JP2004205264A - Current detecting circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a current detecting circuit capable of wide-band current detection from direct currents to high frequencies. <P>SOLUTION: The current detecting circuit comprises both a first current sensor 2 having a current transformer CT for passing a current to be measured through the side of a primary winding N1 and outputting the current to be measured from the side of a secondary winding N2 and a second current sensor 3 for detecting components containing a direct current among the current to be measured. By inputting both the output from the side of the secondary winding N2 and the output from the second current sensor 3 acquired from an error amplifier 5 and connecting a difference signal between the outputs to the secondary winding N2 of the first current sensor 2, it is possible to perform wide-range current detection from low frequencies to high frequencies. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a current detection circuit, and more particularly, to a current detection circuit capable of detecting a wideband current from DC to high frequency.
[0002]
[Prior art]
As a typical circuit of the conventional current detection circuit, a current detection circuit using a shunt resistor or a current transformer (CT) is known. Further, a current detection circuit that enables current detection in a wider band by combining the shunt resistor and the current transformer (CT) has been proposed by the present applicant (for example, see Patent Document 1).
[0003]
[Patent Document 1]
JP 2001-66329 A (pages 3 to 4, FIG. 1)
[0004]
FIG. 4 shows such a current detection circuit. In FIG. 4, a load current I1 is supplied to a load RL from a power supply (V1) 1, a CT (current transformer) 2 as a first current sensor, and a current shunt resistor (Rs) 3 as a second current sensor. Are inserted and connected in series, and the current I1 flowing through the load RL is measured. The current I1 to be measured flows through the primary side (N1) of the first current sensor 2, and a current proportional to the current to be measured I1 is taken out from the secondary-side detection winding (secondary winding: N2). . The output of the secondary winding N2 for detection is input to a current-voltage conversion circuit including an operational amplifier 7 and a resistor Rc, and an output voltage Vac is extracted from the output side of the operational amplifier 7.
[0005]
The voltage generated by the load current I1 flowing through the current shunt resistor 3 as the second current sensor is amplified by the amplifier 4. From the amplified voltage, a low-pass component is extracted by an LPF (low-pass filter) 8 including a resistor and a capacitor, and an output Vdc from the current sensor 3 is obtained.
[0006]
By the way, external noise is added to the current shunt resistor Rs of the second current sensor 3 via the stray capacitance. The transmission path of the external noise has an equivalent circuit via a high-pass filter composed of a stray capacitance (not shown) and a current shunt. The applied external noise component increases as the frequency increases. Become.
[0007]
In order to cancel the DC magnetization of the current transformer CT, the first current sensor 2 includes a correction winding, which is a third winding in addition to the primary winding (N1) and the secondary winding (N2). A line (N3) is provided. The correction winding (N3) is supplied with a current Idc in proportion to the output of the low-pass filter (LPF) 8 and having a phase opposite to that of the load current I1 via a current amplifier 9 in the correction current source. By doing so, the low-frequency component including DC in the load current I1 flows through the current transformer CT as a current having a phase opposite to that of I1, and the DC magnetization of the current transformer CT can be canceled. In this example, the primary winding (N1) of the first current sensor 2 and the third winding (N3) through which the correction current flows are not limited to one turn, and may be plural turns.
[0008]
The outputs Vac and Vdc from the two current sensors 2 and 3 obtained in this manner are combined by the adder / combiner 10 to finally obtain a current detection signal Vo.
[0009]
[Problems to be solved by the invention]
According to the conventional current detection circuit shown in FIG. 4 described above, a current detection circuit that prevents magnetic saturation of CT and reduces the influence of external noise and DC drift over a wide frequency band from DC to high frequency can be obtained. However, there is a problem that it is necessary to additionally wind a correction winding (N3) on the current transformer CT.
[0010]
SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-described problems and to provide a wideband and low-noise current detection circuit that does not require an additional winding.
[0011]
[Means for Solving the Problems]
The present invention relates to a current detection circuit which detects a current by combining a current transformer (CT) and an impedance (resistance), and synthesizes respective signals to enable wideband current detection from a low frequency including DC to a high frequency. Specifically, the following characteristic configuration is adopted.
[0012]
(1) first current detection means having a current transformer in which a current to be measured flows through the primary winding and the current to be measured is output from the secondary winding;
Second current detection means for detecting a component including direct current in the measured current,
An error amplifying unit that receives an output from the secondary winding and an output from the second current detecting unit and outputs a difference signal between the two inputs;
A current detection circuit comprising an output of the error amplification means connected to the secondary winding of the first current detection means.
[0013]
(2) The current detection circuit according to (1), wherein an in-phase amplifier is provided on the secondary winding side of the first current detection means, and a detection signal is output from the in-phase amplifier.
