Current Measurement Circuit and Method

Abstract

A current measurement circuit includes an operational amplifier, an input connector, a gain control sub-circuit, and a measurement output. The operational amplifier includes two inputs and an output. The input connector is for connecting the two inputs across a current-sense resistor. The gain control sub-circuit is connected to the output and includes a switch operable to select a current path from a plurality of current paths. The measurement output is connected to the gain control sub-circuit for outputting a signal indicating a measured current based on a selected current path. The gain control sub-circuit is configured to select a gain across the operational amplifier based on a ratio between the input connector and the selected current path.

Claims

1. A current measurement circuit comprising: an operational amplifier including two inputs and an output; an input connector for connecting the two inputs across a current-sense resistor; a gain control sub-circuit connected to the output and including a switch operable to select a current path from a plurality of current paths; and a measurement output connected to the gain control sub-circuit for outputting a signal indicating a measured current based on a selected current path, wherein the gain control sub-circuit is configured to select a gain across the operational amplifier based on a ratio between the input connector and the selected current path.

2. The current measurement circuit of claim 1 wherein: the input connector includes an input resistor; and the gain control sub-circuit includes a plurality of output resistors and the plurality of current paths connect through one or more of the plurality of output resistors.

3. The current measurement circuit of claim 1 wherein: the gain control sub-circuit includes a high gain current path and a low gain current path; the low gain current path has lower resistance than the high gain current path; and the switch is operable to selectively connect the output to one of the high gain current path and the low gain current path.

4. The current measurement circuit of claim 1 further comprising a controller for controlling the switch based on characteristics indicative of a sensed current amplitude.

5. The current measurement circuit of claim 4 wherein: the controller controls the switch based on the characteristics exceeding a threshold; and the threshold is selected to be indicative of a current amplitude below a saturation amplitude associated with the operational amplifier.

6. The current measurement circuit of claim 4 wherein the characteristics includes a voltage.

7. The current measurement circuit of claim 6 wherein the voltage on the output is measured relative to ground.

8. The current measurement circuit of claim 4 wherein the controller includes a comparator for comparing a voltage on the measurement output to a threshold.

9. The current measurement circuit of claim 8 wherein the controller further includes a voltage follower connected between the measurement output and the comparator for generating an input signal to the comparator based on a voltage on the measurement output.

10. The current measurement circuit of claim 8 wherein the controller further includes a hysteresis for preventing oscillations of a comparator output.

11. The current measurement circuit of claim 1 wherein: the measurement output includes a high gain measurement output and a low gain measurement output; and the switch is operable to select a current path for providing a lower gain signal on the low gain measurement output and a current path for providing a higher gain signal on the high gain measurement output.

12. The current measurement circuit of claim 11 wherein: the gain control sub-circuit includes first and second output resistors connected in series between the output and ground; the switch is operable to bypass the first output resistor; and the high gain measurement output is connected to ground through the first and second output resistors and the low gain measurement output is connected to ground through the second output resistor.

13. A load driver comprising: the current measurement circuit of claim 1.

14. A current protection device comprising: the current measurement circuit of claim 1.

15. A method of controlling the current measurement circuit of claim 1, the method comprising: identifying characteristics exceeding a threshold, wherein the threshold is selected to be indicative of a current amplitude below a saturation amplitude associated with the operational amplifier; and controlling the switch to select a current path for reducing the gain across the operational amplifier.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present disclosure will become more fully understood from the detailed description and the accompanying drawings.

[0026] FIG. 1 shows a current measurement circuit according to a first example arrangement.

[0027] FIG. 2 shows a current measurement circuit according to a second example arrangement.

[0028] FIG. 3 shows an alternative representation of the current measurement circuit 1 shown in FIG. 2.

[0029] FIG. 4 shows a current measurement circuit according to a fourth example arrangement.

[0030] FIG. 5 shows a current measurement circuit according to a fifth example arrangement.

[0031] In the drawings, reference numbers may be reused to identify similar and/or identical elements.

