GB2132447A - Switch device - Google Patents

Abstract

A solid state switch arrangement e.g. for use as a telephone hook switch or as a rectifying element in a bridge network, comprises a field effect transistor T1 forwarded with a current limiter. This current limiter may comprise a bipolar transistor 12 whose base and collector are coupled respectively to the source and gate of the field effect transistor. <IMAGE>

Description

SPECIFICATION Switch device This invention relates to solid state switches and in particular to switches for use in telephony.

There is a need for solid state switches to replace mechanical or electromechanical devices in telephony applications, e.g. in the provision of a solid state hookswitch. Field effect transistors (FET's) have been proposed for such applications as they have a very large ratio of OFF and ON resistance. However FET's suffer from a number of disadvantages. In particular they are easily damaged by surge voltages, e.g. from a lightening strike, and have wide manufacturing tolerances on threshold voltage.

The most stringent specification for an FET switch is when it is used as an electronic hookswitch in a loop impulse signalling system. It is common practice for Telephone Administrations to demand a more severe line transient test when the subset is 'on-hook' than when 'off-hook' and sometimes an even less severe (lower energy or voltage) test when a button is pressed and signalling is taking place. This reduction in severity of the tests reflects the decrease in probability of the subset being in a particular operational state when the largest transients appear on the line.

Where a mechanical hook-switch is fitted in a Strowger system then the loop-impulse switch is 'behind' the hook-switch and is not normally affected by line transients in the 'on-hook' state.

To protect the FET switch kom the line transients it is not just a simple matter of placing a low voltage current shunt across the line such as a zener diode, gas-gap or low voltage varistor. To meet the pulse distortion specifications, when loop impulse signalling, it is necessary for the subset to present a high impedance to the line during the "break" period for all voltages below 120 volts, this means the transient protection circuit must not operate at less than 1 20 volts.

The 120 volts (or more) will appear across the FET switch when it is 'open' or cause a high current to flow when the switch is 'closed'. The high current cannot be reduced by using series resistors since that would degrade the performance of the transmission circuit; it is this need to reduce the series resistance that requires the switch resistance in the closed state to be minimal.

The object of the present invention is to minimise or to overcome the foregoing disadvantages.

According to one aspect of the invention there is provided a switch element, including a field effect transistor providing a switchable current path between its source and drain terminals, and a controlled current path between the gate and source of the transistor whereby, when the transistor is in its conductive state, a current through the source-drain path of the transistor is limited to a predetermined maximum value.

According to another aspect of the invention there is provided a telephone subscriber's instrument provided with a solid state hookswitch, said hook-switch including a field effect transistor and means for controlling the gate potential of the transistor such that, when the hook-switch is closed, the current therethrough is limited to a predetermined maximum value.

According to a further aspect of the invention there is provided a rectifier bridge circuit each area of which includes a rectifier element comprising a field effect transistor provided with means whereby its gate potential is so controlled that, when the element is forward biased, the current therethrough is limited to a predetermined maximum value.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a circuit diagram of one form of solid state switch; Fig. 2 shows an alternative switch construction; Fig. 3 illustrates the use of the switch of Fig. 1 or Fig. 2 as a telephone hook-switch; Fig. 4 illustrates the characteristics of the hookswitch of Fig. 3; Fig. 5 shows a bidirectional switch arrangement formed from two switches as shown in Fig. 1 or 2; Fig. 6 shows a rectifier bridge formed from the switches of Fig. 1 or 2; and Fig. 7 shows a modified form of the bridge of Fig. 6.

Referring to Fig. 1, the switch device includes a field effect transistor T1 whose source drain path carries a current to be switched. A bipolar transistor T2 is connected with its collector and base coupled to the gate and drain respectively of transistor T1 . A resistor R 1, typically 5 to 10 ohms, is connected between the emitter and base of the bipolar transistor.

In Fig. 1, if the drain D and gate G of T1 have a positive voltage, T1 will conduct and T2 will be non-conducting until the voltage across R1 reaches the base-emitter breakdown voltage of T2, whereby T2 conducts and reduces the voltage at the gate of T1 until an equlibrium is reached.