[0014]
(3) The current detection circuit according to (1), wherein the second current detection means is a DC current sensor using a resistor, a Hall element, or the like.
[0015]
(4) The current to be measured flows on the primary winding side, and the primary side of the current transformer having a detection winding for detecting the current to be measured wound on the secondary side, and a shunt resistor are connected in series to the load. And a detection signal from a detection winding of the current transformer and a detection signal from the shunt resistor are connected to an input terminal of an error amplifier, and an output of the error amplifier is connected to the detection winding. circuit.
[0016]
(5) In a current detection circuit for detecting a load current flowing through the load by a current transformer and a shunt resistor connected in series to the load,
A current detection circuit for superimposing and adding each output of the detection current by the current transformer and the detection current by the shunt resistor on the secondary winding side of the current transformer.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, preferred embodiments of the current detection circuit according to the present invention will be described with reference to the drawings.
[0018]
FIG. 1 is a circuit diagram showing a first embodiment of a current detection circuit diagram according to the present invention. In FIG. 1, a load RL is supplied with a load current I1 from a power supply (V1) 1 and a CT (current transformer) as a first current sensor 2 and a current shunt (Rs) as a second current sensor 3 ) Are inserted in series, and the current I1 flowing through the load RL is measured by each.
[0019]
When the current to be measured I1 flows on the primary side of the current transformer CT of the first current sensor 2, if the number of turns (ratio) of the detection winding (secondary winding) N2 is n on the secondary side, the first A current of (1 / n) · I1 flows in the signal pass band of the first current sensor 2. Further, the voltage generated in the current shunt (Rs) as the second current sensor 3 by the load current I1 is amplified by the amplifier 4.
[0020]
An output from the amplifier 4 is input to an amplifier 5 having an error amplification function via a resistor Ra. The amplifier 5 having this error amplification function is composed of an amplifier 51 and a resistor Rf connected between its output and an inverting input (-). The output from the amplifier 4 is input to the inverting input (-) of the amplifier 51. Is done. The non-inverting input (+) of the amplifier 51 is grounded.
[0021]
The output of the error amplifier 5 is connected to one terminal of a secondary winding N2 in the second current sensor, and the other terminal is connected to the inverting input terminal (−) of the amplifier 5 via a resistor Rb. It is grounded by a resistor Ro. At both ends of the resistor Ro, a current detection result (voltage output) Vo is obtained. Here, it is assumed that the load connected to the output terminal OUT has a sufficiently large value.
[0022]
The amplifier 5 is an error amplifier that amplifies an error between outputs from the first current sensor 2 and the second current sensor 3 and is generated by the primary winding N1 by flowing the output through the secondary winding N2. DC and low frequency magnetization are canceled. With such a configuration, the third winding for preventing magnetic saturation of CT in the conventional current detection circuit as shown in FIG. 4 is not required.
[0023]
In the current detection circuit according to the above-described embodiment, the first current sensor 2 is responsible for an AC component equal to or higher than the crossover frequency fc, while the second current sensor 3 using the shunt resistor Rs passes through the error amplifier 5. Accordingly, the signal component from DC to the crossover frequency fc is taken over. The two output signals from the first current sensor 2 and the second current sensor 3 are combined by adding the output of the error amplifier 5 in series to the secondary winding of the first current sensor 2.
[0024]
In the present embodiment, in order to prevent magnetic saturation due to direct current and low frequency of the first current sensor 2, the magnetic field of the primary winding N1 of the current transformer CT in the first current sensor is changed by the inverse magnetic field of the secondary winding N2. I am cancelling. That is, the number of turns on the primary side (N1) of the current transformer CT is 1, the number of turns on the secondary side (N2) is n, and the two windings are wound in the same direction, and 1 is placed on the secondary side of the current transformer CT. By flowing a current of 1 / n on the next side in the reverse direction, magnetic saturation due to DC and low frequency of the current transformer CT is canceled.
[0025]
Now, when a current I1 flows through the primary side of the current transformer CT, a voltage proportional to the current appears at the output of the amplifier 4, and a current flows through the resistor Ra. As a result, negative feedback is provided from the output terminal OUT to the error amplifier 5 via the resistor Rb such that a current (I1 / n) flows from the output of the error amplifier 5 to the secondary winding N2 of the current transformer CT. I'm running.
[0026]
That is, the error amplifier 5 balances the currents flowing through the resistors Ra and Rb, in other words, the current (k1 · I1) flows into the resistor Ra as shown in FIG. k1 · I1) flows in and balances.
