GB2204445A

GB2204445A - Semiconductor switch

Info

Publication number: GB2204445A
Application number: GB08705328A
Authority: GB
Inventors: Andrew Marshall; Derek Colman; Philip John Cavanagh; David Peter Dale
Original assignee: Texas Instruments Ltd
Current assignee: Texas Instruments Ltd
Priority date: 1987-03-06
Filing date: 1987-03-06
Publication date: 1988-11-09
Anticipated expiration: 2007-03-06
Also published as: GB8705328D0; GB2234112B; GB2204445B; GB9011928D0; JPS63308379A; US5289043A; GB2234112A; US4977476A

Description

1 1 0 Semiconductor switch 220444.5 This invention relates to

semiconductor switches.

Traditionally the feeding of electrical pow - er to ancillary devices of a motor vehicle such as lamps for marking the boundaries of the vehicle, lighting the road ahead or indicating an intention to move to the left or to the right, o r electric motors for driving a windscreen wiper, fans, fluid.pumps or raising and lowering windows, for example, has been controlled by switches near the steering wheel connected to the devices by individual wires of sufficient thickness to carry the currents involved. The cost of such wiring and switches is a major component in the --,;,era!! cost of the vehicle and L-heir total weight is considerable.

is Moreover the mechanical switches can be unreliable because of contact fouling and erosion.

The need for a loom of wires and switches capable of carrying the currents can be avoided by the use of relays local to the devices but a complex wiring loom is still required for the operating signals for the relays and the relays are still subject to wear, such as contact erosion.

In order to eliminate the need for the wiring loom and to improve the reliability of the system it has recently been proposed t o provide perhaps only one current carrying conductor for all the ancillary devices, which are connected to the conductor through individual i G local switches and to multiplex control signals for these switches on to a signalling conductor. The switches have decoding circuits for detecting the control signals addressed to them and power switches for implementing the control required. Although relays could be used in such multiplexed control systems for switching the power to the loads, it is-preferable to use semiconductor switches because not only are they more reliable but also they enable the functioning of the system and the load itself to be monitored. Additionally the elimination of mechanical contacts means that the system is safer because no sparks are produced.

Various proposals for a semiconductor switch C for such an application have been made. Circuits composed of discrete devices have been too expensive for wide acceptance. Circuits using a single semiconductor die, though still expensive, would be cheaper in volume prodijction but have suffered from the disadvantage, arising from the use of a DMOS output stage, that they cannot achieve the low voltage drop when on, required with currents up to for instance 6 amps with a heat sink temperature of up to 110 0 C.

It is an object of the present invention to provide a semiconductor switch suitable for a multiplex wiring- system for a motor vehicle in which the above disadvantages are overcome. Although the'invention is of particular value in the motor vehicle application it can also be 1 1 1 used in a wide range of other industrial and consumer application. According to one aspect of the present invention there is provided 5 a semiconductor switch device reponsive to a control signal selectively to pass current to a load and to block the passage of current to the load and including a first semiconductor comDonent containing one or more power I'Dipolar semiconductor switching elements each of vertical construction having a controlled current path extending from one major face to the other of a semiconductor body and arranged for connection in series with the load across a power supply, and control input means connected to the control base of the or each power bipolar semiconductor switching element, and a second semiconductor component containing mea ns responsive to the control signal selectively to produce drive signals which are applied to the control input-means of the first semiconductor component for causing the power bipolar semiconductor switching element or elements to conduct, wherein the control input means includes voltage dropping means formed in the control base region of the or each power bipolar semiconductor switching element.

The power bip olar semiconductor switching element may be a transistor or a darlington multiple and may be i 0 made in different current handling capacities to suit different loads.

The first semiconductor component may include a single power bipolar semiconductor switching element which is connected in series with the load to switch on and off the electric current through the load. Alternatively, the first semiconductor component may include a bridge circuit of power bipolar semiconductor switching elements with the power supply and the load connected so that not only can the electric current feed to the load be switched on and off bu also its polarity can be reversed. The drive signals produced by the second semiconducto-r component are arranged to provide the polarity of ener isation of the load indicated by the control signals. The four switching elements each with its own voltage dropping means may be formed on separate semiconductor bodies or in pairs on two semiconductor bodies. If all four switching elements ate formed on a single semiconductor body steps must be taken to iso]Ate the collectors of one pair of elements from the collectors of the other pair.