The base-emitter voltage is a near constant value for bipolar transistors, hence the voltage across R1 is constant and the drain-to-source current will also be content. If the voltage at the drain is increased, even momentarily as for a transient, the current having reached the limit value will not increase further. An additional diode may be placed in series with the emitter or base of T1 to increase current limit without changing value of R1.

The circuit shown in Fig. 1 uses an n-channel FET with an npn bipolar transistor, but it will be apparent that an equivalent circuit can be constructed for a p-channel FET with a pnp bipolar transistor.

A further modification of the circuit is shown in Fig. 2 where the bipolar transistor T2 is replaced by an n-channel FET T3. This circuit operates in the same way as that of Fig. 1 but lends itself to integration. Again a similar circuit can be constructed from two p-channel FET's.

One application of the switch of Fig. 1 or 2 is as a telephone hook-switch as is shown in Fig. 3. in this circuit transistor T2 provides automatic bias to limit the drain current to 120 mA. With T2 'on' 90% of the line voltage is applied to the gate of the FET, T1, which is then turned 'on'. Until the line reaches 1 20 mA the FET is operating as a saturated switch and for speech signals appears as a linear resistance. When the line current attempts to exceed 120 mA, even for a short time, as when a line high voltage transient occurs, the voltage across resistor R7 will rise sufficiently to turn T2 on and reduce the gate voltage hence restricting the drain current, equilibrium will be reached at 1 20 mA.By restricting the current to 120 mA the voltage across the FET (drain to source) will rise and for a 2 KV 10/700 uS from the CCITT recommended test circuit the voltage will rise to a maximum of 1 53 volts. Under these conditions the transient energy of 25 mJ is within the rating of the FET. No stringent requirements are demanded from T3, most cheap npn transistors are suitable, BC107 or BC238 for example. For these two transistors the base to emitter voltage 'VBE' is 550 mV for a collector current of 200 uA which is the same for most small transistors (200 uA is the current when the subset terminal voltage is 10 V). A 4.7 ohm resistor for R7 will therefore cause T2 to conduct at a drain current of 1 17 mA.

Even with a 200 V transient at 'F' Fig. 4, and the most insensitive FET the voltage at the FET gate cannot exceed 20 volts, which confirms that T2 can be a low voltage transistor and, further, no zener diode is needed to protect the gate thus saving a component.

The resistor R6 is required to provide a reference point for the FET otherwise when T3 is switched off the gate electrode will be 'floating' and excessive leakage currents may flow from drain to source. Due to the very high input impedance of the FET gate it is possible for the gate to get sufficient positive feedback via the gate-to-drain capacitanee to cause oscillation at tens or, most likely, hundreds of MHz. The resistor R4 prevents the oscillation, the value, which is not critical, may be between 390 and 2 K ohms. It will be appreciated that until 1 20 mA is flowing the FET is linear and for normal subset conditions looks like a passive resistor.

One advantage of the arrangement of Fig. 2 is that no series current limiting resistors are required before or after the varistor VAR. These series resistors are usually 1 W carbon composition devices and their elimination means useful space saving. As the line current is limited to 120 mA the current through the transmission circuit cannot exceed 1 20 mA and for static conditions the voltage across the transmission circuit cannot rise above 1 3 volts hence the 1 5 volt zener diode Z will never conduct and is not required. For the dynamic conditions when the subset goes 'off-hook' or during impulse signalling on zero line the line current rises rapidly, as a step function, to 100 mA.The integrated transmission circuits are usually relative slow to turn on, therefore a voltage will develop across the transmission circuit which is higher than the static condition.

An important feature of any switch, especially a hook-switch, is the resistance RS which the switch presents to the circuit. For the switch in Fig. 3 this means the sum of R7 and the 'ON' resistance of the FET. Where the value of RS may vary with current, as in this case, the important area for subsets is the value at low line currents, 20 mA or so. At higher line currents the line impedance is obviously less so more resistance can be tolerated in the hook-switch. The values of RS at varying line currents are shown in Fig. 5 for differing voltages on the FET gate. From Fig. 4 it can be seen that the value of RS is only 12 ohms at 20 mA with 4 volts for a line current of 20 mA.

As a 10 ohm (often 20 ohm) series protection resistor has been eliminated these values for RS can be considered reasonable, for the net effect of the electronic hook-switch on the subset DC or AC characteristics is negligible.

