Shielding for an induction cooking hob

Abstract

The invention relates to a shielding (10) for an induction cooking hob (100) for shielding stray electromagnetic radiation (E), said shielding being arranged on an electrically non-conductive substrate (S). According to the invention, a loop-free pattern (M) of electrically conductive conductor paths (11) is arranged on the electrically non-conductive substrate (S), wherein the conductor paths (11) are conductively connected together in a galvanic manner, preferably outside of a center (Z) or a surface center of gravity, and have an electric contact (12) to a stabilized DC current source or to ground via said connection.

Description

[0001] Shielding for an induction hob

[0002] The invention relates to a shield for an induction hob for shielding electromagnetic scattered radiation, which is arranged on an electrically non-conductive substrate.

[0003] Induction immersion heaters are commonly used for heating liquids. These heaters consist of an induction element placed in a container containing the liquid to be heated, which is then placed on an induction hob. Unlike induction hobs, which are fully covered by a corresponding pot, induction hobs designed for use with induction immersion heaters have the problem that the induction coil emits significant electromagnetic interference. The so-called electromagnetic compatibility according to standard EN61000 for such induction hobs designed for use with induction immersion heaters cannot be achieved without special measures to suppress stray radiation.

[0004] Microwave ovens face a very similar problem. On the one hand, it should be possible to observe the cooking process in the microwave, so a window to the microwave chamber is provided. On the other hand, no electromagnetic interference must escape through the window, and certainly not microwave radiation that could be dangerous to the user. Microwave radiation could be absorbed by the human body, and thus tissue overheating could occur, leading to necrosis. To suppress microwave radiation and electromagnetic interference, perforated grilles are used in microwave door windows, which act like a Faraday cage. However, shielding such as that found in microwave door windows is not suitable for use on induction cooktops, as these perforated grilles allow ring currents to flow, which, if used on an induction coil, would destroy the perforated grille.The perforated grid itself would be heated extremely strongly on the induction coil, causing the perforated grid to burn.

[0005] To use a Faraday cage cover on an induction coil, it is necessary to arrange loop-free paths of metallic conductors. However, even a loop-free arrangement of paths does not guarantee that the metallic conductors remain free of eddy currents. According to experimental studies, the mere formation of bays and partially circular paths leads to partial eddy currents, which therefore significantly heats the metallic conductors. Although this effect is not as strong, it can lead to undesirable heating, which can place thermal stress on the substrate of the Faraday cage.

[0006] The object of the invention is therefore to provide a shield for an induction hob which absorbs interference radiation, whereby the cover in the induction field should not be heated itself.

[0007] The object of the invention is achieved in that a loop-free pattern of electrically conductive conductor tracks is arranged on the electrically non-conductive substrate, wherein the conductor tracks are galvanically conductively connected to one another, preferably outside a center or a center of gravity, and have an electrical contact with a stabilized direct current source or with ground via this connection.

[0008] Further advantageous embodiments are specified in the dependent claims of claim 1. Claims 7 to 10 claim an induction hob equipped with the cover according to the invention. After extensive testing, this conductor pattern results in sufficient shielding against interference, yet prevents the formation of eddy currents of a magnitude that could pose a threat to the conductors themselves through heating.

[0009] It's possible that loops are located in the area where the conductor tracks meet if the conductor tracks are located outside the area of ​​the induction hob to be covered. There, the effect of the induction coil is no longer strong enough for the ring currents generated in the loops to heat the conductor tracks to such an extent that they could be damaged.

[0010] In a particular shielding design, it has proven advantageous to arrange the electrical conductors in a fan-shaped pattern on a circular surface. This fan-shaped pattern, with adjacent conductors at a small angle, results in an optimal compromise between effective shielding and low self-heating.

