EP2380394B1 - Induction cooktop having at least one power inverter - Google Patents

Description

The invention relates to an induction hob with at least one inverter according to the preamble of claim 1.

From the JP 2007 012490 A For example, an induction hob with multiple inverters is known, comprising two semiconductor switches connected to one pole of a DC voltage source. Parallel to each semiconductor switch, a damping capacitor is arranged, which prevents undesired ringing of a resonant circuit comprising the inductors of the hob after switching operations of the semiconductor switches. The ringing leads to scattering losses, which can adversely affect the overall efficiency of the hob.

Other induction hobs of the above type are in the publications JP 2006 114320 A . JP 2007 335274 A and JP 2007 012490 A disclosed.

To minimize power losses while allowing fast switching, the snubber capacitor must be properly sized. The dimensioning of the damping capacitor depends, in particular, on the expected power, which is transmitted via the semiconductor switches before or after the switching operations. Especially in induction hobs of the matrix type, many similar inductors are flexibly grouped into heating zones that are formed depending on a position and / or size of a detected cooking utensil mounted on the hob. A desired heat output determined by a user selected power level is then fed to the various inducers of the heating zone. An inverter can then supply one or more inductors with a suitable heating current. The total power to be generated by an inverter is therefore much more dependent on the situation than in traditional induction hobs in which an inductor is always supplied by a single inverter.

The invention is in particular the object of providing an induction hob with an inverter, which can be operated with a particularly high efficiency.

The object is achieved by an induction hob with the features of independent claim 1. Advantageous embodiments and modifications of the invention will become apparent from the dependent claims.

The invention is based, in particular, on an induction hob with at least one inverter which comprises at least two semiconductor switches connected to different poles of a DC voltage source. Each of the semiconductor switches is associated with a damping arrangement which is arranged parallel to one of the semiconductor switches.

It is proposed to equip the induction hob with at least one means for adjusting a damping constant of at least one of the damping arrangements. This opens up the possibility of adapting the damping constants to an operating situation, so that the damping constant can always be set to an optimum value. This leads to a reduction of scattering losses, so that an overall efficiency of the induction hob can be improved.

Depending on the embodiment of the invention, the poles of the DC voltage source may be a positive and a negative pole or a ground and a pole with a zero voltage value. The DC voltage source may in particular comprise a rectifier for rectifying a mains voltage of a household power grid. In this context, the term "damping constant" is intended to denote a complex-valued damping constant.

The damping arrangement may in particular comprise one or more capacitors and / or one or more resistors.

According to an advantageous embodiment of the invention, the damping arrangement comprises a damping capacitor system, wherein the means for adjusting a total capacity of the damping capacitor system is designed. In this case, the damping capacitor system can either comprise a capacitor with an adjustable capacitance, for example a variable capacitor, or a plurality of capacitors which can be switched on and off. A symmetrical damping of both semiconductor switches can be ensured be, if the means for the simultaneous, in particular symmetrical changing the damping constants of the damping arrangements of both semiconductor switches is designed.

In a structurally simple and flexible embodiment of the invention, it is proposed that the damping arrangement comprises a plurality of switching elements and a plurality of capacitors, which can be connected in each case by closing a switching element.

If the capacitors have different capacitances and can be switched independently of one another, a particularly large variety of possible values of the capacitances can be achieved by the choice of suitable combinations of capacitors.

As already discussed above, the benefits of the invention, in particular in the context of induction hobs come to fruition. Because of the widely varying loads, the inventive concept can be used particularly profitably in cooktops of the matrix type.

An automatic adaptation of the damping arrangement can be made possible if the induction hob comprises a control unit which adjusts the total capacity or damping constant as a function of a nominal power transmitted via the semiconductor switches. Other parameters may be an amplitude of the AC voltage generated by the inverter, a dielectric loss angle of a consumer system powered by the inverter, and / or a total inductance of that system.

Not necessarily a continuous change of the damping constants is necessary. In simpler embodiments of the invention, the attenuation constant may be changed in steps as the relevant parameters reach certain thresholds.

Further advantages emerge from the following description of the drawing. In the drawings, embodiments of the invention are shown. The drawing, the description and the claims contain numerous features in combination. The person skilled in the art will expediently also consider the features individually and combine them into meaningful further combinations.

Fig. 1

ein Kochfeld mit einem Gleichrichter und mehreren Wechselrichtern,

Fig. 2

eine schematische Darstellung eines Schaltkreises mit einem Wechselrichter und Dämpfungsanordnungen,

Fig. 3

einen Schaltkreis mit einem Wechselrichter und einer Dämpfungsanordnung nach einer alternativen Ausgestaltung der Erfindung,

Fig. 4

einen Schaltkreis mit einem Wechselrichter nach einer weiteren alternativen Ausgestaltung der Erfindung, und

Fig. 5

eine Darstellung von Verlustleistungen für verschiedene Werte einer Gesamtkapazität einer Dämpfungsanordnung.

