Method for controlling the supply of electric power to an induction cooking appliance

Abstract

The invention provides a method for controlling the supply of electric power to an induction cooking appliance (1). The induction cooking appliance (1) comprises a circuitry (10) having: an AC-voltage input (11) for receiving an AC-voltage signal (Vin), a resonance circuit (17) with an induction coil (20) and a resonance capacitor (18), at least one switching element (12) for providing pulsed electric power to the resonance circuit (17), and a capacitor (16) being connected in parallel to the series of the switching element (12) and the resonance circuit (17). The method comprises: receiving an AC-voltage signal (Vin) at the AC-voltage input (11), controlling the circuitry (10) of the induction cooking appliance to enter into a discharging phase (D1) to discharge the capacitor (16), after at least partially discharging the capacitor (16), controlling the circuitry (10) to enter into a first stop phase (D2) in which the switching element (12) is in an off-state, and after the first stop phase (D2), controlling the circuitry (10) to enter into a heating phase (D3) in which the switching element (12) provides pulsed electric power to the resonance circuit (17).

Description

[0001] METHOD FOR CONTROLLING THE SUPPLY OF ELECTRIC POWER TO AN INDUCTION COOKING APPLIANCE The present invention relates to a method for controlling the supply of electric power to an induction cooking appliance, a system adapted to control the supply of electric power to an in- duction cooking appliance, and an induction cooking appliance comprising such system. EP 3344 006 A1 discloses a method for controlling an electro- magnetic heating system having a resonance circuit, wherein the method comprises controlling the resonance circuit to enter into a discharging stage, a heating stage, and a stop stage succes- sively. It is an object of the invention to provide an efficient method and system for controlling the supply of electric power to an induction cooking appliance and in particular to provide a con- trol method and system with an improved lifespan of the induc- tion cooking appliance. The invention is defined in the independent claims, respec- tively. Particular embodiments are set out in the dependent claims. According to claim 1, a method for controlling the supply of electric power to an induction cooking appliance is provided. The induction cooking appliance comprises a circuitry having: an AC-voltage input for receiving an AC-voltage signal (in particular from the grid), a resonance circuit with an induction coil and a resonance capacitor, at least one switching element for providing pulsed electric power to the resonance circuit, and a capacitor being connected in parallel to the series of the switching element and the resonance circuit. The method com- prises: receiving an AC-voltage signal at the AC-voltage input, controlling the circuitry of the induction cooking appliance to enter into a discharging phase to discharge the capacitor, after at least partially discharging the capacitor, controlling the circuitry to enter into a first stop phase in which the switch- ing element is in an off-state, and after the first stop phase, controlling the circuitry to enter into a heating phase in which the switching element provides pulsed electric power to the res- onance circuit (in particular to the induction coil of the reso- nance circuit). By discharging the capacitor in the discharging phase before the heating phase, the voltage provided to the switching element in the heating phase is reduced. Preferably, the capacitor is dis- charged in the discharging phase to a voltage of 50 V or lower. Thereby, switching-on of the switching element at high voltage levels can be avoided which reduces power dissipation due to thermal losses within the switching element and acoustic noise generated due to hard switching conditions. However, discharging the capacitor in the discharging phase causes considerable stress on the switching device, in particu- lar the temperature in the switching element rises during dis- charging of the capacitor. This rise in temperature can lead to damage to the switching device and thus reduces the lifespan of the switching device and the induction cooking appliance. By providing the first stop phase between the discharging phase and the heating phase, the appliance and in particular the switching element has time to cool down. Thereby the temperature load on the circuitry and in particular on the switching element is reduced and the lifespan of the induction cooking appliance and in particular of the switching element is increased. By providing the discharging phase and the first stop phase, the operation of the switching element below a maximum operation temperature can be ensured. This increases the lifespan of the switching element. Furthermore, the first stop phase allows the induction cooking appliance and in particular its circuitry to be adapted to the heating phase. In particular, the switching element may be oper- ated with a DC voltage in the discharging phase, while the switching element is operated with a pulsed voltage in the heat- ing phase. By providing the first stop phase, the circuitry has time to switch from providing DC voltage in the discharging phase to pulsed voltage in the first stop phase. Thus, the con- trol efficiency of the induction cooking appliance is improved. In the discharging phase no pulsed electric power may be sup- plied to the resonance circuit. Preferably, pulsed electric power is only supplied to the resonance circuit in the heating phase. The discharging phase, the first stop phase and the heating phase may not be performed simultaneously, but may be performed successively. Thus, the discharging of the capacitor has no det- rimental effects of the heating phase. Preferably in the off-state the switching element is 'switched off', i.e. it is in the non-conductive state. In particular, in the off-state the switching element is driven by a voltage of 0 V or substantially 0 V. Preferably, before the discharging phase, the voltage of the ca- pacitor 16 may be at a certain value, in particular at a maximum value, i.e. the capacitor is fully charged. The circuitry may comprise a single or pair of high frequency switching elements. Preferably, the switching element may be a semiconductor-switching element, particularly an insulated-gate bipolar transistor, IGBT, element. The single switching element preferably forms part of an associated power circuit provided in the form of a or a part of a Quasi Resonant