Circuit with transformer and method applied in the circuit

Abstract

A circuit is provided, comprising a transformer having a first coil arranged on a substrate, a second coil arranged on the substrate above the first coil, and a dielectric between the first coil and the second coil. The circuit further comprises a resonance circuit which is couplable with the first coil and / or the second coil to form a resonance loop, wherein at an output of the resonance circuit, a magnitude of a natural frequency of the resonance loop and / or a magnitude of a current consumption of the resonance loop can be intercepted. Further a method for application in a circuit is provided.

Description

TECHNICAL FIELD

[0001] The present application relates to a circuit with a transformer and a corresponding method, in particular to a circuit and a method which enable a diagnosis of a coil of the transformer. BACKGROUND

[0002] In different applications, for example in automotive applications, signals have to be transmitted between different voltage domains, for example between a circuit portion working at high voltage, for example in the range of > 20 V or > 100 V, for example 400 V, and a circuit portion working at low voltage, for example 5 V or 12 V. Here, the above-mentioned voltage values are to be understood only as examples.

[0003] Here, one possibility for signal transmission between multiple voltage domains is a transformer. A specific type of such a transformer is the so-called coreless transformer, wherein the coils of the transformer are arranged in different metal layers on a substrate, in particular a semiconductor substrate, which have a dielectric between them. Here, the coils are usually surrounded by a guard ring, which is grounded.

[0004] Such a transformer enables signal transmission while providing an insulation barrier which prevents a current flow, in particular a direct current flow, between the multiple voltage domains.

[0005] In the production of such a transformer coil, faults can occur. In safety-critical applications, for example in safety-critical applications in the automotive sector, it is desirable to be able to detect such faults. A conventional method for this is optical inspection, which is, however, time-consuming. SUMMARY

[0006] A circuit according to the application and a method according to the application are provided. Further embodiments are defined in the following description.

[0007] According to one embodiment, a circuit is provided, which comprises a transformer. The transformer has a first coil arranged on a substrate and a second coil arranged on the substrate above the first coil, and a dielectric between the first coil and the second coil. Furthermore, the circuit has a resonance circuit, which is couplable with the first coil and / or the second coil to form a resonance loop. At an output of the resonance circuit, a magnitude of a natural frequency of the resonance loop and / or a magnitude of a current consumption of the resonance loop can be tapped.

[0008] A corresponding method is also provided, which comprises coupling a resonance circuit with the first coil or the second coil of such a transformer. The method further comprises outputting a magnitude of a natural frequency of the resonance loop and / or a magnitude of a current consumption of the resonance loop.

[0009] The overview above provides only a brief overview of some embodiments and should not be construed as limiting, as other embodiments may also have different features than those discussed above. Attached Figure Description

[0010] Figure 1 This is a top view of a circuit according to one embodiment.

[0011] Figure 2 yes Figure 1 A schematic cross-sectional view of a portion of the circuit.

[0012] Figure 3 It is a flowchart used to illustrate a method according to one embodiment.

[0013] Figure 4 This is a partial view of a coil used to illustrate different defects.

[0014] Figure 5 This is a diagram of a circuit according to one embodiment.

[0015] Figure 6 This is a diagram of a circuit according to another embodiment.

[0016] Figure 7 It is a graph used to illustrate the significance of measuring the current consumption of a resonant circuit in some embodiments. Detailed Implementation

[0017] Different embodiments are described in detail below. These embodiments are merely illustrative and should not be construed as limiting. For example, in other embodiments, some of the features, components, or method steps shown may be omitted or replaced by alternative features, components, or method steps. Other features, components, or method steps may be provided in addition to those shown in detail, particularly those used in conventional transformer circuits and therefore not described in detail herein.

[0018] The embodiments described below relate to testing or inspecting coils in a so-called coreless transformer. Figure 1 This is a diagram of a circuit according to one embodiment.

