Induction hob device

Abstract

The invention relates to an induction hob device (10) comprising at least one heating inductor (12, 14, 16, 18) and comprising a detection unit (20) which has at least one self-oscillating oscillator circuit (22) which is provided, in a detection mode, to measure properties of an induction load of the heating inductor (12, 14, 16, 18), and which has a single transistor (24), a feedback network (26), an induction coil (28) and a resistive voltage divider (30), wherein the transistor (24) is connected in common base, wherein the heating inductor (12, 14, 16, 18), in the detection mode, is part of the feedback network (26), wherein the induction coil (28) is connected to a collector terminal (32) of the transistor (24), wherein a base terminal (34) of the transistor (24) is connected to the resistive voltage divider (30) in order to set a direct current operating point of the transistor (24) to a value between 0 V and a supply voltage of the oscillator circuit (22), wherein the oscillator circuit (22) has an electric resistor (38) and a capacitor (40) which are each electrically connected in series to the base terminal (34) of the transistor and are provided to set a base impedance during the alternating current mode in order to ensure an oscillation for a wide range of induction loads.

Description

[0001] Induction hob device

[0002] The invention relates to an induction hob device according to the preamble of claim 1.

[0003] Induction hobs with heating inductors and a detection unit for measuring properties of an induction load of the heating inductor, for example, for pot detection, are already known from the prior art. In many cases, the heating inductors are used as sensors. In detection mode, the heating inductors are exposed to a high-frequency detection signal generated by a self-oscillating oscillator circuit of the detection unit, for example, a Colpitts oscillator. It is important to ensure that the frequencies of the detection signal are sufficiently spaced from the natural resonant frequency of the heating inductor, as otherwise, a loss of oscillation and / or measurement errors may occur, especially if a user touches the pot during the measurement.The self-oscillating oscillator circuit must therefore be able to ensure a large oscillation range in order to generate oscillating detection signals with a sufficient distance from the natural resonant frequency of the heating inductor. For conventional copper heating inductors, the oscillation range of previously known self-oscillating oscillator circuits is just large enough to generate detection signals with a sufficient distance from the natural resonant frequency of the heating inductor. However, if aluminum heating inductors are to be used instead of conventional copper heating inductors, which is desirable due to the significantly lower material costs, the oscillation range of previously known self-oscillating oscillator circuits is too small to maintain a sufficient distance from the natural resonant frequency of the heating inductor, which is significantly lower in the case of aluminum.Measuring the properties of an induction load of a heating inductor using the heating inductor as a sensor is therefore not possible with previously known induction cooktop devices that have conventional self-oscillating oscillator circuits if the heating inductor is made of aluminum. This disadvantageously severely limits flexibility. The object of the invention is, in particular but not limited to, to provide a generic device with improved properties with regard to flexibility. This object is achieved according to the invention by the features of claim 1, while advantageous embodiments and further developments of the invention can be found in the subclaims.

[0004] The invention is based on an induction hob device with at least one heating inductor and with a detection unit which has at least one self-oscillating oscillator circuit which, in a detection mode, is provided for measuring properties of an induction load of the heating inductor, and which has a single transistor, a feedback network, a choke coil and a resistive voltage divider, wherein the transistor is connected in a common base circuit, wherein the heating inductor is part of the feedback network in the detection mode, wherein the choke coil is connected to a collector terminal of the transistor, wherein a base terminal of the transistor is connected to the resistive voltage divider in order to set a DC operating point of the transistor to a value between 0 V and a supply voltage of the oscillator circuit, in particular to half the supply voltage of the oscillator circuit.

[0005] It is proposed that the oscillator circuit comprises an electrical resistor and a capacitor, each electrically connected in series with the base terminal of the transistor and arranged to set a base impedance during AC operation to ensure oscillation for a wide range of inductive loads.

[0006] Such a design advantageously provides an induction hob device with improved properties in terms of flexibility. A larger oscillation range can advantageously be achieved than with previously known oscillator circuits used to measure properties of an induction load of a heating inductor. Furthermore, a higher current can be fed into the heating inductor, thereby advantageously increasing the immunity of signals to noise. Furthermore, it is advantageously possible to distinguish between cooking utensil materials, in particular to distinguish between non-ferromagnetic materials and ferromagnetic materials, since in addition to the frequency, the amplitude of the detection signal can also be measured, which is related to the equivalent resistance of the induction load.

