JP2021001795A - Current detector and current detection method - Google Patents

Abstract

To provide a technique for allowing current detection in a relatively wide range while improving detection accuracy of weak current.SOLUTION: A current detector comprises: a shunt resistor element 18 connected to a path of current flowing through an electrical load; a first amplifier circuit 11 amplifying voltage at both ends of the shunt resistor element; a second amplifier circuit 21 amplifying an output voltage of the first amplifier circuit; and a control section 10 acquiring by calculation a first current value indicative of amplitude of the current on the basis of the output voltage of the first amplifier circuit, and acquiring by calculation a second current value indicative of amplitude of the current on the basis of the output voltage of the second amplifier circuit. The control section selects the first current value as detection result of the current in a first range where the current is relatively large, and selects the second current value as detection result of the current in a second range where the current is relatively small.SELECTED DRAWING: Figure 1

Description

The present invention relates to a technique for detecting an electric current.

For example, when a light emitting element such as an LED is turned on by PWM (Pulse Width Modulation) control and the current flowing at that time is detected, the current is converted into a voltage and amplified by using a shunt resistance element, and this voltage is further increased. Was smoothed by a smoothing circuit, then taken into an analog-digital converter, and the value was converted into a current (average current) value. The technique of smoothing the pulsed voltage and incorporating it into the analog-to-digital converter is described in, for example, FIG. 13 of JP-A-2000-155063 (Patent Document 1).

By the way, when the current is detected by the above method, if the value becomes as small as 0, the influence of the error due to the variation in the characteristics of the circuit element such as the amplifier circuit becomes relatively large, and the current detection accuracy decreases. For example, consider a case where the maximum value of the current to be detected is 1 A and the error at the time of detection is ± 10 mA. The error rate at the time of detection in this case corresponds to ± 1% with respect to the maximum value of the current. On the other hand, considering the case where the current to be detected is 100 mA, the error rate is 10% because the error is ± 10 mA. As described above, when a certain error exists at the time of detection, the smaller the current to be detected, the larger the influence of the error, and the lower the detection accuracy. Further, assuming the same error and considering the case where the current to be detected is 5 mA, if an error of −10 mA occurs, the detected value of the current will be −5 mA. However, since the analog-to-digital converter does not usually correspond to a voltage of 0 V or less, the detected value is 0 A. Since the error affects in this way, it is very difficult to detect a weak current with high accuracy.

Japanese Unexamined Patent Publication No. 2000-155063

One of the specific aspects of the present invention is to provide a technique capable of detecting a current in a relatively wide range and improving the detection accuracy of a weak current.

[1] The current detection device of one aspect according to the present invention is (a) a device for detecting a current flowing through an electric load, and (b) is connected on the path of the current flowing through the electric load. The shunt resistance element, (c) a first amplification circuit that amplifies the voltage across the shunt resistance element, (d) a second amplification circuit that amplifies the output voltage of the first amplification circuit, and (e) the first. 1 The first current value indicating the magnitude of the current is obtained by calculation based on the output voltage of the amplification circuit, and the second current value indicating the magnitude of the current is calculated based on the output voltage of the second amplification circuit. The control unit includes a control unit to be obtained, and (f) the control unit selects the first current value as the detection result of the current in the first range in which the current is relatively large, and the current is relatively small. In the two ranges, it is a current detection device that selects the second current value as the detection result of the current.
[2] One aspect of the current detection method according to the present invention is (a) a method for detecting a current flowing through an electric load, and (b) is connected on the path of the current flowing through the electric load. The first step of amplifying the current across the shunt resistance element, (c) the second step of further amplifying the voltage amplified in the first step, and (d) the control unit are obtained in the first step. The first current value indicating the magnitude of the current is obtained by calculation based on the obtained voltage, and the second current value indicating the magnitude of the current is obtained by calculation based on the voltage obtained in the second step. In the three steps, (e) the control unit selects the first current value as the detection result of the current in the first range in which the current is relatively large, and the control unit selects the first current value as the detection result of the current, and in the second range in which the current is relatively small This is a current detection method including a fourth step of selecting a second current value as the current detection result.

