Tunable inductor

• US 7,202,768 B1
• Filed: 12/10/2004
• Issued: 04/10/2007
• Est. Priority Date: 12/10/2003
• Status: Active Grant

• Current Assignee: DSP Group Incorporated (Synaptics Incorporated)
• Original Assignee: DSP Group Incorporated (Synaptics Incorporated)
• Inventors: Rawat, Prashant, Harvey, Jackson
• Primary Examiner(s): Enad; Elvin
• Assistant Examiner(s): BAISA, JOSELITO SASIS
• Application Number:
  US11/009,512
• Time in Patent Office: 851 Days
• Field of Search: 336/232
• US Class Current: 336/232
• CPC Class Codes: H01F 2017/0053 with means to reduce eddy currents
• CPC Class Codes: H01F 21/08 by varying the permeability of the core, e.g. by varying magnetic bias
• CPC Class Codes: H01F 21/12 discontinuously variable, e.g. tapped

Abstract

In general, the invention is directed to a tunable inductor that makes use of eddy current effect to tune the inductance of an inductor. The tunable inductor may include a spiral or helical inductor in proximity to one or more sets of eddy current coils. Each eddy current coil may be coupled to a corresponding switch that controls whether the eddy current coil is grounded or floating. In operation, a first time-varying current through the inductor induces a first magnetic field that, in turn, induces a time-varying voltage in an eddy current coil. If the eddy current coil is not grounded, an eddy current flows through the eddy current coil. The eddy current, which flows in the opposite direction of the first time-varying current, induces a second magnetic field. The second magnetic field, which opposes the first magnetic field, reduces the inductance of the tunable inductor.

Claims

1. A tunable inductor comprising:

an inductor that induces a magnetic field in response to current flowing through the inductor;

a first eddy current coil in proximity to the inductor;

a first switch to couple the first eddy current coil to ground to prevent eddy current from flowing within the first eddy current coil in response to the magnetic field, and to decouple the first eddy current coil from ground to permit the eddy current to flow within the first eddy current coil in response to the magnetic field, wherein the first eddy current reduces an effective inductance of the inductor

a second eddy current coil in proximity to the inductor, wherein the first eddy current coil and the second eddy current coil are on opposite sides of the inductor;

a second switch to couple the second eddy current coil to ground to prevent eddy current from flowing within the second eddy current coil in response to the magnetic field, and to decouple the second eddy current coil from ground to permit the eddy current to flow within the second eddy current coil in response to the magnetic field, wherein the eddy current flowing in the second eddy current coil reduces an effective inductance of the inductor.

2. The tunable inductor of claim 1, wherein the first switch includes a NMOS transistor.

3. The tunable inductor of claim 1, wherein the first switch includes a microelectromechanical switch.

4. The tunable inductor of claim 1, wherein the inductor is a spiral inductor.

5. The tunable inductor of claim 1, wherein the inductor is a helical inductor.

6. The tunable inductor of claim 1, wherein the inductor, the first eddy current coil, and the second eddy current coil reside on different planes, wherein the planes of the first eddy current coil and the second eddy current coil are parallel with the plane of the inductor.

7. The tunable inductor of claim 1, further comprising a controller to control the first and second switches to selectively couple and decouple the first and second eddy current coils relative to ground to adjust the effective inductance of the inductor.

8. The tunable inductor of claim 1, wherein the first eddy coil includes a conductive ring.

9. A method for tuning an inductor, wherein the inductor induces a magnetic field in response to current flowing through the inductor, the method comprising:

coupling a first eddy current coil to ground to prevent eddy current from flowing within the first eddy current coil in response to the magnetic field;

decoupling the eddy current coil from ground to permit the eddy current to flow within the eddy current coil in response to the magnetic field, wherein the eddy current coil is in proximity to the inductor, and the eddy current reduces an effective inductance of the inductor;

coupling a second eddy current coil to ground to prevent eddy current from flowing within the eddy current coil in response to the magnetic field; and

decoupling the second eddy current coil from ground to permit the eddy current to flow within the second eddy current coil in response to the magnetic field, wherein the second eddy current coil is in proximity to the inductor, the first eddy current coil and the second eddy current coil are on opposite sides of the inductor, and the eddy current flowing in the second eddy current coil reduces an effective inductance of the inductor.

10. The method of claim 9, wherein the inductor is a spiral inductor.

11. The method of claim 9, wherein the inductor is a helical inductor.

12. The method of claim 9, wherein the inductor, the first eddy current coil, and the second eddy current coil reside on different planes, wherein the planes of the first eddy current coil and the second eddy current coil are parallel with the plane of the inductor.

13. The method of claim 9, further comprising:

coupling a third eddy current coil to ground to prevent eddy current from flowing within the third eddy current coil in response to the magnetic field; and

decoupling the third eddy current coil from ground to permit the eddy current to flow within the third eddy current coil in response to the magnetic field,

wherein the third eddy current coil is in proximity to the inductor, and the eddy current flowing in the third eddy current coil reduces an effective inductance of the inductor.

14. The method of claim 9, wherein the first eddy coil includes a conductive ring.