[0007] The induction hob device is designed as a part, in particular as a subassembly, of an induction hob. The induction hob device can also comprise the entire induction hob. The induction hob device comprises at least one heating inductor, which, in heating mode, provides energy in the form of an alternating electromagnetic field to at least one object, in particular to a cooking utensil. The induction hob device could have at least two, in particular at least three, advantageously at least four, particularly advantageously at least five, preferably at least eight, and particularly preferably several heating inductors, each of which could inductively provide energy in heating mode, in particular to a single object or to at least two or more objects. At least some of the heating inductors could be arranged in close proximity to one another, for example in a row and / or in the form of a matrix.The induction hob device can comprise an inverter unit with at least two inverter switching elements for supplying power to the at least one heating inductor. Alternatively, the inverter unit can also be part of an induction hob having the induction hob device. The inverter switching elements of the inverter unit can be designed as semiconductor switching elements, in particular as transistors, for example as a metal oxide semiconductor field-effect transistor (MOSFET) or an organic field-effect transistor (OFET), advantageously as a bipolar transistor with preferably an insulated gate electrode (IGBT).

[0008] The detection unit is provided for measuring properties of an induction load of the heating inductor, for example, and without limitation, for measuring the presence of an object, in particular a cooking utensil or a foreign metal object, above the heating inductor and / or for measuring a degree of coverage of the heating inductor by the object and / or for measuring material properties of the object and / or the like. For this purpose, the detection unit comprises the self-oscillating oscillator circuit, which, in detection mode, is provided for measuring properties of the induction load of the heating inductor. The self-oscillating oscillator circuit is provided for generating a detection signal in detection mode and for applying the detection signal to the heating inductor, wherein the heating inductor is part of the feedback network of the self-oscillating oscillator circuit.The induction load of the heating inductor can be characterized by the equivalent inductance and the equivalent electrical resistance of the heating inductor, whereby the equivalent inductance and the equivalent electrical resistance of the heating inductor depend on the presence of a metallic object, such as a cooking utensil, above the heating inductor, as well as on the shape, size, position, and material of the object. Since the heating inductor is part of the feedback network of the self-oscillating oscillator circuit, the induction load of the heating inductor influences the frequency of the oscillating detection signal, which can be tapped as an output signal at a measurement output of the self-oscillating oscillator circuit. The detection unit preferably has a sampling unit connected to the measurement output of the self-oscillating oscillator circuit and provided for further processing of the output signal.The measurement output of the self-oscillating oscillator circuit is preferably connected to an emitter terminal of the individual transistor of the self-oscillating oscillator circuit. The transistor is connected in a common-base configuration, whereby the transistor advantageously has a low input impedance and a high output impedance. A further advantage of the common-base configuration of the transistor is that, in this configuration, no phase shift occurs between the input and output of the transistor, thus fulfilling the phase condition according to the stability criteria of Barkhausen and Nyquist. The transistor is preferably designed as a bipolar transistor, preferably as an NPN bipolar transistor. Alternatively, the transistor could also be designed as a field-effect transistor, for example as a MOSFET, preferably as an n-channel MOSFET, which is then connected in a gate circuit equivalent to a common-base configuration.

[0009] The self-oscillating oscillator circuit further comprises the resistive voltage divider, which is connected to the base terminal of the transistor and is provided to set the DC operating point of the transistor to a value between 0 V and the supply voltage of the oscillator circuit, in particular to half the supply voltage of the oscillator circuit. The resistive voltage divider has at least two, preferably exactly two, electrical resistors, the values ​​of which are selected to set the DC operating point of the transistor to half the supply voltage of the oscillator circuit. Preferably, the self-oscillating oscillator circuit has a DC voltage source, which is provided to provide the supply voltage and is connected to the resistive voltage divider.

[0010] The self-oscillating oscillator circuit also includes a choke coil connected to the collector terminal of the transistor. The choke coil is preferably connected to the DC voltage source and is designed to open a DC current path to the collector terminal of the transistor and block an AC current path between the collector terminal and the base terminal of the transistor.

[0011] The oscillator circuit includes an electrical resistor and a capacitor, each electrically connected in series with the base terminal of the transistor and designed to adjust the base impedance during AC operation to ensure oscillation for a wide range of inductive loads. The capacitance of the capacitor, which is electrically connected in series with the base terminal of the transistor, is selected such that the capacitor has a high impedance for DC current, so that the DC operating point of the transistor is adjusted solely by the resistive voltage divider.At the same time, the capacitance of the capacitor, which is electrically connected in series with the base terminal of the transistor, is selected such that the capacitor forms a short-circuit path for alternating currents with a frequency within an operating frequency range of the detection unit, so that the base impedance of the transistor during AC operation is determined by the electrical resistance, which is electrically connected in series with the base terminal of the transistor. Preferably, the base terminal of the transistor is connected to the emitter terminal of the transistor via the electrical resistance and the capacitor connected in series with it. In previously known oscillator circuits, however, the base terminal of the transistor is often connected to ground via a capacitor. This, however, leads to a fixed frequency of the detection signal and is therefore unsuitable for applications for measuring properties of the induction load of the heating inductor.Investigations by the applicant have shown that, surprisingly, the oscillation range of the self-oscillating oscillator circuit can be increased by the electrical resistance connected in series with the base terminal of the transistor.