In the present specification, "connection" between a certain circuit element and another circuit element means not only the case where these circuit elements are directly connected to each other, but also the case where another circuit element or the like is between these circuit elements. The case of intervening connection is also included.

According to the above configuration, the current can be detected in a relatively wide range, and the detection accuracy of the weak current can be improved.

FIG. 1 is a diagram showing a configuration of a current detection device according to an embodiment. FIG. 2 is a diagram for explaining the relationship between the current flowing through the light emitting element (measured current) and the voltage input to the microcomputer (detection voltage). 3 (A) and 3 (B) are diagrams for explaining an example of the measured current and the detected voltage at the time of actual current measurement. FIG. 4 is a diagram showing an example of the error rate of the measured current. FIG. 5 is a diagram for explaining the correction of the measured current.

FIG. 1 is a diagram showing a configuration of a current detection device according to an embodiment. The illustrated current detection device 1 detects the current that flows when the light emitting element (LED) 2 is PWM-controlled by the drive circuit 3 to emit light. The current detection device 1 includes a microcomputer 10, operational amplifiers 11, 21, resistance elements 12, 13, 14, 15, 16, 22, 23, 24, 25, 26, capacitive elements 17, 27, shunt resistance elements 18, and three. It is configured to include a reference power supply REF. In the illustrated example, only one light emitting element 2 is used, but an arbitrary number of light emitting elements may be connected in series, parallel, or series-parallel.

The microcomputer (control unit) 10 includes an analog-to-digital converter (AD1) 10a and an analog-to-digital converter (AD2) 10b, each of which amplifies the voltage across the shunt resistance element 18 connected in series with the light emitting element 2. The voltage thus obtained is taken in and converted into digital data, and the value of the current flowing through the shunt resistance element 18 (that is, the current flowing through the light emitting element 2) is obtained based on the value.

In the operational amplifier 11, the non-inverting input terminal (+) is connected to one end (high potential side) of the shunt resistance element 18 via the resistance element 12, and the inverting input terminal (-) is connected to the shunt resistance element 18 via the resistance element 13. It is connected to the other end (low potential side) of the shunt resistance element 18 and amplifies the voltage across the shunt resistance element 18.

In this operational amplifier 11, a resistance element 14 is connected between the non-inverting input terminal and the reference power supply REF, and the potential of the non-inverting input terminal is pulled up to the voltage of the reference power supply REF (for example, + 0.5V). Since the non-inverting input terminal of the operational amplifier 11 is pulled up to the voltage of the reference power supply REF, the output voltage of the operational amplifier 11 is offset to the positive side by a certain voltage, so that the output voltage swings to the negative side due to the detection error. (Or polarity reversal) can be prevented. The resistor element 14 and the reference power supply REF correspond to the "first pull-up circuit".

Further, in the operational amplifier 11, a resistance element 15 for negative feedback is connected between the output terminal and the inverting input terminal. A differential amplifier circuit (first amplifier circuit) is composed of these operational amplifiers 11 and resistance elements 12 to 15. The amplification factor in this differential amplifier circuit is R2 / R1 times when the resistance value of the resistance element 13 is R1 and the resistance value of the resistance element 15 is R2.

The output voltage from the output terminal of the operational amplifier 11 is based on the resistance element 16 connected between the output terminal and the analog-to-digital converter 10a of the microcomputer 10 and the other end (analog-digital converter 10a side) of the resistance element 16. It is smoothed via a smoothing circuit composed of a capacitance element 17 connected to a potential terminal (ground terminal) and input to the analog-to-digital converter 10a. In this case, the microcomputer 10 subtracts (cancels) the voltage pulled up by the resistor element 14 (voltage of the reference power supply REF) from the value of the voltage taken in by the analog-to-digital converter 10a, and subtracts (cancels) the voltage of the voltage. The value is used to calculate the current flowing through the light emitting element 2.