[0019] Figure 1 The circuit includes an integrated high-voltage circuit 10 (high-voltage IC) and an integrated low-voltage circuit 11 (low-voltage IC). Here, the terms "high voltage" and "low voltage" should first be understood relative to each other, meaning that the integrated high-voltage circuit 10 operates at a higher voltage than the integrated low-voltage circuit 11. For example, the integrated high-voltage circuit 10 can operate at voltages >20V or >100V, such as at 400V.

[0020] The integrated low voltage circuit 11 can for example comprise sensor circuits, calculation circuits and / or logic circuits. It is to be noted that any other type of circuits can also be used in the integrated low voltage circuit 11 and in the integrated high voltage circuit 10.

[0021] In some embodiments, the integrated high voltage circuit 10 can comprise power transistors for switching high currents or high voltages, for example for operating a three-phase electric motor, while the integrated low voltage circuit 11 can comprise control circuits for controlling the power transistors.

[0022] In order to be able to transmit signals, for example control signals for the power transistors mentioned above, between the integrated high voltage circuit 10 and the integrated low voltage circuit 11, a transformer arrangement comprising a first transformer 12A and a second transformer 12B is provided. Via the transformers 12A, 12B, signals can be inductively transmitted between the integrated high voltage circuit 10 and the integrated low voltage circuit 11.

[0023] In Figure 1 embodiments, each transformer 12A, 12B has a differential design with two coil pairs. This can improve the quality of the signal transmission. Such a differential design or single pole design of the transformers can be carried out in any conventional manner. The transformers 12A, 12B are coupled to the integrated high voltage circuit 10 by means of bond wires.

[0024] For illustration, a simplified cross-sectional view of such a transformer is shown in Figure 2 Here, in Figure 2 the first coil 21 is arranged in a first metal layer above a substrate 24, while the second coil 20 is arranged in a second metal layer above the substrate 24. Between the coils 20, 21 a dielectric 22 is arranged. The substrate 24 can be a semiconductor substrate, on which, in Figure 1 the case, also the remaining integrated low voltage circuit 11 is constituted. The second coil 20 is then connected to the integrated high voltage circuit 10 by means of a bond wire 25, while the first coil 21 is connected to the remaining integrated low voltage circuit 11 by means of (not shown) metal connections, for example in the first metal layer or in other metal layers of the first coil 21. The other coil pairs in the differential design of the transformer and the other coil pairs of the other transformers (for example the transformers 12A, 12B of Figure 1 ) can be designed correspondingly.

[0025] Around the transformers 12A, 12B of Figure 1 a protection ring 13 is arranged. Said protection ring is separate from a protection ring 14 around the entire integrated low voltage circuit 11. Said separation of the protection rings 13 and 14 is again clearly shown in Figure 3 .

[0026] As inFigure 2 As already indicated with respect to the guard ring 26, the guard ring 13 can be formed, for example, by vertical connections (vias; "vertical interconnect access") of metal sections in a plurality of metal layers. In particular in the case Figure 2 , the metal layers include metal layers in which the first coil 21 and the second coil 20 are also formed.

[0027] Figure 1 The guard ring 13 or Figure 2 the guard ring 26 forms a lateral insulation for the transformer. This arrangement with the transformer and the guard ring results in an insulation barrier between the high-voltage domain of the integrated high-voltage circuit 10 and the low-voltage domain of the integrated low-voltage circuit 11. The insulation barrier is drawn in Figure 2 in dashed lines as an insulation barrier 23.