circuit. Preferably, the circuitry starts the first stop phase immedi- ately after the end of the discharging phase. The duration of the first stop phase may be in the range of 0.2-1 ms, 0.5-1.5 ms, 0.8-2 ms or 1.0-2.5 ms and / or the duration of the first stop phase may be maximum 1 ms, 2 ms, 3 ms or 5 ms. The duration of the first stop phase may depend on at least one of: the charac- teristic of the switching element, the capacity of the capaci- tor, the resistance and / or inductivity of the induction coil. Preferably, the duration of the first stop phase only depends on the characteristic of the switching element. Preferably, the du- ration of the discharging phase is in the range between 0.5 and 6 ms, 1 and 5 ms, 1.5 and 4 ms, preferably between 2 and 3 ms. The duration of the first stop phase and / or of the discharging phase may be independent of a selected power level of the induc- tion cooking appliance. Preferably, the duration of the heating phase and a second stop phase (see below) depends on the power level of the induction cooking appliance selected by the user. Preferably, the discharging phase and / or the first stop phase is started during a period of time in which the slope of the AC- voltage signal is positive. When the AC-voltage signal is recti- fied to receive a rectified voltage signal (see below), the dis- charging phase may be started during a period of time in which the slope of the rectified voltage signal is negative and / or the first stop phase may be started during a period of time in which the slope is positive. 'Slope of a voltage' means the temporal derivative of the voltage. With respect to the non-rectified AC- voltage signal, the discharging phase is started in a positive slope phase of the AC-voltage signal before the zero-crossing. The method may further comprise detecting zero crossing points of the AC-voltage signal at the AC-voltage input, optionally controlling the circuitry to start the discharging phase at a time point before a zero crossing point; and controlling the circuitry to stop the discharging phase at or near to a detected zero crossing point and / or to start the first stop phase at or close to the detected zero crossing point of the AC-voltage sig- nal. Preferably the time point before a zero crossing point is se- lected such that the whole (duration of the) discharging phase is performed before or until reaching the detected zero crossing point. I.e. the end of the discharging phase is before the de- tected zero crossing point or coincides with the detected zero crossing point. The term "near to a detected zero crossing point" may correspond to a time point of 0.5 ms, 1 ms, 1.5 ms, 2 ms, 3 ms or 4 ms from (i.e. before) the detected zero crossing point. Preferably, the end of the discharging phase and / or the start of the first stop phase coincides with or corresponds to the de- tected zero crossing point. At the detected zero crossing point, the voltage provided to the switching element is low and detri- mental effects caused by hard-switching condition can be avoided. Preferably, the circuitry comprises a control unit configured to control the circuitry to stop the first stop phase and to start the heating phase between a first and second detected zero crossing point. The control unit may be configured to apply dif- ferent heating modes with different heating power levels. In particular, the heating modes may have different duty ratios (corresponding to different heating power levels of the induc- tion cooking appliance) such as for example a duty ratio of 1 / 4, 2 / 4 or 3 / 4. In a duty ratio of 1 / 4: the circuitry may be controlled to enter the heating phase for a time period of one half-wave of the AC- voltage signal (i.e. one half of a period of the AC-voltage sig- nal) and after the heating phase to enter into the second stop phase for a time period of three half-waves of the AC-voltage signal. The control unit may be configured to control the cir- cuitry to stop the heating phase and / or to start the second stop phase at or close to the second detected zero crossing point of the AC-voltage signal. In a duty ratio of 2 / 4: the circuitry may be controlled to enter the heating phase for a time period of two half-waves of the AC- voltage signal and after the heating phase to enter into the second stop phase for a time period of two half-waves of the AC- voltage signal. The control unit may be configured to control the circuitry to stop the heating phase and / or to start the sec- ond stop phase at or close to a third detected third crossing point of the AC-voltage signal. In a duty ratio of 3 / 4: the circuitry may be controlled to enter the heating phase for a time period of three half-waves of the AC-voltage signal and after the heating phase to enter into the second stop phase for a time period of one half-wave of the AC- voltage signal. The control unit may be configured to control the circuitry to stop the heating phase and / or to start the sec- ond stop phase at or close to a fourth detected zero crossing point of the AC-voltage signal. Preferably, the duration, the start and / or end time point of the discharging phases and the first stop phases is equal in each heating mode and thus inde- pendent of the heating modes. Preferably, the second zero crossing point corresponds to the next zero crossing point after the first zero crossing point, and / or the third zero crossing point to the next zero crossing point after the second zero crossing point, and / or the fourth zero crossing point to the next zero crossing point after the third zero crossing point. I.e. the first, second, third and fourth zero crossing points are consecutive zero crossing points of the AC-voltage signal without further zero crossing points in between. Preferably, the circuitry starts the heating phase immediately after the end of the first stop phase. The heating phase may end at a time point before the AC-voltage signal reaches a maximum value, in particular at or near a detected zero crossing point of the AC-voltage signal. The method may further comprise controlling, after the heating phase, the circuitry to enter into the second stop phase in which the switching element is entering the off-state. Prefera- bly, the duration of the heating phase and / or the second stop phase are dependent on a selected power level of the induction cooking appliance. For example, when a high power level is se- lected or applied, the duration of the heating phase is longer and the duration of the second stop phase is shorter than when a power level being lower compared to the high power level is se- lected or