In the operational amplifier 21, the non-inverting input terminal (+) is connected to the output terminal of the operational amplifier 11 via the resistance element 16, and the inverting input terminal (-) is connected to the reference power supply REF via the resistance element 23. Amplifies the voltage applied between the inverting input terminal and the inverting input terminal.

In this operational amplifier 21, a resistance element 24 is connected between the non-inverting input terminal and the reference power supply REF, and the potential of the non-inverting input terminal is pulled up to the voltage of the reference power supply REF (for example, + 0.5V). This has the same effect as that of the operational amplifier 11 described above. The resistance element 24 and the reference power supply REF correspond to the "second pull-up circuit".

Further, in the operational amplifier 21, a resistance element 25 for negative feedback is connected between the output terminal and the inverting input terminal. A differential amplifier circuit (second amplifier circuit) is composed of these operational amplifiers 21 and resistance elements 22 to 25. The amplification factor in this differential amplifier circuit is r2 / r1 times, where r1 is the resistance value of the resistance element 23 and r2 is the resistance value of the resistance element 25.

Further, in the operational amplifier 21, the potential of the inverting input terminal is pulled up to the voltage of the reference power supply REF (for example, + 0.5V) via the resistance element 23. As a result, the amount pulled up by the voltage of the reference power supply REF from the output voltage of the operational amplifier 11 can be canceled. If this cancellation is not performed, the voltage for the offset (for example, + 0.5V) is amplified by the operational amplifier 21, so that the voltage obtained by multiplying the voltage for the offset by r2 / r1 is always output from the operational amplifier 21, which is usually the case. In the case of, the permissible range of the input voltage of the microcomputer 10 is exceeded, and the measured value of the current is in a state of being overwhelmed to the maximum value. It is a circuit part for preventing such inconvenience. The resistance element 23 and the reference power supply REF correspond to the "cancellation circuit".

The output voltage from the output terminal of the operational amplifier 21 is based on the resistance element 26 connected between this output terminal and the analog-to-digital converter 10b of the microcomputer 10 and the other end (analog-digital converter 10b side) of the resistance element 26. It is smoothed via a smoothing circuit composed of a capacitance element 27 connected to a potential terminal (ground terminal) and input to the analog-to-digital converter 10b. In this case, the microcomputer 10 subtracts (cancels) the voltage pulled up by the resistor element 24 (voltage of the reference power supply REF) from the value of the voltage taken in by the analog-to-digital converter 10b, and subtracts (cancels) the voltage of the voltage. The value is used to calculate the current flowing through the light emitting element 2.

The voltage input to the analog-to-digital converter 10b is a value obtained by multiplying the value of the voltage amplified by the operational amplifier 11 by the value of the voltage amplified by the operational amplifier 21 and the value obtained by superimposing the pulled-up voltage. The sensitivity of the element 18 to the voltage across it becomes higher. Therefore, the voltage input to the analog-digital converter 10b can be used specifically for a weak current. That is, in the microcomputer 10, the current value is obtained by using the voltage input to the analog-digital converter 10a in the relatively large current range, and is input to the analog-digital converter 10b in the relatively small current range. If the value of the current is obtained using the voltage, the current can be detected in a relatively wide range, and the detection accuracy of the weak current can be improved.

FIG. 2 is a diagram for explaining the relationship between the current flowing through the light emitting element (measured current) and the voltage input to the microcomputer (detection voltage). In FIG. 2, the horizontal axis corresponds to the measured current and the vertical axis corresponds to the detected voltage. For example, assume that the maximum allowable value of the input voltage to the microcomputer 10 is + 5V. In this case, assuming that the voltage of the reference power supply REF is +0.5, the minimum value is set to + 0.5V in consideration of this. That is, a swing-out prevention width of + 0.5 V is provided on the lower limit side. Further, on the upper limit side as well, in consideration of the detection error, for example, a swing prevention width of + 0.5 V is provided. From these, by setting the range of + 0.5V to + 4.5V, that is, the permissible range as 4.0V, it is possible to prevent the detection voltage from swinging over the entire permissible range. This prevents the voltage from swinging out due to a detection error.

