WO1992017892A1

WO1992017892A1 - Inductor

Info

Publication number: WO1992017892A1
Application number: PCT/US1992/002784
Authority: WO
Inventors: Anand K. Upadhyay
Original assignee: Motorola Lighting, Inc.
Priority date: 1991-04-01
Filing date: 1992-04-01
Publication date: 1992-10-15

Abstract

An inductor (2) having a 'double-E' core (4, 6) with an air gap (16) between its center limbs. A bobbin (8) has a hollow interior in which the core's center limbs and gap are positioned, and around which a winding (12) is wound spanning the gap. The bobbin has a shoulder (10) in the region of the gap which spaces the winding from the core in that region. Fringing flux (20, 22) generated in the region of the gap is contained within the transformer and a reduction is achieved in the amount of fringing flux which intersects with the winding and generates eddy current heating and power losses. With the addition of a secondary winding (114), the inductor may form a transformer (102).

Description

INDUCTOR

Field of the Invention

This invention relates to inductors and particularly, though not exclusively, to inductors for use in electronic ballasts for driving gas discharge lamps, e.g. fluorescent lamps.

Background of the Invention

Typically, in an electronic ballast for driving fluorescent lamps, an inductor is used as part of an LC resonant oscillator tank circuit. An inductor is also typically used in a transformer coupling the ballast input to a utility mains electrical supply, and a further transformer is typically used to couple the ballast output to the lamps. Commonly, an additional transformer is used to couple a portion of the ballast output back to an inverter within the ballast.

Typically, the inductors and transformers used have a double "E" core, i.e. two core portions each having an "E" shape with two parallel outer limbs and a parallel center limb therebetween, the center limb and the outer limbs being joined by a single perpendicular limb. The inductor winding (and additionally in the case of a transformer a secondary winding) is wound helically on a hollow bobbin, and the two "E" shaped core portions have their center limbs inserted into the hollow bobbin from opposite ends until the ends of the parallel limbs of the core portions abut. The core and bobbin assembly thus form an inductor (or transformer) assembly in which flux generated by the winding is confined within the core, which forms a closed flux path.

However, because of the two-part nature of the core, there will always be gaps where the limbs of the two core portions abut . Indeed, the center limbs of the core may be deliberately designed to have a predetermined gap. These gaps will give rise to fringing of the flux, in which the flux fringes into laterally adjacent areas as it crosses the gap. This fringing of the flux, particularly at the gap between the center limbs of the core, causes the generation of eddy currents in the windings as the fringing flux lines intersect the conductive material of the windings. The generated eddy currents cause ohrnic heating in the windings, which causes an undesirable temperature rise and represents an unwanted power loss in the inductor/transformer.

In order to reduce this power loss it is known to use thin, multi-strand wires. However, this increases cost and reduces reliability.

Another proposed solution known from U.S. patent no. 4,887,061 is to make one core portion with much longer parallel limbs and the other portion with much shorter parallel limbs . In this way the transformer core is more nearly of the older "E-I" type and the gap between the center limbs of the transformer is moved from the center of the core to one end of the core where it is further away from the windings. Such a solution prevents the flux fringing from causing eddy current heating in the winding, but leaves the fringing flux uncontained, allowing the fringing flux to invade areas adjacent the transformer. Thus, the fringing flux, although no longer causing eddy current heating in the transformer windings, will generate eddy currents in conductive materials adjacent the transformer, e.g. the conductive tracks of a printed circuit board on which the transformer is mounted, or the conductive material of a housing. Such eddy currents may simply generate heat or worse may contribute to electrical noise in the system of which the transformer forms part.

Summary of the Invention

In accordance with the invention there is provided an inductor comprising: a core having first and second portions defining a gap therebetween, the gap giving rise thereat to fringing magnetic flux in use of the inductor; a conductive winding positioned around the core and extending from the first portion to the second portion across the gap, whereby to enhance containment of the fringing magnetic flux; and spacer means spacing the conductive winding from the core by a greater amount in the region of the gap than in adjacent regions, whereby to reduce eddy current generation from the fringing magnetic flux.

By spacing the conductive winding from the core in the region of the gap, intersection by the conductive winding of fringe flux around the gap is reduced, while still containing the fringe flux. In this way eddy current heating and power loss is reduced while still maintaining flux containment.

Brief Description of the Drawings

One inductor and one transformer in accordance with the present invention for use in an electronic ballast for driving fluorescent lamps will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 shows a cross-sectioned elevational view of the inductor; and

FIG. 2 shows a cross-sectioned elevational view of the transformer.

