Cooking hob and method for operating at least one cooking zone of a cooking hob

Abstract

A cooktop with at least one cooking zone (6, 7, 8, 9) comprises at least one heating element (6a, 6b, 7a, 8a, 9a) which can be supplied with electrical energy and whose power output can be controlled via a control unit (10). The control unit (10) has a first electrical switching device (S1a, S1b, S1c, S1d, S1e) and a second electrical switching device (S2a, S2b), which are connected in series with the heating element (6a, 6b, 7a, 8a, 9a). The first electrical switching device (S1a, S1b, S1c, S1d, S1e) is designed as an electromechanical relay, and the second electrical switching device (S2a, S2b) is designed as a semiconductor switch. The second electrical switching device (S2a, S2b) is controlled by the control unit (10) in such a way that the first electrical switching device (S1a, S1b, S1c, S1d, S1e) always switches without load.

Description

[0001] The present invention relates to a cooktop according to the preamble of claim 1. Furthermore, the invention relates to a method according to the preamble of claim 11.

[0002] The power supply for the heating element of a cooking zone on a radiant cooktop is typically provided by a main switch relay to activate the power supply, i.e., to connect it to the mains power supply. Additionally, a timing relay is included, which reduces the maximum power output of the heating element by interrupting the power supply (cycling), according to a user-selected power level.

[0003] A cooktop of this type is known from German patent application DE 10 2010 063 485 A1, which has at least one cooking zone. The cooking zone is connected to a main switch relay and to a timing relay, which can be operated in a clocked manner via a control unit. The control unit is also designed to operate the main switch relay with a main switch timing signal.

[0004] The invention is based on the objective of improving the aforementioned state of the art.

[0005] This problem is solved by a cooktop having the features of claim 1. Furthermore, the problem is solved by a method having the features of claim 11. Advantageous embodiments can be found in the dependent claims, individually or in combination.

[0006] According to the characterizing part of claim 1, the second electrical switching device is designed as a semiconductor switch and can be controlled by the control unit such that the first electrical switching device always switches without a load. Because the first electrical switching device, which is designed as an electromechanical relay, always switches without a load, a more cost-effective electromechanical relay can be used, which nevertheless functions reliably for the entire service life of the cooktop. Furthermore, the electromechanical relay can reliably interrupt current flow through the heating element at any time, or reliably disconnect the circuit even under load. Designing the second electrical switching device as a semiconductor switch makes it possible to switch high load currents without causing wear on electromechanical contacts.Frequent switching operations under load do not lead to increased contact resistance or potential failures such as the contacts sticking together. The switching relays used in conventional technology are subject to high wear because they frequently switch high currents. To ensure their functionality over the lifespan of the cooktop, very expensive relays, which are also only available from a few manufacturers, are typically used. The semiconductor switch, also known as a solid-state relay, uses electronic components for switching operations and therefore does not require moving contacts. It preferably has at least one semiconductor switching element, such as a transistor, a MOSFET, a thyristor, an IGBT, or a triac.

[0007] The heating device is preferably designed as a radiant heater. Radiant heaters are defined as heaters whose heating effect is based on the emission of infrared radiation. Such heaters can, for example, be designed as resistance heaters and have a heating wire or coil. Halogen heaters or quartz heating elements also fall into this category.

[0008] The use of a semiconductor switch enables highly flexible switching operations. For example, the duty cycle of a pulse-width or pulse-duration modulation can depend on the required heating power or a selected cooking level, with corresponding values ​​for each selectable heating power or cooking level stored in a memory unit of the control unit. Furthermore, it is possible for the switching operations for power control of the heating device to be mains-synchronized, similar to phase-angle or phase-cut control. It is also possible to control the power of the heating device by switching individual half-waves or full-waves of the sinusoidal supply voltage, with the switching operation always occurring at the zero crossing of the supply voltage.

[0009] The flexible switching capabilities of a semiconductor switch make it possible, for example, to create a continuous glow pattern on the radiant heating elements by using appropriately short or adjusted cycle times. This glow pattern remains visible even at low power levels. In contrast, conventional cooktops, with their long cycle times at lower power levels, produce a flickering glow pattern, meaning that even when the cooking zone is switched on and hot, the heating element may not be visible at times.