3 (A) and 3 (B) are diagrams for explaining an example of the measured current and the detected voltage at the time of actual current measurement. For example, assume that the maximum value of the measured current is 500 mA. When the measured current is 500 mA, the microcomputer 10 takes in the output voltage of the differential amplifier circuit by the operational amplifier 11 which is a standard sensitivity by the analog-digital converter 10a, and uses this to flow to the light emitting element 2. Obtain the value of the measured current (see FIG. 3 (A)). At this time, the value of the output voltage of the differential amplifier circuit by the operational amplifier 21, which has high sensitivity, is in a state of swinging to the upper limit side when it is taken into the microcomputer 10, so that the microcomputer 10 does not use this output voltage. Similarly, for example, even when the measured current is 200 mA, the value of the measured current can be obtained using the output voltage of the differential amplifier circuit by the operational amplifier 11 (see FIG. 3A).

On the other hand, for example, when the measured current is 50 mA, the microcomputer 10 takes in the output voltage of the differential amplifier circuit by the highly sensitive operational amplifier 21 with the analog-digital converter 10b, and obtains the value of the measured current using this. (See FIG. 3 (B)).

In this case, since the output voltage of the differential amplifier circuit by the operational amplifier 11 is also within the range in which the detection accuracy does not decrease so much, the microcomputer 10 uses the output voltage of the differential amplifier circuit by the operational amplifier 11 to determine the value of the measured current. It can also be obtained (see FIG. 3 (A)). Further, the microcomputer 10 compares the values of the measured currents obtained from the outputs of the differential amplifier circuits by the operational amplifiers 11 and 21, and if the difference is larger than the predetermined reference, a failure occurs in any of the circuits. You may judge that it is.

Further, for example, when the measured current is 5 mA, the microcomputer 10 takes in the output voltage of the differential amplifier circuit by the high-sensitivity operational amplifier 21 with the analog-digital converter 10b, and obtains the value of the measured current using this. (See FIG. 3B).

As described above, the microcomputer 10 obtains the measured current by using the output voltage of the differential amplifier circuit by the operational amplifier 11 in the first range (for example, the range of 80 mA to 500 mA in this example) in which the measured current is relatively large. In the second range (for example, in the range of 0 to 80 mA in this example) where the measured current is relatively small, it is adopted (selected) as the detection result, and the measured current is obtained using the output voltage of the differential amplifier circuit by the operational amplifier 21. Is adopted (selected) as the detection result. As a result, it is possible to cover the entire range of the measured current, and the detection accuracy can be improved even in the range where the measured current is weak. The term "adopted (selected)" as used herein means, for example, that the microcomputer 10 is used as a detection result of a current output to an external device or the like, or that the microcomputer 10 controls other controls (for example, brightness control of a light emitting element). ) Refers to using the detection result of the current.

FIG. 4 is a diagram showing an example of the error rate of the measured current. As shown in the curves a1 and a2 in the figure, the error rate of the measured current based on the output voltage of the differential amplifier circuit by the operational amplifier 11 increases as the value of the measured current decreases, and rapidly increases as it approaches 0. On the other hand, as shown by curves b1 and b2 in the figure, the error rate of the measured current based on the output voltage of the differential amplifier circuit by the operational amplifier 21 increases as the value of the measured current decreases, and as it approaches 0, it increases. The tendency to increase rapidly is the same, but the absolute value of the error rate is small. Therefore, as described above, the output voltage of the differential amplifier circuit by the operational amplifier 11 is used in the first range L1 where the measured current is relatively large, and the differential by the operational amplifier 21 is used in the second range L2 where the measured current is relatively small. By using the output voltage of the amplification circuit, the detection accuracy can be further improved in the entire range of the measured current, particularly in the range of the weak current. For example, in the illustrated example, the detection accuracy is within ± 8% in the entire range of the measured current.

FIG. 5 is a diagram for explaining the correction of the measured current. When the current measuring device of the above embodiment is applied to a mass-produced product, individual differences may occur in the detection accuracy of the measured current due to variations in parts and the like. In order to absorb such individual differences, it is advisable to make the corrections described below at the time of manufacturing the product. As an example, it is assumed that the maximum value of the measured current is 500 mA, and the voltage of the reference power supply REF is + 0.5 V.

