Current Transducer

Abstract

An open-loop current converter (1) for surface mounting on an external circuit board, comprising: an integrated circuit (IC) chip (2) including a magnetic field sensing portion (10) and connection terminals (11) on a first active surface (9a) of the IC chip (2), a leadframe structure (3) including a primary conductor (4) and a plurality of secondary conductors (5) including IC connection portions (15), a plurality of bond wires (6) interconnecting the IC connection portions (15) to the connection terminals (11) of the IC chip (2), and an insulating overmolded housing (7) overmolded onto a portion of the IC chip (2), the bond wires (6), and the leadframe structure (3). The leadframe structure (3) is formed from sheet metal having a base sheet with a thickness B, at least a portion of the leadframe structure (3) having a thickness equal to the base sheet thickness B. The first active surface (9a) of the IC chip (2) faces the lead frame structure (3), and at least a portion of the lead frame structure (3) includes a thin portion having a thickness T that is 20% to 80% of the base sheet thickness B, and the thin portion having a thickness T includes the IC connection portion (15) of the secondary conductor (5).

Description

[Technical field]

[0001] The present invention relates to an open loop current transformer that is surface mounted on a circuit board. [Background technology]

[0002] It is known to provide an open loop current transducer having a surface mount electrical connection for mounting on a circuit board. In many applications it is advantageous to have a very compact, low height current transducer having a primary conductor integrated in an overmolded insulating housing. Such a coreless current transducer may include an integrated circuit (IC) chip having an active sensing layer thereon that detects the magnetic field induced by the current to be measured flowing in a primary conductor integrated in the current transducer, the integrated circuit chip being electrically connected to a secondary contact that provides power and allows the transmission of the measurement signal to an external circuit board.

[0003] It is well known to form the primary and secondary conductors from a leadframe, which is a strip of sheet metal that is stamped to cut out the primary and secondary contacts from the sheet metal, the contacts and integrated circuitry are overmolded with an insulating polymer, and the connecting ends of the contacts are exposed and configured for solder or weld connection to circuit traces on a circuit board.

[0004] The electrical interconnection between the integrated circuit and the secondary contacts may be connected by so-called "flip-chip" connections or by wire bonding. The active parts and the connection interfaces on a semiconductor substrate, e.g., a CMOS IC, are usually provided on the same single side of the IC. A flip-chip structure, e.g., as described in U.S. Patent Application Publication No. 2006 / 219436 A1, allows the active side to face and be adjacent to the leadframe conductor structure, allowing electrical interconnection between the leadframe and the connection parts on the integrated circuit active surface while at the same time allowing the active sensing parts of the integrated circuit to be very close to the primary conductors. This allows for a very small distance between the sensing layer and the primary conductors, resulting in high sensitivity for measuring the primary current. However, flip-chip connections can be expensive and complicated to manufacture due to the small distance on the IC chip between the connection points that need to be directly connected to the secondary conductors of the leadframe structure. They also have the disadvantage that electrical creepage distances are more difficult to manage than wirebonding interconnects.

[0005] Wire bonding is a very common and widely used technique for interconnecting integrated circuits to conductors that form connection terminals for IC chips. However, in conventional current transducers, a drawback of wire bonding interconnections between an IC chip and a lead frame structure is the increased height compared to flip chip structures due to the height of the interconnect wire loops between the IC substrate and the lead frame conductors. Also, in conventional current transducers with wire bonding between the lead frame and the IC chip, such as described in U.S. Patent Application Publication No. 2016 / 282388 A1, the active sensor surface faces away from the lead frame conductor structure to make the active surface accessible for wire bonding between the lead frame terminals and connection points on the IC circuit. Therefore, there is a lower sensitivity compared to flip chip structures. Summary of the Invention [Problem to be solved by the invention]

[0006] In view of the foregoing, it is an object of the present invention to overcome the shortcomings of both conventional flip-chip and conventional wire-bonded structures in an open-loop current transducer for surface mounting on a circuit board. In particular, it is an object of the present invention to provide an open-loop current transducer for surface mounting on a circuit board that has a low profile, high measurement sensitivity, and is economical to manufacture.

