Abstract:
A current sensor configured to be employed in an array of current sensors and an array of parallel connected current sensors is disclosed. In one embodiment the current sensors comprise integrated circuit current sensors and a plurality of the current sensors are connected in parallel in a number that is selected to at least accommodate the maximum magnitude of a current to be monitored. When configured in parallel as an array of sensors, at least one of the current sensors of the array of current sensors provides an output signal that represents an average of the currents measured by the plurality of current sensors in the array.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of U.S. provisional application 62/245,032 filed Oct. 22, 2015 and titled Scalable Average Current Sensor System. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Current sensors are provided as an integral part of many electronic systems and employ different methods for the current sensing process. Generally, a current sensor is a device that provides a current path from a current input to a current output and that generates an output signal that is representative of the magnitude of the current flowing through the current path. Common current sensing methods include resistive shunt measurements, measurements based on the direct current resistance of a magnetic element, transformer based measurements, MOSFET RDS on or ratiometric measurements, Hall Effect measurements and Magneto Resistive measurement techniques. Each method has various advantages and disadvantages. 
         [0003]    Resistive shunt sensors are one of the simplest techniques and potentially most accurate methods for sensing current. However, when measuring currents greater than approximately 10 amps, I 2 R ohmic power losses become significant and limit the application of this approach. Additionally, the resistive shunt technique is not galvanic plate isolated and thus becomes inappropriate for systems sensing voltages above 24 V and certainly above 60 V which is considered the maximum safe voltage that a human can directly touch. A resistive shunt sensor also has a limited dynamic range. A shunt resistor has to be scaled to give the right amount of voltage drop to be amplified and measured but not so high a resistance as too cause too large a voltage drop. 
         [0004]    Hall Effect sensors are available in several configurations for the measurement of higher currents in the range of 50-20,000 amps. These configurations generally require that the current to be sensed pass through a large magnetic element. Transformer type current sensors employed in the sensing of currents in excess of 200 amps tend to be bulky devices. A conductor carrying the current to be measured typically passes through an opening in the transformer type current sensor which in turn is electrically coupled to an associated integrated circuit for processing. The transformer type current sensor and the integrated circuit are separate devices which are often mounted to a common substrate, such as a printed circuit board. Consequently, the end user must provide an external magnetic sensor and conductor associated with the sensor that is interconnected with the integrated circuit. 
         [0005]    It is often desirable to sense currents which are greater than the maximum current rating of an integrated circuit sensor. This can be done by splitting the total current into two or more paths to and assuring that the current measured on each path with an integrated circuit does not exceed the maximum current rating for the respective device. However, due to practical considerations, it is difficult to know the current divide ratio and different currents may flow in different current paths. This approach therefore can lead to reduced accuracy or require a post-assembly calibration. 
         [0006]    One current sensor that is capable of parallel interconnection for high current measurement is available from Texas Instruments™ under model number INA250. Each INA250 current sensor includes a shunt resistor. A portion of the current to be sensed passes through each of the shunt resistors when the INA250 devices are used in parallel. In this device, the resulting current that is sensed is the sum of the currents sensed by each of the current sensors and required bias and output voltages disadvantageously increase with an increasing number of parallel connected sensors, necessitating the addition of larger power voltages as the number of current sensors increase. 
         [0007]    It would therefore be desirable to have a current sensor that was fabricated as a small integrated circuit (IC) that allowed the IC based current sensor to be used in high current measurement applications. Additionally, it would be desirable to have a current sensor that was scalable so as to permit the IC based current sensor to meet a wide range of application requirements, including high current measurement requirements, while avoiding the need to provide increasing bias power supplies and output voltages with an increasing number of current sensors. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    In accordance with the present invention an IC based current sensor is disclosed. The disclosed current sensor is configured to permit multiple IC based current sensors to be connected in parallel as an array of current sensors. When configured as a plurality of current sensor interconnected in parallel as an array, a portion of the current to be measured passes through each one of the plurality of current sensors in the array. The maximum permissible current specification for the array is thus approximately the maximum current specification for each current sensor multiplied by the number of current sensors in the array. The array of current sensors provides as an output a signal that represents the average of the currents sensed by the plurality of current sensors in the array. Since any number of the IC based current sensors may be connected in parallel, a current sensing solution is provided that is scalable to satisfy any current sensing requirement. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0009]    The invention will be more fully understood by reference to the following Detailed Description of the Invention in conjunction with the Drawings of which: 
           [0010]      FIG. 1  is a simplified schematic diagram of a current sensor in accordance with the present invention that is configured to permit parallel connection of the current sensor with one or more like current sensors; 
           [0011]      FIG. 2  is a simplified schematic diagram depicting a plurality of parallel interconnected current sensors in accordance with the present invention; and 
           [0012]      FIG. 3  is a schematic diagram of the current sensor of  FIG. 1  that includes circuitry for offset and gain adjustments. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    U.S. provisional application 62/245,032 filed Oct. 22, 2015 and titled Scalable Average Current Sensor System is hereby incorporated herein by reference it its entirety. 
