EP4241538A1

EP4241538A1 - Circuit arrangement for an induction cooker, induction cooker and method for operating an induction cooker

Info

Publication number: EP4241538A1
Application number: EP20803538.6A
Authority: EP
Inventors: Marcel HOEFSMIT
Original assignee: Intell Properties BV; Gorenje d o o
Current assignee: Intell Properties BV; Gorenje d o o
Priority date: 2020-11-06
Filing date: 2020-11-06
Publication date: 2023-09-13
Also published as: WO2022096122A1; CN116889098A

Abstract

A circuit arrangement (1) for an induction cooker (10) comprises a rectifier (RT) which is connectable to an alternating mains voltage (MS); at least one intermediate circuit capacitor (Cf), wherein the intermediate circuit capacitor (Cf) is configured to buffer the rectified mains voltage; at least one resonant circuit (LC) comprising an induction coil (L) and a resonant circuit capacitor (Cr); a switching element (T1) which is connected to the resonant circuit (LC); a controller (CT), which is connected to the switching element (T1) and which is configured to operate the switching element (T1); a separate discharge circuit (SDC) comprising a switch (SW), wherein the separate discharge circuit (SDC) is connected to the at least one intermediate circuit capacitor (Cf) and which is separate to the resonant circuit (LC), wherein the switch controller (SE) is configured to operate the switch (SW) such that the at least one intermediate circuit capacitor (Cf) is discharged at a predetermined time period (TP) before the switching element (T1) in the resonant circuit (LC) is operated.

Description

Circuit arrangement for an induction cooker, induction cooker and method for operating an induction cooker

FIELD OF THE INVENTION

The present invention relates to a circuit arrangement for an induction cooker, an induction cooker and a method for operating such an induction cooker.

BACKGROUND OF THE INVENTION

When operating a cooking zone for an induction cooktop a resonant circuit can be coupled to a switch, for example an igb (insulated bipolar gate) transistor switch, which is used to be operated by a pulsed signal in order to generate heating power at an induction coil of the resonant circuit. A frequency converter can be used to generate a control voltage for operating the induction coil, wherein a low frequent voltage from mains can be transformed to a high frequent voltage for driving the induction coil.

Typically on/off cycles are implemented to achieve an average of low power which are in the range of seconds.

In a commonly used single switch topology using an igb transistor (igbt) a power regulation area can be limited. A typical power regulation can range between 900W - 2100W, wherein power can be regulated by adjusting an on-time of the igbt, wherein the larger the on-time, the more energy can be transferred to the resonant circuit. In order to make the control in a lower power control area more precise it is possible to apply a control method such that the resonant circuit is activated only at several periods of a mains frequency, what can be an improvement compared to switching on and off the resonant circuit in the range of seconds. As a side effect it happens that every time the igbt switch for operating the induction zone is disabled for a longer period, an intermediate circuit capacitor (DC bus capacitor), which is used for buffering, is fully charged to approximately the maximum value of the mains voltage. For the next time when the igbt switch is activated then, the energy stored in the intermediate capacitor has to be discharged, wherein in known circuit arrangements the intermediate capacitor can only be discharged via the induction zone by switching the igbt switch properly. When performing this switching an acoustic noise can appear depended on the cookware material.

The document EP 1 935 213 B1 describes a method for operating an induction heating device, wherein a capacitor for buffering a voltage can be discharged over an igbt switch of a resonant circuit in its linear mode.

SUMMARY OF THE INVENTION

It is an object of the invention to improve the discharging of an intermediate circuit capacitor for induction coils and to lower peak currents in the circuit during the discharging.

The object is solved by the subject-matter of the independent claims.

It is possible to provide a circuit arrangement for an induction cooker, an induction cooker and a method for operating such an induction cooker, wherein an intermediate circuit capacitor for buffering voltage and for operating the induction coil can be discharged with a lowered or prevented dissipation of energy from the intermediate circuit capacitor to the induction coil. The way of discharging the intermediate circuit capacitor can thus be improved. Thereby, the load on the induction coil and on remaining circuit components can be lowered during a low power regime and an acoustic noise resulting from cookware at high peak currents at the induction coil can be lowered or prevented by lowering or preventing high peak currents at the induction coil. In the following the term "induction zone" is used but the mentioned features, properties and corresponding advantages are also valid for an induction hob cooking zone, an induction cooktop or for an induction cooker and for all in general without specifying the zone where currents are induced.

The present invention pertains a circuit arrangement for an induction cooker according to claim 1, an induction cooker according to claim 7 and a method for operating an induction cooker according to claim 8.

Preferred embodiments are subject of the dependent claims.

