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 charged wirelessly by NFC (“Near Field Communication”) transmitters.

[0003] The charging cycle is carried out by placing the electrical device near the transmitter, so as to be able to cause a charging current by induction inside the electrical device.

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

[0005] With the use of MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) in the circuits of NFC transmitters, it is possible to charge electrical devices with a power typically reaching around 3W, thus expanding the scope of application of NFC technology for charging electrical devices.

[0006] A disadvantage inherent in the use of NFC technology to recharge electrical devices is the possible intrusion of a foreign metallic body into the interface separating the NFC transmitter from the device to be recharged.

[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 can present a risk of burns for the user, or even a risk of fire.

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

[0010] Different solutions have been explored to detect the presence of a foreign metallic body, in particular 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 to be 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 up to scratch.

[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 drawbacks of the prior art.

[0014] This aim 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 experimental works have in fact made it possible to demonstrate 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 charged.

[0016] Thus, by measuring the drain-source voltage and applying appropriate digital processing to it, it is possible to extract information making it possible to discriminate between 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 hardware 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 characteristics of the method according to the invention:

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

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

[0021] - we also use the product of the fundamental frequency of the drain voltage- 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 characteristics and advantages of the invention will emerge on reading the description which follows, with reference to the appended figures, which illustrate:

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

[0025] [Fig.2]: a series of curves of the drain-source voltage of a MOSFET forming part of an NFC transmitter according to the invention, for different positions relative to the electrical device to be charged, 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.l], the NFC transmitter according to the invention 1 typically comprises a direct current input 3, supplying in particular at least one MOSFET 5 having a drain-source voltage Vds 7 itself supplying at least one coil 9 capable of inductively charging an electrical device 11 forming a receiver arranged opposite the NFC transmitter 1 and itself provided with its own induction coil 13.

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

[0029] The NFC transmitter 1 also comprises memory means, as well as a microcomputer making it possible 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 it will have been possible to ensure through prior experiments that it is 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 transmitter 1.

[0034] This calibration, carried out using different types of foreign metallic bodies (for example coin, piece of metallic paper, etc.), is carried out once once and for all for a given NFC transmitter, and the coefficients a, b,....., x are stored in the memory of the 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 precision, a term PFOm representative of 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 microcomputer of the NFC transmitter 1, is compared at each calculation cycle to a threshold value OEseuii, which can be determined beforehand and once and for all, during calibration operations, as being 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] As an 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 charged (“receiver”) and of a foreign metallic body relative to the NFC transmitter 1: • curve 1: electrical device 11 centered in relation to transmitter 1, without foreign metallic body, • curve 2: electrical device 11 on the side of transmitter 1, without foreign metallic body, • curve 3: electrical device 11 centered in relation to the transmitter 1, with foreign metallic body centered in relation to the transmitter, • curve 4: electrical device 11 centered in relation to transmitter 1, with foreign metallic body on the side of the transmitter, • curve 5: electrical device 11 on the side of transmitter 1, with foreign metal body centered in relation to the transmitter, • curve 6: electrical device 11 on the side of transmitter 1, with metal body on the side of the transmitter.

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

[0041] Graphs 1 to 6 of [Fig.3] respectively illustrate 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 include 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 charging.

[0043] When the value OE; reaches the value OEseuii, this indicates with a high confidence interval that a foreign metal body is located 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 of the foreign metal body and / or incorrect charging of the electrical device 11.

[0044] As an illustration, in a particular case we could have used the following calculation formula for the OE value:

[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 a 93.6% success rate.

[0047] As can be understood in light of the preceding description, the method according to the invention makes it possible, solely by implementing an appropriate algorithm and without any particular hardware modification of an NFC transmitter with MOFSET, 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 allowing relatively low power electrical devices to be recharged by induction, and thanks to an algorithm developed specifically for this purpose, the MOFSET can fulfill the function of detecting the presence of a metallic object interposed between the NFC transmitter and an electrical device to be recharged.

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

Claims

Claims

1. 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 (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).

2. Method according to claim 1, in which the representative value (OE;) of the presence of the foreign metallic body is determined from the harmonics (Hb..., H;) of said drain-source voltage (VÆ

3. J- Method according to claim 1, in which said drain-source voltage (VÆ) is analyzed by Fourier transform.

4. Method according to any one of the preceding claims, in which the product (PF0M) of the fundamental frequency of the drain-source voltage (V*) by the input current (3) of the NFC transmitter (1) is further used to determine said representative value (OE;).

5. NFC transmitter (1) comprising at least one MOSFET (5), memory means in which a threshold value (OEseuii) has been pre-recorded, means for measuring the drain-source voltage (Vds), and means for calculating a representative value (OE;) and comparing it with said threshold value (OEseuU), said transmitter being programmed to emit a signal or to stop the charging cycle when said value (OEi) reaches a predetermined threshold (OEseuil).

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