The voltage dropping means may consist of a series connected resistor or constant current circuit in the base lead of the or each power bipolar semi- conductor switching element.

The second semiconductor component may be an integrated circuit of relatively small current handling i capacity dissipating relatively little heat since the drive signal for the power bipolar semiconductor switching element can be derived from the power supply through a transistor or transistors in very low or very high resistance states only, because the voltage dropping means for determining the drive current of the power switching element is provided in the first semiconductor component.- The second semiconductor component may be arranged to monitor the current through the load and.the voltage applied to it to detect, for example, open- and shortcircuit conditions of the load, and produce an output indication of such abnormal status of the load. Timing means may be provided in the second semiconductor compo- "ch an excessive current nent to determine the time for w.A is drawn by the load, and to terminate the drive current to the first semiconductor component if the excessive current is drawn for more than a predetermined time.

The first semiconductor com ponent may include a temperature sensor such as a diode or Schottky diode integrated with the power bipolar semiconductor switching element(s). The forward conduction voltage or the reverse bias leakage current of such a diode would indicate the temperature component. The sensor may be connected to divert current from the base of the switching element to tend to terminate its conduction if the temperature becomes too'high.

The two semiconductor components may be assembled C) 1 6 - into separate packages or a single package. Since most of the heat dissipation will be from the first component the packaging may be arranged so that this component is attachable to a heat sink, and no particular arrangements need be made to dissipate the heat from the second component.

According to a second aspect of the invention there is provided a semiconductor component having a power bipolar semiconductor switching element of vertical construction in a semiconductor body with a controlled current path extending from one major face to the other of the semiconductor body and arranged for connection in series with a load across a power supply and a conji.-rol inpu-L terminal connected to the control base of the semiconductor switching element through voltage dro-pping means formed in the control base region of the semiconductor switching element.

The semiconductor switching element may be a -20 transistor or a darlington multiple.

The voltage dropping means may be a resistor or a constant current circuit connected in a series path from the control input terminal to the control base of the semiconductor switching element.

An over-temperature sensor may be provided in the semiconductor body to detect excessive current through the switching element for a period of time. The sensor may be a diode or a Schottky diode of which i 0 either the voltage across it in forward conduction or the reverse bias leakage current is monitored. The sensor may be connected to divert current from the base of the switching element to terminate its conduction if the temperature of the semiconductor body becomes too high.

According to a third aspect of the invention there is provided a semiconductor component having a power bipolar semiconductor switching element of vertical.construction in a semiconductor body with a controlled current path extending from one major face to the other of the semiconductor body and arranged for connection in series with a load across a power supply, a control input terminal connected to the control base of the semiconductor switch- ing element through voltage dropping means and an overtemperature sensor in the semiconductor body responsive to excessive temperature of the semiconductor body to divert drive current from the control base Of the switching element and terminate its conduction if the temperature of the semiconductor body becomes too high.

The semiconductor switching element may be a transistor or a darlington multiple.

The voltage dropping means may be a series resistor or a constant current source circuit and may be formed in the control base region of the semicondctor body.

The-overtemperature sensor may be either a bipolar diode or a Schottky diode of which either thd voTtage:--azross 0 8 it in forward conduction or the reverse bias leakage current is used to control the diversion of the drive current.

According to a further aspect of the invention there is provided a switching system for enabling electrical power to be applied selectively to a plurality of loads in which control signals are transmitted fr om a central controller to a plurality of local switches respectively connected to the loads, each local. switch including a first semiconductor component containing one or more power bipolar semiconductor switching element each of vertical construction and having a controlled current path extending from one major face to another of a semiconductor body arranged for connection in series with the load across a power supply, and control input me 1 ans connected to the control base of the or each power bipolar semiconductor switching element through voltage dropping means formed in the control base regions, and a second semiconductor component containing means responsive to the control.signal selectively to produce drive signals which are applied to the control input means of the first semiconductor component for causing' the power bipolar semiconductor switching element or elements to conduct.

