GB2181613A - Switching circuit - Google Patents

Abstract

The invention relates to a switching circuit for controlling the application of a d.c. voltage from power supply lines (2,3) to output terminals (4,5), to which a load, such as an armament fuze, is connected. The circuit comprises a number of first transistors (6,7,8) connected in a series string between one power supply line (2) and one output terminal (4), and a number of second transistors (11,12) connected between the junction points (9,10) of the first transistors and the other power supply line (3). A further second transistor (13) is connected between the output terminals. The second transistors are of the opposite conductivity type to the first transistors. When the load is to be energised, switching signals are applied to all of the transistors from secondary windings (16,17,18) of a transistor (14), in response to optical pulses fed to the transformer primary winding (15) over an optical fibre (34). A high-permeability screen (23), located between the primary and second windings as a safety device, must be removed in order to be able to apply the switching signals to the transistors. The switching signals ensure that the second transistors are turned hard off and that the first transistors are conductive for energising the load. In the event of a short-circuit occurring in one of the first transistors when the circuit is quiescent, conduction in that transistor will apply a large forward voltage across the associated second transistor, causing it to conduct heavily. The virtual short-circuit across the d.c. supply will cause a fuse (33) to blow, thereby preventing unwanted energisation of the load. <IMAGE>

Description

SPECIFICATION Switching circuit This invention relates to a switching circuit, and parti cularly to a high integrity switching circuit for use, for example, in control circuits for armaments.

Modern military aircraft, naval vessels, and landbased static and mobile artillery rely in the majority of casesforthe correctness and efficiency ofthe release, ejection, and fuzing of stores or projectiles, upon either electrical or electro-mechanical switching systems.

It is essential in the control of these armament circuits that all of the electrical switches and associated mechanical, optical, or chemical components are of the utmost reliability and highest integrity.

Paramount in the design and operation of such devices that in the event ofcircuitorcomponent malfunction or breakdown,the associated circuit or system remains in a safe condition.

Until relatively recently, it has been possible to use only mechanically-operated switches orelectromechanical relays to switch d.c. currents of any appreciable magnitude. Switches and relays which need to operate underthe stringent military and airborne environmental conditions, although not necessarily bulky individually, require considerable space when a multiplicity of such devices are connected in circuit configurations which provide the high degree of redundancy which is required.

Mechanically-operated electrical switches and electro-mechanical relays also suffer from the problem of chatter and contact bounce. As a consequence, this involves the extra complication of having to introduce delay, inhibit, or paralysis circuits in orderto prevent inadvertent operation through spurious sig nals or signals of too short du ration. Furthermore, mechanical devices of the necessary quality and reliability have very limited useful lives in modern armament release, initiation, and fuzing systems.

It is an object of the present invention to provide an improved high-integrity switching circuit.

According to the invention, a switching circuit for controlling the application ofa d.c. voltagefrom a pair of power supply input lines to a load comprises a plurality offirsttransistors of one conductivity type interconnected at junction points to form a series circuit between one of the power supply input lines and a load connection point; means operable to apply a switching signal to each firsttransistorto cause all of said transistors to conduct for connecting the pow ersupplytothe load; and a pluralityofsecondtran- sistors ofthe opposite conductivity type, each connected between a respective one of said points and the other power supply input line and operative to conduct if a voltage is applied thereacross from the power supply input line in the absence of the switching signals.

The transistors are preferably field-effect transis- tors, but may, alternatively, be bi-polartransistors.

Preferably, the means to apply a switching signal comprises transformer means having a primary winding and a plurality of secondary windings each connected to the gate or base electrode of a respective one of said first transistors; and means to apply a switching command signal to said primary winding.

An embodiment ofthe invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure lisa circuit diagram of a switching circuit in accordance with the invention, and Figure2 is a diagram of waveforms appearing in the circuit.

Referring to Figure 1, a switching circuit 1 is connected between a positive line 2 and a negative line 3 of a d.c. power supply. A load (not shown), such as an armament fuze, can be connected to a pair of output terminals4,5.

