Method for voltage limitation for a transponder

p0001The invention relates to a method for voltage limitation in accordance with the preamble of claim 1. Moreover, the invention relates to a circuit for voltage limitation in a transponder according to the preamble of patent claim 6.

p0002Such a method and such a circuit are known per se, in particular through RFID applications (RFID = Radio Frequency Identification). Under an RFID application is here understood to mean any application in which a transmitting resonant circuit provides an inductively coupled receiving resonant circuit with energy and reads via the receiving resonant circuit data. Such compounds are used for example for object identification, wherein a transmitting resonant circuit of a reader (reader) via a receiving resonant circuit a with a mark (tag) responds to an object marked and retrieves information.

p0003To contact the transmitting resonant circuit of the reader generates a high frequency magnetic field induced in an inductance of a receiving resonant circuit, which is located near the reader, an AC voltage. The voltage induced in the receiving resonant circuit AC voltage is rectified and used for example for supplying energy to a resonant circuit connected to the receiving integrated circuit. In addition, a clock frequency can be derived, which will put the integrated circuit, that is, for example, a microprocessor and / or memory element as the system clock can also available from the induced alternating voltage. With the addition of the inductance of the transmitting resonant circuit and / or receiving resonant circuit with capacities, particularly with parallel capacity to resonant circuits, resonance effects are achieved, which greatly improve the efficiency of energy transfer.

p0004A transmission of data from the reader to the receiving resonant circuit (downlink) can be effected for example by switching on and off of the magnetic field. For a data transport in the reverse direction from the receiving resonant circuit to the reading device the so-called load modulation is used, which requires a sufficient proximity (distance less than 0.16 wavelength) of the transmitting and receiving oscillator circuit. With sufficient proximity, the transformer coupling, in which the power consumption of the receiving coil depicting by a feedback to the transmitting resonant circuit in voltage changes at the transmitting resonant circuit. Controlled modulation of the load, ie the impedance of the receive oscillating circuit, therefore, elicit changes in voltage in the transmission resonant circuit, which can be evaluated for data transmission.

p0005With increasing quality of the inductors used in the receiving resonant circuit, that is with an increasing ratio of reactance to resistance to reduce the effective damping of the resonant circuit and the width of the resonance curve. The use of coil higher quality thus causes a higher frequency selectivity and, at the same voltage on the reader side, a higher voltage on the day-side, which increases the range of the communication link. At small distances between reader and tag such large voltages can be induced depending on the transmission power in the day that an existing in the tag integrated circuit can be destroyed.

p0006From the <patcit id="pcit0001" dnum="DE102004020816A1"><text>DE 10 2004 020 816 A1</text></patcit> It is known to reduce the voltage at the receiving resonant circuit to certain values ​​or limit, which is hereinafter referred to as a first terminal voltage. Further, the load modulation between a first voltage is switched ie the first terminal voltage and a second lower voltage in the frame. For this depletion layer elements between resonant circuit terminals and a reference or ground potential can be switched. A lower terminal voltage is, for example, realized in that above the barrier layer components drops whose forward voltage, the voltage drop due to the exponential dependence of the current of the voltage is independent of power supplies in a first approximation.

p0007As a result, the depletion layer elements also act at high coil currents as a reliable limitation of the resonance circuit voltage to a corresponding value. This is for systems with high-Q inductors important that otherwise can cause undesirable high voltages in the proximity of Sendesehwingkreis and receiving resonant circuit.

p0008The upper clamp voltage can be realized by a series connected with reverse conducting direction Zener diode, the short-circuit is controlled or switched. In the short-circuited state takes place, the described limitation on the lower clamping voltage, while in the non-short-circuited state, the breakdown voltage of the Zener diode for a voltage offset additive ensures that defines an upper terminal voltage in the sum with said forward voltages. In the state with short-circuited Zener diode, a comparatively large current from the receiving resonant circuit flows out, corresponding to the loaded state of the resonant circuit. Accordingly, the current drain from the resonant circuit and the load on the resonant circuit by opening the short circuit is reduced across the Zener diode.