S J 0 - 9 The control signals, may be transmitted from the central controller to the local switches via a common control line to a plurality of local controlle;cs each connected to one or more local switches via individual lines, the local controllers including decoding means responsive to codes contained in the control signals to detect and forward the control signals to the local switches to which they are addressed.

The local switches may be arranged to generate status signals indicating the functional states of the loads to which they are connected, the status signals being returned to the control controller where a display of the status, particularly if it is a fault, may be produced. Where a local controller is provided it may additionally or alternatively be connected to one or more local sensors and generate report signals indicating the sensed conditions which are transmitted to the central controller for display or monitoring.

The switching system may be used in a vehicle such as a motor vehicle, the central controller being arranged to respond to inputs from the driver to produce control signals and to produce displays visible to him.

In order that the invention may be fully understood and readily carried into effect, embodiments of it will now be described with reference to the accompanying drawings, of which:

Figure 1 is a diagram of an example of a system incorporating switches according to an example of the invention; Figure 2 is a diagram of the switch used in Figure 1; Figure 3 shows the circuit of a power transistor suitable for use in the switch of Figure 2 with a resistor integrated into the base lead producing a parasitic transistor; Figure 4 is a cross-sectional diagram of the transistor shown in Figure 3; Figure 5 is the circuit of an alternative powertransiStor suitable -for use in the switch of Figure 2 with a constant &urrent source integrated into the base lead in place of the resistor shown in Figure 3; Figure 6 is a cross-sectional diagram of the transistor shown in Figure 5; Figure 7 shows the circuit of a power transistor with one form of over- temperature protection; Figure 8 shows the transistor of Figure 5 with the over-temperature protection shown in Figure 7; Figure 9 is a cross-sectional diagram of parts of the transistor s ' hown in Figure 8; Figure 10 is a diagram of an alternative form of switch for energising a load with currents of different polarities; and Figure 11 shows a modification to the switch shown in Figure 10.

0 1 - 11. - The system shown in Figure 1 could be used in a motor vehicle or in an industrial or domestic installation where remote control of the energisation of a number of electrical devices is required. A central controller 1 is connected to receive inputs from a unit 2, which may, for example, comprise a keyboard or a plurality of manually operable switches, and supplies outputs to indicators and displays 3, including, for example, warning lamps, light-emitting'diode and/or liquid crystal displays, and digital to analogue converters connected to meters.

The central controller 1 is connected via a bus line 4 to a plurality of local controllers of which only one is shown having the reference 5. The local controller 5 is energised via power supply conductors 6 and 7, for example connected to the motor vehicle battery, and these conductors with the bus line 4 may conveniently beformed into a single cable which is routed about the vehicle as required.

Alternatively the earth return may be through the bodychassis structure of the vehicle in which case the earthed conduc tor may be omitted. The local controller -5 is connected via individual conductor pairs 8,9 to local switches of which only 2 are shown havihg the references 11 and 12. The local controller 5 is also connected via conductors 13 to receive inputs from _. local sensors of.which only one is shown.

k c z Each local switch, 11 or 12, includes a linear. chip 15 to which the conductor pair 8,9 is connected, a power chip 16 and a resistor 17. The power chip 16 and the resistor 17 are connected in series with each-other and in series with a load 18, the energisation of which the local switch is to control, across the power supply conductors 6 and 7. The linear chip 15, which is also connected to the power supply conductors 6 and 7, is connected to control the power chip 16 via a conductor 19 and has two other leads 20 and 21 Connected respectively to the end terminals of the resistor 17.