Show it:

Fig. 1

a hob with a rectifier and several inverters,

Fig. 2

a schematic representation of a circuit with an inverter and damping arrangements,

Fig. 3

a circuit comprising an inverter and a damping arrangement according to an alternative embodiment of the invention,

Fig. 4

a circuit with an inverter according to another alternative embodiment of the invention, and

Fig. 5

a representation of power losses for different values of a total capacity of a damping arrangement.

Fig. 1 FIG. 5 shows an induction hob with a matrix of inductors 30 and a control unit 32 which can detect a cookware element 34 placed on the hob and determine the size and shape of the bottom of the cookware element 34 or several cookware elements. Depending on the size, shape and position of the cookware element 34, the control unit 32 summarizes a plurality of inductors 30 to form a flexibly definable heating zone 36.

Via a user interface 38, a user can set a power level for the heating zone 36 or for a plurality of heating zones 36. The control unit 32 interconnects a plurality of inverters 10 of a power supply unit 40 with the inducers 30 of the heating zone 36 in such a way that the inducers 30 of the heating zone 36 couple a total heating power dependent on the set power level into the cookware element 34. For this purpose, the inverters 10 or an inverter 10 generate a high-frequency alternating current with a frequency in the order of 50-100 kHz, which flows through the inductors 30 and generates a magnetic field which changes with the same frequency. The magnetic field, in turn, generates alternating currents in the ferromagnetic bottom of the cookware element 34, thereby heating that bottom.

The power supply unit 40 comprises a DC voltage source 12 which comprises a rectifier 42 and a filter circuit 44. The rectifier 42 is connected to a phase of a household power network 46. Even if in Fig. 1 only one power supply unit 40 is shown, the hob is fed in practice by two power supply units, which are connected to two phases of the household electricity network 46.

Between the power supply unit 40 and the inductors 30, a likewise controlled by the control unit 32 circuitry 48 is provided, can be made and separated via the flexibly configurable connections between the inverters 10 and the inductors 30.

Fig. 2 shows a circuit with an inverter 10, the two semiconductor switches 14, 16 includes. Each of the semiconductor switches 14, 16 consists of a transistor 50, in particular an insulated gate bipolar transistor (IGBT) and a diode 52.

The inverter 10 with the two semiconductor switches 14, 16 is constructed in a half-bridge topology. An upper contact point 54 is connected to a positive output of the DC voltage source 12, and a lower contact point 56 of the circuit is connected to the ground potential.

In order to generate an alternating voltage in the inductor 30, the semiconductor switches 14, 16 are switched on and off alternately by the actuation of the transistors 50. The inductor 30 is supplemented by resonant capacitors 58 to a resonant circuit. Scatter losses are in the circuit diagram in Fig. 2 symbolized by a resistor 60. In order to dampen a ringing of this resonant circuit as a result of a switching operation of one of the semiconductor switches 14, 16, a damping arrangement 18, 20 is provided parallel to each of the semiconductor switches 14, 16. In the in Fig. 2 illustrated embodiment, each of the damping assemblies 18, 20 comprises two capacitors 28a, 28b. A first capacitor 28a is hardwired to the corresponding semiconductor switch 14, 16. The two capacitors 28a of the upper and lower semiconductor switches 14, 16 are each connected to a center of the inverter 10, to which the inductor 30 is also connected.

Each of the damping arrangements 18, 20 additionally comprises a further capacitor 28b. The two capacitors 28b are connected in common with an input of a switching element 26, which may be formed, for example, as an electromechanical relay. By closing the switching element 26, the two capacitors 28b can also be connected to the center of the inverter 10, so that the capacitors 28b are connected in parallel with the corresponding semiconductor switch 14, 16. The switching element 26 therefore forms, in interaction with the capacitors 28a, 28b, a means 22 for varying a damping constant of the damping arrangements 18, 20. By closing the switching element 26, the damping constants of both damping arrangements 18, 20 are changed simultaneously in the same way. The capacitors 28a, 28b together form a flexibly configurable snubber capacitor system 24.

The control unit 32 ( Fig. 1 ) can control the switching element 26 via a control line, not shown here. In order to match the total capacitance of the damping assemblies 18, 20 to a load of the inductor 30, the control unit 32 closes the switching element 26 when a load of the inductor or a power transmitted through the semiconductor switches 14, 16 exceeds a threshold.