[0028] In the production of the coils of the transformer, different defects can occur. In order to be able to detect such defects, Figure 1 the circuit has a resonant circuit 15, which can be coupled with one or more of the coils of the transformer 12A, 12B. Here, the resonant circuit is a circuit which can form a resonant circuit, in particular an LC resonant circuit, together with the coil of the transformer 12A, 12B with which it is coupled, in which resonant circuit the coil(s) form(s) the inductance. The resonant circuit can also include components in order to supply the resonant circuit thus formed with energy, so that an oscillator circuit is formed. Such a resonant circuit, also called an oscillation circuit, has an inherent frequency f0. Furthermore, in particular if the resonant circuit has an amplitude adjustment device, the resonant circuit has a current consumption due to losses. This is explained in detail. The value of the inherent frequency f and / or the value of the current consumption I can be tapped at the resonant circuit 15. In particular if the thus determined value of the inherent frequency or the current consumption deviates from the expected value, a fault of the coil can be detected on the basis of the value of the inherent frequency and the value of the current consumption. Thus, a defect of the coil can change, for example, the inductance of the coil or the ohmic resistance of the coil, which can influence not only the inherent frequency of the resonant circuit but also the current consumption of the resonant circuit.

[0029] Here, the value of the inherent frequency is understood to be a variable which allows the inherent frequency to be inferred. In some embodiments which are explained in detail later, the value is a signal which is produced from the output signal of the resonant circuit by frequency division. Correspondingly, the value of the current consumption is a variable which allows the current consumption to be inferred. This can be, for example, an adjustment parameter of a current adjustment device or a value which is produced from a current measurement.

[0030] A flowchart of the corresponding method is shown in Figure 3 . Figure 3 The method can be used in Figure 1 and Figure 2The circuit is used in this way, and for simplicity, it is described with reference to the above explanation. However, Figure 3 This method can also be applied to other circuits, including those with corresponding transformers.

[0031] In 30, the resonant circuit, such as Figure 1 The resonant circuit 15 or referred to below Figure 5 and Figure 6 The described resonant circuit is coupled to one or more coils of a transformer. This coupling is particularly possible in diagnostic operating modes. In normal operating mode, the resonant circuit can be decoupled from the coils, and / or the resonant circuit can be deactivated. Then, in normal operating mode, the signal can be transmitted via the transformer, for example, in [the specified location]. Figure 1 Transmission between the integrated low-voltage circuit 11 and the integrated high-voltage circuit 10.

[0032] Then, in step 31, the inherent frequency and / or current consumption of the resonant circuit formed together with the coil through the resonant circuit are output. Based on these values, defects in the coil can then be identified as described.

[0033] Different faults that may occur in the coil and can be identified using the techniques described herein Figure 4 The images 1 through 5 are shown. Figure 4 Images 1 through 5 show top views of a portion of the coil. In image 1, the upper (farthest from the substrate) portion of the coil (e.g.) Figure 2 The winding of coil 20 is short-circuited due to incorrect handling. In Figure 2, the lower (closer to the base) coil (e.g.) Figure 2 A high-ohm circuit exists in coil 21. In image 3, there is an interruption in the winding of the upper coil. In image 4, the outer winding of the upper coil is short-circuited. In image 5, there is a short circuit within the upper coil.

[0034] Such faults, for example, alter the inductance of the coil, which is reflected in the resonant frequency of the resonant circuit formed by means of resonant circuit 15. This fault also affects the ohmic resistance of the coil, and consequently the current consumption of the resonant circuit. Therefore, this fault can be detected based on the magnitude of the resonant frequency and / or the magnitude of the current consumption. It should be noted that faults related to… Figure 4 The faults shown are different, as long as the faults correspondingly affect the ohmic resistance and / or inductance of the coil.

[0035] Figure 5 A circuit according to another embodiment is shown. Figure 5In the circuit shown, a signal is transmitted from a communication circuit 50, which is used here as a transmitting circuit (TX), to a communication circuit 51, which is used here as a receiving circuit (RX), via a transformer. Here, the transformer is a differential transformer, which is used for differential signal transmission by means of a first coil pair 53A, 53B and a second coil pair 54A, 54B. The first coil 53A of the first coil pair and the first coil 54A of the second coil pair are connected to the communication circuit 51 as shown. The first terminal of the first coil 53A and the second terminal of the first coil 54A are connected to the communication circuit 51, while the second terminal of the first coil 53A and the first terminal of the first coil 54A are connected to ground. The second coil 53B of the first coil pair and the second coil 54B of the second coil pair are connected to the communication circuit 50 in a corresponding manner, wherein the first terminal of the second coil 53B and the second terminal of the second coil 54B are in turn used to receive a signal from the communication circuit 50, and the second terminal of the second coil 53B and the first terminal of the second coil 54B are connected to ground.