According to the invention a circuit arrangement for an induction cooker comprises a rectifier which is connectable to an alternating mains voltage and which is configured to rectify the mains voltage; at least one intermediate circuit capacitor which is coupled between output terminals of the rectifier, wherein the intermediate circuit capacitor is configured to buffer the rectified mains voltage and to provide a buffered voltage from the rectified mains voltage; at least one resonant circuit comprising an induction coil and a resonant circuit capacitor; a switching element which is connected to the resonant circuit, wherein in an active state of the switching element the resonant circuit is configured to operate the induction coil by the buffered voltage to provide a heating power and/or to perform a cookware detection; a controller, which is connected to the switching element and which is configured to operate the switching element by providing a pulsed switching signal; and a separate discharge circuit comprising a switch and a switch controller, wherein the separate discharge circuit is connected to the at least one intermediate circuit capacitor and which is separate to the resonant circuit, wherein the switch controller is configured to operate the switch such that the at least one intermediate circuit capacitor is discharged at a predetermined time period before the switching element is operated.

The induction coil can be included to an induction zone for cooking and detecting cookware. In the context of this invention it is mentioned that the switching element can be operated in order to generate heating power at the induction coil, wherein this description is general and means that (induced) magnetic fields are generated by the coil when a corresponding voltage signal is applied, for example switched in intervals from the buffer voltage. The magnetic fields can then cause a heating power in the cookware.

The active state of the switching element represents a state where the switching element is electrically conducting for providing heat power/magnetic fields at the induction zone, and to operate the coil. The active state can also describe a switching period over which a pulsed signal of the buffered voltage is generated and over which the switching is done/operated. The induction coil can be operated by the buffered voltage when using a pulsed mode of the switching element to generate a switched signal of the buffered voltage.

The circuit arrangement can be also operated at a mains frequency of about 60 Hz, or lower, what can be detected by the controller and/or by the switch controller. By discharging the intermediate circuit capacitor an adjustable heating capacity in the circuit arrangement can be provided. The mentioned circuit arrangement can be used for quasi resonant topologies but it is not limited thereto and it is possible to apply it also to other circuit arrangements, in particular to other induction heating topologies with an intermediate circuit capacitor because the discharge circuit is separated from the resonant circuit.

The separate discharge circuit can be built such to comprise several switches or only one single switch, what can be provided much easier than a full bridge or a half bridge. The separate discharge circuit and its components can be independent of the dimensions and characteristic parameters of the induction coil, whereas known discharge methods and/or circuits, using the switching igbt of the resonant circuit, need components balanced with the induction coil, resulting in limited numbers of available components for higher peak currents, faster switching frequencies and higher dissipation (voltages, loads).

The rectifier, the intermediate circuit capacitor and the switching element of the resonant circuit can act as a frequency converter which can produce a control voltage for the induction coil, in particular a buffered voltage applied to the induction coil at active phases of the switching element. The intermediate circuit capacitor can equalize the rectified mains voltage to a predetermined level, depending on its capacitance, and provide it as the buffered voltage.

When the intermediate circuit capacitor is discharged to a threshold voltage, for example 0V or up to about 20V, the resonant circuit can be used for cookware detection without producing acoustic noise or with a significantly lowered noise from the cookware. By this improvement it is not required to limit the frequency of a cookware detection pulse, in particular a frequency of the signal for the switching of the igb transistor when detecting cookware, for reasons of lowering noise. A pulse at the igb transistor can be 1 |is, for example when detecting cookware. The cookware detection can be therefore performed also at high frequency without producing acoustic noise that can else be recognized by the user or at least to significantly lower this noise. The quiet and high frequent cookware detection can be done at a low voltage from the intermediate circuit capacitor of about 20 V, for example.

The circuit arrangement according to the invention can be used in a low power regime, for example in the range of about 100 W, what can result in such a pulsewidth signal at the switching element of the resonant circuit that there can be only some periods of the rectified mains signal at which the induction coil is operated. Between these active periods it is possible to have no switching by the switching element and no load on the induction coil or that cookware detection is performed.

Advantageously, the acoustic noise can therefore be mitigated or fully prevented and also a lifetime of electronic components can be enlarged and EMC emissions can be lowered.

By using the separate discharge circuit instead of the igbt switch (switching elements) of the resonant circuit) the occurrence of high peak currents when discharging the intermediate circuit capacitor can be prevented or lowered when compared to discharging the intermediate circuit capacitor with the igbt via the induction zone. By discharging via the igbt of the resonant circuit it is possible that such peak currents also occur at the resonant capacitor or circuit as well, wherein also these peak currents can be lowered or prevented by discharging over the separate discharge circuit (by the mains signal), as done by the invention. Separate can mean that the discharge circuit can be an own circuit, not belonging to the resonant circuit or to the igbt switch(es) and can be connected to the intermediate circuit capacitor at corresponding connections/terminals.

According to a further embodiment of the circuit arrangement for an induction cooker the switching element comprises at least one igb transistor and the switch comprises at least one mosfet.

The mosfets used can be much smaller and cheaper than igbts.

According to a further embodiment of the circuit arrangement for an induction cooker, the circuit arrangement comprises a first resonant circuit connected to a first intermediate circuit capacitor and a second resonant circuit connected to a second intermediate circuit capacitor, wherein both the first intermediate circuit capacitor and the second intermediate circuit capacitor are connected to and are dischargeable by the separate discharge circuit.