applied. Preferably, the capacitor can be loaded to its maximum voltage value in the second stop phase. The discharging phase, the first stop phase, the heating phase, and optionally the second stop phase may be repeated periodi- cally with the periodicity or a multiple of the periodicity of the AC-voltage signal. Preferably said phases are repeated every second period of the AC-voltage signal. Preferably, after the end of the second stop phase, another discharging phase starts immediately. Preferably, the switching element is driven: in the discharging phase, by a first driving voltage being a DC voltage signal, and / or in the heating phase, by a second driving voltage being a pulsed voltage signal and / or having a maximum voltage being higher than the first driving voltage, and / or in the first and / or the second stop phase, by a third driving voltage config- ured to keep the switching element in an off-state. The first driving voltage applied to the switching element in the discharging phase may be a DC voltage being in a range be- tween 5 V and 14.5 V, between 7 V and 13.5 V, between 8 V and 13 V, or between 10 V and 12 V. The second driving voltage applied to the switching element in the heating phase may have a maximum voltage which is higher than the first driving voltage by at least 0.5 V, 1 V, 2 V, 3 V or 5 V or which may be equal to 15 V. The third driving voltage applied to the switching element in the first and / or second stop phase may be ground voltage, in particular being 0V or substantially 0V. Preferably the control unit is adapted to supply to the switch- ing element at least one, two or all of the first driving volt- age, the second driving voltage and the third driving voltage. The control unit may comprise a user interface at least for re- ceiving a command input of a user. The user interface allows the user to control at least one operational parameter of the induc- tion cooking appliance, particularly a heating power level (with a specific duty ratio) of the induction cooking appliance. More- over, the control unit, and particularly the user interface, may be operatively connected to other appliances or interfaces, e.g. a suction hood, a voice control device, a server, a remote in- terface, a cloud-computing source or the like. The second driving voltage applied in the heating phase to the switching element may have a switching frequency or a periodic- ity in the range of 15 kHz to 35 kHz, 18 kHz to 32 kHz, 20 kHz to 30 kHz or 23 kHz to 27 kHz, preferably the switching fre- quency is 25 kHz. The method may further comprise rectifying the AC-voltage signal received at the input such that a rectified or at least par- tially rectified voltage signal is supplied to the resonance circuit. For example, the rectified voltage signal may be de- rived by rectifying a sinusoidal AC-voltage signal with a cer- tain frequency. The rectified voltage signal may comprise the double frequency of the sinusoidal AC-voltage signal. For exam- ple, if the sinusoidal AC-voltage signal has a frequency of 50 Hz, the rectified voltage signal may have a frequency of 100 Hz. Furthermore, a system adapted to control the supply of electric power to an induction cooking appliance is provided. The system comprises a circuitry having: an AC-voltage input for receiving an AC-voltage signal, a resonance circuit with an induction coil and a resonance capacitor, at least one switching element for providing pulsed electric power to the resonance circuit, a ca- pacitor being connected in parallel to the series of the switch- ing element and the resonance circuit, and a control unit. The control unit may be configured to control the switching element operation such that the circuitry: enters the discharging phase in which the capacitor is discharged, after discharging the ca- pacitor, enters into the first stop phase in which the switching element is in an off-state, and after the first stop phase, en- ters into the heating phase in which the switching element pro- vides pulsed electric power to the resonance circuit. Preferably the circuitry and in particular the control unit com- prises a control chip and / or a switching element driving module adapted to supply the first driving voltage (in particular in the discharging phase) to the switching element and / or to supply the second driving voltage (in particular in the heating phase) to the switching element and / or the third driving voltage (in particular in the first and / or second stop phase). The first stop phase may allow the induction cooking appliance and in par- ticular its circuitry to be adapted to the heating phase. In particular, the switching element is operated with a DC voltage in the discharging phase, while the switching element is oper- ated with a pulsed voltage in the heating phase. The first stop phase provides time for the circuitry of the appliance to switch either to provide the DC voltage in the discharging phase or the pulsed voltage instead of the DC voltage in the heating phase. Preferably, the switching element driving module comprises a first switching element driving module adapted to supply the first driving voltage (in particular in the discharging phase) to the switching element and a second switching element driving module adapted to supply the second driving voltage (in particu- lar in the heating phase) to the switching element. Preferably the second switching element driving module comprises an oscillator element adapted to generate the pulsed second driving signal. Preferably, the system further comprises a zero crossing point detecting unit which is coupled to the control unit and which is configured to detect zero crossing points of the AC-voltage sig- nal at the AC-voltage input, and wherein, the control unit is configured to control the circuitry to stop the discharging phase at the or near to the detected zero crossing point and / or to start the first stop phase at or close to the detected zero crossing point of the AC-voltage signal. Preferably, the system comprises a rectifier connected to the AC-voltage input and being configured to rectify the input volt- age to receive the rectified or at least partially rectified voltage signal. Furthermore, an induction cooking appliance comprising a system as disclosed herein is provided. The induction cooking appliance (e.g. an induction cooking hob) may comprise, preferably consists of, a cooking support for sup- porting foodstuff or cookware items and a lower casing. An open top side of the lower casing may be covered by at least a part of the cooking support. The cooking support may be provided par- ticularly as at