Patent Description:
Electrical current sensors are used in a large variety of applications for monitoring or controlling electrical devices and system and in many applications there is an important advantage in reducing the manufacturing cost of such components and also the costs of implementing and using the components in an electrical circuit.

Although certain current transducers are provided without a magnetic core for cost and/or size reasons, this generally reduces reliability and/or sensitivity and/or accuracy and/or operating range of the transducer compared to one provided with a magnetic core surrounding the primary conductor. Therefore, many electrical current transducers for current sensing applications comprise a magnetic core made of a high permeability magnetic material, surrounding a central aperture through which passes a primary conductor carrying the current to be measured. The difficultly however is to provide a particularly compact current transducer with magnetic core in order to miniaturize and/or reduce the weight of the devices in which the components are mounted. There are also many applications in which the current transducer is mounted on a circuit board and needs to respect a pre-defined connection footprint or a surface area limitation or a height limitation that requires a particularly compact arrangement. Depending on the voltage amplitude of the primary conductor, this can lead to difficulties in achieving the required electrical creepage distances between the primary conductor and the electrical conductors of the magnetic field detector circuit.

A current transducer mountable on a PCB that meets the aforementioned requirements of compactness, accurate measurement and economical manufacturing is described in <CIT>. In this transducer, a leadframe conductor arrangement is overmolded with a housing base, and the magnetic field sensing cell is then mounted on the conductor arrangement and connected thereto by wire bonding. The magnetic core is inserted over the primary conductor and magnetic field sensor before a housing cover is mounted over the core and detector and clipped to the housing base. The interior of the housing may be unfilled (contain air) or filled with an insulating potting material. Despite the holding of the magnetic core by the housing cover and base, under conditions of vibration or mechanical shock, small movements between the core and housing may occur that impart stresses in the transducer that could lead to failure, or to movement of the airgap relative to the magnetic field sensor that affect the precision of the transducer. Moreover, arc tracking between the primary conductor and the magnetic core or the magnetic field detectors, in view of the small separation distances, may be difficult to avoid especially for high current measurement applications. Deformation of the housing base and the cover may also augment such problems under thermal and mechanical stresses.

<CIT> relates to a current measuring device using a magnetic field concentrator ring positioned around a current carrying conductor. The conductor is fixed at a constant radial distance from the magnetic field concentrator ring via a holder and connected to an electrical installation network at its opposite ends, the measuring device contained in a network termination housing.

<CIT> relates to a current detection device using a magnetic sensor and a method of manufacturing the same.

<CIT> relates to an electrical current transducer including a housing, a magnetic core comprising a central passage and a magnetic circuit gap, a magnetic field detector positioned in the magnetic circuit gap, and a leadframe conductor arrangement comprising a primary conductor for carrying the current to be measured and magnetic field detector conductors for connecting the magnetic field detector to an external circuit. The housing base comprises a primary conductor overmold portion that surrounds the central portion of the primary conductor providing a dielectric insulating layer between the magnetic core and the primary conductor. The housing further comprises a cover mounted over the base and covering the magnetic core and magnetic field sensor receiving cavity.

<CIT> relates to an electric current detector comprising a Hall effect sensor, a plurality of lead terminals electrically connected to said Hall effect sensor, a plastic package for encapsulating said Hall effect sensor and each end of said lead terminals, and a conductor loosely disposed with a gap in an opening formed in said plastic package and in spaced relation to said Hall effect sensor to pass a detected electric current through said conductor.

An object of the invention is to provide an electrical current transducer for mounting on a circuit board, with integrated primary conductor, magnetic field detector and magnetic core, which is accurate and reliable, yet very compact and robust, especially when subject to mechanical and thermal stresses.

It is advantageous to provide an electrical current transducer that has a high electrical tracking resistance between the primary conductor and conductors of the magnetic field detector.

It is advantageous to provide an electrical current transducer that has a large operating range.