First, 0 point correction is performed. Specifically, when no current is flowing through the light emitting element 2, the value of the voltage based on the measured current is + 0.5V in an ideal state. However, in reality, the value deviates from + 0.5V due to the influence of variation. In the 0-point correction, the voltage when no current is actually flowing is measured by the microcomputer 10 and the value is stored. Since this value is an individual difference, this stored value is used when actually obtaining the voltage.

Next, full-scale correction is performed. Specifically, when the measured current is the maximum value (500 mA in this case), the value of the voltage based on the measured current is + 4.5 V in an ideal state. However, in reality, the value deviates from + 4.5V due to the influence of variation. In full-scale correction, a current of 500 mA with high accuracy is actually passed, the voltage at that time is measured by the microcomputer 10, and the value is stored. Since this value is an individual difference, this stored value is used when actually calculating the voltage.

As shown in FIG. 5, the voltage values (indicated by triangles in the figure) with respect to the measured current obtained by each of the 0-point correction and the full-scale correction are connected by a straight line. Then, the current measurement formula can be expressed by a linear function. Assuming that the measured current of the graph is the X-axis and the detected voltage is the Y-axis, it can be expressed as Y = AX + B (A: graph slope, B: 0 point). Next, a correction element is incorporated into this. B can substitute the result of the above-mentioned 0 point correction as it is. For A, after the value of B is determined, the value of the slope A can be calculated from the equation obtained by applying the results of the current value and the detected voltage at the time of full-scale correction to this equation. Using these A and B, the current calculation formula can be expressed as I = (VB) / A. V here is the detection voltage. This expresses Ohm's law, where the I part applies to the current, the (VB) part applies to the voltage, and A applies to the resistor. Since the coefficients (A, B) of this formula are different for each product due to the influence of component variation, the above correction is performed for each product, this formula is retained for each product, and is applied at the time of actual voltage calculation. As a result, individual differences can be absorbed. This formula is prepared for each of the standard sensitivity and high sensitivity operational amplifiers.

Specific numerical examples of each circuit element of the current measuring device will be described below.
The shunt resistance element 18 is 1.3Ω, the full-scale current value is 500mA, and the voltage of the reference power supply REF is + 0.5V. The resistance elements 12 and 13 are 1 kΩ, respectively, and the resistance elements 14 and 15 are 6.2 kΩ, respectively. As a result, the gain (amplification factor) of the operational amplifier 11 is 6.2 times (6.2 k / 1 k), so that the voltage when a current of 500 mA flows is about 4.00 V (= 1.3 Ω × 500 mA × 6. 2). Since the reference voltage at 0 A is superimposed, the output voltage of the operational amplifier 11 becomes 4.5 V at this time. In the case of PWM current, this output voltage becomes PWM behavior, which is smoothed by a smoothing circuit and taken in as direct current by the analog-to-digital converter 10a. Immediately before the analog-digital converter 10a, a CR circuit may be further provided to have a function as a noise filter and a function of suppressing charge transfer to the internal capacitance of the analog-digital converter 10a.

The output voltage of the differential amplifier circuit by the operational amplifier 11 is converted into direct current by the smoothing circuit and input to the operational amplifier 21. Assuming that the resistance elements 22 and 23 are 10 kΩ and the resistance elements 24 and 25 are 75 kΩ, the gain of the operational amplifier 21 is 7.5 times (75 k / 10 k). Since the reference voltage is superimposed on the inverting input terminal, 0.5V is canceled. Therefore, the voltage obtained by subtracting this 0.5V is amplified 7.5 times by the operational amplifier 21.