1 _f 0 _i The central controller 1, which may be constructed as a conventional microcomputer having a microprocessor connected by data and address buses to random access memory,' read-only memory and input-output circuitry, repeatedly scans the switches in the input unit 2 and reacts whenever there is. a change in the state of the switch. The status of the switches are stored in the random access memory of the controller 1, and depending on the change of state of the switches in the unit 2, the controller 1 generates coded -control signals representing the changes of energisation, off or on, required for the various loads 18. These coded control signals are multiplexed onto the conductor 4 and each includes a code identifying to which of the loads 18 the control signal is addressed.

A local controller 5, which may also include a conventional microcomputer, detects those of the coded control signals which are addressed to loads under its. command and records the energisation-status of those loads, changing the status in response to the coded control signals as they are received. The local controller 5 generates an -output on the conductor 9 which is connected to the local switch, say 11, controlling.the energisation of the load 18 to which the change relates. In the local switch the linear chip 15 contains a multivibrator the state of which is set or reset depending on whether or not the corresponding load 18 is to be energised or not.' The linear chip 15 provides a drive signal via the conductor 19 to switch on and off-a power transistor included in the power chip 16, thereby t C 14 switching the energisation of the load. 18 on and off. The resistor 17 included in series with the load 18 will develop a-voltage due to the current through the load, which voltage is.conveyed by the conductors 20 and 21 to the linear chip 15. In addition, the voltage across the load 18 is also provided-y the conductor 21. The linear chip 15 contains circuitry for detecting short and open circuit conditions of the load 18 and a signal representing these conditions is conveyed by the conductor 8 back to the local controller 5..Preferably, the signal on the conductor 8 from the linear chip 15 which represents satisfactory operation of the load is an active one so that should the local switch fail an indication of its failure will be conveyed to the local controller 5.

The status information obtained from the local switches together with data from local sensors connected to the controller 5 are stored in it and communicated to the central controller 1 between the multiplexed coded control signals emitted by the controller 1. Typically, a local sensor may include a variable resistor the resistance of which is controlled by the c juantity being sensed, and an analogue to digital converter is used to change a voltage proportional to that resistance to a digital value which is used by the local controller 5. The indicators and displays 3 are energised by the central controller 1 to provide information to the driver of the quantities sensed by the vaious local sensors, the energisation-of the various loads 18 and whether any of those loads has failed.

1 0 1 -15 Figure 2 shows in some more detail the construction of a local switch and in this figure the reference numerals used correspond to those used in Figure 1.

As shown in Figure 2, a power chip 16 includes a PNP power transistor 25 and a resistor 26 connected in series in its base connection. It is a feature of the present invention that the local switch consists of two semiconductor dies, the linear chip 15 and the power chip 16, which may be assembled into two packages or combined into a single.package. As mentioned above, the linear chip contains all the logic to communicate with the central controller via the local controller, to detect fault conditions and to control the power chip. The power chip reacts to the control signals from the linear chip and switches the current to the load 18. The PNP transistor 25 in the power chip is constructed so as to be capable of handling the current required by the load 18 with a sufficiently low voltage drop through its controlled current path when on.

In a modification the PNP transistor may be replaced by a darlington pair or triple.

The PNP transistor 25 has the resistor 26 connected in series in its base lead, which is integrated into the semiconductor die on which the power transistor is formed, to provide a voltage drop so that the drive signal applied to the conductor 19 by the linear chip is connected through low' resistance switches either to the negative supply conductor 6 or the positive supply conductor 7, so that power diss.ipation in the linear chip 15 arising from the control of the drive JO 16 - current for the transistor 25 is very low, and a major part of the heat dissipation of the switch is from the power chip 16 rather than the linear chip 15. This means that the heat sinking requirements of the local switch as a whole can be concentrated on the power chip 16, with very relaxed assembly requirements for the linear chip 15 which can therefore be mounted in a low cost package.

The use of a PNP transistor as a switch element of the power chip provides an optimum solution for a motor vehicle environment due to its very low on-voltage capability. As mentioned above, the transistor could be repiaced bv a darlington multiple, such as a darlington pair, which would give a much greater gain and-allow the switching of higher load currents using a low drivd current from the linear chip 15.

