Overcurrent protection device

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 (1) that controls the line current, (b) a control transistor (4) that controls the base or gate voltage of the switching transistor and is responsive to an overcurrent through the switching transistor, and (c) a voltage source, for example a battery (3), a dc-dc converter (58/59) or a Seebeck device (43), applied to the base or gate of the switching transistor which biases the switching transistor into or toward conduction in normal operation; and (3) is capable of being remotely reset into a conducting state by stopping current in the line. The arrangement enables the initial voltage drop that is required to turn the switching transistor (1) on to be reduced or eliminated while requiring relatively little current from the voltage source. The arrangement can be reset remotely by briefly removing the voltage source or load from the circuit.

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 
switching transistor to be due to its collector resistance or its channel 
resistance. The voltage source may have any value up to or even higher 
than that required to bias the switching transistor into conduction, the 
initial voltage drop across the switching transistor reducing as the 
voltage source potential rises. 
The voltage source may be provided by any of a number of devices the 
particular choice depending on a number of factors including the current 
that will be drawn from the voltage source. For example, it may comprise a 
battery. The battery will be in series with the control transistor and a 
resistor (the value of which determines at least partly the leakage 
current of the arrangement). This resistor can have a relatively high 
value, for example 1 M.OMEGA. or more, often 10 M.OMEGA. or more, in which 
case the maximum current that will be drawn from the battery will be in 
the order of 5 .mu.A and preferably in the order of 500 nA or less when 
the arrangement has tripped. In the normal state of the arrangement when 
the control transistor is off the current drawn from the battery will 
usually be in the order of picoamps, eg. less than 100 pA. Thus, the 
battery may be formed as a small lithium cell having only a very small 
capacity, eg. in the order of lmAh which can be incorporated into an 
integrated circuit package and will have a lifetime of a number of years. 
The voltage source is preferably connected in series with a current 
limiting resistor, especially where a battery is employed, in order to 
prevent discharging of the battery when an overcurrent is experienced and 
the control transistor is turned on. 
Another form of voltage source that may be employed is a thermoelectric 
device such as a Seebeck device. Such a device is advantageously located 
in thermal contact with the switching transistor so that heat generated by 
the switching transistor flows through the device. This arrangement has 
the advantage that the thermoelectric device provides a feedback 
arrangement in which an increase in heat generation in the switching 
transistor caused by the voltage drop across the switching transistor 
increases the base or gate offset voltage and so reduces the voltage drop. 
For relatively low frequency changes in the circuit current, this feedback 
can effectively reduce the switching transistor channel resistance. 
Yet another voltage source that can be used is a dc-dc voltage converter. 
Such converters are two-port networks which take a low voltage dc input 
and produce a higher dc voltage output. The converter may be used to 
increase the voltage from another voltage source such as a Seebeck device 
mentioned above, or it may be connected across the switching transistor so 
that the voltage drop across the switching transistor is multiplied and 
fed into its base or gate. 
Other forms of voltage source that may be employed include photovoltaic 
devices and capacitors that are charged up, for example by voltage 
multiplication or by top-up charging when the switching transistor is off. 
Alternatively, a separate supply may be employed for the voltage source, 
for example a rectified mains supply. 
If desired the base or gate voltage of the control transistor may be 
determined by a voltage divider that spans the switching transistor so 
that the trip current of the arrangement is determined by the switching 
transistor channel resistance and the proportion of the voltage drop 
across the switching transistor that is fed into the gate of the control 
transistor. Alternatively, the base or gate of the control transistor may 
be connected directly to the collector or drain of the switching 
transistor so that the arrangement will trip if the overcurrent voltage 
drop across the switching transistor exceeds the turn-on voltage of the 
control transistor. 
Where the arrangement is intended to be employed with ac circuits, it may 
be connected to the line via a rectifying bridge circuit. Alternatively a 
pair of equivalent circuit protection arrangements according to the 
invention may be employed, the two arrangements handling different cycles 
of the ac signal. This latter arrangement has the advantage that the 
overall voltage drop across the arrangement is reduced due to a reduction 
in the number of diodes employed. 
The overcurrent protection arrangement may employ either bipolar junction 
transistor or field effect transistors, although FETs are preferred since 
a bipolar switching transistor will require a significant base current to 
be provided by the voltage source. In addition, the term "transistor" 
includes circuit elements employing more than one transistor that can 
emulate the switching properties of a transistor, for example a number of 
transistors in a Darlington configuration. In the case of bipolar 
arrangements, Darlington configurations are preferred in order to reduce 
the switching transistor base current. Not only does this base current 
load the voltage source, but it must be supplied via a resistor connected 
between the base and collector of the switching transistor. When the 
circuit switches to its blocking state the switching transistor base 
current is diverted through the control transistor (which is now on) and 
becomes a leakage current. However, since the voltage drop across the 
resistor is much higher when the arrangement is in its blocking state, the 
leakage current is larger than the switching transistor base current. If a 
Darlington pair or triplet is employed as the switching transistor, the 
effective dc current gain will be increased considerably so that a much 
higher resistance can be used. 
