Abstract:
An electromagnetic transponder includes an oscillatory circuit, a battery and a first rectifier bridge. Alternating current input terminals of the rectifier bridge are connected to the terminals of the oscillatory circuit, and at least two rectifier elements of the rectifier bridge are controllable on the basis of the voltage supplied by the battery.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure generally relates to electronic circuits and, more specifically, to electromagnetic transponders equipped with a battery. 
         [0003]    A field of application of the present invention relates to so-called NFC (Near Field Communication) devices, which are most often mobile telecommunication devices equipped with near-field communication circuits. 
         [0004]    2. Description of the Related Art 
         [0005]    Most devices provided with a battery and with a near-field communication router are designed to be powered by the battery when available or to be remotely supplied by extracting power from an electromagnetic field radiated by a terminal in the absence of a battery or when said battery is not available or discharged. 
         [0006]    A transmission from a terminal to a transponder is generally performed in amplitude modulation of a high-frequency carrier (for example, at 13.56 MHz). Since the carrier must be able to supply power to the transponder when it is not powered by its battery, the modulation index is generally smaller than 100% so that the carrier is always present. 
         [0007]    The modulation index is defined as being the ratio of the variation of the message amplitude to the variation of the carrier amplitude. In practice, the modulation index generally is 0.8, which means that the difference between the respective amplitudes of a high state and of a low state does not exceed 20% of the carrier amplitude. This modulation index conditions the sensitivity necessary for the demodulator on the transponder side so that said transponder can make out high-state transmissions from low-state transmissions. 
         [0008]    Problems may arise when the charge of the transponder battery is at a level such that the transponder attempts to draw power from the electromagnetic field during high levels of the transmission and to draw power from the battery during low levels. In such a case, the modulation index is no longer respected, which may cause signal interpretation errors. 
       BRIEF SUMMARY 
       [0009]    Thus, an embodiment of the present invention aims at overcoming all or part of the disadvantages of usual electromagnetic transponders equipped with a battery. 
         [0010]    An embodiment of the present invention aims at preserving the modulation index of the terminal, whether the transponder operates in battery mode or in remote-supply mode. 
         [0011]    An embodiment of the present invention aims at simplifying the switching from one mode to the other. 
         [0012]    An embodiment of the present invention aims at providing a solution which can be easily integrated in low-voltage technology. 
         [0013]    To achieve all or part of these and other objects, the present invention provides an electromagnetic transponder comprising: 
         [0014]    an oscillating circuit; 
         [0015]    a battery; and 
         [0016]    a first rectifying bridge having A.C. input terminals connected across the oscillating circuit and having at least two rectifying elements controllable with the voltage supplied by the battery. 
         [0017]    According to an embodiment of the present invention, the transponder further comprises a second rectifying bridge having an output connected to the input of an amplitude demodulator. 
         [0018]    According to an embodiment of the present invention, said two controllable rectifying elements are two first MOS transistors. 
         [0019]    According to an embodiment of the present invention, the gate of each first MOS transistor is connected to the junction point of a series association of a resistive element and of a second MOS transistor between one of the A.C. input terminals of the first rectifying bridge and a reference potential. 
         [0020]    According to an embodiment of the present invention, each second transistor receives, on its gate, a voltage representative of the battery charge level. 
         [0021]    According to an embodiment of the present invention, said representative voltage is the voltage supplied by the battery. 
         [0022]    According to an embodiment of the present invention, the transponder comprises a comparator of the level of the battery voltage with a threshold for controlling the first rectifying bridge. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0023]    The foregoing and other features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
           [0024]      FIG. 1  is a partial simplified representation of an example of a usual electromagnetic transponder; 
           [0025]      FIGS. 2A ,  2 B,  2 C, and  2 D illustrate, in the form of timing diagrams, the operation of the transponder of  FIG. 1 ; 
           [0026]      FIG. 3  is a simplified electric diagram of an embodiment of an electromagnetic transponder; and 
           [0027]      FIG. 4  shows a detail of the circuit of  FIG. 3 . 
