Semiconductor protection against high energy transients

Abstract: A switching circuit is provided for protecting semiconductors such as switching transistors against transients occurring in system voltage provided by an associated source of electrical energy. The circuit uses a switching device (18), including any number of avalanche transistors, to apply electrical energy from the source thereof to a load (20) with which it is in series. A voltage sensor (12) monitors the system voltage and generates a transient detection signal whenever it detects a voltage transient. A processing device (16) is used to process the transient detection signal and generate an output control signal that switches the avalanche transistors into a state of conduction so that the system voltage, including any voltage transients, appears substantially entirely across the load. Using avalanche transistors as switching transistors provides a significant margin of time for the detection, processing and switching actions to occur without damage being done to the switching transistors by a voltage transient.

Description: This invention relates to voltage transient protection devices that detect, and protect semiconductors such as switching transistors from, voltage transients. Electrical transient protection devices have been known for many years. For example, United States Patent No. 4,525,765, issued on June 25, 1985 to Brajder, discloses a protective circuit for a switching transistor. The protective circuit includes a comparator that compares the base-emitter voltage of the switching transistor with a reference voltage. Any increase in the collector current of the switching transistor will be reflected by an increase in base-emitter voltage, the base current remaining constant. Whenever the base-emitter voltage exceeds the reference voltage, the comparator generates an output signal. Driving pulses that switch the switching transistor into and out of a state of conduction are controlled by a device responsive to the output signals from the comparator so that the switching transistor is switched off when its collector current increases above a predetermined level, thereby preventing the flow of excessive current through the transistor. United States Patent No. 4,816,963, issued on March 28, 1989 to Eden, discloses an apparatus for protecting a transistor when an associated load is shorted. The circuit compares a command voltage with a reference voltage; and the transistor is turned on if the command voltage exceeds the reference voltage and is turned off if the reference voltage exceeds the command voltage. The reference voltage is developed across a capacitor that is charged quickly when the load is shorted and is discharged slowly. This keeps the transistor turned on for short periods and turned off for comparatively long periods, the former being short enough to ensure the protection of the transistor while the load is shorted. United States Patent No. 4,589,049, issued on May 13, 1986 to Krumrein, discloses a protection circuit for integrated circuits. The protection circuit is connected across the integrated circuit to be protected and includes a first transistor in series with a measuring resistor, the series combination thereof also being connected across the integrated circuit. A zener diode, or its equivalent, is connected between the base and collector of the first transistor so that the latter will conduct if an excessive amount of voltage is applied across the integrated circuit. Whenever the first transistor conducts, a voltage is developed across the measuring resistor. This voltage is compared to a reference voltage by a comparator the output of which is connected to the base of a second transistor, the collector of which is connected to the base of the first transistor. When the voltage is developed across the measuring resistor, the output of the comparator turns on the second transistor, which renders the first transistor more conductive. With the first transistor turned on, the series circuit of the first transistor and the measuring transistor shunts current flow therethrough, developing thereacross a relatively low voltage, which is also reflected across the integrated circuit to which it is connected in parallel. A limiting resistor is used between a source of operating potential and the first transistor to develop thereacross a major portion of voltage applied across the series circuit including the limiting resistor, the first transistor and the measuring resistor. United States Patent No. 4,849,845, issued on July 18, 1989 to Schmitt, discloses a transient suppressor for dissipating overvoltages developed across a power supply connected to a load. The transient suppressor senses an overvoltage condition with respect to a reference voltage and turns on a transistor connected across the power supply. The conducting transistor and a resistor connected in series with it across the power supply shunt current flow from the power supply, developing a relatively low voltage across the series circuit and across the integrated circuit to which it is connected in parallel, the resistor dissipating excess electrical energy while the transistor is conducting. When the voltage drops to a level below that of the overvoltage, the transistor is turned off. If the overvoltage continues, the transistor is turned on and off repeatedly. The number of transistor transitions are counted; and, if a predetermined number is exceeded, the operation of the transient suppressor is inhibited and the supply of electrical power is discontinued. The protective circuits disclosed in the first two patents, to Brajder and Eden respectively, are of the type that turn off the transistors to be protected when an electrical transient is detected. The circuits each require added current-sensing, decision and control circuitry and protect single devices. The last two patents, to Krumrein and Schmitt respectively, provide clamping circuits that shunt current around circuits to be protected. The Krumrein circuit requires an added overvoltage detector, current measuring circuit, clamping transistor and limiting resistor and protects a single device. The Schmitt patent requires an added overcurrent detector, decision and control circuitry, and clamping hardware. EP-A-0 294 887 discloses an arrangement for driving a semiconductor device having an input for receiving a control signal and a control output for connection to the control electrode of a power semiconductor device which is connected in series with a load across a power supply. The arrangement includes an active turn-off device to achieve rapid turn-off of the power semiconductor device. Furthermore, a threshold detector operates to turn on the power semiconductor device in the event of a high voltage transient on the power supply to reduce power dissipation within the device during transients. A second threshold detector is also provided to disable the operation of the active turn-off device during transients to prevent damage to the