It is advantageous to provide an electrical current transducer that is lightweight.

It is advantageous to provide an electrical current transducer that is easy to implement and economical to use.

Objects of the invention have been achieved by providing a current transducer according to claim <NUM> and a method of producing a current transducer according to claim <NUM>. Preferred embodiments are also provided in dependent claims <NUM> to <NUM>, <NUM> and <NUM>.

Disclosed herein is an electrical current transducer including an insulating body, a magnetic core comprising a central passage and a magnetic circuit gap, a magnetic field detector positioned in the magnetic circuit gap, and a sheet metal leadframe conductor arrangement comprising a primary conductor for carrying the current to be measured and secondary conductors for connecting the magnetic field detector to an external circuit. The primary conductor comprises a central portion extending through the central passage of the magnetic core, lateral extension arms extending from opposite ends of the central portion, and connection ends for connection to an external conductor. The secondary conductors comprising a plurality of conductors, each conductor comprising a sensing cell connection pad substantially aligned in a same plane with the central portion of the primary conductor and a connection end for connection to the external circuit. The insulating body comprises an inner overmold portion surrounding a central portion of the primary conductor and forming a core guide positioning and insulating the magnetic core with respect to the leadframe conductor arrangement.

The insulating body further comprises an outer overmold portion molded over the inner overmold portion, the magnetic core, magnetic field sensor, and a central portion of the leadframe conductor arrangement.

Also disclosed herein is a method of manufacturing an electrical current transducer comprising:.

In an advantageous embodiment, the inner overmold portion is formed by injection molding of a thermoplastic polymer.

In an advantageous embodiment, the outer overmold portion is formed by transfer molding of a thermosetting polymer.

The inner overmold portion (<NUM>) consists of thermoplastic polymer.

In an advantageous embodiment, the thermoplastic polymer is selected from a group including PPS (polyphenylenesulphide), LCP (liquid crystal polymer), PA (polyamide).

The outer overmold portion consists of a thermosetting polymer.

In an advantageous embodiment, the thermosetting polymer is a semiconductor grade epoxy mold compound.

In an advantageous embodiment, the inner overmold portion comprises a primary portion molded over the central portion of the primary conductor, the overmold portion comprising the core guide lateral guide edges engaging opposite lateral sides of said magnetic core branch for laterally positioning the magnetic core branch with respect to the leadframe conductor arrangement. The core guide may further comprise a base layer arranged to position a lateral branch of the magnetic core thereagainst.

In an advantageous embodiment, the inner overmold portion consists of said primary portion and said core guide.

In an embodiment, the core guide further comprises a base layer extending from the primary portion and partially over the secondary conductors, on one side of the conductor lead arrangement opposed to a side against which the sensing cell of the magnetic field detector is mounted and connected.

In an advantageous embodiment, on a side of the leadframe conductor arrangement against which the sensing cell of the magnetic field detector is mounted, the magnetic core branch is separated by a gap from the inner overmold portion, said gap being filled with material of the outer overmold portion.

In an advantageous embodiment, the outer overmold portion completely encapsulates the magnetic core and inner overmold portion.

In an advantageous embodiment, the sensing cell is connected to the secondary conductor via bond wire connections.

In an embodiment, the sensing cell is mounted on the leadframe conductor arrangement on an underside of the leadframe conductor arrangement facing a mounting surface of the current transducer. In another embodiment, the sensing cell is mounted on the leadframe conductor arrangement on an upperside of the leadframe conductor arrangement facing away from a mounting surface of the current transducer.

Referring to the figures, an electrical current transducer according to embodiments of the invention is shown, the current transducer comprising an insulating body <NUM>, a magnetic core <NUM> comprising a central passage <NUM> and a magnetic circuit gap <NUM>, a magnetic field detector <NUM> positioned in the magnetic circuit gap <NUM>, and a conductor arrangement <NUM> made from a leadframe. The leadframe conductor arrangement <NUM> comprises a primary conductor <NUM> for carrying the current to be measured, and conductors <NUM> for connecting to the magnetic field detector <NUM>. The current transducer of the present invention is particularly well suited for open loop current measurement.