         [0014]    A scalable IC based current sensor  200  in accordance with the present invention and an array of such sensors interconnected in parallel are depicted in  FIGS. 1-3 . 
         [0015]    The disclosed current sensor may be provided as a fully integrated bi-directional current sensor that deliver both high accuracy and high bandwidth. In one embodiment Anisotropic Magneto Resistive (AMR) current sensing is employed which provides low noise, excellent linearity and repeatability. Any other suitable current sensing technology may also be utilized. 
         [0016]    A fully isolated current path is provided by a low resistance copper conductor integrated into the package making it suitable for both high-side and low side bi-directional current sensing. The current sensor has a high bandwidth which makes it suitable for feedback loops in motor control and power supply applications. 
         [0017]    Referring to  FIG. 1 , the current sensor includes a current sensing element  202  which, in the illustrated embodiment is an Anisotropic Magneto Resistive (AMR) sensor. While the illustrated sensing element  202  is depicted as an AMR sensing element, the current sensing element may a shunt resistive element, DC Resistance (DCR), a Hall Effect sensor, a transformer, or any other suitable current sensing element. The current sensor  200  provides an output signal that is representative of the current I 1  traversing a current path  210  between IP+ and IP−. The output of the current sensing element  202  is coupled to the input of a gain stage amplifier  230  which in turn is coupled to an output stage amplifier  240 . The output stage gain is determined by resistors R 5 , R 6 , R 7  and R 8 . A unity gain voltage reference buffer  250  is provided with a reference input (V ref  input) that provides a bias reference for the output stage amplifier  240 . The output from the output stage amplifier  240  is a voltage signal that represents and is proportional to the current I 1  traversing the current path  210 . The output stage amplifier  240  output is coupled to a SHARE connection through a resistor R 9  and the SHARE connection is connected to an output buffer  260  input. In the illustrated embodiment, the output buffer is shown as an amplifier  260  that provides an output signal V out . The gain of the amplifier  260  is determined by the resistors R 10  and R 11 . The presently described circuit may be fabricated using discrete electronic components, as an integrated circuit or, as a combination of discrete components and one or more integrated circuit components. The SHARE connection is an external connection when the current sensor  200  is fabricated using one or more integrated circuits that include the relevant circuitry to the permit the SHARE connections of multiple current sensors  200  to be bussed together and thus electrically interconnected one to the other. 
         [0018]    More specifically, the AMR sensor  202  monitors the magnetic field generated by the current I 1  flowing through a U shaped current pathways from IP+ to IP− in an integrated circuit package lead frame. The AMR sensor  202  produces a voltage proportional to the magnetic field created by the positive or negative current in the IP+ to IP− current loop  210  while rejecting external magnetic interference. The current sensor  202  output voltage is coupled to a differential amplifier  230  whose gain is temperature compensated. The differential amplifier  230  output is in turn coupled to an output stage an amplifier  240 . The output stage amplifier  240  produces an output voltage that is representative of the current passing through the IP+ to IP− pathway  210 . To provide both positive and negative current data, the V out output pin is referenced to the V ref  output pin. The voltage on the V ref  output is typically about one half of the full scale positive and negative range of the V out  output signal. With no current flowing through the IP+/IP− pins, the voltage on the V out  output will typically equal the voltage on the V ref  output. Positive IP+/IP− current causes the voltage on V out  to increase relative to V ref  while negative IP+/IP− current will cause it to decrease. 