[0007] It would be advantageous to provide an open loop surface mount current transducer that is highly reliable, in particular one that ensures good insulation properties between the IC chip and the integrated primary conductor. [Means for solving the problem]

[0008] The object of the present invention is achieved by providing an open loop current converter as claimed in claim 1.

[0009] Disclosed herein is an open loop current transformer for surface mounting on an external circuit board, the open loop current transformer comprising an integrated circuit (IC) chip having: a magnetic field sensing portion and a connection terminal on a first active surface of the IC chip; a lead frame structure including a primary conductor and a plurality of secondary conductors including an IC connection portion; a plurality of bond wires interconnecting the IC connection portions to connection terminals of the IC chip; an insulating overmolded housing overmolded over the IC chip, the bond wires, and a portion of the leadframe structure; Includes.

[0010] The leadframe structure is formed from sheet metal having a base sheet of thickness B, at least a portion of the leadframe structure having a thickness equal to said base sheet thickness B. A first active surface of an IC chip faces the leadframe structure, and at least a portion of the leadframe structure includes a thinned portion of thickness T that is 20%-80% of base sheet thickness B, said thinned portion of thickness T including said IC connection portion of a secondary conductor.

[0011] In an advantageous embodiment, the IC chip is mounted partially overlapping a portion of the primary conductor including the sensing portion of the primary conductor, an insulating sheet mounted between the IC chip and the primary conductor is made of a material different from the material forming the insulating overmolded housing, and the connection terminals of the IC chip are provided on the portion of the IC chip that is not overlapping the primary conductor.

[0012] In an advantageous embodiment, the insulating foil extends beyond the portion of the IC chip that overlaps the primary conductors, with a perimeter of non-zero width W, to increase the electrical creepage distance between the primary conductors and the IC chip.

[0013] In an advantageous embodiment, the insulating foil extends beyond the edges of the primary conductors over which the IC chip also extends, increasing the creepage distance between the IC chip and the primary conductors.

[0014] In an advantageous embodiment, the sensing portion has a reduced current-carrying cross-section compared to the surface-mounted connection terminals of the primary conductors between which the sensing portion is arranged and interconnected.

[0015] In an advantageous embodiment, the surface mounted connection terminal of the primary conductor includes a plurality of raised portions.

[0016] In an advantageous embodiment, the surface mounted connection terminals of the secondary conductors are arranged in a row along a first edge of the overmolded housing and the surface mounted connection terminals of the primary conductors are arranged adjacent an opposing edge of the overmolded housing.

[0017] In an advantageous embodiment, the thinned portion thickness T is in the range of 30% to 70% of the base sheet thickness B. Preferably, the thinned portion thickness T is in the range of 40% to 60% of the base sheet thickness B.

[0018] In an advantageous embodiment, the primary conductor includes one or more heat sink extensions that function as heat sinks extending from its surface mounted connection terminals.

[0019] In an advantageous embodiment, one or more heat sink extensions extend from the primary conductor surface mount connection terminals towards the secondary conductor surface mount connection terminals.

[0020] In an advantageous embodiment, there are a pair of heat sink extensions, one for extending from each surface mounted connection terminal of the primary conductor.

[0021] Further objects and advantageous features of the present invention will become apparent from the appended claims, detailed description and accompanying drawings. [Brief description of the drawings]

[0022] [Figure 1a] FIG. 1 is a perspective view of a surface mounted open loop current transformer in accordance with an embodiment of the present invention. [Figure 1b] FIG. 1 is a perspective view of a surface mounted open loop current transformer in accordance with an embodiment of the present invention. [Figure 1c] FIG. 2 is a perspective view of a surface mount open loop current transformer according to an embodiment of the present invention, showing an overmolded housing portion partially cut away. [Diagram 2] 1 is a schematic cross-sectional view of a current transducer according to an embodiment of the present invention; [Figure 3a] FIG. 2 is a perspective view of the current transformer of FIGS. 1a-1c with the overmolded housing removed. [Figure 3b] FIG. 2 is a perspective view of the current transformer of FIGS. 1a-1c with the overmolded housing removed. [Figure 3c] FIG. 2 is a plan view of the current transformer of FIGS. 1a-1c with the overmolded housing removed. [Figure 4a] 1 is a top view of another embodiment of a current transformer according to the present invention with the overmolded housing removed. FIG. [Figure 4b] 1 is a perspective view of another embodiment of a current transformer according to the present invention with the overmolded housing removed. FIG. [Figure 4c] FIG. 4b is an enlarged view of a portion of FIG. 4a. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Referring to the figure, a current transducer 1 for surface mount connection on an external circuit board includes a magnetic field sensor 2 connected to a lead frame structure 3 housed within an insulating overmolded housing 7 .