p0009The load modulation from the <patcit id="pcit0002" dnum="DE102004020816A1"><text>DE 10 2004 020 816 A1</text></patcit> is the following problem has been observed: When the switching of the modulation, so when limiting the resonant circuit voltage on the lower terminal voltage, particularly a high coil current is induced, so these flows may via the bridging of the Zener diode and, connected in the conducting direction other barrier components from, wherein the resonant circuit voltage may fall below the lower terminal voltage and also below a threshold value, which is used for the detection of oscillations (pulses) of the resonant circuit voltage. So it can happen when the load with unfavorable phase conditions, the voltage at the transmitting resonant circuit decreases due to the reaction of one or more periods below a detection threshold, which distorts the transmission of information. This can result in data loss when transmitting information to the reader.

p0010Namely switched modulation at a high induced Spulensrom, the depletion layer elements for a limitation of the resonant circuit voltage care to a predetermined by the depletion layer elements value. The diodes act in this phase as a DC voltage source and thus do not provide sufficient damping to the coil current counter, so that the induced oscillation is changed. The result is a widening of the present input clock phase (pulse broadening), which leads to at least partial cancellation of the sequential oscillation. It occurs thus in appearance, that at least one oscillation in the amplitude is too small for a predetermined detection threshold.

p0011The publication <patcit id="pcit0003" dnum="US5815355A"><text>US 5815355</text></patcit> discloses a circuit and method for limiting the voltage for a transponder. Here, the voltage to the resonant circuit coil by the control of a parallel transistor is limited.

p0012In the conference report "<nplcit id="ncit0001" npl-type="s"><text>Circuitry for a wireless micorsystem for neural re-cording micro probes "veröffentllicht in Annual International Conference of IEEE Engineering in The M, Vol. Vol. 1 of 4. Conf. 23, October 25, 2001 Pages 761- 764</text></nplcit> XP010593486 is a voltage limiter for an inductively coupled system is disclosed that the attached circuit protection at high voltages induced from destruction. A disadvantage is that the circuit is used only for a rectified signal.

p0013Against this background, the object of the invention to provide a method and a circuit arrangement for voltage limitation in a transponder, which reduces the disadvantages of the prior art, and in the claims is described.

p0014Further advantages result from the description and the accompanying figures. It is understood that the features mentioned above and those yet to be explained not only in the particular combination indicated but also in other combinations or alone without departing from the scope of the present invention.

p0015Embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. The drawings show in schematic form:<dl id="dl0001"><dt>Fig. 1</dt><dd>a transmitting and receiving resonant circuit with elements for load torque modulation for a first voltage half-wave</dd><dt>FIG. 2</dt><dd>the transmitting and receiving oscillator circuit of <figref idrefs="f0001">Fig.1</figref> with elements for load modulation for a second voltage half-wave</dd><dt>Fig. 3</dt><dd>another transmitting and receiving resonant circuit with elements for load modulation for a first voltage half-wave</dd><dt>Fig. 4</dt><dd>the transmitting and receiving oscillator circuit of <figref idrefs="f0003">Fig.3</figref> with elements for load modulation for a second voltage half-wave</dd><dt>Fig. 5</dt><dd>the course of the resonant circuit voltages U_L, U_C1: U_C2</dd><dt>Fig. 6</dt><dd>a desired modulation behavior</dd></dl>

p0016In the embodiments according to the <figref idrefs="f0001">figure 1</figref> and the <figref idrefs="f0002">figure 2</figref> not it is a part of the invention which is the subject of this claim desire. The<figref idrefs="f0001">Fig. 1</figref> and <figref idrefs="f0002">FIG. 2</figref> show a transmitting and receiving resonant circuit 10 of a receiving part or tag. Parallel to the resonant circuit 10 including an inductor L 12 and a capacitor C 13, is located between a first oscillator circuit connection C₁ 25 and a second resonant circuit terminal C₂ 26, a barrier member 27, preferably an NMOS transistor. At the exit of the junction member 27 is a reference potential terminal 28 which provides a ground potential of the circuit is. Since the barrier member 27 between the source (source) and drain (drain) can switch, the circuit can be used in the present form both for a first and for a second voltage half-wave.