A bipolar control transistor may advantageously be employed in conjunction 
with a field effect switching transistor. This arrangement has the 
advantage that the trip voltage is reduced to a pn junction voltage drop, 
thereby allowing a switching transistor having a lower channel resistance 
to be used with a consequent reduction of power dissipation. 
Where field effect transistors are employed, enhancement mode MOSFETs 
should be employed. The arrangement may be produced as an integrated 
circuit, in which case the resistors employed in the switching circuit may 
be provided by MOSFETs, for example with their gates and drains connected 
as in nMOS logic. Alternatively, the current limiting resistor may be 
replaced by a further FET that forms a complementary pair with the control 
transistor. 
According to a further aspect, the invention provides an electrical circuit 
which comprises a circuit voltage or current supply, a load and a 
current-carrying line connecting the supply and load, the circuit 
including a two terminal circuit protection arrangement that: (1) is 
series connected in the current-carrying line; (2) comprises: (a) a series 
switching transistor that controls the line current; (b) 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) 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; and (3) is capable of being remotely reset 
into a conducting state by removing the circuit voltage or current source 
or the load.

Referring to the accompanying drawings, the circuit of a two-terminal 
arrangement for protecting a circuit from an overcurrent is shown in FIG. 
1. The arrangement comprises an n-channel enhancement mode switching 
MOSFET 1 that is connected between the terminals 2 and 2' of the device so 
that it passes the entire circuit current. The gate of switching 
transistor 1 is connected to its drain via battery 3 which offsets the 
gate voltage from the drain by the battery voltage and current limiting 
resistor 5. 
An enhancement mode control MOSFET 4 is connected across the gate source 
junction of the switching transistor 1 in order to switch the switching 
transistor off when an overcurrent is experienced, the gate of the control 
transistor being connected directly to the drain of the switching 
transistor 1. 
In normal operation of the circuit, if there is no current in the line, the 
switching transistor 1 will be on or off depending on whether battery 3 
offsets the transistor gate by more or less than the threshold voltage of 
the transistor. When the line is loaded the voltage across the switching 
transistor 1 will increase as the current increases as shown in FIG. 2 
curve A, the slope of the curve depending on the switching transistor 
channel resistance. The voltage will continue to rise with increasing 
loading of the circuit until the trip voltage V.sub.T is reached at which 
point drain source voltage of the switching transistor 1 is equal to the 
threshold voltage of the control transistor 4, and the control transistor 
"shorts" the gate and source terminals of the switching transistor. 
Once the arrangement has switched it will remain latched in its high 
resistance state even after the overcurrent has subsided because the 
resistance of transistor 1 is such that the entire circuit voltage is 
dropped across it. Thus, the arrangement must be disconnected from the 
circuit supply or load before it will reset itself. Current limiting 
resistor 5 prevents rapid draining of the battery 3 when the arrangement 
has tripped. 
By way of comparison the I-V characteristic of a protection arrangement in 
accordance with German Application No. P 37 25 390 (employing FETs) is 
shown as curve B. This curve has the same form as curve A but is offset to 
a higher voltage drop. This is due to the fact that an initial voltage 
drop V.sub.I must occur across the switching transistor before the gate of 
the control transistor reaches the threshold voltage. The I-V 
characteristic of a protection arrangement in accordance with German 
Application No. P 37 05 177 is similar to that of the present invention 
(curve A) until an overcurrent occurs, whereupon the device latches in its 
high resistance state with the I-V curve lying on the voltage axis until 
manually reset. 
FIG. 3 shows an alternative form of two terminal protection arrangement in 
which an enhancement mode MOSFET 1 passes the operating current of the 
circuit. The gate of switching transistor 1 is connected to its drain via 
a battery 3 (eg. about 1.5V) and 1 Mohm current limiting resistor 5, and a 
control MOSFET 4 is connected across the gate-source junction of 
transistor 1, as described with reference to FIG. 1. However, in this 
circuit the gate voltage of the control transistor 4 is held by a voltage 
divider formed from 1 Mohm resistance 6 and 1.22 Mohm resistance 7 which 
span the switching transistor. In operation this arrangement will perform 
in the same manner as that shown in FIG. 1 with the exception that the 
magnitude of the current required to cause it to switch is determined by 
the potential divider resistors 6 and 7 in addition to the threshold 
voltage of transistor 4 and channel resistance of switching transistor. 