Pesc-rxptJ-on of he Pre erred Sibodiment

Referring now to FIG. 1, an inductor 2 for use in an electronic ballast (not shown) for driving fluorescent lamps (also not shown) has a two-part core constituted by core portions 4 and 6. The core portions 4 and 6 are made of E375 cores of ferrite material and are of identical ^WE" shape, each having three parallel limbs 4A, 4B & 4C and 6A, 6B & 60 respectively joined by a perpendicular limb 4D and 6D respectively. The outer limbs 4A & 40 and 6A & 60 are of the same length, and the center limb 4B, 6B of each core portion is slightly shorter than the outer limbs .

The inductor 2 also includes a hollow bobbin 8. For reasons which will be explained below, the bobbin 8 is formed with an circumferential shoulder 10 protruding radially outwardly by approximately 30mil around the center of the bobbin. The bobbin 8 has wound helically on it a winding 12 formed by 155 turns of AWG 27 gauge bifilar wire . The winding 12 is wound on the bobbin 8 using a conventional winding machine (not shown) .

The inductor 2 is assembled by inserting the center limbs 4B and 6B of the core portions .4 and 6 into opposite ends of the bobbin's hollow interior and pushing the core portions towards each other until the core portions' outer limbs 4A & 6A and 40 & 60 abut. With the outer limbs abutting, the center limbs (because of their shorter length) define an air gap 16 therebetween. The difference in lengths of the core portions' center and outer limbs is chosen so that the gap 16 has a width of approximately 80mil. The inductor winding 12 has an inductance of approximately 2.2mH. The inductor 2 is held with the core portions' outer limbs in abutment by adhesive or other suitable fixative (not shown) .

In use of the inductor 2 current in the winding 12 produces flux, which circulates in the core in the directions indicated by the arrows 18 and 20. As the flux in the center of the core crosses the air gap 16 between the center limbs 4B and 6B, it produces flux fringing which extends laterally away from the hollow interior of the bobbin as shown by the flux lines 20 and 22.

It will be appreciated that by providing the shoulder 10 in the bobbin 8, the winding 12 is spaced away from the core's center limbs 4B, 6B in the region adjacent the gap 16 where flux fringing is concentrated. Thus, little of the fringing flux 20, 22 intersects the conductive winding 12 and causes eddy current losses. It will also be appreciated that since the winding 12 extends from one side of the gap 16 to the other side, the fringing flux 20, 22 is contained within the winding and so is prevented from straying out of the inductor 2 where it could otherwise cause generation of heat or electrical noise.

In a practical example of the above described inductor 2, the temperature rise in operation was reduced to approximately 30°C, compared with a temperature rise of 70°C in an otherwise identical transformer not having the shoulder 10.

Referring now to FIG. 2,- a transformer 102 for use in an electronic ballast (not shown) for driving fluorescent lamps (also not shown) has a two-part core constituted by core portions 104 and 106. The core portions 104 and 106 are made of E375 cores of ferrite material and are of identical "E" shape, each having three parallel limbs 104A, 104B & 104C and 106A, 106B & 106C respectively joined by a perpendicular limb 104D and 106D respectively. The outer limbs 104A & 1040 and 106A & 1060 are of the same length, and the center limb 104B, 106B of each core portion is slightly shorter than the outer limbs.

The transformer 102 also includes a hollow bobbin 108. Like the bobbin 8, the bobbin 108 is formed with an circumferential shoulder 110 protruding radially outwardly by approximately 30ir.il around the center of the bobbin. The bobbin 108 has wound helically on it a primary winding 112 formed by 155 turns of AWG 27 gauge bifilar wire. A secondary winding 114 is wound on the same bobbin 108 around the primary winding 110. The primary and secondary windings 112 and 114 are wound on the bobbin 108 using a conventional winding machine (not shown) .

Like the inductor 2, the transformer 102 is assembled by inserting the center limbs 104B and 106B of the core portions 104 and 106 into opposite ends of the bobbin's hollow interior and pushing the core portions towards each other until the core portions' outer limbs 104A & 106A and 104C & 106C abut. With the outer limbs abutting, the center limbs (because of their shorter length) define an air gap 116 therebetween. The difference in lengths of the core portions' center and outer limbs is chosen so that the gap 116 has a width of approximately 80n.il. The transformer primary winding 112 has an inductance of approximately 2.2mH. The transformer 102 is held with the core portions' outer limbs in abutment by adhesive or other suitable fixative (not shown) .

In use of the transformer 102 current in the primary winding 112 produces flux, which circulates in the core in the directions indicated by the arrows 118 and 120. As the flux in the center of the core crosses the air gap 116 between the center limbs 104B and 106B, it produces flux fringing which extends laterally away from the hollow interior of the bobbin as shown by the flux lines 120 and 122.