[0011] With the typical dimensions of an induction hob for a household, namely with a power density of 500W to 3kW on an area between approx. 75 cm 2 up to approx. 1,200 cm 2The best compromise between shielding effectiveness and low self-heating has been found to be when the distance between two adjacent conductor tracks is between 0.5 mm and 2 mm, with the width of the conductor tracks being between 0.08 mm and 0.5 mm. In yet another embodiment of the shielding according to the invention, it has proven particularly advantageous when the angle α between two conductor tracks is between 15' (0.25°) and 2.5°, with the area to be shielded, which is covered by the conductor tracks 11, being between approximately 75 cm 2 and approx. 1,200 cm 2 amounts.

[0012] The shielding can be constructed like a flexible circuit board consisting of a non-conductive substrate and conductor tracks. The electrically non-conductive substrate can be made of polyimide or polyethylene terephthalate, with the conductor tracks being made of clad copper or deposited or vapor-deposited gold. After extensive experiments, it has proven advantageous for the aforementioned household-typical energy density and actual size ranges if the layer thickness of the conductor tracks is between 5 μm and 100 μm. If the layer thickness is thinner, the electrical resistance increases, so that the electrical resistance leads to self-heating. If the layer thickness is thicker, microscopic eddy currents with a larger surface area form in the conductor track, which also leads to self-heating. This also reduces the shielding effect.

[0013] In order to prevent eddy currents from forming in the conductor tracks, it can be provided that in an induction hob with shielding the galvanic connection of the conductor tracks is arranged at the edge of the induction hob.

[0014] In order to avoid harmonics with high frequencies in the interference radiation, it can be provided that a low-pass filter is arranged in front of an induction coil of an induction cooking plate with the shielding according to the invention, which low-pass filter filters out harmonics from an excitation current, wherein the filtered-out harmonic frequency of the excitation current is greater than 250 kHz and extends up to approximately 1 MHz.

[0015] The shielding presented here is suitable for induction hobs with a nominal heat output of an induction coil between 500 W and 3 kW and a diameter of between 10 cm and 40 cm. The shielding conductors can be galvanically connected to the induction coil on the ground side. Galvanic contact with the induction coil is not absolutely necessary, as the currents generated in the conductors can be diverted via a galvanic contact with a stabilized DC power source. An electrical ground contact is a stabilized contact with a stabilized DC power source with a potential of ideally 0 V. A ground contact ensures that the shielding potential cannot become dangerous.

[0016] The invention is explained in more detail with reference to the following figures. Fig. 1 shows an exemplary induction hob without shielding,

[0017] Fig. 2 a shield according to the invention,

[0018] Fig. 3 a cup with liquid and induction body submerged in it on an unshielded induction cooking plate,

[0019] Fig. 4 a cup with liquid and induction body submerged in it on a shielded induction cooking plate,

[0020] Fig. 5.1 a graph of an excitation voltage with harmonic content,

[0021] Fig. 5.2 an exemplary low-pass filter as an RC element,

[0022] Fig. 5.3 a graph of an excitation voltage with filtered out harmonics,

[0023] Fig. 5.4 a sketched induction coil,

[0024] Fig. 6.1 a shielding from the PRIOR ART, as used in door windows of microwave ovens, with the path of a possible eddy current drawn in bold,

[0025] Fig. 6.2 the shielding from Fig. 6.1 from the PRIOR ART in a perspective view with drawn magnetic field lines,

[0026] Fig. 7 shows an alternative arrangement of conductor tracks in a further embodiment of the shield,

[0027] Fig. 8 shows an alternative arrangement of conductor tracks in a further embodiment of the shield,

[0028] Fig. 9 shows an alternative arrangement of conductor tracks in another square embodiment of the shield,

[0029] Fig. 10 shows an alternative arrangement of conductor tracks in a further rectangular embodiment of the shield,

[0030] Fig. 11 shows an alternative arrangement of conductor tracks in a further hexagonal embodiment of the shield, Fig. 12 shows an alternative arrangement of conductor tracks in a further hexagonal embodiment of the shield.