[0012] In this document, “at least substantially” is to be understood as meaning that a deviation from a predetermined value deviates in particular by less than 25%, preferably less than 10% and particularly preferably less than 5% of the predetermined value.

[0013] In this document, numerals such as "first" and "second," which precede certain terms, serve only to distinguish between objects and / or to correlate objects with each other and do not imply a total number and / or ranking of the objects. In particular, a "second object" does not necessarily imply the presence of a "first object."

[0014] "Intended" should be understood as specifically programmed, designed, and / or equipped. The fact that an object is intended for a specific function should be understood as meaning that the object fulfills and / or performs this specific function in at least one application and / or operating state.

[0015] It is further proposed that the electrical resistance, which is electrically connected in series with the base terminal of the transistor, has a value which is at least a factor of 2 smaller than the values ​​of the electrical resistances that form the resistive voltage divider. This can advantageously further increase flexibility. In particular, the oscillation range of the self-oscillating oscillator circuit can advantageously be further enlarged. The electrical resistance, which is electrically connected in series with the base terminal of the transistor, has a value which is advantageously at least a factor of 3, preferably at least a factor of 4, and more preferably at least a factor of 5 smaller than the values ​​of the electrical resistances that form the resistive voltage divider.For example, the electrical resistors forming the resistive voltage divider can each have a value of 1 kΩ, and the electrical resistor connected in series with the base terminal of the transistor can have a value of 200 Ω. Furthermore, it is proposed that the electrical resistor connected in series with the base terminal of the transistor has a value of at least 40 Ω and at most 250 Ω. This advantageously further increases flexibility. Oscillation can advantageously be ensured for a particularly wide range of cooking utensils with different material properties.The electrical resistance, which is electrically connected in series with the base terminal of the transistor, has in particular a value of at least 45 Q and at most 240 Q, advantageously a value of at least 50 Q and at most 230 Q, particularly advantageously a value of at least 55 Q and at most 230 Q, preferably a value of at least 60 Q and at most 220 Q, preferably a value of at least 65 Q and at most 210 Q and particularly preferably a value of 200 Q.

[0016] Furthermore, it is proposed that the capacitor, which is electrically connected in series with the base terminal of the transistor, has a capacitance of 100 nF. Such a configuration can advantageously increase the reliability of the induction hob device. In particular, it can be ensured that the capacitor, which is electrically connected in series with the base terminal of the transistor, has a high impedance for direct current and a low impedance for alternating currents with a frequency in the operating frequency range of the detection unit. Alternatively, the capacitor, which is electrically connected in series with the base terminal of the transistor, can also have a capacitance of less than 100 nF or more than 100 nF, as long as it has a sufficiently high impedance for direct current and a sufficiently low impedance for alternating currents with a frequency in the operating frequency range of the detection unit.

[0017] It is further proposed that the oscillator circuit comprise two capacitors arranged between the collector terminal and the emitter terminal of the transistor, forming a capacitive voltage divider. Such a configuration advantageously allows for a simple design of the oscillator circuit. The oscillator circuit can be configured as a Colpits oscillator if it comprises the two capacitors arranged between the collector terminal and the emitter terminal of the transistor, forming the capacitive voltage divider. Preferably, the measurement output of the oscillator circuit is connected to a center point of the capacitive voltage divider. This advantageously improves measurement accuracy, since the capacitive voltage divider also acts as a high-pass filter, thus filtering out low-frequency noise originating from the heating inductor that may otherwise overlay the output signal.The capacitances of the two capacitors forming the capacitive voltage divider are selected to ensure oscillation in the operating frequency range of the detection unit, i.e., the amplitude condition according to the stability criteria of Barkhausen and Nyquist is met.

