Current sensor

A current sensor for measuring an electric current flowing through a conductor includes a ring shaped magnetic core, a bare semiconductor chip, and a case. The magnetic core has a gap and surrounds the conductor. The bare semiconductor chip has a front surface and a vertical Hall effect element formed on the front surface. The bare semiconductor chip is arranged in the gap of the magnetic core to detect a magnetic field generated by the electric current. The magnetic core and the bare semiconductor chip are accommodated in the case. A back surface of the bare semiconductor chip is fixed in the case in such a manner that the front surface of the bare semiconductor chip is parallel to a direction of the magnetic field.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and incorporates herein by reference Japanese Patent Application No. 2006-229477 filed on Aug. 25, 2006.

FIELD OF THE INVENTION

The present invention relates to a current sensor.

BACKGROUND OF THE INVENTION

A current sensor has been proposed that measures an electric current by using a Hall effect integrated circuit (IC) made of a semiconductor material such as silicon (Si), indium arsenide (InAs), indium antimonide (InSb), or gallium arsenide (GaAs). Nowadays, a Hall effect IC, in particular, made of silicon is provided as a molded IC chip package. Specifically, as shown inFIGS. 10A-10C, a sensor chip101mounted on a lead frame100is encapsulated in a molding resin102so that a sensor package103is provided.

A current sensor disclosed, for example, in U.S. Pat. No. 7,084,617 corresponding to JP 2005-308526 and JP 2005-308527 uses the sensor package103. As shown inFIG. 11, the conventional current sensor includes a case110, a magnetic core111accommodated in the case110, the sensor package103placed in a gap112of the magnetic core111, a capacitor120for preventing noise from entering the sensor package103, and a thermistor121for temperature detection.

In such a sensor package, a Hall effect element is encapsulated with molding material such as epoxy or plastic. Therefore, the Hall effect element may be subjected to stress from the molding material, in particular, thermal stress caused by a thermal strain due to a change in temperature. The stress causes a reduction in accuracy of an output signal of the Hall effect element. Accordingly, the conventional current sensor cannot accurately measure an electric current. Further, the conventional current sensor still has room for improvement in assembly. In short, the conventional current sensor is relatively difficult to assemble.

SUMMARY OF THE INVENTION

In view of the above-described problem, it is an object of the present invention to provide a current sensor that accurately measures an electric current and assembles easily.

A current sensor for measuring an electric current flowing through a conductor includes a ring shaped magnetic core, a bare semiconductor chip, and a case. The magnetic core having a gap and surrounds the conductor. When the electric current flows through the conductor, a magnetic field generated by the electric current is concentrated by the magnetic core and appears in the gap. The bare semiconductor chip has a front surface and a vertical Hall effect element formed on the front surface. The bare semiconductor chip is arranged in the gap of the magnetic core to detect the magnetic field in the gap. The case includes a case body, a case cover, and a conductive terminal. The case body has an inner room for holding the magnetic core and the bare semiconductor chip. The case cover is attached to the case body to seal the inner room. The conductive terminal is supported in the case body. The conductive terminal has a first end exposed to the inner room and electrically connected to the bare semiconductor chip by a bonding wire and a second end exposed to outside the case body to be connectable to an external device. A back surface of the bare semiconductor chip is fixed to the inner room of the case body in such a manner that the front surface of the bare semiconductor chip is parallel to a direction of the magnetic field generated by the electric current flowing through the conductor.

The bare semiconductor chip having the vertical Hall effect element is not encapsulated with a molding material. Therefore, the vertical Hall effect element can avoid stress from the molding material so that the current sensor can accurately detect the electric current.

The bare semiconductor chip uses a vertical Hall effect element instead of a conventional lateral Hall effect element. In such an approach, the bare semiconductor chip can be arranged parallel to the direction of the magnetic field in the gap of the magnetic core. Thus, the bare semiconductor chip can be easily connected to the conductive terminal by wire bonding.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown inFIG. 1, a current sensor1according to an embodiment of the present invention includes two sensor sets and a case30. Each sensor set has a ring-shaped magnetic core10and a sensor chip20. Each sensor set measures an electric current IF flowing through a different conductor90. For example, each conductor90is connected to a different one of three outputs of an inverter (not shown) that drives a three-phase alternating-current (AC) motor for a hybrid electric vehicle (HEV) or an electric vehicle (EV). Thus, the current sensor1measures two of three output currents of the inverter at a time.