The use of only one discharge circuit for several intermediate circuit capacitors for several induction zones can save elements and costs and simplifies the circuit arrangement.

According to a further embodiment of the circuit arrangement for an induction cooker the at least one intermediate circuit capacitor has a capacitance between about 3 pF and 20 pF and/or wherein the separate discharge circuit is looped in between an input terminal and an output terminal of the rectifier.

The mentioned small values of the capacitors have the advantage that the capacitors and also other components of the discharge circuit can be robust and very small in their characteristic values when compared to known discharge switches. ■ o ^_

According to a further embodiment of the circuit arrangement for an induction cooker the switch controller is configured to monitor the mains voltage and to identify a positive period when the mains voltage is positive between two zero crossings of the mains voltage, and wherein the switch of the separate discharge circuit is operated to discharge the intermediate circuit capacitor during the positive period.

A discharge during the positive period is more stable for the remaining components than a negative period.

According to a further embodiment of the circuit arrangement for an induction cooker the switch controller is configured to identify whether the intermediate circuit capacitor is charged at least to a local maximum voltage and afterwards to operate the switch to connect the intermediate circuit capacitor for discharge to the rectified mains voltage at a moment when the rectified mains voltage has a maximum value.

The at least local maximum value of the charged capacitor can be connected to the mains and then the discharge can pull the whole energy from the capacitor which has been stored or which can be stored in the capacitor. The discharge can then follow the mains signal behaviour. The decrease in voltage in the circuit can therefore correspond to a removal of energy from the circuit by the mains (source) without or nearly without energy dissipation at the electronic elements of the circuit when discharging the intermediate circuit capacitor.

The separate discharge circuit can be a completely separated implementation from the resonant circuit and its switching element and therefore independent of the applied type of the switching element, for example an igbt, therefore the separate discharge circuit and its elements can be independent of variations in the manufacturer process and have the option to implement different types of transistor switches without altering parameters of the discharge circuit.

It is further possible to use this discharge circuit for cookware detection as well, since almost no power is dissipated due to discharging, therefore it is possible to increase frequency of the pot detection to have a responsive way of pot detection.

A total circuit dissipation (energy consumption) per discharge cycle can be held around 10 mJ independent whether there is cookware placed on the induction zone or not. In this sense the dissipation of energy is rather a removal of energy from the circuit when the voltage signal from mains and linked thereto also the voltage at the intermediate circuit capacitor drops. So the main part of the energy from the intermediate circuit capacitor is removed by the voltage source when the mains signal drops and only a very little part is consumed by the components of the circuit arrangement.

According to the invention an induction cooker comprises a circuit arrangement for an induction cooker as according to the invention.

According to the invention a method for operating an induction cooker comprises the step of providing an alternating mains voltage at a rectifier and rectifying the mains voltage by the rectifier; a step of buffering the rectified mains voltage by at least one intermediate circuit capacitor which is coupled between output terminals of the rectifier and providing a buffered voltage from the rectified mains voltage; a step of operating a resonant circuit of an induction coil of the induction cooker with the buffered voltage, wherein a switching element for the resonant circuit is operated by a pulsed switching signal and/or operating the resonant circuit to detect cookware; characterized in that, the at least one intermediate circuit capacitor is controlled by a separate discharge circuit which is connected to the at least one intermediate circuit capacitor and which is separate to the resonant circuit, wherein a switch of the separate discharge circuit is operated by a switch controller such that the at least one intermediate circuit capacitor is discharged at a predetermined time period before the switching element for the resonant circuit is switched to operate the induction coil to generate a heating power and/or before a cookware detection is performed.

The switch can be operated such that the intermediate circuit capacitor can be charged and/or discharged at predetermined times, representing the control of the capacitor. Therefore, the switch can be in a conducting or non-conducting mode.

According to a further embodiment of the method the switch controller monitors the mains voltage and identifies a positive period when the mains voltage is positive between two zero crossings of the mains voltage, and wherein the switch of the separate discharge circuit is operated to discharge the intermediate circuit capacitor during the positive period.

The coincidence can prove that the discharge is performed at the positive period and immediately before the switching or cookware detection at the resonant circuit is intended to be performed.

According to a further embodiment of the method the switch controller monitors the voltage at the intermediate circuit capacitor, in particular at the moment or after a zero crossing of the rectified mains signal appears. In this case it can be checked whether the remaining voltage at the intermediate circuit capacitor after discharge is below or equal to a predetermined threshold, for example the threshold is about or equal to 10 V or 20 V. By this check it can be proven that the discharge was successful and that subsequently a cookware detection or an operation of the induction zone (cooking) can be performed by operating the igb transistors) after the intermediate circuit capacitor has been discharged to said threshold or below it.

According to a further embodiment of the method the predetermined time period coincides at least partly with the positive period and ends at a zero crossing of the mains voltage.

