Method for determining the presence of a foreign metallic body at the interface between an NFC transmitter and an electrical device to be charged

Abstract

The invention relates to a method for determining the presence of a foreign metallic body at the interface between an NFC transmitter (1) comprising at least one MOSFET (5) and an electrical device to be charged (11), comprising the steps of: - measuring the drain-source voltage (Vds) of the MOSFET (5), - deducing from this measurement a value (OEi) representative of the presence of said body, - emitting a signal and / or interrupting the charging cycle when said value (OEi) reaches a predetermined threshold (OEseuil). (Fig. 1)

Description

Title of the invention: Method for determining the presence of a foreign metallic body at the interface between an NFC transmitter and an electrical device to be charged

[0001] The present invention relates to the field of wireless charging of electrical devices.

[0002] Electrical devices requiring low charging power, such as in-ear headphones, key fobs or smartwatches, can be wirelessly charged by NFC (“Near Field Communication”) transmitters.

[0003] The charging cycle is carried out by placing the electrical device close to the emitter, so as to be able to induce a charging current inside the electrical device.

[0004] Until recently, this NFC technology only allowed the charging of electrical devices with very low power, i.e. less than 1W.

[0005] With the use of MOSFETs (“Metal-Oxide-Semiconductor Field-Effect Transistor”) in the circuits of NFC transmitters, it is possible to charge electrical devices whose power typically reaches the order of 3W, thus expanding the field of application of NFC technology for the charging of electrical devices.

[0006] An inherent disadvantage of using NFC technology to charge electrical devices is the possible intrusion of a foreign metallic body into the interface separating the NFC transmitter from the device to be charged.

[0007] Such a foreign body may be, for example, a coin, a piece of metallic paper, etc.

[0008] Under the effect of NFC radiation, this foreign body can heat up, which may present a risk of burns to the user, or even a risk of fire.

[0009] Moreover, this foreign body disrupts the transmission of power to the electrical device to be recharged, thus degrading the expected quality of service.

[0010] Various solutions have been explored to detect the presence of a foreign metallic body, including power balance methods and current and voltage measurement methods, these methods being able to be used alone or in combination.

[0011] In practice, these methods have proven unreliable - particularly at frequencies above 1 MHz where the measurement of electrical parameters is not precise enough and / or the power balance is not sufficient on its own.

[0012] It was then considered to use measurements of non-electrical parameters such as pressure and temperature, requiring the use of expensive sensors whose reliability was not always guaranteed.

[0013] The present invention thus aims in particular to provide a method for determining the presence of a foreign metallic body at the interface between an NFC transmitter comprising at least one MOSFET and an electrical device to be charged which makes it possible to overcome these disadvantages of the prior art.

[0014] This objective of the invention is achieved with a method for determining the presence of a foreign metallic body at the interface between an NFC transmitter comprising at least one MOSFET and an electrical device to be charged, comprising the steps of: • Measure the drain-source voltage of the MOSFET. • deduce from this measurement a value representative of the presence of said body, • emit a signal and / or interrupt the charging cycle when said value reaches a predetermined threshold.

[0015] Various research and experimentation work has indeed shown that the drain-source voltage (i.e. between the drain and the source) of the MOSFET contained information strongly correlated with the presence of a metallic body in the interface between the NFC transmitter and the electrical device to be loaded.

[0016] Thus, by measuring the drain-source voltage and applying appropriate digital processing to it, information can be extracted to discriminate situations in which the charging power is at least partly transmitted to a foreign metallic body instead of being transmitted entirely to the electrical device to be charged.

[0017] Such a method, requiring no material modification of an NFC transmitter, but simply algorithmic modifications, makes it possible to considerably increase the reliability of detection of a foreign metallic body in the interface between the NFC transmitter and the electrical device to be charged, and this at a lower cost.

