Abstract:
A contactless card including an antenna coil, a resonant capacitor coupled between both end terminals of the antenna, a plurality of capacitors coupled in parallel with the terminals of the antenna correspondingly through switches, a shunt transistor coupled between the terminals of the antenna, forming a bypassing current path, a rectifier coupled between the terminals of the antenna, generating a DC voltage, and a control circuit sensing the DC voltage and controlling a gate voltage of the shunt transistor and on/off conditions of the switches in accordance with the sensed DC voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This U.S. non-provisional patent application claims priority under 35 U.S.C. § 119 of Korean Patent Application No. 2005-05037 filed on Jan. 17, 2006, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND 
       [0002]    The present disclosure relates to contactless smart cards, contactless identification devices such as radio frequency identification tags (RFIDs), and contactless identification systems. 
         [0003]    Until recently, in the field of systemic identification technology, barcode and magnetic card systems have been widely used for credit cards, public telephone cards, and traffic cards. Such magnetic or barcode identification systems have a problem of degrading a rate of identification over time due to weakening magnetic force, physical damages, or other destructions. Thus, smart cards and radio frequency identification (RFID) tags are increasingly employed as advanced identification systems for overcoming the problems found in conventional systems. 
         [0004]    The smart card, which is a so-called IC card, is a kind of plastic card about the size of a credit card having embedded therein an IC chip that is designed to conduct a given transaction and that includes a microprocessor, a card operating system, a security module, and a memory. The smart card is generally distinguished as a contact type, a contactless type, and a combined type in accordance with the manner employed in reading data therefrom. 
         [0005]    The RFID tag includes information for identifying an object and is often called a smart tag having a microcomputer chip equipped with an antenna. The RFID tag functions as an identifying body or a memory. The technology for RFID is provided by merging electromagnetic and electrostatic coupling effects in the field of radio frequency (RF) with the electromagnetic spectrum for the purpose of differentiating products, animals, or persons. The RFID tag is currently of interest as a means capable of being a substitute for the barcode system, being convenient in use because there is not need of direct contact or optical scanning in the visual bandwidth. 
         [0006]    In a system with the RFID tag, a device or apparatus for writing data in the RFID tag or reading data from the RFID tag is called an identifier or reader. 
         [0007]    A contactless identification system, in which the RFID tag or smart card communicates with the reader by way of radio frequency, may be classified into contact, proximity, and vicinity types. According to the specification defined by ISO/IEC 14443, the proximity contactless identification system is operable in the communication range of 0˜10 cm. On the other hand, ISO/IEC 15693 defines the vicinity contactless identification system to be operable in the communication range of 0˜70 cm. 
         [0008]    In the proximity/vicinity contactless identification system using a smart card there might be supplied an excessive voltage into the smart card during a proximity operation mode (in 5 cm). Since such an excessive voltage causes damage to a central processor unit or IC chip in the smart card, a technique to block the excessive voltage that is generated during the proximity mode is required. 
         [0009]      FIGS. 1A and 1B  are graphic diagrams showing amplitude patterns of modulated/demodulated subcarrier signals during the proximity mode of a general contactless card. A conventional solution to this problem uses a shunt transistor to prevent the excessive voltage effect. The contactless card operates the shunt transistor to have a small resistance during the proximity mode, thereby bypassing a current flowing from an antenna coil. In this way, it prevents the excessive voltage, which is induced by the proximity operation, from being supplied into an internal circuit. During the proximity operation mode, however, when the small resistance of the shunt transistor is coupled in parallel with a modulated load resistance portion of an internal transformer, as shown in  FIG. 1B . As a result, the intensity of the data signal transferred into an internal demodulator of the card reader becomes lower so as to cause communication errors thereby. 
       SUMMARY OF THE INVENTION 
       [0010]    Exemplary embodiments of the present invention are directed to a contactless identification device and a system capable of preventing circuit defects and communication errors due to an excessive voltage transferred to a smart card or tag during a proximity operation mode. 
         [0011]    According to an exemplary embodiment of the present invention, a contactless card includes an antenna, a resonant capacitor coupled between both terminals of the antenna; pluralities of capacitors coupled in parallel with the terminals of the antenna correspondingly through switches; a shunt transistor coupled between the terminals of the antenna and forming a bypassing current path; a rectifier coupled between the terminals of the antenna and generating a DC voltage, and a control circuit sensing the DC voltage and controlling a gate voltage of the shunt transistor and on/off conditions of the switches in accordance with the senses DC voltage. 
         [0012]    In an exemplary embodiment, the contactless card further includes: a nonvolatile memory supplied with the DC voltage; and a digital circuit supplied with the DC voltage, processing data to be transceived through the antenna. 