DETAILED DESCRIPTION

[0032] FIG. 1 shows a current measurement circuit 1 according to a first example arrangement. The circuit 1 comprises a shunt resistor 21 connected on the line between a power supply 11, such as a battery, and a load 10. A bypass 2 is connected around the shunt resistor 21 and functions as an input for connecting to an operational amplifier (op-amp) 3 in a differential amplifier configuration. An input resistor 4 is connected on the line to the inverting input of the op-amp 3.

[0033] A gain control sub-circuit 5 is connected to the output line 6 from the op-amp 3. The gain control circuit 5 comprises a first output resistor 51 connecting between the output line 9 and ground, and a parallel second output resistor 52 switchably connecting between the output line 6 and ground. The second output resistor 52 is switchably connected under the control of switch 53. As such, the operation of switch 53 controls the gain of the current sense circuit based on varying the resistance of the gain control sub-circuit 5.

[0034] In this connection, switch 53 may be implemented as a MOSFET controllable to adjust the gain of the current measurement circuit 1 based on the combination of gain setting resistors used. That is, the overall gain is equal to the resistance provided by the gain control sub-circuit 5 divided by the resistance provided by the input resistor 4. As such, when switch 53 is closed, the second output resistor 52 is connected, lowering the resistance of the gain control sub-circuit 5. As such, the gain is reduced. This allows higher currents to be measured without saturation. Conversely, when switch 53 is open, thereby disconnecting the second output resistor 52, the output resistance is increased, providing an increased gain. This allows lower currents to be measured with a higher resolution.

[0035] The switch 53 is controlled by a controller 54. The controller 54 may be implemented using, for example, a microcontroller or a comparator which provides a signal for controlling the switch 53 such that the switch 53 is open when the voltage being measured exceeds a threshold and closed when the voltage being measured is below the threshold. Accordingly, at higher currents, where a voltage exceeds a predetermined threshold, the gain control sub-circuit 5 is set to a lower gain to avoid saturation. Conversely, at lower currents, where the voltage is below the predetermined threshold, the gain control sub-circuit 5 is set to a higher gain for improved resolution.

[0036] FIG. 2 shows a current measurement circuit 1 according to a second example arrangement. This arrangement is similar to the first example, except that gain control sub-circuit 5 comprises the first and second output resistors 51 and 52 connected in series between the output line of the op-amp 3 and ground, with the output line 6 being divided into a high-gain measurement output line 61 and low-gain measurement output line 62. Again, the operation of switch 53 is used to control the gain of the current sense circuit 1 based on varying the resistance of the gain control sub-circuit 5.

[0037] In this connection, with the switch 53 open, the output resistance of the gain control sub-circuit 5 is the sum of the first and second output resistors 51, 52, resulting in a high-gain output on high-gain measurement output line 61. As such, the high-gain measurement output line 61 provides a high gain output which can be used for measuring low currents. However, as the current increases, saturation will occur. To address this, switch 53 may be closed to thereby bypass the first output resistor 51. As such, the gain is based on the resistance of the second output resistor 52 divided by the input resistor 4, resulting in a low-gain output being applied on low-gain measurement output line 62. Accordingly, the closing of switch 53 acts to disable the high gain, low current measurement chain outputted on high-gain measurement output line 61.

[0038] The switch 53 in this second example is similarly controlled by a controller 54, as with the first example. Accordingly, when the voltage being measured is below a threshold, the switch 53 is closed, providing for a higher gain. Conversely, when the voltage exceeds a predetermined threshold, the switch 53 is opened, providing for a lower gain, for thereby avoiding saturation.

[0039] FIG. 3 shows an alternative representation of the current measurement circuit 1 shown in FIG. 2, with corresponding features numbered the same. In this case, the controller 54 operates by comparing a threshold voltage input 55 to controller input 56 taken from the low-gain measurement output line 62. As such, when the voltage on the controller input 56 exceeds the threshold voltage applied to threshold voltage input 55, the switch 53 is opened to provide for a lower gain in the current measurement circuit. The low-gain measurement output line 62 is used for the voltage measurement because this is used for high current measurements and the gain on this line is stable and hence is not affected by gain changes implemented using switch 53.