A number of N-channel field-effecttransistors 6,7 and 8 are connected in series between the line 2 and the output terminal 4, the source ofthe transistor 6 being connected to the drain ofthe transistor 7 at a point 9. Similarly, the transistor 7 and 8 are interconnected at a point 10. Although three transistors are shown, there may be any number of such transistors (in excess of one), depending upon the degree of redundancy required.

P-channel field-effect transistors 11, 12 and 13 are connected between the line 3 and the points 9, 10 and 4, respectively. In each case, the drain ofthetransistor is connected tothe line 3 and the source is connected to the respective point 9, 10 and 4.

Switching signals for the transistors are provided buy a radio-frequency attenuating coupling, comprising a transformer 14. The transformer has a primary winding 15 mounted on a core (not shown), and three centre-tapped secondarywindings 16,17 and 18 mounted on a common core 19, spaced from the core of the primary winding. In the gap between the cores are positioned grounded diaphragms 20 and 21 for screening thewindings, an insulating barrier22 and a removable, high-permeable, shutter 23.

Thecentretapofthesecondarywinding 16 is con- nected to the point 9, and the ends ofthe winding are connected to the anodes of diodes 24 and 25, respectively. The cathodes of the diodes are connected together, so that they provide a biphase half-wave rectified output at a point 26. That output is fed to the gate ofthe transistor 6 via a diode 27 and a resistor 28 connected in series.A resistor 29 is connected between the gate and the point 9, so that the resistors 29 and 28 and the diode 27 form a voltage divider circuit for biasing the transistor 6 and for feeding the switch ingsignalsthereto.Asimilarnetwork,comprisinga diode 30 and a resistor 31 connected in series between the point 26 and the gate of the transistor 11, together with a resistor 32 connected between the gate and the negative line 3, is provided for biasing the transistor 11 and forfeeding the switching signals thereto. The point9 is the common switching signal connection for both of the transistors 6 and 11. Simi lar arrangements are provided at the secondary wind- ings 17 and 18 for feeding the gates ofthe transistors 7,12and8,13.

In use ofthe circuit, under normal standby conditions no signal is fed to the primary winding 15, so the transistors 6,7 and 8 remain non-conductive, even when a d.c. voltage is applied to the lines 2 and 3.

Hence, the load remains unenergised. Suppose, now, thatthe transistor 6 develops a fault, so that it becomes short-circuited. The point 9 would rise substantiallyto the potential of the line 2, so that a high forward voltage would be applied across the transistor 11.

Furthermore, due to the substantially changed voltage now applied to the gate ofthattransistorvia the network 30,31,32, the transistor would become for- ward biased. The transistorwould conduct, and would thereby apply a short-circuit across the d.c.

supply, causing a fuse 33to blow. Undesired energisation of the load would thereby be prevented.

It will be seen that, with the arrangement shown, all ofthe transistors 6,7 and 8 wouid have to become short-circuited beforethe powersupplyvoltage would be applied to the load, and that as each such short-circuit occurs, the corresponding shunttransis tor 11,12 or 13 will also become short-circuited when the supplyvoltage appears across it.Averysafe arrangement is therefore provided.

When the load is to be energised, a pulse train is applied across the primary winding 15, either by direct application of an electrical signal from a remote position or by transmission of optical pulses along a fibre optic34,the optical pulses being translated into electrical pulses by a circuit (not shown) adjacent the primary winding.

With the grounded shutter 23 in place, there will be very little coupling between the primary and second windings, and no gating signals will be applied to the transistors. This shuttertherefore provides another safetyfeature, in case the primary winding isener- gised inadvertently. The shutter is particularly importantwhen the high integrity solid state switch is used in situations where explosive or inflammable pow ders, liquids, or gases are present. It is most effective when during primary initiation (i.e. application ofthe pulse train to the primarywinding) the secondary winding must remain in the de-energized state until a predetermined amount of relative movement between two bodies in a given direction has taken place.

When the shutter is removed, the primary flux links with the secondarywindings, and the rectified signals from the diodes are fed to the gates ofthetransistors.

