JP2013253841A - Current sensing circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a current sensing circuit having a simple configuration, which is capable of accurately sensing current without being influenced by temperature characteristics of a shunt resistor.SOLUTION: A current sensing circuit includes: an integrator circuit comprising an operational amplifier with an input resistor whose temperature coefficient is same as that of a shunt resistor which converts current being measured into voltage and a capacitor on a feedback part; a comparator with hysteresis characteristics, which compares output voltage from the integrator circuit with first and second threshold values and sets the output signal thereof either to High or Low; a charge switch which allows voltage across the shunt resistor to be fed to the operational amplifier via the input resistor to charge the capacitor when the comparator output signal is High; a discharge switch which allows electrical charge accumulated in the capacitor to be discharged via a discharge resistor when the comparator output signal is Low; and a counter which counts the time during which the comparator output signal is High.

Description

  The present invention relates to a current detection circuit that eliminates temperature dependency in current detection using a shunt resistor.

  As a technique for detecting and quantizing a current to be measured, for example, as shown in FIG. 6, a current I to be measured is passed through a shunt resistor R1, and a voltage generated in the shunt resistor R1 is a differential amplifier (operational amplifier). There is known a current detection circuit that detects an OP and quantizes an output voltage by an A / D converter and outputs the quantized output voltage (see, for example, Non-Patent Document 1). In FIG. 6, Ra and Rb are input resistors of the operational amplifier OP, Rc is a bias resistor, and Rd is a feedback resistor. The resistor Re and the capacitor C form a low pass filter (LPF) for preventing aliasing for the A / D converter.

  Incidentally, the shunt resistor R1 has a temperature characteristic that its resistance value changes with temperature, and there is a problem that the temperature characteristic affects current detection accuracy. Therefore, it has been proposed to use a resistor having a small temperature coefficient such as a metal film resistor as the shunt resistor R1. It has also been proposed that the temperature characteristic of the shunt resistor R1 is obtained in advance and the current detection value is temperature-corrected according to the temperature measured using a temperature sensor such as a thermistor (see, for example, Patent Document 1).

JP 2011-125130 A

Renesas μPD166015GR data sheet (2012.01.19)

  However, the metal film resistor is expensive, and when the entire current detection circuit including the shunt resistor R1 is to be integrated, it is necessary to integrate a resistor having a small temperature coefficient equivalent to the metal film resistor on the chip. Its realization is difficult. Furthermore, it is necessary to form a low-pass filter (LPF) in front of the A / D converter, which increases the number of components, which hinders cost reduction of the current detection circuit.

  By the way, the temperature characteristics of the differential amplifier OP itself can be offset by aligning the temperature characteristics of the input resistor Rb and the feedback resistor Rd that determine the gain of the differential amplifier (operational amplifier) OP. However, in order to compensate for the temperature characteristics of the shunt resistor R1, for example, if the temperature characteristics of the input resistors Ra and Rb of the differential amplifier OP and the shunt resistor R1 are aligned, the temperature characteristics of the differential amplifier (operational amplifier) OP itself is a problem. I can't deny that.

  The present invention has been made in consideration of such circumstances, and an object of the present invention is to provide a current detection circuit having a simple configuration capable of performing current detection with high accuracy without depending on the temperature characteristics of the shunt resistor. There is.

In order to achieve the above-described object, a current detection circuit according to the present invention includes:
A shunt resistor that converts the current to be measured into a voltage;
An integration circuit including an input resistor having the same temperature coefficient as the shunt resistor and an operational amplifier including a capacitor in the feedback unit;
A comparator that compares the first and second threshold values set with hysteresis characteristics with the output voltage of the integrating circuit and inverts the output signal to [H] or [L];
The capacitor is interposed between the shunt resistor and the input resistor and is controlled to be turned on / off by the output of the comparator, and a voltage generated in the shunt resistor is input to the operational amplifier via the input resistor. Charging switch to charge,
A discharge switch that is ON / OFF controlled complementarily to the charge switch by an output signal of the comparator, and discharges the charge charged in the capacitor via a discharge resistor;
And a counter that measures the number of times that the output signal of the comparator is inverted in a certain period of time, or an output period of an output signal that turns on the charging switch of the comparator.