           [0028]      FIG. 5  shows an alternate embodiment of a detail of the circuit of  FIG. 3   
       
    
    
     DETAILED DESCRIPTION 
       [0029]    The same elements have been designated with the same reference numerals in the different drawings. For clarity, only those elements which are useful to the understanding of the embodiments which will be described have been shown and will be detailed. In particular, the generation of the signals to be transmitted by a terminal has not been detailed, the described embodiments being compatible with usual transmissions. Further, circuits for using the signals received by the transponder have not been detailed either, the described embodiments being here again compatible with the current use of such transmissions. 
         [0030]      FIG. 1  very schematically shows an example of circuit forming a usual electromagnetic transponder. 
         [0031]    Such a transponder is based on the use of an oscillating circuit  10 , for example, parallel, formed of an inductive element forming an antenna and of a capacitive element C 1  in parallel. This oscillating circuit is intended to detect a high-frequency electromagnetic field radiated by a terminal or reader (not shown). This type of system is intended to operate in near field communication, or near field inductive coupling, or NFC. 
         [0032]    The transponders to which the embodiments which will be described apply are devices equipped with a battery  11  capable of powering the transponder circuits in certain operating modes. Such circuits have been schematically shown by a block  4  and comprise the different electronic circuits necessary for the use of the data received from the terminal and for the possible generation of transmissions in the transponder-to-terminal direction. The transponder power supply may also be extracted from the electromagnetic field radiated by the terminal having the transponder within its range. In this case, a rectifying bridge  2  having A.C. input terminals connected to terminals  12  and  13  of the oscillating circuit and having output terminals  21  and  22  respectively defining terminals of provision of a positive voltage and a terminal connected to a reference voltage, generally the ground, is used. In the example of  FIG. 1 , rectifying bridge  2 , called power supply bridge, is a fullwave bridge comprising four diodes  24  to  27 . A filtering and voltage step-down cell  23  is generally interposed between output  21  of rectifying bridge  2  and a terminal  31  for powering electronic circuits  4  internal to the transponder. Cell  23  for example is a pi-shaped filter comprising two capacitive elements C 23  and C 23 ′ respectively grounding terminals  21  and  31 , a resistor R 23  being interposed between terminals  21  and  31 . The excursion across the oscillating circuit may reach several tens of volts while the battery is intended to power circuits  4  with a voltage of a few volts. Cell  23  is thus used as a step-down transformer. Power supply terminal  31  receives a voltage originating either from cell  23  or from battery  11 . In practice, a diode-type rectifying element D 11  is interposed between the output of the battery and terminal  31  to avoid for a current to flow towards the battery. 
         [0033]    Relating to data transmissions, the communications received from a terminal are interpreted by means of a demodulator  41  (DEMOD) comprised in block  4 . This demodulator generally is an amplitude demodulator, transmissions in a terminal-to-transponder direction being performed in amplitude modulation of the radio frequency carrier generated by the terminal. Thus, a second rectifying bridge  5  is interposed between oscillating circuit  10  and an input  42  of demodulator  41 . A.C. input terminals of rectifying bridge  5  are connected to terminals  12  and  13  of oscillating circuit  10  while its rectified output terminals  51  and  52  are respectively connected to terminal  42  and to the reference voltage. A filtering capacitive element C 53  generally grounds terminal  42 . Rectifying bridge  5 , called demodulation bridge, for example is a fullwave bridge formed of 4 diodes  54  to  57 . 
         [0034]    The fact for the power supply of the transponder circuits to be drawn either from the battery or from the oscillating circuit according to the operating mode may be a problem, especially in the case where the battery charge is in a particular situation. 
         [0035]      FIGS. 2A ,  2 B,  2 C, and  2 D illustrate this problem. These drawings are timing diagrams respectively illustrating, in their left-hand portion I, a battery-powered operation, in their central portion II, an operation where the transponder circuits are exclusively remotely supplied by a field detected by oscillating circuit  10 , and in their right-hand portion III, a mixed operation capable of raising an issue. 
