Current detection structure

A current detection structure includes a busbar to flow a current therethrough, a magnetic detection element to detect a strength of a magnetic field generated by the current flowing through the busbar, and a current detecting portion to determine the current flowing through the busbar based on the magnetic field detected by the magnetic detection element. A through-hole is formed penetrating the busbar such that a current path is formed on both sides of the through-hole. The magnetic detection element is disposed in the through-hole. The current detecting portion determines the current flowing through the busbar based on a strength of a synthetic magnetic field detected by the magnetic detection element. The synthetic magnetic field is produced by combining a magnetic field that is generated by a current flowing through the current path on the both sides of the through-hole.

The present application is based on Japanese patent application No.2014-008775 filed on Jan. 21, 2014, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a current detection structure.

2. Description of the Related Art

A magnetic detection element is used for detecting the strength of a magnetic field generated by a current flowing through a busbar. From the magnetic field strength detected by the magnetic detection element, it is possible to derive the current flowing through the busbar by calculation.

MR (Magneto Resistance) sensors and GMR (Giant Magneto Resistive effect) sensors are known as the magnetic detection element.

Prior art related to the invention of the present application may include e.g. JP-B-5153481 and JP-A-2013-170878.

SUMMARY OF THE INVENTION

It is desired to use a higher sensitivity magnetic detection element such as GMR sensor so as to conduct a more accurate measurement.

However, if a large current flows through the busbar, e.g., in case of detecting a current flowing in each phase of a three-phase motor, the strength of a magnetic field generated by the current flowing through the busbar may be too large. Thus, it is difficult to use a high sensitivity magnetic detection element such as GMR sensor.

It is an object of the invention to provide a current detection structure that allows the use of a high sensitivity magnetic detection element even if a large current flows through the busbar so as to conduct a more accurate measurement.

(1) According to one embodiment of the invention, a current detection structure comprises:

a busbar to flow a current therethrough;

a magnetic detection element to detect a strength of a magnetic field generated by the current flowing through the busbar; and

a current detecting portion to determine the current flowing through the busbar based on the magnetic field detected by the magnetic detection element,

wherein a through-hole is formed penetrating the busbar such that a current path is formed on both sides of the through-hole,

wherein the magnetic detection element is disposed in the through-hole, wherein the current detecting portion determines the current flowing through the busbar based on a strength of a synthetic magnetic field detected by the magnetic detection element, and

wherein the synthetic magnetic field is produced by combining a magnetic field that is generated by a current flowing through the current path on the both sides of the through-hole.

In the above embodiment (1) of the invention, the following modifications and changes can be made.

(i) The magnetic detection element is arranged such that a detection axis thereof is along a thickness direction of the busbar.

(ii) The magnetic detection element comprises a GMR sensor.

(iii) The magnetic detection element is arranged at a position where a magnetic flux density of the synthetic magnetic field is more than 0 and not more than 5 mT.

(iv) The magnetic detection element is arranged at a position where a magnetic flux density of the synthetic magnetic field is more than 0 and not more than 2 mT.

(v) The though-hole is formed to have a symmetrical shape with respect to a central axis of the busbar, wherein the current path is formed symmetrically on the both sides of the though-hole, and the magnetic detection element is arranged off the central axis of the busbar.

(vi) The magnetic detection element is arranged at a center of the through-hole in a longitudinal direction of the busbar.

(vii) The current path on the both sides of the through-hole is linearly formed along a longitudinal direction of the busbar.

(viii) The current flowing through the busbar has a frequency of not more than 100 kHz, wherein the busbar comprises copper or a copper alloy, and wherein the current path on the both sides of the through-hole has a width of not more than 0.5 mm.

(ix) The magnetic detection element comprises a GMR sensor, and wherein a center of the magnetic detection element in a thickness direction of the busbar coincides with a center of the busbar in the thickness direction thereof.

EFFECTS OF THE INVENTION

According to one embodiment of the invention, a current detection structure can be provided that allows the use of a high sensitivity magnetic detection element even if a large current flows through the busbar so as to conduct a more accurate measurement

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention will be described below in conjunction with the appended drawings.

FIGS. 1A and 1Bare diagrams illustrating a current detection structure in the present embodiment, whereinFIG. 1Ais a perspective view andFIG. 1Bis a plan view.FIG. 2is a cross sectional view taken on line2A-2A inFIG. 1Band a diagram illustrating magnetic flux density distribution on the2A-2A line cross section.