[0031] Figure 1 shows an exemplary induction cooking plate 100 without a shield 19 according to the invention and a pot T standing thereon. Such an induction cooking plate 100 as an accessory device is available in retail stores, and this induction cooking plate 100 is designed to accommodate corresponding pots T that have approximately the base area of ​​the roof surface of the induction cooking plate 100 shown here. An edge of the pot T runs approximately along the edge 110 of the induction cooking surface shown here. Since the pot T completely covers the induction cooking plate, the problem of stray electromagnetic radiation leaving the induction cooking plate 100 and appearing in the surrounding area as a strong source of electromagnetic interference does not arise.

[0032] Figure 2 outlines a shield according to the invention. Such a shield can be positioned between an induction coil 120 (Figure 5.4) and the surface of an induction cooking plate 100. There, it is protected from environmental influences in the kitchen (humidity, water, grease) and, in turn, covers the induction coil 120, so that no stray or only significantly attenuated stray electromagnetic interference leaves the induction cooking plate. The shield can be made of polyimide like a flexible circuit board and laminated with copper as conductor tracks 11. Alternatively, polyethylene terephthalate can also be considered. If the shield can become hot, polyimide, which is resistant to temperatures up to 400°C, is suitable. Any highly conductive metal or other electrical conductors such as carbon fiber, but preferably copper, can be used as the conductor track material, as well as vapor-deposited gold.Gold has the advantage that it won't corrode in a kitchen environment, even if a protective layer over the circuit board is damaged. Finally, gold's high ductility is valuable because it allows the evaporated layer to be very thin without mechanical stress causing the conductor track 11 on the flexible substrate to tear or crack.

[0033] Figure 3 shows a cup containing liquid 150 and an induction body I submerged in the cup, i.e., in the liquid 150, on an unshielded induction cooking plate 100. Since the diameter of the cup shown here is considerably smaller than the diameter of the induction cooking plate 100, a portion of the induction coil 120, located beneath the surface of the roof of the induction cooking plate 100, is exposed, allowing electromagnetic interference to escape from the induction cooking plate 100. The interference is shown here as an electric field E, which changes its deflection in a wave-like manner over time t and along its path along the radius r. The amplitude of the electric field E represents the intensity of the unwanted scattered radiation or stray electromagnetic radiation. The frequency fß of the induction coil 120 in the induction cooking plate 100 is approximately 30 kHz to approximately 50 kHz.This frequency fß generates a magnetic field through the induction coil 120 with the oscillating magnetic field lines B shown here. These oscillating magnetic field lines B generate eddy currents in an induction body I, which heat the induction body I considerably. The energy transfer between the induction coil 120 in the induction cooking plate 100 thus takes place via an inductive coupling, so that a certain proportion of the energy in the induction coil 120 leaves the induction coil 120 as an electric field E. The frequencies of the fundamental oscillation in the range from 30 kHz to 50 kHz are less problematic than the frequencies fß beyond 250 kHz up to approximately 1 MHz. Harmonics above this are no longer very pronounced and can be attenuated by a low-pass filter in the control electronics at the device's mains connection.

[0034] Figure 4 depicts a cup containing liquid 150 and an induction body I submerged therein on an induction cooking plate 100 shielded with a shield 10 according to the invention. The linear conductor tracks 11 in the shield 10 act like a Faraday cage, so that stray electromagnetic radiation no longer leaves the induction cooking plate 100; at least, this interference radiation with a frequency fE in the range of 250 kHz to 1 MHz is strongly attenuated. Nevertheless, the magnetic field lines B leave the induction cooking plate 100 almost unfiltered. The shield 10 according to the invention is therefore transparent to the oscillating magnetic field lines B. In contrast, the shield 10 according to the invention is opaque to stray electromagnetic radiation or at least attenuates the unwanted stray electromagnetic radiation.

