Apparatus and method for detuning of RFID tag to regulate voltage

• US 6,229,443 B1
• Filed: 06/23/2000
• Issued: 05/08/2001
• Est. Priority Date: 06/23/2000
• Status: Expired due to Term

• Current Assignee: Smartrac Technology Fletcher, Inc. (Avery Dennison Corporation)
• Original Assignee: Single Chip Systems Corporation (OEP Technologie BV)
• Inventors: Roesner, Bruce B.
• Primary Examiner(s): Hofsass, Jeffery
• Assistant Examiner(s): Nguyen, Phung
• Application Number:
  US09/602,280
• Time in Patent Office: 319 Days
• Field of Search: 340/572.1, 340/662, 340/663, 340/825.54, 361/56, 342/44, 343/5
• US Class Current: 340/572.1
• CPC Class Codes: G06K 19/0701 at least one of the integrated circuit chips comprising an arrangement for power management
• CPC Class Codes: G06K 19/0723 the record carrier comprising an arrangement for non-contact communication, e.g. wireless communication circuits on transponder cards, non-contact smart cards or RFIDs
• CPC Class Codes: H04B 5/26 using coils

Abstract

Overload protection is provided to the tuned receiving front end circuitry of an RFID tag by selectively detuning the tuned receiving front end in response to sensed voltage overload generated across the tuned receiving front end by a high field. The tuned receiving front end is or is modeled as a tuned parallel inductor and capacitor tank circuit. A parallel capacitance is added to the tank circuit by means of a control transistor which is controlled by a voltage sensing circuit. The voltage sensing circuit senses the voltage being applied across the tank circuit and turns on the control transistor to a degree to sufficiently detune the tank circuit so that the operating voltage across the tank circuit remains at or below a desired operating value or safe limit regardless of the value of the field in which the RFID tag is placed.

Claims

1. An improvement in a circuit for avoiding overload of RFID tags in high fields comprising:

a tuned receiving circuit;

a voltage sensing circuit coupled to said tuned receiving circuit to sense voltage developed across said tuned receiving circuit from said high fields;

a transistor coupled to and controlled by said voltage sensing circuit; and

a circuit element selectively and controllable coupled by said transistor to said tuned receiving circuit to detune said tuned receiving circuit so that overload of said RFID tag in high fields is avoided.

2. The improvement of claim 1 where said circuit element is a capacitor.

3. The improvement of claim 1 where said receiving tuned circuit comprises an inductor and capacitor in parallel circuit and where said circuit element is a capacitor.

4. The improvement of claim 1 where said capacitor coupled by said transistor to said tuned circuit has a capacitance of the same order of magnitude as said capacitor in said tuned circuit.

5. The improvement of claim 1 where said capacitor coupled by said transistor to said tuned circuit has a capacitance approximately equal to that of said capacitor in said tuned circuit.

6. The improvement of claim 1 where said transistor is capable of being turned on gradually by said voltage sensing circuit.

7. A method for avoiding overload of RFID tags in high fields comprising:

providing a tuned receiving circuit;

sensing a voltage generated across said tuned receiving circuit from said high fields; and

selectively detuning said tuned receiving circuit in response to said sensed voltage so that overload of said RFID tag in high fields is avoided.

8. The method of claim 7 where said tuned receiving circuit comprises a inductor and capacitor in parallel and where selectively detuning said tuned receiving circuit in response to said sensed voltage comprises increasing capacitance in said tuned receiving circuit.

9. The method of claim 8 where increasing capacitance in said tuned receiving circuit comprises coupling a detuning capacitor in parallel to said capacitance in said tuned receiving circuit.

10. The method of claim 9 where coupling said detuning capacitor in parallel to said capacitance in said tuned receiving circuit comprises gradually turning on and off a transistor in series with said capacitor.

11. The method of claim 8 where increasing capacitance in said tuned receiving circuit comprises increasing said capacitance in said tuned receiving circuit by a factor of approximately two.

12. The method of claim 7 where selectively detuning said tuned receiving circuit in response to said sensed voltage detunes said tuned receiving circuit by a degree to keep said sensed voltage across said tuned receiving circuit at a predetermined operating voltage or less.

13. The method of claim 7 where selectively detuning said tuned receiving circuit in response to said sensed voltage detunes said tuned receiving circuit by a degree to keep said sensed voltage across said tuned receiving circuit at five volts or less.

14. The method of claim 12 where keeping said sensed voltage across said tuned receiving circuit at a predetermined operating voltage or less comprises detuning said tuned receiving circuit to a degree in proportion to how much said sensed voltage across said tuned receiving circuit is different than said predetermined operating voltage.

15. The method of claim 14 where said tuned receiving circuit is comprises of an inductor and capacitor in parallel and where detuning said tuned receiving circuit comprises coupling a capacitance in parallel to said inductor by means of a series coupled transistor which is gradually turned on and off.