[0036] The communication circuit 50 can be provided, for example, in a low-voltage domain, such as an integrated low-voltage circuit 11, and receives a signal di to be transmitted, which is then transmitted to the communication circuit 51 via the transformer and output by the communication circuit as a received signal do. In the example shown, the communication circuit 50 is designed as a transmitting circuit and the communication circuit 51 is designed as a receiving circuit. In other embodiments, the roles of the communication circuits 50, 51 can be reversed, or the communication circuits can be designed for bidirectional data transmission. Figure 5 Figure 5 In the example shown, the connections between the communication circuits 50, 51 and the respective coils 53A, 53B, 54A, 54B are furthermore via diodes 55A, 55B, 56A, 56B, as shown. The diodes 55A, 55B, 56A, 56B are used to protect the communication circuits 50, 51 from overvoltages, for example, in the event of a short circuit. Figure 5 The parasitic capacitances resulting from the components and the layout of the circuit are connected to ground.

[0037] Figure 5 ​The circuit further has a first resonant circuit 52A, which is coupled with the first coils 53A, 54A as shown, and a second resonant circuit 52B, which is coupled with the second coils 53B, 54B as shown. The coupling can be designed as an optional coupling, for example by means of a corresponding switch, so that the resonant circuits 52A, 52B are coupled with the respective coils only for diagnostic purposes and are decoupled from the coils when a signal is transmitted from the communication circuit 50 to the communication circuit 51 in normal operation. In some embodiments, the resonant circuits 52A, 52B are also coupled with the respective coils 53A, 54A or 53B, 54B only during product testing and are permanently deactivated thereafter. In further embodiments, the coupling can be repeated during the service life of the circuit in order to identify occurring effects of the coils. Instead of decoupling, in further embodiments the oscillator circuit 52A or 52B can also be connected in a currentless manner in normal operation, for example decoupled from the supply.

[0038] The resonant circuit 52A forms a resonant loop together with the coils 53A, 54A, the natural frequency of which is determined by the inductance of the coils 53A, 54A and by the capacitance of the resonant loop 52A. The resonant loop is then oscillated at its natural frequency for diagnostic purposes, for which purpose a current is fed to the resonant loop. The output signal having the natural frequency is then also divided by a frequency divider 55A in order to output an output signal whose frequency is the magnitude of the natural frequency. In some applications, the natural frequency can occur in the gigahertz range, for example at approximately 8 GHz. The frequency is reduced by the frequency divider 55A so that the output signal f can be more easily processed by subsequent circuit parts. However, in further embodiments, the frequency divider 55A can also be omitted. Furthermore, the oscillator circuit 52A outputs the magnitude of the current consumption I.

[0039] For example, if a short circuit is present in the transformer winding, the effective inductance of the respective coil is smaller and the oscillator frequency rises, since the natural frequency is

[0040]

[0041] Where L is the inductance of the coil, and C is the capacitance of the resonant circuit (which can also be composed of the capacitances of multiple capacitors). Deviations from the desired frequency can be detected. If the resistance of the coil winding is greater than it should be, the quality of the resonant circuit deteriorates, and current consumption increases. Furthermore, a short circuit in the transformer winding can also affect current consumption, and / or a high-resistance winding can affect inductance and thus frequency. Therefore, such faults can be detected by evaluating the inherent frequency and / or current consumption. For this purpose, in the diagnostic circuit 56, for example, the magnitude of the inherent frequency (signal f) and / or the magnitude of the current consumption (I) can be compared with expected values, and a fault signal can be output if the deviation exceeds a preset tolerance threshold.