[0018] Furthermore, it is proposed that the oscillator circuit have a further capacitor which is electrically connected in series with the collector terminal of the transistor and which, in detection mode, is electrically connected in series with the heating inductor. This advantageously allows flexibility to be further increased. The oscillator circuit can be designed as a Clapp oscillator if it has the further capacitor which is electrically connected in series with the collector terminal of the transistor and which, at least in detection mode, is electrically connected in series with the heating inductor. The further capacitor advantageously allows a frequency of the detection signal to be set more precisely. The further capacitor also advantageously enables capacitive decoupling of the detection unit from the heating inductor in heating mode using particularly simple technical means.This can be achieved by selecting a capacitance of the further capacitor such that the further capacitor represents a high impedance for alternating currents in the range of up to 100 kHz, with which the heating inductor is supplied with energy by the inverter unit in the heating mode, and a low impedance for alternating currents in the operating frequency range of the detection unit, which includes frequencies above 500 kHz.

[0019] In a particularly advantageous embodiment of the invention, it is proposed that the additional capacitor has a capacitance that is at least a factor of 10 smaller than the capacitances of the capacitors that form the capacitive voltage divider. This advantageously allows the frequency of the detection signal to be set even more precisely, since a dependence of the frequency of the detection signal on the capacitances of the two capacitors that form the capacitive voltage divider is further reduced. In particular, the additional capacitor has a capacitance that is at least a factor of 20, advantageously at least a factor of 50, particularly advantageously at least a factor of 100, preferably at least a factor of 500, more preferably at least a factor of 1,000, and particularly preferably at least a factor of 10,000 smaller than the capacitances of the capacitors that form the capacitive voltage divider.The capacitors forming the capacitive voltage divider may, for example and without limitation, each have a capacitance of 1 nF and the further capacitor may, for example and without limitation, have a capacitance of 10 pF.

[0020] It is further proposed that the heating inductor be formed at least largely from aluminum. Such a configuration can advantageously improve efficiency, in particular cost-effectiveness, since aluminum is significantly more inexpensive to obtain than copper, which is conventionally used as a material for heating inductors in induction hobs. In combination with the induction hob device according to the invention, a particularly inexpensive induction hob can be provided that simultaneously offers a particularly high degree of operating convenience, since measuring properties of an induction load of the heating inductor is now also possible when the heating inductor is formed at least largely from aluminum.The heating inductor is in particular made of aluminum to a large extent of more than 50 wt.%, advantageously at least a large extent of more than 60 wt.%, particularly advantageously at least a large extent of more than 70 wt.%, preferably at least a large extent of more than 80 wt.%, and preferably at least a large extent of more than 90 wt. For example, the heating inductor could be made of an alloy of aluminum and copper with an aluminum content of more than 50 wt.%. Particularly preferably, the heating inductor is made entirely of aluminum.

[0021] Furthermore, it is proposed that the heating inductor have an equivalent inductance of at least 20 pH and at most 120 pH, an equivalent electrical resistance of at most 200 Ω, and a parasitic capacitance of at most 150 pF. This advantageously allows flexibility to be increased even further. In particular, for all commonly available cooking utensils on the market, which can be made of a wide variety of metallic materials, it can be ensured that oscillation occurs, thus enabling measurement of properties of the induction load of the heating inductor if the heating inductor has an equivalent inductance of at least 20 pH and at most 120 pH, an equivalent electrical resistance of at most 200 Ω, and a parasitic capacitance of at most 150 pF.

[0022] Furthermore, it is proposed that the choke coil has an inductance which is selected such that the choke coil represents a low impedance for direct currents and a high impedance for alternating currents with a frequency in an operating frequency range of the detection unit. This can advantageously ensure reliable operation of the oscillator circuit. The choke coil has in particular an inductance of at least 0.5 mH and at most 1.5 mH, advantageously an inductance of at least 0.6 mH and at most 1.4 mH, particularly advantageously an inductance of at least 0.7 mH and at most 1.3 mH, preferably an inductance of at least 0.8 mH and at most 1.2 mH, preferably an inductance of at least 0.9 mH and at most 1.1 mH and particularly preferably an inductance of 1.0 mH.

[0023] It is further proposed that the oscillator circuit include a protective diode whose anode is connected to an emitter terminal and whose cathode is connected to the base terminal of the transistor. This advantageously increases safety. In particular, damage to the transistor due to overvoltages can be prevented. The protective diode is connected in the forward direction from the emitter terminal toward the base terminal and is provided to provide overvoltage protection for the transistor.

[0024] The invention further relates to an induction hob with at least one induction hob device according to one of the previously described embodiments. Such an induction hob is characterized in particular by its advantageous properties with regard to high flexibility and efficiency, which are achieved by the induction hob device.

[0025] The induction hob device is not intended to be limited to the application and embodiment described above. In particular, the induction hob device may have a number of individual elements, components, and units that differs from the number stated herein to fulfill a functionality described herein.