The primary conductor <NUM> comprises a central portion <NUM>, lateral extension arms <NUM> extending from opposite ends of the central portion <NUM> and connection ends <NUM> at the free ends of the extension arms for connection to an external conductor through which flows the current to be measured. The external conductor may in particular be connected to a circuit board (not shown) provided with conductive contacts for connection to the connection ends <NUM>. The conductive contacts may for instance be in the form of conductive contact pads for surface mount connection of the connection ends <NUM>. The central portion <NUM> of the primary conductor <NUM> extends through the central passage <NUM> of the magnetic core.

The magnetic core has a general U-shape formed by an end branch 6a and lateral branches 6b, 6c extending therefrom to free ends <NUM>, the magnetic circuit gap <NUM> being formed between the lateral branches 6b, 6c proximate the free ends <NUM>.

The magnetic core <NUM> acts as a magnetic flux concentrator for the magnetic field detector <NUM> positioned in the magnetic circuit gap <NUM>. The magnetic flux generated by the current flowing in the primary conductor is concentrated through the magnetic circuit gap <NUM>. The magnetic core is made of a material with a high magnetic permeability, examples of such magnetic materials being FeSi or NiFe alloys, MnZn or other ferrites, nanocrystalline materials, and amorphous materials. The magnetic core according to embodiments of the invention increases the signal level and provides good immunity against the external fields in comparison to current transducers that are not provided with a magnetic core, for instance in which the magnetic field detector is positioned proximate the primary conductor without a magnetic flux concentrator.

In an advantageous embodiment, the free end <NUM> of the lateral branches is provided with a chamfer <NUM> on an outer side of the lateral branches, the chamfer reducing the amount of magnetic material needed and reducing fringe fields.

The leadframe conductor arrangement <NUM> is stamped or etched and formed out of a single piece of sheet metal, whereby the central portion <NUM> of the primary conductor and the major portions of the magnetic field detector conductors <NUM> are substantially aligned and extend in a same major plane and comprise substantially identical thicknesses corresponding to the sheet metal thickness from which the conductor arrangement is formed. The connection ends <NUM> of the primary conductor <NUM> and connection ends <NUM> of the magnetic field detector conductors <NUM> may be bent out of the major plane to provide terminals for connection to an external circuit, in particular for connection to contact pads of the external circuit board.

The magnetic field detector conductors <NUM> comprise a plurality of conductors, each conductor comprising a connection end <NUM> for connection to the external circuit and a sensing cell connection pad <NUM> in the leadframe major plane for connection to the magnetic field detector <NUM>. The magnetic field detector conductors comprise at least a pair of supply terminals for instance one being at a supply voltage Vc and the other at ground GND, and at least one signal out terminal Vout. The magnetic field detector conductors may further comprise a reference terminal Vref, a ground terminal and optionally supplementary signal out terminals, for instance for an overcurrent detection signal OCD.

The connection pads <NUM> may be provided with different shapes, surface areas and positions optimized for connection to the sensing cell <NUM>. The magnetic field detector <NUM> comprises a sensing cell <NUM> and connection means, for instance in the form of bond wires <NUM>. In the illustrated embodiments, the connection pads <NUM> are connected to the magnetic field detector by means of bond wire connections <NUM>. Other interconnection means that are per se known in the art can however be provided between the sensing cell <NUM> and connection pads <NUM> of the leadframe conductor arrangement <NUM>. For instance, the interconnection means may comprise a so called "flip chip" connection arrangement between a semiconductor substrate and metal contact pads, whereby for example solder beads interconnect connection areas on the sensing cell <NUM> to the connection pads <NUM> of the leadframe conductor arrangement <NUM>.