It will be appreciated that, as in the inductor 2, by providing the shoulder 110 in the bobbin 108, the primary winding 112 is spaced away from the core's center limbs 104B, 106B in the region adjacent the gap 116 where flux fringing is concentrated. Thus, little of the fringing flux 120, 122 intersects the conductive primary winding 112 and causes eddy current losses. It will also be appreciated that since the primary winding 112 extends from one side of the gap 116 to the other side, the fringing flux 120, 122 is contained within the winding and so is prevented from straying out of the transformer 102 where it could otherwise cause generation of heat or electrical noise.

It will be appreciated that although the invention has been described above in relation to an inductor and a transformer for use in an electronic ballast for driving fluorescent lamps, the invention can be applied to any core-gapped inductor/transformer in which the containment of flux-fringing and the reduction of its effects is desirable.

It will also be appreciated that various other modifications or alternatives to the above described embodiment will be apparent to the person skilled in the art without departing from the inventive concept of extending a winding in a core-gapped inductor across the gap and spacing the winding from the core in the region of the gap, so as to contain fringing flux around the gap and to reduce its heat generating effect in the winding.

Claims

1. An inductor comprising: a core having first and second portions defining a gap therebetween, the gap giving rise thereat to fringing magnetic flux in use of the inductor; a conductive winding positioned around the core and extending from the first portion to the second portion across the gap, whereby to enhance containment of the fringing magnetic flux; and spacer means spacing the winding from the core by a greater amount in the region of the gap than in adjacent regions, whereby to reduce eddy current generation from the fringing magnetic flux.

2. An inductor according to claim 1 wherein the core comprises first and second members each having first and second parallel limbs of substantially equal length, a shorter third parallel limb therebetween and a perpendicular limb joining the first, second and third parallel limbs, the first and second parallel limbs of the first member abutting respectively the first and second parallel limbs of the second member, whereby the the third parallel limbs form the first and second portions of the core and define the gap therebetween.

3. An inductor according to claim 1 or 2 further comprising a bobbin having a hollow interior in which the first and second portions of the core and the gap are positioned, the winding being wound on the bobbin.

4. An inductor according to claim 3 wherein the spacer means comprises a shoulder on the bobbin and extending away from the bobbin's hollow interior.

5. An inductor according to any preceding claim wherein the gap is approximately 80mil wide.

6. An inductor according to claim 4 wherein the shoulder is approximately 30m.il high..

7. An inductor according to any preceding claim wherein the winding has an inductance of approximately 2.2mH.

8. An inductor according to any preceding claim further comprising an additional conductive winding positioned around the core so as to form a transformer.

9. An inductor comprising: a core including first and second members each having first and second parallel limbs of substantially equal length, a shorter third parallel limb therebetween and a perpendicular limb joining the first, second and third parallel limbs, the first and second parallel limbs of the first member abutting respectively the first and second parallel limbs of the second member, whereby the third parallel limbs define a gap therebetween, the gap giving rise thereat to fringing magnetic flux in use of the inductor; a bobbin having a hollow interior in which the third parallel limbs of the core and the gap are positioned and having thereon a shoulder in the region of the gap and extending away therefrom; and a conductive winding positioned around the core and extending from one of the third parallel members to the other of the third parallel members across the gap, whereby the shoulder spaces the winding from - li ¬ the core by a greater amount in the region of the gap than in adjacent regions, thereby to reduce eddy current generation from the fringing magnetic flux, and the extending of the conductive winding from one of the third parallel members to the other of the third parallel members across the gap enhances containment of the fringing magnetic flux.

10. A transformer comprising: a core including first and second members each having first and second parallel limbs of substantially equal length, a shorter third parallel limb therebetween and a perpendicular limb joining the first, second and third parallel limbs, the first and second parallel limbs of the first member abutting respectively the first and second parallel limbs of the second member, whereby the third parallel limbs define a gap therebetween, the gap giving rise thereat to fringing magnetic flux in use of the transformer; a bobbin having a hollow interior in which the third parallel limbs of the core and the gap are positioned and having thereon a shoulder in the region of the gap and extending away therefrom; and a first conductive winding positioned around the core and extending from one of the third parallel members to the other of the third parallel members across the gap, whereby the shoulder spaces the first conductive winding from the core by a greater amount in the region of the gap than in adjacent regions, thereby to reduce eddy current generation from the fringing magnetic flux, and the extending of the conductive winding from one of the third parallel members to the other of the third parallel members across the gap enhances containment of the fringing magnetic flux; and a second conductive winding positioned around the core.