Patent Publication Number: US-2007108297-A1

Title: Adaptation of transponder card performance to available power

Description:
TECHNICAL FIELD  
      The present invention relates to high frequency electromagnetic transponders. More particularly, the invention relates to a device and method for detecting an amount of power transmitted and adjusting a performance level of a load device to correspond to a magnitude of available power received.  
     BACKGROUND ART  
      Smart card technology is a means of communication, detection, and power transmission between a sending unit and a receiving unit. Smart cards are generally considered as part of radio frequency identification (RFID) or electronic article surveillance (EAS) technologies. Radio frequency or microwave electromagnetic fields are generated by a reader (detector) and received by a transponder (i.e., a smart card). The combination of the reader and transponder may be used for identification tags, anti-theft devices, article surveillance, and security cards. Generally, smart card technology allows for the reader to monitor the transponder through electromagnetic coupling.  
      A coil and capacitor form a receiver section of the smart card. A coil and capacitor connected in parallel is commonly known as a “tank circuit.” When the tank circuit is energized, subsequent oscillations of current and electromagnetic fields result in an energy exchange back and forth between inductor and capacitor. Energizing may occur by exposing the tank circuit to an electromagnetic field at either radio or microwave frequencies. A particular value of inductance and capacitance allows the exchange of energy at a single frequency known as the resonant frequency. The resonant frequency of oscillation is given by the equation  
           f   Resonant     =     1     2   ⁢   π   ⁢     LC           ,       
 
 where L is a value of inductance of the coil and C is an amount of capacitance of the capacitor. 
 
      The reader generates an electromagnetic field with a generator frequency, f G . When the generator frequency, f G  equals the resonant frequency, f Resonant , of the transponder, the energy within the generated field is coupled to the transponder. The tank circuit of the transponder will develop a “sympathetic oscillation” at the generator frequency, f G .  
      With reference to  FIG. 1 , a coupling coil  112  and a tuning capacitor  114  are connected in parallel to form a receiver section  110  in a prior art circuit diagram  100 . An input of a rectifying section  120  is connected to the receiver section  110 . The rectifying section  120  is made from a full-wave rectifier bridge  125 . A regulating section  130  connects to an output of the rectifying section  120  and contains a shunt regulator  135  connected across the inputs to the regulating section  130 . A load section  140  connects to an output of the regulating section  130  and contains a smart card load  145  across the inputs of the load section  140 .  
      Values of inductance and capacitance for the coupling coil  112  and the tuning capacitor  114  respectively, are selected for an appropriate resonant frequency of operation. For optimal coupling of energy, the resonant frequency of the smart card corresponds to the generated frequency, f G , of a reader (detector). When the smart card is exposed to a generated field with a generator frequency, f G , that equals the resonant frequency, f Resonance , energy from the generated field is coupled to the receiver section  110 .  
      On each oscillation of the receiver section  110 , energy is supplied to the rectifying section  120 . A positive phase of oscillation produces a conduction path from a first terminal  116  of the receiver section  110  and continues on through a first branch  122  of the rectifier bridge  125 , the regulating section  130 , the load section  140 , a first return path branch  124  of the rectifier bridge  125 , and back to a second (complementary) terminal  118  of the receiver section  110 . A negative phase of oscillation produces a conduction path from the second terminal  118  of the receiver section  110  and continues on through a second branch  126  of the rectifier bridge  125 , the regulating section  130 , the load section  140 , of a second return path branch  128  of the rectifier bridge  125 , and back to the first terminal  116  of the receiver section  110 .  
      An amount of power available to supply a transponder load device varies proportionately with the strength of the electromagnetic field received. Where the power for a transponder load depends on the amount of power received from an electromagnetic field, coordinating available power and the performance of the load is highly desirable. Optimally, there would be a way for the transponder to detect the amount of power available from the generated field and adjust the performance of the load device to consume an amount of energy appropriate to the level of power received from the generated field. Additionally, the same power detection mechanism could be available to regulate the power supplied to the load to prevent saturation of the load device when ample available power exists.  
     SUMMARY  
      A smart card operates within an electromagnetic field produced at radio or microwave frequencies and rectifies the received field to provide power for operation of a load device. The field strength received by the smart card varies due to differing strengths of generation sources, proximity of the card to a source, and a presence of other cards in the field. The amount of transmitted power available to supply an operation of the smart card varies in proportion to the strength of the field received by the card. The smart card detects the amount of power available and adjusts a performance point of the smart card load so that an amount of power consumed is appropriate to the amount of power received. In the event that more than adequate power is available from the generated field, a shunting device is employed to regulate voltage supplied to the smart card load. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1  is a prior art circuit diagram of a contactless smart card.  
       FIG. 2  is an exemplary circuit diagram of a contactless smart card with a field strength detector and performance adapting load.  
       FIG. 3  is an exemplary process flow diagram of a method to adapt transponder performance to available power. 
    