The use of a simple series resistor as the voltage dropping structure connected in the base lead of the PNP transistor has the advantage that it can be produced as part of the emitter diffusion process or by a separate diffusion step during the fabrication of the transistor. The resistor has the disadvantage that it results in a parasitic transistor being formed which can bypass a fraction of base drive current when the PNP power transistor is in saturation. Figure 3 is a circuit diagram showing the parasitic transistor as T2 connected to the power transistor T1. The bypassing of.the base drive current can be reduced by including an extra heavy base diffusion ar'ound the region forming the resistor. This extra diffusion is shown in _f 1 c 17 Figure 4, which shows a diagr-ammatic cross-section of the PNP power transistor and the series re'sistor connected in its base lead.

in Figure 4, an epitaxial layer 31 of P-type conductivity is formed on the surface of a P +_ conductivity substrate 30. A base region 32 of N- type conductivity is formed in the epitaxial layer 31, and an emitter region of P +_ type conductivity is formed in the base region. This constitutes the PNP power transistor.- The resistor is formed by a P +_ conductivity region 34 in an extra heavy base diffusion region 35, itself within the base region 32. As mentioned above, the extra heavy base region 35 serves to reduce the gain of the parasitic inverse transistor shown as T2 in Figure 3.

A constant current source circuit can be used instead of the simple resistor in the base lead of the transistor switch element. Figure 5 shows such a circuit connected in series in the base lead of the powertransistor TI and includes an NPN transistor T3 having its emitter lead connected through a resistor R4 to a terminal A and having its collector connected to the base of the transistor'Tl. A series chain of resistors R3 and two diodes D2 and D3 is 'also connected from the base of the 'transistor Tl to the terminal A with the junction between the resistor R3 and the diode D2 being connected to the base of a transistor T3. The operation of the constant current circuit is conventional in that the forward conduction voltage of the diodes D2 a:nd D3 provides a bias voltage for the transistor T3 to conduct just A- Q f: - 18 sufficient current through the resistor R4 to maintain the transistor T3 in conduction. Although the fabrication of the con,stant current source requires an extra diffusion step as compared with the fabrication of the simple resistor shown in Figure 4, the use of a constant current source has the advantage that the base current of the power transistor T1 is independent of the supply voltage which in a motor vehicle can vary between 7 and 16 volts. Since the base current of the power transistor T1 is kept constant, the drive current produced by the linear chip 15 can be kept more nearly to the minimum necessary to operate the power transistor with the result that the size and cost of the linear chip can be considerably reduced.

Figure 6 is a cross-secItional diagram of the circuit shown in Figure 5 formed in a semiconductor body. The parts of the structure which constitute the elements of the circuit shown in Figure 5 are indicated in Figure 6. The constant current source circuit also produces a parasitic tansistor in a similar way to the resistor, and the effects of this parasi-tic transistors are reduced as described above by_the provision of N regions containing the resistors and diodes of the circuit.

The power chip may be provided with internal selfprotection against excessive current through it due to shortcircuits, for example, and to excessively high temperatures which could lead to damage to the semiconductor devices. As mentioned above, the linear chip can be used to detect shortcircuiting of the load 18 by monitoring the voltage on the tl 4 19 conductor 21. Because of the nature of the loads in certain instances, for example incandescent lamps or motors, the load itself.may appear as a short-circuit when first turned on and only reach its normal running resistance after a delay of a fraction of a second. It follows therefore that any protection against excessive current through the power chip must have a built-in delay before conduction is terminated to protect the semiconductor devices. To provide this delay the linear chip may be provided with a timer which prevents any protective action being taken until an appropriate delay of, say, 0.3 seconds has elapsed following the start of an a pparent short-circuit. If the voltage across the load or the current through it has not reached an acceptable value 4 4k. 1_ within the delay period, then the linear cl-JJ4.D would be arranged to remove the drive current from the power chip to provide a controlled shut down of the circuit. The load current may alternatively be monitored by measuring the voltage across the resistor 17 connected in series with the load. This resistor need not be a discrete element or a thin or thick film deposited layer, but may simply be a section of printed circuit board conductor which is already included in the system.