  Preferably, the operational amplifier constitutes an inverting amplification type integration circuit that lowers its output voltage as the capacitor is charged, for example. The comparator inverts the output signal to [L] when the output voltage of the integration circuit decreases to the first threshold with charging of the capacitor, and the integration with discharge of the capacitor. When the output voltage of the circuit rises to the second threshold value, it has a hysteresis characteristic that inverts the output signal to [H].

  Further, the counter counts a clock signal having a predetermined frequency over a period in which the output signal of the comparator is [H], and outputs the counted value as a signal inversely proportional to the current to be detected. Alternatively, the number of times the output signal of the comparator is inverted to [H] or [L] over a predetermined time is counted, and the counted value is output as a signal proportional to the current to be detected. Composed.

  According to the current detection circuit having the above configuration, since the temperature coefficients of the input resistance and shunt resistance of the operational amplifier forming the integration circuit are uniform, the time change rate (slope) of the output voltage of the operational amplifier depends on the temperature. It becomes constant without. The time for the output voltage of the operational amplifier to change by a constant voltage is inversely proportional to the current flowing through the shunt resistor and does not depend on the temperature. Therefore, by measuring the time during which the output voltage of the operational amplifier changes by a certain voltage, current detection can be performed with high accuracy without depending on the temperature coefficient of the shunt resistor.

  Further, according to the above configuration, the conventional A / D converter is unnecessary, and a low pass filter (LPF) for preventing aliasing for the A / D converter is also unnecessary, so that the configuration can be simplified. it can. Furthermore, it is possible to use an inexpensive resistor having a large temperature coefficient as the shunt resistor, and there is an advantage that an integrated circuit of the entire current detection circuit including the shunt resistor can be easily obtained.

The figure for demonstrating the electric current detection principle of this invention. The figure which shows the time and output characteristic of the operational amplifier which formed the integration circuit shown in FIG. 1 is a schematic configuration diagram of a current detection circuit according to an embodiment of the present invention. FIG. 4 is a signal waveform diagram showing an operation of the current detection circuit shown in FIG. 3. The schematic schematic block diagram of the current detection circuit which concerns on another embodiment of this invention. The schematic block diagram of the conventional general current detection circuit.

  The current detection circuit according to the present invention includes an input resistor R2 having the same temperature coefficient k as the shunt resistor R1 that converts the current I to be measured into a voltage as shown in FIG. An integrating circuit constituted by an operational amplifier OP provided with a capacitor C is used, and the current I is detected from the time change of the output voltage Vo of the integrating circuit (operational amplifier OP).

Here, when the current I flows through the shunt resistor R1, the voltage generated at the node A, which is the input voltage of the integrating circuit, becomes [r1 · I] with the resistance value of the shunt resistor R1 as r1. The voltage at node B, which is the inverting input terminal of the operational amplifier OP, is ground potential (0V) because an imaginary short circuit is established, and therefore the current flowing through the input resistor R2 has the resistance value of the input resistor R2 as r2. r1 · I / (r1 + r2)]. However, since r2 >> r1, it can be approximated as [(r1 / r2) · I / (r1 / r2 + 1) ≈r1 · I / r2]. Since the capacitor C of the integration circuit is charged by this current, the output voltage Vo of the operational amplifier OP is expressed by Vo = − {(I · r1) / (C · r2)} · t (1) where t is time.
It becomes.

On the other hand, if the temperature coefficients of the shunt resistor R1 and the input resistor R2 are equal and the temperature coefficient is k, the resistance values depending on the temperature T of the shunt resistor R1 and the input resistor R2 are the resistances R1, R2 at the reference temperature To. When the reference resistance values of R2 are r1o and r2o, respectively, r1 = r1o {1 + k (T−To)} (2)
r2 = r2o {1 + k (T-To)} (3)
Can be expressed as

Therefore, when the expressions (2) and (3) are substituted into the expression (1), Vo = − {(I · r1o) / (C · r2o)} · t (4)
Thus, there is no term dependent on the temperature T. That is, the time change rate (slope) of the output voltage Vo of the operational amplifier OP is a constant value that does not depend on the temperature T as shown in FIG. The time Δt during which the output voltage Vo changes by a constant voltage ΔVo is
Δt = − {(C · r2o) / (I · r1o)} · ΔVo
It becomes. In other words, the time Δt during which the output voltage Vo of the operational amplifier OP changes by the constant voltage ΔVo is inversely proportional to the current I flowing through the shunt resistor R1, and does not depend on the temperature T.

