GB2025180A - Transistor electronic switching circuits - Google Patents

Abstract

A single chip Darlington transistor pair device TR1, TR2 has a fourth contact effected to the base of the output transistor of the pair. This permits independent current control of the two transistors. A switching circuit including such a device is shown wherein feedback to the fourth contact reduces the effective storage time. <IMAGE>

Description

SPECIFICATION Improvements in or relating to electronic switching circuits This invention relates to semiconductor devices and in particular to a modified Darlington pair device and to switching circuits employing such a device.

Electronic switching circuits, such as are employed e.g. in motor vehicle ignition systems, frequently use a Darlington transistor pair as the output device. Such devices have three terminals, a input terminal contacting the base of the input transistor, and output terminals contacting the emitter of the output transistor and the collectors of both transistors. Conventionally such devices are switched on by the application of a control current to the input terminal and are switched off by discontinuing that current. With such an arrangement it is difficult to switch of quickly and at the same time to control the switch off time.

The object of the invention is to minimise or to overcome this disadvantage.

According to one aspect of the invention there is provided a single chip Darlington transistor pair device in which external contacts is effected to the base of each transistor of the pair.

According to another aspect of the invention there is provided an electronic switching circuit, including an input trigger stage and an output switching stage, in which the output stage includes a Darlington transistor pair device in which external contact is effected to the base of each transistor of the pair, a feedback loop being provided between the collector and base of the output transistor of the pair.

Figure 1 is a circuit diagram of the Darlington pair four terminal device.

Figure 2 shows a switching circuit employing the device of Fig. 1, and Figure 3 shows current and voltage waveforms across an inductor coupled to the output of the circuit of Fig. 2.

Referring to Fig. 1 the Darlington pair single chip device includes two transistors TR 1 and TR2. To effect external control of the device contact is made to the base metallisation of transistor T2, this contact being brought out to a fourth terminal 11 on the device package. In use, the device is operated by a control current applied to the base terminal 12 of the input transistor TR1, this current switching on both transistors. When the control current is switched off the input transistor TR1 turns off. In this condition independent control of the output transistor TR2 can be effected by the application of a current via the terminal 11.

Referring now to Fig. 2 the circuit shown, which may for example be employed as an ignition circuit for a motor vehicle, comprises a trigger stage including a conventional three terminal Darlington pair TR3, and an output stage including a four terminal Darlington pair device of the type shown in Fig. 1. An input to the trigger stage is provided e.g. via a phototransistor TR4 and LED D1 arrangement.

When the first Darlington pair is in its 'on' condition current is supplied to the input 12 of the output Darlington pair which is thus held 'on' and in this condition operates in the conventional manner.

When the trigger stage is switched off the output transistor of the Darlington pair TR3 becomes conductive and removes base charge from transistor TR1. After a period determined by the storage time of transistor TR2 that transistor turns off causing inductor L1 to induce a corresponding positive back E.M.F.

at the collector of TR2. This increasing positive going voltage causes base current to be passed by TR2 via capacitor C1 thus causing that transistor to become partially conductive.

This negative feedback limits the rate of the rise of voltage across the inductor. Zener diode ZD1 protects the arrangement against excessive voltages and in some applications may be omitted.

As shown in Fig. 3 as the current, after a delay corresponding to the storage time of TR2, falls, the inductor voltage rises to a maximum value V max at which level the voltage is clamped by the negative feedback through capacitor C1. When the inductor current has fallen substantially to zero the inductor voltage then falls away, a slight oscillation being caused by the small self capacitance of the inductor.

In other applications the device can be used in the following mode.

With a control current applied to the base of TOR 1 such that the device is turned on, a further control current can be applied via the fourth terminal 11 to turn the transistor TR2 on, off, or partially on.

Claims (7)

1. A single chip Darlington pair transistor device in which external contact is effected to the base of each transistor of the pair.

2. A single chip Darlington pair transistor device substantially as described herein with reference to Fig. 1 of the accompanying draw nags.

3. An electronic switching circuit, including an input trigger stage and an output switching stage, in which the output stage includes a Darlington transistor pair device in which external contact is effected to the base of each transistor of the pair, a feedback loop being provided between the collector and base of the output transistor of the pair.

4. A circuit as claimed in claim 3, and in which said feedback loop includes a capacitor.

5. An electronic switching circuit, inciuding a Darlington transistor pair device, and means for providing current contact to the base of each transistor of the pair thereby providing independent control of each transistor.

6. An electronic switching circuit substantially as described herein with reference to the accompanying drawings.

7. A motor vehicle spark ignition system provided with a circuit as claimed in any one of claims 3 to 7.