If desired, the battery can be connected between the gate and source of the 
switching transistor provided its polarity is changed. 
FIG. 4 shows a similar arrangement to that shown in FIG. 3 in which the 
gate voltage of switching transistor 1 is controlled by control transistor 
4 whose gate voltage is set by a voltage divider formed from resistors 6 
and 7 that span the switching transistor 1. 
In this arrangement the gate of the switching transistor is connected to 
its drain via a Seebeck device 43 that is in thermal contact with the 
switching transistor 1 so that any heat generated in the switching 
transistor will cause a temperature difference between the junctions of 
the Seebeck device. 
In operation, when current first flows along the circuit line, the 
switching transistor is cold and no voltage is generated by the Seebeck 
device 43 so that an initial voltage drop of 1 to 2 volts occurs across 
the switching transistor 1 as shown in FIG. 2 curve B. However, the heat 
generated in the switching transistor 1 by virtue of this voltage drop 
will cause a voltage to be generated by the Seebeck device 43 which will 
bias the switching transistor's gate toward its drain and so reduce the 
voltage drop across the switching transistor. Thus, a feedback mechanism 
is established that reduces power dissipated in the switching transistor. 
As with the arrangement shown in FIG. 3, if an overcurrent occurs the 
arrangement will switch to its non-conducting state when the voltage drop 
across the switching transistor 1 is sufficient to raise the gate source 
voltage of transistor 4 to its threshold value. 
In an alternative arrangement the output of the Seebeck device may be 
connected in parallel with resistor 5, which is preferably being used with 
a bipolar switching transistor so that sufficient initial base current can 
be provided. The Seebeck output may instead be connected between the gate 
and source of the switching transistor. 
FIG. 5 shows yet another form of arrangement according to the invention. In 
this arrangement switching transistor 1 is series connected in a line of 
the circuit and its gate and source are connected together via control 
transistor 4. 
A dc-dc converter is to be included in order to convert 71 a low voltage 
appearing across the switching transistor 1 or part of that voltage, to a 
higher voltage to offset the gate of switching transistor 1. The input 58 
for the dc-dc converter 71 is between resistor 57 and the source of the 
transistors 1 and 4, and the output of the dc-dc converter 71 is to be 
connected at 59, namely between the gate and source of the switching 
transistor 1, in series with current-limiting resistor 60. It is quite 
possible, however, to connect the output between the gate and drain of the 
switching transistor 1. 
In use, any voltage that appears across the switching transistor 1 will be 
multiplied and fed back by the dc-dc converter 71 to offset the gate of 
switching transistor 1. This has the effect that for all currents up to 
the trip current the switching transistor has a relatively constant low 
resistance and has an initial voltage drop (V.sub.I in FIG. 2) of zero 
volts. Resistor 60 prevents the control transistor 4 in its on state from 
loading the output 58 for the dc-dc converter 71. 
If the arrangement is subject to an overcurrent, it will trip into its high 
resistance state when the voltage drop across the switching transistor 1' 
causes the gate source voltage of control transistor 4 to rise to its 
threshold value, whereupon the current flowing through the switching 
transistor 1 falls to substantially zero. 
This form of arrangement has the advantage that all the components employed 
are relatively reliable and do not need to be replaced, and in addition, 
all the components are capable of integration to form a monolithic device. 
Some dc-dc converters will require a short period of time to generate a 
voltage output after experiencing a current input. This delay will mean 
that the control transistor 4 will switch on before the switching 
transistor 1 and so short out the source gate junction of the switching 
transistor, thereby causing the circuit to latch in its tripped state as 
soon as it is switched on. This problem may be overcome by the arrangement 
as shown in FIG. 6 which incorporates a start-up circuit comprising a FET 
60 that is connected across the source and gate terminals of the control 
transistor 4 and whose gate is held in an RC voltage divider formed by 
capacitor 61 and resistor 62. When the circuit current is switched on the 
RC voltage divider acts as a differentiator, causing the gate of FET 60 
immediately to go high and then to fall to its source voltage as capacitor 
61 charges. FET 60 will therefore initially be on, forcing the control 
transistor 4 to be off while the dc-dc converter begins to operate. 
As shown in FIG. 6 the input for the dc-dc converter is regulated by a 
Zener diode 63, and a resistor 64 of typically 500.OMEGA. will limit the 
input current to about 2 mA. Alternatively a current limiting diode 
circuit may be employed in place of the resistor 64 and Zener diode 63.