         [0036]      FIG. 2A  illustrates examples of the shape of signal V RF  detected by the oscillating circuit, that is, the voltage between terminals  12  and  13 .  FIG. 2B  illustrates examples of charge levels V 11  of the battery.  FIG. 2C  illustrates the corresponding shapes of voltage V 51  obtained at the output of demodulation bridge  5 .  FIG. 2D  shows the resulting modulation index MI. 
         [0037]    In the three situations, I, II, and III, it is assumed that the carrier transmitted by the terminal is amplitude-modulated with a modulation index of approximately 50%, that is, the level of voltage V RF  for high states of the data signal is twice the level of the low states of this signal. 
         [0038]    For simplification, the voltage drop in diode D 11  is not taken into account. In practice, voltage V 11  shows the battery charge level minus the forward voltage drop in diode D 11 . 
         [0039]    In portion I, level V batI  of voltage V 11  is assumed to be greater than the voltage level extracted from the oscillating circuit and provided by cell  23 . Block  4  is powered by the battery. Voltage V 51  reflects the 50% modulation rate between the two levels of the radio frequency signal. It can be considered that, since the transponder is powered by its battery, it only forms (at least in receive mode) a negligible load on the generated electromagnetic field. As a result ( FIG. 2D ), the modulation index is respected on the transponder side, thus reflecting the modulation of the electromagnetic field carrier. 
         [0040]    In operating mode II, it is considered that the transponder is integrally remote-supplied. This for example corresponds to the absence of any battery (V batII =0) or to a very discharged battery. In this case, all the power necessary to the operation of the electromagnetic circuit of the transponder is drawn from the electromagnetic field. The load then formed by the transponder on the field causes a modification (decrease) of the amplitude of signal V RF . However, this decrease occurs in the same way for high-level periods and low-level periods of the modulation (see  FIGS. 2A and 2C ). Accordingly, the modulation index remains unmodified and approximately equal to 50%. 
         [0041]    In operating mode III, battery  11  is partially discharged and is at a level V batIII  intermediate between the high and low levels of the voltage extracted from the electromagnetic field. As a result, circuits  4  draw the power supply necessary for their operation from the electromagnetic field during high levels H ( FIG. 2B ) of the electromagnetic signal, and from the battery during low levels L of the modulation. With such a situation, voltage V RF  across the oscillating circuit no longer respects a 50% modulation index. Accordingly, voltage V 51  recovered for the demodulator no longer has a 50% modulation index, but a lower index ( FIG. 2D ). In the shown example, the modulation index is assumed to have dropped to a value of 25%. 
         [0042]    Such an instability of the modulation index may be prejudicial to the reliability of the data decoding since an amplitude demodulator generally operates with respect to thresholds. There thus are battery charge level situations where the demodulator is no longer capable of discriminating high states from low states of the signal modulating the carrier. 
         [0043]    The example of a 50% modulation index is an arbitrary example. This line of argument transposes whatever the modulation index, since it will be altered anyway in certain battery charge situations. 
         [0044]    It could have been devised to insert a switch between terminals  21  and  31 , to disconnect rectifying bridge  2  when the battery is present and thus avoid altering the modulation index. However, the transponder circuits have a high power consumption and a large switch would thus be necessary to avoid too strong an increase of remote-supply mode losses. Further, the control of such a switch risks being a problem, since it would require a voltage of a level higher than that of the battery, or even higher than the highest level between the field level and the battery level (case of a P-channel MOS transistor, for example). 
         [0045]      FIG. 3  is a simplified electric diagram of an embodiment of a transponder having, as a rectifying bridge for providing the power supply voltage, a bridge  2 ′ controllable with a signal different from the voltage across the oscillating circuit (a diode bridge can indeed be considered as automatically controllable according to the voltage level extracted from the oscillating circuit), that is, having at least two of its rectifying elements comprising control terminals or electrodes. The other elements of the circuit of  FIG. 3  are identical to those described in relation with  FIG. 1 . Since the rectifying elements must anyway be provided according to the required remote supply power, replacing them with switches adapted to this power is not a problem. 
