Space saving surface-mounted inductors

An inductive element and corresponding method for making an inductive element for surface mounting on an adjacent structure for providing improved heat transfer characteristics. Specifically, a core and winding of the inductive element define coplanar surfaces that can then be mated to an adjacent structure, preferably a printed circuit board. Devices such as inductors or transformers including the inductive element have multiple, low thermal resistance conductive paths for removing heat from the core, and thereby enhance the heat transfer characteristics of the devices. The inductive element is particularly well suited for power electronics, such as for use as a power choke or as part of a power transformer.

FIELD OF THE INVENTION

The present invention relates to structures and methods for augmenting space utilization and heat transfer in electronic devices and, in particular, to structures and methods that provide configurations of a core and winding so as to improve both space utilization and heat flow from a surface mounted inductor formed from these components.

BACKGROUND OF THE INVENTION

The performance, reliability and lifetime of electronic circuits are affected by the temperature of the various circuit components. Power electronics, in particular, usually have one or more components that generate large amounts of heat, and thus may require heat transfer augmentation structures to dissipate this heat and thereby maintain acceptable operating temperatures. Constraints on the maximum size of a circuit can further increase the difficulty of removing heat, making thermal management an important aspect of power electronics design.

A particularly difficult problem is the removal of heat from printed circuit board (PCB) mounted inductors and transformers. The operation of the circuit results in the generation of large amounts of heat within the core and a resulting increase in core temperature. Due to the desirability of having the inductor occupy a small surface area of the PCB, and the normal inductor geometry of having a core surrounded by a winding, it can be difficult to transfer heat from the core to the surrounding environment.

More specifically, prior art transformers, and in particular surface-mounted transformers for power electronics, typically include windings around torroidal cores. The core in such a transformer is surrounded by the winding wires, which wrap around a significant portion, if not the entire outer surface of the torroid. Heat within the core can therefore only be removed by conduction through the wires of the winding to an adjacent heat sink. The torroidal core also wastes space because it includes a large unoccupied hole in its center.

What is needed is an improved surface-mountable design that provides for improved space utilization as well as enhanced heat transfer from the core of a transformer or inductor to a heat sink. Such an inductor or transformer should have a small footprint, be efficient, inexpensive, and compatible with conventional surface mount technology, such as enabling reflow soldering of the inductor or transformer to a PCB.

SUMMARY OF THE INVENTION

The present invention solves the above-identified problems of known surface mountable inductors and transformers by providing a winding and core structure that fully occupies available space and provides for coplanar contact of both the core ends and the winding with an adjacent heat sink.

In a preferred embodiment of the present invention, a surface mountable inductive element comprises a magnetic core having a central elongated portion and two end portions, each said end portion defining an end portion planar surface, and a winding wound about said elongated portion, where the outer surface of said winding defines a planar surface that is coplanar with each said end portion planar surface so as to facilitate surface mounting of said magnetic core and winding on an adjacent structure.

According to another aspect of the present invention, the inductive element comprises a first magnetic core having a central elongated portion and two end portions, each said end portion defining an end portion planar surface, a first winding wound about said elongated portion, where the outer surface of said first winding defines a planar surface that is coplanar with each said end portion planar surface, and a mounting frame to secure the wire ends of said first winding and to enable said first core and first winding to be surface mounted on an adjacent structure such that a portion of the planar surface of said adjacent structure is in contact with said end portion planar surfaces and said first winding planar surface to enhance heat transfer to said adjacent structure from said inductive element. The inductive element preferably also includes a second magnetic core having an elongated portion and two end portions, each said end portion of said second core defining an end portion planar surface, a second winding wound about said elongated portion of said second core, where the outer surface of said second winding defines a planar surface that is coplanar with each said second core end portion planar surface, and wherein said mounting frame further secures the wire ends of said second winding and is shaped to affix said first core end portions against corresponding second core end portions and such that the coplanar surfaces of said first core end portions and said first winding are coplanar with the coplanar surfaces of said second core end portions and said second winding.

According to yet another aspect of the present invention, a transformer comprises first and second magnetic cores each having a central elongated portion and two end portions, each said end portion defining an end portion planar surface, a winding about each said elongated portion of said two magnetic cores, where the outer surface of each winding defines a planar surface that is coplanar with each said end portion planar surface of its respective core, and a material for affixing the end portions of said first and second magnetic cores together such that the end portion planar surface of each end portion of said first core is coplanar with the end portion planar surface of each end portion of said second core. The transformer preferably also includes a mounting frame surrounding said first and second cores, wherein said mounting frame secures the wire ends of each said winding and enables said first and second core to be surface mounted on an adjacent structure such that a portion of the planar surface of said adjacent structure is in contact with said end portion planar surfaces and the planar surfaces of each said winding to enhance heat transfer to said adjacent structure from said transformer.