[0026] Further advantages will become apparent from the following description of the drawings. The drawings illustrate an exemplary embodiment of the invention. The drawings, the description, and the claims contain numerous features in combination. Those skilled in the art will also expediently consider the features individually and combine them into useful further combinations.

[0027] They show:

[0028] Fig. 1 is a schematic representation of an induction hob with an induction hob device comprising at least one heating inductor and a detection unit,

[0029] Fig. 2 is a schematic electrical equivalent circuit diagram of the heating inductor, Fig. 3 is a schematic diagram showing a natural resonance frequency of the heating inductor of the induction hob device compared to a natural resonance frequency of a conventional heating inductor,

[0030] Fig. 4 is a schematic diagram showing quality factors of the heating inductor of the induction hob device compared to quality factors of the conventional heating inductor,

[0031] Fig. 5 is a schematic electrical circuit diagram of a self-oscillating oscillator circuit of the detection unit and

[0032] Fig. 6 is a schematic diagram showing a frequency and phase response of the self-oscillating oscillator circuit.

[0033] Figure 1 shows a schematic representation of an induction hob 60. The induction hob 60 comprises an induction hob device 10. The induction hob device 10 comprises at least one heating inductor 12. In the present case, the induction hob device comprises the heating inductor 12 and three further heating inductors 14, 16, 18. The heating inductors 12, 14, 16, 18 are arranged below a hob plate 64 of the induction hob 60 and are intended to provide energy in the form of an alternating electromagnetic field to at least one object (not shown), for example, a cooking utensil, on the hob plate 64 during heating operation. The induction hob device 10 is not limited to the number of four heating inductors 12, 14, 16, 18, but could alternatively have any number of heating inductors 12, 14, 16, 18 greater than or equal to one without departing from the scope of the present invention.

[0034] The induction hob device 10 comprises a detection unit 20. The detection unit 20 has a self-oscillating oscillator circuit 22 (see Figure 5), which, in a detection mode, is provided for measuring properties of an induction load of the heating inductor 12. The self-oscillating oscillator circuit 22 is also provided here for measuring properties of induction loads of the additional heating inductors 14, 16, 18. The induction hob device 10 may comprise a multiplexer circuit (not shown) which is provided to connect the detection unit 20 sequentially to the heating inductors 12, 14, 16, 18 for the detection operation and to disconnect the connection between the detection unit 20 and the heating inductors 12, 14, 16, 18 for a heating operation of the heating inductors 12, 14, 16, 18.In the heating mode, the heating inductors 12, 14, 16, 18 are each operated by an inverter unit (not shown) of the induction hob device 10.

[0035] Figure 2 shows a schematic electrical equivalent circuit diagram of the heating inductor 12. The heating inductor 12 is formed at least largely from aluminum. In the present case, the heating inductor 12 is formed entirely from aluminum. The heating inductor 12 has an equivalent inductance 54, an equivalent electrical resistance 56, and a parasitic capacitance 58, which are shown in the equivalent circuit diagram of Figure 2. In the present case, the heating inductor 12 has the equivalent inductance 54 of at least 20 Ω and at most 120 Ω. In the present case, the heating inductor 12 has the equivalent electrical resistance 56 of at most 200 Ω. In the present case, the heating inductor 12 has the parasitic capacitance 58 of at most 150 pF. The other heating inductors 14, 16, 18 have essentially identical properties to the heating inductor 12.

[0036] Figure 3 shows a schematic diagram illustrating a natural resonant frequency 66 of the heating inductor of the induction cooktop device 10 compared to a natural resonant frequency 68 of a conventional heating inductor (not shown). A frequency of a detection current in megahertz is plotted on an abscissa 70 of the diagram. A self-inductance in millihenries is plotted on an ordinate 72 of the diagram. A series of measurements 74 in the diagram shows a curve of the self-inductance of the heating inductor 12 upon excitation with detection currents having frequencies in the megahertz range. A series of measurements 76 in the diagram shows a curve of the self-inductance of the conventional heating inductor upon excitation with detection currents having frequencies in the megahertz range.In contrast to the heating inductor 12, the conventional inductor is essentially made of copper, but otherwise has a substantially identical geometry and substantially identical dimensions to the heating inductor 12. The measurements shown in the diagram were each performed using the same cooking utensil (not shown), which was placed above the heating inductor 12 or above the conventional inductor. As can be seen from the diagram, the natural resonant frequency 66 of the heating inductor 12 is significantly lower than the natural resonant frequency 68 of the conventional heating inductor, which is due to the use of the different materials.If the detection current has a frequency that is in the range of the natural resonant frequency 66 of the heating inductor 12 or in the range of the natural resonant frequency 68 of the conventional heating inductor, this can lead to various problems, including a loss of oscillation and / or measurement errors if the pot is touched by a user during the measurement, thereby increasing the parasitic capacitance of the system. It is therefore desirable for the detection current to have a frequency that is sufficiently far away from the respective natural resonant frequencies 66, 68. For conventional heating inductors, this is easily possible with previously known oscillator circuits due to the higher natural resonant frequency 68.For inductors which, like the heating inductor 12, are made at least largely of aluminum, however, a larger oscillation spectrum is required to enable reliable detection, which was not possible with previously known oscillator circuits.