least one panel, wherein preferably the panel is a glass ceramic panel. Preferably, at least one or more heating power transferring elements (e.g. the induction coil(s)) are ar- ranged below the panel. The lower casing may be manufactured from different material comprising plastics or metal, e.g. alu- minum. In particular, such casing may include a bottom wall and at least one sidewall. It is preferred that the casing is made of metal, e.g. aluminum or steel, and / or plastics, wherein pref- erably the casing made of metal is grounded. The induction cooking appliance may comprise at least one cool- ing means. Particularly, the cooling means is adapted for cool- ing down the electric and / or electronic elements. The cooling means may comprise at least one of a fan, a cooling channel, a cooling body, preferably from a metal, particularly aluminum, cooling air-guiding means, cooling air deflection means and the like. Particularly, the induction cooking appliance may comprise the cooling means for cooling the switching element(s). For conducting the cooking process, particularly in the heating phase, the induction cooking appliance, particularly the lower casing, comprises the induction coil which is associated with one heating zone. The cooking appliance may comprise more than one induction coil each associated with one heating zone. The induction coil is provided for transferring heating power to the foodstuff or cooking liquid, preferably contained in the cookware item. In particular, the induction coil is configured to provide the heating power by a heat-generating magnetic field, more particularly an induction field. The induction coil may be driven with different power levels for providing differ- ent amount of heating power. The induction coil may comprise a planar conductive winding wire, particularly a copper wire. Preferably, the induction coil comprises at least one magnetic field supporting element, e.g. a ferrite element. Preferably, the at least one magnetic field supporting element, particularly at least one ferrite element, is arranged below the plane of the conductive winding wire. Said at least one magnetic field supporting element, particularly ferrite element, is preferably in establishing and / or supporting the high frequent alternating magnetic field of the induction coil. The magnetic field supporting element, particularly if ar- ranged below the conductive winding wire, may be glued to or supported by ferrite support elements, e.g. snap fit connectors or the like. Preferably, the induction coil comprises a shielding element, e.g. a mica sheet. The shielding element preferably is adapted to the form of the planar conductive winding wire or the form of at least two planar conductive winding wires of at least two ad- jacently arranged coils. The shielding element preferably is provided above the at least one magnetic field supporting ele- ment, particularly at least one ferrite element. The shielding element preferably in its main function is a support for the planar conductive wire windings of the coil. The shielding element, particularly mica sheet, may also shield temperature radiated from the above, e.g. resulting from a heated up pot bottom. The lower casing and the cooking support may form a closed unit comprising all essential parts of the induction cooking appli- ance. The lower casing may comprise fastening means for fas- tening and / or arranging the cooking hob on top of or in a cutout of a work plate. The induction coil may be arranged below the cooking support, preferably in an upper portion of the lower casing. The induction coil may be arranged and supported by one or more heating power transferring element carrier or heating power transferring element support. Preferably the induction coil is attached and / or arranged on said carrier or support. Each individual feature of the system and / or the induction cook- ing appliance can be combined with the method, or any sub-group of features (e.g. any of the dependent claims) of the system and / or the induction cooking appliance can be individually com- bined with the method. Vice versa any individual (functional) feature or sub-group of (functional) features of the method can be combined with the system and / or the induction cooking appli- ance as a functional feature of the machine. Any feature disclosed herein (for the above embodiments and / or configurations and from the below described detailed embodiments and modifications) can be combined with the claimed subject in- dividually or in any sub-combination. If herein the conjunction "and / or" is used all logical elements and combinations are indi- vidually disclosed. E.g. a, b and / or c discloses the ele- ments / combinations a, b, c, ab, ac, bc as well as abc. Reference is made in detail to preferred embodiments of the in- vention, examples of which are illustrated in the accompanying figures, which show: Fig. 1 a top view of an induction cooking appliance, Fig. 2 a schematic diagram of a circuitry of a system for con- trolling the supply of electric power to an induction cooking appliance, and Fig. 3 a schematic diagram illustrating different phases during controlling of an induction cooking appliance in differ- ent heating modes and an exemplary switching element voltage curve during one of the heating modes. Fig. 1 is a top view of an induction cooking appliance 1, in the present example an electric induction hob. The induction cooking appliance 1 comprises one or more heating zones 2. In the exam- ple shown in Fig. 1, the induction cooking appliance 1 comprises four different heating zones 2. However, any other number of heating zones is possible. Each heating zone 2 may be associated with one or more heating power transferring elements, specifi- cally, one or more induction coils 20 (see: Fig. 2). The induc- tion cooking appliance 1 may be configured to combine two or more heating zones 2 in order to form larger-sized cooking zones. The induction cooking appliance 1 may comprise a user interface 3, based on which a user may control the induction cooking ap- pliance 1. For example, based on the user interface 3, the user may control the power level of the heating zones 2. The power level may be chosen between a minimum power level and a maximum power level. For example, a heating mode A may correspond to a minimum power level, a heating mode B to a medium power level and a heating mode C to a maximum power level (see Fig. 3). Fig. 2 shows a schematic diagram of a circuitry of a system for controlling the supply of electric power to the induction cook- ing appliance 1 and in particular to an induction