Typically, the maximum mains value is reached at a quarter period of the cycle before a second zero crossing of a positive period. Further actions (cookware detection or providing heating power by the induction zone, for example) can be done immediately subsequent to the discharge to prevent a new charging of the intermediate capacitor before further actions are performed.

According to a further embodiment of the method the predetermined time period equals a quarter of a full cycle of the mains voltage and wherein the switching element for the resonant circuit is operated to generate a heating power at the induction coil and/or the cookware detection is performed immediately subsequent when the predetermined time period ends.

According to a further embodiment of the method during discharge of the intermediate circuit capacitor a dissipation of energy from the intermediate circuit capacitor at the resonant circuit is prevented or lowered and the mains voltage is used to remove the energy from the intermediate circuit capacitor. According to a further embodiment of the method the switch controller identifies whether the intermediate circuit capacitor is charged at least to a local maximum voltage and afterwards operates the switch to connect the intermediate circuit capacitor for a discharge to the rectified mains voltage at a moment when the rectified mains voltage has a maximum value.

According to a further embodiment of the method the switch controller identifies whether the intermediate circuit capacitor is charged to the maximum value of the rectified mains voltage and connects the intermediate circuit capacitor to the rectified mains voltage at the maximum value of the rectified mains voltage such that the intermediate circuit capacitor is discharged according to a subsequent temporal behaviour of the rectified mains voltage and disconnected from the rectified mains signal/voltage when being discharged to a predetermined voltage value.

The switch controller and/or the controller of the switching element can be operated by software and provide a triggering signal to open and close the switch and/or switching element. In other words, the start for discharging the intermediate capacitor is triggered by a software and the discharge itself and its end is done by the hardware configuration, wherein the discharge follows the mains voltage behaviour and the discharge can be stopped when the rectified mains voltage has a zero crossing. The triggering signal can be represented by (at least) one pulse, for example of 5 ms, and can have a duration at least until the zero crossing in the rectified mains signal appears. The software can thus switch on said pulse for the discharge and the hardware which produces the zero cross can stop the discharge, even if the trigger signal is still present when the zero crossing appears and after it. The zero crossing can therefore ensure a stop of the discharging and can overrule the triggering signal when operating the switch for the discharge. The controller can identify the zero crossing and operate the switch in order to stop the discharge.

The discharge is forced by the behaviour of the rectified mains signal, in case the intermediate capacitor is connected to the rectified mains signal at its (both) maximum, the rectified mains signal will decrease until reaching a zero crossing of the mains. Since the intermediate capacitor is connected to the rectified mains signal, at this time it will be forced that the voltage at the intermediate circuit capacitor behaves similar to the rectified mains voltage, for example as a decreasing sinus curve. In case the rectified mains voltage drops it drops very gently and no rapid peak behaviour is produced, the same is valid for the discharge voltage of the intermediate circuit capacitor. Therefore, the dropping rectified mains voltage will pull the energy from the intermediate circuit capacitor to the rectifier and back to mains without dissipating power anywhere in the circuit, except for little losses at resistivities of the conducting paths in the circuit or at other electronic elements. This represents a significant difference to commonly known methods and/or circuits where the discharge of the intermediate capacitors can be done by dissipating energy at the induction coil and/or at the igbt. The circuit arrangement according to the invention can prevent a dissipation of energy at the induction coil or at least lower it to almost zero. The forced discharge can gently follow the rectified mains signal and can prevent the generation of excessive stress at the components in the separate discharge circuit and/or in the circuit of the rectifier thus enabling the use of small electronic components. In this sense the wording "small" considers the characteristic values of the particular component, for example a low capacitance of a capacitor (used in addition to the intermediate circuit capacitor), low resistivities, switches which need to sustain only currents in the range up to 1 A, for example. Advantageously, the use of such components is cheap and by absent or lowered peak currents it is possible to increase the lifetime of the used compo- nents significantly. In the separated circuit it is easy to change the type of the elements of the circuit since they can be independent of high peak currents, independent of high switching frequencies and almost no dissipation happens at these components, what makes the separate discharge circuit topology flexible, robust and cheap when compared to known discharge concepts.

According to a further embodiment of the method the resonant circuit is operated by the pulsed switching signal only over predetermined cycles of the mains voltage.

The operation conducted morely over a predetermined period allows to operate the coil at a low power level.

The method can be also characterized by the features and advantages of the circuit arrangement for the induction cooker and vice versa. The same is valid for the induction cooker.

BRIEF DESORPTION OF THE DRAWINGS

The invention will be explained in greater detail with reference to exemplary embodiments depicted in the drawings as appended.

The accompanying drawings are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification. The drawings illustrate a comparative embodiment and embodiments of the present invention and together with the description serve to explain the principles of the invention. Other embodiments of the present invention and many of the intended advantages of the present invention will be readily appreciated as they become better understood by reference to the following detailed description.

The elements of the drawings are not necessarily to scale relative to each other.

