Abstract:
A power supply circuit and a transponder having a circuit for rectifying an A.C. voltage and two power storage elements, the rectifying circuit providing a rectified voltage to at least one of the storage elements and an output voltage being provided by at least one of the storage elements, and at least one switching element for switching the circuit operation between a state of provision of a relatively high voltage and a state of provision of a relatively low voltage, the second state configuring the rectifying circuit in halfwave operation.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the field of power supply circuits extracting their power from an A.C. voltage source having a dynamically-varying amplitude. The present invention applies to systems supplied by a fixed voltage source as well as to mobile systems recovering their power from a source remotely transmitting power, and thus a variable voltage.  
         [0003]     An example of application of the present invention relates to remotely supplied transponders which extract the power necessary to their operation from the electromagnetic field radiated by an antenna of a read/write terminal in the vicinity of which they are present. Electromagnetic transponders are based on the use of a parallel LC-type oscillating circuit, across which an A.C. voltage having an amplitude varying according to the distance between the transponder and the terminal is generated.  
         [0004]     2. Discussion of the Related Art  
         [0005]      FIG. 1  very schematically illustrates in the form of blocks a terminal  1  for reading electromagnetic transponders and a conventional transponder  10  intended to communicate with this terminal.  
         [0006]     On the read terminal side, a series oscillating circuit  2  formed of an inductance L 1  forming an antenna can generally be found, in series with a capacitor C 1  connected between an output terminal  3  of an amplifier or antenna coupler (not shown) and a reference terminal  4  (generally the ground). The antenna coupler belongs to one or several circuits  5  (LECT) for controlling the oscillating circuit and exploiting the received data and comprises, among others, a modulator-demodulator and a microprocessor for processing the control and data signals.  
         [0007]     Circuit  5  of the terminal generally communicates with different input/output circuits (keyboard, screen, means of transmission to a central server, etc.) and/or processing circuits not shown. These circuits extract the power necessary to their operation from a power supply circuit (not shown) connected, for example, to the electric system or to a battery.  
         [0008]     On the side of transponder  10 , an inductance L 2 , in parallel with a capacitor C 2 , forms a parallel oscillating circuit (called a resonant circuit), intended to sense the electromagnetic field generated by the series oscillating circuit (L 1 , C 1 ) of terminal  1 . The resonant circuit (L 2 , C 2 ) of transponder  10  is tuned to the frequency of a carrier of excitation of the oscillating circuit (L 1 , C 1 ) of terminal  1 .  
         [0009]     Terminals  11 ,  12  of the resonant circuit (L 2 , C 2 ), corresponding to the terminals of capacitor C 2 , are connected to two A.C. input terminals of a rectifying circuit  13  formed of a bridge of four diodes D 1 , D 2 , D 3 , and D 4  of full-wave rectification type. The A.C. input terminals are formed by the midpoints of the branches formed by respective series associations of diodes (D 1 , D 3 ) and of diodes (D 2 , D 4 ).  
         [0010]     In a first transponder type (not shown), a capacitor is connected to output terminals  14  and  15  of circuit  13  to store the power and smooth the rectified voltage.  
         [0011]     In a second transponder type such as shown in  FIG. 1 , it is provided to increase the range by allowing a voltage doubler operation. A series association of two capacitors C 3  and C 4  is then connected to rectified output terminal  14  and  15  (GND) of circuit  13 . Terminal  15  forms the ground of transponder  10 .  
         [0012]     When transponder  10  enters the electromagnetic field of terminal  1 , a high-frequency A.C. voltage VE is generated across the resonant circuit (L 2 , C 2 ). This voltage, rectified by circuit  13  and smoothed by capacitors C 3  and C 4 , becomes a voltage VS on terminal  14 . Voltage VS is applied to the input of a regulator  19  (REG) having the function of providing a regulated voltage VR to a circuit  20  (CTL). Circuit  20  essentially comprises a microprocessor and a memory (not shown).  
         [0013]     The junction point of diodes (D 2 , D 4 ) is connected to a first terminal  17  of a selector (SEL) having a second terminal connected to ground GND. Terminal  14  is connected to a first input of a comparator  18  (COMP) having a second input receiving a voltage threshold VSLIM. Selector SEL switches a terminal  16  connected to the junction point of capacitors C 3  and C 4  on one or the other of terminals  15  and  17 . The output of comparator  18  controls selector SEL according to voltage VS with respect to threshold VSLIM.  