coil 20. The circuitry 10 may comprise an AC-voltage input 11 for receiv- ing an AC-voltage input signal Vin (cf. AC-voltage signal Vin in Fig. 3), in particular from the grid. The circuitry 10 may com- prise a rectifier 14 connected to the input 11 and being config- ured to rectify the input voltage Vin to receive a rectified or at least partially rectified voltage signal. For example, the rectified voltage signal may be derived by rectifying a sinusoi- dal AC-voltage signal with a certain frequency. The rectified voltage signal may comprise the double frequency of the sinusoi- dal AC-voltage signal. For example, if the sinusoidal AC-voltage signal has a frequency of 50 Hz, the rectified voltage signal may have a frequency of 100 Hz. The circuitry 10 may further comprise a resonance circuit 17 having the induction coil 20 and a resonance capacitor 18 which are electrically connected. The resonance capacitor 18 may be connected in parallel to the induction coil 20. The resonance circuit 17 may be electrically connected in series to said rectifier 14 such that the rectified or at least partially rectified voltage signal is supplied to the resonance circuit 17. The circuitry 10 may comprise a switching element 12 which is electrically connected in series to the resonance circuit 17. In particular, the switching element 12 is electrically coupled with the induction coil 20 in order to provide electric power to the induction coil 20 to heat foodstuff contained in a cookware item 30. The switching element 12 may be a semiconductor-switch- ing element, particularly an IGBT element. The circuitry 10 may comprise a (bus) capacitor 16. The capaci- tor 16 is preferably connected in parallel to the switching ele- ment 12. In particular, the capacitor 16 may be connected in parallel to the serial connection of the switching element 12 and the resonance circuit 17. The capacitor 16 stabilizes the voltage across the resonance circuit 17 at least in a certain time span during a period of the input voltage Vin. In the non- heating phases the voltage from capacitor 16 is applied over temporally non-conducting switching element 12 as indicated with voltage Vr in Fig. 2. The switching element 12 may be electrically connected with a control unit 22 for receiving a switching element voltage VS to control an operation of the switching element 12. In particular, the control unit 22 is adapted to control the switching element operation such that the circuitry 10 enters a discharging phase D1 in which the capacitor 16 is discharged, after discharging the capacitor 16, enters into a first stop phase D2 in which the switching element 12 is in an off-state, and after the first stop phase D2, enters into a heating phase D3. In the heating phase D3, the switching element 12 may provide pulsed electric power to the resonance circuit 17 and in particular to the in- duction coil 20 to cause an induction heating process (cf. Fig. 3) to heat food contained in the cookware item 30. Before the discharging phase D1, the voltage of the capacitor 16 may be at a certain value, in particular at a maximum value, i.e. the capacitor 16 is fully charged. The system and in particular the circuitry 10 may further com- prise a zero crossing point detecting unit 26 which is coupled to the control unit 22 and to the AC-voltage input 11. The zero crossing point detecting unit 26 may be configured to detect zero crossing points Z1 to Z4 of the AC-voltage signal Vin at the AC-voltage input 11 (see Fig. 3). The control unit 22 may be configured to control the circuitry 10 in dependence of the de- tected zero crossing points Z1 to Z4. In particular, the control unit 22 is configured to control the circuitry 10 to stop the discharging phase D1 at the or near to the first detected zero crossing point Z1 and / or to start the first stop phase D2 at or close to the first detected zero crossing point Z1 of the AC- voltage signal (see Fig. 3). Fig. 3 is a schematic diagram illustrating different phases dur- ing controlling of an induction cooking appliance in different heating modes A, B, C. An exemplary curve of the input voltage Vinand an exemplary curve of a switching element voltage VSdur- ing the heating mode A each in dependence of the time are shown in Fig. 3. Heating modes A, B, C comprise different duty ratios for con- trolling the supply of electric power to the induction cooking appliance 1. Each heating mode corresponds to a different power level of the induction cooking appliance. In particular, the heating mode B may correspond to a higher power level than heat- ing mode A, and heating mode C may correspond to a higher power level than heating mode B. In each of the heating modes A, B and C, the circuitry 10 may be controlled to enter into at least three different phases succes- sively, namely the discharging phase D1, the first stop phase D2 after the discharging phase D1, and the heating phase D3 after the first stop phase D2. Preferably, and as shown in Fig. 3, in each of the heating modes A, B, C, the circuitry 10 may be con- trolled to enter into a second stop phase D4 in which the switching element 12 is entering the off-state (i.e. stops heat- ing). The capacitor 16 may be loaded to its maximum voltage value in the second stop phase D4. The first stop phase D2 between the discharging phase D1 and the heating phase D3 allows the appliance and in particular the switching element 12 to cool down, which reduces the temperature load on the switching element and increases its lifespan. By discharging the capacitor 16 in the discharging phase D1 be- fore the heating phase D3, the voltage Vr applied at collector of the switching element 12 is reduced before starting the heat- ing phase D3. Thereby, switching-on of the switching element 12 at high voltage levels can be avoided which reduces power dissi- pation due to thermal losses within the switching element and acoustic noise generated due to hard switching conditions. In particular, by providing the discharging phase D1 and the first stop phase D2, the operation of the switching element 12 below a maximum operation temperature can be ensured. This in- creases the lifespan of the switching element 12. By providing the first stop phase D2 the heat generated in switching element 12 during the DC discharge in the discharge phase D1 can dissi- pate out of the switching element before the current is flowing again in the heating phase D3. The different heating modes A, B, C differentiate from each other in the duration of the heating phase D3 and the second stop phase D4. In each heating mode, the control unit 22 is con- figured to control