Planar transformer assembly including non-overlapping primary and secondary windings surrounding a common magnetic flux path area

• US 5,598,327 A
• Filed: 01/28/1994
• Issued: 01/28/1997
• Est. Priority Date: 11/30/1990
• Status: Expired due to Term

• Current Assignee: Burr-Brown Corporation (Texas Instruments, Inc.)
• Original Assignee: Burr-Brown Corporation (Texas Instruments, Inc.)
• Inventors: Somerville, Thomas A., Meinel, Walter B., Stitt, R. Mark II
• Primary Examiner(s): Sterrett, Jeffrey L.
• Application Number:
  US08/188,159
• Time in Patent Office: 1,096 Days
• Field of Search: 336/200, 336/212, 336/232, 336/83, 336/96, 29/602.1, 29/606, 363/121
• US Class Current: 363/131
• CPC Class Codes: H01F 17/0006 Printed inductances printed coils for dynamo-electric machines H02K3/26; printed circuits H05K
• CPC Class Codes: H01F 19/04 Transformers or mutual inductances suitable for handling frequencies considerably beyond the audio range resonant circuits H03H
• CPC Class Codes: H02J 2207/20 Charging or discharging characterised by the power electronics converter
• CPC Class Codes: H02J 7/007184 in response to battery voltage gradient
• CPC Class Codes: H02J 7/02 for charging batteries from ac mains by converters
• CPC Class Codes: H02M 3/33507 with automatic control of the output voltage or current, e.g. flyback converters H02M3/33561, H02M3/33569 take precedence
• CPC Class Codes: H04M 19/08 with current supply sources at the substations generating ringing current H04M19/04
• CPC Class Codes: H05K 1/165 incorporating printed inductors
• CPC Class Codes: H05K 2201/086 for inductive purposes, e.g. printed inductor with ferrite core
• CPC Class Codes: H05K 2201/09063 Holes or slots in insulating substrate not used for electrical connections
• CPC Class Codes: Y10T 29/49073 by assembling coil and core

Abstract

A planar transformer assembly includes an insulative layer, a first spiral winding thereon circumscribing a magnetic flux path, a second spiral winding thereon in non-overlapping relation to the first spiral winding circumscribing the magnetic flux path, and a ferrite core assembly including first and second core sections defining a shallow gap or passage within which the spiral windings are disposed. In one embodiment, a plurality of laminated insulative layers are provided with a primary winding including a plurality of series-connected spiral subwindings and a non-overlapping secondary winding formed on the various insulative layers. The non-overlapping structure and the order of the various windings minimize electric field gradients and thereby minimize electric field coupled noise currents.

Claims

1. A planar transformer assembly comprising in combination:

(a) an insulative layer having a top surface and a bottom surface;

(b) a first spiral winding on the top surface, surrounding a magnetic flux path area;

(c) a second spiral winding on the bottom surface, surrounding the magnetic flux path area, concentric with the first spiral winding, in non-overlapping relation to the first spiral winding;

(d) a ferrite core assembly including

i. a first core section including a thin, flat first ferrite plate disposed on the top surface and subtending the first and second spiral windings, and

ii. a second core section including a thin, flat second ferrite plate disposed on the bottom surface and subtending the first and second spiral windings.

2. The planar transformer assembly of claim 1 wherein the insulative layer includes a plurality of insulative sublayers, the first spiral winding being disposed on a first insulative sublayer, the second spiral winding being disposed on a second insulative sublayer, the planar transformer assembly including a conductive shield disposed between the first and second insulative sublayers and surrounding the magnetic flux path area and generally coextensive with one of the first and second spiral windings to reduce capacitive coupling between the first and second spiral windings and thereby reduce noise generation during operation of the planar transformer assembly.

3. The planar transformer assembly of claim 2 wherein the guard ring includes a gap to interrupt field coupled currents in the guard ring, and wherein the guard ring is coupled through a low impedance to a fixed voltage.

4. The planar transformer assembly of claim 1 wherein a portion of the insulative layer extends beyond the first and second core sections.

5. The planar transformer assembly of claim 4 including adhesive attaching the first core section to the top surface and adhesive attaching the second core section to the bottom surface.

6. The planar transformer assembly of claim 4 including an elongated first ferrite spacer section separating first ends of the first and second ferrite plates, and an elongated second ferrite spacer section separating second ends of the first and second ferrite plates, and material securing the first and second ferrite plates in fixed relation to the insulative layer.

7. The planar transformer assembly of claim 6 wherein the material includes an elastic clip engaging the first and second core sections to clamp them together.

8. The planar transformer assembly of claim 6 wherein the first ferrite spacer section includes abutting first and second parts integral with the first ends of the first and second ferrite plates, respectively, and the second ferrite spacer section includes abutting third and fourth parts integral with the second ends of the first and second ferrite plates, respectively.

9. The planar transformer assembly of claim 8 wherein the insulative layer includes a plurality of laminated insulative sublayers, wherein the first spiral winding is a subwinding of a primary winding including a plurality of series-connected spiral subwindings formed on various surfaces, respectively, of the various insulative sublayers, the spiral subwindings being connected in series by conductive vias passing through the various insulative sublayers.