The sensing cell <NUM> may, in a preferred embodiment, be in the form of a Hall sensor, per se well-known in the art of current transducers, that is formed in a semiconductor substrate (for instance a silicon substrate). Other sensing cell technologies may however also be adopted in the present invention, for instance fluxgate type of magnetic field detectors or giant magneto resistive type of magnetic field sensors. Hall sensors that are formed in a substantially planar semiconductor substrate are advantageous in view of their low cost, compactness, and robustness.

The sensing cell <NUM> may also be an arrangement of more than one semiconductor chip, for example a highly sensitive Hall chip adjacent to a signal processing chip.

The magnetic field detector conductors <NUM> and primary conductor central portion <NUM> are advantageously in the same plane (the major plane), or essentially the same plane, and are held together by an inner overmold portion <NUM> of the insulating body <NUM>.

In the embodiment illustrated in <FIG>, the inner overmold portion <NUM> of the insulating body <NUM> is overmolded over portions of the secondary conductors of the leadframe conductor arrangement <NUM> while exposing connection pads <NUM> of the magnetic field detector conductors <NUM>. The inner overmold portion <NUM> and leadframe conductor arrangement <NUM> present an essentially planar mounting surface with the connection pads <NUM> of the magnetic field detector conductors <NUM>.

In an embodiment, for instance as illustrated in <FIG>, the sensing cell <NUM> is mounted on an an underside of the lead frame arrangement, facing a mounting plane of the conductor connection ends <NUM>, <NUM>.

The inner overmold portion <NUM> is overmolded around the central portion <NUM> of the primary conductor <NUM>. The inner overmold portion <NUM> provides a dielectric insulating layer between the magnetic core and the primary conductor and optionally may provide in addition a positioning guide between the central passage of the magnetic core and the primary conductor, also ensuring proper positioning of the sensing cell <NUM> within the magnetic circuit gap <NUM>.

The magnetic core <NUM> and leadframe conductor arrangement <NUM> with the inner overmold portion <NUM> of the present invention provides a particularly compact arrangement yet allowing the primary conductor to be well separated and insulated from the magnetic field detector conductors <NUM>. The primary conductor central portion <NUM> can be provided with a large and unreduced cross section and at the same time have a good insulation separation distance from the sensing cell of the magnetic field detector in a compact footprint (i.e. surface area occupied by the electrical current transducer when positioned on an external circuit board).

The inner overmold portion <NUM> may advantageously comprise a core guide <NUM> comprising lateral guide edges 32b flanking opposite lateral sides of the magnetic core <NUM> to laterally position the magnetic core with respect to the leadframe arrangement <NUM> and sensing cell <NUM>.

The inner overmold portion <NUM> comprises a primary portion <NUM> that is molded over the central portion <NUM> of the primary conductor <NUM>.

In a first embodiment, as illustrated in <FIG>, the inner overmold portion <NUM> comprises a secondary portion <NUM> that extends from the primary portion <NUM> and is partially overmolded over portions of the sensing cell connection pads <NUM> of the secondary conductors <NUM>. The core guide <NUM> is substantially molded on one side 4a of the leadframe conductor arrangement <NUM>, opposite the side 4b on which the sensing cell <NUM> of the magnetic field detector is mounted.

In the illustrated embodiment of <FIG>, the core guide <NUM> comprises a base wall or layer 32a on an upper side 4b of the leadframe conductor arrangement, presenting a surface 32a against which an inner side <NUM> of an upper branch 6c of the magnetic core is positioned against. The base layer 32a provides an insulation layer that defines the distance between the magnetic core and the primary and secondary conductors <NUM>, <NUM>. As best seen in <FIG>, on the opposite side of the base layer 32a, the opposite lateral branch 6b of the magnetic core <NUM> is spaced with a certain gap <NUM> from the leadframe conductor arrangement <NUM> and the inner overmold portion <NUM>. The sensing cell <NUM> and the bond wire connections <NUM> are positioned in the magnetic field circuit gap <NUM> between the free ends of the lateral branches 6b, 6c, this gap being filled with the material of the outer overmold portion <NUM>.