[0010] The flexible control options of the second electrical switching device also make it possible to even out the cooktop's power consumption by appropriately staggering the on-times of the different heating elements. This allows, in particular, short-term power peaks to be reduced. This offers particular advantages for power management when using photovoltaic systems, possibly in conjunction with their associated storage systems.

[0011] In a preferred embodiment, the second electrical switching device is provided for power cycling of the heating element. Power cycling refers to the periodic switching on and off of the current flow through the heating element, thereby allowing an average heating power to be set that is lower than the rated power of the heating element. The average heating power of the heating element is calculated by multiplying the rated power of the heating element by the duty cycle, i.e., the ratio between the on-time and the period. For example, with a cycling pattern where the on-time equals the off-time, a duty cycle and thus a heating power of 50% result. By designing the second electrical switching device as a semiconductor switch, the duty cycle can be freely selected within wide limits. Very fast switching operations are also possible with the semiconductor switch without a noticeable increase in wear.Since previously standard electrical switching devices were designed as electromechanical clock relays, they had to be equipped with expensive components in order to last the lifetime of the cooktop.

[0012] To enable complete isolation of the heating element from the power grid, the heating element is positioned between two electrical switching devices. For example, the first electrical switching device is located between one of the live conductors L1, L2, or L3 and the heating element, allowing the connection between the heating element and the live conductor to be interrupted. Furthermore, the second electrical switching device is located between the heating element and the neutral conductor, allowing the heating element to be completely disconnected from the grid by opening both electrical switching devices.

[0013] In an advantageous embodiment of the invention, the control unit interrupts the current flow through the heating element for a switching operation of the first electrical switching device with the second electrical switching device. Due to the series connection of the heating element with the two electrical switching devices, the current flow through the first electrical switching device is also interrupted. The interruption time is less than 70 ms, particularly less than 50 ms, and most preferably less than 30 ms. The interruption time of the second electrical switching device, which is designed as a semiconductor switch, is essentially determined by the duration of a switching operation of the first electrical switching device. A safety margin must also be taken into account to compensate for typical component variations and other disruptive influences, such as fluctuations in the supply voltage.Thus, the interruption time is sufficiently long to allow for safe, de-energized switching of the first electrical switching device. Furthermore, the interruption time is sufficiently short to briefly disconnect all heating devices, including those not intended to be switched, from the power supply in the case of parallel-connected heating elements with their associated first electrical switching devices, without the brief interruption noticeably affecting the heating output of the heating elements not intended to be switched. This is due to the long periods or cycle times of several tens of seconds that are typical, especially for radiant heaters, where a brief interruption in the range of a few tens of milliseconds is negligible.

[0014] If at least two heating devices, together with their respective first electrical switching devices, are connected to each other in a parallel branch and are jointly connected with only a single, second electrical switching device, a simpler structure with fewer components results; in particular, at least one second electrical switching device is saved.

[0015] Preferably, for each switching operation of one of the first electrical switching devices, the second electrical switching device briefly de-energizes the entire parallel branch. This ensures that the first electrical switching devices always switch without a load.

[0016] In such a design, it is further preferred that the interruption times of short-term power interruptions in heating devices not intended for switching operations are summed and correspondingly taken into account when balancing the power cycling of the respective heating device. Thus, during power cycling, the switch-on time can be slightly increased at defined intervals to compensate for the power interruptions that occurred but were not required for these heating devices.

[0017] To achieve a high level of operational reliability, each of the two electrical switching devices is assigned a sensor that monitors the current switching state.

[0018] In a preferred embodiment, the control unit evaluates the sensor data and, upon detecting a faulty switching state in one of the two electrical switching devices, switches off the other electrical switching device. Specifically, if a switching state is detected as faulty, all electrical switching devices of the affected branch are switched off. In the case of heating devices connected in parallel, all heating devices are switched off accordingly.