An alternative to measuring the excessive current after a delay time, is to integrate into the power.chip an over-temperature sensor. The use of such a sensor has the advantage that the necessary time delay is automatically provided by the heat capacity of the chip itself and, in addition, monitors the temperature of the. chip which is a f -r, -WI.

certain indicator of the likelihood of damage to the chip. In one example of the over-temperature sensor, a forward biassed diode is provided on the power chip and receives a small current from the linear chip. The voltage drop across the diode is monitored by the linear chip to provide information to that chip relating to the temperature of the power chip. In another example, the diode on the power chip is reverse biassed and the leakage current from the diode is monitored by the linear chip to provide information about the temperat ure of the power chip. The sensitivity of the measurement provided by the latter example would be enhanced by the use of a Schottky diode having a higher reverse bias leakage current. A disadvantage of this arrangement is that it requires two additional connections Lrom the linear c1hilp to the power chip.

A further example which requires no alditional connections from the linear chip to the power chip involves proViding a diode or other temperature sensor struture on the power chip together with necessary extra circuitry for diverting the base drive current of the power transistor away from the base so as to starve the transistor of base drive and turn it off. Figure 7 shows the circuit of an example in which the leakage current of a Schottky diode Dl fed through a resistor R2 from the base drive circuit for the power-chip is amplified by a darlington pair consisting of transistors T4 and T5.

c.

21 The use of an over-temperature sensor obviates the -need for a timer on the linear chip, thereby giving a considerable saving in die area and therefore cost.

The over-temperature sensor just described may be combined with the constant current circuit shown in Figure 5,. giving the circuit shown in Figure 8. The individual circuit elements are shown in the cross-sectionof a semiconductor body in Figure 9. To produce the circuit of Figure 8 it is necessary to produce appropriate numbers of components required by the circuit and interconnect them as shown in the figure.

As described above, the local switches are suitable for single ended loads with one terminal grounded. Many electric motors in motor vehicles, however, require switching of the direction of the current flow to reverse the motor. An example of this is the use of a motor to open and close the windows of the vehicle. Reversal of the current flow could be achieved by the use of four.semiconductor switching elements, such as transistors each with additional circuit components as described above, connected in a bridge configuration with the power supply connected across one diagonal of the_bridge and the load connected across the other diagonal. Obviouslyi the linear chip controlling such a power chip or chips would have to be able to turn on diagonally opposite pairs of. transistors depending on the direction of current flow required through the load. Figure 10 is a diagram of a suitable circuit for such a local' switch. The transistors together with their base series < L.. 22 resistors.or other circuits may be formed on separate chips or combined on one or two chips.

Figure 11 shows a modification of Figure 10 in which two transistors are connected to be driven directly from-the linear chip and the other two transistors are driven by the output currents of the first two transistors. This arrapgement has the advantage of reducing the dissipation from the linear chip and also simplifying it.

Although the invention has been described with reference to specific examples concerned with the particular problems encountered in motor vehicles, it will be appreciated that the same techniques would be applicable to the energisation of a plurality of remotely located loads from a central control position both in domestic and industrial environments.

23

Claims

WHAT WE CLAIM IS:-

1. A semiconductor switch device responsive to a control signal selectively to pass current to a load and to block the passage of current to the load and including a first semiconductor component containing one or more power bipolar semiconductor switching elements each of vertical construction having a controlled current path extending from one major face to the other of a semiconductor body and arranged for connection in series with the load across a power supply, and control input means connected to the control base of the or each power bipolar semiconductor switching element, and a second semiconductor component containing means responsive to the control signal selectively to produce drive signals which are applied to the control input means of the first semiconductor component for causing the power bipolar semiconductor switching element or elements to conduct, wherein the control input means includes voltage dropping means formed in the control base region of the or each power bipolar semiconductor switching element.

2. A device according to claim 1 wherein the or each bipolar semiconductor switching element is a transistor or a darlington multiple transistor.t

3. A device according to claim 1 or claim 2 wherein the first semiconductor component includes a single power bipolar switching element arranged for connection in series with the load to enable the current through the load to be switched on and off.