Like reference numerals designate corresponding similar parts.

Fig. 1 shows a circuit arrangement in an induction cooker according to an embodiment of the invention.

Fig. 2 shows voltages and a trigger signal for triggering discharge of the intermediate circuit capacitor during a method for operating an induction cooker according to an embodiment of the invention.

Fig. 3a shows a discharge of an intermediate circuit capacitor and a corresponding energy dissipation in an induction cooker according to a comparative embodiment.

Fig. 3b shows a discharge of an intermediate circuit capacitor and a corresponding energy dissipation in an induction cooker according to an embodiment of the invention.

Fig. 4 shows a flowchart of method steps of a method for operating an induction cooker according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. Generally, this application is intended to cover any adaptations or variations of the specific embodiments discussed herein.

The circuit arrangement 1 in an induction cooker 10 comprises a rectifier RT which is connectable to an alternating mains voltage MS and which is configured to rectify the mains voltage MS. Regarding the embodiment shown in Fig. 1 two induction zones/coils L1 and L2 with a first resonant circuit LC1 and a second resonant circuit LC2 are illustrated, wherein the first resonant circuit LC1 is connected to a first intermediate circuit capacitor Cf 1 and the second resonant circuit LC2 is connected to a second intermediate circuit capacitor Cf2, both intermediate capacitors comprised in the circuit arrangement 1, wherein both the first intermediate circuit capacitor Cf 1 and the second intermediate circuit capacitor Cf2 are connected to and are dischargeable by a separate discharge circuit SDC. The intermediate circuit capacitors Cfl and Cf2 are coupled between output terminals of the rectifier RT, wherein the intermediate circuit capacitors Cfl and Cf2 are configured to buffer the rectified mains voltage for their particular induction zone/coil and to provide a buffered voltage from the rectified mains voltage.

The circuit arrangement 1 further comprises a first switching element T1 for the first resonant circuit LC1 and a second switching element T2 (or more of them) which are connected to the particular resonant circuits LC1 and LC2, wherein in an active state of the switching elements T1 and T2 the corresponding resonant circuits LC1 and LC2 are configured to generate a heating power/ magnetic fields at their induction coils L1 and/or L2 by the buffered voltage and/or to perform a cookware detection.

The circuit arrangement 1 further comprises a controller CT, which is connected to the switching elements T1 and T2 and which is configured to operate the switching elements T1 and T2 by providing a pulsed switching signal PWM.

The circuit arrangement 1 further comprises a separate discharge circuit SDC, preferably one which can also discharge several intermediate circuit capacitors Cf 1 and Cf2 and which comprises a switch SW and a switch controller SE, wherein the separate discharge circuit SDC is connected to the intermediate circuit capacitors Cf 1 and Cf2 and which is separate to the resonant circuits LC1 and LC2. The switch controller SE is configured to operate the switch SW such that the intermediate circuit capacitors Cf 1 and Cf2 are discharged at a predetermined time period each before the switching elements T1 and/or T2 is/are operated.

The separate discharge circuit SDC can be looped in between an input terminal and an output terminal of the rectifier RT. Since the separate discharge circuit SDC can be used for discharging the intermediate circuit capacitors Cf 1 and Cf2 of both resonant circuits it is sufficient to provide only one separate discharge circuit SDC what helps to lower costs for providing more than one separate discharge circuit.

The first and second switching elements T1 and T2 comprise each an igb transistor (insulated gate bipolar transistor) and the switch SW comprises in the embodiment of Fig. 1 a first mosfet M1, a second mosfet M2 and a third mosfet M3. The switch controller SE is configured to provide a trigger signal for operating the switching of the mosfets M1, M2 and M3. In the exemplary ranges of a capacitance of 4.7 pF of the intermediate circuit capacitors it can be estimated that a maximum current flowing through the mosfets to discharge the intermediate circuit capacitors can be only in the range of 1 A, for example for two capacitors of 4.7 pF which in coupling equal 10 pF, wherein 1 A is a very low value when compared to known concepts, for example a linear discharge by an igbt over the resonant circuit will need components which are able to withstand much higher currents and energy dissipations, what can increase the costs for providing electronic elements for higher currents.

Therefore, the separate discharge circuit SDC can be configured such to implement two (or three) cost effective SOT223 (smd 3.7mm x 4.6mm) package transis- tors/mosfets, for example. The mosfets M1, M2 and M3 can be relatively slow, because an operation of the discharge and of the generation of heating power can happen lower than the resonant frequency (30KHz) of the induction zone. Instead, the separate discharge circuit SDC has only to conduct during the mains frequency in a millisecond range, therefore a high switching speed is not regarded as a highly important parameter for the separate discharge circuit SDC. Further, the switching in the separate discharge circuit SDC can be performed different from hard- switching, in particular the mosfets can be activated for conduction when the intermediate circuit capacitors voltage equals mains voltage, therefore almost no (near zero) switching losses happen in the separate discharge circuit SDC and the discharging happens very gently, for example following the sinus curve of the decreasing mains voltage MS. The mains voltage MS can for example have a maximum value of 325 V.