turn-off device. While each of these protection circuits functions with a certain degree of efficiency, none discloses a device for transistor protection against high energy transients wherein so few elements need to be added to the device to be protected, wherein switching transistors can have relatively low voltage ratings, wherein relatively long applications of high energy transients can be tolerated, wherein a number of devices can be protected by one protection device, and wherein the functions of a microcomputer, which may be used for other purposes in an associated circuit, can be shared by the protection device as does the present invention as is hereinafter more fully described. An object of the present invention is to provide improved voltage transient protection that requires a minimum of elements be added to devices to be protected. According to the present invention, there is provided a circuit for protecting a semiconductor switching device having a conductive state and a nonconductive state, the switching device switching a system voltage across a load in series with the switching device, the circuit comprising means for generating a transient detection signal as a function of a transient appearing in the system voltage and means for processing the transient detection signal to switch the switching device into its conductive state so that the system voltage, including any voltage transients, appears substantially entirely across the load, characterised in that the means for generating a transient detection signal comprises a source for providing a reference voltage that maintains a level equal to that of normal system voltage, a voltage divider having a first voltage divider resistor and a second voltage divider resistor connected in series between the source of system voltage and ground; a sensor transistor having base, emitter and collector elements, of which the base element is connected to the source of reference voltage, the emitter element is connected to a common connection between the first and second voltage divider resistors, and the collector element is connected to ground by way of a sensor resistor; and in that the processing means are connected to the collector element of the sensor transistor for processing the transient detection signal at the collector element of the sensor transistor and providing an output control signal to switch the switching device to its conductive state. The present invention provides an improved voltage transient protection that requires the use of switching transistors having lower voltage ratings than those typically required by previous devices and that accommodates a longer application of voltage transients without damaging protected devices. Further the present invention provides an improved voltage transient protection that can accommodate a number of devices to be protected. Further in the improved voltage transient protection, a microcomputer used for other purposes in an associated circuit, can be used as a part of a protection circuit thereby minimising the number of additional elements required for the protection circuit. A system embodying the invention includes transistorised switching circuit that incorporates protection against voltage transients occurring in system voltage provided by an associated source of electrical energy. The circuit uses any number of avalanche transistors as a switching device to apply system voltage to a load with which it is in series. The circuit includes a sensor to monitor the system voltage and generate a transient detection signal whenever it detects a voltage transient. A processing device, or microcomputer, is used to process the transient detection signal and generate an output control signal that switches the avalanche transistors into a state of conduction. The microcomputer can be used for other processing tasks in addition to its application in processing the transient detection signals from the sensor. With the avalanche transistors in a state of conduction and the resistance thereof being negligible compared to that of the load, the system voltage applied across the series circuit including the load and the avalanche transistors is developed almost entirely across the load. Since the switching transistors are avalanche transistors, sufficient time is provided for the detection, processing and switching actions to occur without damage being done to the switching transistors by the voltage transient. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which : Figure 1 is a block diagram of a switching circuit incorporating protection against voltage transients and embodying features of the invention; and Figure 2 is a schematic diagram of a circuit similar to that shown by Figure 1 and having a plurality of protected switching transistors controlling the application of electrical current to a like number of loads. With reference to Figure 1 of the drawings, shown is a switching circuit, generally indicated by the reference numeral 10, that includes protection against high energy electrical transients. The switching circuit 10 includes a voltage sensor 12 connected to a source of electrical energy that provides system voltage, as at reference numeral 14. A processing device, or microcomputer, 16 is connected between the voltage sensor 12 and a switching device, generally indicated by the reference numeral 18 and including any number of avalanche switching transistors. The switching device 18 is connected in series with a load, generally indicated by the reference numeral 20, to control the application of system voltage to the load 20. The voltage sensor 12 is also connected to a source of reference voltage, as at reference numeral 22. The microcomputer 16 need not be one provided exclusively for the switching circuit 10 or exclusively dedicated thereto. Indeed, an advantage of the present method for protecting a switching circuit is that it requires that a minimum of elements be added to existing system circuits to provide the voltage transient protection disclosed. Accordingly, the functions of an existing microcomputer already used in other parts of the system can be shared. Shown by Figure 2 of the drawings is a more detailed diagram of the switching circuit 10. It is basically the switching circuit 10 shown by Figure 1 but shows a number of individual switching transistors 24, 26 and 28 controlling the application of system voltage to a like number of individual loads 30, 32 and 34 respectively. In the preferred embodiment of the present invention, the switching transistors 24, 26 and 28 are avalanche transistors or other semiconductor devices having similar cascading characteristics when operated under relatively high reverse bias voltage conditions. It should be understood that the circuit could include a greater or lesser number of switching transistors 18 and associated loads 20 than shown by Figure 2. Figure 2 also shows the voltage sensor 12 in detail. It includes a voltage divider having a first voltage divider resistor 36 and a