[0024] The magnetic field sensor 2 is in the form of an integrated circuit (IC) chip having a first active face 9a and a second face 9b, at least the first active face 9a having an active portion including electrical connection terminals 11 for connecting to power and signal transmission conductors, and a magnetic field sensor portion 10 disposed on or adjacent to the surface of the first face. The chip may be attached to a lead frame by an adhesive layer between face 9a and an insulating layer 8. The insulating layer 8 may be attached to the lead frame by an adhesive layer.

[0025] The magnetic field sensing portion 10 may include a Hall effect sensor, or a giant magnetoresistance sensor, or other forms of magnetic field sensors known per se in the art. The IC chip 2 may be in the form of, for example, a CMOS chip, but other types of semiconductor integrated circuit chips having a magnetic field sensing portion known per se in the art may also be used. The semiconductor chip does not need to be described further, and such semiconductor chips having a magnetic field sensing portion and connection terminals are known per se in the art.

[0026] The leadframe structure 3 includes a primary conductor 4 and a plurality of secondary conductors 5. The leadframe structure is formed from a sheet of conductive metal, more specifically in the form of a band or strip that is stamped and formed in a stamping and forming process in which the sheet metal is cut, bent, and / or embossed to form the primary and secondary conductors. Stamping and forming leadframe structures from sheet metal is a technique known per se. The sheet metal used in such structures initially has a constant thickness, which shall be referred to herein as the "base sheet thickness" having a thickness B. In conventional leadframe structures stamped and formed from sheet metal, the primary and secondary conductors generally include essentially the base sheet thickness B throughout, except for minor variations due to bending operations when portions of the conductors are bent out of the original plane of the sheet metal, for example to form contact terminals. The base sheet thickness B is an important characteristic of the mechanical stiffness of the contact terminals, especially when the contact terminals protrude from an overmolded housing. Even in structures where the contact terminals are flush with or recessed in the overmolded housing with only the contact pad surfaces exposed, a base thickness may be required at least around the connection end for secure fixation to the housing and bonding to the external circuit board conductive pads to which it is welded or soldered. The thickness of the sheet metal also affects the cross-section of the conductors, particularly the primary conductors, and thus the current carrying capability of the conductors in the leadframe structure.

[0027] The IC chip 2 has a certain thickness, and the overmolded housing 7 molded around the leadframe conductor structure 3 and the IC chip 2 also requires a certain thickness to insulate these components from the outside environment.

[0028] However, in many applications, it is desirable to have a transducer with a very low height H for incorporation onto a circuit board, since the overall height of the electronic components, including the circuit board, must be as small as possible.

[0029] According to the present invention, at least a portion of the secondary conductor 5 has a thinned portion having a thickness T in the range of 20% to 80% of the base thickness B, preferably in the range of 30% to 70%, and more preferably in the range of 40% to 60%, for reasons that will be explained in more detail below.

[0030] The primary conductor includes a pair of surface mounted connection terminals 13 configured for surface mounting connection to conductive traces on an external circuit board, and a sensing portion 12 interconnecting the pair of surface mounted connection terminals 13. The sensing portion 12 has a reduced conductive cross-sectional area to concentrate the current to be measured (the primary current) and to increase the strength of the magnetic field around the sensing portion 12. The reduced cross-sectional area of ​​the sensing portion 12 may be formed, for example, by providing a slot 24 between the connection terminals 13 of the primary conductor.

[0031] The IC chip 2 is positioned in a partially overlapping relationship over the sensing portion 12. In particular, the magnetic field sensing portion 10 of the IC chip is preferably positioned in an overlapping relationship with the sensing portion 12 of the primary conductor such that the magnetic field sensing portion 10 is positioned very close to the sensing portion 12 of the primary conductor.