the circuitry 10 to stop the first stop phase D2 and to start the heating phase D3 between the first and sec- ond detected zero crossing points Z1, Z2. In heating mode A, the control unit 22 is configured to control the circuitry 10 to stop the heating phase D3 and / or to start the second stop phase D4 at or close to the second detected zero crossing point Z2 of the AC-voltage signal. In heating mode B, the control unit 22 is configured to control the circuitry 10 to stop the heating phase D3 and / or to start the second stop phase D4 at or close to a third detected third crossing point Z3 of the AC-voltage signal. In heating mode C, the control unit 22 is configured to control the circuitry 10 to stop the heating phase D3 and / or to start the second stop phase D4 at or close to the fourth detected zero crossing point Z4 of the AC-voltage signal (see Fig. 3). Prefer- ably, the duration and the start and end time point of the dis- charging phases D1 and the first stop phase D2 is equal in each heating mode and thus independent of the heating modes A, B, C. In the heating mode A, for example, the duty ratio may be 1 / 4. In particular, the circuitry 10 may be controlled to enter the heating phase D3 for a time period of one half-wave of the AC- voltage signal Vin(i.e. one half of a period T1, T2 of the AC- voltage signal) and after the heating phase D3 to enter into a second stop phase D4 for a time period of three half-waves of the AC-voltage signal Vin. The single half-waves of the AC-sig- nal Vin are numbered from 1 to 10 in Fig. 3 for better under- standing. In the heating mode B, for example, the duty ratio may be 2 / 4. In particular, the circuitry 10 may be controlled to enter the heating phase D3 for a time period of two half-waves of the AC- voltage signal Vinand after the heating phase D3 to enter into the second stop phase D4 for a time period of two half-waves of the AC-voltage signal Vin. In the heating mode C, for example, the duty ratio may be 3 / 4. In particular, the circuitry 10 may be controlled to enter the heating phase D3 for a time period of three half-waves of the AC-voltage signal Vin and after the heating phase D3 to enter into a second stop phase D4 for a time period of one half-wave of the AC-voltage signal Vin. Fig. 3 shows an exemplary curve of the switching element voltage VS during the heating mode A in dependence of the time. The switching element 12 may be driven by different driving voltages V1, V2, V3 to enter the discharging phase D1, the first stop phase D2 or the heating phase D3. Preferably, the switching element 12 is driven in the discharging phase D1 by the first driving voltage V1 being a DC voltage signal which is preferably in a range between 5 V and 14.5 V, between 7 V and 12 V or between 8 V and 10 V. Preferably, the switching element 12 is driven in the heating phase D3 by the second driving V2 voltage having a maximum voltage being higher than the first driving voltage V1 and / or being a pulsed voltage signal. The second driving voltage is preferably equal to 15 V. The switching element 12 may be driven in the first stop phase D2 (and / or in the second stop phase D4) by the third driving voltage V3 configured to keep the switching element 12 in an off-state. The third driving voltage is preferably ground voltage being equal to 0V (cf. Fig. 3 curve of the switching element voltage VSfor the first heating mode A). The first, second and third driving voltage values V1, V2 and V3 may be respectively applicable in the discharging phase D1, the first stop phase D2, the heating phase D3 and / or the second stop phase D4 of the respective heating modes B and C. The circuitry 10 and in particular the control unit 22 may com- prise a control chip 23 and / or a switching element driving mod- ule adapted to supply the first driving voltage V1 (in particu- lar in the discharging phase) to the switching element 12 and to supply the second driving voltage V2 (in particular in the heat- ing phase) to the switching element 12. The first stop phase D2 may allow the induction cooking appliance and in particular its circuitry 10 to be adapted to the heating phase D3. In particu- lar, the switching element 12 is operated with a DC voltage in the discharging phase D1, while the switching element 12 is op- erated with a pulsed voltage in the heating phase D3. The first stop phase D2 provides time for the circuitry 10 of the appli- ance to switch from the DC voltage V1 applied in the discharging phase D1 to the pulsed voltage V2 applied in the heating phase D3. Preferably, the switching element driving module comprises a first switching element driving module 24a adapted to supply the first driving voltage V1 (in particular in the discharging phase D1) to the switching element 12 and a second switching element driving module 24b adapted to supply the second driving voltage V2 (in particular in the heating phase D3) to the switching ele- ment 12 (see Fig. 2). Preferably the second switching element driving module 24b comprises an oscillator element (not shown) adapted to generate the pulsed second driving signal V2. The second driving voltage V2 applied in the heating phase D3 to the switching element 12 may have a switching frequency or a pe- riodicity in the range of 20 kHz to 30 kHz, specifically 25 kHz. The switching frequency may depend on the electrical dimension- ing of the resonance circuit 17. As shown in Fig. 3, the circuitry 10 preferably starts the first stop phase D2 immediately after the end of the discharging phase D1 and / or starts the heating phase D3 immediately after the end of the first stop phase D2. As shown in Fig. 3, the discharging phase D1 and the first stop phase D2 are preferably started during a period of time in which the slope of the AC-voltage signal Vin is positive. The duration of the first stop phase D2 may be in the range of 0.8-2 ms. The duration of the discharging phase D1 may be in the range between 2 and 3 ms. The duration of the first stop phase D2 may depend on at least one of: the characteristic of the switching element 12, the capacity of the capacitor 16, the resistance and / or inductivity of the induction coil 20. Preferably, the discharging phase D1, the first stop phase D2, the heating phase D3, and the second stop phase D4 are repeated periodically with the periodicity or a multiple of the periodicity of the AC-voltage signal Vin. As shown in Fig. 3, said phases D1, D2, D3 and D4 are repeated every second period T1, T2 of the AC-voltage signal Vin(i.e. the phases D1, D2, D3 and D4 extend over a time period of two periods T1, T2 of the AC-voltage signal Vin).