The magnetic core10and the sensor chip20are accommodated in the case30. The magnetic core10has a gap11and a center opening. The magnetic core10may be, for example, made of a nickel iron magnetic alloy, i.e., permalloy. Specifically, the magnetic core10is formed by laminating multiple ring-shaped plates, each of which is made of permalloy and has a thickness of about 1 millimeter (mm).

As shown inFIG. 2, the conductor90is placed in the center opening of the magnetic core10to be surrounded by the magnetic core10. When the current IF flows through the conductor90, a magnetic field generated by the current IF is concentrated by the magnetic core10and appears in the gap11, as shown inFIG. 6. The strength of the magnetic field changes with the amplitude of the current IF. As shown inFIG. 5, the sensor chip20has two vertical Hall effect elements21and placed in the gap11to detect the magnetic field generated by the current IF.

The case30includes a case body31and a case cover32. As shown inFIG. 3, the case body31has a rectangular frame31aand a bottom31b. The bottom31bis integrally formed with the rectangular frame31ato cover a first opening defined by the frame31a. The case cover32covers a second opening defined by the frame31aso that the case30has a sealed inner room33for accommodating the magnetic core10and the sensor chip20. The case cover32is attachable to and detachable from the case body31. A connection terminal34is insert-molded with the case body31.

The case body31has two through holes S1. Each conductor90is inserted through a corresponding one of the through holes S1. Also, the case cover32has two through holes S2. When the case cover32is attached to the case body31, each of the through holes S2communicates with a corresponding one of the through holes S1.

The magnetic core10and the sensor chip20are fixed to the inner room33of the case body31. For example, the magnetic core10is fixed to the case body31by an adhesive (e.g., silicone adhesive), snap-fitting, or thermal welding. As shown inFIGS. 5,6, the case body31has a base portion36. The sensor chip20is fixed to the base portion36of the case body31in such a manner that a surface20aof the sensor chip20is parallel to a direction the magnetic field in the gap11. Unlike the sensor package103shown inFIGS. 10A-10C, the sensor chip20is a bare semiconductor chip. In short, the sensor chip20is not encapsulated with a molding material such as epoxy resin or plastic.

As shown in, for example,FIG. 4, the connection terminal34has a first end exposed to the inner room33. The first end of the connection terminal34extends close to the sensor chip20to be parallel and approximately coincident with the surface20aof the sensor chip20. The sensor chip20and the first end of the connection terminal34are electrically connected by a bonding wire35made of gold, aluminum, or the like.

Before the case cover32is attached to the case body31to cover the second opening of the case body31, the magnetic core10and the sensor chip20are placed in the inner room33from the side of the second opening. Then, wire bonding is performed from the side of the second opening to electrically connect the sensor chip20and the connection terminal34by the bonding wire35. After the wire bonding is finished, the case cover32is attached to the case body31. Thus, the magnetic core10and the sensor chip20can be assembled into the current sensor1.

As shown inFIG. 5, the sensor chip20has a signal processing circuit22in addition to the vertical Hall effect elements21and is constructed as a Hall effect integrated circuit (IC). The signal processing circuit22may be, for example, formed with metal oxide semiconductor (MOS) transistors. As shown inFIG. 4, a gel40is placed over the sensor chip20so that the sensor chip20is coated (i.e., sealed) with the gel40. The gel40may be, for example, a silicon gel.

As shown inFIGS. 5,6, the case body31has a dam portion37that defines the base portion36where the sensor chip20is fixed. The dam portion37surrounds the base portion36and projects with respect to the base portion36. The dam portion37prevents the gel40from flowing out of the base portion36, when the gel40is applied to the sensor chip20. In short, the dam portion37limits the gel40within the base portion36. Alternatively, the dam portion37may be recessed with respect to the base portion36to prevent the gel40from flowing out of the base portion36.