[0035] The actual dimensions and orientation of the conductor tracks 11 in the shielding 10 are very important for attenuating electromagnetic radiation. The induction cooking plates 100 in question here have a rated power between 500 W and 3 kW. The induction cooking plates 100, with the induction coil 120 installed within them, have a diameter between 10 cm and 40 cm. Induction cooking plates 100 with a diameter of 10 cm are intended as add-on appliances in an office kitchen or as add-on appliances in a private household. With a rated power of up to 3 kW, the small induction cooking plates 100 have a relatively high energy density for household appliances. Induction cooking plates with a diameter of up to 40 cm are conceivable as cooking plates in fitted kitchens.In order to shield stray electromagnetic interference from such an induction hob 100 in such a way that it complies with the requirements of legal regulations on electromagnetic compatibility, as defined in the EN61000 standard, it has proven advantageous if the spacing of the conductor tracks is between 0.5 mm and 2 mm, the distance between two adjacent conductor tracks 11 of the shielding, with the width of the conductor tracks 11 being between 0.08 mm and 0.5 mm. This coverage ratio, coupled with the actual dimensions of the width of the gaps between the conductor tracks and the conductor track width, ensures, on the one hand, that electromagnetic compatibility is met in the range between 250 kHz and 1 MHz.At the same time, it is ensured that eddy currents that can form in the conductor tracks 11, due to the dimensions of the width of the conductor tracks 11, only cover such a small area at the given magnetic field strength, around 500 W to 3 kW on an area between approx. 75 cm. 2 , based on a radius of the induction cooking plate 100 of 5 cm (area: TT r 2 , i.e. approximately 3.14 x 5 cm x 5 cm ) up to an area of ​​approximately 1,200 cm 2 , based on a radius of the induction cooking plate 100 of 20 cm (area: TT r 2, i.e. approximately 3.14 x 20 cm x 20 cm) can form so that the energy dissipation through heat does not destroy the conductor tracks 11 with the usual conductor track thicknesses that are common in circuit board construction and also the substrate is not overheated. Conductor track thicknesses between 5 pm and 100 pm are advantageous here. In order to ensure that the shielding effect does not vary too much in all spatial directions emanating from the surface, the angle between the conductor tracks 11 should be between 15' (0.25°) and 2.5°. In this way, the clear width between individual conductor tracks 11 does not change so much that the coverage ratio between conductor track 11 and the free space in between becomes so small that the effectiveness of the Faraday cage does not fall below a critical value.These specific dimensions are advantageous for ensuring electromagnetic compatibility with the dimensions of the induction hob 100, which range from 5 cm to 20 cm in radius and have a rated power between 500 W and 3 kW. If the conductor tracks 11 are wider, a larger surface area is available for eddy currents, so that the conductor tracks 11 dissipate energy from the induction field into heat and heat themselves in the process. If the spacing between the conductor tracks 11 becomes too large, the effectiveness, the damping effect, of the resulting Faraday cage is reduced. If the conductor tracks 11 have loops, a ring current can form along the loops; therefore, it is important that the conductor tracks 11 do not have closed loops in the shielding 10. Ideally, the conductor tracks should not even form bays in which partial eddy currents can form. Figure 5.Figure 1 shows a graph of an excitation voltage for an induction coil 120 with a harmonic component. Harmonics in a current or voltage used to drive induction coils 120 have a variety of causes and can arise due to imperfections in the electronics used, deviations in the parameters of individual electronic components within a certain tolerance range, and even due to the wiring in the control electronics. Finally, the switching behavior of semiconductors in the device used to generate the induction voltage with a frequency of 30 kHz to 50 kHz is also responsible for the creation of unwanted harmonics. Harmonics manifest themselves in the fact that the AC voltage curve deviates from the ideal sinusoidal shape.Since every regularly repeating waveform can be represented by a linear combination of sine curves of different frequencies, a deviation from the sinusoidal shape leads to a harmonic component of voltages with higher frequencies (a mathematical Fourier transformation for analyzing the waveform reveals the components of the different frequencies). These harmonics that appear in a Fourier analysis can be the cause of electromagnetic incompatibility, which means that the induction hob 100 emits electromagnetic radiation in the frequency range of the harmonics. To attenuate high frequencies that only occur with a small proportion in the frequency analysis, such as frequencies above approximately 1 MHz, it is possible to connect a low-pass filter 130 in front of the induction coil 120. The low-pass filter 130 short-circuits the high frequencies, so that the high-frequency current is attenuated via a capacitor.The low-pass filter here consists not of an RC but of an LC element, because the entire load current would have to flow through the resistor, which would lead to very high losses. Using a filter choke L as a low-pass filter also attenuates higher frequency components, whereby the attenuated power is not converted into heat but is temporarily stored in the magnetic field. Frequency components in the range from 250 kHz to 1 MHz with an excitation frequency of the induction coil 120 between 30 kHz and 50 kHz can be so high that a low-pass filter would have to dissipate larger currents. Furthermore, not only does driving the induction coil 120 with a distorting signal (a signal consisting of more than one frequency) cause harmonics, but the induction coil 120 itself, in conjunction with the control electronics, can force such harmonics into a signal distortion, resulting in unwanted harmonics.