4. A device according to claim 1 or claim 2 wherein the first semiconductor component includes a bridge circuit of four power bipolar semiconductor switching elements connectible to the power supply and the load so that the electric current feed to the load can be switched on and off and reversed in polarity, the drive signals producible by the second semiconductor component being arranged to cause the first semiconductor component to provide a desired polarity of energisation of the load in response to the control signals.

f k 24

5. A device according to claim 4 wherein the four power bipolar semiconductor switching elements each with its own voltage dropping means are formed on separate semiconductor bodies.

6. A device according to claim 4 wherein the four power bipolar semiconductor switching elements each with its own voltage dropping means are formed in pairs, respectively on two separate semiconductor bodies.

7. A device according to any pr-eceding claim wherein the voltage dropping means consists of a series connected resistor in the lead to the control base region.

8. A device according to any one of claims 1 to 6 wherein the voltage dropping means consists of a constant current circuit connected in the lead to the control base region.

9. A device according to any preceding claim wherein the second semiconductor component is arranged to produce the or each drive signal from the power supply through one or more transistors, the conductivity of each of which is switched between a very low resistance state and a very high resistance state in producing the drive signal,so that the second semiconductor component dissipates relatively little heat.

10. A device according to any preceding claim including means responsive to the current through the load and the voltage applied to the load and connected to the second semiconductor component to enable it to detect an abnormal status of the load, such as an open circuit or a short-circuit condition, and produce a corresponding output indication.

11. A device according to claim 10 wherein the second semiconductor component includes timing means for measuring the time for which a current in excess of a threshold value is drawn by the load, and means responsive to the timing means to terminate the drive current to the first semiconductor component if the excessive current is drawn for more than a predetermined time.

12. A device according to any one of the preceding claims in which the first semiconductor component includes temperature sensing means integrated with the power bipolar semiconductor switching element or elements.

el 1 f J f

13. A device according to claim 12 wherein the temperature sensing means is a bipolar diode or a Schottky diode of which the forward conduction vvItage or the reverse bias leakage current is dependent on the temperature of the diode.

14. A device according to claim 12 or claim 13 wherein the temperature sensing means is connected to divert current from the control base region of any power bipolar semiconductor switching element which is conducting at the time so as to terminate its conduction if the sensed temperature becomes too high.

15. A device according to any preceding claim wherein the first and second semiconductor components are mounted in separate packages, the package containing the first semiconductor component being arranged to be attachable to a heat sink.

16. A device according to any one of claims 1 to 14 wherein the first and second semiconductor components are mounted in the same package in such a way that the first semiconductor component is attachable to a heat sink.

17. A semiconductor component having a power bipolar semiconductor switching element of vertical construction in a semiconductor body with a controlled current path extending from one major face to the other of the semiconductor body and arranged for connection in series with a load across a power supply and a control input terminal connected to the control base of the semiconductor switching element through voltage dropping means formed in the control base region of the semiconductor switching element.

18 A component according to claim 17 wherein the semiconductor switching element is a transistor or a darlington multiple transistor.

19. A component according to claim 17 or 18 wherein the voltage dropping means includes a resistor connected in a series path from the control input terminal to the control base of the semiconductor switching element.

20. A component according to claim 17 or claim 18 wherein the voltage dropping means includes a constant current circuit connected in a series path from the control input terminal to the control base of the semiconductor switching element.

26

21. A component according to any one of claims 17 to 20 further including temperature sensing means on the semiconductor body responsive to the temperature of the semiconductor switching element.

22. A component according to claim 21 wherein the temperature sensing means includes a bipolar diode or a Schottky diode and means responsive to either the forward conduction voltage or the reverse bias leakage current of the diode.

23. A component according to claim 21 or claim 22 wherein the temperature sensing means is connected to divert current from the control base of the semiconductor switching element to terminate its conduction if the temperature of the semiconductor body becomes too high.