         [0014]     In the example of  FIG. 1 , voltage VE recovered between terminals  11 ,  12  of the transponder in the field of terminals  1  depends on the distance which separates the transponder from the terminal and on the coupling between the respective oscillating circuits of the terminal and of the transponder. To have a system with a relatively large range (on the order of from 20 to 50 centimeters), it must be switched from a fullwave rectification at short distance to a voltage doubler rectification when the transponder is distant from the terminal. The rectification mode switches when voltage VS reaches threshold VSLIM. Voltage VSLIM represents the minimum supply voltage of circuits  19  and  20 . Threshold VSLIM also corresponds to the maximum distance between terminal  1  and transponder  10  from which the remotely supplied power provided to the transponder becomes insufficient to supply circuits  19  and  20 .  
         [0015]     When transponder  10  is close to terminal  1 , voltage VS is greater than threshold VSLIM. The output of comparator  18  connects terminal  16  of selector SEL to terminal  15 , thus short-circuiting capacitor C 4 . Circuit  13  operates in fullwave rectification. Diode pairs (D 1 , D 4 ) and (D 2 , D 3 ) are alternately turned on at the frequency of voltage VE. Only capacitor C 3  stores the power and is charged to voltage VS with a frequency which is twice that of A.C. voltage VE. Voltage VS is on average equal to once rectified input voltage VE.  
         [0016]     As transponder  10  is moved away from terminal  1 , voltage VS becomes smaller than voltage VSLIM. The output of comparator  18  switches terminal  16  of selector SEL to terminal  17 . This switching configures circuit  13  in voltage doubler rectification mode. Only diodes D 1  and D 3  are alternately turned on at the frequency of voltage VE. The power is alternately stored in each of capacitors C 3  and C 4 . The voltage present across each capacitor C 3  and C 4  is in average equal to once rectified input voltage VE. Voltage VS is then equal, in average, to twice rectified input voltage VE. A disadvantage is that, when a transponder is closer to the terminal, in fullwave rectification, it receives too high a power as compared to its needs.  
         [0017]     A problem which is then posed when the oscillating circuits of the terminal and of the transponder are very close to each other is that, if they are tuned, the power transmitted from the terminal to the transponder is such that said transponder heats up. This thermal effect may have as a consequence a deformation of the plastic card containing the transponder.  
         [0018]     More generally, a disadvantage of systems supplied by A.C. voltage sources with a very high dynamic variation is that they modify the amplitude of the rectified voltage without adapting the power storage frequency to the needs of the load, formed by the transponder in the case of  FIG. 1 .  
         [0019]     Another disadvantage of the system of  FIG. 1  is that it requires means of protection against overcharges, compatible with a storage resulting from a fullwave rectification, and which often are of dissipative nature.  
       SUMMARY OF THE INVENTION  
       [0020]     The present invention aims at providing a novel solution which overcomes the disadvantages of conventional solutions, especially in the case of systems supplied by A.C. voltage sources with a dynamic variation.  
         [0021]     The present invention also aims, in the case of transponders, at reducing the storage of the power remotely supplied at short distance from the read/write terminal.  
         [0022]     The present invention further aims at providing a solution which is particularly simple to implement.  
         [0023]     To achieve these and other objects, the present invention provides a power supply circuit comprising a circuit for rectifying an A.C. voltage and two power storage elements, the rectifying circuit providing a rectified voltage to at least one of the storage elements and an output voltage being provided by at least one of the storage elements, and at least one switching element for switching the circuit operation between a state of provision of a relatively high voltage and a state of provision of a relatively low voltage, the second state configuring the rectifying circuit in halfwave operation.  
         [0024]     According to an embodiment of the present invention, the switching element short-circuits one of the storage elements.  
         [0025]     According to an embodiment of the present invention, the rectifying circuit and the storage elements are respectively formed of series associations of two diodes and of two capacitors, the A.C. voltage being applied to the respective midpoints of said series associations connected in parallel.  
         [0026]     According to an embodiment of the present invention, the capacitors are of same value.  
         [0027]     According to an embodiment of the present invention, the power supply circuit further comprises a comparator of data representative of the dissipation in the supplied load with respect to a threshold.  
         [0028]     According to an embodiment of the present invention, said comparator compares the output voltage with a predetermined threshold, greater than the maximum that can be reached by said relatively high voltage.  
         [0029]     According to an embodiment of the present invention, the threshold is settable.  