Another source of unwanted high energy signals on the line is ring-trip failure. For normal ringing voltages and conditions with the hookswitch in the 'on-hook' state the FET switch will see the ringing voltage only and no current. As the FET voltage rating of 200 V is greater than the peak ringing voltage no problem arises. If, when going 'off-hook' the ringing continues, current will flow through the switch but limited to 120 mA. If we assume a 100 V rms ringing voltage from a 400 ohm source, a current of 120 mA will flow and produce 52 volts across the switch and dissipate 6.24 Watts in the FET, which the FET can tolerate as the FET maximum dissipation is 6.25 Watts. The situation is actually better than this as voltage will be dropped across the bridge and R7 and some current may still be flowing in the ringing circuit.

The switches of Figs. 1 and 2 may be used in the construction of a bidirectional switch as shown in Fig. 5. In Fig. 7 with a positive voltage at A relative to B and a positive voltage at C, T3 1 and T41 will conduct, when the voltage at the source S of T3 1 reaches a sufficient voltage to cause base current to flow in T1 1 automatic bias takes place at T3 1 such that the current from A to B remains constant. Until the current reaches its constant value the circuit is essentially linear. The parameters of T41 and the value of R2 which may be emitted determine the current. The diode D2 can also be used to change the current limit value, D2 may be a transistor or an FET.

If the terminal B is made positive with respect to A, then T2 1 controls the voltage on the gate of T41 and hence the current limit. Diode D1 may be used as an additional element for setting current limit. As before a similar arrangement can be constructed with p-channel FET's and pnp transistors or from four FET's.

Using the FET switches in Fig. 1 a rectifier bridge may be formed as shown in Fig. 6. With a positive voltage at A relative to B, T5 1 and T7 1 conduct, and T61 and T81 are biased to a non conducting state, producing a voltage across the +Ve and -Ve output. Should the current through R1 1 or R31 tend to increase beyond a predetermined level T21 orT41 conduct and automatically control the voltage on the gate of T51 such that the current through T51 and T71 remains constant.

If the terminal B becomes positive relative to A then T61 and T81 conduct, again causing a voltage to appear across the +Ve and -Ve output.

The current through R21 and R41 is used to limit the current as before.

A number of variations of the basic bridge are possible. The switch of Fig. 2 may be used instead of the bipolar circuit. Normal bipolar diodes may be used instead of any one switch in Fig. 6 or instead ofT51 and T71 or instead of T61 andT81 The current limiting feature of two of the switches can be omitted, for example T21, R1 1 and T31, R21,orT11,R41 andT41,R31 may be removed, in either case the remaining two arms of the bridge limit the current and protect any circuit between +Ve and -Ve from transient voltages or currents. It should be noted that in Fig. 6 the drain and source connections of the FET's may be reversed.

Fig. 7 shows an aiternative method of connecting the bridge which allows any arm or arms of the bridge to be switched off, as in a telephone signalling system or an electronic hookswitch. The symmetrical circuit of Fig. 3 can be used in one or more arms of the bridge. This is of advantage when using VMOS or DMOS transistors.

Claims (9)

1. A hook switch device for a telephone subscriber's instrument, the device including a pair of series connected field effect transistors, and means for controlling the gate potential of each said transistor such that, in use when the hook switch is closed, the current through the transistor is limited to a predetermined maximum value.

2. A switch device as claimed in claim 1, wherein the control means comprises a bipolar transistor, one for each field effect transistor, the collector and base of which are coupled separately to the gate and drain of that transistor.

3. A switch device as claimed in claim 2, wherein a diode is connected in series with the emitter or with the base of the bipolar transistor.

4. A hook switch device for a telephone subscriber's instrument substantially as described herein with reference to Figs. 1, 2 and 3 of the accompanying drawings.

5. A telephone instrument provided with a hook switch device as claimed in any one of claims 1 to 4.

6. A switch element -- see lines 21- 27 page 2.

7. A rectifier bridge circuit each arm of which includes a rectifier element comprising a field effect transistor provided with means whereby its gate potential is so controlled that, when the element is forward biased, the current therethrough is limited to a predetermined maximum value.

8. A rectifier bridge circuit substantially as described herein with reference to Figs. 6 to 7 of the accompanying drawings.

9. A telephone instrument provided with a rectifier bridge as claimed in claim 7 or 8.