Temperature compensated current limiting mechanism

The present disclosure is directed to a current limiting circuit. The current limiting circuit may include a load, a first switch that controls current supplied to the load, and a first resistive network. The current limiting circuit may further include a voltage divider connected across the first resistive network and including a thermistor. The current limiting circuit may further include a first bipolar junction transistor that controls switching of the first switch. The output terminal of the voltage divider may be connected to a base junction of the first bipolar junction transistor.

TECHNICAL FIELD

The present disclosure relates generally to electrical systems, and more particularly to electrical systems including a temperature compensated current limiting mechanism.

BACKGROUND

Traditional locomotives are known to use several on-board electrical systems to drive an output or load. Similarly, other electronic devices such as portable hand-held or wireless devices include load-driving circuitry. Typically, such electrical systems and devices include mechanisms to limit the amount of current that may be supplied to the load or output circuit. One such mechanism is a current limiter, which limits the current that may be supplied to the load or output circuit.

A conventional current limiting circuit200is illustrated inFIG. 2that limits the current supplied to load201. Circuit200uses the base-to-emitter voltage, or VEB, of PNP transistor Q1to limit the current when load201attempts to draw current that exceeds a predetermined limit value. As the load201draws increasing current, the VEBof transistor Q1increases from 0 mV to 650 mV based on the voltage drop across resistor RSENSE. A VEBof 650 mV begins to “turn on” the PNP transistor Q1, which increases the voltage drop across resistor RGATE. This increase in the voltage across resistor RGATEbegins to “turn off” PMOS (or p-type MOSFET) Q2because the gate voltage of PMOS Q2begins to increase. This turning off of Q2continues until the current limit is reached. The above configuration actively limits the load current in this manner. The arithmetic expression of the current limit value (ILIM) at room temperature may be given by:
ILIM=VBE÷RSENSE≈0.650V÷RSENSE(1)

Equation (1) above shows that one may select a current limit value by means of selecting an appropriate RSENSEresistor. InFIG. 2, circuit200limits the current deliverable to load201at approximately 217 mA (217 mA≈650 mV÷3Ω).

Circuit200, however, suffers from variation over temperature, since the VEBof Q1depends upon temperature for a given emitter-base current. An emitter-base current that produces a VEBof 650 mV at 25° C. produces approximately 800 mV at −40° C. and 420 mV at 150° C. That is, the threshold VEBvalue at which Q1“turns on” increases with a decrease in temperature. It will be apparent from equation (1) that such a change in the VEBthreshold value from 650 mV at room temperature will result in a large variation in the current limit for load201. Specifically, the load current will limit at too high for low temperatures, and too low at high temperatures.

U.S. Pat. No. 5,587,649 discloses a scheme that recognizes the variation in the VEBthreshold value for transistor Q1in a current limiting circuit. The '649 patent suggests replacing sense resistor RSENSEwith a combination of resistors including a thermistor having a negative temperature coefficient.

While the '649 patent may disclose a current limiting circuit that may take into account the variation in the VEBthreshold value for transistor Q1, the disclosed current limiting circuit does not attempt to maintain a low variation in the current limit over a wide temperature range. Instead, the disclosed current limiting circuit assumes that the maximum current demand of load201changes over temperature and the current limit tracks this change in current demand over temperature. Accordingly, the disclosed current limiting circuit may not be able to provide a low variation in the current limit over a wide temperature range.

Further, the disclosed current limiting circuit may not be useful for systems in which a high load current is required. This is because commercially available thermistors have a large resistance value as a result of which the combination of resistors including the thermistor will have a large effective resistance. As a result, a low load current (of the order of milli-amps) will cause a large voltage drop across the resistor combination sufficient to “turn on” transistor Q1, thereby limiting the load current to the milli-amps range.

The presently disclosed current limiting circuit and system including the same is directed to overcoming one or more of the problems set forth above and/or other problems in the art.

SUMMARY

In accordance with one aspect, the present disclosure is directed to a current limiting circuit. The current limiting circuit may include a load, a first switch that controls current supplied to the load, and a first resistive network. The current limiting circuit may further include a voltage divider connected across the first resistive network and including a thermistor. The current limiting circuit may further include a first bipolar junction transistor that controls switching of the first switch. The output terminal of the voltage divider may be connected to a base terminal of the first bipolar junction transistor.

According to another aspect, the present disclosure is directed to a locomotive. The locomotive may include a first electrical module and a second electrical module driven by the first electrical module. The second electrical module may include a load, a first switch that controls current supplied to the load, and a first resistive network. The second electrical module may further include a voltage divider connected across the first resistive network and including a thermistor. The second electrical module may further include a first bipolar junction transistor that controls switching of the first switch. The output terminal of the voltage divider may be connected to a base terminal of the first bipolar junction transistor.

DETAILED DESCRIPTION

FIG. 1illustrates a consist100comprising a plurality of locomotives120, the plurality including at least a first and a last locomotive120. Each locomotive120may include a locomotive engine140. In one embodiment, locomotive engine140may comprise a uniflow two-stroke diesel engine system. Those skilled in the art will also appreciate that each locomotive120may also, for example, include an operator cab (not shown), facilities used to house electronics, such as electronics lockers (not shown), protective housings for locomotive engine140(not shown), and a generator used in conjunction with locomotive engine140(not shown).