The 3 and 4 show alternative embodiments of the invention. The following description is essentially limited to differences from those in Fig. 1 and 2 illustrated embodiment of the invention to avoid repetition. With regard to consistent features, reference is made to the description of the Fig. 1 and 2 directed.

Fig. 3 shows a circuit with three pairs of capacitors 28a, 28b, 28c, wherein each part of the pair is associated with one of the semiconductor switches 14, 16 and one of the damping assemblies 18, 20. The capacitors 28a are hardwired to the semiconductor switches 14, 16, and the capacitors 28b and 28c can be switched on or off by switching elements 26a, 26b, respectively. The control unit 32 can thereby choose between three possible values of the total capacitance of the damping arrangements 18, 20, between the capacitance of the capacitors 28a, between the sum of the capacitances of the capacitors 28a and 28b and between the sum of the capacitances of all three capacitors 28a, 28b, 28c. In the in Fig. 3 illustrated embodiment, the capacitances of the capacitors 28b and 28c have the same value, so that the total capacity of the damping capacitor system can be increased starting from a basic value in two stages with the same distance.

Fig. 4 shows a further alternative embodiment of the invention, in the switching elements 26a, 26b for connecting capacitors 28b, 28c of the damping capacitor system 24 not as in the embodiment after Fig. 3 are connected in series in series, but are connected in parallel with the center of the inverter 10. The capacitors 28b, 28c preferably have different capacitances, so that a different total capacitance can be achieved by closing the switching element 26a when the switching element 26b is open than by closing the switching element 26b when the switching element 26a is open. Altogether four different values of the total capacity are adjustable.

By adjusting the total capacity of the damping assemblies 18, 20, the control unit 32 can prevent leakage during operation. The total capacitance, and hence the attenuation constant, of the attenuator assemblies 18, 20 can be matched to the load, such that substantial ringing of the resonant circuits formed by the resonant capacitors 58 and the inductor 10 and discharge of oversized snubber capacitors of one attenuator assembly through the transistor 50 of another Semiconductor circuit can be avoided. At the same time too great inertia of the circuit can be avoided by an over-dimensioned at low power attenuation capacitor.

Fig. 5 shows power losses in an exemplary embodiment. The power losses are composed of power losses, which are represented by a horizontal hatching and switching losses, which are represented by a vertical hatching together. In the Fig. 5 left column shows the losses at a comparatively high power with open switching element 26 in the embodiment according to Fig. 2 , and those in Fig. 5 right column shows the losses at the same power with closed switching element 26 and switched capacitors 28b. It can be seen that the switching losses can be significantly reduced. reference numeral 10 inverter 56 contact point 12 DC voltage source 58 Resonant capacitor 14 Semiconductor switches 60 resistance 16 Semiconductor switches 18 damping arrangement 20 damping arrangement 22 medium 24 Snubber capacitor-system 26 switching element 28a capacitor 28b capacitor 28c capacitor 30 inductor 32 control unit 34 Cookware element 36 heating zone 38 User interface 40 Power supply unit 42 rectifier 44 Filter circuit 46 Household power grid 48 circuitry 50 transistor 52 diode 54 contact point

Claims (7)

1. Induction hob having at least one inverter (10), which comprises at least two semiconductor switches (14, 16) connected to different poles of a direct voltage source (12) and at least two damping arrangements (18, 20), which are arranged in parallel with one of the semiconductor switches (14, 16) in each case, and having at least one means (22) for adjusting a damping constant of at least one of the damping arrangements (18, 20),
   characterised in that
   the means (22) is designed to change the damping constants of the damping arrangements (18, 20) of both semiconductor switches (14, 16) simultaneously.
2. Induction hob according to claim 1, characterised in that the damping arrangement (18, 20) comprises at least one damping capacitor system (24), wherein the means (22) is designed to adjust an overall capacitance of the damping capacitor system (24).
3. Induction hob according to one of the preceding claims, characterised in that the damping arrangement (18, 20) comprises a number of switching elements (26) and a number of capacitors (28), which can each be connected by closing a switching element (26).
4. Induction hob according to claim 3, characterised in that the capacitors (28) have different capacitances and can be connected independently of one another.
5. Induction hob according to one of the preceding claims, characterised in that the domestic appliance is an induction hob and that the inverter (10) generates a heating current for energising inductors (30).
6. Induction hob according to one of the preceding claims, characterised by a control unit (32), which is designed to adjust the overall capacitance independently of a target power transmitted by way of the semiconductor switches (14, 16).
7. Induction hob according to claim 6, characterised in that the control unit (32) increases the overall capacitance in stages if the target power or a target current exceeds a threshold value.

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