         [0013]    In an exemplary embodiment, the contactless card further includes: a demodulator operating to demodulate data received through the antenna; and a load modulator operating to modulate data to be transmitted. 
         [0014]    In an exemplary embodiment, the switches are NMOS transistors. 
         [0015]    In an exemplary embodiment, the control circuit includes: a detector determining whether the DC voltage is an excessive voltage; and a selector adjusting the gate voltage of the shunt transistor in accordance with a result from the detector and generating selection signals to turn on/off the switches. 
         [0016]    In an exemplary embodiment, the shunt resistor is an NMOS transistor. 
         [0017]    In an exemplary embodiment of the present invention, a contactless card system includes: a contactless card; and a card reader communicating with the contactless card in a radio mode. The contactless card includes: a nonvolatile memory; an analogue circuit generating a DC voltage form data transferred in the radio mode; a digital circuit controlling the nonvolatile memory and processing the data transceived to/from the card reader; and a control circuit determining whether the DC voltage generated by the analogue circuit is an excessive voltage. 
         [0018]    In an exemplary embodiment the analogue circuit is composed of an antenna; a resonant capacitor coupled between both terminals of the antenna; pluralities of capacitors coupled in parallel with the terminals of the antenna correspondingly through switches; a shunt transistor coupled between the terminals of the antenna, forming a bypassing current path, and a rectifier coupled between the terminals of the antenna, generating a DC voltage. In this case, the control circuit regulates a gate voltage of the shunt transistor and on/off conditions of the switches in accordance with the sensed DC voltage. 
         [0019]    In an exemplary embodiment, the analogue circuit further includes: a demodulator operating to demodulate data received through the antenna; and a load modulator operating to modulate data to be transmitted. 
         [0020]    In an exemplary embodiment, the control circuit includes: a detector determining whether the DC voltage is an excessive voltage; and a selector adjusting the gate voltage of the shunt transistor in accordance with a result from the detector and generating selection signals to turn on/off the switches. 
         [0021]    A further understanding of the nature and advantages of the inventions herein may be realized by reference to the remaining portions of the specification and the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Exemplary embodiments of the present invention will be understood in more detail from the following descriptions taken in conjunction with the attached drawings, in which: 
           [0023]      FIGS. 1A and 1B  are graphic diagrams showing amplitude patterns of modulated/demodulated subcarrier signals during a proximity operation of a general contactless card. 
           [0024]      FIG. 2  is a schematic block diagram illustrating a contactless identification system in accordance with an exemplary embodiment of the present invention; 
           [0025]      FIG. 3  is a detailed block diagram illustrating the contactless identification system shown in  FIG. 2 ; 
           [0026]      FIG. 4  is a circuit diagram illustrating in more detail the contactless identification system shown in  FIG. 3 ; and 
           [0027]      FIG. 5  is a circuit diagram illustrating a variable capacitor used in the system shown in  FIG. 4  in accordance with an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0028]    Exemplary embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. The invention may, however, be embodied in different forms and should not be constructed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
         [0029]    Hereinafter, will be described an exemplary embodiment of the invention in conjunction with the accompanying drawings. 
         [0030]      FIG. 2  is a schematic block diagram illustrating a contactless identification system in accordance with an exemplary embodiment of the present invention. Referring to  FIG. 2 , the contactless identification system is comprised of a contactless card reader  10 , and a contactless smart card or tag, hereinafter, referred to as ‘smart card’  20 . The contactless card reader  10  continuously radiates an electronic wave with a constant frequency. Thus, the smart card  20  is powered up by RF when it is close to a frequency range of the contactless card reader  10 . Such a kind of smart card  10 , which operates with power supplied form the contactless power reader  20 , is referred as a ‘passive’ type. Otherwise, a kind of smart card that has its own power is referred as an ‘active’ type. The smart card  20  upon being activated sends a responding signal to the contactless card reader  10  when there is an input command from the contactless card reader  10 . During the operation, the contactless card reader  10  interrupts the communication if there is no response form the smart card  20  after a predetermined delay time (defined by ISO/IEC 14446 and ISO/IEC 15693) following the sending of the command. 
         [0031]    Because the passive type of contactless smart card  20  conducts an RF signal processing operation with power supplied form the contactless card reader  10 , the rate of power supplied is greatly affected by a communication distance from the contactless card reader  10 . Therefore, exemplary embodiments of the present invention adopt an advanced contactless identification system, described as follows, in order to overcome the troubles due to variation of power supply rate. 