[0032] An insulating sheet 22 is positioned between the portion of the primary conductor 4 surrounding the sensing portion 12 and the IC chip 9. The insulating sheet 8 may be in the form of a sheet of polymeric material, such as polyimide, in single or multiple layers, or may be a small sheet of glass that defines a gap between the IC chip active sensing surface 9a and the primary conductor and provides insulation between the primary conductor and the IC chip. The insulating sheet 8 extends over the primary conductor 4 to a greater extent than the IC chip 2, forming a border of minimum width W around it and increasing the creepage distance between the IC chip and the primary conductor. The semiconductor IC chip only partially overlaps the primary conductor, and a portion of the IC chip including the connection terminals 11 extends beyond the inner edge 25 of the primary conductor 4, and the connection portions do not overlap the primary conductor and are accessible for connection to the bond wires 6. The insulating sheet 8 also advantageously extends beyond the inner edge 25 of the primary conductor, and the foil only partially overlaps the primary conductor, again increasing the electrical creepage distance between the IC chip 2 and the primary conductor 4. The IC chip is oriented so that the active sensing portion faces the primary conductor as close as possible to the primary conductor to maximize signal measurement.

[0033] The secondary conductors 5, which serve to interconnect the IC chip 2 to the power and signal connections of the external circuit to which the current transformer is intended to be connected, comprise a plurality of conductors formed from a lead frame structure as discussed above, each secondary conductor comprising an IC connection portion 15 at one end and a surface mount connection terminal 16 at the other end. The surface mount connection terminals 16 may, for example, be arranged in a row, presenting contact pads intended for surface mount solder or weld connection, as known per se in the art, to circuit traces of an external circuit board. Thus, the surface mount connection terminals 16 are positioned close to the outer edge of the transformer overmolded housing 7, and the IC connection portions 15 are positioned close to the IC chip 2, allowing for bond wire connection between the IC chip 2 and the IC connection portions 15 of the secondary conductors. The IC connection portions 15 of the secondary conductors are spaced closer to each other than the surface mount connection terminals 13, which fan outwardly from the IC connection portions.

[0034] A bond wire 6 is connected between the IC connection portion 15 of each secondary conductor 5 and a connection terminal 11 on the IC chip 2. It should be noted that certain secondary conductors may not have connections to the IC chip, forming a redundant secondary conductor for use with semiconductor chips that have many connections. In other words, the number of connections depends on the functionality to be incorporated into the IC chip. While the leadframe conductor structure 3 may have a fixed configuration, the number of wire bond interconnects between the secondary conductors and the IC chip may depend on the configuration of the IC chip and the desired functionality to be incorporated into the transducer.

[0035] According to the present invention, at least a portion of the secondary conductor surrounding the IC connection portion 15 has a thinned portion of reduced thickness T, which is in the range of 20% to 80%, preferably in the range of 30% to 70%, more preferably in the range of 40% to 60% of the base thickness B, as described above. The reduced thickness of the IC connection portion 15 allows the bond wire 6 to have a lower overall height H of the overmolded housing 7 because the bond wire 6 has a reduced loop height relative to the outermost surface 26 of the leadframe structure 3 facing away from the IC chip 2. In the embodiment shown diagrammatically in FIG. 2, the bond wire loop height is entirely contained within the recess formed by the thinned portion T, but the bond wire 6 may have a portion that extends above the recess. Nevertheless, the portion that extends above the recess will be less than in conventional structures where the bond wire loop is further away from the outer surface of the leadframe. Also, advantageously, bond wires 6 are mounted and connected to IC chip 2 and secondary conductor 5 on the same side of the transducer as surface mounted connection terminals 13 and 16 of primary conductor 4 and secondary conductor 5.

[0036] The surface mount connection terminals 13, 16 may be embossed or stamped in a plane offset from the plane of the leadframe structure containing the IC connection portion 15 of the secondary conductor to present terminals that are flush with or extend beyond the mounting side 27 of the overmolded housing 7. The leadframe may be plated with any suitable wire bondable layer according to the bond wire used.

[0037] The thinned portion thickness T of the leadframe structure surrounding at least the IC connecting portion 15 of the secondary conductor may be formed by an etching process, such as a chemical etching process or a laser etching process, which are known per se in the field of etching metallic conductive materials. Other example processes may include coining or embossing.