The same advantages and behaviour can also be provided for a single resonant circuit and are not limited to the description of using two or more induction zones/circuits. Fig. 2 shows voltages and a trigger signal for triggering discharge of the intermediate circuit capacitor during a method for operating an induction cooker according to an embodiment of the invention.

As can be seen from Fig. 2 the rectified mains voltage MS-RT shown in the middle plot can follow a sin-signal composed of the rectified part of the negative mains signal MS-N and of the positive part of the mains signal MS-P. The rectified mains voltage MS-RT can oscillate between a maximum value of 325 V and a minimum value of 0 V or another predetermined threshold value, for example 10 V or 20 V in case cookware detection is desired. Zero crossings ZC occur in case the positive part of the mains signal MS-P and the negative mains signal MS-N equal each other.

The upper plot shows the buffered voltage VB of the intermediate circuit capacitor which can be discharged several times. Between the discharging periods a maximum value of about 325 V can remain as nearly constant as the buffered voltage of the rectified mains voltage MS-RT.

The lower plot shows the trigger signal ST for discharging the intermediate discharge capacitor as a pulse signal from the switch controller or from an external mcu.

A switch controller monitors the mains voltage parts MS-N and MS-P and identifies a positive period PP when the mains voltage is positive between a first zero crossings ZC1 and a second zero crossing ZC2 (middle plot) of the rectified mains voltage MS-RT and of the positive part of the mains signal MS-P. Then the switch of the separate discharge circuit is operated to discharge the intermediate circuit capacitor during the positive period PP. A predetermined time period TP can coincide partly with the positive period PP and ends at the second zero crossing ZC2.

The predetermined time period TP can equal a quarter D/4 of a full cycle D of the mains voltage (or of the rectified mains voltage MS-RT) and wherein the switching element in the resonant circuit is subsequently operated to generate a heating power at the induction coil and/or the cookware detection is performed immediately subsequent when the predetermined time period TP ends and starting at the second zero crossing ZC2 when the voltage VB is zero or at a threshold value.

When reaching the second zero crossing ZC2 the intermediate circuit capacitor can be disconnected from the rectified mains voltage again and the switch controller can be configured to detect the second zero crossing ZC2 and to drop the switching signal ST for the switch of the separate circuit to zero (or to disconnect status). The triggering of the closing or opening of the switch can be performed by a signal from an external microcontroller unit (mcu) or from the switch controller itself. The trigger signal ST can be a pulse having a duration at least until the discharge is intended to be stopped by the second zero crossing ZC2. Also a check whether the mains voltage is at a time period of positive values between two zero crossings can be performed by the external mcu and/or by the switch controller itself. The switch controller can be connected to an external mcu.

During discharge of the intermediate circuit capacitor a dissipation of energy from the intermediate circuit capacitor at/to the resonant circuit can be prevented or kept at a minimum range and the mains voltage can be used to remove the energy from the intermediate circuit capacitor. It can be seen that during the period TP the decreasing voltage VB of the intermediate circuit capacitor nearly or fully equals to the decreasing voltage of the positive part of the mains signal MS-P until at the second zero crossing ZC2 the intermediate circuit capacitor is disconnected from the rectified mains signal again. Thus a smooth and gently discharge behaviour can be forced by the mains signal and the energy from the intermediate capacitor can be removed from the circuit by the mains signal and high dissipation currents lowered or even prevented. The switch controller can identify whether the intermediate circuit capacitor is charged and in case being on 325 V, for this example, the trigger ST can operate the switch to connect the intermediate circuit capacitor for discharge to the rectified mains voltage at a moment when the rectified mains voltage has a maximum value, in particular a quarter D/4 of a full cycle D before the second zero crossing ZC2. It can further be seen in Fig. 2 that between 0 and 25 ms a longer period is shown during which no switching is performed and then the intermediate circuit capacitor has no discharging in between. Since the operation frequency for the induction coil can be low enough the intermediate capacitor can be fully charged between the next operation of the coil. Therefore, a discharge can be needed.

Fig. 3a shows a discharge of an intermediate circuit capacitor and a corresponding energy dissipation or consumption in an induction cooker according to a comparative embodiment.

The shown case corresponds to a linear mode of an igbt switch of a resonant circuit. The upper plot shows a rectified branch of the mains voltage MS-RT and the voltage at the intermediate capacitor Uc over a time period, wherein at the maximum voltage a discharge is triggered. The lower plot shows the dissipated power P in the circuit of the switching element (igbt in linear mode) during dissipa- tion/discharge, wherein a high peak current and a high peak of dissipated power P can be recognized immediately when the igbt switches. The intermediate circuit capacitor is discharged only to a threshold value in this case. The energy from the discharge can be consumed and in this sense dissipated at the induction zone/coil. Fig. 3b shows a discharge of an intermediate circuit capacitor and a corresponding energy dissipation or consumption in an induction cooker according to an embodiment of the invention.