         [0030]     The present invention also provides a transponder comprising:  
         [0031]     a resonant circuit providing a variable voltage from an electromagnetic circuit radiated by a terminal; and  
         [0032]     a power supply circuit providing an output voltage.  
         [0033]     According to an embodiment of the present invention, said relatively high voltage is selected to be greater than the minimum operation voltage at the range limit.  
         [0034]     The foregoing objects, 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. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]      FIG. 1 , previously described, is intended to show the state of the art and the problem to solve;  
         [0036]      FIG. 2  very schematically shows a first embodiment of the circuit for rectifying and regulating the stored power according to the present invention; and  
         [0037]      FIG. 3  very schematically shows a second embodiment of the circuit for rectifying and regulating the stored power according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0038]     The same elements have been designated with the same reference numerals in the different drawings. For clarity, only those elements necessary to the understanding of the present invention have been shown in the drawings and will be described hereafter.  
         [0039]     A feature of the present invention is to reduce the power stored from an A.C. rectified voltage source when it exceeds the load needs, without using a specific dissipation system.  
         [0040]      FIG. 2  schematically shows an embodiment of a circuit for rectifying and regulating the power stored from a dynamically-variable voltage source. The example of  FIG. 2  will be described in relation with an application to electromagnetic transponders having their A.C. voltages dynamically varying according to the distance between the transponder and the terminal.  
         [0041]     In this example, when transponder  10  ( FIG. 1 ) enters the electromagnetic field of terminal  1 , a high-frequency A.C. voltage VE is generated between terminals  11  and  12  of resonant circuit (L 2 , C 2 ). A rectifying circuit  13 ′ formed by a series association of two diodes D 1  and D 3  is in parallel with two capacitors C 3  and C 4  in series, which connect the cathode of diode D 1  to the anode of diode D 3 . Input terminals  11  and  12  are formed by respective junction points of diodes D 1  and D 3  and of capacitors C 3  and C 4 . A.C. voltage VE, rectified by diodes D 1  and D 3  and smoothed by capacitors C 3  and C 4 , becomes a voltage VS between a terminal  14  corresponding to the cathode of diode D 1  and a terminal  15  corresponding to the anode of diode D 3 . Voltage VS is applied to a first terminal of a comparator  18  (COMP), having a second terminal receiving a voltage threshold (VCLIM). Output  21  of comparator  18  controls a switch S 1 , having its terminals connecting junction point  22  (here, confounded with terminal  12 ) of capacitors C 3  and C 4  to ground  15 . Voltage VS is applied to an input of a regulator  19  (REG) having an output providing a voltage VR to a circuit  20  (CTL). As previously, circuits  19  and  20  are also connected to ground  15  of the transponder.  
         [0042]     The comparison between voltages VS and VCLIM is referenced with respect to the voltage present on terminal  22 . This amounts to comparing the voltage between terminals  14 ,  22  of capacitor C 3  with threshold VCLIM.  
         [0043]     When transponder  10  ( FIG. 1 ) is far from terminal  1 , the voltage across capacitor C 3  is smaller than threshold VCLIM, output  21  controls the turning-off of switch S 1 . Only diodes D 1  and D 3  are alternately turned on at the frequency of voltage VE. The power is alternately stored in each of capacitors C 3  and C 4 . The voltage present across each capacitor C 3  or C 4  is in average equal to once rectified input voltage VE. Voltage VS is equal, in average, to twice rectified input voltage VE. Circuit  13 ′ then operates in voltage doubler mode. Storage elements C 3  and C 4  are generally sized to maximize the remote-supply distance of transponder  10  in voltage doubler rectification.  
         [0044]     When transponder  10  is close to terminal  1 , the voltage across capacitor C 3  is greater than or equal to threshold VCLIM, and output  21  controls the turning on of switch S 1 , connecting terminal  22  to ground  15 . Switch S 1  then short-circuits capacitor C 4 . Only diode D 1  is turned on at half the frequency of voltage VE. The power is stored in the sole capacitor C 3  at the rate of one halfwave out of two of voltage VE. The voltage present across C 3  is in average equal to once rectified input voltage VE, minus the voltage drop resulting from the power consumption of circuits  19  and  20  during the halfwaves of voltage VE where diode D 1  is not on. Voltage VS is equal, in average, to less than once rectified input voltage VE. Circuit  13 ′ is then configured in halfwave rectification.  