In a preferred embodiment, the adjacent structure on which the inductor or transformer according to the present invention is mounted is a printed circuit board.

It is yet another aspect of the present invention to provide a method of forming an inductive element from a core having a central elongated portion and two end portions for surface mounting to an adjacent structure, e.g., the surface of a printed circuit board or the like. The method includes the steps of forming a winding of wire about the central elongated portion of the core to create a surface on said winding that is coplanar with a surface on each end of said core, and mounting said core in a mounting frame and securing the wire ends of said winding to posts on said mounting frame.

It is another aspect of the present invention to provide an inductive element having a small footprint with improved heat transfer from the transformer core to an adjacent structure, thereby enabling lower temperature rated materials to be used that in the past for the same applications.

A further understanding of the invention can be had from the detailed discussion of the specific embodiment below. For purposes of clarity, this discussion refers to devices, methods, and concepts in terms of specific examples. However, the method of the present invention may be used to connect a wide variety of types of devices. It is therefore intended that the invention not be limited by the discussion of specific embodiments.

Reference symbols are used in the Figures to indicate certain components, aspects or features shown therein, with reference symbols common to more than one Figure indicating like components, aspects or features shown therein.

DETAILED DESCRIPTION OF THE INVENTION

To facilitate its description, the invention is described below in terms of a core and winding design for a surface-mounted inductive element, such as a PCB mountable inductor, power choke, or transformer. In general, the present invention is a core and winding configuration that facilitates removal of heat from the core to the exterior of the inductive element by creating a second heat transfer path from the core to the external environment. The device permits the coplanar mounting of the core and the winding to the PCB so that heat in the core can be conducted to the PCB through both the core ends and the winding. It is understood that the inventive device can be used to improve the heat flow from inductors and transformers in general and is particularly useful in removing heat from power converter transformers mounted on printed circuit boards or the like. The scope of the invention is not limited by the following embodiments.

The present invention will now be described in more detail with reference to the Figures.FIGS. 1–3are several views of an inductive element100structured as a transformer or power choke, according to the present invention, having a surface101for surface mounting the inductive element on a printed circuit board (PCB)10, whereFIG. 1is a perspective view of the inductive element mounted the surface of the PCB,FIGS. 2A and 2Bare bottom and side views of inductive element100, respectively, andFIG. 3is a sectional view3—3. Power choke/transformer100has conventional electrical operating characteristics that are a function of its configuration, as is well known in the field of power electronics. In addition, the mounting and use of surface-mounted transformers is similarly well known in the field and will not be repeated here.

Inductive element100may include a single core having a single winding wound around it. Preferably, it includes a pair of matching cores110, individually denoted110aand110b, and a winding120about each core, indicated as winding120aand120b. More specifically, each core110has a pair of ends111and an elongated portion113that supports the corresponding winding120. Each winding120is bounded on the inside by elongated portion113and defines an outer surface121, as shown inFIGS. 2 and 3. In addition, each end111has a surface115. Surfaces115a,115b,121a, and121bare approximately coplanar to define surface101. Inductive element100also has a surface103opposite surface101that may be coplanar with the surfaces of core ends111and windings120that are similarly opposite to surfaces115and121, respectively.

Inductive element100also includes a mounting frame130having posts131adjacent to surface101. Mounting frame130is preferably an insulating material, such as plastic. As shown more clearly inFIGS. 2 and 3, each winding120includes a conducting material, such as a metallic ribbon or wire, wound at least once about elongated portion113. In addition, windings120terminate in wires123at posts131. More specifically, each of wires123wraps about one of posts131and has an exposed portion near surface101to facilitate electrical connections between inductive element100and conductive traces (not shown) on PCB10.FIG. 2Cis an enlarged view of an exemplary post131having a wire123wrapped around it.

Surface101of inductive element100is mounted on PCB10by soldering posts131to the PCB, preferably by reflow soldering. As noted previously, surface115of core110and surface121of winding120are approximately coplanar and define surface101. A thin thermally conductive adhesive or paste140applied between surfaces115and121and the surface142of PCB10, provides enhanced thermal coupling between the core110and winding120and the PCB10. Heat within core110can thus conduct to PCB10either through windings120or through the ends111of core110.

In addition, a heat sink150may be provided on the side of PCB10opposite inductive element100. The use of heat sinks to remove heat and thus reduce the temperature of heat generating components is well known in the art. Heat sink150includes, but is not limited to, the illustrated heat sink, and may include fins, enlarged surfaces that extend beyond the footprint of the inductive element100, or may include active elements such as thermionic cooling or heat pipes, or any other configuration or devices that effectively promote the transfer of heat from the inductive element to the external environment.