24. A semiconductor component having a power bipolar semiconductor switching element of vertical construction in a semiconductor body with a controlled current path extending from one major face to the other of the semiconductor body and arranged for connection in series with a load across a power supply, a control input terminal connected to the control base of the semiconductor switching element through voltage dropping means and an over-temperature sensor in the semiconductor body responsive to excessive temperature of the semiconductor body to divert drive current from the control base of the switching element and terminate its conduction if the temperature of the semiconductor body becomes too high.

25. A component according to claim 24 wherein the power bipolar semiconductor switching element is a transistor or a darlington multiple transistor.

26. A component according to claim 24 or claim 25 wherein the voltage dropping means includes a resistor formed in the control base region of the semiconductor body.

27. A component according to claim 24 or claim 25 wherein the voltage dropping means includes a constant current source circuit formed in the control base region of the semiconductor body.

28. A component according to any one of claims 24 to 27 wherein the over-temperature sensor includes a bipolar diode A( 27 or a Schottky diode of which either the forward conduction voltage or the reverse bias leakage current is connected to control the diversion of the drive current.

29. A switching system for enabling electrical power to be applied selectively to a plurality of loads in which control signals are transmitted from a central controller to a plurality of local switches respectively connected to the loads, each local switch including a first semiconductor component containing one or more power bipolar semiconductor switching elements each of vertical construction and having a controlled current path extending from one major face to another of a semiconductor body arranged for connection in series with the load across a power supply, and control input means connected to the control base of the or each power bipolar semiconductor switching element through voltage dropping means formed in the control base region, and a second semiconductor component containing means responsive to the control signal selectively to produce drive signals which are applied to the control input means of the first semiconductor component for causing the power bipolar semiconductor switching element or elements to conduct.

30. A system according to claim 29 including a plurality of local controllers connected to receive the control signals from the central controller via a common control line, each local controller being connected to one or more local switches via individual lines, the local controller including decoding means responsive to code contained in the control signals to detect and forward the control signals to the local switches to which they are addressed.

31. A system according to claim 30 wherein a local controller is also connected to one or more local sensors and is responsive thereto to generate report signals indicating the sensed conditions, the report signals being transmitted to the central controller which is arranged to monitor the sensed conditions.

32. A system according to claim 29, claim 30 or claim 31, wherein the local switches are arranged to generate status 28 signals indicating the functional states of the loads to-which they are connected.

33. A system according to claim 32 wherein the status signals are returned to the central controller, the central controller including means responsive to the status signals to produce a display representing the status.

34. A switching system substantially as described herein with reference to, and as shown in, the accompanying drawings.

35. A motor vehicle incorporating a switching system according to any one of claims 29 to 34 connected to control the energisation of at least one part of the vehicle.

36. A semiconductor switch device substantially as described herein with reference to, and as shown in, Figure 2 or 10 or 11. together with Figures 3 and 4 or Figures 5 andfi, of the accompanying drawings, with the modification described with reference to Figure 7.

37. A semiconductor switch device substantially as described herein with reference to, and as shown in, Figur.e 2 or 10 or 11, together with Figures 3 and 4 or Figures 5 and 6, without the modification described with reference to Figure 7 of the accompanying drawings.

38. A semiconductor component substantially as described herein with reference to, and as shown in, Figures 3 and 4 of the accompanying drawings, with the modification described with reference to Figure 7.

39. A semiconductor component substantially as described herein with reference to, and as shown in, Figures 3 and 4, without the modification described with reference to Figure 7 of the accompanying drawings.

40. A semiconductor component substantially as described herein with reference to, and as shown in, Figures 5 and 6 of the accompanying drawings, with the modification described with reference to Figures 8 and 9.

41. A semiconductor component substantially as described herein with reference to, and as shown in, Figures 5 and 6, without the modification described with reference to Figures 8 and 9 of the accompanying drawings.

Publibhed 1988 at The Patent Office, State House. 68171 High Holborn, London WC IR 4TP. Further copies maybe obtained from The Patent Office, Sales Branch, St MaTy Cray, Orpington, Kent BRS 3RD. Printed by Multiplex techniques ltd, St MaTy Cray, Kent. Con. 1/87.

1