The current flowing through the 1.3Ω shunt resistance element 18 passes through the operational amplifiers 11 and 21, and the voltage obtained by multiplying the respective gains becomes the output voltage of the differential amplifier circuit by the operational amplifier 21. The full-scale current at this time is 4.00 V / (1.3 × (6.2 × 7.5)) ≈66.17 mA, where the full-scale voltage is 4.0 V. That is, if the full scale range of the operational amplifier 21 is set to 66.17 mA, the 4.0 V range can be used to the maximum. Since the reference voltage of 0.5 V is similarly superimposed here as well, the output voltage of the operational amplifier 21 becomes 4.5 V when 66.17 mA flows. This voltage is taken in by the analog-digital converter 10b. Immediately before the analog-digital converter 10b, a CR circuit may be further provided to have a function as a noise filter and a function of suppressing charge transfer to the internal capacitance of the analog-digital converter 10b.

In the current detection device according to the above numerical example, the microcomputer 10 obtains the current based on the output voltage of the differential amplifier circuit by the highly sensitive operational amplifier 21 for the current in the range of less than 66.17 mA, and detects the current. For currents in the range of 66.17 mA or more, the current is obtained based on the output voltage of the differential amplifier circuit by the operational amplifier 11 with standard sensitivity, and this is used as the detection result to improve the detection accuracy. be able to.

According to the above-described embodiment, the current can be detected in a relatively wide range, and the detection accuracy of the weak current can be improved.

The present invention is not limited to the contents of the above-described embodiment, and can be variously modified and implemented within the scope of the gist of the present invention. For example, in the above-described embodiment, the shunt resistance element is connected in series with the light emitting element, but the shunt resistance element may be connected on one of the branched paths of the current flowing through the light emitting element. Further, in the above-described embodiment, the light emitting element has been illustrated as an example of the electric load, but other electric loads may be used. Further, the driving method of the light emitting element is not limited to PWM control.

1: Current detector, 10: Microcomputer 10, 10a, 10b: Analog-to-digital converter, 11, 21: Operational amplifier, 11, 12, 13, 14, 15, 16, 21, 22, 23, 24, 25, 26: Resistance element, 17, 27: Capacitive element, 18: Shunt resistance element, REF: Reference power supply

Claims (7)

A device for detecting the current flowing through an electrical load.
A shunt resistor element connected on the path of the current flowing through the electrical load,
A first amplifier circuit that amplifies the voltage across the shunt resistor element and
The second amplifier circuit that amplifies the output voltage of the first amplifier circuit and
The first current value indicating the magnitude of the current is obtained by calculation based on the output voltage of the first amplifier circuit, and the second current indicating the magnitude of the current is calculated based on the output voltage of the second amplifier circuit. The control unit that calculates the value and
Including
The control unit selects the first current value as the detection result of the current in the first range where the current is relatively large, and selects the second current value as the current in the second range where the current is relatively small. Select as the detection result of
Current detector. A first pull-up circuit that raises the output voltage of the first amplifier circuit by a first predetermined value,
Including
The control unit cancels the first predetermined value at the time of the calculation to obtain the first current value.
Current detector. A cancel circuit that subtracts the voltage corresponding to the first predetermined value from the input voltage to the second amplifier circuit.
Including,
The current detection device according to claim 2. A second pull-up circuit that raises the output voltage of the second amplifier circuit by a second predetermined value.
Including
The control unit cancels the second predetermined value at the time of the calculation to obtain the second current value.
The current detection device according to any one of claims 1 to 3. A smoothing circuit connected between the first amplifier circuit and the control unit,
The current detection device according to any one of claims 1 to 4, further comprising. The electrical load is at least one light emitting element.
The current detection device according to any one of claims 1 to 5. A method for detecting the current flowing through an electrical load.
The first step of amplifying the voltage across the shunt resistor element connected on the path of the current flowing through the electrical load, and
In the second step of further amplifying the voltage amplified in the first step,
The control unit obtains a first current value indicating the magnitude of the current by calculation based on the voltage obtained in the first step, and calculates the magnitude of the current based on the voltage obtained in the second step. The third step of obtaining the second current value indicating the value, and
The control unit selects the first current value as the detection result of the current in the first range where the current is relatively large, and selects the second current value as the current in the second range where the current is relatively small. The fourth step to select as the detection result of
Current detection methods, including.

Images