[0038] It should be noted that the thinned portion thickness T may extend over most of the leadframe structure, including all of the leadframe structure except for the surface mount connection terminals 13, 16 of the primary and secondary conductors.

[0039] In an embodiment, the surface-mounted connection terminal 13 of the primary conductor may advantageously include a plurality of raised portions 21, which may be embossed, for example, by stamping the sheet metal or may be formed in an etching process from the base thickness of the sheet metal. The raised portions 21 advantageously improve the solder connection to the complementary circuit traces on the external circuit board and improve the mechanical and electrical connection between the primary conductor and the circuit traces on the external circuit board. The improved connection may also improve heat transfer if the current transducer is used to measure high-intensity currents (relative to the size of the transducer). Nevertheless, in a variant, the connection portion 13 may include only a few or only one connection pad per connection surface.

[0040] The primary conductor 4 may further include an extension 23 extending from the surface mounted connection terminal 13, which acts as a heat sink and may optionally be configured for thermal coupling to an external heat sink. Also, the heat sink may simply be integrated into the current transformer to provide a large surface area for improved heat transfer from the primary conductor when large currents (relative to the size of the transformer) are flowing through the transformer.

[0041] [List of reference symbols used] Current transducer 1 Magnetic field sensor 2 (integrated circuit (IC) chip) First surface (active sensing surface) 9a Magnetic field sensing part 10 Connection terminal 11 Second face 9b Lead frame structure 3 Primary conductor 4 Sensing part 12 Slot 24 Inner edge 25 Surface mount connection terminal 13 Raised part 21 Secondary conductor 5 IC connection part 15 Surface mount connection terminal 16 First (bonding wire) surface 17a Second (die mounting) face 17b Base sheet part 18 Thin part 19 Bond wire connection recess 20 Etching recess Insulation sheet 8 Bond Wire 6 Overmolded Housing 7 Mounting side 27 Base sheet thickness B Thin part thickness T Recess thickness R

[0042] [Embodiment] (1) An open-loop current converter for surface mounting on an external circuit board, comprising: an integrated circuit (IC) chip (2) including a magnetic field sensing portion (10) and a connection terminal (11) on a first active surface (9a) of the IC chip; A lead frame structure (3) including a primary conductor (4) and a plurality of secondary conductors (5) including IC connection portions (15); a plurality of bond wires (6) interconnecting the IC connection portion (15) to the connection terminals (11) of the IC chip; an insulating overmolded housing (7) overmolded onto the IC chip, the bond wires, and a portion of the leadframe structure; Including, the leadframe structure being formed from sheet metal having a base sheet of thickness B; At least a portion of the leadframe structure has a thickness equal to the base sheet thickness B; an open loop current transformer, characterized in that the first active surface of the IC chip faces the lead frame structure, and at least a portion of the lead frame structure includes a thinned portion having a thickness T that is 20% to 80% of the base sheet thickness B, and the thinned portion having a thickness T includes the IC connection portion (15) of the secondary conductor (5). (2) the IC chip is mounted so as to overlap partially with the sensing portion (12) of the primary conductor; A current converter as described in embodiment 1, wherein an insulating sheet (8) mounted between the IC chip and the primary conductor is made of a material different from the material forming the insulating overmolded housing, and the connection terminal (11) of the IC chip is provided on a portion of the IC chip that does not overlap the primary conductor. (3) A current transformer as described in embodiment 2, wherein the insulating sheet extends beyond the portion of the IC chip that overlaps the primary conductor with a boundary line of non-zero width W to increase the electrical creepage distance between the primary conductor and the IC chip. (4) The current transformer of embodiment 3, wherein the insulating foil extends beyond an edge (25) of the primary conductor that also extends over the IC chip, increasing the creepage distance between the IC chip and the primary conductor. (5) A current transducer as described in embodiment 2, wherein the sensing portion (12) has a reduced current-carrying cross-sectional area compared to the surface-mounted connection terminal (13) of the primary conductor.