In a second embodiment, as illustrated in <FIG>, the inner overmold portion <NUM> does not extend over portions of the sensing cell connection pads <NUM> of the secondary conductors <NUM> and is separated therefrom by a gap that is filled with the outer overmold portion <NUM>. An advantage of this configuration is to ensure a long electrical creepage distance between the primary conductor <NUM> and secondary conductors <NUM>. In the interface between the inner overmold portion <NUM> and the outer overmold portion <NUM>, in particular if two different materials are used, chemical bonding between the materials of the inner overmold portion and outer overmold portion may be incomplete or such that internal creepage currents can breakdown the insulation barrier. In this embodiment, the inner overmold portion surrounds only the primary conductor and does not have an interface with the secondary conductors <NUM>, and the outer overmold portion <NUM> surrounds the secondary conductors <NUM> without interfacing with the inner overmold portion along a secondary conductor. There is therefore no interface between the inner and outer overmold portions along a portion of secondary conductor <NUM> thereby avoiding a creepage along an interface between overmold portions.

The inner overmold portion <NUM> serves to assemble the magnetic core <NUM> to the conductor lead arrangement <NUM>, providing structural rigidity, positioning, and insulation between the conductor lead arrangement <NUM> and magnetic core <NUM>, and between the primary and secondary conductors.

In a preferred embodiment, the inner overmold portion <NUM> is formed of a thermoplastic polymer that is preferably injection molded on the lead frame conductor arrangement <NUM> while it is still formed as a blank and connected to a lead frame <NUM> during the manufacturing process. This process is very advantageous for mass production in view of the rapidity of the injection molding process, including the rate of hardening of the thermoplastic material after injection, and the ease of passing the leadframe through a die for injection molding in a an economical manner. The overmolding of the inner overmold portion <NUM> prior to mounting and interconnection of the magnetic field detector <NUM> on the secondary conductors can be automated very well (in contrast to applying adhesive tape for example) and provides an insulating layer completely surrounding the primary lead in the vicinity of the magnetic field detector chip and other conductive parts of the secondary side.

The leadframe <NUM> is formed of a strip of sheet metal <NUM> that is stamped in a stamping die to cut out portions of the strip of metal to form the blank shapes of the leadframe conductor arrangement and attachments thereof to the strip of metal during the stamping and forming process, such stamping processes being per se known in the art; for relatively low production quantities an etching process can be more economic.

The magnetic core <NUM> and the sensing cell <NUM> are assembled to the leadframe conductor arrangement while it is still attached to the leadframe <NUM>. The leadframe <NUM> may thus pass through an injection molding, transfer molding or compression molding die for forming the inner overmold portion <NUM>. The inner overmold portion in an embodiment may advantageously be made of a thermoplastic material.

As best seen in <FIG>, in a first embodiment the inner overmold portion <NUM> comprises an opening <NUM> in the base layer 32a. The opening <NUM> is positioned opposite the sensing cell <NUM> mounted on the opposite side 4a of the leadframe conductor arrangement <NUM>. The opening <NUM> may serve to avoid material overflowing during the molding process on the surface portion of the leadframe conductor arrangement <NUM> against which the sensing cell <NUM> is positioned and bonded.

