Patent Description:
An apparatus as defined by the claims relates to a circuit for detecting an overcurrent condition in a load.

Apparatus and associated methods relate to detection of an overcurrent condition by determining if a voltage across a current-sense resistor exceeds a predetermined voltage threshold. The voltage at each side of the current-sense resistor is sensed indirectly, through a diode network. The diode networks through which the voltages on each side of the current-sense resistor are biased differently from one another. Such differently-biased diode networks translate the voltages at each side of the current-sense resistor by different amounts, the biasing of these diode networks is such that a voltage difference between the second terminals of the first and second diode networks is of a first polarity during normal current conditions, and the voltage difference between the second terminals of the first and second diode networks is of a second polarity during overcurrent conditions.

<FIG> is a schematic view of an overcurrent detection circuit monitoring current provided by an aircraft engine to an electric motor. In <FIG>, aircraft <NUM> includes engine <NUM>, electrical generator <NUM>, electric motor <NUM>, interrupt switch <NUM>, and overcurrent protection circuit <NUM>. Aircraft engine <NUM> acts as the prime mover for electric generator <NUM>. Electric generator <NUM> generates electrical power for various electrical systems aboard aircraft <NUM>, including electric motor <NUM>. Electric motors and solenoids, such as electric motor <NUM>, perform various mechanical operations on aircraft <NUM>. Interrupt switch <NUM> is configured to controllably interrupt power provided to electric motor <NUM> by electric generator <NUM>. Operation of interrupt switch <NUM> is performed by overcurrent protection circuit <NUM>.

Overcurrent protection circuit <NUM> includes current-sensing resistor <NUM>, first and second diode networks <NUM> and <NUM>, first and second biasing circuits <NUM> and <NUM>, and comparator <NUM>. Current-sensing resistor <NUM> is in series with electric motor <NUM>. In such a series configuration, current-sensing resistor <NUM> is configured to develop a voltage difference thereacross that is indicative of the current provided therethrough (and thus the current conducted by electric motor <NUM>). The voltage difference across current-sensing resistor <NUM> is translated through first and second diode networks <NUM> and <NUM> to comparator <NUM>.

First and second diode networks <NUM> and <NUM> are biased by first and second biasing circuits <NUM> and <NUM>, respectively, thereby establishing biasing conditions for first and second biasing circuits <NUM> and <NUM>. The biasing conditions established by first and second biasing circuits <NUM> and <NUM> for first and second diode networks <NUM> and <NUM>, respectively, result in differential voltages across first and second diode networks <NUM> and <NUM> that are dissimilar one to another. For example, a first voltage difference across first diode network <NUM> might be greater than a second voltage across second diode network <NUM>. This differential voltage difference ΔV between the voltage differences Vi and V<NUM>, established across first and second diode networks <NUM> and <NUM>, respectively, is configured to be equal to a predetermined voltage threshold VTHRESHOLD: <MAT>.

The threshold voltage is set to be equal to a sense voltage VSENSE across current-sensing resistor <NUM> that results from an overcurrent condition IOVERCURRENT flowing through current-sensing resistor <NUM>. These biasing and current conditions result in the differential voltage at the inputs of comparator <NUM> being zero when the current flowing through current-sensing resistor <NUM> is equal to IOVERCURRENT. Thus, the output signal of comparator <NUM> will be in a fist state when the current flowing through current-sensing resistor <NUM> is less than the overcurrent condition IOVERCURRENT, and the output signal of comparator <NUM> will be in a second state when the current flowing through current-sensing resistor <NUM> is greater than the overcurrent condition IOVERCURRENT. Such an output signal causes interrupt switch <NUM> to open during an overcurrent condition. In some embodiments, interrupt switch <NUM> will latch open in response to such an overcurrent condition.

<FIG> is a schematic circuit drawing of an exemplary high-side overcurrent protection circuit, which uses dissimilar biasing of diode networks. In <FIG>, electrical source <NUM> is providing power to load <NUM>. The current provided by electrical source <NUM> to load <NUM> is monitored by high-side overcurrent protection circuit <NUM>. Overcurrent protection circuit <NUM> includes current-sensing resistor <NUM>, first and second diode networks <NUM> and <NUM>, biasing circuits <NUM> and <NUM> comparator <NUM>, feedback resistor <NUM> and output biasing resistor <NUM>. Current-sensing resistor <NUM> is in series with and on the higher-voltage side of load <NUM>, thereby designating overcurrent protection circuit <NUM> as being a high-side protection circuit.