p0017Also in parallel to the resonant circuit 10 and between the first C₁ 25 and the second resonant circuit terminal C₂ 26, is a series circuit of polarity reversal circuit 14 and diodes 19, 20, 21, 22, 23, 24 which are used to control the depletion layer component 27th In this case, the diodes may be implemented as NMOS transistors, which are connected as diode. Parallel to the barrier member 27 is located further comprises a rectifier circuit 29 with a further reference potential terminal 30th

p0018The polarity switching circuit 14 consists of two parallel series circuits each of an ohmic resistor R₁ 15 or R₂ 16 and a diode 17 or 18 include, in which the flow direction of the diodes 17, 18 is different.

p0019The diodes 19 to 24 can switch S₁ 31, S₂ 32, S₃ 33 are bridged by the control circuit 34 via cascaded MOD signals MOD₀, MOD₁ and MOD₂ are actuated. The switches 31, 32, 33 are preferably implemented as transistors, in particular as MOS transistors.

p0020The barrier member 27 is used initially to limit the resonant circuit voltage U_L 1 between the terminals 25 and 26 of the transmitting and receiving oscillator circuit 10 with open switches 31, 32 and 33. In this state the barrier member 27 is limited in each case when the potential difference between the reference potential terminal 28, 30 and one of the terminals 25 or 26, the forward voltage the depletion layer component exceeds 27th This value defines the upper limit voltage or first clamp voltage U_KL 2.2.

p0021When bypassing the diodes 19 to 24 by the switches 31, 32 and 33, which are not closed simultaneously to stepped modulation of the voltage sequentially clamps the depletion layer component 27 the resonant circuit voltage U_L 1 to a lower value UL_MOD2.1.

p0022By opening and closing the switch 31, 32, 33 modulates the control circuit 34 the value of the resonant circuit voltage U_L 1 corresponding to the data to be transmitted sequence. As described, this modulation of transmitting and receiving resonant circuit 10 forms the day. As a modulation of the load of the transmitting and receiving resonant circuit of the reader provided a transformer coupling

p0023<figref idrefs="f0003">FIGS. 3 and Fig. 4</figref> show another transmitting and receiving resonant circuit 10 of a receiving part or tag. This shows<figref idrefs="f0003">Fig. 3</figref> a circuit for a first voltage half-wave and <figref idrefs="f0003">Fig. 4</figref> the circuit of <figref idrefs="f0003">Fig. 3</figref> for a second voltage half-wave. Parallel to the resonant circuit 10 including an inductor L 12 and a capacitor C 13, is located between a first oscillator circuit connection C₁ 25 and a second resonant circuit terminal C₂ 26 is a Sperrschichlbauteil 35, preferably a PMOS transistor. At the output of 35 is a Sperrschtchtbauteits Bezugspotentiatanschtuss 28, which is a ground potential for the circuit. To control the depletion layer component 35 are a current source I_e, in turn, a discharge for discharging a parallel capacitance 7 makes available, as well as a voltage source U_ref 3. In this case, between the capacitance 7 and the voltage source U_ref 3, a switch S₁ 9 is arranged, which is actuated by means of an inverse signal NMOD 4th

p0024The barrier member 35 serves to limit the resonant circuit voltage U_L 1 to the value of the upper clamping voltage U_KL 2.2 between the terminals 25 and 26 of the transmitting and receiving oscillator circuit 10. When the switch S₁ 9 is closed, is located at the junction member 35, the voltage U_ref 3, which by the predetermined voltage U_G 6 is limited at the gate and thus can not go beyond the Schweilspannung the PMOS transistor. When the switch S₁ 9 by means of 4 NMOD signal of the control circuit 34 is closed, the capacitor 7 to the value U_ref 3aufgeladen and then by means of the discharge current of the current source I_E2 8 discharged. The consequence of this is that the voltage U_G 6 at the gate linearly decreases to zero. This is the resonant circuit voltage U_L 1 to a second, smaller terminal voltage U_LMOD 2.1 is limited, corresponding to the threshold voltage of the PMOS Translstors.