During the manufacturing process, with the leadframe conductor arrangement <NUM> connected to the leadframe <NUM>, after connection of the sensing cell <NUM> to the secondary conductors <NUM> and mounting of the magnetic core <NUM> over the inner overmold portion, the aforementioned assembly is fed into a transfer molding die having a die cavity corresponding substantially to the outer envelope of the outer overmold portion <NUM>, and a thermosetting polymer is injected into the die cavity to fill said cavity and form the outer overmold portion. Thermosetting polymers in comparison to thermoplastic polymers may generally have a much lower viscosity and advantageously fill the spaces around the sensing cell <NUM>, between the bonding wires <NUM>, between the conductor lead arrangement conductors <NUM>, <NUM> and also in the spaces between a magnetic core <NUM> and conductors, with relatively low hydrodynamic forces. The thermosetting polymer thus forms an excellent insulation barrier between the leadframe conductor arrangement, in particular between the primary and secondary conductors and between the primary conductor and magnetic core and sensing cell, in a compact and robust arrangement. In particular the thermosetting outer overmold portion <NUM> provides a particularly robust and stable securing of the magnetic core <NUM> to the leadframe conductor arrangement <NUM>. Moreover, the formation of the inner overmold, the connection of the sensing cell to the leadframe, the molding of the magnetic core to the leadframe arrangement and the subsequent overmolding of the outer overmold portion may advantageously be performed while the leadframe conductor arrangement is formed as a blank still attached to the leadframe to facilitate accurate assembly of components and efficiency of the production process.

As best seen in <FIG> and <FIG>, the leadframe conductor arrangement is in a planar shape with the primary and secondary conductors in a plane of the strip of sheet metal <NUM>. After forming of the outer overmold portion <NUM>, the primary conductor <NUM> and secondary conductors <NUM> may be severed from the strip of sheet metal <NUM> (<FIG> and <FIG>) and bent into their final shapes (<FIG>) before the electric current transducer <NUM> is severed from the leadframe metal strip <NUM>.

As shown in <FIG>, the leadframe conductor arrangement is connected to the leadframe <NUM> via a plurality of bridge attachments 42a, 42b, 42c, including primary conductor bridge attachments 42a, secondary current conductor bridge attachments 42b and transducer support attachments 42c. Furthermore, the leadframe conductor arrangement is connected to the leadframe via the conductor end attachments 43a, 43b, namely the primary conductor end attachments 43a and secondary conductor end attachments 43b. In a subsequent step, as illustrated in <FIG>, the primary and secondary conductors are cut away from the leadframe strip <NUM> by various associated cutting dies <NUM> as best illustrated in <FIG>.

The various bridge attachments 42a, 42b between secondary conductors and leadframe strip <NUM> and between primary conductors and leadframe strip <NUM> serve to stabilize and provide secure position of the conductor leadframe arrangement during the stamping and subsequent molding and assembly steps mentioned above.

The primary and secondary conductors may then be bent into their final shapes as shown in <FIG> for connection to an external circuit board while the transducer remains connected to the leadframe strip <NUM>. In a subsequent step, the transducer may be cut off the leadframe strip <NUM> by severing the transducer support attachments 42c.

Claim 1:
Electrical current transducer including an insulating body (<NUM>), a magnetic core (<NUM>) comprising a central passage (<NUM>) and a magnetic circuit gap (<NUM>), a magnetic field detector (<NUM>) positioned in the magnetic circuit gap, and a sheet metal leadframe conductor arrangement (<NUM>) comprising a primary conductor (<NUM>) for carrying the current to be measured and secondary conductors (<NUM>) for connecting the magnetic field detector to an external circuit, the primary conductor comprising a central portion (<NUM>) extending through the central passage of the magnetic core, lateral extension arms (<NUM>) extending from opposite ends of the central portion, and connection ends (<NUM>) for connection to an external conductor, the secondary conductors comprising a plurality of conductors, each conductor comprising a sensing cell connection pad (<NUM>) aligned with the central portion of the primary conductor and a connection end (<NUM>) for connection to the external circuit, the insulating body comprising an inner overmold portion (<NUM>) surrounding the central portion (<NUM>) of the primary conductor, characterized in that the insulating body further comprises an outer overmold portion (<NUM>) molded over the inner overmold portion, the magnetic core, magnetic field sensor, and a central portion of the leadframe conductor arrangement and in that the inner overmold portion (<NUM>) consists of a thermoplastic polymer and the outer overmold portion (<NUM>) consists of a thermosetting polymer.