Current detector

Abstract

A current detector (100) includes: a magnetic core (104) that converges a magnetic field generated by the conduction of a detected current (IP); a Hall element (108) that outputs a detection signal corresponding to the intensity of the magnetic field converged on the magnetic core (104); and a feedback circuit (116) that applies a feedback current to a secondary winding (118) based on the detection signal from the Hall element (108) to perform magnetic balance. Furthermore, the current detector (100) includes a coupling circuit (124) that couples a supply path (123, 123) of a DC power supply (122) reaching the feedback circuit (116) with an application path (117) for applying a feedback current to the secondary winding (118) via capacitor elements (C1, C2).

Description

Technical Field

[0001] The present invention relates to a current detector, and in particular to a magnetic balance current detector. Background Art

[0002] For example, a publication (JP2014-228418A1) issued by the Japan Patent Office discloses a magnetic balance current sensor. This current sensor generates an opposing magnetic field using a feedback coil placed on a magnetic core. The output signal is obtained using a feedback current of a magnitude that cancels out the magnetic field generated by the current being detected. Specifically, this prior art includes a digital amplifier that amplifies the detection signal from a magnetic detection element. By using the output filter of the digital amplifier as a feedback coil, the entire device is miniaturized.

[0003] The miniaturization of current sensors, as seen in the aforementioned prior art, not only reduces product cost and heat generation but also increases flexibility in device configuration. Therefore, the theme of "miniaturization of current sensors" itself can be said to point to a desirable direction for technological development.

[0004] However, when looking at the interior of a current sensor, miniaturization limits the insulation distance that can be maintained, leading to undeniable influences at various locations. For example, this influence manifests as a disturbance in the output signal from the current sensor associated with a sudden change in the detected current. Summary of the Invention

[0005] One object of the present invention is to provide a technique for reducing the disturbance of an output signal. To achieve this object, the present invention adopts the following solution.

[0006] The current detector of the present invention couples a supply path for supplying power to a magnetic balance circuit and an application path for applying current to a winding (feedback coil) via a capacitive element.

[0007] The output signal disturbances that this invention addresses are thought to arise from, for example, the high-frequency components of the detected current's rapid switching (sudden changes) acting as noise, adversely affecting the output current. While it's often difficult to assume that the frequent, high-speed switching of the detected current directly affects the output signal, the inventors of this invention discovered capacitive coupling within the current sensor.

[0008] Specifically, a current sensor typically includes components such as a magnetic core, a magnetic detection element mounted on the core, feedback windings, a magnetic balancing circuit, and other electronic components, including a control IC and a drive circuit. These components are housed in a resin casing, with the interior sealed with a resin filler. Within the device, the primary conductor that conducts the detected current is insulated from the magnetic core and other electronic components by a necessary distance. This creates parasitic capacitance between them, inevitably leading to capacitive coupling. Consequently, the high-frequency noise component can interfere with the output signal through the parasitic capacitance, causing adverse effects.

[0009] The inventors of the present invention have come up with the idea that if such high-frequency noise components have an impact through parasitic capacitance, they can be actively released through other capacitive elements to a path different from the output signal to reduce the occurrence of adverse effects.

[0010] The coupling circuit included in the current sensor of the present invention functions based on this concept. By discharging noise components that may be generated during high-speed switching of the detected current into the power supply path, the coupling circuit minimizes any adverse effects on the feedback current application path and, consequently, the output signal.

[0011] The concept of the coupling circuit in the present invention includes multiple aspects.

[0012] (1) The first aspect is that the coupling circuit has a capacitance element larger than the capacitance between the primary conductor that conducts the current to be detected and the conductor (sensor output line) that outputs the feedback current applied to the winding as a signal.

[0013] (2) The second aspect is that the coupling circuit includes a capacitance element having a size determined based on the insulation distance between a primary conductor that conducts the current to be detected and a conductor (sensor output line) that outputs the feedback current applied to the winding as a signal.

[0014] In either case (1) or (2) above, the capacitive element actively configured in the coupling circuit divides the parasitic capacitance, thereby directing the output destination of the noise component generated in the winding due to the high-frequency component of the detected current to the supply path. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a circuit diagram of a current detector according to one embodiment.

[0016] Figure 2 is a circuit diagram of a current detector according to a first comparative example.

[0017] Figure 3 is a circuit diagram of a current detector according to a second comparative example.

[0018] Figure 4 It is a graph showing the characteristics of the current detector of the first comparative example.

[0019] Figure 5 It is a graph showing the characteristics of the current detector of the second comparative example.

[0020] Figure 6 This is a diagram showing the characteristics of a current detector according to one embodiment. DETAILED DESCRIPTION

[0021] Hereinafter, embodiments will be described with reference to the drawings. Figure 1 1 is a circuit diagram of a current detector 100 according to an embodiment.