As shown inFIGS. 1A to 2, a current detection structure1is provided with a busbar2for carrying a current along a longitudinal direction thereof and a magnetic detection element3for measuring strength of a magnetic field generated by the current flowing through the busbar2. The current detection structure1is to detect a current flowing through the busbar2which is provided on, e.g., an inverter of a vehicle.

The busbar2is a plate-shaped conductor and serves as a current path for carrying a current. The current flowing through the busbar2is, e.g., up to about 200 A in a steady state and up to about 800 A as inrush current in an abnormal state, etc., and has a frequency of, e.g., up to about 100 kHz.

The magnetic detection element3is configured to output a voltage output signal according to magnetic field strength (magnetic flux density) in a direction along a detection axis D. In the present embodiment, a GMR sensor having a high sensitivity is used as the magnetic detection element3.

In the current detection structure1of the present embodiment, a through-hole4is formed on the busbar2so as to penetrate therethrough and the magnetic detection element3is arranged in the through-hole4. The magnetic detection element3is arranged so that the detection axis D thereof is along a thickness direction of the busbar2. In other words, the magnetic detection element3is arranged so that the detection axis D thereof is orthogonal to the surface of the busbar2. When the magnetic detection element3is a GMR sensor, the magnetic detection element3has two or more detection axes and one specific axis is perpendicular to the surface of the busbar2. The detection axis D of the magnetic detection element3may be inclined about −10° to 10° with respect to a direction orthogonal to the surface of the busbar2.

The through-hole4is formed in the middle of the busbar2so as to penetrate therethrough and is thus surrounded by the busbar2. In other words, the through-hole4is not a cutout which partially opens to a lateral side of the busbar2. By forming such a through-hole4, current paths5and6are formed on the both sides of the through-hole4.

Since the current paths5and6are formed on the both sides of the through-hole4, magnetic field components in a thickness direction generated by the two current paths5and6cancel each other out in the through-hole4. Here, what the magnetic detection element3arranged in the through-hole4detects is strength of a synthetic magnetic field produced by combining magnetic fields respectively generated by currents flowing through the current paths5and6on the both sides of the through-hole4, i.e., strength of the magnetic fields generated in the current paths5and6and then cancelled each other out. Therefore, by adjusting a position of the magnetic detection element3, the degree of cancellation can be adjusted and strength of the magnetic field to be detected can be thereby adjusted to the optimum level.

The current detection structure1is also provided with a current detecting portion7. The current detecting portion7is to detect a current flowing through the busbar2based on magnetic field strength which is the strength of the synthetic magnetic field produced by combining magnetic fields respectively generated by currents flowing through the current paths5and6on the both sides of the through-hole4and is detected by the magnetic detection element3. The current detecting portion7is provided on, e.g., an ECU (Electronic Control Unit) of a vehicle.

That is, in the current detection structure1, the magnetic detection element3can detect the magnetic field with appropriate strength even when the current flowing through the busbar2is high and strength of magnetic field generated by each of the current paths5and6is large since the large magnetic fields cancel each other out. Therefore, a high sensitivity GMR sensor can be used as the magnetic detection element3for measurement.

In case of using the GMR sensor as the magnetic detection element3, it is desirable that the magnetic detection element3be arranged at a position where a magnetic flux density of the synthetic magnetic field produced by combining magnetic fields generated in the current paths5and6is more than 0 and not more than 5 mT. This is because output of a general GMR sensor is saturated at a magnetic flux density of more than 5 mT and it makes measurement difficult. The magnetic flux density here is the magnitude in a steady state and the case of temporality exceeding 5 mT in an abnormal state or a transient state is taken as an exception.

More desirably, the magnetic detection element3be arranged at a position where a magnetic flux density of the synthetic magnetic field produced by combining magnetic fields generated in the current paths5and6(the magnetic flux density in a steady state) is more than 0 and not more than 2 mT since a range in which the magnetic flux density can be accurately detected (a range in which the magnetic flux density and output voltage are linear) is generally not more than 2 mT in the GMR sensor.

Arranging the magnetic detection element3in the through-hole4herein means that at least a portion of the magnetic detection element3is housed in the through-hole4, in other words, at least a portion of the magnetic detection element3overlaps the busbar2as viewed on a horizontal cross section (or as viewed from a side). In the current detection structure1, the magnetic detection element3is arranged so that the center thereof (the center of the magnetic detection element3in the thickness direction of the busbar2(vertical direction inFIG. 2)) coincides with the center of the busbar2in the thickness direction thereof. Thus, when the GMR sensor is used as the magnetic detection element3, the magnetic fields enter the magnetic detection element3only in a direction parallel to the detection axis D of the magnetic detection element3and this allows currents to be easily detected with high accuracy.