[0019] If the control unit has a special operating mode in which the two electrical switching devices are controlled in such a way that the first electrical switching device switches under load for a limited period, its relay contacts can be cleaned. This cleaning is specifically aimed at removing oxide layers and / or deposits of dirt or dust that form over time. These deposits are removed or burned off by switching under load or by the flow of high currents. A defined duration for this limited period is stored in the cooktop's control unit. Alternatively, a number of switching cycles sufficient to clean the relay contacts can be stored in the control unit.

[0020] Furthermore, the invention relates to a method for operating at least one cooking zone of a cooktop, in which a heating element of the cooking zone is supplied with current via a control device, and the power supply is provided via a first electrical switching device, which is designed as an electromechanical relay, and a second electrical switching device, which is designed as a semiconductor switch, and which electrical switching devices are each connected in series with the heating element, comprising the following steps: Switching the first electrical switching device, pulsed switching of the second electrical switching device for the duration of the cooktop operation to control the heating power according to a setpoint, deactivating the second electrical switching device when the cooktop operation is terminated so that the current flow through the heating device is interrupted, and switching the first electrical switching device so that the heating device is disconnected from the mains power supply.

[0021] Preferably, several heating devices are connected in parallel, each with its associated first electrical switching device, with the second electrical switching device arranged in series with this parallel connection. When several heating devices are operated simultaneously, the individual heating devices are clocked via the first electrical switching device. To switch a particular first electrical switching device, the second electrical switching device briefly de-energizes the parallel branch, ensuring that the first electrical switching devices can always be de-energized.

[0022] Advantages and features described with regard to the cooktop apply - insofar as applicable - accordingly to the method according to the invention and vice versa.

[0023] Further features and advantages of the invention will become apparent from the following description of an exemplary embodiment with reference to the accompanying figures. These show: Fig. 1 the cooktop according to the invention in a top view; Fig. 2 a circuit diagram for controlling the heating equipment in accordance with the state of the art; Fig. 3 a circuit diagram for controlling the heating devices according to the inventive design; Fig. 4 a control scheme for two heating devices or cooking zones; Fig. 5 a detail a of the control scheme from Fig. 4 in an enlarged view; Fig. 6 a control scheme for three heating devices or cooking zones and Fig. 7 A control scheme for a heating system in partial load operation.

[0024] In the figures, identical or functionally equivalent elements are given the same reference symbols.

[0025] Fig. 1 Figure 2 shows a cooktop 2, which has a glass-ceramic plate 4 with cooking zones 6, 7, 8, and 9. Cooking zone 6, located at the rear left, is a dual-circuit cooking zone and accordingly has two separately switchable heating elements 6a and 6b beneath the glass-ceramic plate 4. A user can, for example, activate only heating element 6a at a predefined power level. This applies to the inner, smaller section of cooking zone 6, which is intended for small-diameter cookware. For larger-diameter cookware, a second section of cooking zone 6, or rather the associated heating element 6b, can be activated. The combination of these two sections of cooking zone 6 creates a larger cooking area for larger-diameter cookware. The second section cannot be used independently. In combined operation, heating elements 6a and 6b can be controlled or pulsed independently to...B. to accommodate the deflection of a cooking vessel base. The other cooking zones 7, 8, and 9 each have only one associated heating element 7a, 8a, and 9a. The cooktop 2 also has a control unit 10, which is located below the glass-ceramic plate 4 and supplies the heating elements 6a, 6b, 7a, 8a, and 9a with power as needed and regulates their output. The control unit 10 is also connected to a control panel 12 of the cooktop, allowing a user to operate the cooktop and, for example, assign corresponding power levels to the cooking zones 6, 7, 8, and 9.