[0018] According to other optional features of the process according to the invention:

[0019] - the representative value of the presence of the foreign metallic body is determined at starting from the harmonics of said drain-source voltage;

[0020] - said drain-source voltage is analyzed by Fourier transform;

[0021] - the product of the fundamental frequency of the drain- voltage is also used source by the input current of the NFC transmitter to determine said representative value.

[0022] The present invention also relates to an NFC transmitter comprising at least one MOSFET, memory means in which said threshold value has been pre-recorded, means for measuring the drain-source voltage, and means for calculating said representative value and comparing it with said threshold value.

[0023] Other features and advantages of the invention will become apparent from the following description, with reference to the accompanying figures, which illustrate:

[0024] [Fig-1]: Schematically, an NFC transmitter according to the invention arranged in vis- with regard to an electrical device to be charged;

[0025] [Fig.2]: a series of drain-source voltage curves of a MOSFET forming part of an NFC transmitter according to the invention, for different positions relative to the electrical device to be loaded, and with or without a foreign metallic body; and

[0026] [Fig.3]: a series of drain-source voltage / frequency graphs corresponding respectively to the curves of [Fig.2], after application of a Fourier transform.

[0027] As can be seen in [Fig.1], the NFC transmitter according to the invention 1 typically comprises a DC current input 3, supplying in particular at least one MOSFET 5 having a drain-source voltage Vds 7 supplying itself at least one coil 9 capable of inductively charging an electrical device 11 forming a receiver arranged opposite the NFC transmitter 1 and itself equipped with its own induction coil 13.

[0028] Typically, this type of MOFSET transmitter makes it possible to obtain a class E amplifier, suitable for charging electrical devices whose power can be on the order of 3 W: these small rechargeable electrical devices include for example in-ear headphones, electronic key fobs, smartwatches, etc.

[0029] The NFC transmitter 1 also includes memory means, as well as a microcomputer enabling, in particular, from the measurement of the drain-source voltage Vds 7 of the MOFSET 5 and the supply current 3 of the transmitter, to carry out the operations which will be described below.

[0030] According to the invention, a value extracted from the measurement of the drain-source voltage Vds 7 is calculated in real time, which has been verified through prior experiments to be representative of the presence of a foreign metallic body in the interface between the NFC transmitter 1 and the electrical device to be charged 11.

[0031] In this case, it appeared that a highly representative value could be expressed in the form:

[0032] OEi = aH12 + bH22+ + xHi2(l)

[0033] where OEi is the indicative value of the presence of a foreign metallic body, Hl, .... Hi are the harmonic frequencies of the drain-source voltage Vds 7, and the coefficients a, b, .... x result from a prior calibration of the NFC 1 transmitter.

[0034] This calibration, carried out using different types of foreign metallic bodies (for example, a coin, a piece of metallic paper...), is performed a once and for all for a given NFC transmitter, and the coefficients a, b, ..., x are stored in the memory of NFC transmitter 1.

[0035] The harmonic frequencies Hb....., H; are calculated in real time by the microcomputer of the NFC transmitter 1, by Fourier transform.

[0036] Preferably, and to gain in precision, a term PFOm representing the power consumed by the MOFSET 5 is added to the formula referenced above, obtained by multiplying the intensity of the current 3 supplying the NFC transmitter by the fundamental frequency of the drain-source voltage Vds:

[0037] OEi = aH12 + bH22+ + xHi2 + PFOM (2)

[0038] This value OE;, calculated in real time by the NFC transmitter's microcomputer 1, is compared at each calculation cycle to a threshold value OEseuii, which we will have been able to determine beforehand and once and for all, during calibration operations, that it is indicative of the presence of a foreign metallic body in the interface between the NFC transmitter 1 and the electrical device to be charged 11.