  As described above, the current detection circuit according to the present invention includes the input circuit R2 having the temperature coefficient k equal to that of the shunt resistor R1, and uses an integration circuit including an operational amplifier OP including a capacitor C in the feedback unit. This is based on the measurement principle that the time Δt during which the output voltage Vo of the (operational amplifier OP) changes by a constant voltage ΔVo is measured, and thereby the current I flowing through the shunt resistor R1 is detected.

  FIG. 3 shows a schematic configuration of a current detection circuit according to an embodiment of the present invention. In FIG. 3, IS indicates a current source for supplying a current to be measured (current to be measured) I, and R1 is a shunt resistor. U is an integrating circuit including an input resistor R2 having the same temperature coefficient k as the shunt resistor R1 and an operational amplifier OP including a capacitor C in a feedback unit. Specifically, the input resistor R2 is connected to the inverting input terminal (−) of the operational amplifier OP whose non-inverting terminal (+) is grounded, and the capacitor C is connected to the output terminal of the operational amplifier OP and the inverting input terminal. And an inversion type integration circuit U is configured.

  One end of the shunt resistor R1 and between the node A where the current flows from the current source IS to the shunt resistor R1 and the input resistor R2 are on / off controlled by an output signal of a comparator COMP described later. A charge switch S1 is interposed. When the charging switch S1 is on, the voltage at the node A is applied to the inverting input terminal of the operational amplifier OP to charge the capacitor C.

  The inverting input terminal of the operational amplifier OP is connected via a discharge resistor R3 to a discharge switch S2 that is ON / OFF controlled complementarily to the charge switch S1 by the output signal of the comparator COMP. When the discharge switch S2 is on, the inverting input terminal of the operational amplifier OP is connected to the negative power source V1 through the discharge resistor R3, and the charge charged in the capacitor C is discharged.

  Here, the comparator COMP receives the output voltage Vo of the operational amplifier OP, which is the output of the integrating circuit U, and the first threshold value Vth1 and the second threshold value Vth2 (> Vth1) set in advance and the output. It comprises a so-called hysteresis comparator having a hysteresis characteristic that compares the voltage Vo and inverts its output signal to [H] or [L]. The first threshold Vth1 is set by the first reference voltage source V2, and the second threshold Vth2 is set by the second reference voltage source V3.

  Specifically, the comparator COMP changes its output signal to [L] when the output voltage Vo of the integrating circuit U (operational amplifier OP) drops to the first threshold value Vth1 as the capacitor C is charged. It has a hysteresis characteristic in which the output signal is inverted to [H] when the output voltage Vo of the operational amplifier OP rises to the second threshold value Vth2 as the capacitor C is discharged.

  The charge switch S1 described above is turned on when the output signal of the comparator COMP is [H], and the voltage generated in the shunt resistor R1 is applied to the inverting terminal of the operational amplifier OP via the input resistor R2. Then, the capacitor C is charged. The charging switch S1 is turned off when the output signal of the comparator COMP is [L], and disconnects the shunt resistor R1 from the operational amplifier OP.

  The discharge switch S2 is ON / OFF controlled by an output signal of the comparator COMP via a logic inverter INV. Accordingly, the discharge switch S2 is turned on when the output signal of the comparator COMP is [L], discharges the charge charged in the capacitor C through the discharge resistor R3, and the output signal of the comparator COMP. Is [H], it is turned off and the discharge path of the capacitor C is cut off.

  In other words, the charge switch S1 and the discharge switch S2 are complementarily turned on / off depending on whether the output signal of the comparator COMP is [H] or [L]. The capacitor C is charged when the charging switch S1 is on (when the discharging switch S2 is off), and the charging charge of the capacitor C is discharged when the discharging switch S2 is on (when the charging switch S1 is off). The

  On the other hand, when the output signal is [H], the counter COUNT that inputs the output signal of the comparator COMP counts the clock signal CP of a predetermined frequency input to the clock terminal, and the count value N is output as a signal inversely proportional to the current I. An output signal (count value N) of the counter COUNT is given as digitized (quantized) current data to, for example, a microcomputer (not shown).