First diode network <NUM> includes diode 42D and resistor 42R in series with one another. First biasing circuit <NUM> is simply a resistor in the depicted embodiment. In other embodiments first biasing circuit <NUM> could be another type of biasing circuit, such as, for example, a transistor current source. Second diode network <NUM> is simply a diode in the depicted embodiment. Second biasing circuit <NUM> is also simply a resistor in the depicted embodiment. In other embodiments second biasing circuit <NUM> could be another type of biasing circuit, such as, for example, a transistor current source.

To establish different biasing conditions for first and second diode networks <NUM> and <NUM>, first and second diode networks are dissimilar to one another and/or first and second biasing circuits are dissimilar to one another. In the depicted embodiment, both are dissimilar to one another. A bias current determined by first biasing circuit <NUM> is one-tenth a bias current determined by second biasing circuit <NUM> in the depicted embodiment. Other ratios of resistance (and/or currents sourced thereby) can be used to establish different biasing conditions for first and second diode networks <NUM> and <NUM>. For example, a ratio of currents sourced by first and second biasing circuits <NUM> and <NUM> can be <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, <NUM>:<NUM>, etc. In the depicted embodiment the diodes of first and second diode networks <NUM> and <NUM> are identical to one another. Although these diodes are identical to one another, voltage differences will be dissimilar across the diodes of first and second diode networks <NUM> and <NUM>, because the current sourced by first and second biasing circuits <NUM> and <NUM> are dissimilar to one another.

In other embodiments, the currents sourced by first and second biasing circuits <NUM> and <NUM> can be substantially equal to one another, but first and second diode networks <NUM> and <NUM> then must be dissimilar to one another. For example, first diode network might have a single diode of a type, while second diode network might have <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, etc. of such type diodes in parallel. Such embodiments will also result in voltage differences being dissimilar across the diodes of first and second diode networks <NUM> and <NUM>. In such embodiments, these voltage differences are because the diodes of first and second diode networks <NUM> and <NUM> are dissimilar to one another. This embodiment is not the preferred embodiment, in that current hogging by one of the parallel-connected diodes can result. Such current hogging can be exacerbated due to a negative temperature coefficient of forward barrier voltages of a diode.

The configuration of first and second diode networks <NUM> and <NUM>, first and second biasing circuits <NUM> and <NUM>, and current-sensing resistor <NUM> results in the voltages VPOS and VNEG at the positive and negative inputs of comparator <NUM> as approximately (e.g., assuming resistor <NUM> is infinitely large and comparator <NUM> is ideal) given by: <MAT> <MAT> where: VLOAD is the voltage across load <NUM>; ILOAD is the current supplied by electrical source <NUM> via current-sensing resistor <NUM> to load <NUM>; RSENSE is the resistance of current-sensing resistor <NUM>; VDN1 is the voltage across the diode 42D of first diode network <NUM>; VDN2 is the voltage across the diode of second diode network <NUM>; RCS1 is the resistance of first biasing circuit <NUM>; and RDN1 is the resistance of the resistor 42R of first diode network <NUM>.

Such input voltages result in a voltage difference ΔVINPUT between the positive and negative inputs of comparator <NUM> as given by: <MAT> The output of comparator <NUM> will be of a first polarity or value in response to the voltage difference ΔVINPUT between the positive and negative inputs of comparator <NUM> being positive, and the output of comparator <NUM> will be of a second polarity or value in response to the voltage difference ΔVINPUT between the positive and negative inputs of comparator <NUM> being negative. Thus, a voltage difference ΔVINPUT between the positive and negative inputs of comparator <NUM> of approximately zero volts is the switching voltage of comparator <NUM> (e.g., assuming resistor <NUM> has a resistance much greater than the resistance of resistor 42R and comparator <NUM> is nearly ideal). Solving equation (<NUM>) for ISUPPLY when ΔVINPUT is zero volts results in the overcurrent threshold IOVERCURRENT, at which the comparator switches output state: <MAT>.