[0042] The above description of the resonant circuit 52A with frequency divider 55A also applies in a corresponding manner to the resonant circuit 52B and frequency divider 55B, to test the second coils 53B and 54B.

[0043] An example of this design scheme for a resonant circuit is shown in Figure 6 As shown in the image. Here, corresponding to... Figure 5 The components of the drawing have the same reference numerals and will not be described again.

[0044] Figure 6 A resonant circuit is shown, which is coupled to the second coils 53B and 54B and can be used as... Figure 5 An example of the resonant circuit 52B. A correspondingly designed resonant circuit can also (corresponding to...) Figure 5 The resonant circuit 52A is coupled to the first coils 53A and 54A. The communication circuit 50 is designed to simplify... Figure 6 Not shown in the image.

[0045] Figure 6 The resonant circuit includes a pair of transistors 60A and 60B, which are cross-coupled via capacitors 63A and 63B and are energized by a bias voltage V. bias A bias voltage is applied via a resistor as shown. Furthermore, capacitors 64A and 64B are coupled in parallel with the second coils 53B and 54B. The resonant circuit thus formed is connected via a supply voltage V. supply A controllable current source 65 is supplied. In this case, the resonant circuit therefore also includes components for transferring energy to the resonant circuit, as already present. Figure 1 As described in the feasibility section, the resonant circuit can be designed via transistor 66 in a manner that separates it from the supply voltage. Therefore, the resonant circuit can, for example, be switched off in normal operating mode and activated only for diagnostic purposes.

[0046] The natural frequency of the resonant circuit thus formed is determined by the inductance of the second coils 53B and 54B and the capacitance of capacitors 63A, 63B, 64A, and 64B according to the above formula. The output signal of the resonant circuit with the stated frequency can be intercepted at nodes 61A and 61B, and can then be obtained again as referred to Figure 5 As already described, the signal is also fed to the frequency divider. The output signal of the resonant circuit is also fed to the amplitude adjustment device 62 (AGC, Automatic Gain Control). The amplitude adjustment device adjusts the current I supplied through the current source 65 so that the oscillation amplitude of the output signal of the resonant circuit has a preset value. Here, any conventional amplitude adjustment device can be used. The current consumption thus derived from the resonant circuit formed by the resonant circuit and the second coils 53B and 54B can be taken into account for analysis, as also described above. The increased resistance of the coils causes even higher current consumption. Therefore, the current I can be measured in a conventional manner, such as via a current mirror, a measuring transistor, a measuring resistor, etc., or the adjustment parameters of the amplitude adjustment device 62 can be taken into account as a value of current consumption. The effect of resistance on current consumption will now be briefly referred to. Figure 7 Explanation.

[0047] As already explained, the coil and the capacitor of the resonant circuit together form a resonant circuit. Figure 7 A simplified equivalent circuit diagram for the resonant circuit is shown on the left. Here, the coil is connected by an ideal inductance L (i.e., an inductance without ohmic resistance) and a series resistance R. S The series circuit is shown. The capacitor is represented by capacitor C. The natural frequency f0 of this oscillating circuit is as mentioned above.

[0048]

[0049] Figure 7 The equivalent circuit diagram on the left can be obtained through Figure 7 The equivalent circuit diagram on the right is used to replace it, where the series resistor R connected in series with the inductor L is replaced. S There is a parallel resistance R P In this context, it applies to:

[0050] R p =L / (R) s x C)

[0051] Therefore, the amplitude of the oscillation is A = gm × R. p Where gm is the amplification factor determined by cross-coupled transistors 60A and 60B, and this amplification factor is related to the magnitude of the current I. The ohmic resistance R of the coil is then... S The larger R is, the better. pThe smaller, and for a given amplitude, the larger the amplification factor gm. Thus, a larger current consumption is required here, and the current consumption can indicate an increased resistance of the coil.

[0052] Thus, a fault in the coil can be determined by different embodiments.