    
     DETAILED DESCRIPTION  
      With reference to  FIG. 2 , a coupling coil  112  and a tuning capacitor  114  are connected in parallel to form a receiver section  110  of an exemplary circuit diagram  200  of a contactless smart card. A rectifying section  120  input is connected to the receiver section  110  and is made from a rectifier bridge  125 . A regulating section  230  connects to an output of the rectifying section  120  and contains a detector  237  connected to an input of the regulating section  230 . A shunt regulator  235  connects from the output of the regulating section  230  to ground. A load section  240  connects to an output of the regulating section  230  and contains a smart card load  245  across the inputs of the load section  240 .  
      Values of inductance and capacitance for the coupling coil  112  and the tuning capacitor  114  respectively, are selected for an appropriate resonant frequency of operation. For optimal coupling of energy, the resonant frequency of the smart card corresponds to the generated frequency, f G , of a reader (detector). When the smart card is exposed to an electromagnetic field with a generator frequency, f G , that equals the resonant frequency, f Resonance , of the transponder, energy from the generated field is coupled to the receiver section  110 .  
      On each oscillation of the receiver section  110 , energy is fed to the rectifying section  120 . Continuous exposure to the electromagnetic field replenishes the energy supplied to the rectifying section  120 . A positive phase of oscillation produces a conduction path from a first terminal  116  of the receiver section  110 , through a first branch  122  of the rectifier bridge  125 , through the regulating section  230  and load section  240 , through a first return path  124  portion of the rectifier bridge  125 , and back to a second, complementary terminal  118  of the receiver section  110 . A negative phase of oscillation produces a conduction path through the second terminal  118  of the receiver section  110 , through a second branch of the rectifier bridge  125 , through the regulating section  230  and load section  240 , through a second return path  128  portion of the rectifier bridge  125 , and back to the first terminal  116  of the receiver section  110 .  
      The amount of energy available to be supplied to the rectifying section  120  is dependent upon and directly proportional to the strength of the generated field. For instance, in a typical application, an amount of available power from the generated field varies from 4 milliamps (mA) to 80 mA. A stronger generated field produces more energy for the load section  240 . The strength of the generated field at the smart card depends on the strength of the field produced, the proximity of the smart card to the generating source, and the presence of other cards in the field. The load section  240  will have varying amounts of energy available for the smart card load  245  depending upon the strength of the field to which the smart card is exposed.  
      The detector  237  senses the amount of power supplied from the generated field and provided by the rectifier section  120  by monitoring the current coming from the rectifier bridge  125 . The monitored current at the output of the detector  237  produces an indicator signal proportional to the amount of power produced by the rectifier section  120 . Since the amount of power provided by the rectifier section  120  comes from the electromagnetic field, the indicator is a field strength signal. The indicator signal is supplied to a clock frequency regulator  247  within the smart card load  245 . The clock frequency regulator  247  determines the clock frequency provided to a microprocessor (not shown) in the smart card load section  240 . The clock frequency regulator  247  may be implemented, for example, by a voltage-controlled oscillator. A relatively higher indicator signal produces a proportionately higher clock frequency output to the microprocessor. An alternative exemplary embodiment of the clock frequency regulator  247  is a digital clock divider controlled by an analog-to-digital converter monitoring the current through the rectifier.  
      The microprocessor and smart card load  245  will consume power proportionate to the power available from the generated field based on the frequency output by the clock frequency regulator  247 . The clock frequency regulator  247 , therefore, acts as a performance adapter for matching an operating point of the smart card load  245  to available power from the generated field. The operating point is set by the indicator signal supplied by the detector  237  to the frequency regulator  247 . A typical smart card may consume between 2 mA and 20 mA when supplied with a field of sufficient strength to supply a corresponding performance level. The combination of the detector  237 , the indicator signal, and the clock frequency regulator  247  produce a performance level in a microprocessor appropriate to the power available in the generated electromagnetic field available to the transponder.  
      A feedback signal (not shown) is provided to the shunt regulator  235 . The feedback signal is produced by devices (not shown) monitoring, for example, a voltage produced by the smart card. The shunt regulator  235  controls the voltage supplied to the load section  240 . The greater the magnitude of power provided by the rectifier section  120 , the greater the feedback signal and the greater the amount of shunting provided by the shunt regulator  235 . For a sufficiently low magnitude of electromagnetic field the shunt regulator  235  receives a low enough feedback signal to decrease the amount of shunting by the shunt regulator  235  to a level of effectively no shunting.  
      With reference to  FIG. 3 , an exemplary process flow diagram  300  of a method to adapt transponder performance commences with monitoring  305  the generated electromagnetic field. The method continues with producing  310  a field strength indication proportionate to the strength of the field received and is followed by a step of generating  315  a clock frequency proportional to the field strength. A next step in the process is a shunting  320  of power proportional to the field strength indication.  
      While various portions of a transponder card have been depicted with exemplary components and configurations, an artisan in the electromagnetic field would readily recognize alternative embodiments for accomplishing a similar result. For instance, a detector has been presented as a single series device with an indicator signal output. An artisan in the field would recognize a possibility for various networks of field effect transistor devices to conduct the current from the rectifier section  120  ( FIG. 2 ) and produce voltage drops across an on-channel resistance of one of the transistor devices proportionate to a rectifier current. Similarly, an artisan familiar with the field would also recognize the capability of an input gate of a field effect transistor to detect a voltage drop across a series resistor to effect the same detection capability.  
      Additionally, a clock frequency regulator  247  has been represented by a voltage-controlled oscillator. One skilled in the art could readily conceive of a phase-locked-loop frequency synthesizer employing phase detection, clock dividers, and prescalers to provide a frequency to produce an appropriate performance level of a microprocessor. The smart card load  245  has been represented by a microprocessor. An artisan in the field of transponders would conceive of other process specific circuitry to be incorporated into a smart card. For instance, an LED, counter, or signal transmitter could be triggered and powered by the electromagnetic field the transponder is immersed in to provide surveillance, tracking, or inventory capabilities. The specification and drawings are therefore to be regarded in an illustrative rather than a restrictive sense.