Electricity: electrical systems and devices – Safety and protection of systems and devices – Impedance insertion
Patent
1993-07-07
2000-09-12
Leja, Ronald W.
Electricity: electrical systems and devices
Safety and protection of systems and devices
Impedance insertion
H02H 900
Patent
active
061186414
DESCRIPTION:
BRIEF SUMMARY
This invention relates to arrangements for protecting electrical circuits from overcurrents, for example from overcurrents caused by equipment faults, electrostatic discharge or other threats, and to circuits thus protected.
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 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.
Another circuit protection arrangement is described in German patent application No. 37 05 177 dated Feb. 18, 1987 to Siemens AG. This arrangement includes a power MOSFET in a line of the circuit, the gate of the MOSFET being biased on by a battery. A thyristor is connected between the gate of the transistor and the circuit line on the source side of the transistor and senses the voltage drop in the line across a resistor connected in series with the MOSFET. When the voltage drop across the resistor is greater than 0.7V the thyristor fires and the MOSFET is switched off. This circuit has the advantage that there is no initial voltage drop before current can flow through the MOSFET. However, once the arrangement has switched into its blocking state it will remain latched in that state and can be reset to its conducting state only by means of a switch that shorts the anode and cathode of the thyristor. Furthermore, the battery needs to be able to supply a current in the order of lmA to maintain the thyristor in its on state the whole time from when the arrangement trips to its blocking state until it is manually reset.
According to the present invention, there is provided a two terminal circuit protection arrangement that: (1) is intended to be series connected in a line of the circuit; (2) comprises: (a) a series switching transistor that controls the line current; (b) and a control transistor that controls the base or gate voltage of the switching transistor and is responsive to an overcurrent through the switching transistor; and (c) the arrangement including a voltage source applied to the base or gate of the switching transistor which biases the switching transistor into or toward conduction in normal operation; (3) is being capable of being remotely reset into a conducting state by stopping current in the line.
The arrangement according to the invention has the advantage that it is possible to form an arrangement that reduces, or even eliminates the initial voltage drop across the switching transistor before it conducts, while at the same time drawing a relatively low current from the voltage source under all conditions. Furthermore, it is possible to reset the arrangement remotely for example by briefly removing the source or the load. As soon as the source or load is removed the arrangement will reset itself to its low resistance state.
Because the initial voltage drop can be reduced or eliminated it is possible, in normal operation, for the only voltage drop across the switc
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Atkins Ian Paul
Pryor Dennis Malcolm
Williams David Mansel
Leja Ronald W.
Raychem Limited
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