[0040] FIG. 4 shows a current measurement circuit 1 according to a third example arrangement. This arrangement is similar to the circuit shown in FIG. 3, except that the controller 54 is shown in further detail. In this case, the signal applied to switch input 58 is controlled by a comparator 57, which includes an op-amp with positive feedback for comparing the voltage on input line 56 with a voltage on threshold voltage input 55. The input line 56 is connected to a voltage follower 58 which simply follows the voltage on the low-gain measurement output line 62, but acts to prevent the comparator 57 from disturbing the signal on the low-gain measurement output line 62. In particular, the provision of a voltage follower 58 may act to mitigate the influence of temperature changes on the switching threshold of the controller 54.

[0041] In addition, the controller 54 in this arrangement also incorporates a hysteresis around the comparison threshold to avoid oscillations. That is, the threshold set by the input 55 of comparator 57 is complemented by a feedback resistor 581 from the output 58 to the positive input below comparator 57. This thereby provides a hysteresis which acts to avoid oscillations of the output 58 of the comparator 57 and thereby avoids the successive quick tuning-on and -off of the switch 53 in scenarios where there is a low amplitude oscillation of the measured current around the threshold value.

[0042] Accordingly, with the above arrangement, the comparator 57 generates a switch command signal on the switch input 58 when the voltage exceeds a predetermined threshold in order to open the switch 53 for providing a lower gain. As such, saturation of the measurement signal may be avoided.

[0043] It will be understood that current measurement circuits may be implemented to provide more than two gains. In this respect, FIG. 5 shows a current measurement circuit 1 according to a fourth example arrangement. In this case, the gain control sub-circuit 5 comprises two gain control switches 531, 532 which are independently controlled by controller 54 in order to vary the output resistance. This provides additional gain levels which can be used to vary the amplification of the current measurement circuit.

[0044] Accordingly, with the above arrangements, a variable gain current measurement amplifier may be provided which allows for current to be accurately measured over a wide range of amplitudes. In particular, saturation of the measurement amplifier can be avoided at high currents, whilst still providing for high resolution at lower currents.

[0045] It will be understood that the embodiments illustrated above show applications only for the purposes of illustration. In practice, embodiments may be applied to many different configurations, the detail of which being straightforward for those skilled in the art to implement.

[0046] For example, a microcontroller may be used to monitor the measurement chain and control the switching of the gain control sub-circuit. However, the measurement circuit can go into saturation between two polling periods of the microcontroller if the current varies abruptly. As such, controlling the gain change using a comparator allows rapid gain switching based on logic functions.

[0047] It will also be understood that in some arrangements, the low gain (high current) measurement line may be monitored continually, by providing a dedicated low-gain measurement output line 62 with the two output resistors in series.

[0048] Finally, it will be understood that various arrangements may be implemented to incorporate the hysteresis principle described above in relation to FIG. 4. As such, the controller may be reinforced using a hysteresis around the comparison threshold in order to avoid oscillations. In arrangements, this hysteresis may be implemented on an analogue comparator with a feedback of the output to the positive input, or digitally in a microcontroller by adjusting the threshold accordingly once a crossing of the initial threshold has been detected.

[0049] The term non-transitory computer-readable medium does not encompass transitory electrical or electromagnetic signals propagating through a medium (such as on a carrier wave). Non-limiting examples of a non-transitory computer-readable medium are nonvolatile memory circuits (such as a flash memory circuit, an erasable programmable read-only memory circuit, or a mask read-only memory circuit), volatile memory circuits (such as a static random access memory circuit or a dynamic random access memory circuit), magnetic storage media (such as an analog or digital magnetic tape or a hard disk drive), and optical storage media (such as a CD, a DVD, or a Blu-ray Disc).

[0050] The term set generally means a grouping of one or more elements. The elements of a set do not necessarily need to have any characteristics in common or otherwise belong together. The phrase at least one of A, B, and C should be construed to mean a logical (A OR B OR C), using a non-exclusive logical OR, and should not be construed to mean at least one of A, at least one of B, and at least one of C. The phrase at least one of A, B, or C should be construed to mean a logical (A OR B OR C), using a non-exclusive logical OR.