[0022] 〔Magnetic core and primary conductor〕

[0023] The current detector 100 includes, for example, a resin case 102, and a magnetic core 104 is housed in the resin case 102. Figure 1 In the figure, the magnetic core 104 is schematically shown using a two-dot chain line. However, the magnetic core 104 is preferably in the shape of an angular ring, a circular ring, or the like. A space for conducting current, known as a current conducting portion, is formed inside the magnetic core 104 (the inner circumference of the ring). Therefore, a primary conductor 106, such as a bus bar, is disposed inside this current conducting portion. It should be noted that the detected current IP is conducted through the primary conductor 106.

[0024] In this manner, current detector 100 detects the current flowing through primary conductor 106, and magnetic core 104 is arranged in a ring shape along the direction of the magnetic field generated when the detected current IP flows through primary conductor 106. It should be noted that if the detected current IP is relatively low (weak), primary conductor 106 can be wound around magnetic core 104.

[0025] 〔Magnetic detection element〕

[0026] The current detector 100 includes a Hall element 108 (which may also be an MR element or an MI element) as an example of a magnetic detection element. As described above, the magnetic core 104 is annular, with an air gap formed by partially cutting away the center. The Hall element 108 is attached to the magnetic core 104 so as to be inserted into the air gap. The Hall element 108 outputs a voltage signal (Hall voltage) corresponding to the strength (magnetic flux) of the magnetic field generated in the air gap.

[0027] Dedicated circuit

[0028] The current detector 100 also includes a driver IC 110. This driver IC 110 is an electronic component (discrete product) that has a structure optimized for the servo-type current detector 100, such as that of the present embodiment, and houses a specially designed circuit. In addition to supplying a drive voltage (e.g., +5V) from a power supply circuit (Vcc) 112 to the driver IC 110, the driver IC 110 also receives a voltage signal output from the Hall element 108. Furthermore, the driver IC 110 distributes and supplies drive power to the Hall element 108, or reverses its polarity.

[0029] 〔Magnetic Balance Circuit〕

[0030] As described above, the current detector 100 of this embodiment is a servo type and therefore includes a component serving as a feedback circuit 116 (magnetic balance circuit). The feedback circuit 116 includes, for example, a differential amplifier 114 connected to the driver IC 110, a switching element (reference numerals omitted in the figure), and a secondary winding 118 wound around the magnetic core 104. The secondary winding 118 is formed, for example, in a state of being wound around the outer periphery of the magnetic core 104. Figure 1 108 . Feedback circuit 116 drives a switching element based on the voltage signal from Hall element 108, generating a feedback current that is applied to secondary winding 118. The feedback current is applied to secondary winding 118 via application path 117. The application (supply) of the feedback current to secondary winding 118 generates a countermagnetic field that counteracts the magnetic field generated by detected current IP, thereby balancing the internal magnetic field of magnetic core 104.

[0031] Output voltage Vout

[0032] The current detector 100 outputs the feedback current of the secondary winding 118 to the outside. The output current is converted into an output voltage Vout through, for example, an external detection resistor 120. It should be noted that the detection resistor 120 can also be built into the current detector 100.

[0033] [Power supply path]

[0034] Current detector 100 can be operated by connecting to an external DC power supply 122, for example. Power is supplied from DC power supply 122 to feedback circuit 116 via supply paths 123, 123. DC power supply 122 is the source (Vcc(+) and Vcc(-)) of the feedback current generated by feedback circuit 116.

[0035] In addition, the DC power supply 122 has a voltage of, for example, ±24 V. It should be noted that the DC power supply 122 may also be built into the current detector 100 .

[0036] In addition, the current detector 100 also incorporates various other circuit elements and protective elements (resistors, diodes, Zener diodes, etc.), but their description is omitted.

[0037] Coupling circuit

[0038] The current detector 100 of this embodiment includes a coupling circuit 124. The coupling circuit 124 includes two capacitors C1 and C2, and couples the secondary winding 118 to supply paths 123 and 123, which supply power from the DC power supply 122 to the feedback circuit 116, via these capacitors C1 and C2. One of the capacitors C1 and C2 is connected to the supply path 123 on the positive (Vcc (+24V)) side of the DC power supply 122, while the other capacitor C2 is connected to the supply path 123 on the negative (Vcc (-24V)) side. This coupling circuit 124 can be implemented by mounting, for example, chip capacitors serving as the capacitors C1 and C2 on a circuit substrate (not shown), thereby minimizing manufacturing costs, including wiring pattern production.