[0026] Fig. 2 Figure 1 shows a circuit diagram of a currently common control system for heating devices 6a, 6b, 7a, 8a, and 9a. An electrical switching device S1a is connected to one of the power supply's phase conductors L1. This device acts as a main switching relay for the downstream heating devices 6a and 6b. This main switching relay is an electromechanical component and can interrupt the current flow through all downstream heating devices 6a and 6b by means of the control unit 10, thus disconnecting them from the power supply. Each of the heating devices 6a and 6b is also assigned a timing relay T1 and T2, which are likewise electromechanical components. These relays enable partial load operation of the assigned heating devices 6a and 6b by means of individual power switching. The corresponding control signals for the timing relays T1 and T2 are provided by the control unit 10.The respective switching relays T1-T5 connect the corresponding heating elements 6a, 6b, 7a, 8a, and 9a to the neutral conductor. For even load distribution across the power grid, the remaining heating elements 7a-9a can be connected to the live conductor L2 via a separate electrical switching device S1b. Heating elements 7a-9a are connected to the neutral conductor via switching relays T3-T5. The switching relays T1-T5 must be designed to switch the high currents flowing through heating elements 6a, 6b, 7a, 8a, and 9a frequently and reliably, and throughout the entire service life of the cooktop 2. The main switch relays, or the electrical switching devices S1a and S1b, must also be designed to switch high currents reliably.

[0027] Fig. 3 Figure 1 shows the circuit diagram of the embodiment according to the invention. The two heating devices 6a and 6b are again assigned to the live conductor of the power supply L1. A first electrical switching device S1a, S1b is connected in series upstream of each of the heating devices 6a and 6b. A common second electrical switching device S2a provides a switchable connection to the neutral conductor. The load current flowing through the heating devices 6a and 6b is switched exclusively by the second electrical switching device S2a, which is designed as a semiconductor switch. Due to its design as a semiconductor switch, there is no wear on mechanical contacts – reliable switching is always ensured. The first electrical switching devices S1a and S1b serve to select the corresponding heating device 6a or 6b.For example, it can be determined whether only the inner circuit with its associated heating element 6a is used in cooking zone 6, or whether both the inner and outer circuits are used by activating both heating circuits 6a and 6b. If both heating circuits of cooking zone 6 are used, for example, to heat a large-diameter cooking vessel, it may still be necessary to operate the heating elements 6a and 6b at different frequencies or power levels to achieve optimal cooking results. To achieve this, the individual heating elements 6a and 6b are controlled by the first electrical switching devices S1a and S1b. These devices are always de-energized, as the second electrical switching device S2a briefly interrupts the current through both heating elements 6a and 6b for the duration of each switching operation by the first electrical switching devices S1a and S1b.The first electrical switching devices S1a and S1b are hardly stressed despite the numerous switching operations, as they always switch without current, thus achieving a long service life despite the use of inexpensive components.

[0028] Each of the additional heating elements 7a, 8a, and 9a (cooking zones 7, 8, and 9) is connected to a first electrical switching device S1c, S1d, and S1e, respectively, allowing them to be connected to phase L2. The heating elements 7a, 8a, and 9a, with their associated first electrical switching devices S1c, S1d, and S1e, are connected in parallel. Connection to the neutral conductor is made via a common second electrical switching device S2b, which is connected in series with the parallel connection of the heating elements 7a, 8a, and 9a. The operating principle is analogous to that used for controlling the heating elements 6a and 6b.

[0029] An example of a corresponding control scheme for the heating devices 6a and 6b is shown in Fig. 4 The diagram shows the switching states of the electrical switching devices S1a, S1b, and S2a, plotted against time. The switching states can be "On" (top line) or "Off" (bottom line). In the control diagram, the first electrical switching device, S1a, switches with equal on and off times, which, with respect to the heating element 6a, which heats the inner circle of cooking zone 6, means a halving of the maximum possible heating power. The outer circle of cooking zone 6, which is controlled by the electrical switching device S1b, has a different switching frequency than the first electrical switching device S1a, which, for example, results in a higher surface heating power. This can be advantageous, for example, if a large cooking vessel has a curved bottom area that only rests on the glass-ceramic plate 4 at its edges.