         [0032]      FIG. 3  is a detailed block diagram illustrating the contactless identification system shown in  FIG. 2 . Referring to  FIG. 3 , the contactless smart card  20  is comprised of an analogue circuit  21 , a digital circuit  23 , a memory, for example, a nonvolatile memory,  25 , and a control circuit  27 . The analogue circuit  21  includes a voltage generator  210 , a demodulator  220 , and a load modulator  230 . The analogue circuit  21  generates a power source voltage at the time of transceiving data by RF signals in a contactless mode. The voltage generator  210  of the analogue circuit  21  generates voltages, which are to be applied to the digital circuit  23  and the memory  25 , from RF signals received from the contactless card reader  10 . Simultaneously, the demodulator  220  of the analogue circuit  21  provides the digital circuit  23  with reception data that is demodulated from the data contained in subcarrier signals. The load modulator  230  treats data, which is transferred from the digital circuit  23 , in a load modulation mode, and then transmits the load-modulated data to the contactless card reader  10 . 
         [0033]    The digital circuit  23  processes data received from the contactless card reader  10 , and includes a receiver, a transmitter, a modulator, and a central processor unit (not shown), and controls data input/output operations into/from the memory  25 . Furthermore, the digital circuit  23  first modulates the data and transfers the modulated data to the analogue circuit  21  for transmission. 
         [0034]      FIG. 4  is a circuit diagram illustrating in more detail the contactless identification system shown in  FIG. 3 . Referring to  FIG. 4 , the voltage generator  210  is comprised of an antenna coil  211 , a variable capacitor  213 , a shunt resistor  214 , and a rectifier  215 . The variable capacitor  213 , the shunt transistor  214 , and the rectifier  215  are all coupled in parallel with the two terminals of the antenna  211 . 
         [0035]    The contactless card reader  10  is comprised of a signal processor  11  and an antenna coil  13  transceiving RF signals. When the contactless smart card  20  accepts the RF signals from the contactless card reader  10 , an AC voltage, also called a subcarrier signal, is generated at both terminals of the antenna coil  211 . The AC voltage is transformed in to DC voltage through the rectifier  215  and supplied to each internal block of the countless smart card  20  as an output voltage Vout. Data accepted by the contactless smart card  20  from the contactless card reader  10  is contained in AC voltage or subcarrier signal and then is input to the demodulator  220 . The demodulator  220  transfers the demodulated reception data Rx_DATA to the digital circuit  23 . The digital circuit  23  operates to store the reception data Rx_DATA into the memory  25 . 
         [0036]    Hereinafter, the features of the resonant circuit  211  and the variable capacitor  213 , the shunt transistor  214 , and the modulator  230  will be described in detail.  FIG. 5  is a circuit diagram illustrating the variable capacitor  213  shown in  FIG. 4  according to an exemplary embodiment of the present invention. Generally, a resonant circuit is a unit for passing a signal in a predetermined frequency bandwidth. In an exemplary embodiment of the present invention, the resonant circuit is composed of the antenna coil  211  and the variable capacitor  213 . In this contactless identification system, a frequency of the RF signal transmitted from the card reader  10  is defined by the communication protocol, for example, 13.56 MHz as defined by ISO/IEC 14443. A resonant frequency ƒ is established by the parameters that are the inductance L of the antenna coil  211 , and the capacitance C of the variable capacitor  213 , as follows. 
         [0000]    
       
         
           
             
               
                 
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                     1 
                     
                       2 
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                        
                       π 
                        
                       
                         LC 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
         [0037]    As can be seen from Equation 1, the voltage generator  210  can be improved in efficiency when the frequency ƒ of the resonant circuit consisting of the antenna coil  211  and the variable capacitor  213  matches a frequency, for example, 13.56 MHz, of the RF signal provided from the card reader  10 . More specifically, this condition assures the highest voltage from the voltage generator  210 . 
         [0038]    The modulator  230 , as illustrated in  FIG. 4 , is comprised of resistors R 1  and R 2 , and an NMOS transistor MN 5  having a current path coupled in parallel with the resistor R 2  and a gate coupled to an output from the digital circuit  23 . The NMOS transistor MN 5  is turned on or off in response to variation of logical level (high or low) in the transmission data Tx_DATA output from the digital circuit  23 . According to the on/off condition of the NMOS transistor MN 5 , the resistance between the antenna coil  211  and the variable capacitor  213  varies to change the amount of current flowing through the antenna coil  211 . Thereby, a signal processed by the modulator  230  is transferred to the contactless card reader  10 . 