The shown case corresponds to a discharge by a separate discharge circuit according to the invention. The upper plot shows a rectified branch of the mains voltage MS-RT and the voltage at the intermediate capacitor Uc over time t, wherein at the maximum voltage a discharge is triggered. The lower plot shows the dissipated or consumed power P (in the separate discharge circuit and/or in general) during dis- sipation/discharge, wherein the energy contained in the circuit varies with the varying mains signal over time (P = U * current). The energy from the discharge is returned to the mains (source) and therefore almost no consumption of energy in the circuit happens except of some small losses in the circuit. In average over one oscillating period it can be seen that the average power in the circuit is close to zero, also the local peak values at switching can be estimated to be of a factor 100 - 1000 lower than for the linear igbt mode from Fig. 3a. The occurring peak in dissipation as energy consumption in the circuit can happen when the discharge is triggered and results from local resistivities in the circuit and can be much lower as in the case of Fig. 3a. Consequently, the energy discharged (consumed) in the circuit and further to the coil can be of a factor 100 to 1000 lower than compared to a discharge with linear igbt regime (Fig. 3a). The simulation of Fig. 3b has been made with an intermediate capacitor of 4.7 pF, and over a duration of 10 ms for charge and discharge and 1 ohm of cookware resistance. Finally, such a dissipated energy in the circuit returns 6 mJ. The case of Fig. 3a with same sized components leads to a dissipated (consumed in the circuit) energy of 665 mJ, being a much bigger load for the components and the coil. Hence, compared to the case of Fig. 3a it is possible by using the separate discharge circuit to (better) implement low and continuous power of approximately 100 W what can lead to a good energy efficiency rate since the discharge does not count as energy waste and also a noise reduction from the cookware is possible.

The method for operating an induction cooker comprises the steps of providing S1 an alternating mains voltage at a rectifier and rectifying S2 the mains voltage by the rectifier; buffering S3 the rectified mains voltage by at least one intermediate circuit capacitor which is coupled between output terminals of the rectifier and providing a buffered voltage from the rectified mains voltage; operating S4 a resonant circuit of an induction coil of the induction cooker with the buffered voltage, wherein a switching element for the resonant circuit is operated by a pulsed switching signal and/or operating S4a the resonant circuit to detect cookware. The method is further characterized in that, the at least one intermediate circuit capacitor is controlled S5 by a separate discharge circuit which is connected to the at least one intermediate circuit capacitor and which is separate to the resonant circuit, wherein a switch of the separate discharge circuit is operated S5a by a switch controller such that the at least one intermediate circuit capacitor is discharged at a predetermined time period before the switching element in the resonant circuit is operated to generate S5b a heating power at the induction coil and/or before a cookware detection S4a is performed. Regarding the cookware detection various steps an approaches can be applied, also such which are already known to the skilled reader. The feature that the intermediate circuit capacitor is controlled by the separate discharge circuit means that the discharge can be performed. Further, controlling can in this sense also comprise the step of charging the intermediate capacitor(s).

In the foregoing detailed description, various features are grouped together in one or more examples or examples with the purpose of streamlining the disclosure. It is to be understood that the above description is intended to be illustrative, and not restrictive. It is intended to cover all alternatives, modifications and equivalents.