The scope of the present invention is not limited to transformers or power chokes as shown inFIGS. 1–3. In general, the inventive inductive element is not limited to the size or number of windings on the individual cores, or to the number of cores. It is within the scope of the present invention to provide an inductive element having more than two cores110and windings120, or an inductor having only a single core and winding. In addition, it within the scope of the present invention, for multi winding inductive elements, to provide a post for attaching each end of each winding to a PCB via an appropriately sized and shaped mounting frame. Also, it is within the scope of the present invention to mount a heat sink on surface103of inductive element100.

A first embodiment of an inductive element according to the present invention is shown at400inFIGS. 4A–4C, whereFIG. 4Ais a top view,FIG. 4Bis a side view, andFIG. 4Cis sectional view4C—4C of a core410and a winding420.FIG. 4Calso shows a second core410b(in phantom next to a first core410a), arranged as an inductive element of the type shown at100inFIGS. 1 and 2. Each core410includes an elongated portion413of length L between a pair of ends411, and a wire420having ends423that are wrapped along length L.FIGS. 4A and 4Bshow elongated portion413in phantom through the center of winding420. Side viewFIG. 4Bshows coplanar core and winding surfaces, specifically a core surface415and a winding surface421.

FIG. 4Cshows a sectional view of core410aplaced side-by-side with core410bshown in phantom. Surface101of inductive element400includes the coplanar surface defined by surfaces415a,415b,421a, and421b.

Elongated portion413has dimensions, indicated as a width W and a depth D, that provide the required electromagnetic properties of windings420, as is known in the field. Core ends411protrude from elongated portion413by a height D resulting in ends having rectangular shapes with a depth E, width W+2D, and height H+2D. The elongated portion413has an approximately rectangular cross-sectional shape. The dimensions of ends411are selected to provide mounting surfaces for winding420. Specifically, winding420is wrapped along length L to occupy a thickness D, such that the wire fills in the space between the elongated portion413and ends411. Alternatively, winding420could have another shape that presents a pair of approximately planar surfaces, not necessarily parallel, for mounting to a PCB and, if desired, an additional heat sink.

A second embodiment of an inductive element according to the present invention is shown at500inFIGS. 5A–5C, whereFIG. 5Ais a top view,FIG. 5Bis a side view, andFIG. 5Cis sectional view5C—5C of a core510and a winding520.FIG. 5Calso shows a second core510b(in phantom next to a first core510a), arranged as an inductive element of the type shown at100inFIGS. 1 and 2. Each core510includes an elongated portion515between a pair of ends511, and a wire520having ends523that are wrapped along the elongated portion.FIGS. 5A and 5Bshow elongated portion515in phantom through the center of winding520. Side viewFIG. 5Bshows the coplanar core and winding surfaces, specifically a core surface513and a winding surface521.

FIG. 5Cshows a sectional view of core510aplaced side-by-side with core510bshown in phantom. Surface101of inductive element500includes the coplanar surface defined by surfaces513a,513b,521a, and521b.

Cores410include an elongated member413that terminates in the center of ends411. In contrast, cores510include an elongated member515that does not terminate in the center of end511. As is shown inFIG. 5C, winding520thus protrudes beyond the edge in that core's ends511, as seen at525. It is not necessary for each of the cores to have the same geometry. Thus, for example, an alternative embodiment for a two winding power choke could have one core410and one core510.

The inventive inductive element100,400,500differs from the prior art in that it provides two paths for heat conduction from the core to a heat sink.

FIG. 6is a schematic view of the present invention showing the flow of heat from the inductive element100. Heat core110is provided with two conductive paths to heat sink150according to the present invention as follows. The novel path, indicated by the arrows labeled Q1does not pass through winding120. Core110, as well as core400,500, has ends111that protrude from elongated portion113, which is the central portion of winding120. The flow of heat as indicated by arrows Q1is thus from elongated portion113, through ends111, and across to the surface142of PCB10(and optionally on to a heat sink150), where it can be transferred away from inductive element100. The second path is the conventional path, indicated by the arrows labeled Q2, that passes through winding120. Heat generated in elongated portion113is conducted through winding120, to PCB10(and again optionally to a heat sink150). The added paths for heat transfer, and in particular path Q1that bypasses the winding, greatly increase the amount of heat that can be removed from inductive element100, thereby enabling the ability to better control the temperature of the inductive element.

In addition to the alternative windings discussed herein, it would be apparent to one skilled in the art that other core configurations, within the scope of the present invention, can be provided to allow the conduction of heat from the core. The inventive inductive element greatly increases the heat flow by providing a significantly enhanced lower thermal resistance pathway from the core to the heat sink, as opposed to prior art inductive elements where the flow of heat is only across the wires of the winding to the heat sink. The improved heat transfer characteristics of the inventive inductive element also allows for a smaller PCB footprint that prior art inductive elements.

The invention has now been explained with regard to specific embodiments. Variations on these embodiments and other embodiments may be apparent to those of skill in the art. It is therefore intended that the invention not be limited by the discussion of specific embodiments. It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.