[0053] Some embodiments are defined by the examples below:

[0054] Example 1. A circuit, the circuit comprising:

[0055] a transformer, the transformer having:

[0056] a first coil, the first coil being arranged on a substrate,

[0057] a second coil, the second coil being arranged on the substrate above the first coil, and

[0058] a dielectric between the first coil and the second coil, and

[0059] a resonance circuit, the resonance circuit being couplable with the first coil and / or the second coil to form a resonance loop, wherein at an output of the resonance circuit, a magnitude of a natural frequency of the resonance loop and / or a current consumption of the resonance loop can be tapped.

[0060] Example 2. The circuit according to example 1, wherein the circuit is arranged for, in a normal mode of operation, transmitting a signal via the transformer, and decoupling the resonance circuit from the first coil and the second coil, and / or deactivating the resonance circuit.

[0061] Example 3. The circuit according to example 1 or 2, wherein the resonance circuit comprises a current regulating means of a current flowing through the resonance loop, the current regulating means being arranged for regulating an oscillation amplitude of the resonance loop.

[0062] Example 4. The circuit according to any one of examples 1 to 3, wherein the resonance circuit comprises a first resonance circuit couplable with the first coil to form a first resonance loop, and at an output of the first resonance circuit, a first natural frequency of the first resonance loop can be tapped, and a second resonance circuit couplable with the second coil to form a second resonance loop, and at an output of the second resonance circuit, a second natural frequency of the second resonance loop can be tapped.

[0063] Example 5. The circuit according to any one of examples 1 to 4, wherein the resonance circuit comprises a capacitance, wherein the natural frequency is determined by the capacitance and an inductance of the first coil or the second coil.

[0064] Example 6. The circuit according to any one of examples 1 to 5, wherein the resonance circuit comprises a pair of cross-coupled transistors.

[0065] Example 7. The circuit according to any one of examples 1 to 6, further comprising a diagnostic circuit, the diagnostic circuit being configured to detect a fault of the first coil or the second coil based on the natural frequency and / or the current.

[0066] Example 8. The circuit according to any one of examples 1 to 7, wherein the first coil comprises a first pair of differential coils and the second coil comprises a second pair of differential coils.

[0067] Example 9. The circuit according to any one of examples 1 to 8, wherein the resonant circuit further comprises a frequency divider for dividing the natural frequency, wherein at the output the divided natural frequency is available as a magnitude of the natural frequency.

[0068] Example 10. A method, the method comprising:

[0069] coupling a resonant circuit with the first coil or the second coil of the transformer so as to form a resonant loop, wherein the first coil is arranged on a substrate, the second coil is arranged on the substrate above the first coil, and a dielectric is arranged between the first coil and the second coil, and

[0070] outputting a magnitude of a natural frequency of the resonant loop and / or a magnitude of a current consumption of the resonant loop.

[0071] Example 11. The method according to example 10, further comprising, in a normal operation mode, transmitting a signal via the transformer and decoupling the resonant circuit from the first coil and the second coil and / or deactivating the resonant circuit.

[0072] Example 12. The method according to example 10 or 11, further comprising adjusting a current flowing through the resonant loop so as to adjust an oscillation amplitude of the resonant loop.

[0073] Example 13. The method according to any one of examples 10 to 12, wherein the resonant circuit comprises a capacitance, wherein the natural frequency is determined by the capacitance and an inductance of the first coil or the second coil.

[0074] Example 14. The method according to any one of examples 10 to 13, further comprising detecting a fault of the first coil or the second coil based on the natural frequency and / or the current.

[0075] Example 15. The method according to any one of examples 10 to 14, further comprising dividing the natural frequency, wherein the magnitude of the natural frequency is the divided natural frequency.

[0076] While particular embodiments have been illustrated and described in the present specification, it will be appreciated that alternative and / or equivalent implementations can be employed without departing from the scope of the application illustrated. It is intended that the application encompass all solutions discussed herein. Therefore, it is intended that the application not be limited to the particular embodiments disclosed in the specification.