As described above, the sensor chip20has the two vertical Hall effect elements21and the signal processing circuit22. As shown inFIG. 7, a power supply voltage Vcc is applied to each of the vertical Hall effect elements21. The signal processing circuit22has two operational amplifiers45. The operational amplifiers45amplify output voltages of the vertical Hall effect elements21, respectively. The sensor chip20outputs the amplified voltages through output terminals Vout1, Vout2, respectively.

The vertical Hall effect elements21are detailed inFIGS. 9A-9C. An n-type well layer51is formed at a surface portion of a p-type silicon substrate50. An impurity diffusion layer52for element isolation is formed at the surface portion of the p-type silicon substrate50to surround the n-type well layer51. N+ contact regions53a-53eare formed at a surface portion of the n-type well layer51. The contact regions53a-53eare electrically connected to terminals S, G1, G2, V1, and V2, through electrodes, respectively.

P-type impurity diffusion layers54a,54bare formed at the surface portion of the n-type well layer51to divide the surface portion of the n-type well layer51in first, second, and third regions51a-51c. The contact regions53a,53d, and53eare formed in the first region51a. The contact region53bis formed in the second region51b. The contact region53cis formed in the third region51c.

The contact region53ais formed between the contact regions53d,53e. Further, the contact region53ais formed between the contact regions53b,53c. Specifically, a line connecting the contact regions53d,53eperpendicularly intersects with a line connecting the contact regions53b,53cat the contact region53a. The contact region53ais opposite to the contact regions53b,53cacross the impurity diffusion layers54a,54b, respectively. A region between the contact regions53d,53eacts as a magnetic sensing portion HP.

A control current flowing from the terminal S passes below the impurity diffusion layers54a,54bvia the magnetic sensing portion HP and reaches the terminals G1, G2, respectively. In this case, the control current flows perpendicular to the surface of the silicon substrate50at the magnetic sensing portion HP. Therefore, when the magnetic field B applied to the magnetic sensing portion HP contains a component parallel to the surface of the silicon substrate50, a Hall voltage VH appears between the terminals V1, V2due to Hall effect. The Hall voltage VH changes with the parallel component contained in the magnetic field B. Therefore, the magnetic field B can be measured by detecting the Hall voltage VH.

As shown inFIGS. 1-4, the case30further includes a connector portion38integrally formed with the case body31. The connector portion38is tube-shaped and has a bottom. A second end34aof the connection terminal34is exposed to an inner surface of the bottom of the connector portion38to act as a male terminal. As shown inFIG. 4, when the connector portion38is mated with a female connector60of an external device, the connection terminal34is electrically connected to a female terminal61of the female connector60. Thus, the sensor chip20is electrically connected to the external device.

As described above, the magnetic core10and the sensor chip20are placed in the inner room33from the side of the second opening of the case body31, and the wire bonding is also performed from the side of the second opening. Thus, the magnetic core10and the sensor chip20are assembled into the current sensor1from the same side to facilitate the assembly of the current sensor1. The first end of the connection terminal34extends close to the sensor chip20to be parallel and approximately coincident with the surface20aof the sensor chip20. As shown, for example, inFIG. 3, the sensor chip20is connected to the connection terminal34at one side by the bonding wire35so that there is no need to rotate the sensor chip20in the wire bonding. Thus, the number of manufacturing steps of the current sensor1can be reduced. The sensor chip20is a bare semiconductor chip and is not encapsulated with the molding material. Since the sensor chip20is subjected to no stress from the molding material, the sensor chip20can produce an accurate output.

FIG. 8shows a result of an experiment conducted to determine a relationship between an offset voltage of a Hall effect element and stress applied to the Hall effect element. The experiment is conducted under conditions where a temperature is 27 degrees Celsius (° C.), a control current of the Hall effect element is 1 milliampere (mA), and a magnetic field applied to the Hall effect element is 0.1 tesra (T). A first graph F1represents a packaged conventional (i.e., lateral) Hall effect element, and a second graph F2represents a bare vertical Hall effect element.

Whereas a vertical Hall effect element responds to a magnetic field parallel to its surface, a lateral Hall effect element responds to a magnetic field perpendicular to its surface. As can be seen fromFIG. 8, when the same stress is applied to the bare vertical Hall effect element and the packaged lateral Hall effect element, an offset voltage of the bare vertical Hall effect element is smaller than an offset voltage of the packaged lateral Hall effect element.