According to the current detection circuit configured as described above, when the output signal of the comparator COMP is [H], the charge switch S1 is turned on, and at the same time, the discharge switch S2 is turned off. Therefore, as described above, the capacitor C is charged by the current [r1 · I / r2] flowing into the operational amplifier OP via the input resistor R2. As a result, the output voltage Vo of the operational amplifier OP is represented by Vo = −Q / C = − {(I · r1) / (C · r2)} · t when the amount of charge charged in the capacitor C is Q. (5)
As shown in FIGS. 4B and 4D, it decreases with the passage of time t. 4 (b) shows the change in the output voltage Vo when the current I flowing into the shunt resistor R1 is small as shown by the solid line X in FIG. 4 (a), and FIG. 4 (a) shows a change in the output voltage Vo when the current I flowing into the shunt resistor R1 is large as indicated by a broken line Y.

  When the output voltage Vo drops to the first threshold value Vth1, the output signal of the comparator COMP is inverted to [L] as shown in FIGS. 4 (c) and 4 (e). 4C shows a change in the output signal of the comparator COMP when the current I flowing into the shunt resistor R1 is small, and FIG. 4E shows the current I flowing into the shunt resistor R1. This shows a change in the output signal of the comparator COMP when there is a large amount.

  Then, with the inversion of the output signal of the comparator COMP, the charging switch S1 is turned off and the discharging switch S2 is turned on at the same time. As a result, since the electric charge charged in the capacitor C is discharged through the discharge resistor R3 as described above, the output voltage Vo of the operational amplifier OP rises. When the output voltage Vo rises to the first threshold value Vth1, the output signal of the comparator COMP is inverted to [H]. Accordingly, the charging switch S1 is turned on again, and at the same time, the discharging switch S2 is turned off, and charging of the capacitor C is started again. Thereafter, charging and discharging of the capacitor C are repeated alternately.

Here, the charging period of the capacitor C, that is, the period ΔTh in which the output signal of the comparator COMP is [H] is the voltage of the first and second threshold values Vth1 and Vth2 set in the comparator COMP. When the difference is ΔVth (= Vth2−Vth1) ΔTh = {(C · r2) / (I · r1)} · ΔVth (6)
And is inversely proportional to the current I flowing through the shunt resistor R1.

  Specifically, when the current I flowing through the shunt resistor R1 is small (solid line X), the charging speed of the capacitor C is slow, so that the change in the output voltage Vo of the operational amplifier OP as shown in FIG. As shown in FIG. 4C, the period ΔTh in which the output signal of the comparator COMP is [H] is long. On the other hand, when the current I flowing through the shunt resistor R1 is large (broken line Y), the charging speed of the capacitor C increases, so that the output voltage Vo of the operational amplifier OP as shown in FIG. As shown in FIG. 4C, the period ΔTh during which the output signal of the comparator COMP is [H] is shortened.

  Therefore, as described above, the counter COUNT counts the clock signal CP over the period ΔTh during which the output signal of the comparator COMP is [H]. If the count value N is obtained as data indicating the period ΔTh in which the output signal is [H], the count value N inversely proportional to the current I can be obtained as current data. In addition, the count value N obtained by quantizing the current data can be obtained.

In particular, according to the above configuration, since the temperature coefficients of the shunt resistor R1 and the input resistor R2 are equal, if the above-described equations (2) and (3) are substituted into the equation (6),
ΔTh = {(C · r2o) / (I · r1o)} · ΔVth (7)
Thus, there is no term dependent on the temperature T. Therefore, the current I flowing through the shunt resistor R1 can be detected with high accuracy without depending on the temperature T.

  Further, according to the above configuration, as in the conventional current detection circuit shown in FIG. 6, the current value (voltage) detected using the differential amplifier (operational amplifier OP) is digitized using the A / D converter ( There is no need for quantization, and no low-pass filter (LPF) for preventing aliasing for the A / D converter is required, so that the configuration can be simplified. Furthermore, it is not necessary to use a metal film resistor having a small temperature coefficient as the shunt resistor R1, in other words, it is possible to use an inexpensive resistor having a large temperature coefficient, and the entire current detection circuit including the shunt resistor can be integrated into an integrated circuit. There is an advantage that it becomes easy.

  By the way, in the above-described embodiment, the period ΔTh in which the output signal of the comparator COMP is [H] is obtained by counting the clock signal CP using the counter COUNT, thereby calculating the period inversely proportional to the current I. The numerical value N was obtained as current data. However, as shown in FIG. 5 as a modification of the present invention, the number of inversions of the output signal of the comparator COMP within a predetermined time, specifically, the number of times the output signal of the comparator COMP becomes [H] or [L]. It is also possible to measure using the counter COUNT.

