Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2000-06-23
2001-05-08
Hofsass, Jeffery (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S662000, C340S663000, C340S870030, C361S056000
Reexamination Certificate
active
06229443
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of radiofrequency identification (RFID) tags and in particular to protecting RFID tags against high field burn out or overload.
2. Description of the Prior Art
In the normal operation of an RFID label, a voltage is generated in a field by applying an AC signal to a tuned transmitting antenna in a reader (or coil for low frequency operation). An RFID tag receives the power through a tuned antenna (or coil) when placed in the proximity of the generated field. When the tag (receiver) is placed in close proximity to a reader (transmitter), the tag can receive so much power that the voltage on the RFID chip must be limited so as not to damage the RFID chip. Normally, this is accomplished by placing a DC regulator on the RFID chip. The regulator must be of sufficiently low impedance so as to control the voltage even when the power supplied from the reader might generate many volts.
For high frequency operation utilizing E field transmission, the voltages supplied to a chip might reach 15 or 20 volts while the chip is trying to regulate below 5 volts. Unfortunately, for low frequencies where coils are used to transmit energy utilizing H fields, the voltage on the front end of the chip can easily reach voltages in excess of 50 volts. Regulation by limiting the voltage to only a few volts can cause severe problems. For a typical coil impedance of 200 ohms and regulation at 5 volts means that the power sustained on the chip becomes greater than 1 watt. Without any heat sinking capabilities, a small RFID chip will literally burn up.
There are a few techniques which one can implement to minimize the problem. First, the chip can allow for higher voltage operation without drawing excessive currents. Unfortunately, more state of the art integrated circuit technologies allow only very low voltage operation. Placing a low current limited in series with the main circuitry of the chip is a possibility, but once again an active device such as a transistor cannot be built into the chip and sustain the high voltages. The other option is to place a passive device such as a resistor in series with the chip. However, operating the chip at low voltages (long read distances) becomes quite difficult if not impossible.
What is needed is an apparatus and method which overcomes the inadequacies of the prior art solutions.
BRIEF SUMMARY OF THE INVENTION
The invention is an improvement in a circuit for avoiding overload of RFID tags in high fields comprising a tuned receiving circuit, and a voltage sensing circuit coupled to the tuned receiving circuit to sense voltage developed across the tuned receiving circuit from the high fields. A transistor is coupled to and controlled by the voltage sensing circuit. A circuit element is selectively and controllable coupled by the transistor to the tuned receiving circuit to detune the tuned receiving circuit so that overload of the RFID tag in high fields is avoided.
In the illustrated embodiment the circuit element is a capacitor, particularly where the receiving tuned circuit comprises an inductor and capacitor in parallel circuit. However, it must be understood that any circuit element which is capable of or useful for detuning the tuned circuit may be substituted. The capacitor, which is coupled by the transistor to the tuned circuit, has a capacitance of the same order of magnitude as the capacitor in the tuned circuit. In particular, the capacitor has a capacitance approximately equal to that of the capacitor in the tuned circuit. The transistor is capable of being turned on gradually by the voltage sensing circuit.
The invention is also characterized as a method for avoiding overload of RFID tags in high fields comprising the steps of providing a tuned receiving circuit, and sensing a voltage generated across the tuned receiving circuit from the high fields. The tuned receiving circuit is selectively detuned in response to the sensed voltage so that overload of the RFID tag in high fields is avoided.
The step of selectively detuning the tuned receiving circuit in response to the sensed voltage comprises the step of increasing capacitance in the tuned receiving circuit. In the illustrated embodiment this step of increasing capacitance in the tuned receiving circuit comprises the step of coupling a detuning capacitor in parallel to the capacitance in the tuned receiving circuit. It must be understood, however, that many other equivalent methods of detuning the RFID tag could be substituted for the illustrated embodiment.
In the illustrated embodiment the step of coupling the detuning capacitor in parallel to the capacitance in the tuned receiving circuit comprises the step of gradually turning on and off a transistor in series with the capacitor. In particular, the step of increasing capacitance in the tuned receiving circuit comprises increasing the capacitance in the tuned receiving circuit by a factor of approximately two.
The step of selectively detuning the tuned receiving circuit in response to the sensed voltage detunes the tuned receiving circuit by a degree to keep the sensed voltage across the tuned receiving circuit at a predetermined operating voltage or less, which in the illustrated embodiment is five volts or less. In general the step of keeping the sensed voltage across the tuned receiving circuit at a predetermined operating voltage or less comprises detuning the tuned receiving circuit to a degree in proportion to how much the sensed voltage across the tuned receiving circuit is different than the predetermined operating voltage. In the case where the tuned receiving circuit is comprised of an inductor and capacitor in parallel, the step of detuning the tuned receiving circuit comprises the step of coupling a capacitance in parallel to the inductor by means of a series coupled transistor which is gradually turned on and off.
Although the illustrated invention has been briefly summarized as a method of steps, it is to be expressly understood that such reference is made only for the purpose of grammatical ease of expression and that the invention is not limited by the “step limitations” of 35 USC 112. The invention can be better visualized by turning to the following drawing.
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Dawes, Esq. Daniel L.
Hofsass Jeffery
Myers Dawes & Andras LLP
Nguyen Phung
Single Chip Systems
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