Currents with a component in a width direction flow in the vicinity of longitudinal end portions of the through-hole4, which causes an error. Therefore, it is desirable to arrange the magnetic detection element3at a position spaced from the longitudinal end portions of the through-hole4so as not to be affected by the currents with a component in a width direction, and it is preferable to arrange the magnetic detection element3at the center of the through-hole4in the longitudinal direction of the busbar2. Taking the magnitude of the current flowing through the busbar2into calculation, a length Lh of the through-hole4is determined so that the magnetic detection element3can be arranged at a position not affected by magnetic fields generated in the vicinity of the longitudinal end portions of the through-hole4.

In present embodiment, the though-hole4is formed to have a symmetrical shape with respect to a central axis O of the busbar2and the current paths5and6are formed symmetrically on the both sides of the though-hole4. By such a configuration, magnetic fields are generated symmetrically from the current paths5and6.

As shown inFIG. 2, in the though-hole4, distribution of a magnetic flux density B1generated by the current path5and distribution of a magnetic flux density B2generated by the current path6are substantially inversely proportional to distances from the current paths5and6and directions of the magnetic flux densities B1and B2generated by the current paths5and6are opposite to each other. By symmetrically forming the current paths5and6on the both sides of the though-hole4, the magnetic fields generated in the two current paths5and6completely cancel each other out on the central axis O of the busbar2, resulting in magnetic flux density (B1+B2) of 0. In the graph ofFIG. 2, distribution of the magnetic flux density B1which is generated by the current path5shown on the left side of the drawing is indicated by a thin dotted line, distribution of the magnetic flux density B2which is generated by the current path6shown on the right side of the drawing is indicated by a thin dash-dot line, and distribution of the magnetic flux density (B1+B2) produced by combining the magnetic flux densities generated by the two current paths5and6is indicated by a thick solid line.

Therefore, the magnetic detection element3when arranged at an appropriate position off the central axis O of the busbar2can detect the optimal level of magnetic flux density (B1+B2) and it is thus possible to carry out measurement with high accuracy. Arranging the magnetic detection element3at a position off the central axis O of the busbar2here means that the center of the magnetic detection element3in the width direction of the busbar2is shifted from the central axis O in the width direction. Therefore, a portion of the magnetic detection element3may be on the central axis O.

In addition, in the current detection structure1, the distribution of the magnetic flux density (B1+B2) produced by combining the magnetic flux densities generated by the two current paths5and6is relatively close to flat in the vicinity of the central axis O of the busbar2, which allows an error due to disturbance to be reduced and provides excellent robustness. Since the distribution of the magnetic flux density (B1+B2) in the vicinity of the central axis O becomes flatter with an increase in a width Wh of the through-hole4, the width Wh of the through-hole4is desirably as large as possible from the viewpoint of improving the robustness.

Preferably, taking the frequency of the current flowing through the busbar2into calculation, a width W of the current paths5and6is determined so that the influence of the skin effect can be suppressed. Since a skin depth at a frequency of 100 kHz is about 0.2 mm when using copper or copper alloy for the busbar2, the width W of the current paths5and6in the present embodiment is desirably not more than 0.5 mm, more desirably not more than 0.2 mm. At a frequency of 10 kHz, the skin depth is about 1 mm and the width W of the current paths5and6in such a case is desirably not more than 2 mm, more desirably not more than 1 mm.

In this regard, however, if the width W of the current paths5and6is reduced too much, resistance is increased due to a decrease in the cross sectional area of the current paths5and6, resulting in increases in loss and heat generation. Therefore, the width W of the current paths5and6and the width Wh of the through-hole4should be appropriately set in view of the influence of the skin effect and acceptable loss and heat generation, etc.

Preferably, also taking the frequency of the current flowing through the busbar2into calculation, a thickness of the current paths5and6is determined so that the influence of the skin effect can be suppressed. When copper or copper alloy is used for the busbar2and the frequency of the current flowing through the busbar2is not more than 100 kHz, the thickness of the current paths5and6is desirably not more than 0.5 mm, more desirably not more than 0.2 mm. Meanwhile, when the frequency of the current flowing through the busbar2is not more than 10 kHz, the thickness of the current paths5and6is desirably not more than 2 mm, more desirably not more than 1 mm.

In the present embodiment, the though-hole4is formed to have a symmetrical shape with respect to the central axis O of the busbar2and the current paths5and6are formed symmetrically on the both sides of the though-hole4. However, the current paths5and6on the both sides of the though-hole4may be formed asymmetrically as shown inFIG. 3or the though-hole4may have an asymmetrical shape.FIG. 3shows an example in which a width W1of the current path5on the left side of the drawing is larger than a width W2of the current path6on the right side of the drawing and, in such a case, it is possible to arrange the magnetic detection element3on the central axis O of the busbar2or on the widthwise center of the though-hole4.