Claims

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CLAIMS Circuit arrangement (1) for an induction cooker (10) comprising: a rectifier (RT) which is connectable to an alternating mains voltage (MS) and which is configured to rectify the mains voltage (MS); at least one intermediate circuit capacitor (Cf) which is coupled between output terminals of the rectifier (RT), wherein the intermediate circuit capacitor (Cf) is configured to buffer the rectified mains voltage and to provide a buffered voltage from the rectified mains voltage; at least one resonant circuit (LC) comprising an induction coil (L) and a resonant circuit capacitor (Cr); a switching element (T1) which is connected to the resonant circuit (LC), wherein in an active state of the switching element (T1) the resonant circuit (LC) is configured to operate the induction coil (L) by the buffered voltage to provide a heating power and/or to perform a cookware detection; a controller (CT) which is connected to the switching element (T1) and which is configured to operate the switching element (T1) by providing a pulsed switching signal (PWM); a separate discharge circuit (SDC) comprising a switch (SW) and a switch controller (SE), wherein the separate discharge circuit (SDC) is connected to the at least one intermediate circuit capacitor (Cf) and which is separate to the resonant circuit (LC), wherein the switch controller (SE) is configured to operate the switch (SW) such that the at least one intermediate circuit capacitor (Cf) is discharged at a predetermined time period (TP) before the switching element (T1) is operated. Circuit arrangement (1) according to claim 1, wherein the switching element (T1) comprises at least one igb transistor and the switch (SW) comprises at least one mosfet. Circuit arrangement (1) according to claim 1 or 2, which comprises a first resonant circuit (LC1) connected to a first intermediate circuit capacitor (Cfl) and a second resonant circuit (LC2) connected to a second intermediate circuit capacitor (Cf2), wherein both the first intermediate circuit capacitor (Cfl) and the second intermediate circuit capacitor (Cf2) are connected to and are dischargeable by the separate discharge circuit (SDC). Circuit arrangement (1) according to any of the claims 1 to 3, wherein the at least one intermediate circuit capacitor (Cf) has a capacitance between 3 pF and 20 pF and/or wherein the separate discharge circuit (SDC) is looped in between an input terminal and an output terminal of the rectifier (RT). Circuit arrangement (1) according to any of the claims 1 to 4, wherein the switch controller (SE) is configured to monitor the mains voltage (MS) and to identify a positive period (PP) when the mains voltage (MS) is positive between two zero crossings (ZC) of the mains voltage (MS), and wherein the switch (SW) of the separate discharge circuit (SDC) is operated to discharge the intermediate circuit capacitor (Cf) during the positive period (PP). Circuit arrangement (1) according to any of the claims 1 to 5, wherein the switch controller (SE) is configured to identify whether the intermediate circuit capacitor (Cf) is charged at least to a local maximum voltage and afterwards to operate the switch (SW) to connect the intermediate circuit capacitor (Cf) for discharge to the rectified mains voltage at a moment when the rectified mains voltage has a maximum value. Induction cooker (10) comprising a circuit arrangement (1) according to any of the claims 1 to 6. Method for operating an induction cooker (10), comprising the following steps: providing (S1) an alternating mains voltage (MS) at a rectifier (RT) and rectifying (S2) the mains voltage (MS) by the rectifier (RT); buffering (S3) the rectified mains voltage by at least one intermediate circuit capacitor (Cf) which is coupled between output terminals of the rectifier (RT) and providing a buffered voltage from the rectified mains voltage; operating (S4) a resonant circuit (LC) of an induction coil (L) of the induction cooker (10) with the buffered voltage, wherein a switching element (T1) for the resonant circuit (LC) is operated by a pulsed switching signal (PWM) and/or operating (S4a) the resonant circuit (LC) to detect cookware; characterized in that the at least one intermediate circuit capacitor (Cf) is controlled (S5) by a separate discharge circuit (SDC) which is connected to the at least one intermediate circuit capacitor (Cf) and which is separate to the resonant circuit (LC), wherein a switch (SW) of the separate discharge circuit (SDC) is operated (S5a) by a switch controller (SE) such that the at least one intermediate circuit capacitor (Cf) is discharged at a predetermined time period (TP) before the switching element (T1) for the resonant circuit (LC) is switched to operate (S5b) the induction coil (L) for generating a heating power and/or before a cookware detection (S4a) is performed. - 28 -

9. Method according to claim 8, wherein the switch controller (SE) monitors the mains voltage (MS) and identifies a positive period (PP) when the mains voltage (MS) is positive between two zero crossings (ZC) of the mains voltage (MS), and wherein the switch (SW) of the separate discharge circuit (SDC) is operated to discharge the intermediate circuit capacitor (Cf) during the positive period (PP).

10. Method according to claim 9, wherein the predetermined time period (TP) coincides at least partly with the positive period (PP) and ends at a zero crossing (ZC) of the mains voltage (MS).

11. Method according to claim 10, wherein the predetermined time period (TP) equals a quarter (D/4) of a full cycle (D) of the mains voltage (MS) and wherein the switching element (T1) for the resonant circuit (LC) is operated to generate (S5b) a heating power at the induction coil (L) and/or the cookware detection is performed immediately subsequent when the predetermined time period (TP) ends.

12. Method according to any of the claims 8 to 11, wherein during discharge of the intermediate circuit capacitor (Cf) a dissipation of energy from the intermediate circuit capacitor (Cf) at the resonant circuit (LC) is prevented or lowered and the mains voltage (MS) is used to remove the energy from the intermediate circuit capacitor (Cf).

13. Method according to any of the claims 8 to 12, wherein the switch controller (SE) identifies whether the intermediate circuit capacitor (Cf) is charged at least to a local maximum voltage and afterwards operates the switch (SW) to connect the intermediate circuit capacitor (Cf) for a discharge to the - 29 - rectified mains voltage at a moment when the rectified mains voltage has a maximum value. Method according to claim 13, wherein the switch controller (SE) identifies whether the intermediate circuit capacitor (Cf) is charged to the maximum value of the rectified mains voltage and connects the intermediate circuit capacitor (Cf) to the rectified mains voltage at the maximum value of the rectified mains voltage such that the intermediate circuit capacitor (Cf) is discharged according to a subsequent temporal behaviour of the rectified mains voltage and disconnected from the rectified mains voltage when being discharged to a predetermined voltage value. Method according to any of the claims 8 to 14, wherein the resonant circuit (LC) is operated by the pulsed switching signal (PWM) only over predetermined cycles of the mains voltage (MS).