RELATED APPLICATION

This application claims priority to earlier filed European patent application serial number EP19188995 entitled “POWER SUPPLY CIRCUIT THAT SOURCES ENERGY FROM AN NFC ANTENNA,” filed on Jul. 30, 2019, the entire teachings of which are incorporated herein by this reference.

TECHNICAL FIELD

The following disclosure relates to the field of power supplies and Near Field Communication (NFC) and particularly to circuits and devices used in connection with Near Field Communication (NFC).

BACKGROUND

Near Field Communication (NFC) denotes a standardized data transmission technique for the contactless exchange of data across relatively short distances (e.g. a few centimeters) using electromagnetically coupled inductors. NFC is based on the well-known RFID technique and allows data rates of several hundred kBits/s. Currently, NFC is mainly used in the field of “Micropayment” (non-cash payment of small amounts) as well as in the field of access control. Further applications include, for example, the transmission of authentication data in the process of establishing a communication link via, for example, Bluetooth, Wireless LAN or the like. Recently, sensor applications have been developed which allow retrieving measured data, e.g. by a mobile device, using NFC. NFC is standardized in ISO/IEC 18092 (Near Field Communication Interface and Protocol-1) and ISO/IEC 21481 (Near Field Communication Interface and Protocol-2).

With regard to the mentioned payment function many mobile devices such as smart phones are equipped with NFC transceivers (NFC Reader/Writer). Such devices are usually referred to as NFC enabled devices. An NFC chip—often referred to as NFC tag or NFC transponder—usually does not have its own energy supply and sources the electrical energy required for its operation from the electromagnetic field used for NFC (NFC field). The NFC field is usually generated by the NFC enabled device (e.g. a smart phone). That is, energy is transferred via the NFC field from the NFC enabled device to an NFC chip/transponder, whereas data transmission is possible in both directions. Many NFC enabled devices are not capable of controlling the power of the NFC field and operate with a preset transmission power which may depend on the type and the manufacturer of an NFC enabled device.

The antennas of NFC enabled devices and NFC chips/transponders are usually simple conductor loops which may be formed, e.g., on a printed circuit board (PCB). In antenna circuits such conductor loops represent respective inductors which form, in connection with corresponding capacitors, resonant circuits. For an efficient energy transfer from an NFC enabled device to an NFC transponder the corresponding antenna circuits are usually operated at the same resonance frequency, which results in the electromagnetic coupling being at its maximum. As a consequence, the voltage induced in the antenna of the NFC transponder is at its maximum. For standard applications the resonance frequency is usually at 13.56 MHz.

If the inductive coupling between the antenna of an NFC enabled device and the corresponding antenna of an NFC transponder (e.g. when the NFC transponder is placed very closely to the NFC enabled device) is good, situations can occur in which more energy is transferred to the NFC transponder than required by the NFC transponder for its normal operation. In these situations the surplus energy needs to be dissipated in the NFC transponder, for example in a shunt transistor.

SUMMARY

In some applications it might be desirable to not dissipate the surplus energy in the NFC transponder as the energy could be used for purposes other than data transmission.

A power supply circuit is described herein which is capable of sourcing energy from an NFC antenna. In one embodiment, the circuit comprises a rectifier circuit configured to be coupled to an NFC antenna for receiving an antenna voltage, a filter coupled to an output of the rectifier circuit and configured to provide the rectified and smoothed (filtered) antenna voltage as supply voltage, and a current limiting device coupled between the filter and an output node and configured to limit an output current provided at the output node dependent on a control signal. Further, the power supply circuit comprises a control circuit configured to receive the supply voltage and a reference voltage and to generate the control signal dependent on a difference between the reference voltage and the supply voltage.

Furthermore, a circuit arrangement is described herein that, in accordance with one embodiment, includes an NFC antenna and an NFC circuit connected to the NFC antenna, wherein the NFC circuit includes a supply circuit configured to generate a DC supply voltage for the NFC circuit. The circuit arrangement further includes one or more of the mentioned above power supply circuits for supplying further circuitry.

Further embodiments herein include a circuit comprising: an NFC (Near Field Communication) antenna; an NFC circuit connected to the NFC antenna, the NFC circuit including a supply circuit operative to generate a DC supply voltage for the NFC circuit; and supplemental power supply circuitry coupled to the NFC antenna, the supplemental power converter circuitry including: a rectifier circuit coupled to an NFC antenna for receiving an antenna voltage; a filter coupled to an output of the rectifier circuit, the filter operative to provide a rectified and filtered antenna voltage as a supply voltage; a current limiting device coupled between the filter and an output node, the current limiting device operative to limit an output current provided at the output node dependent on a control signal; and a control circuit operative to receive the supply voltage and a reference voltage and to generate the control signal dependent on a difference between the reference voltage and the supply voltage.

In one embodiment, the current limiting device has a controllable resistance which depends on the control signal; the control circuit is operative to generate the control signal such that the supply voltage does not fall below the reference voltage.

In still further example embodiments, wherein the current limiting device includes a transistor.

In yet further example embodiments, the control circuit includes a difference amplifier operative to amplify a signal representing the difference between the reference voltage and the supply voltage.

If desired, the rectifier circuit includes a bridge rectifier and the filter includes a capacitor.

In one embodiment, the reference voltage is controlled in accordance with a configuration signal.

Circuitry as discussed herein can be configured in any suitable manner, in one embodiment, the NFC circuit is an integrated circuit arranged in a first chip; and the supplemental power supply circuitry is an integrated circuit disposed in a second chip. The first chip, the second chip, and the NFC antenna are optionally disposed on the same circuit board.

Further embodiments herein include a method comprising: generating a supply voltage by rectifying and filtering an antenna voltage received from an NFC antenna to supply power to a load, and via the supply voltage, providing an output current to the load; and controlling the output current provided by the supply voltage depending on a difference between a reference voltage and the supply voltage.

One method embodiment as described herein includes generating the reference voltage by an NFC circuit that is coupled to the NFC antenna.

Further embodiments herein include limiting the output current provided to the load based on a control signal. The control signal is produced based on the difference between the reference voltage and the supply voltage.

Still further embodiments herein controlling the output current based upon a respective control signal applied to a current limiter that provides conductivity between the supply voltage and the load.

In one embodiment, controlling the output current includes applying the control signal to a respective switch.

Yet further example embodiments herein include deriving a control signal based upon a magnitude of the supply voltage.

Controlling the output current optionally includes applying the control signal to a respective switch controlling conveyance of the supply voltage to the load.

Moreover, a method for providing a supply voltage is described herein. In accordance with one embodiment, the method includes generating the supply voltage by rectifying and smoothing an antenna voltage received from an NFC antenna, and providing a respective output current to a load. Further, the method includes controlling the output current dependent on a difference between a reference voltage and the supply voltage.

