Electricity: electrical systems and devices – Safety and protection of systems and devices – Voltage regulator protective circuits
Patent
1993-06-10
1995-11-07
Deboer, Todd
Electricity: electrical systems and devices
Safety and protection of systems and devices
Voltage regulator protective circuits
361 58, 361 91, H02H 320
Patent
active
054651883
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
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.
2. Introduction to the Invention
One circuit protection arrangement of relatively simple form is described in German Patent Application No. 37 25 390 dated Jul. 31, 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 device is particularly simple, it suffers from the disadvantage that its performance can vary considerably with temperature. For example, in many cases the current required to open the switching transistor (the "trip current") will fall to a small fraction of the ambient trip current when the temperature rises to 100.degree. C. or so. Thus, in normal operation of the circuit, it is usually only possible to pass a very small fraction, eg. about 20%, of the trip current through the switch. If larger currents are allowed to pass, heating of the switching transistor (due to a voltage drop across it that is always in the order of a few volts) will cause the trip current level to fall, with the result that tripping of the arrangement will at some stage occur with the normal circuit current.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a circuit protection arrangement that is intended to be series connected in a line of an electrical circuit, which comprises 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, the comparator circuit being powered by the voltage drop that occurs across the transistor switch.
Thus the arrangement according to the invention can be used as a two terminal device, that is to say, as a device that requires no separate power supply. If desired additional terminals may be included for example a third terminal to shunt any overcurrent across the load, for example as described in our copending international application No. PCT/GB91/01760 the disclosure of which is incorporated herein by reference.
DETAILED DESCRIPTION OF THE INVENTION
The arrangement according to the present invention 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.
The simplest form of arrangement may comprise a comparator circuit, for example in the form of an open loop operational amplifier, having one input terminal that is connected to a voltage reference and another terminal that samples the voltage difference across the switch by means of a voltage divider. The voltage reference should have a relatively temperature stable performance, preferably having a temperature coefficient of not more than .+-.0.5%K.sup.-1, more preferably not more than .+-.0.2%K.sup.-1 and especially not more than 0.1%K.sup.-1. Normally a Zener diode or band gap device will be employed as the voltage regulator.
The circuit protection arrangement according to the i
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"Various Types of Power Supply Protection Circuits", Switching Power Supply Units, Chapter 8, pp. 209-229.
Challis Michael
Pryor Dennis M.
Burkard Herbert G.
DeBoer Todd
Gerstner Marquerite E.
Raychem Limited
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