[0002] Reference Numeral List1 induction cooking appliance2 heating zone3 user interface10 circuitry11 AC input (from grid)12 switching element14 rectifier16 (bus) capacitor17 resonance circuit18 resonance capacitor20 induction coil22 control unit23 control chip24a first switching element driving module (DC)24b second switching element driving module (pulsed voltage)26 zero crossing point detection unit30 cookware itemVSswitching element voltageVr voltage at the collector of switching elementV1 first driving voltage of switching elementV2 second driving voltage of switching elementV3 third driving voltage of switching elementVinAC-voltage input signalD1 discharging phaseD2, D4 (first / second) stop phaseD3 heating phaseZ1, Z2, Z3, Z4 zero crossing point A, B, C first / second / third heating mode T1, T2 first / second period of AC signal

Claims

Claims:

1. Method for controlling the supply of electric power to an induction cooking appliance (1), the induction cooking appliance (1) comprising a circuitry (10) having: an AC-voltage input (11) for receiving an AC-voltage signal (Vin), a resonance circuit (17) with an induction coil (20) and a resonance capacitor (18), at least one switching element (12) for providing pulsed electric power to the resonance circuit (17), and a capacitor (16) being connected in parallel to the series of the switching element (12) and the resonance circuit (17); the method comprising: - receiving an AC-voltage signal (Vin) at the AC-voltage input (11), - controlling the circuitry (10) of the induction cooking appliance to enter into a discharging phase (D1) to discharge the capacitor (16), - after at least partially discharging the capacitor (16), controlling the circuitry (10) to enter into a first stop phase (D2) in which the switching element (12) is in an off-state, and - after the first stop phase (D2), controlling the circuitry (10) to enter into a heating phase (D3) in which the switching element (12) provides pulsed electric power to the resonance circuit (17).