In the special case when no resistor is used in first diode network <NUM> (i.e., RDN1=<NUM>), equation (<NUM>) reduces to: <MAT> One purpose of including a resistor, such as resistor 42R, in first diode network <NUM> is to fine tune the overcurrent threshold IOVERCURRENT. Such fine tuning can be better understood if equation (<NUM>) is rewritten as: <MAT> Thus, the overcurrent threshold IOVERCURRENT is increased from that current threshold given by equation (<NUM>) by the amount given by: <MAT> Finally, feedback resistor <NUM> and output biasing resistor <NUM> establish hysteresis so that the comparator does not chatter (i.e., oscillate) during a threshold crossing event.

<FIG> is a schematic circuit drawing of an exemplary low-side overcurrent protection circuit, which uses dissimilar biasing of diode networks. In <FIG>, electrical source <NUM> is providing power to load <NUM>. The current provided by electrical source <NUM> to load <NUM> is monitored by low-side overcurrent protection circuit <NUM>'. Overcurrent protection circuit <NUM>' includes current-sensing resistor <NUM>, first and second diode networks <NUM> and <NUM>, biasing circuits <NUM> and <NUM> comparator <NUM>, feedback resistor <NUM> and output biasing resistor <NUM>. Current-sensing resistor <NUM> is in series with and on the lower-voltage side of load <NUM>, thereby designating overcurrent protection circuit <NUM>' as being a low-side protection circuit.

The <FIG> embodiment differs from the <FIG> embodiment in the relative ordering of load <NUM> and current-sensing resistor <NUM>, as well as the biasing of biasing circuits <NUM> and <NUM>. Operation of low-side overcurrent protection circuit <NUM>', as depicted in <FIG>, is nearly identical to operation of overcurrent protection circuit <NUM>, as depicted in <FIG>.

<FIG> is a graph depicting forward-voltage/temperature relations of a diode at various biasing condition. In <FIG>, graph <NUM> includes horizontal axis <NUM>, vertical axis <NUM> and forward-voltage/temperature relations 62A-62D. Horizontal axis <NUM> is indicative of temperature, and vertical axis <NUM> is indicative of forward voltage across an exemplary diode, such as those that can be used in first and second diode networks of an overcurrent protection circuit. Relation 62A is indicative of a forward-voltage/temperature relation for the diode, when biased at <NUM> mA. Relation 62B is indicative of a forward-voltage/temperature relation for the diode, when biased at <NUM> mA. Relation 62C is indicative of a forward-voltage/temperature relation for the diode, when biased at <NUM> mA. And relation 62D is indicative of a forward-voltage/temperature relation for the diode, when biased at <NUM>µA.

Each of forward-voltage/temperature relations 62A-62D indicates a forward-voltage that decreases with increasing temperature. Although each of forward-voltage/temperature relations 62A-62D indicate a temperature dependent forward voltage, the temperature dependency of a voltage difference between adjacent pairs of forward voltage/temperature relations 62A-62D is much more modest. For example, the voltage difference between forward-voltage/temperature relation 62C and forward-voltage/temperature relation 62D is <NUM> mV at -<NUM> and <NUM> mV at <NUM>. Thus, the voltage difference between forward-voltage/temperature relations 62C and 62D only varies by <NUM> mV over a <NUM> temperature swing. This similarity of temperature dependencies for differently biased diodes is exploited in the overcurrent protection circuits <NUM>, <NUM> and <NUM>', as depicted in <FIG>. To exploit this similarity of temperature dependencies for differently biased diodes, such diodes are typically located nearby one another so that any temperature gradient that exists across an assembly results in close temperature matching of adjacent or nearby diodes.

<FIG> is a graph depicting an output-detection-signal/current relations for the overcurrent detection circuit depicted in <FIG> at various temperatures. In <FIG>, graph <NUM> includes horizontal axis <NUM>, vertical axis <NUM>, and output-voltage/load-current relations 70A-70C. Horizontal axis <NUM> is indicative of load current ILOAD, and vertical axis <NUM> is indicative of output voltage VOUT. Relation 70A is indicative of an output-voltage of overcurrent protection circuit <NUM> (as depicted in <FIG>) as a function of load current ILOAD, when operating at a j unction temperature of -<NUM>. Relation 70B is indicative of an output-voltage of overcurrent protection circuit <NUM> as a function of load current ILOAD, when operating at a junction temperature of +<NUM>. Relation 70A is indicative of an output-voltage of overcurrent protection circuit <NUM> as a function of load current ILOAD, when operating at a junction temperature of +<NUM>. A ratio of the delta load current ΔILOAD and the nominal load current ILOAD(NOM) is low, at least in part due to the modest temperature dependency of the forward-voltage difference between differently biased diodes. In some embodiments this ratio of the delta load current ΔILOAD and the nominal load current ILOAD(NOM) can be less than <NUM>%, <NUM>%, <NUM>%, or <NUM>%.