The transistors 6,7 and 8will be switched on simultaneously, and the transistors 11, 12 and 13 will be switched off. The d.c. supply istherefore appliedto the load. If necessary, respective delay circuits 35,36 and 37 may be interposed in the gate circuits ofthe transistors 6,7 and 8, that those transistors turn on some microseconds after the transistors 11, 12 and 13 have turned off.

Diodes may be provided for clamping the gate drive voltage so that it does not exceed the power supply voltage, if considered necessary. Smoothing components may be provided for smoothing the rectified outputs ofthe secondarywindings, if this is warran-ted by circuit requirements.

An open circuit in any of the series transistors 6,7 or 8 our a short circuit in any of the shunt transistors 11,12 orl3would rendertheswitch unserviceable, but effectively in the open state. The only way in which the switch could fail in the closed state is for all the series transistors to fail in a short circuit condition and all the shunt transistors to fail in an open circuitcondi- tion.

In the circuit described above with reference to Figure 1, the switching transistors 6,7,8, 11, 12 and 13 are field-effecttransistors. However, it will be appre ciated that a circuit including suitable bi-polartransistors in the appropriate P and N configurations could alternatively be used.

Although a transformer/rectifier arrangement is shown as a very suitable arrangementforsupplying gate signals to the transistors, clearly it would be possibleto apply d.c. voltages directlytothe gates from aswitched d.c.supplyorfrom aTTLorother logic circuit.

Figure 2 ofthe drawings indicates waveforms occurring at various points in the circuit, as follows:- (a)the pulse train appliedtothe primary winding 15, (b) the zero output from each secondary winding when the shutter 23 is in place, (c) the voltage on one half of a secondary winding 16,17 or 18 when the shutter is removed, (d) the voltage on the other half of the secondary winding, (e) the resultant rectified secondary winding output, and (f) the conductive and non-conductive states of the transistors.

Claims (11)

1. Aswitching circuitfor controlling the applica- tion of a d.c. voltage from a pair of power supply input linestoa load,comprising a pluralityoffirsttransis- tors of one conductivity type interconnected atjunction points to form a series circuit between one ofthe power supply input lines and a load connection point; means operable to apply a switching signal to each firsttransistorto cause all of transistors to conduct for connecting the power supply to the load; and a plurality of second transistors ofthe opposite conductivitytype, each connected between a respective one of said points and the other power supply input line and operative to conduct if a voltage is applied thereacross from the power supply input line in the absence ofthe switching signals.

2. Acircuitasclaimed in claim 1, wherein the transistors are field-effect transistors.

3. A circuit as claimed in claim 1, wherein the transistors are bi-polartransistors.

4. A circuit as claimed in any preceding claim, wherein the means to apply a switching signal com prisestransformer means having a primary winding and having a plurality ofsecondarywindings each connected to the control electrode of a respective one of said first transistors; and means to apply a switching command signal to said primary winding.

5. A circuit as claimed in claim 4, wherein the meansto applya switching commandsignal com- prises optical fibre means to receive an optical signal fortransmission therealong; and means to convert the optical signal transmitted along the optical fibre means into an electrical signal for application to the transformer primary winding.

6. A circuit as claimed in ciaim 4 or claim 5, wherein the transformer means includes a highpermeability screen positioned between the magnetic paths of the primary and secondary windingsto substantially prevent coupling between the windings, the screen being removable when the switching signal is to be applied to the transistors.

7. A circuit as claimed in any one of claims 4-6, including electrostatic screening means located between the primary winding and the second windings.

8. A circuit as claimed in any one of claims 4-7, wherein each secondary winding is centre-tapped, and each end of the secondary winding is connected toa respectivediodetoform a biphasehalf-wave rectifier circuit.

9. A circuit as claimed in any preceding claim, wherein the switching signal is applied to each transistorvia a respective network which provides a bias voltage forthattransistor.

10. A circuit as claimed in any preceding claim, including respective delay means for delaying the application ofthe switching signal to each firsttransistor relative to the application of the switching signal to each second transistor.

11. A switching circuit for controlling the application of a d.c. voltage from a pair of power supply input lines to a load, substantially as hereinbefore described with reference to the accompanying drawings.