[0027]
Now, considering the DC component, a current k1 · I1 flows through the resistor Ra in the direction of the arrow as shown in FIG. Then, this current causes a current I1 / n to flow through the secondary winding N2 of the current transformer CT connected to the output of the error amplifier 5, and Vo = Ro · (I1 / n) across both ends (output terminals) of the resistor Ro. Output voltage. This output voltage Vo is input to the error amplifier 5 via the resistor Rb, and is controlled so that a current of (I1 / n) always flows through the secondary winding N2 of the current transformer CT.
[0028]
The value of the current (correction current) flowing through the secondary winding N2 of the current transformer CT needs to be (I1 / n). By the way, the current flowing through the resistor Rb is Vo / Rb, and the absolute value of this current is the same as the current (k1 · I1) flowing through Ra. Therefore, the relationship Vs / Ra = Vo / Rb, that is, Vs / Vo = Ra / Rb.
[0029]
In designing, first, Vs and Vo when a rated current is passed through the current transformer CT are determined, and if an arbitrary Ra is given, the value of the resistor Rb is obtained from the above relational expression as Rb = (Vo / Vs) · Ra. .
[0030]
Then, a difference current obtained by subtracting a current (k1 · I1 = Vs / Ra) flowing through the resistor Rb from the correction current (I1 / n) flows through the resistor Ro, and the value of the output voltage Vo is divided by the value of the difference current. Thus, the resistance Ro can be calculated. Since the current flowing through the resistor Rb is usually designed to be sufficiently smaller than the current (I1 / n) flowing through the current transformer CT, the current flowing through the resistor Rb can be neglected, and Ro = Vo / (I1 / n).
[0031]
The determination of the resistance Rf connected between the inverting input (−) and the output of the error amplifier 5 corresponds to the characteristics of the current transformer CT, and further includes the error amplifier 5, the secondary winding N2 of the current transformer CT, It is desirable that the phase margin of the negative feedback loop is determined to be sufficiently large in order to secure the stability of the negative feedback loop including the resistor Rb.
[0032]
By the way, as described above, external noise is added to the current shunt resistor Rs as the second current sensor 3 via the stray capacitance, and an equivalent circuit of the transmission path of the external noise is constituted by the stray capacitance and the current shunt. It can be regarded as a high-pass filter, and the external noise component increases as the frequency increases. In the present embodiment, the high frequency component of the detection signal due to Rs is not attenuated and used, so that the influence of this external noise can be reduced. Since the current sensor (resistance Rs) 3 uses a resistor having a low resistance value (for example, 1 mΩ or less) generally called a shunt resistor, the inductance component of the resistor cannot be ignored in current detection using only the shunt resistor. However, according to the present invention, it is possible to prevent the high-frequency characteristics from deteriorating.
[0033]
FIG. 2 is a diagram showing a simulation result of frequency characteristics of the current detection circuit according to the embodiment shown in FIG. In this simulation, a capacitor Cf was inserted in series with the resistor Rf. The main circuit constants in this simulation are
Vs = 0.1V when I1 = 1A
Turn ratio of current transformer CT = 1: 250
Vo = 40mV
Ra = 22kΩ
Rb = 8.8 kΩ
Rf = 4.7 MΩ
Cf = 0.039 μF
Ro = 10Ω
It is.
[0034]
The characteristic A in FIG. 2 is a frequency characteristic from the current sensor Rs3 to the output terminal OUT, and forms a low-pass filter. The characteristic B is a frequency characteristic from the current transformer CT of the first current sensor 2 to the output terminal OUT. As can be seen from the fact that the first current sensor (current transformer CT) does not pass DC, a high-pass filter is used. Has formed. As is clear from FIG. 2, these characteristics A and B intersect at a frequency of ≒ 456 Hz, and the combined characteristics thereof, that is, the current sensor combining the current sensor (current transformer CT) of the present circuit and the shunt resistor Rs. Becomes flat as shown in a characteristic C.
In the present embodiment, the primary winding N1 of the current sensor (current transformer CT) 2 has one turn, but it goes without saying that the primary winding N1 may have a plurality of turns.
[0036]
In the present embodiment, the connection point between the secondary winding N2 of the current sensor (current transformer CT) 2 and the resistor Ro is used as an output for current detection, but an in-phase amplifier is connected between this connection point and the output terminal OUT. In addition, the resistor Rs may be connected to the output side. FIG. 3 shows a current detection circuit of another embodiment having such a configuration. In FIG. 3, components denoted by the same reference numerals as those in FIG. 1 indicate components having the same functions, and thus detailed description thereof will be omitted. Referring to FIG. 3, an amplifier 6 having a gain G is inserted between the output side of the current sensor 2 and the output terminal OUT. This increases the number of components, but has the effect of increasing the degree of freedom in design, for example, the output voltage Vo can be set independently of Ro.
