This invention relates to arrangements and devices for protecting electrical circuits from overcurrents, for example from overcurrents caused by equipment faults, electrostatic discharge or other threats.

One circuit protection arrangement of relatively simple form is described in German Patent Application No. 37 25 390 dated 31st July 1987 to Wickmann-Werke GmbH. This arrangement comprises a series switching transistor that controls the circuit current and a control transistor that controls the base or gate voltage of the switching transistor. The base or gate voltage of the control transistor is set by a voltage divider that spans the switching transistor, so that, if the arrangement experiences an overcurrent, the control transistor will be biased into conduction and will turn the switching transistor OFF. Although this arrangement is particularly simple, it suffers from the disadvantage that in normal operation there will always be a significant voltage drop across the arrangement before it will conduct current, this voltage drop being due to the base-emitter junction voltage of the switching transistor added to the voltage drop across the base resistor in the case of bipolar arrangements. In the case of arrangements based on enhancement

mode FETs, the voltage drop will be due to the threshold voltage of the switching transistor. The voltage drop prevents this form of circuit protection arrangement being used in a number of applications and can lead to heat generation problems in high current applications.

According to one aspect, the present invention provides an arrangement that is intended to be series connected in a line of an electrical circuit for protecting the circuit from an overcurrent, which comprises a depletion mode FET that switches the line current and a control device that is connected across a resistance in the line and will bias the gate of the FET in response to the voltage difference across the resistance in order to switch the FET off when the arrangement is subjected to an overcurrent in the line.

The invention has the advantage that, because the series connected FET is normally in the on state there is no initial voltage drop across the arrangement before it becomes conductive so that the arrangement can be substantially linear. The arrangement according to the invention can exhibit a "foldback" characteristic, that is to say, one in which the current that passes through the arrangement increases with increasing voltage difference across it until a certain voltage, referred to as the threshold voltage, is reached whereupon the current through the device decreases to a lower value. Normally the ratio of the maximum leakage current of the device in its off state to the maximum current of the device in its on state (trip current) is not more than 0.5, more preferably not more than 0.1 and especially not more than 0.01. In many cases the ratio can be lower than 10 ^-4 . It is possible, depending on the mechanism of operation of the device that biases the FET, for the arrangement to have a "slow" or a "fast" foldback characteristic. If the arrangement switches quickly from its on state to its off state, for example in less than 100 μs, then it can be said to exhibit a fast foldback characteristic, whereas if the transition between the on state and the off state takes longer it can be said to exhibit a slow foldback characteristic. Whichever characteristic is preferred will depend on the application of the circuit. For example an arrangement that

exhibits a fast foldback characteristic will generally let through less energy to the load when subjected to a current transient, whereas arrangements exhibiting slower foldback characteristics may be preferred if the circuit has a load having a significant inductance or if the arrangement needs to be insensitive to short current transients due, for example to equipment being switched on. The switching speed of the arrangement may be altered by including a capacitor and or a resistor between the source and gate of the FET. Including a small capacitor will slow the switching speed of the arrangement while including a resistor will decrease the abruptness of the switching of the arrangement as a function of the voltage across the FET. Normally a parallel combination of a capacitor and resistor will be employed to allow the capacitor to discharge through the resistor.

The term "depletion mode FET" as used herein is intended to include any FET that is normally in the on state and which will switch off only when the gate is biased. Thus, for example the term includes junction FETs, depletion mode MOSFETS and the like. Depletion mode MOSFETS will normally be employed for relatively high power applications in d.c. circuits or where an a.c. signal is superimposed on a d.c. offset so that there is no reverse current. JFETs, however, may be employed for a.c. signal lines in general. Normally arrangements employing a single JFET will only be able to handle relatively small currents, eg. up to 50 mA (although it is now possible to purchase JFETs having a current rating of 700 mA). However, more than one FET may be placed in parallel with their gates connected to a common control device in order to increase the current rating of the arrangement.

As stated above, the control device that biases the gate of the FET does so in response to the voltage drop that occurs across a resistance in the line. Although it is possible, at least in the broadest aspect of the invention, for the line to include a separate series resistance that causes the voltage drop, it is preferred for the series

resistance to be provided at least in part by the FET itself so that the device is connected across the FET. Most preferably the resistance is provided entirely by the FET. In this case it is possible for the arrangement to include no resistive components series connected in the line other than the FET.