2. Method according to claim 1, wherein the circuitry (10) starts the first stop phase (D2) immediately after the end of the discharging phase (D1), and / or wherein the duration of the first stop phase (D2) is in the range of 0.2-1 ms, 0.5-1.5 ms, 0.8-2 ms or 1.0-2.5 ms, and / or wherein the duration of the first stop phase (D2) is maximum 1 ms, 2 ms, 3 ms or 5 ms.

3. Method according to claim 1 or 2, wherein the duration of the discharging phase (D1) is in the range between 0.5 and 6 ms, 1 and 5 ms, 1.5 and 4 ms, preferably between 2 and 3 ms.

4. Method according to claim 1, 2 or 3, wherein the discharging phase (D1) and / or the first stop phase (D2) is started during a period of time in which the slope of the AC-voltage signal (Vin) is positive.

5. Method according to any of the preceding claims, the method further comprising: detecting zero crossing points (Z1-Z4) of the AC-voltage signal (Vin) at the AC-voltage input (11), optionally controlling the circuitry (10) to start the discharging phase (D1) at a time point before a zero crossing point (Z1); and controlling the circuitry (10) to stop the discharging phase (D1) at or near to a detected zero crossing point (Z1) and / or to start the first stop phase (D2) at or close to a detected zero crossing point (Z1) of the AC-voltage signal.

6. Method according to any of the preceding claims, wherein the circuitry (10) starts the heating phase (D3) immediately after the end of the first stop phase (D2).

7. Method according to any of the preceding claims, wherein the method further comprises controlling, after the heating phase(D3), the circuitry (10) to enter into a second stop phase (D4) in which the switching element (12) is entering the off-state.

8. Method according to any of the preceding claims, wherein the discharging phase (D1), the first stop phase (D2), the heating phase (D3), and optionally the second stop phase (D4) are repeated periodically with the periodicity or a multiple of the periodicity of the AC-voltage signal (Vin), preferably repeats the phases every second period (T1, T2) of the AC-voltage signal (Vin).

9. Method according to any of the preceding claims, wherein the switching element (12) is driven: in the discharging phase (D1), by a first driving voltage (V1) being a DC voltage signal, and / or in the heating phase (D3), by a second driving voltage (V2) voltage having a maximum voltage being higher than the first driving voltage (V1) and / or being a pulsed voltage signal, and / or in the first and / or a or the second stop phase (D2, D4), by a third driving voltage (V3) configured to keep the switching element (12) in an off-state.

10. Method according to claim 9, wherein the first driving voltage (V1) applied to the switching element (12) in the discharging phase (D1) is a DC voltage being in a range between 5 V and 14.5 V, between 7 V and 12 V or between 8 V and 10 V, and / or wherein the second driving voltage (V2) applied to the switching element (12) in the heating phase (D3) has a maximum voltage which is higher than the first driving voltage (V1) by at least 0.5 V, 1 V, 2 V, 3V or 5 V or which is equal to 15 V, and / or wherein the third driving voltage (V3) applied to the switching element (12) in the first and / or second stop phase(D2, D4) is ground voltage, in particular being 0V or substantially 0V.

11. Method according to claim 9 or 10, wherein the second driving voltage (V2) applied in the heating phase (D3) to the switching element (12) has a switching frequency or a periodicity in the range of 15 kHz to 35 kHz, 18 kHz to 32 kHz, 20 kHz to 30 kHz or 23 kHz to 27 kHz, preferably the switching frequency is 25 kHz.

12. Method according to any of the preceding claims, further comprising rectifying the AC-voltage signal received at the input (11) such that a rectified or at least partially rectified voltage signal is supplied to the resonance circuit (17).

13. System adapted to control the supply of electric power to an induction cooking appliance (1), the system comprising a circuitry (10) having: an AC-voltage input (11) for receiving an AC-voltage signal (Vin), a resonance circuit (17) with an induction coil (20) and a resonance capacitor (18), at least one switching element (12) for providing pulsed electric power to the resonance circuit (17), a capacitor (16) being connected in parallel to the series of the switching element (12) and the resonance circuit (17), and a control unit (22) configured to control the switching element operation such that the circuitry (10): - enters a discharging phase (D1) in which the capacitor (16) is discharged, - after discharging the capacitor (16), enters into a first stop phase (D2) in which the switching element (12) is in an off-state, and- after the first stop phase (D2), enters into a heating phase (D3) in which the switching element (12) provides pulsed electric power to the resonance circuit (17).

14. System according to claim 13, wherein the circuitry further comprises a zero crossing point detecting unit (26) which is coupled to the control unit (22) and which is configured to detect zero crossing points (Z1-Z4) of the AC-voltage signal (Vin) at the AC-voltage input (11), and wherein, the control unit (22) is configured to control the circuitry (10) to stop the discharging phase (D1) at the or near to the detected zero crossing point (Z1) and / or to start the first stop phase (D2) at or close to a detected zero crossing point (Z1) of the AC-voltage signal.

15. Induction cooking appliance (1) comprising a system according to claim 13 or 14.

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