second voltage divider resistor 38 connected in series between the source of electrical energy and ground. A sensor transistor 40, having base, emitter and collector elements, is also included in the voltage sensor 12. The base element of the sensor transistor 40 is connected to the source of reference voltage as at reference numeral 22, and the emitter element thereof is connected to a common connection between the first and second voltage divider resistors 36 and 38 respectively. The collector element of the sensor transistor 40 is connected to ground through a sensor resistor 42. System voltage is divided by the first and second voltage divider resistors, 36 and 38 respectively; and the voltage developed across the latter is input to the emitter of the sensor transistor 40. When the system voltage includes a voltage transient of sufficient amplitude, the emitter-base junction of the sensor transistor 40 becomes forward biased, the sensor transistor 40 conducts, and a transient detection signal is developed across the sensor resistor 42 and is communicated to the microcomputer 16. During this period, the voltage transient is also felt across the switching transistors 24, 26 and 28, which begin to increase in temperature and to avalanche. While the switching transistors 24, 26 and 28 are reverse biased, their resistance is high; and a substantial portion of the system voltage is developed across them. Accordingly, a substantial portion of the voltage of the transient is also developed across the switching transistors 24, 26 and 28. The microcomputer 16 converts the analog transient detection signal into a digital signal for processing, generating in response a digital signal that is converted into an analog output control signal, having a duration of 80 milliseconds, that biases the switching transistors 24, 26 and 28 into a state of conduction before the latter have time to become deleteriously overheated. When the switching transistors 24, 26 and 28 begin to conduct, the resistance across each, and therefore the proportional voltage developed across each, decreases. Since the associated loads 30, 32 and 34 now represent essentially all the resistance in each series, load-transistor circuit, substantially all the voltage of the voltage transient is developed across the loads 30, 32 and 34. After the voltage transient has passed, the voltage developed across the second voltage divider resistor 38 and applied to the emitter element of the sensor transistor 40 is reduced. The emitter-base junction of the sensor transistor 40 becomes reverse biased, and the sensor transistor 40 ceases to conduct. With the sensor transistor 40 in a state of nonconduction, no current flows through the sensor resistor 42. Consequently, no voltage is developed across the sensor resistor 42; and no transient detection signal is communicated to the microcomputer 16. After a period of 80 milliseconds, if the microcomputer 16 is not receiving the same or a subsequent transient detection signal, it discontinues sending an output control signal to the base elements of the switching transistors 24, 26 and 28. As a result, the switching transistors 24, 26 and 28 are biased into a state of nonconduction. When the switching transistors 24, 26 and 28 cease conducting, their resistance increases; and a substantial portion of the system voltage is once again developed across them. At this point, the microcomputer 16 need not process any more signals from the voltage sensor 12 until another voltage transient is detected. Other circuits, using standard transistors, that might attempt to use some of the principles of the present switching circuit protection would require a number of additional circuit components, at additional cost, interconnecting transistor control gates because standard transistors fail quickly if excessive voltage is applied across them. Conversely, the use of avalanche transistors alone would not provide adequate protection either because the voltage of a voltage transient applied thereacross for more than several hundred microseconds could damage even an avalanche transistor. Required is the combination of the present circuit using avalanche transistors. The avalanche properties of the switching transistors 24, 26 and 28 provide sufficient time for the voltage sensor 12 to sense the voltage transient and generate a transient detection signal and for the microcomputer 16 to process the transient detection signal and bias the switching transistors 24, 26 and 28 into states of conduction before the latter are damaged by the voltage transient.

Claims: A circuit for protecting a semiconductor switching device (18) having a conductive state and a nonconductive state, the switching device (18) switching a system voltage across a load (20) in series with the switching device (18), the circuit comprising means (12) for generating a transient detection signal as a function of a transient appearing in the system voltage and means (16) for processing said transient detection signal to switch the switching device (18) into its conductive state so that the system voltage, including any voltage transients, appears substantially entirely across the load (20), characterised in that the means (12) for generating a transient detection signal comprises : a source (22) for providing a reference voltage that maintains a level equal to that of normal system voltage, a voltage divider (36,38) having a first voltage divider resistor (36) and a second voltage divider resistor (38) connected in series between the source of system voltage and ground; a sensor transistor (40) having base, emitter and collector elements, of which the base element is connected to the source of reference voltage (22), the emitter element is connected to a common connection between the first and second voltage divider resistors (36,38), and the collector element is connected to ground by way of a sensor resistor (42); and that the means (16) for processing are connected to the collector element of the sensor transistor (40) for processing the transient detection signal at the collector element of the sensor transistor (40) and providing an output control signal to switch the switching device (18) to its conductive state. A circuit as claimed in claim 1, for use with a plurality of switching devices (24,26,28), wherein the switching devices (24,26,28) are semiconductor devices of a type that initiates a chain generation of hole-electron pairs when operated at a relatively high reverse bias voltage. A circuit as claimed in claim 1, for use with a plurality of switching devices (24,26,28), wherein the switching devices (24,26,28) are avalanche transistors. A circuit as claimed in any preceding claim, wherein the means (16) for processing the transient detection signal comprises a microcomputer.

Inventors: WEBER CHARLES F, WEBER, CHARLES F.
Applicants: FORD FRANCE, FORD MOTOR CO, FORD WERKE AG, FORD FRANCE S. A., FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1992
Main classification: H03K17
IPC codes: H03K17