The sensor chip20includes the two vertical Hall effect elements21. As shown inFIG. 7, the two vertical Hall effect elements21operate independently of each other, and the sensor chip20has the corresponding output terminals Vout1, Vout2. Therefore, even when one of the vertical Hall effect elements21is broken, the sensor chip20can work normally. Thus, the current sensor1has redundancy to ensure reliability without an increase in the number of parts. Alternatively, the sensor chip20may include three or more vertical Hall effect elements21for higher redundancy. In contrast, in the case of the conventional current sensor shown inFIG. 11, two sensor packages103are required to be placed in the gap112for redundancy. The number of parts is increased, when redundancy is given to the conventional current sensor.

As shown inFIG. 5, the sensor chip20further includes a capacitor23for noise reduction (elimination) and a thermistor24for temperature detection. Since the capacitor23and the thermistor24are integrated in the sensor chip20, the external capacitor120and the external thermistor121ofFIG. 11can be unnecessary. Thus, the number of parts can be reduced. Accordingly, the number of manufacturing steps can be reduced. For example, the thermistor24monitors deterioration of a vehicle battery by detecting a temperature of the battery.

As described above, the case30includes the case body31and the case cover32. The case body31has the inner room33for accommodating the magnetic core10and the sensor chip20having the surface20a, where the vertical Hall effect elements21are formed. The case cover32is attached to the case body31to seal the inner room33. The sensor chip20is a bare semiconductor chip and is fixed to the base portion36in the inner room33in such a manner that the surface20ais parallel to the direction the magnetic field in the gap11. The sensor chip20is electrically connected to the connection terminal34by the bonding wire35.

According to the embodiment, the sensor chip20is a bare semiconductor chip, i.e., is not encapsulated with the molding material. Therefore, the vertical Hall effect elements21formed on the sensor chip20can avoid stress from the molding material, in particular, thermal stress caused by a thermal strain due to a change in temperature. As a result, the current sensor1can accurately measure the electric current IF.

The sensor chip20uses a vertical Hall effect element instead of a conventional lateral Hall effect element. As described previously, whereas the vertical Hall effect element responds to the magnetic field parallel to its surface, a lateral Hall effect element responds to the magnetic field perpendicular to its surface. Since the sensor chip20uses the vertical Hall effect element, the surface20aof the sensor chip20can be positioned parallel to the first end of the connection terminal34. Therefore, it is easy to perform the wire bonding so that the current sensor1can assemble easily. If the sensor chip20uses the lateral Hall effect element, the surface20aof the sensor chip20needs to be positioned perpendicular to the first end of the connection terminal34so that it is difficult to perform the wire bonding.

The gel40is placed over the sensor chip20so that the sensor chip20is coated (sealed) with the gel40. In such an approach, the sensor chip20can be surely protected from moisture or the like. It is preferable that the gel40be a silicone gel, which has high moisture resistance and low elasticity. The dam portion37surrounds the base portion36, where the sensor chip20is fixed. The dam portion37projects or is recessed with respect to the base portion36so that the gel40placed over the sensor chip20can be prevented from flowing out of the base portion36.

The sensor chip20has multiple vertical Hall effect elements21. In such an approach, the current sensor1can have redundancy without the increase in the number of parts, i.e., without an increase in size.

The case30includes the connector portion38integrally formed with the case body31. The second end34aof the connection terminal34connected to the sensor chip20extends to the connector portion38to act as the male terminal. Thus, the sensor chip20can be electrically connected to the external device via the connector portion38.

The current sensor1has two sensor sets, i.e., two magnetic cores10and two sensor chips20. Therefore, the current sensor1can measure two of three output currents of the inverter at a time.

The capacitor23and the thermistor24are integrated in the sensor chip20. In such an approach, the external capacitor120and the external thermistor121ofFIG. 11can be unnecessary. Accordingly, manufacturing cost and size of the current sensor1can be reduced.

The embodiment described above may be modified in various ways. For example, the current sensor1may have one sensor set and measure one of three output currents of the inverter, or the current sensor1may have three sensor sets and measure all the three output currents of the inverter. Alternatively, the current sensor1may measure electric currents other than the output currents of the inverter. The signal processing circuit22may be eliminated from the sensor chip20.