[0039] Parasitic capacitance

[0040] In the structure of current sensor 100, the interior of resin case 102 is sealed with a resin filler (e.g., PU). Therefore, the filler acts as a dielectric, resulting in parasitic capacitance Cs (stray capacitance) between primary conductor 106 and secondary winding 118. The inventors of the present invention have determined that this parasitic capacitance Cs acts in a direction that capacitively couples primary conductor 106 and secondary winding 118. Therefore, when the detected current IP undergoes abrupt changes (high-speed switching), the high-frequency components are transferred to secondary winding 118 as noise, adversely affecting output voltage Vout.

[0041] The inventors' improved solution achieves this by incorporating the aforementioned coupling circuit 124 into the current detector 100. Specifically, coupling circuit 124 couples secondary winding 118 to power supply paths 123 and 123(±) from DC power supply 122 to feedback circuit 116 via capacitors C1 and C2. This allows problematic noise components to be actively released to a location other than secondary winding 118. This prevents adverse effects on output voltage Vout and maintains high quality and accuracy of the detection signal obtained by current detector 100.

[0042] The usefulness of the current detector 100 of this embodiment is further illustrated by comparing it with the following two comparative examples. It should be noted that the comparative examples are not embodiments but related technologies and are not prior arts.

[0043] [First Comparative Example]

[0044] Figure 2 This is a circuit diagram of the current detector 200 of the first comparative example. A major difference from this embodiment is that the current detector 200 of the first comparative example does not include the coupling circuit 124 used in this embodiment. More specifically, the differential amplifier 114 of the first comparative example is not connected to the negative side of the DC power supply 122, which is different from this embodiment. Regarding the other configurations that are the same as those of this embodiment, Figure 2 The same reference numerals are used to represent them and their repeated descriptions are omitted.

[0045] The current detector 200 of the first comparative example does not specifically address parasitic capacitance Cs. Consequently, the aforementioned problem identified by the inventors significantly arises, and the output voltage Vout is significantly adversely affected by noise components. Specific verification results regarding this point will be presented and further described below.

[0046] [Second Comparative Example]

[0047] Figure 3 This is a circuit diagram of a current detector 300 according to a second comparative example. In this second comparative example, a physical shielding member 302 (e.g., a copper plate) is interposed between the primary conductor 106 and the secondary winding 118, and the shielding member 302 is grounded to the negative side of the DC power supply 122. While this theoretically eliminates parasitic capacitance, it is not a sufficient countermeasure in practice. Specific verification results will be presented and described further below.

[0048] Figure 4 is a diagram showing the characteristics of the current detector 200 of the first comparative example, Figure 5 is a diagram showing the characteristics of the current detector 300 of the second comparative example, and Figure 6 1 is a diagram showing the characteristics of each current detector 100 according to the present embodiment.

[0049] 〔 Figure 4 : Characteristics of the first comparative example]

[0050] The upper part shows the input waveform of the detected current IP. Here, for example, at three times t10, t12, and t13, the detected current IP undergoes high-speed switching (all of which are sudden changes).

[0051] As can be seen, in the first comparative example where no countermeasures are taken, the high-frequency component of the detected current IP becomes a noise component with respect to the feedback current through the parasitic capacitance Cs between the primary conductor 106 and the secondary winding 118, thereby having an extremely adverse effect on the output voltage Vout shown in the lower section (times t10, t12, and t13).

[0052] 〔 Figure 5 : Characteristics of the Second Comparative Example]

[0053] As seen in the previous section, the same is true here. For example, at three times t20, t22, and t23, the detected current IP switches at high speed (all of them are sudden changes).

[0054] As can be seen in the following paragraph, while the second comparative example theoretically implements a countermeasure by configuring shielding member 302, actual verification fails to fully eliminate the adverse effects of noise components on output voltage Vout. For example, while waveform disturbances in the positive direction of output voltage Vout are suppressed at times t20, t22, and t23, disturbances in the negative direction are not. Furthermore, disturbances in output voltage Vout also occur in the period after times t20, t22, and t23 (not shown in the figure), achieving the opposite effect.

[0055] 〔 Figure 6 : Characteristics of this embodiment]

[0056] As described in the previous section, in this embodiment as well, the detected current IP undergoes high-speed switching (all changes are sudden) at, for example, three times t1 , t2 , and t3 .

[0057] However, as seen in the lower section, the fluctuation of the output voltage Vout at each time t1 , t2 , and t3 is significantly reduced compared to the first and second comparative examples, and the adverse effects of the noise components are suppressed to a minimum.