[0036] Figure 5.2 shows an example low-pass filter 130 as an RC element. A low-pass filter 130 can also consist of coils and power resistors (LC element) or of coils, capacitors, and resistors (RLC element). The expert is free to determine the correct low-pass circuit. An input voltage Ue with harmonics flows through the low-pass filter 130, which consists of a resistor R and a capacitor C connected in parallel. This voltage exits the low-pass filter 130 as the output voltage Ua.

[0037] Finally, Figure 5.3 shows a graph of an excitation voltage with the harmonic component filtered out. The remaining oscillation is very close to an ideal sine wave.

[0038] An induction coil is shown in Figure 5.4. This can be driven by the output voltage Ua of the low-pass filter 130 shown in Figure 5.3.

[0039] Figure 6.1 shows a shield 50 from the PRIOR ART, as used in microwave oven door windows, with the path of a possible eddy current indicated by the circular bold print of the pattern. While the shields 50 in microwave door windows also act as Faraday cages, these shields 50 would be unusable in the application presented here. The strong magnetic field lines used to excite eddy currents in the induction body I with a power between 500 W and 3 kW would, in shields 50 for microwaves, lead to the formation of ring currents that would simply destroy the shield 50, as the electrical ring current would become so large that the shield 50 would burn. Figure 6.2 shows the shield from Fig. 6.1 from the PRIOR ART in a perspective view with the magnetic field lines drawn in.In the center of the closed magnetic field lines B, whose strength changes over time, an electric current forms which flows along the path shown in bold and would destroy the shielding.

[0040] In order to design the shielding 10 of an induction hob 100 with the dimensions and thermal / electrical power typical for a household so that it is durable and effectively suppresses interfering radiation in accordance with the European standard EN61000, specific dimensions are required. These dimensions cannot be derived a priori but were determined through testing. This resulted in a multitude of arrangements in which the protection sought here offers optimal shielding.

[0041] Figure 7 shows an alternative arrangement of the conductor tracks 11 in a further embodiment of the cover according to the invention as cover 10'. In this embodiment, the conductor tracks 11 form partially circular path paths that are concentrically curved towards the center. Ring currents can form in these paths, even though the conductor tracks 1T in this embodiment of the cover are not closed and do not form loops. In these conductor tracks 1T that form partial loops, eddy currents can form at high powers of the induction coil 120, which can significantly heat the conductor tracks 1T. This arrangement of the conductor tracks 1T is not so well suited for shielding the induction coil 120 with high powers of the induction coil 120.

[0042] Figure 8 shows an alternative arrangement of the conductor tracks 11 in a further embodiment of the cover according to the invention as cover 10'. In this embodiment, the conductor tracks 11 form narrow, sideways, U-shaped bays along the central vertical connecting line. Partial ring currents can form in these bays, even though the conductor tracks 11" in this embodiment of the cover are not closed and do not form loops. In these partial loop-forming conductor tracks 11" with narrow bays, eddy currents can form at high powers of the induction coil 120, which can significantly heat the conductor tracks 11". This arrangement of the conductor tracks 11' is not well suited for shielding the induction coil 120 at high powers.