[0037] Another challenge when using aluminum instead of copper as the material for the heating inductor 12 is the quality factor. The quality factor is calculated from the following formula 1 : where Q is the quality factor, w is the angular frequency, L eq for the equivalent inductance, R eq stands for the electrical equivalent resistance. The angular frequency w is calculated using the following formula 2: iü = 2'rcjf (2) where TT stands for the angular number and f for the frequency of the detection current.

[0038] Figure 4 shows a schematic diagram illustrating quality factors of the heating inductor 12 of the induction hob device 10 compared to quality factors of the conventional heating inductor. A frequency in megahertz is plotted on an abscissa 78 of the diagram. The quality factor is plotted as a dimensionless parameter on an ordinate 80 of the diagram. A series of measurements 82 shows a curve of the quality factor of the heating inductor 12 as a function of the frequency of the detection current. A series of measurements 84 shows a curve of the quality factor of the conventional heating inductor as a function of the frequency of the detection current. As can be seen from the diagram, the conventional heating inductor consistently has a higher quality factor than the heating inductor 12 across the entire frequency range.The amplitude of a sinusoidal output signal of the oscillator circuit used depends, among other things, on the quality factor, so that the amplitude of the output signal when measuring the properties of an inductive load of the heating inductor 12 is lower than with a conventional heating inductor when a conventional oscillator circuit is used. This can lead to a signal-to-noise ratio that is too low, and the measurements become correspondingly inaccurate.

[0039] The disadvantages described above with reference to Figures 3 and 4, which arise when using aluminum inductors as heating inductors in induction hobs with regard to the measurement of properties of induction loads, can be overcome by the self-oscillating oscillator circuit 22 of the detection unit 20 of the induction hob device 10.

[0040] Figure 5 shows a schematic electrical circuit diagram of the self-oscillating oscillator circuit 22. The oscillator circuit 22 comprises a single transistor 24, a feedback network 26, a choke coil 28, and a resistive voltage divider 30. The transistor 24 has a collector terminal 32, a base terminal 34, and an emitter terminal 36. The transistor 24 is connected in a common-base configuration. The oscillator circuit 22 here has a protective diode 62, the anode of which is connected to the emitter terminal 36 and the cathode of which is connected to the base terminal 34 of the transistor 24. The protective diode 62 is thus connected in the forward direction, starting from the emitter terminal 36 toward the base terminal 34. The protective diode 62 is provided for overvoltage protection of the transistor 24.

[0041] The heating inductor 12 is part of the feedback network 26 during detection operation. In the electrical circuit diagram of Figure 5, the heating inductor 12 is represented by its equivalent inductance 54 and its equivalent electrical resistance 56. The parasitic capacitance 58 (see Figure 2) has been omitted in Figure 5 for simplicity.

[0042] The choke coil 28 is connected to the collector terminal 32 of the transistor 24. The choke coil 28 has an inductance selected such that the choke coil 28 represents a low impedance for direct currents and a high impedance for alternating currents with a frequency within an operating frequency range of the detection unit 20. In the present case, the choke coil 28 has an inductance of 1.0 mH.

[0043] The oscillator circuit 22 has two electrical resistors 42, 44 that form the resistive voltage divider 30. The base terminal 34 of the transistor 24 is connected to the resistive voltage divider 30 in order to set a DC operating point of the transistor 24 to half a supply voltage of the oscillator circuit 22. The electrical resistors 42, 44 therefore have the same values. In the present case, the induction cooktop device 10 has a DC voltage source 86 for providing the supply voltage for the oscillator circuit 22. The transistor 24 is connected to the DC voltage source 86 via its collector terminal 32 and via the choke coil 28. The transistor 24 is also connected to the DC voltage source 86 via its base terminal 34 via the resistive voltage divider 30.