         [0046]    According to the embodiment of  FIG. 3 , it is provided to control (signal CTRL) controllable rectification elements of bridge  2 ′ from the voltage level of battery  11 . Bridge  2 ′ may thus be only activated if the battery voltage level is not sufficient to guarantee a fully remote-supplied operation such as described in portion III of  FIGS. 2A to 2D . 
         [0047]    To simplify the representation of  FIG. 3 , the battery disconnection switch has not been illustrated. It may for example be a MOS transistor replacing diode D 11  and having its parasitic diode used to block any current flow towards the battery when the transponder is powered from the electromagnetic field. 
         [0048]      FIG. 4  shows a more detailed electric diagram of an embodiment of a controllable bridge  2 ′. 
         [0049]    As compared with bridge  2  of  FIG. 1 , diodes  25  and  26  are replaced with MOS transistors  65  and  66 . The control of these transistors is performed by connecting their respective gates to junction points  61  and  62  of series associations of resistors R 63 , respectively R 64 , and of MOS transistors  67 , respectively  68 , between respective A.C. input terminals  12  and  13  of bridge  2 ′ and the reference potential (the ground). Transistors  67  and  68  have their gates interconnected, for example, to output terminal  43  of battery  11  or, as a variation, to a terminal of application of a voltage representative of the voltage across the battery. 
         [0050]    Resistors R 63  and R 64  form resistive dividing bridges with the respective on-state drain-source resistors of transistors  67  and  68 . This protects transistors  67  and  68 , which can thus have a decreased size with respect to a switch having to withstand the remote-supply power and voltage. 
         [0051]    When the battery voltage is sufficient, transistors  67  and  68  are turned on (N-channel transistors are assumed), which grounds the respective gates of transistors  65  and  66  and, accordingly, turns off (disconnects) rectifying bridge  2 ′. The power is then exclusively supplied by the battery. Conversely, when the battery voltage is lower than a threshold, transistors  67  and  68  are off. As a result, transistors  65  and  66  are non-conductive. In this case, this amounts to connecting their respective gates to terminals  12  and  13 , which makes them diode-assembled. The rectifying bridge is then active. 
         [0052]    The turn-on threshold of transistors  67  and  68  is set by the gate-source voltage to be applied thereto. In a simplified embodiment such as shown in  FIG. 4 , their gates are directly connected to the battery output (terminal  43 ). It is then considered that, when the battery is present, bridge  2 ′ has to be disconnected, whatever the battery charge level. 
         [0053]    Preferably, a voltage dividing bridge, for example, resistive, is interposed between a terminal (for example,  43 ) for providing the battery voltage level and the gates of transistors  67  and  68 . A minimum battery charge level can thus be taken into account to disconnect the bridge. It will then be ascertained to select this threshold so that it is sufficiently low to avoid creating a new conflict between the remote-supply voltage and the battery voltage. The selection of this threshold depends on the demodulator sensitivity and on the power consumption of the transponder circuits. 
         [0054]    The operation of the transponder described in relation with  FIGS. 3 and 4  amounts, in the timing diagrams of  FIGS. 2A to 2D , to only having situations I and II. Situation I corresponds to that where bridge  2 ′ is disconnected. 
         [0055]    As an alternative to the circuit of  FIG. 4 , as shown in  FIG. 5 , a comparator  70  may be provided to compare the battery voltage level with a threshold and to accordingly control transistors  67  and  68 . However, the embodiment of  FIG. 4  is a preferred embodiment since it minimizes the number of necessary components. 
         [0056]    An advantage of the provided control circuit is that transistors  67  and  68  are controllable with a signal having a relatively low voltage level as compared with the excursion of more than 10 volts that can be seen by transistors  65  and  66 . It is then possible to provide it with a smaller size, which makes the integration easier. For example, the control voltage that they are to receive is compatible with the level of a few volts (in practice, less than 4 volts) of the battery. 
         [0057]    Various embodiments have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. In particular, it may be provided to control (to replace with transistors) diodes  24  and  27  instead of or in addition to the replacing of diodes  25  and  26 . Further, the practical implementation of the described embodiments is within the abilities of those skilled in the art based on the function indications given hereinabove and by using current integrated circuit manufacturing technologies.