         [0039]    The variable capacitor  213 , as illustrated in  FIG. 5 , is comprised of several capacitors C 1 ˜C 4  coupled in parallel with both terminals of the antenna coil  211 , and NMOS transistors MN 1 ˜MN 3  operating as switches coupled in series with each of the capacitors C 2 ˜C 4 .  FIG. 5  shows the three NMOS transistors MN 1 ˜MN 3  coupled each to the capacitors C 2 ˜C 4 , in addition to the antenna coil  211  and the capacitor C 1  that may constitute a general resonant circuit. It will be apparent to those skilled in this art that the number of capacitors and transistors is variable based upon design factors for this system. 
         [0040]    Gates of the NMOS transistors MN 1 ˜MN 3  are supplied with selection signals SEL 1 ˜SEL 3  that are output signals from a selector  275  of the control circuit  27 . Thus, the capacitors C 1 ˜C 4  and the NMOS transistors MN 1 ˜MN 3  form the step-type variable capacitor  213 , regulated by the control circuit  27 . The step-type variable capacitor  213  is convenient in implementing its circuit pattern and alterably adjusting the total capacitance C in Equation 1. 
         [0041]    The shunt transistor  214  is coupled in parallel with the step-type variable capacitor  213 . A gate of the shunt transistor  214  is coupled to the selector  273 , to which a control signal CON 1  is applied. The shunt transistor  214  forms a current path bypassing an excessive current caused by an excessive voltage, so as to prevent the generation of excessive voltage during the proximity operation. 
         [0042]    If the contactless smart card  20  receives an RF signal from the contactless card reader  10  that is in the proximate distance, for example, within 5 cm, an AC voltage is generated at the antenna coil  211 , and the rectifier  215  transforms the AC voltage into a DC voltage as the output voltage Vout. The detector  271  of the control circuit  27  determines the presence of the excessive voltage by comparing the output voltage Vout with a reference voltage. If an excessive voltage has been generated, the selector  273  of the control circuit  27  outputs selections signals SEL 1 ˜SEL 3 , in response to an output signal from the detector  271 , to turn-on/off the transistors MN 1 ˜MN 3  of the step-type variable capacitor  213 . Thereby, the total capacitance Ctot of the step-type variable capacitor  213  is changed. More specifically, when the selection signal SEL 1  is generated with a logically high level, the total capacitance Ctot of the variable capacitor  213  increases to C 1 +C 2 . When the selection signals SEL 1  and SEL 2  are generated with a logically high level, the total capacitance Ctot of the variable capacitor  213  rises up to C 1 +C 2 +C 3 . Because the alteration of the total capacitance Ctot causes the resonant frequency ƒ to vary in accordance with Equation 1, it changes the AC voltage transferred to the rectifier  215 . Thus, it is possible to adjust the AC voltage by way of a simple control operation. 
         [0043]    At the same time, the selector  273  controls a gate voltage of the shunt transistor  214 . According to a rise/fall of the gate voltage, an amount of current  1  flowing through the bypassing current path also increases or decreases. By altering the gage voltage of the shunt transistor  214 , it is possible to minutely adjust an amount of the current  1 , thereby making voltage variations of the step-type variable capacitor  213  be linear. 
         [0044]    Therefore, according to exemplary embodiments of the present invention, even when the contactless smart card  20  is operating in the proximate distance of less than 5 cm in the contactless identification system, it prevents damage to the internal circuits due to the excessive voltage that would be generated by the voltage generator  210 . Furthermore, exemplary embodiments of the present invention provide the voltage control by varying the resonant capacitance along with the voltage control provided by the shunt resistor. Thus, during the proximity operation, it is permissible for the resistance of the shunt transistor, which was relatively low, to be maintained higher than the conventional case, assuring the intensity of the subcarrier wave according to the load modulation and hence transferring the effective substrate intensity to the contactless card reader  10 . Thereby, the exemplary embodiment solves communication errors that have occurred in the conventional system. 
         [0045]    According to exemplary embodiments of the present invention, it is possible to control the output voltage of the voltage generator in a linear form by providing the contactless smart card  20  with the variable capacitor and the shunt transistor that operate a stepping control for capacitance, and the control circuit for regulating the capacitor and transistor. Thus, an excessive voltage during the proximity mode is prevented from being generated, in order to prevent damage to the internal circuits of the contactless smart card. 
         [0046]    Moreover, by minutely regulating an amount of the current passing through the shunt transistor, it is possible to minimize degradation in the intensity of the modulated signal by the load modulator by utilizing the resistance of the shunt transistor Thereby, it is possible to lessen communication errors that would be caused by weak modulation signals. 
         [0047]    The above-disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all such modifications, enhancements, and other embodiments, which fall within the true spirit and scope of the present invention. Thus, to the maximum extend allowed by law, the scope of the present invention is to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing detailed description.