[0058] The above is summarized as follows:

[0059] (1) In the closed-loop current sensors of this embodiment, the first and second comparative examples, and others, the primary conductor 106 and the secondary winding 118 are coupled to each other via parasitic capacitance Cs via the resin case 102 and the filler. Therefore, when a high-speed switching voltage is applied to the primary conductor 106, a noise waveform is output as an error (malfunction) in the sensor's output signal (Comparative Example 1).

[0060] (2) To suppress this, in the second comparative example, a shielding member 302 was provided around the primary conductor 106 and positioned on the negative side of the DC power supply 122 to suppress the error in the output signal. However, in practice, this did not produce very good results.

[0061] (3) In the method as in the second comparative example, when the shield member 302 is made of a copper flexible board, it is expected that the cost per product will increase by several hundred yen.

[0062] (4) In this embodiment, capacitors C1 and C2, connected to Vcc(+) and Vcc(-), respectively, are connected to the connection between the feedback current output line and the inductor (secondary winding 118). This reduces the amount of malfunction in the sensor output signal. Because capacitor elements C1 and C2 are simply two chip capacitors as mounting components, a reliable countermeasure can be implemented at a very low cost.

[0063] (5) In the verification results for this embodiment, it was confirmed that the amount of malfunction was reduced by approximately 50% compared to the first comparative example and the second comparative example.

[0064] [Capacitance Setting Criteria]

[0065] Here, the setting of the capacitive elements C1 and C2 in the coupling circuit 124 will be described. In this embodiment, the capacitance can be set according to the following criteria, for example.

[0066] (1) The capacitance elements C1 and C2 are set to have a capacitance larger than the parasitic capacitance Cs. For example, if the parasitic capacitance Cs is several hundred pF, a capacitance of 1000 pF is set, which is larger than the parasitic capacitance Cs.

[0067] (2) The capacitance elements C1 and C2 can be set based on the insulation distance between the primary conductor 106 and the sensor output line (output conductor) from the secondary winding 118 .

[0068] However, if capacitors C1 and C2 are as large as 0.1 μF, the transistor's output line drive capability will be insufficient, preventing the output of a suitable step response waveform. Furthermore, if capacitors C1 and C2 are connected to the ends of secondary winding 118, oscillation will occur between the L component and the capacitance component of secondary winding 118. Therefore, such a connection should be avoided.

[0069] In either case, according to the current detector 100 of this embodiment, the capacitor elements C1 and C2 arranged in the coupling circuit 124 divide the parasitic capacitance Cs, thereby releasing the noise components caused by the high-speed switching of the primary conductor 106 to the positive and negative sides of the DC power supply 122, thereby minimizing the impact of high-frequency components on the output voltage Vout.

[0070] The above embodiment can be modified in various ways. For example, the shape of the magnetic core 104 can be not only the square ring shape or circular ring shape listed in the embodiment, but also other polygonal ring shapes, or even an elliptical shape. In addition, the magnetic core 104 can also be made of permalloy or other magnetic materials (ferrite, silicon steel plate, iron-nickel alloy, etc.), and the magnetic core 104 can adopt an annular structure or a stacked structure.

[0071] The current detector 100 may include a degaussing circuit that eliminates residual magnetic flux in the magnetic core 104 (so-called hysteresis removal). Therefore, hysteresis can be removed using a degaussing coil wound separately from the secondary winding 118 .

[0072] In addition, with the attached Figure 1 The above-mentioned current detector 100 or a part thereof is merely a preferred example, and various components may be added to the basic structure or a part may be replaced.

Claims

1. A current detector for detecting a current to be detected, comprising: The magnetic core converges the magnetic field generated by the conduction of the detected current; a magnetic detection element that outputs a detection signal corresponding to the intensity of the magnetic field converged on the magnetic core; and a magnetic balancing circuit that applies a feedback current to a winding provided on the magnetic core based on a detection signal from the magnetic detection element, thereby causing the winding to generate a magnetic field in a direction opposite to the magnetic field generated by the conduction of the detected current for balancing. The current detector is characterized in that: The current detector further includes a coupling circuit that couples a supply path for supplying power to the magnetic balance circuit and an application path for applying a feedback current to the winding via a capacitive element.

2. The current detector according to claim 1, wherein: The coupling circuit includes a capacitance element larger than the capacitance between the primary conductor that conducts the current to be detected and the winding.

3. The current detector according to claim 1 or 2, characterized in that: The coupling circuit guides the output destination of the noise component generated in the winding due to the high-frequency component of the detected current to the supply path.

4. The current detector according to claim 1 or 2, characterized in that: The coupling circuit includes a capacitance element whose size is determined based on an insulation distance between a primary conductor that conducts a current to be detected and the winding.

5. The current detector according to claim 3, wherein: The coupling circuit includes a capacitance element whose size is determined based on an insulation distance between a primary conductor that conducts a current to be detected and the winding.

Images