Overcurrent protection device

Details Overcurrent protection device Overcurrent protection device

1995-02-23

2002-06-11

Sherry, Michael J. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

With specific current responsive fault sensor

C361S093400

Reexamination Certificate

active

06404608

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the protection of electrical circuits from overcurrents, for example from overcurrents caused by equipment faults or transient overcurrents caused by lightning, electrostatic discharge, equipment induced transients or other threats.
2. Introduction to the Invention
Many electrical circuits, for example telephone systems and other information distribution systems, are subjected both to transient overcurrents and to overcurrents of long duration for instance due to equipment failure or short circuits caused by damage to equipment. In such cases it would be desirable to incorporate in the system a device that would protect the system against both types of overcurrent but would automatically allow the system to continue functioning after a transient overcurrent has passed.
One such arrangement is described in Australian Patent Application No. 48128/85 to Glynn et al in which a pair of switching transistors in Darlington configuration is connected in series with a circuit line, the base of which is controlled by a silicon controlled rectifier (SCR) that senses the voltage drop across a resistor in series with the switching transistor. In addition, resetting circuitry is provided to reset or attempt to reset the switching arrangement periodically in the event that it trips. Another overcurrent protection circuit that will reset itself into normal operation after a transient overcurrent is described in U.S. Pat. No. 4,202,023 to Sears. However, both these circuits have a number of drawbacks. For example, the presence of a series resistor adds to the voltage drop across the device in use and will increase the difficulty and cost of manufacturing the arrangement in integrated circuit form because the resistor will have to carry load current in normal use. Also, both circuits will attempt to reset themselves indefinitely when the system is subject to a long-duration overcurrent such as caused by equipment failure with the result that it may be necessary to switch the system off before the fault can be repaired. Furthermore, in the case of the Glynn et al circuit, when the arrangement has tripped into its OFF state there will remain a relatively high leakage current through the SCR in the order of 10 to 20 mA.
SUMMARY OF THE INVENTION
Thus, according. to the invention there is provided an overcurrent protection arrangement, which comprises a switching circuit that is intended to be series connected in a line of the circuit to be protected and which will allow normal circuit currents to pass but will open when subjected to an overcurrent, the arrangement including a pulse generator which, when the switching circuit has opened, will generate pulses to a predetermined finite maximum number or for a predetermined time that reset, or attempt to reset, the switching circuit to its conducting state, the pulse generator and any other components of the arrangement taking their power supply from the voltage difference across the switching circuit, optionally after appropriate voltage regulation, for example by means of a Zener diode.
DETAILED DESCRIPTION OF THE INVENTION
The invention has the advantage that the number of pulses that is generated in order to reset or to attempt to reset the arrangement, or the time for which they are generated, is limited so that, for example, in the case of equipment failure the source is not continually switched into the faulty equipment. Thus, the protection can discriminate between transients and persistent system faults. In the case of an overcurrent the switching circuit will rapidly switch off and will then reset itself or attempt to reset itself one or more times in case the overcurrent is due to an externally induced transient. However, if the overcurrent persists, for example if it is caused by a fault in the load circuit, the switching circuit will immediately revert to its OFF state as soon as the resetting pulse ends. Once this has occurred for the predetermined number of pulses the arrangement will remain in its OFF state indefinitely.
The switching circuit preferably comprises a switching transistor that is intended to be series connected in the circuit line, and a control transistor that determines the base or gate voltage of the switching transistor, and whose base or gate voltage depends on the voltage drop across the switching circuit. For example, the control transistor may form one arm of a voltage divider which spans the switching transistor and which sets the base or gate bias of the switching transistor, the control transistor being connected in parallel with the base and emitter or gate and source of the switching transistor. The base or gate bias of the control transistor may also be determined by a voltage divider that spans the switching transistor. In normal operation of this form of switching circuit, when no current passes along the circuit line both the switching and the control transistor are off. As the voltage on the line increases the base or gate forward bias of the switching transistor rises due to the relatively high resistance of the control transistor in its off state, until the switching transistor turns on. In normal operation the arrangement will allow the circuit current to pass with a small voltage drop across the switching transistor of about 1.5 V in the case of an enhancement mode MOSFET or about 0.65 V in the case of a single bipolar junction transistor.
When the line is subjected to an overcurrent, the voltage drop across the switching transistor increases, hence the base or gate forward bias of the control transistor increases until the control transistor turns ON, thereby shorting the base and emitter or the gate and source of the switching transistor and turning the switching transistor OFF. As this occurs the voltage across the switching transistor increases, so increasing the forward bias of the control transistor base or gate and locking the arrangement in the OFF state even if the overcurrent transient passes.
This form of circuit has the advantage that it does not require any series resistor to be provided in the line of the electrical circuit for determining the existence of an overcurrent, so that the voltage drop across the switching circuit is solely due to the collector-emitter or drain-source voltage drop of the switching transistor. In addition, the absence of a series resistor reduces the number of load current carrying components which allows easier integration of the device.
If the switching circuit has this configuration, the pulse generator is preferably arranged to short the base and emitter or gate and source of the control transistor, thereby turning it OFF which in turn will turn the switching transistor ON. This may be achieved by providing a resetting transistor for “shorting” the base and emitter or gate and source of the control transistor, the base or gate voltage of the resetting transistor being taken from the pulse generator.
Another form of switching circuit may be provided by a transistor switch that controls the circuit current and has a control input, and a control arrangement that controls the voltage of the control input and is responsive to an overcurrent through the switch, the control arrangement comprising a comparator circuit that compares a fraction of the voltage across the switch with a reference voltage and opens the switch if the fraction is greater than the reference voltage.
This arrangement has the advantage that it enables much flatter performance variations with respect to temperature to be obtained. In addition, it is possible to run the circuit protection arrangement according to the invention at considerably higher circuit currents without the danger of it tripping under the normal circuit current. In many cases the arrangement can be operated with up to 80% of the trip current without danger of it tripping.
Preferably the comparator circuit is powered by the voltage drop that occurs across the transistor switch, thereby obviating the need for any separate power supply.
The simple

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