One form of control device that may be employed is a negative voltage generator or dc-dc converter. Such devices will in general lead to an arrangement that exhibits foldback characteristics with the maximum leakage current normally being not more than 0.1 times the trip current and typically about 0.01 times the trip current.

Alternatively an opto-electronic coupling arrangement may be employed as the control device. Such an arrangement may comprise a light source that is powered by the voltage drop in the line and which is optically coupled to a photovoltaic element which is connected to the gate of the FET. The light source may generate visible or invisible radiation. It may for example comprise a light- emitting diode, an electroluminescent device or a fluorescent device, while the photovoltaic device may comprise a photodiode connected in the photovoltaic mode.

Although the arrangement described above is a two terminal arrangement, it is possible also to form three terminal arrangements according to the invention where the third terminal switches on when an overcurrent is experienced in order to shunt the current across the load or to the earth terminal. Such arrangements can advantageously be formed so that they do not draw power from the circuit supply line. The third (shunt) terminal may be triggered electrically to shunt the overcurrent, but in the case of arrangements that employ opto-electronic couplers the third terminal is preferably triggered optically. For example, the third terminal may include an opto-triac that is triggered by the control device.

Five terminal protection arrangements for protecting a pair of lines, as commonly used in the telephone protection industry, may be formed employing a pair of overcurrent protection devices according to the invention which can employ either two devices for shunting the overcurrent to the earth terminal, or a single shunting device across the lines. In the case of optically coupled arrangements the light source in either of the overcurrent protection devices can be used to trigger all the protection components of the arrangement, thus comprehensively protecting the load.

Preferably all components of the arrangement take their power from the current in the lines so that no separate power supply is needed.

Several arrangements in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a circuit diagram of one arrangement in accordance with the invention;

Figure 2 is a circuit diagram of a modification of the arrangement of Figure 1 which can be employed in a.c. circuits;

Figure 3 is a circuit diagram of a form of another arrangement that employs an optoelectronic coupler to control the FET;

Figure 4 is a circuit diagram of a three-terminal arrangement which is a modification of the arrangement as shown in Figure 3 ; and

Figure 5 is a circuit diagram of a modification of the arrangement of Figure 1 that has an increased voltage rating

Referring to the accompanying drawings, Figure 1 shows a two-terminal circuit protection device for a d.c. circuit which comprises a depletion mode MOSFET or a JFET 1 that is series connected between the terminals 2 and 3, and a negative voltage generator 4 whose input is taken across the FET 1 and whose output is connected to the gate of the FET 1, so that, when the negative voltage generator operates, the gate-source voltage of the FET is approximately equal to the negative of its drain-source voltage.

In use the FET 1 is in its ON state during normal operation of the circuit, and the only voltage drop caused by the circuit protection arrangement is that caused by the FET channel resistance. Since the channel resistance is generally constant under normal conditions the protection arrangement is linear and acts like a small resistance in the line. If the arrangement is subjected to an overcurrent, the voltage drop across the FET will increase and thus the input voltage to the negative voltage generator 4 increases. At some stage the input of the negative voltage generator will be sufficient to cause it to generate an output at which stage the negative voltage generator will switch the FET off. Once this has occurred, it will remain latched in its OFF state until the power supply or the load has been disconnected, since even when the overcurrent passes the entire circuit voltage occurs across the FET and is fed back into its gate.

Figure 2 shows a modification of the circuit of Figure 1 which is suitable for use in an a.c. circuit. In this circuit a JFET 1 is still connected directly in the line of the circuit between the terminals 2 and 3, but the negative voltage generator 4 is located within a rectifying diode bridge that is formed from diodes 5 and that is connected across the JFET. The JFET 1 will pass current in either direction in normal operation while its gate is held below the

threshold voltage of the JFET, and a rectified version of the signal voltage across the JFET is fed into its gate by the negative voltage generator. When the arrangement is subjected to an overcurrent the gate bias will increase so as to switch the JFET 1 OFF irrespective of the polarity of the overcurrent.