[0043] (6) A current transformer as described in embodiment 5, wherein the surface mounted connection terminal (13) of the primary conductor includes a plurality of raised portions (21). (7) A current transformer as described in embodiment 5, wherein the surface mounted connection terminals (16) of the secondary conductors (5) are arranged in a row along a first edge of the overmolded housing (7) and the surface mounted connection terminals (13) of the primary conductors are arranged adjacent to an opposite edge of the overmolded housing. (8) The current converter according to embodiment 1, wherein the thin portion thickness T is in the range of 30% to 70% of the base sheet thickness B. (9) The current converter according to embodiment 8, wherein the thin portion thickness T is in the range of 40% to 60% of the base sheet thickness B. (10) A current transformer as described in embodiment 5, wherein the primary conductor (4) includes one or more heat sink extensions (23) that function as a heat sink extending from the surface mounted connection terminal (13) thereof.

[0044] (11) The current transformer of embodiment 10, wherein the one or more heat sink extensions extend from the primary conductor surface mounted connection terminal (13) toward the surface mounted connection terminal (16) of the secondary conductor (5). (12) A current transformer as described in embodiment 10, wherein there are a pair of heat sink extensions, one for extending from each surface mounted connection terminal (13) of the primary conductor.

Claims

1. An open loop current transformer (1) for surface mounting on an external circuit board, comprising: an integrated circuit (IC) chip (2) including a magnetic field sensing portion (10) and a connection terminal (11) on a first active surface (9a) of the IC chip; a leadframe structure (3) including a primary conductor (4) and a plurality of secondary conductors (5) including IC connection portions (15); a plurality of bond wires (6) interconnecting the IC connection portion (15) to the connection terminals (11) of the IC chip; an insulating overmolded housing (7) overmolded onto the IC chip, the bond wires, and a portion of the leadframe structure; Including, The leadframe structure is formed from sheet metal having a base sheet of thickness B; At least a portion of the leadframe structure has a thickness equal to the base sheet thickness B; an open-loop current transformer, characterized in that the first active surface of the IC chip faces the leadframe structure, and at least a portion of the leadframe structure includes a thinned portion having a thickness T that is 20% to 80% of the base sheet thickness B, the thinned portion having a thickness T including the IC connection portion (15) of the secondary conductor (5).

2. The IC chip is mounted to overlap a sensing portion (12) of the primary conductor; 2. The current transformer of claim 1, wherein an insulating sheet (8) mounted between the IC chip and the primary conductor is made of a material different from the material forming the insulating overmolded housing, and the connection terminals (11) of the IC chip are provided on portions of the IC chip that do not overlap the primary conductor.

3. 3. The current transformer of claim 2, wherein the insulating sheet extends beyond the portion of the IC chip that overlaps the primary conductor with a perimeter of non-zero width W to increase the electrical creepage distance between the primary conductor and the IC chip.

4. 4. The current transformer of claim 3, wherein the insulating foil extends beyond an edge (25) of the primary conductor that also extends over the IC chip, increasing the creepage distance between the IC chip and the primary conductor.

5. 3. A current transformer as claimed in claim 2, wherein the sensing portion (12) has a reduced current-carrying cross-sectional area compared to the surface-mounted connection terminal (13) of the primary conductor.

6. 6. The current transformer of claim 5, wherein the surface mounted connection terminal (13) of the primary conductor includes a plurality of raised portions (21).

7. 6. The current transformer of claim 5, wherein the surface mounted connection terminals (16) of the secondary conductors (5) are arranged in a row along a first edge of the overmolded housing (7) and the surface mounted connection terminals (13) of the primary conductors are arranged adjacent to an opposite edge of the overmolded housing.

8. The current transformer of claim 1 , wherein the thinned portion thickness T is in the range of 30% to 70% of the base sheet thickness B.

9. The current transformer of claim 8 , wherein the thinned portion thickness T is in the range of 40% to 60% of the base sheet thickness B.

10. 6. A current transformer as claimed in claim 5, wherein the primary conductor (4) includes one or more heat sink extensions (23) that function as a heat sink extending from the surface mounted connection terminals (13) thereof.

11. 11. The current transformer of claim 10, wherein the one or more heat sink extensions extend from the primary conductor surface mounted connection terminal (13) towards the surface mounted connection terminal (16) of the secondary conductor (5).

12. 11. A current transformer as claimed in claim 10, wherein there is a pair of said heat sink extensions, one for extending from each surface mounted connection terminal (13) of said primary conductor.

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