[0043] To illustrate the geometric dependence of the area to be shielded and the angle a between individual conductor tracks 11, Figure 9 shows a square induction cooking plate 100', with the conductor tracks 11 meeting at one corner of the square. This allows the evenly distributed conductor tracks 11 to span a total angle of 90°.

[0044] Figure 10 shows that when the conductor tracks 11 are arranged at the edge of the rectangle of a rectangular induction cooking plate 100", the conductor tracks 11 must span an angle of 180°. This requires a higher number of conductor tracks 11 over the spanned angle of 180°.

[0045] The effect is more clearly demonstrated in Figure 11. For a hexagonal 100' induction hob, where the conductor tracks 11 meet at the edge of the hexagon, an angle of 180° must be spanned to also shield the areas adjacent to the connection point.

[0046] Figure 12 shows that if the meeting point of all conductor tracks 11 is arranged in a corner of a hexagonal induction cooking plate 100"", only 120° span is required, which allows the density of the conductor tracks 11 to be reduced.

[0047] 10 Shielding C Capacitor

[0048] 11 Conductor track E Scattered radiation

[0049] 12 electrical contact E electric field fß frequency of

[0050] 50 grid induction coil

[0051] G handle

[0052] 100 induction hob I induction body

[0053] 100' induction hob K circular area

[0054] 100" Induction Cooktop L Coil

[0055] 100"' Induction hob M pattern

[0056] 100"" induction hob r radius, distance

[0057] 110 Edge R Resistance

[0058] 120 Induction coil S substrate

[0059] 130 low-pass Ue input voltage

[0060] Ua output voltage

[0061] 150 liquid t time

[0062] T Pot x Direction y Direction a Angle

[0063] Z Center

[0064] B magnetic field line

Claims

Shielding for an induction hob PATENT CLAIMS Shielding (10) for an induction cooktop (100, 100', 100", 100'', 100"") for shielding electromagnetic stray radiation (E) arranged on an electrically non-conductive substrate (S), characterized in that a loop-free pattern (M) of electrically conductive conductor tracks (11) is arranged on the electrically non-conductive substrate (S), wherein the conductor tracks (11) are galvanically connected to one another, preferably outside a center (Z) or a centroid, and have an electrical contact (12) with a stabilized DC power source or with ground via this connection. Shielding according to claim 1, characterized in that the electrical conductor tracks (11) are arranged fan-shaped on a circular area (K). Shielding according to claim 1 or 2, characterized in that the distance between two adjacent conductor tracks (11) is between 0.5 mm and 2 mm, wherein the width of the conductor tracks (11) is between 0.08 mm and 0.5 mm. Shielding according to any one of claims 1 to 3, characterized in that the angle (a) between two conductor tracks is between 15° and 2.5°, wherein the area to be shielded, which is covered by the conductor tracks 11, is between approximately 75 cm². 2 and approximately 1,200 cm 2 Shielding according to any one of claims 1 to 4, characterized in that the electrically non-conductive substrate (S) consists of polyimide or polyethylene terephthalate, wherein the conductor tracks (11) consist of carbon fibers, laminated copper, or deposited or vapor-deposited gold. Shielding according to any one of claims 1 to 5, characterized in that the layer thickness of the conductor tracks 11 is between 5 pm and 100 pm. An induction cooktop (100) for heating liquids (150) in household quantities, comprising a shield (10) according to any one of claims 1 to 6, characterized in that the galvanic connection of the conductor tracks (11) is arranged at the edge (110) of the induction cooktop (100). An induction cooktop according to claim 7, characterized in that a low-pass filter (130) is arranged upstream of an induction coil (120), which filters out harmonics from an excitation current, wherein the filtered harmonic frequency of the excitation current is greater than 250 MHz and extends to approximately 1 MHz. An induction cooktop according to claim 7 or 8, characterized in that the nominal heating power of an induction coil (120) is between 500 W and 3 kW, and the induction coil (120) has a diameter between 10 cm and 40 cm.Induction hob according to one of claims 7 to 9, characterized in that the conductor tracks (11) of the shielding (10) are galvanically connected to the ground side of the induction coil (120).

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