1. Field of the Invention
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.
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.
2. Discussion of the Related Art
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.
On the read terminal side, a series oscillating circuit 2 formed of an inductance L1 forming an antenna can generally be found, in series with a capacitor C1 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.
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.
On the side of transponder 10, an inductance L2, in parallel with a capacitor C2, forms a parallel oscillating circuit (called a resonant circuit), intended to sense the electromagnetic field generated by the series oscillating circuit (L1, C1) of terminal 1. The resonant circuit (L2, C2) of transponder 10 is tuned to the frequency of a carrier of excitation of the oscillating circuit (L1, C1) of terminal 1.
Terminals 11, 12 of the resonant circuit (L2, C2), corresponding to the terminals of capacitor C2, are connected to two A.C. input terminals of a rectifying circuit 13 formed of a bridge of four diodes D1, D2, D3, and D4 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 (D1, D3) and of diodes (D2, D4).
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.
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 C3 and C4 is then connected to rectified output terminal 14 and 15 (GND) of circuit 13. Terminal 15 forms the ground of transponder 10.
When transponder 10 enters the electromagnetic field of terminal 1, a high-frequency A.C. voltage VE is generated across the resonant circuit (L2, C2). This voltage, rectified by circuit 13 and smoothed by capacitors C3 and C4, 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).
The junction point of diodes (D2, D4) 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 C3 and C4 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.
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.
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 C4. Circuit 13 operates in fullwave rectification. Diode pairs (D1, D4) and (D2, D3) are alternately turned on at the frequency of voltage VE. Only capacitor C3 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.
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 D1 and D3 are alternately turned on at the frequency of voltage VE. The power is alternately stored in each of capacitors C3 and C4. The voltage present across each capacitor C3 and C4 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.
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.
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.
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.