[0030] The second electrical switching device S2a, which is designed as a semiconductor switch, switches on shortly after the activation of the first electrical switching device S1b, or establishes a connection to the neutral conductor. Subsequently, the second electrical switching device S2a remains switched on until a switching operation of one of the two first electrical switching devices S1a or S1b is imminent. Shortly before the switching operation of the first electrical switching device S1a or S1b, the second electrical switching device S2a interrupts the current flow in the entire parallel branch of the heating devices 6a and 6b, so that the switching operation of the corresponding first electrical switching device S1a or S1b can take place without current flow. Shortly after the switching operation of the first electrical switching device S1a or S1b, the second electrical switching device S2a re-establishes the connection to the neutral conductor.This allows the current to flow through the heating devices 6a, 6b again. The second electrical switching device S2a thus briefly interrupts the current flow through the heating devices 6a, 6b for each switching operation of one of the first electrical switching devices S1a, S1b. The frequent, short switching operations of the second electrical switching device S2a, which is designed as a semiconductor switch, have only a negligible effect on its service life.

[0031] Fig. 5 shows detail a of the control scheme from Fig. 4 , which represents a time-stretched switching point of the first electrical switching device S1b. In order for the first electrical switching device S1b to switch off the current, the current flow through the associated heating devices 6a, 6b is interrupted for a short period t by the second electrical switching device S2a shortly before the switching operation. This short period t is only 10 ms, which is sufficient to perform a switching operation of the first electrical switching device S1b without current.

[0032] Fig. 6 Figure 1 shows an example of a control scheme for the heating elements 7a, 8a, and 9a, which are assigned to cooking zones 7, 8, and 9. The heating elements 7a, 8a, and 9a are switched by means of first electrical switching devices S1c, S1d, and S1e, as well as by means of a second electrical switching device S2b designed as a semiconductor switch. The basic principle corresponds to that shown in the Fig. 4 However, three heating devices are controlled here, which results in an increased switching frequency of the second electrical switching device S2b. Here too, the first electrical switching devices S1c, S1d, S1e always switch off.

[0033] Fig. 7 This illustrates the case where only the inner part of cooking zone 6 is heated by the heating element 6a. In this case, the first electrical switching device S1a switches on and establishes a connection to the live conductor L1. The first electrical switching device S1a is de-energized because the second electrical switching device S2a switches on after a time delay and enables the current flow through the heating element 6a. The first electrical switching device S1a remains switched on throughout the entire heating process. The cycling of the heating power is achieved by corresponding switching operations of the second electrical switching device S2a. Due to its design as a semiconductor switch, the switching frequency can be varied within wide limits without causing a noticeably increased wear of the semiconductor switch. Fig. 7The on-times correspond to the off-times, resulting in an average power output of the heating device 6a of 50% compared to the maximum power output (continuous current). Reference symbol list:

[0034] 2Cooking surface 4Glass ceramic plate 6Cooking zone 6aHeating device 6bHeating device 7Cooking zone 7aHeating device 8Cooking zone 8aHeating device 9Cooking zone 9aHeating device 10Control device 12Control panel L1; L2Line conductor NNeutral conductor S1a-S1eFirst electrical switching device T1-T5Clock relay S2a; S2bSecond electrical switching device tInterruption time

Claims

1. Hob with at least one cooking zone (6, 7, 8, 9), comprising at least one heating element (6a, 6b, 7a, 8a, 9a) which can be supplied with electrical energy and whose power output can be controlled via a control device (10), wherein the control device (10) has a first electrical switching device (S1a, S1b, S1c, S1d, S1e) and a second electrical switching device (S2a, S2b) which are connected in series with the heating element (6a, 6b, 7a, 8a, 9a), and wherein the first electrical switching device (S1a, S1b, S1c, S1d, S1e) is designed as an electromechanical relay, characterized by the fact that the second electrical switching device (S2a, S2b) is designed as a semiconductor switch and is controlled by the control device (10) in such a way that the first switching device (S1a, S1b, S1c, S1d, S1e) always switches without load.

2. Cooktop according to one of the preceding claims, characterized by the fact thatthe second electrical switching device (S2a, S2b) is provided for power switching of the heating device (6a, 6b, 7a, 8a, 9a).

3. Cooktop according to one of the preceding claims, characterized by the fact that the heating device (6a, 6b, 7a, 8a, 9a) is arranged in terms of circuit technology between the two electrical switching devices (S1a, S1b, S1c, S1d, S1e, S2a, S2b).