An apparatus relates to a circuit for detecting an overcurrent condition in a load as defined in independent claim <NUM>.

The system of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components:.

A further embodiment of any of the foregoing systems, wherein the first terminals of the first and second diode networks can be cathode terminals of the first and second diode networks.

A further embodiment of any of the foregoing systems, wherein at least one of the first and second diode networks can consist of a single diode.

A further embodiment of any of the foregoing systems, wherein at least one of the first and second diode networks can include a sensing diode in series with a sensing resistor.

A further embodiment of any of the foregoing systems, wherein the first and second diode networks can include diodes of a single type.

A further embodiment of any of the foregoing systems, wherein a ratio of currents sourced by the first and second biasing circuits can be greater than <NUM>:<NUM>.

A further embodiment of any of the foregoing systems, wherein the first diode network can include a first number of parallel diode elements, and the second diode network can include a second number of parallel diode elements. The first number can be different from the second number.

A further embodiment of any of the foregoing systems, wherein a ratio of currents sourced by the first and second biasing circuits can be equal to one another.

A further embodiment of any of the foregoing systems, wherein the first terminal of the current-sense resistor can be connected to the power source and the second terminal of the current-sense resistor can be connected to the load, which is also connected to a voltage reference.

A further embodiment of any of the foregoing systems, wherein the first terminal of the current-sense resistor can be connected to the load, which is also connected to the power source, and the second terminal of the current-sense resistor can be connected to a voltage reference.

A further embodiment of any of the foregoing systems, wherein the first and second diode networks can be directly coupled to a corresponding one of the first and second biasing circuits.

A further embodiment of any of the foregoing systems can further include a comparator having positive and negative input terminals and an output terminal. The positive and negative input terminals can be conductively coupled to the second terminals of the first and second diode networks. The comparator can generate an output signal indicative of the overcurrent condition on the output terminal.

A further embodiment of any of the foregoing systems can further include a power-interrupt switch configured to interrupt power to the load in response to the output signal indicative of the overcurrent condition.

A further embodiment of any of the foregoing systems, wherein the positive and negative input terminals can be directly coupled to the second terminals of the first and second diode networks.

A further embodiment of any of the foregoing systems, wherein the power-interrupt switch can be latched open in response to the output signal indicative of the overcurrent condition.

A further embodiment of any of the foregoing systems, wherein each of the first and second biasing circuits can include a resistor.

A further embodiment of any of the foregoing systems, wherein each of the first and second biasing circuits can consist of a resistor.

A further embodiment of any of the foregoing systems, wherein a ratio of resistances of the resistors of the first and second biasing circuits can exceed <NUM>:<NUM>.

A further embodiment of any of the foregoing systems, wherein the power source can be an aircraft's power source.

Claim 1:
A circuit for detecting an overcurrent condition in a load (<NUM>), the circuit comprising:
a current-sense resistor (<NUM>) having first and second terminals, the current-sense resistor is configured to be connected in series with the load, the current-sense resistor and the load provided power by a power source;
a first biasing circuit (<NUM>);
a second biasing circuit (<NUM>);
a first diode network (<NUM>) having anode and cathode terminals, the anode terminal of the first diode network connected to the first terminal of the current-sense resistor and the cathode terminal of the diode network conductively coupled to the first biasing circuit (<NUM>) so as to bias the first diode network in forward-voltage operation;
a second diode network (<NUM>) having anode and cathode terminals, the anode terminal of the second diode network connected to the second terminal of the current-sense resistor and the cathode terminal of the diode network conductively coupled to the second biasing circuit (<NUM>) so as to bias the second diode network in forward-voltage operation,
characterised in that
the first and second biasing circuits are configured to provide current to the first and second diode networks such that a voltage difference between the cathode terminals of the first and second diode networks is of a first polarity during normal current conditions of the load, and the voltage difference between the cathode terminals of the first and second diode networks is of a second polarity during overcurrent conditions of the load.