         [0045]     As transponder  10  ( FIG. 1 ) is brought closer to terminal  1 , voltage VS reaches threshold VCLIM. Switch S 1  is turned on, configuring rectifying circuit  13 ′ in halfwave rectification. Capacitor C 3  is then the only one to be recharged at the rate of half the frequency of voltage VE and at a voltage smaller than once VE. The power stored in capacitor C 3  is then decreased by at least a factor two with respect to a fullwave rectification mode.  
         [0046]     Threshold VCLIM is selected to be at a voltage beyond which the power stored in doubling mode in capacitors C 3  and C 4  becomes such that is causes an overheating by dissipation in transponder  10 .  
         [0047]     As an alternative, it may be provided to replace the measurement of voltage VS at terminal  14  by a temperature measurement, threshold VCLIM then corresponding to a maximum temperature threshold not to be exceeded.  
         [0048]     It may also be provided to vary voltage threshold VCLIM according to a control signal provided, for example, by circuit  20 . Threshold VCLIM may also be obtained in digital fashion.  
         [0049]     In the example of  FIG. 2 , capacitors C 3  and C 4  are preferably selected to be identical. However, it may be provided to select capacitors C 3  and C 4  of different values. The series association of capacitors C 3  and C 4  then forms a divider of voltage VS. This results in generating different voltages across each of capacitors C 3  and C 4  to, for example, provide different supply voltages for the needs of circuits  20 .  
         [0050]     According to a variation of the present invention, switch S 1  is connected in parallel on capacitor C 3 , and comparator  18  is placed between terminal  22  and ground  15 . Comparator  18  then compares the voltage present on terminal  22  with threshold VCLIM. In this case, only capacitor C 4  stores the power in halfwave rectification.  
         [0051]      FIG. 3  very schematically shows a second embodiment of the circuit for rectifying and regulating the power stored from a dynamically-variable voltage source. Only the differences with respect to the first embodiment of  FIG. 2  will be described.  
         [0052]     In the example of  FIG. 3 , switch S 1  ( FIG. 2 ) is replaced with a switch S 2  between terminals  12  and  22 . The control terminal of switch S 2  is connected to output  21  of comparator  18 . In parallel with switch S 2 , a diode D 5  is connected to terminal  12  by its cathode, and to terminal  22  by its anode.  
         [0053]     Conversely to the example of  FIG. 2 , as long as voltage VS is smaller than voltage VCLIM, switch S 2  is maintained on by output  21  of comparator  18 . Circuit  13 ′ is then configured in voltage doubler rectification mode.  
         [0054]     When transponder  10  ( FIG. 1 ) comes closer to terminal  1  and voltage VS is equal to threshold VCLIM, output  21  controls the turning-off of switch S 2 . Diodes D 1  and D 5  are then on. Only capacitor C 3  stores the power at half the frequency of voltage VE. As described in  FIG. 2 , voltage VS is equal, in average, to less than once rectified input voltage VE. Circuit  13 ′ is then configured in halfwave rectification, decreasing the power stored in capacitor C 3  by at least a factor two with respect to a fullwave rectification mode. Preferably, switch S 2  is formed by an N-type (or P-type) MOS transistor having its parasitic diode forming diode D 5 . MOS transistor gate control techniques are well known by those skilled in the art and here pose no specific problem.  
         [0055]     As an alternative, it may also be provided to reverse the direction of conduction of diode D 5  by connecting its anode to terminal  12  and its cathode to terminal  22 . When voltage VS is equal to VCLIM, the switch is off and diodes D 3  and D 5  are then on. Only capacitor C 4  stores the power, according to the same principle as that described in  FIG. 3 .  
         [0056]     An advantage of the present invention is that it adapts the power stored in the storage elements to the needs of the load of a power supply circuit having a dynamically-varying voltage source.  
         [0057]     Another advantage of the present invention is that it reduces the power stored from an alternately rectified voltage source when it exceeds the needs of the load, without using a specific dissipation system.  
         [0058]     Another advantage of the present invention is that it avoids an overheating of the transponder when it is in close coupling with the terminal.  
         [0059]     The present invention enables increasing the sensitivity of the reader in close coupling, while decreasing the risk of saturation of the reader demodulator consecutive to too high a rectified voltage of the transponder.  
         [0060]     Although the present invention has been described in relation with the measurement of rectified voltage VS across the storage elements, it may be provided to switch rectifying modes based on any other information or signal linked to this rectified voltage.  
         [0061]     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the sizing of the storage elements, as well as the rectification mode switching threshold depend on the application and, in particular, on the frequency of the A.C. voltage source.  
         [0062]     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.