4. Cooktop according to one of the preceding claims, characterized by the fact that The control device (10) for a switching operation of the first electrical switching device (S1a, S1b, S1c, S1d, S1e) with the second electrical switching device (S2a, S2b) interrupts the current flow through the heating device (6a, 6b, 7a, 8a, 9a) for an interruption time (t) of less than 70 ms, in particular less than 50 ms and preferably less than 30 ms.

5. Cooktop according to one of the preceding claims, characterized by the fact thatat least two heating devices (6a, 6b, 7a, 8a, 9a) together with their respective first electrical switching devices (S1a, S1b, S1c, S1d, S1e) are connected to each other in a parallel branch and are connected together with only a single, second electrical switching device (S2a, S2b).

6. Cooktop according to claim 5, characterized by , for each switching operation of one of the first electrical switching devices (S1a, S1b, S1c, S1d, S1e) the second electrical switching device (S2a, S2b) briefly switches off the entire parallel branch.

7. Cooktop according to claims 4 and 6, characterized by the fact that The interruption times (t) of the short-term power interruptions in the heating devices (6a, 6b, 7a, 8a, 9a) not intended for a switching operation are summed and taken into account accordingly in the power cycling of the respective heating device (6a, 6b, 7a, 8a, 9a).

8. Cooktop according to one of the preceding claims, characterized by the fact that Each of the two electrical switching devices (S1a, S1b, S1c, S1d, S1e, S2a, S2b) is assigned a sensor that monitors the current switching state.

9. Cooktop according to claim 8, characterized by , the control unit (10) evaluates the sensor data of the sensors, and when a faulty switching state of one of the two electrical switching devices (S1a, S1b, S1c, S1d, S1e, S2a, S2b) is detected, switches off the other electrical switching device (S1a, S1b, S1c, S1d, S1e, S2a, S2b).

10. Cooktop according to one of the preceding claims, characterized by the fact that the control device (10) is set up in a special operating mode to control the two electrical switching devices (S1a, S1b, S1c, S1d, S1e, S2a, S2b) in such a way that the first electrical switching device (S1a, S1b, S1c, S1d, S1e) switches under load for a limited period of time in order to clean its relay contacts.

11. Method for operating at least one cooking zone (6, 7, 8, 9) of a cooktop 2, wherein a heating element (6a, 6b, 7a, 8a, 9a) of the cooking zone (6, 7, 8, 9) is supplied with current via a control device (10), and the power supply is provided via a first electrical switching device (S1a, S1b, S1c, S1d, S1e), which is designed as an electromechanical relay, and a second electrical switching device (S2a, S2b), which is designed as a semiconductor switch, and which electrical switching devices (S1a, S1b, S1c, S1d, S1e, S2a, S2b) are each connected in series with the heating element (6a, 6b, 7a, 8a, 9a), comprising the following steps: • Switching the first electrical switching device (S1a, S1b, S1c, S1d, S1e), • pulsed switching of the second electrical switching device (S2a, S2b), for the duration of the hob operation, in order to control the heating power according to a setpoint,• When the cooktop is switched off, deactivate the second electrical switching device (S2a, S2b) so that the current flow through the heating element (6a, 6b, 7a, 8a, 9a) is interrupted, and • switch the first electrical switching device (S1a, S1b, S1c, S1d, S1e) so that the heating element (6a, 6b, 7a, 8a, 9a) is disconnected from the mains power supply.

12. Method according to claim 11, characterized by the fact thatSeveral heating devices (6a, 6b, 7a, 8a, 9a) with their associated first electrical switching devices (S1a, S1b, S1c, S1d, S1e) are connected in parallel, and the second electrical switching devices (S2a, S2b) are arranged in series with this parallel connection, wherein, when several heating devices (6a, 6b, 7a, 8a, 9a) are operated simultaneously, the clocking of the individual heating devices (6a, 6b, 7a, 8a, 9a) is carried out via the first electrical switching devices (S1a, S1b, S1c, S1d, S1e), and to switch a particular first electrical switching device (S1a, S1b, S1c, S1d, S1e), the second electrical switching device (S2a, S2b) briefly de-energizes the parallel branch, thereby de-energizing the first electrical switching devices (S1a, S1b, S1c, S1d, S1e) are always switched off.

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