[0039] By way of example, curves 1 to 6 of [Fig.2] illustrate the drain-source voltage Vds of the MOSFET 5 over time, for different respective positions of an electrical device to be loaded ("receiver") and a foreign metallic body relative to the NFC emitter 1: • Curve 1: Electrical device 11 centered with respect to emitter 1, without foreign metallic body, • Curve 2: Electrical device 11 on the side of the emitter 1, without any foreign metallic body, • Curve 3: Electrical device 11 centered with respect to emitter 1, with foreign metallic body centered with respect to emitter, • Curve 4: electrical device 11 centered with respect to emitter 1, with a foreign metallic body on the side of the emitter, • Curve 5: electrical device 11 on the side of emitter 1, with a foreign metallic body centered relative to the emitter, • curve 6: electrical device 11 on the side of the emitter 1, with metallic body on the side of the emitter.

[0040] In these examples, the situations considered critical, i.e. undesirable because they are likely to lead to a situation of heating of the foreign metallic body and / or incorrect charging of the electrical device 11, are those of figures 3, 5 and 6.

[0041] Graphs 1 to 6 of [Fig.3] illustrate respectively the Fourier transforms of curves 1 to 6 of [Fig.2],

[0042] As can be seen in this [Fig.3], graphs 3, 5 and 6 are those which have harmonics with the highest amplitudes, which makes it possible to understand why formulas (1) and (2), using the harmonics of the aforementioned drain-source voltage Vds, can translate the situations considered critical in terms of heating of a foreign metallic body and / or incorrect load.

[0043] When the value OE; reaches the value OEseuii, this indicates with a high confidence interval that a foreign metallic body is in the interface between the NFC transmitter 1 and the electrical device to be charged 11, and the transmitter 1 is then programmed to emit a signal to the user or to stop the charging cycle, in order to avoid heating the foreign metallic body and / or incorrect charging of the electrical device 11.

[0044] By way of illustration, in a particular case the following calculation formula could have been used for the value OE:

[0045] OEi = 4.8H32 + H52+ 1.8(H4 / H3)2 + 3PFOM

[0046] and with a threshold value OEseuil between 1.86 and 2.04, this formula made it possible to detect critical situations (heating of a foreign metallic body and / or incorrect charging of the electrical device 11) with 93.6% success.

[0047] As can be understood from the preceding description, the method according to the invention makes it possible, solely by implementing an appropriate algorithm and without any particular material modification of a MOFSET NFC transmitter, to detect the presence of a foreign metallic body in the interface between the transmitter and the electrical device to be charged.

[0048] This algorithm proves to be very reliable in practice, particularly in frequency ranges above 1 MHz, where the detection methods of the prior art are lacking.

[0049] In the present invention, the MOSFET therefore combines two functions: in addition to its basic function of inductively charging relatively low power electrical devices, and thanks to an algorithm developed specifically for this purpose, the MOSFET can fulfill the function of a presence sensor for a metallic object interposed between the NFC transmitter and an electrical device to be recharged.

[0050] Naturally, the invention is described above by way of example. It is understood that a person skilled in the art is able to carry out different embodiments of the invention without departing from the scope of the invention.

Claims

Demands

1. A method for determining the presence of a foreign metallic body at the interface between an NFC transmitter (1) comprising at least one MOSFET (5) and an electrical device to be loaded (11), comprising the steps of: - measuring the drain-source voltage (VÆ) of the MOSFET (5), - deducing from this measurement a value (OE;) representative of the presence of said body, - emitting a signal and / or interrupting the charging cycle when said value (OE;) reaches a predetermined threshold (OEseuii), the method being characterized in that the product (PFOM) of the fundamental frequency of the drain-source voltage (Vds) by the input current (3) of the NFC transmitter (1) is further used to determine said representative value (OEi).

2. A method according to claim 1, wherein the representative value (OE;) of the presence of the foreign metallic body is determined from the harmonics (Hi,..., H;) of said drain-source voltage (VJ.

3. Method according to claim 1, wherein said drain-source voltage (VJ) is analyzed by Fourier transform.