However, the widths W1and W2need to be adjusted so as not to be significantly different from each other since, if the difference between the width W1of the current path5and the width W2of the current path6is increased, reverse current, etc., occurs and causes an error.

In addition, in case that the current paths5and6are formed asymmetrically or the though-hole4has an asymmetrical shape, a difference between currents flowing through the two current paths5and6is generated and causes a difference in strength between magnetic fields generated in the two current paths5and6. This causes the magnetic flux density distribution to be uneven unlike the uniform distribution shown inFIG. 2, which may cause susceptibility to disturbance in a specific direction. Therefore, from the viewpoint of increasing the robustness, though-hole4formed to have a symmetrical shape with respect to the central axis O of the busbar2and the current paths5and6formed symmetrically on the both sides of the though-hole4are more desirable.

In addition, although the though-hole4in the present embodiment is formed in a rectangular shape in a plan view, the shape of the though-hole4is not limited thereto and the though-hole4may have, e.g., an ellipse shape as shown inFIG. 4Aor a polygonal shape as shown inFIG. 4B. In case that the though-hole4has the shape as shown inFIG. 4A or 4B, however, currents with a component in a width direction are generated in the current paths5and6and cause an error. Therefore, the current paths5and6linearly formed on the both sides of the though-hole4along the longitudinal direction of the though-hole4are more preferable.

As described above, in the current detection structure1of the present embodiment, the through-hole4is formed to penetrate the busbar2so that the current paths5and6are formed on the both sides of the through-hole4, the magnetic detection element3is arranged in the through-hole4, and the current detecting portion7detects the electric current flowing through the busbar2based on magnetic field strength which is the strength of the synthetic magnetic field produced by combining magnetic fields respectively generated by the currents flowing through the current paths5and6on the both sides of the through-hole4and is detected by the magnetic detection element3.

Due to such a configuration, the magnetic detection element3can detect strength of the magnetic fields generated in the current paths5and6and then cancelled each other out, and strength of the magnetic field to be detected can be adjusted to the optimum level by adjusting a position of the magnetic detection element3. As a result, it is possible to use a high sensitivity magnetic detection element3such as GMR sensor even when a large current flows through the busbar2, and it is thus possible to carry out measurement with high accuracy.

In addition, in the current detection structure1, by adjusting the width W or thickness of the current paths5and6according to the frequency of the current flowing through the busbar2, it is possible to suppress the influence of the skin effect and thereby to reduce frequency dependence. In an example of the conventional art in which the magnetic detection element3is arranged in the vicinity of the busbar2without forming the through-hole4on the busbar2, the magnetic flux density increases with increasing frequency as shown inFIG. 5, hence, high frequency dependence. On the other hand, in the current detection structure1of the present embodiment (the present invention), variation in magnetic flux density due to the frequency is very small and is reduced by about 1.4% in a frequency range of 1 Hz to 100 kHz in the example shown in the drawing.

Next, another embodiment of the invention will be described.

A current detection structure61shown inFIG. 6is based on the current detection structure1ofFIG. 1and is provided with protruding portions52and62which are integrally formed with the current paths5and6so as to protrude the lateral sides of the busbar2(to protrude outward in the width direction). It is possible to increase the cross sectional area of the current paths5and6by forming the protruding portions52and62, which allows an increase in resistance due to forming the through-hole4to be suppressed and a resulting increase in loss or heat generation to be reduced.

The total cross sectional area of the current paths5and6should be not less than 10 mm2per 1 A of current flowing through the busbar2. This is because, if the total cross sectional area of the current paths5and6is less than 10 mm2per 1 A of current flowing through the busbar2, a non-negligible increase in loss occurs and heat generation is also increased.

When the protruding portions52and62are formed, the increased width W of the current paths5and6may cause deterioration in frequency dependence but does not cause any problems if the frequency of the current flowing through the busbar2is small. In other words, the current detection structure61is effective particularly when the frequency of the current flowing through the busbar2is small enough to eliminate necessity of considering the skin effect and when a problem of heat generation occurs due to large current.

Although the protruding portions52and62are formed to increase the cross sectional areas of the current paths5and6in the current detection structure61, it is not limited thereto. The cross sectional areas of the current paths5and6may be increased by increasing the thickness thereof.

As such, the invention is not intended to be limited to the embodiments, and it is obvious that the various kinds of modification can be implemented without departing from the gist of the present invention.