p0025<figref idrefs="f0003">Fig. 5</figref> shows the course of the resonant circuit voltage U_L 1, the course of the respective half-wave voltage U_C1 and U_C2 at the two resonant circuit terminals C 25 and C₂ 26 The resonant circuit voltage U_L 1 is the difference from the voltage values ​​at the terminals. U_L = U_C1-U_C2,

p0026If UL 1 is positive, according to the circuit <figref idrefs="f0001">Fig.1</figref> and <figref idrefs="f0003">Fig. 3</figref> the resonant circuit terminal C₂ connected via the rectifier 29 with an internal mass 30th In the opposite case, if U_L 1 is negative, as shown in the <figref idrefs="f0002">FIG. 2</figref> and <figref idrefs="f0003">Fig. 4</figref> the resonant circuit terminal C₁ 26 connected to the internal ground 30th

p0027<figref idrefs="f0004">Fig. 6</figref> showing a desired profile of the resultant resonant circuit voltage UL in the receiving resonant circuit under the influence of a controlled load modulation according to the present invention over time t. The large amplitudes 2.2 arise with open switches 31, 32, 33 according to<figref idrefs="f0001">Fig. 1</figref> and <figref idrefs="f0002">Fig.2</figref> and the small amplitudes 2.1 arise in closed switches 31, 32, 33 a, which are bridged diodes 19, 20, 21, 22 a.

LIST OF REFERENCE NUMBERS

p0028<dl id="dl0002" compact="compact"><dt>1</dt><dd>U_L Resonant circuit voltage</dd><dt>2.1</dt><dd>UL_MOD lower clamping voltage (modulated voltage)</dd><dt>2.2</dt><dd>U_KL upper clamping voltage</dd><dt>3</dt><dd>U_ref voltage reference</dd><dt>4</dt><dd>n_MOD inverse signal</dd><dt>5.1</dt><dd>Modulation signal MOD₀</dd><dt>5.2</dt><dd>Modulation signal MOD₁</dd><dt>5.3</dt><dd>Modulation signal MOD₂</dd><dt>6</dt><dd>U_G gate voltage</dd><dt>7</dt><dd>C₂ capacitor</dd><dt>8th</dt><dd>IE₂ Current source discharge</dd><dt>9</dt><dd>switch S₁</dd><dt>10</dt><dd>Sending - and receiving resonant circuit</dd><dt>11</dt><dd>day</dd><dt>12</dt><dd>inductance L</dd><dt>13</dt><dd>capacitance C</dd><dt>14</dt><dd>Polarity reversal circuit</dd><dt>15</dt><dd>Ohmic resistance R1</dd><dt>16</dt><dd>Ohmic resistance R2</dd><dt>17</dt><dd>Diode at R1</dd><dt>18</dt><dd>Diode at R2</dd><dt>19</dt><dd>diode</dd><dt>20</dt><dd>diode</dd><dt>21</dt><dd>diode</dd><dt>22</dt><dd>diode</dd><dt>23</dt><dd>diode</dd><dt>24</dt><dd>diode</dd><dt>25</dt><dd>First resonant circuit terminal C₁</dd><dt>26</dt><dd>Second resonant circuit terminal C₂</dd><dt>27</dt><dd>Depletion layer component M1</dd><dt>28</dt><dd>Reference potential terminal</dd><dt>29</dt><dd>Rectifier circuit</dd><dt>30</dt><dd>Reference potential terminal</dd><dt>31</dt><dd>switch S₁</dd><dt>32</dt><dd>switch S₂</dd><dt>33</dt><dd>switch S₃</dd><dt>34</dt><dd>Control circuit (digital to analog converter)</dd><dt>35</dt><dd>Depletion layer component P₁</dd></dl>