[0044] The oscillator circuit 22 has an electrical resistor 38 and a capacitor

[0045] 40, each of which is electrically connected in series with the base terminal 34 of the transistor. The electrical resistor 38 and the capacitor 40 are provided to set a base impedance during AC operation in order to ensure oscillation for a wide range of inductive loads. The electrical resistor 38, which is electrically connected in series with the base terminal 34 of the transistor 24, has a value that is at least a factor of 2 smaller than the values ​​of the electrical resistors 42, 44 that form the resistive voltage divider 30. The electrical resistor 38, which is electrically connected in series with the base terminal 34 of the transistor 24, has a value of at least 40 Ω and at most 250 Ω. In the present case, the electrical resistor 38 has a value of 200 Ω.The electrical resistances 42, 44 in the present case each have a value of 1 kΩ, so that the value of the electrical resistance 38 in the present case is smaller by a factor of 5 than the values ​​of the electrical resistances 42, 44.

[0046] Capacitor 40, which is electrically connected in series with base terminal 34 of transistor 24, has a capacitance of 100 nF. For direct current, capacitor 40 therefore has a high impedance, so that the direct current operating point of transistor 24 is set solely by resistive voltage divider 30. For alternating currents with a frequency within the operating frequency range of detection unit 20, however, capacitor 40 forms a short-circuit path, so that the base impedance of the transistor during AC operation is determined by electrical resistance 38.

[0047] The oscillator circuit 22 has two capacitors 46, 48 arranged between the collector terminal 32 and the emitter terminal 36 of the transistor 24 and forming a capacitive voltage divider 50. The capacitors 46, 48 are part of the feedback network 26. A measurement output 88 of the oscillator circuit 22 is arranged between the capacitor 46 and the capacitor 48. This arrangement of the measurement output 88 filters low-frequency noise. The capacitances of the capacitors 46, 48 are selected such that the amplitude condition according to the stability criteria of Barkhausen and Nyquist, which are known to those skilled in the art, is met. Since the transistor 24 is connected in a common base circuit and the base as a common reference point causes a phase shift of 0°, the feedback network 26 also causes a phase shift of 0°, so that the phase condition according to the stability criteria of Barkhausen and Nyquist is also fulfilled.

[0048] The oscillator circuit 22 also has a further capacitor 52, which is electrically connected in series with the collector terminal 32 of the transistor 24 and which, at least in the detection mode, is electrically connected in series with the heating inductor 12. The oscillator circuit 22 is thus a variant of a Clapp oscillator. The oscillation frequency of the oscillator circuit 22 is determined in this case by the further capacitor 52. The further capacitor 52 has a capacitance that is at least a factor of 10 smaller than the capacitances of the capacitors 46, 48, which form the capacitive voltage divider 50.

[0049] Figure 6 shows a schematic diagram illustrating an example frequency and phase response of the self-oscillating oscillator circuit 22. A frequency in megahertz is plotted on an abscissa 90 of the diagram. A signal strength in decibels is plotted on a first ordinate 92 of the diagram. A phase position in ° is plotted on a second ordinate 94 of the diagram. A curve 96 in the diagram shows a profile of the phase response. A curve 98 in the diagram shows a profile of a signal strength of a signal in the oscillator circuit 22 as a function of frequency. At the beginning of detection operation, the capacitors 46, 48, 52 are discharged. As soon as the voltage source 86 is activated, the capacitors 46, 48, 52 are charged via the choke coil 28. It is not necessary for the capacitors 46, 48, 52 to be fully charged before oscillation can begin.Transistor 24 operates in amplifier mode at the beginning of detection operation. As in any electronic circuit, a small noise signal also exists in oscillator circuit 22, which is amplified by transistor 24, but only at a specific frequency, which is shown in the diagram in Figure 6 as the operating frequency 100 of detection unit 20. For oscillation to begin, the stability criteria of Barkhausen and Nyquist must be met, the gain factor of transistor 24 must be greater than 1, and a phase shift between the input, in this case the emitter terminal 36, and the output, in this case the collector terminal 32, of transistor 24 must be 0°, which is the case for the operating frequency 100. In this case, the operating frequency 100 is, for example, 0.915 MHz.Since the heating inductor 12 is part of the feedback network 26, the magnitude of the operating frequency 100 depends, among other things, on the properties of the induction load of the heating inductor 12, for example on the material properties, the shape and size, as well as the positioning of a cooking utensil placed above the heating inductor 12.