         [0019]    The current sensor  200  may optionally include a voltage regulator  220  to provide a regulated bias voltage to the current sensing element  202  and to provide fixed gain from the sensor resistors R 1 -R 4 . When a voltage regulator  220  is employed, the sensor resistors R 1 -R 4  are biased with a fixed voltage so as to immunize the current sensing circuitry  202  from changes in the V cc  supply voltage. 
         [0020]    When the voltage regulator  220  is omitted, the sensor resistors R 1 -R 4  are biased to the Vcc supply voltage and produce a differential voltage that is ratiometric to V. This configuration is suited to applications where analog-to-digital converter (A-to-D) circuitry receiving the current sensor output signal from V out  are biased by, and ratiometric to, the same supply voltage as the current sensor. The ratiometric configuration provides increased gain and enhanced supply rejection compared to the embodiment that includes the regulator  220 . 
         [0021]    Power is provided to the current sensor  200  between V cc  and Gnd. 
         [0022]    In  FIGS. 1 and 2 , input signals for offset and gain adjustments that may be provided for purposes of temperature compensation or component variations have been omitted to more clearly describe the operation of the current sensor  200  individually and when employed in an array. Such components, however, are illustrated and discussed below in connection with  FIG. 3 . 
         [0023]    When the current sensor  200  is used as a single sensor, the output signal V out  is a voltage output that is representative of the current I 1  through the current path  210  of the current sensing element  202 . Additionally, when the current sensor  200  is used singularly, the maximum current that can be accommodated and measured by the device is limited to the maximum current rating of the respective sensor  200 . 
         [0024]    As illustrated in  FIG. 2 , current sensors  200  may be interconnected and arrayed in parallel to extend the measurement capability of current sensor fabricated as an integrated circuit to high current applications. 
         [0025]    Referring to illustrative  FIG. 2 , three current sensors  200   a - 200   c  are connected in parallel with the SHARE connections of the three current sensors electrically connected to one another. A total current I Total  which is the sum of currents I 1 , I 2  and I 3 , passes through the current sensor array, with a first portion of the total current, I 1 , passing through a first current sensor  200   a,  a second portion of the total current, I 2 , passing through a second current sensor  200   b  and a third portion of the total current, I 3 , passing through a third current sensor  200   c.  While three current sensors are illustrated in the parallel interconnected array depicted  FIG. 3 , any number of current sensors  200  may be connected in parallel via the SHARE connection. It is further noted that all IP+ connections of current sensors are bussed together and all IP− connections of current sensors are bussed together so that portions of the total current I Total  pass through each of the current sensors in the array. 
         [0026]    Since it is difficult to fabricate multiple current splitting paths so that the currents passing through each individual path are all exactly equal, the currents I 1 , I 2  and I 3  carried by the current pathways of the respective sensors may be mismatched. Thus, the output voltages from the output amplifiers  240  (See  FIG. 1 ) in the respective current sensors may differ. By bussing the SHARE connections of the current sensors together, and setting each resistor R 9  to be the same value within an acceptable and defined tolerance, the voltage on the SHARE terminal represents the average of the voltages on the output of the output stage amplifiers  240  of the various current sensors and thus, the average of the currents flowing through the current pathways of the three current sensors. Since the number of current splitting paths and the number of sensors are known in advance, the average of the currents conveys the same information as the total current. More specifically, the total current is the average current times the number of current splitting paths. Additionally, while ideally, the value of the resistors are equal, it should be recognized, that, in practice, it is extremely difficult to perfectly match any two electrical components. The value of the resistors R 9  are equal within a defined tolerance and, in this context, are substantially equal. The resistor R 9  may be preselected or trimmed during production to a desired value within a specified tolerance. For example, the resistor R 9  may be trimmed during fabrication of an integrated circuit to within 1% of the specified value. Alternatively, the resistor R 9  may be provided as a controllable resistance which may be adjusted to achieve a desired value as illustrated in  FIG. 3 . 
         [0027]    The SHARE terminal is connected to the input of the V out  Buffer. The V out  Buffer provides a voltage output corresponding to the average of the voltage outputs of the Output Amplifiers  240  of the current sensors. An output from one of the V out  Buffers is employed, as illustrated in  FIG. 2 , although each of the output buffers in the illustrated embodiment produces the same output voltage. The outputs from the other V out . Buffers are not used as illustrated in  FIG. 2  by an “X”. 