[0037]
In this case, first, the voltages Vs and Vo when the rated current is applied to the current sensor (current transformer CT) 2 are determined, and if an arbitrary resistance Ra is given, the resistance is calculated from the equation: Rb = (Vo / Vs) · Ra. Rb is determined. Next, Vo = (I1 / n) · Ro · G, and the current (I1 / n) flowing through the secondary winding N2 of the current sensor CT2 at the rated current is uniquely determined if the number of turns n is determined. Then, Ro and G may be appropriately determined.
[0038]
In the present embodiment, the output of the current sensor (current transformer CT) is passed through the resistor Ro and converted into a voltage, and the voltage output is taken out from both ends of the resistor Ro. May be applied to an IV (current-voltage) conversion circuit. In this case, a normal IV conversion circuit using an operational amplifier has a configuration in which a resistor is connected between the output of the operational amplifier and an inverting input, and the input / output phase is inverted. Signal needs to be added.
[0039]
The resistor Rf in the error amplifier 5 in FIGS. 1 and 3 may be a CR network composed of a resistor and a capacitor.
[0040]
Although the resistor Ra on the current sensor Rs3 side is connected to the output of the amplifier 4, the resistor Ra may be directly connected to the hot side of the current sensor Rs3.
[0041]
In the present embodiment, a resistor (shunt resistor) is used as the second current sensor 3. However, since it is only necessary to detect the DC component of the measured current, the second current sensor is limited to a resistor. However, for example, a Hall element or the like may be used.
[0042]
The configuration and operation of the preferred embodiment of the present invention have been described above in detail. However, such an embodiment is merely an example of the present invention, and does not limit the present invention in any way. It will be readily apparent to those skilled in the art that various modifications and changes can be made in accordance with the particular application without departing from the spirit of the invention.
[0043]
【The invention's effect】
As described above, according to the present invention, since the outputs of the current detection by the current sensor CT and the current detection by the shunt resistor are superimposed and added in the secondary winding of the current sensor CT, a special correction winding is provided. Can be prevented from being added to the magnetic field, magnetic saturation of the current sensor CT can be prevented, and a wide-band, low-noise current sensor obtained by adding the respective frequency bands can be obtained.
[0044]
Further, even if the measured current includes a DC component, the DC component is detected and added to the current sensor CT as a correction current, so that the DC magnetization of the current sensor CT can be prevented, and the accuracy of current detection is significantly improved. I do.
[0045]
Furthermore, the current sensor CT can be inserted at an arbitrary position on a target line, and DC detection is performed by a shunt resistor, so that DC stability is high.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing an embodiment of a current detection circuit according to the present invention.
FIG. 2 is a diagram showing a simulation result of a frequency characteristic of the current detection circuit according to the embodiment shown in FIG. 1;
FIG. 3 is a circuit diagram showing another embodiment of the current detection circuit according to the present invention.
FIG. 4 is a diagram of a conventional current detection circuit.
[Explanation of symbols]
Reference Signs List 1 power supply 2 first current sensor 3 second current sensor 4 amplifier 5 error amplifier 6 amplifier 7 amplifier 8 LPF (low-pass filter)
9 Current amplifier 51 Amplifier RL Load

Claims (5)

First current detection means having a current transformer through which a current to be measured flows through the primary winding and the current to be measured is detected from the secondary winding;
Second current detection means for detecting a component including direct current in the measured current,
An error amplifying unit that receives an output from the secondary winding and an output from the second current detecting unit and outputs a difference signal between the two inputs;
A current detection circuit, wherein an output of the error amplification means is connected to the secondary winding of the first current detection means. The current detection circuit according to claim 1, wherein an in-phase amplifier is provided on a secondary winding side of the first current detection means, and a detection signal is output from the in-phase amplifier. The current detection circuit according to claim 1, wherein the second current detection means is a DC current sensor using a resistor, a Hall element, or the like. A current to be measured flows on the primary winding side, and a primary side of a current transformer having a detection winding wound thereon for detecting the measured current and a shunt resistor are connected in series to the load on the secondary side. A detection signal from a detection winding of the current transformer and a detection signal by the shunt resistor are connected to an input terminal of an error amplifier, and an output of the error amplifier is connected to the detection winding. Current detection circuit. In a current detection circuit for detecting a load current flowing through the load by a current transformer and a shunt resistor connected in series to the load,
A current detection circuit, wherein each output of the detection current by the current transformer and the detection current by the shunt resistor is superimposed and added on the secondary winding side of the current transformer.

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