10. The planar transformer assembly of claim 9 wherein the second spiral winding is a secondary winding all turns of which surround a region within which all turns of the subwindings of the primary windings are disposed.

11. The planar transformer assembly of claim 10 wherein the sublayers are laminated together to form a multilayer printed circuit board, the planar transformer assembly including a primary current supply circuit disposed on the printed circuit board and connected to a first terminal of the primary winding, and also including a switching circuit disposed on the printed circuit board and connected to a second terminal of the primary winding.

12. The planar transformer assembly of claim 11 wherein the primary supply circuit supplies a relatively low first frequency rectified line voltage to the first terminal of the primary winding and the switching circuit couples and decouples the second terminal of the primary winding to a current return path at a relatively high second frequency equal to a resonant frequency of a primary winding circuit including a flyback capacitor, the second frequency being much greater than the first frequency.

13. The planar transformer assembly of claim 12 wherein the first frequency is approximately 60 hertz and the second frequency is approximately 500 kilohertz to one megahertz.

14. The planar transformer assembly of claim 12 wherein the information controls the switching circuit causing it to adjust the amount of time during which the second terminal of the primary winding is decoupled from the current return path in accordance with the amount of current loading of the secondary winding by the secondary load circuit so as to ensure zero-voltage switching of the primary winding despite changes in inductance of the primary winding caused by variations in the current loading of the secondary winding, thereby reducing noise generation of the planar transformer assembly.

15. The planar transformer assembly of claim 11 wherein the first terminal of the primary winding is connected to a first subwinding thereof, and wherein the secondary winding is disposed on a top insulative sublayer adjacent to another insulative sublayer on which the first subwinding is disposed, and wherein the second terminal of the primary winding is connected to a second subwinding thereof disposed on a bottom insulative sublayer, whereby the secondary winding is substantially separated from the second subwinding, to thereby reduce electrical field gradients and resulting electromagnetic radiation in the planar transformer assembly.

16. The planar transformer assembly of claim 15 wherein the primary winding has a predetermined number of turns in order to achieve a predetermined inductance, and wherein the number of subwindings is selected to allow a diameter of the primary winding to be small enough that all of the predetermined number of turns can be contained within the region surrounded by the turns of the secondary winding.

17. The planar transformer assembly of claim 15 including first and second isolation barrier capacitors adapted to couple information between a secondary load circuit coupled to a terminal of the secondary winding and an input of the switching circuit, wherein conductive plates of each of the first and second isolation barrier capacitors and electrical conductors connected thereto are formed on various laminated insulative layers outside of a magnetic flux path area surrounded by the primary and secondary windings.

18. A method of achieving reduced noise generation in a planar transformer assembly including a multilayer printed circuit board having a plurality of insulative layers, the method comprising the steps of:

(a) forming a secondary winding by forming a first spiral winding surrounding a flux path area on a surface of a first insulative layer,

(b) forming a primary winding having first and second terminals by forming a plurality of subwindings on a plurality of insulative layers, respectively, and connecting the subwindings in series by means of conductive vias, the first terminal being adapted to receive a supply line voltage, the second terminal being adapted to be coupled to a switching circuit, all turns of the primary winding being contained in a region surrounded by the secondary winding, whereby the primary winding and secondary winding are non-overlapping and thereby reduce capacitive coupling and thereby reduce harmonic noise generation during operation of the planar transformer assembly.

19. The method of claim 18 wherein the secondary winding has a predetermined secondary winding inductance, the method including selecting the number of subwindings so that a predetermined number of primary winding turns necessary to produce a predetermined primary winding inductance all can be included in the region surrounded by the secondary winding.

20. The method of claim 18 wherein the first insulative layer is adjacent to a second insulative layer, the method including forming one of the subwindings connected to the first terminal on the second insulative layer, and forming another of the subwindings connected to the second terminal on a third insulative layer, the first and third insulative layer being adjacent to opposite faces of the printed circuit board to thereby separate the secondary winding as much as possible from the second terminal, whereby voltage gradients between the primary and secondary windings are reduced and therefore generation of noise due to such gradients is reduced.

21. The method of claim 18 including supplying the line voltage as a rectified line voltage at a relatively low first frequency and operating the switching circuit to couple and decouple the second terminal to a current return path at a relatively high second frequency approximately equal to a resonant frequency of a primary winding circuit to provide improved transformer efficiency and further reduced harmonic noise generation.

22. The method of claim 21 including measuring information indicative of an amount of current loading of the secondary winding, coupling that information to the switching circuit, and operating the switching circuit in response to the information to adjust an amount of time during which the second terminal of the primary winding is decoupled from the current return path in accordance with the amount of current loading of the secondary winding so as to ensure zero-voltage switching of the primary winding despite changes in inductance of the primary winding due to variations in the current loading of the secondary winding, thereby reducing noise generation of the planar transformer assembly.