation causing the voltage drop across R1 to be equal to V CESAT (Q3). The change in the voltage across R1 from just before to just after the onset of regeneration may be defined as V HYST , where, V HYST ≈ {[R1/R2](VBE2)} - V CESAT (Q3) The resulting decrease in voltage across R1 increases the voltage applied to the series circuit formed by Z1, R2 in parallel with the base-emitter junction of Q2, and R3. This results in an increase in the current passing through each of the elements. Because of the saturation of Q3 involving collector CO1, the supply voltage must be reduced below VOV before the voltage applied to the base-emitter junction of Q2 is reduced to less than VBE2. Let the supply voltage which must be applied for the voltage drop across R2 to be equal to VBE2 after regeneration has occurred, be VON, where VON = VOV - V HYST When the supply voltage equals VON, conduction through Q2 and Q3 is substantially decreased. The collector current of Q3 which is supplied to the junction of R3 and Z1 can no longer supply enough current for regeneration to continue. Therefore, Q3 returns to the cutoff mode and the voltage drop across R1 increases by an amount equal to V HYST . When Q3 enters the cutoff region of operation, the voltage drop across R5 decreases below that required for Q4 to remain in the active region. Therefore, Q4 enters the cutoff region and the circuitry which it controls is allowed to return to the normal operation conditions which existed prior to Vs increasing to VOV. Because of the regenerative nature of this circuit, the turn-on and turn-off characteristics of Q4 are sharp with respect to the supply voltage, not gradual as in the prior art. Also, by appropriate choice of values for V HYST , oscillations are eliminated when the supply voltage is near VOV. As described above, circuits embodying the invention enjoy one or more of the following features: 1. Overvoltage shutdown with hysteresis provides operation without oscillation due to noise near the control voltage. 2. Hysteresis provided by regenerative action which changes the operating point of the circuit. 3. Hysteresis provided by regenerative action which is activated primarily by a Zener or other reference diode(s). 4. Circuit draws only leakage current when the supply voltage is lower than the predetermined control voltage. 5. Circuit does not allow current flow when reverse biased. In the circuit of FIGURE 2, the reference setting element was a Zener diode. However, it should be evident that the Zener could be replaced by a number of forward biased diodes or by a circuit having a Zener-diode like characteristic. It should also be evident that other types of transistors and other arrangements of complementary transistors may be used to practice the invention.

Claims: An overvoltage sensing circuit comprising: first and second nodes (20,22) at which first and second voltages are provided; a current flow path formed of a first resistor (R2), a second resistor (R1) and a voltage reference circuit element (Z1) coupled in series between said first and second nodes (20, 22); a first bipolar transistor (Q2) having a base, an emitter and a collector, the base and emitter of said first transistor (Q2) being connected across said first resistor (R2), said first transistor (Q2) being operative to sense current flow through said current flow path in response to current flow through said first resistor (R2) forward biasing the base-emitter of said first transistor (Q2) and causing current flow in the collector of said first transistor (Q2); and a second bipolar transistor (Q3) having a base, an emitter and a first collector (CO1), the first collector (CO1) and emitter of said second transistor (Q3) being connected across said second resistor (R1) and the base of said second transistor (Q3) being connected to the collector of said first transistor (Q2), so that current flow in the collector of said first transistor (Q2) is operative to turn on said second transistor (Q3) and reduce the voltage drop across said second resistor (R1), the first and second bipolar transistors (Q2,Q3) being of complementary polarity types such that when the first and second transistors (Q2,Q3) are rendered conducting, regenerative feedback causes them to go into a latch condition; which circuit is characterised in that the first collector (CO1) of said second transistor (Q3) is connected to the base of the first transistor (Q2) via said voltage reference element (Z1); the second transistor (Q3) has a second collector (CO2); and a controllable switching element (Q4) is arranged to be coupled in with a load and has a control input coupled to the second collector (CO2) of said second transistor (Q3). An overvoltage sensing circuit as claimed in Claim 1, comprising a third resistor (R4) connected in circuit across the base and emitter of said second transistor (Q3). An overvoltage sensing circuit as claimed in Claim 1 or 2, wherein said controllable switching element (Q4) comprises a third bipolar transistor having a base, collector and emitter, and said third transistor (Q4) has its collector-emitter current flow path coupled in circuit with said load, and its base coupled to said second collector (CO2) of said second transistor (Q2). An overvoltage sensing circuit as claimed in any one of Claims 1 to 3, wherein said voltage reference element (Z1) is a Zener diode.

Inventors: DESHAZO THOMAS TYAN JR, DESHAZO, THOMAS TYAN, JR.
Applicants: HARRIS CORP, HARRIS CORPORATION
Application year: 1992
Main classification: G05F1
IPC codes: G05F1, H02H3, G01R19