Claims

1. A circuit, the circuit comprising: a transformer (12A; 12B) having a first coil (21) arranged on a substrate, a second coil (20) arranged on the substrate above the first coil (21), and a dielectric (22) between the first coil (21) and the second coil (20), and a resonance circuit (15), wherein the circuit is set up in a diagnostic operating mode, the resonance circuit (15) is coupled with the first coil (21) and / or the second coil (20) to form a resonance loop, wherein at an output of the resonance circuit (15) a magnitude of an eigenfrequency of the resonance loop and a magnitude of a current consumption of the resonance loop can be intercepted, wherein the resonance circuit forms an oscillator circuit which defines the eigenfrequency and the current consumption, and in a normal operating mode, a signal is transmitted via the transformer (12A; 12B) and the resonance circuit (15) is decoupled from the first coil (21) and the second coil (20) and / or deactivated, wherein the resonance circuit (15) comprises an automatic gain control of a current flowing through the resonance loop in order to adjust an oscillation amplitude of the resonance loop, and the circuit further comprises a diagnostic circuit (56) set up for detecting a fault of the first coil (21) and / or the second coil (20) based on the magnitude of the eigenfrequency and the magnitude of the current consumption.

2. The circuit according to claim 1, wherein the resonant circuit (15) comprises: a first resonance circuit (52A) which can be coupled with the first coil (21) to form a first resonance loop and at an output of which a first eigenfrequency of the first resonance loop can be intercepted, and a second resonance circuit (52B) which can be coupled with the second coil (20) to form a second resonance loop and at an output of which a second eigenfrequency of the second resonance loop can be intercepted.

3. The circuit according to claim 1 or 2, wherein the resonance circuit (15) comprises a capacitance (63A, 63B; 64A; 64B), wherein the eigenfrequency is determined by the capacitance (63A, 63B; 64A, 64B) and an inductance of the first coil (21) or of the second coil (20).

4. The circuit according to claim 1 or 2, wherein the resonance circuit (15) comprises a pair of cross-coupled transistors (60A, 60B).

5. The circuit according to claim 1 or 2, wherein the first coil (21) comprises a first pair of differential coils (53B, 54B) and the second coil (20) comprises a second pair of differential coils (53A, 54A).

6. The circuit according to claim 1 or 2, wherein the resonant circuit (15) further comprises a frequency divider (55A, 55B) for dividing the natural frequency, wherein at the output the divided natural frequency is available as a magnitude cut of the natural frequency.

7. A method applied in a circuit according to any one of claims 1 to 6, the method comprising: in a diagnostic operating mode, coupling a resonant circuit (15) with a first coil (21) or a second coil (20) of a transformer (12A; 12B) so as to form a resonant tank, wherein the first coil (21) is arranged on a substrate, the second coil (20) is arranged on the substrate above the first coil (21), and a dielectric (22) is arranged between the first coil (21) and the second coil (20), and outputting a magnitude of a natural frequency of the resonant tank and a magnitude of a current consumption of the resonant tank, wherein the resonant circuit forms an oscillator circuit which defines the natural frequency and the current consumption, in a normal operating mode, transmitting a signal via the transformer (12A; 12B) and decoupling the resonant circuit (15) from the first coil (21) and the second coil (20) and / or deactivating the resonant circuit, wherein the method further comprises an automatic gain control of a current flowing through the resonant tank so as to adjust an oscillation amplitude of the resonant tank, and the method further comprises detecting a fault of the first coil (21) and / or the second coil (20) based on the magnitude of the natural frequency and the magnitude of the current consumption.

8. The method according to claim 7, wherein the resonant circuit (15) comprises a capacitance (63A, 63B; 64A, 64B), wherein the natural frequency is determined by the capacitance (63A, 63B; 64A, 64B) and an inductance of the first coil (21) or the second coil (20).

9. The method according to claim 7 or 8, the method further comprising dividing the natural frequency, wherein the magnitude of the natural frequency is the divided natural frequency. ​

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