  In FIG. 5, the same parts as those of the above-described current detection circuit shown in FIG. 3 are denoted by the same reference numerals, and redundant description is omitted. In the current detection circuit shown in FIG. 5, instead of counting the clock signal CP in the counter COUNT, the output signal of the comparator COMP is input to the clock terminal of the counter COUNT, and the number of inversions of the output signal is calculated. In this case, the frequency of the output signal is directly counted.

That is, when the output voltage Vo of the operational amplifier OP rises from the first threshold value Vth1 to the second threshold value Vth2, that is, when the discharge time of the capacitor C is Td, the output voltage Vo of the operational amplifier OP increases or decreases 1 The period Top is the sum of the charging time ΔTh of the capacitor C and the discharging time Td described above. Top = {(C · r2o) / (I · r1o)} · ΔVth + Td (8)
Can be expressed as

If the resistance value of the discharge resistor R3 is small, the discharge time Td is sufficiently shorter than the charge time ΔTh of the capacitor C, and one period Top in which the output voltage Vo of the operational amplifier OP increases or decreases is reduced. Top≈ {(C · r2o) / (I · r1o)} · ΔVth (9)
As an approximation. Therefore, the frequency fc of the output signal is fc = 1 / Top.
= (I · r1o) / (C · r2o · ΔVth) (10)
This frequency fc is proportional to the current I flowing through the shunt resistor R1.

  Therefore, as described above, if the counter COUNT is used to count the number of inversions of the output signal of the comparator COMP over a certain period of time, the count value N indicates the frequency fc of the output signal. It becomes possible to measure the current I flowing through the shunt resistor R1. In other words, the time average value of the current I can be measured. Therefore, the same effect as the above-described embodiment can be obtained.

  The present invention is not limited to the embodiment described above. For example, the resistance value of the shunt resistor R1, the integration constant of the integration circuit U configured using the operational amplifier OP, and the first and second threshold values Vth1 and Vth2 set in the comparator COMP are to be measured. What is necessary is just to set according to measurement specifications, such as the magnitude | size of the electric current I, its change width. Further, a non-inverting type integration circuit U is configured, and the charge switch S1 and the discharge switch S2 are controlled to be turned on / off by using the output signal of the comparator COMP in reverse, and the charge / discharge of the capacitor C is controlled. It is also possible to measure the charging time. In addition, the present invention can be variously modified and implemented without departing from the scope of the invention.

R1 Shunt resistance R2 Input resistance R3 Discharge resistance OP Operational amplifier C Capacitor U Integration circuit COMP Comparator (hysteresis comparator)
INV logic inverter COUNT counter

Claims (4)

A shunt resistor that converts the current to be measured into a voltage;
An integration circuit including an input resistor having the same temperature coefficient as the shunt resistor and an operational amplifier including a capacitor in the feedback unit;
A comparator that compares the first and second threshold values set with hysteresis characteristics with the output voltage of the integrating circuit and inverts the output signal;
The switch is interposed between the shunt resistor and the input resistor, and is controlled to be turned on / off by the output signal of the comparator, and the voltage generated in the shunt resistor is input to the operational amplifier via the input resistor. A charge switch for charging the capacitor;
A discharge switch that is ON / OFF controlled complementarily to the charge switch by an output signal of the comparator, and discharges the charge charged in the capacitor via a discharge resistor;
A current detection circuit comprising: a counter for measuring an output time of an output signal for turning on the charging switch of the comparator or a number of times the output signal of the comparator is inverted during a certain period. The operational amplifier constitutes an inverting amplification type integration circuit that lowers its output voltage as the capacitor is charged,
The comparator inverts the output signal to [L] when the output voltage of the integration circuit decreases to the first threshold value with charging of the capacitor, and the integration circuit with discharge of the capacitor. 2. The current detection circuit according to claim 1, wherein the current detection circuit has a hysteresis characteristic that inverts the output signal to [H] when the output voltage of the first output voltage rises to the second threshold value.   The counter counts a clock signal having a predetermined frequency over a period in which the output signal of the comparator is [H], and outputs the count value as a signal inversely proportional to the current to be detected. Item 2. The current detection circuit according to Item 1.   The counter counts the number of times the output signal of the comparator is inverted to [H] or [L] over a predetermined time, and outputs the count value as a signal proportional to the current to be detected. The current detection circuit according to claim 1.

Images