[0050] Reference symbol

[0051] 10 Induction hob device

[0052] 12 Heating inductor

[0053] 14 additional heating inductors

[0054] 16 additional heating inductors

[0055] 18 additional heating inductors

[0056] 20 detection units

[0057] 22 self-oscillating oscillator circuit

[0058] 24 transistors

[0059] 26 Feedback network

[0060] 28 Choke coil

[0061] 30 resistive voltage divider

[0062] 32 collector connection

[0063] 34 Basic connection

[0064] 36 Emitter connection

[0065] 38 electrical resistance

[0066] 40 Capacitor

[0067] 42 electrical resistance

[0068] 44 electrical resistance

[0069] 46 Capacitor

[0070] 48 Capacitor

[0071] 50 capacitive voltage divider

[0072] 52 additional capacitor

[0073] 54 Equivalent inductance

[0074] 56 electrical equivalent resistance

[0075] 58 parasitic capacitance

[0076] 60 induction hob

[0077] 62 Protection diode

[0078] 64 Hob plate natural resonance frequency

[0079] Natural resonance frequency

[0080] abscissa

[0081] ordinate

[0082] series of measurements

[0083] series of measurements

[0084] abscissa

[0085] ordinate

[0086] series of measurements

[0087] series of measurements

[0088] DC voltage source

[0089] Measuring output

[0090] Abscissa first ordinate second ordinate

[0091] curve

[0092] curve

[0093] Working frequency

Claims

Claims 1. Induction hob device (10) with at least one heating inductor (12, 14, 16, 18) and with a detection unit (20) which has at least one self-oscillating oscillator circuit (22) which, in a detection mode, is provided for measuring properties of an induction load of the heating inductor (12, 14, 16, 18), and which has a single transistor (24), a feedback network (26), a choke coil (28) and a resistive voltage divider (30), wherein the transistor (24) is connected in a common base circuit, wherein the heating inductor (12, 14, 16, 18) is part of the feedback network (26) in the detection mode, wherein the choke coil (28) is connected to a collector terminal (32) of the transistor (24), wherein a base terminal (34) of the transistor (24) is connected to the resistive voltage divider (30),to set a DC operating point of the transistor (24) to a value between 0 V and a supply voltage of the oscillator circuit (22), characterized in that the oscillator circuit (22) comprises an electrical resistor (38) and a capacitor (40), each electrically connected in series with the base terminal (34) of the transistor and intended to set a base impedance during AC operation in order to ensure oscillation for a wide range of inductive loads.

2. Induction hob device (10) according to claim 1, characterized in that the electrical resistance (38) which is electrically connected in series with the base terminal (34) of the transistor (24) has a value which is at least a factor of 2 smaller than the values of the electrical resistances (42, 44) which form the resistive voltage divider (30).

3. Induction hob device (10) according to claim 1 or 2, characterized in that the electrical resistance (38) which is electrically connected in series with the base terminal (34) of the transistor (24) has a value of at least 40 Ω and at most 250 Ω.

4. Induction hob device (10) according to one of the preceding claims, characterized in that the capacitor (40), which is electrically connected in series with the base terminal (34) of the transistor (24), has a capacitance of 100 nF.

5. Induction hob device (10) according to one of the preceding claims, characterized in that the oscillator circuit (22) has two capacitors (46, 48) which are arranged between the collector terminal (32) and an emitter terminal (36) of the transistor (24) and form a capacitive voltage divider (50).

6. Induction hob device (10) according to one of the preceding claims, characterized in that the oscillator circuit (22) has a further capacitor (52) which is electrically connected in series with the collector terminal (32) of the transistor (24) and which is electrically connected in series with the heating inductor (12, 14, 16, 18) at least in the detection mode.

7. Induction hob device (10) according to claims 5 and 6, characterized in that the further capacitor (52) has a capacitance which is at least a factor of 10 smaller than the capacitances of the capacitors (46, 48) which form the capacitive voltage divider (50).

8. Induction hob device (10) according to one of the preceding claims, characterized in that the heating inductor (12, 14, 16, 18) is formed at least largely from aluminum.

9. Induction hob device (10) according to claim 8, characterized in that the heating inductor (12, 14, 16, 18) has an equivalent inductance (54) of at least 20 pH and at most 120 pH, an equivalent electrical resistance (56) of at most 200 Ω, and a parasitic capacitance (58) of at most 150 pF.

10. Induction hob device (10) according to one of the preceding claims, characterized in that the choke coil (28) has an inductance which is selected such that the choke coil (28) represents a low impedance for direct currents and a high impedance for alternating currents having a frequency in an operating frequency range of the detection unit (20).

11. Induction hob device (10) according to one of the preceding claims, characterized in that the oscillator circuit (22) has a protective diode (62) whose anode is connected to an emitter terminal (36) and whose cathode is connected to the base terminal (34) of the transistor (24).

12. Induction hob (60) with at least one induction hob device (10) according to one of the preceding claims.

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