         [0028]    The array of current sensors thus serves as a current sensor having a theoretical maximum amperage specification equal to the number of current sensors in the array times the maximum amperage specification of each of the current sensors. In practice, since the currents may not split evenly among multiple current paths, the actual maximum amperage specification will be less than the theoretical maximum amperage specification since no current path may exceed the maximum current rating for the respective current sensor and some current paths may carry less than the maximum current for which the respective sensors are rated. The disclosed system provides several advantages over known prior art systems using parallel connected current sensors to accommodate current measurements in excess of the maximum current specification of a single current sensor. 
         [0029]    When a current sensor as described above is fabricated as an integrated circuit, a current sensing solution can be provided that is much smaller in size when compared to existing solutions used for sensing 50 amps or greater. Additionally, by sensing the average current sensed by the array of sensors, an accurate current measurement may be obtained even if the total current I Total  being measured is not divided equally among all of the individual sensors in the sensor array. Furthermore, since any number of current sensors may be connected in parallel, the array of current sensors formed upon interconnection can accommodate any level of current. Additionally, unlike known systems which require voltage supplies having higher voltages as the number of stages increase, the presently disclosed system employs a single Vcc supply voltage irrespective of the number of current sensors employed in the array. Thus, the need for multiple power supplies of different voltages is avoided. Lastly, thermal management is simplified since current sensors may be physically spread out to minimize local heating. 
         [0030]      FIG. 3  illustrates the current sensor of  FIG. 1  but includes components for providing offset and gain adjustments for bias and temperature compensation. More specifically, as illustrated in  FIG. 3 , the current sensor  200  also includes a temperature sensor  310 , an arithmetic logic unit (ALU)  320  which is interfaced to a processor (not shown) and an oscillator  330  providing a clock for the ALU  320 . The ALU  320  includes digital outputs that are coupled to Digital to Analog Converters (DACs)  360 ,  370 ,  380 ,  390 ,  395  which in turn have analog outputs coupled to the Output State Amplifier  240 , Gain Stage Amplifier  210 , V ref  Buffer  250 , optionally to R9 if R9 is adjustable and to the V out  Buffer  260  to permit gain, offset or value adjustments to the respective components, as applicable. A control signal I Ready  is provided as an output from the ALU that is coupled to an input of the processor to permit the processor to detect when the ALU has powered up after a power up sequence. 
         [0031]    A digital compensation scheme allows for compensation due to variations of sensor sensitivity and offset with temperature. Both the offset and gain of the entire signal path are adjustable using the digital to analog converters (DACs). The high resolution (16 bit) digital temperature sensor  310  measures the temperature of the sensor  200 . The arithmetic logic unit (ALU)  320  calculates trim codes for the offset and gain of the amplifiers  230 ,  240 ,  250 ,  260  based on the temperature sensor  310  inputs. When there is a change in these codes there will be a step at the output that provides a correction in gain or offset should such be necessary. The DACs have a small step size to provide a fine adjustment capability in sensor output voltage. In one embodiment, the temperature readings are collected and output codes are re-calculated at a rate of approximately 2 kHz although any suitable rate may be employed. The control codes do not change by more than 1 LSB at a time which guarantees a small step at the outputs. Filtering is used on the temperature sensor  310  output to minimize noise on the temperature sensor  310  output signal. Initial accuracy may be pre-programmed into a one-time programmable (OTP) memory through the two TST pins. 
         [0032]    While the disclosed embodiment utilizes digital techniques for controlling temperature compensation and offset adjustments, it will be recognized by those of ordinary skill in the art that analog techniques for such control may alternatively be employed. 
         [0033]    While the illustrated current sensor  200  provides an analog output, it should be recognized that an analog to digital converter (A-to-D) may be employed to convert the analog output to a digital output representative of the total current I Total . 
         [0034]    As described above, the disclosed current sensor and method of use permit like current sensors to be interconnected in parallel in a scalable manner to provide for the measure of large currents. When interconnected in parallel, the system provides an output that is the average of the currents flowing through the respective interconnected current sensors. It will be appreciated by those of ordinary skill in the art that variations of and modifications to the above-described current sensor and method may be made without departing from the inventive concepts disclosed herein. 
         [0035]    Accordingly, the invention should not be viewed as limited except by the scope and spirit of the appended claims.