While not shown inFIG. 1, consist100may comprise more than two locomotives120. Additionally, consist100may also comprise a variety of other railroad cars, such as freight cars or passenger cars, and may employ different arrangements of the cars and locomotives to suit the particular use of consist100. In an embodiment, the locomotives within consist100communicate with each other through, for example, wired or wireless connections between the locomotives. Particular examples of such connections may include, but are not limited to, a wired Ethernet network connection, a wireless network connection, a wireless radio connection, a wired serial or parallel data communication connection, or other such general communication pathway that operatively links control and communication systems on-board respective locomotives of a consist.

FIG. 3illustrates an exemplary temperature compensated current limiting circuit300limiting the current supplied to load301. Circuit300operates from a power supply of 12V. It will be apparent, however, that the power supply value can be any other value, which is large enough to properly bias the components to the right of PNP transistor Q1. For example, the power supply could be 2.5V, 3.5V, 5.5V, etc.

Like circuit200, circuit300also includes PMOS Q2, capacitors C1, C2, and resistors RGATEand RSENSE. While the embodiment discloses capacitors C1and C2as having a value of 10 μF, it will be understood that any suitable value may be used for capacitors C1and C2. Similarly, the value of RGATEas 15 kohms is arbitrary and any other suitable value may be used. The value of RSENSEis also arbitrary (here 3 ohms) and can be adjusted based on the desired current limit.

Compared to circuit200, circuit300may include a voltage divider formed by resistor R1, negative temperature coefficient thermistor TH1, and resistor R2. One end of the voltage divider may be connected to one end of RSENSEand the other end of the voltage divider may be connected to the other end of RSENSE. Exemplarily, R1and R2may have a value of 49.9K and thermistor TH1may have a resistance value of 100K at room temperature. It will be understood that these values are only exemplary, and that R1, R2, and TH1may take on other values. The base terminal of the PNP transistor Q1may be connected to the output of the voltage divider such that thermistor TH1is connected between the base and emitter terminals of PNP transistor Q1. Next, the temperature compensation aspect of circuit300will be explained.

As discussed in the background section, the threshold VEBvalue for transistor Q1increases with a decrease in temperature and decreases with an increase in temperature. As a result, for example, when the temperature increases, a lower load current will be sufficient to create a voltage drop across RSENSEthat is enough to “turn on” transistor Q1. In circuit300, since thermistor TH1has a negative temperature coefficient, TH1's resistance value decreases with an increase in temperature. Accordingly, the decrease in the VEBthreshold is compensated by a decrease in the output voltage of the voltage divider, where the output voltage of the voltage divider equals VEB. Similarly, if the temperature decreases, the resistance of TH1increases to compensate for an increase in the VEBthreshold for transistor Q1.

FIG. 4illustrates the variation in current limit between the conventional circuit200and the disclosed exemplary circuit300across a wide range of temperatures. Line401is the variation of the current limit for the conventional circuit200and line402is the variation of the current limit for the disclosed circuit300. As can be seen fromFIG. 4, the variation in the current limit from the room temperature current limit decreased to about ±10% overall for circuit300compared to roughly +23% at −40° C. and −35% at 120° C. for circuit200. The graphical illustration of the current limit variation employed the Murata part number NCP15WF104F03RC for the negative temperature coefficient thermistor TH1.

Various modifications can be made to circuit300. For example, in a functionally equivalent circuit300, the p-type transistor Q1could be replaced by an n-type transistor Q1. Similarly, the p-type MOSFET Q2may be replaced by an n-type MOSFET Q2. Moreover, Q1may be replaced by a comparator circuit.

FIG. 5illustrates another exemplary temperature compensated current limiting circuit500limiting the current supplied to load201. Like the circuit inFIG. 3, a voltage divider formed by a negative temperature coefficient resistor (TH1) with resistances R1and R2drives the base of transistor Q1. An increase or decrease in resistance of TH1with a change in temperature compensates for the corresponding change in VEBthreshold for transistor Q1. Further, MOSFET Q2in circuit300has been replaced with a bipolar junction transistor Q2.

Additionally, resistor RGATEin circuit300has been replaced with resistor RBIASwhich is connected with the base and collector junctions of transistor Q2. It will be apparent to a skilled artisan that circuit500could also serve its purpose if resistor RGATEwas provided in circuit500like in circuit300.

INDUSTRIAL APPLICABILITY

The disclosed current limiting circuit may provide a low variation in the current limit over a wide temperature range. By providing a negative temperature coefficient thermistor in the base drive of transistor Q1, the temperature variance of the threshold VEBvalue can be compensated. Moreover, the disclosed current limiting circuit may have an advantage over conventional circuits in that the current limiting circuit300may be operable for both small and large load currents. This operation over a wide load current range is made possible by the provision of two separate paths to transistor Q2—a first low resistance path through RSENSEand a second high resistance path through the voltage divider. Most of the load current will flow through RSENSE, whose value can be adjusted based on the desired load current operating range.

It will also be understood that circuit300can be employed in an electrical module of locomotive100that is being driven by another electrical module of locomotive100. Moreover, it will be apparent that circuit300may be utilized in any electrical system where a current limiting mechanism is desired. For example, circuit300may be utilized in a battery pack. Circuit300may be utilized, for example, in a handheld device where a current limiting mechanism is desired.