This arrangement has the advantage that since the diodes in the bridge are not series connected in the line they do not cause any cross-over distortion and the arrangement is linear. As with the arrangement shown in Figure 1, the only series resistance in the line is the channel resistance of JFET1. In addition, because the diodes do not pass load current, small signal diodes can be employed.

Figure 3 shows a further form of arrangement that is suitable for use in an a.c. circuit. In this arrangement a JFET 1 is connected in series in the line, and a pair of light-emitting diodes 8 and 9 are connected across the JFET 1, the LEDs 8 and 9 having opposite polarity. A resistor 10 may be connected in series with the LEDs 8 and 9 in order to provide a degree of freedom in determining at what level of overcurrent the LEDs will turn on and to limit the current through the LEDs. The LEDs 8 and 9 are optically coupled to a light sensitive diode array 11 connected in the photovoltaic mode to the gate of the JFET 1 at its cathode terminal and to a pair of back-to-back steering diodes 12 and 13 at its anode terminal, the diodes 12 and 13 spanning the JFET 1.

As with the other arrangements described above, the arrangement will conduct current in normal operation with no initial voltage drop occurring across the JFET 1, the only voltage drop occurring being that due to the channel resistance of the JFET. When the arrangement is subject to an overcurrent the increase in voltage across the JFET 1 will cause either LED 8 or LED 9 to light up depending on the polarity of the overcurrent and cause the light sensitive diode array 11 to bias the gate of JFET 1 negatively. As the gate bias increases the current through the JFET reduces until the

gate bias exceeds the threshold voltage of the JFET whereupon the line current will approach zero. The increase in voltage across the JFET 1 in its off state will cause the gate bias to increase and the FET to be latched in its off state.

The purpose of the steering diodes 12 and 13 is to ensure that the voltage of the anode terminal of the photovoltaic diode array 11 does not rise above one diode voltage drop of whichever of terminals 2 and 3 has the lowest voltage. This ensures that when LED 8 or 9 lights up, the gate of JFET 1 will be biased negatively with respect to whichever terminal of JFET 1 is acting as the source.

In a modification of the circuit, a current regulator, eg. a current regulating diode, may be included instead of resistance 10 or in series with it in order to ensure that excessive current does not flow through the LEDs 8 and 9 at high voltages.

Figure 4 shows a modification of the circuit shown in Figure 3 which is a three terminal device that will protect a circuit from an overcurrent and at the same time shunt the current across the load. This arrangement is constructed in the same way as that shown in Figure 3 but includes an opto-triac 15 that is optically coupled to the LEDs 8 and 9. One terminal of the opto-triac is connected to the circuit line and the other terminal is connected to the gate of a power triac 16 that is connected between the line and the earth terminal 17.

When a transient is experienced one of LEDs 8 and 9 lights up and causes the photovoltaic diode array to switch JFET 1 off, as described above. At the same time opto-triac 15 briefly shorts the gate of the power triac 16 to the circuit line, thereby turning the power triac on and shorting the transient across the circuit load.

Figure 5 shows a circuit that employs a pair of series connected MOSFETS 51 and 52 for switching the line. Unfortunately the maximum voltage rating of readily available depletion mode

transistors tend to be in the order of 200V, yet many higher voltage applications exist of reasonable current rating. The use of a pair of series connected transistors will halve the voltage drop across each transistor.

A pair of negative voltage generators 53 and 54 are series connected together and take their input voltage from the voltage drop across the two FETs 51 and 52, the output of each of the negative voltage generators being fed into the gate of one of the FETs. A pair of current limiting circuits formed from FETs 55 and 56 and resistors 57 and 58 limit the input current to the negative voltage generators 53 and 54 when an overcurrent is experienced.

It is important that both FETs 51 and 52 switch off at the same time in order that the voltage drop is evenly distributed over them and the voltage rating of each of the transistors is not exceeded. In order to ensure that this occurs, the node between the negative voltage generators 53 and 54 is held at a voltage mid way between the voltages on either side of the two FETs 51 and 52 by means of a voltage divider formed by resistors 59 and 60.

Capacitors 61 to 64 are employed to hold charge in the negative voltage generators 53 and 54, and, in combination with 1MΩ 65 and 66, reduce noise from the output of the negative voltage generators.