CN116889098A

CN116889098A - Circuit arrangement for an induction hob, induction hob and method for operating an induction hob

Info

Publication number: CN116889098A
Application number: CN202080106985.XA
Authority: CN
Inventors: M·霍夫斯密特
Original assignee: Guloni Co ltd; Hisense Guangdong Kitchen and Bath System Co Ltd; Intell Properties BV
Current assignee: Guloni Co ltd; Hisense Guangdong Kitchen and Bath System Co Ltd; Intell Properties BV
Priority date: 2020-11-06
Filing date: 2020-11-06
Publication date: 2023-10-13
Also published as: EP4241538A1; WO2022096122A1

Abstract

A circuit arrangement (1) for an induction hob (10) comprises: a Rectifier (RT) connectable to an alternating mains voltage (MS); at least one intermediate circuit capacitance (Cf), wherein the intermediate circuit capacitance (Cf) is configured to buffer the rectified mains voltage; at least one resonant circuit (LC) comprising an induction coil (L) and a resonant circuit capacitance (Cr); a switching element (T1) connected to the resonant circuit (LC); a Controller (CT) connected to the switching element (T1) and 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 capacitance (Cf) and separated from the resonant circuit (LC), wherein the switch controller (SE) is configured to operate the Switch (SW) such that the at least one intermediate circuit capacitance (Cf) is discharged for a predetermined period of Time (TP) before the switching element (T1) in the resonant circuit (LC) is operated.

Description

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

Technical Field

The application relates to a circuit arrangement for an induction hob, an induction hob and a method of operating such an induction hob.

Background

When operating the cooking zone of the induction hob, the resonant circuit may be coupled to a switch, for example a igb (insulated gate bipolar) transistor switch, which is operated by a pulse signal to generate heating power at the induction coil of the resonant circuit. The frequency converter may be used to generate a control voltage for operating the induction coil, wherein a low frequency voltage from the mains may be converted into a high frequency voltage for driving the induction coil.

An on/off cycle is typically implemented to achieve an average low power in the range of seconds.

In a common single-switch topology using igb transistors (igbt), the power regulation region may be limited. A typical power regulation range may be between 900W and 2100W, where the power may be regulated by adjusting the on-time of igbt, where the larger the on-time the more energy may be transferred to the resonant circuit.

In order to make the control of the low power control region more accurate, a control method may be applied such that the resonant circuit is activated only for a few cycles of the mains frequency, which is an improvement compared to switching the resonant circuit in the second range. As a side effect, it happens that each time the igbt switch for operating the sensing region is disabled for a long time, the intermediate circuit capacitance (dc side capacitance) for buffering will be fully charged to about the maximum value of the mains voltage. For the next time, when the igbt switch is activated, the energy stored in the intermediate capacitance has to be discharged, wherein in the known circuit arrangement the intermediate capacitance can only be discharged via the sensing region by appropriately switching the igbt switch. When such switching is performed, noise may occur depending on the cooker material.

Document EP 1935213 B1 describes a method for operating an induction heating device, in which a capacitor for damping a voltage can be discharged in its linear mode via an igbt switch of a resonant circuit.

Disclosure of Invention

It is an object of the application to improve the discharge of intermediate circuit capacitances for induction coils and to reduce peak currents in the circuit during the discharge.

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

A circuit arrangement for an induction hob, an induction hob and a method of operating such an induction hob may be provided, wherein intermediate circuit capacitances for buffering voltages and for operating an induction coil may be discharged with reduced or prevented energy dissipation from the intermediate circuit capacitances to the induction coil. The way in which the intermediate circuit capacitance discharges can be improved. Thus, the load on the induction coil and the remaining circuit components can be reduced during low power regimes, and the acoustic noise generated by the cookware at the induction coil at high peak currents can be reduced or prevented by reducing or preventing high peak currents at the induction coil. The term "induction zone" is used hereinafter, but the mentioned features, properties and corresponding advantages also apply to the cooking zone of an electromagnetic hob, to the electromagnetic cooktop or to the electromagnetic oven, and to all general cases, without specifying the area in which the current is induced.

The present application relates to a circuit arrangement for an induction hob according to claim 1, an induction hob according to claim 7 and a method for operating an induction hob according to claim 8.

Preferred embodiments are the subject matter of the dependent claims.

According to the application, a circuit arrangement for an induction hob comprises a rectifier connectable to an alternating mains voltage and configured to rectify the alternating mains voltage; at least one intermediate circuit capacitor coupled between the 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 capacitance; a switching element connected to the resonant circuit, wherein in an activated state of the switching element, the resonant circuit is configured to operate the induction coil by buffering the voltage to provide heating power and/or perform cookware detection; a controller connected to the switching element and configured to operate the switching element by providing a pulse switching signal; 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 capacitance and is separated from the resonant circuit, wherein the switch controller is configured to operate the switch such that the at least one intermediate circuit capacitance discharges for a predetermined period of time before operating the switching element.

An induction coil may be included to an induction zone for cooking and detecting cookware. In the context of the present application, reference is made to the fact that the switching element can be operated to generate heating power at the induction coil, wherein such description is generic and refers to the coil generating (induction) magnetic fields when the respective voltage signal is applied (e.g. switched from buffer voltage intervals). The magnetic field then generates heating power in the cooker.

The activated state of the switching element means a state in which the switching element is conductive to provide thermal power/magnetic field in the induction zone and operate the coil. The active state may also describe a switching cycle that generates a pulse signal of the buffer voltage and completes/operates switching. When the switching signal of the snubber voltage is generated using the pulse pattern of the switching element, the induction coil may be operated by the snubber voltage.

The circuit arrangement may also operate at mains frequencies of about 60Hz or less, which may be detected by the controller and/or the switch controller.

By discharging the intermediate circuit capacitance, an adjustable heating capacity can be provided in the circuit arrangement. The mentioned circuit arrangement can be used for quasi-resonant topologies, but is not limited thereto and can also be applied to other circuit arrangements, in particular other inductive heating topologies with intermediate circuit capacitances, since the discharge circuit is separated or separated from the resonant circuit.

The separate discharge circuit may be constructed to include several switches or to include only one single switch, which is easier to provide than a full bridge or a half bridge. The separate discharge circuit and its components may be independent of the size and characteristic parameters of the induction coil, whereas known discharge methods and/or circuits using switches igbt of the resonant circuit require components for equalizing the induction coil, resulting in a limited number of available components for higher peak currents, faster switching frequencies and higher dissipation (voltage, load).

The rectifier, the intermediate circuit capacitance and the switching element of the resonant circuit may act as a frequency converter which may generate a control voltage for the induction coil, in particular a snubber voltage applied to the induction coil during the activation phase of the switching element. The intermediate circuit capacitance may equalize the rectified mains voltage to a predetermined level according to its capacitance and provide it as a buffer voltage.

When the intermediate circuit capacitance discharges to a threshold voltage, e.g., 0V or up to about 20V, the resonant circuit can be used for cookware detection without generating acoustic noise or with a significant reduction in the noise of the cookware. With this improvement, it is not necessary to limit the frequency of the cooker detection pulse, particularly the frequency of the signal for switching the igb transistor when detecting the cooker for the reason of reducing noise. The pulse of the igb transistor may be 1 mus, for example when detecting cookware. Thus, the cooker detection can also be performed at high frequency without generating noise that would otherwise be identifiable by the user or at least significantly reducing the noise. For example, quiet and high frequency cookware detection can be accomplished at low voltages of approximately 20V intermediate circuit capacitance.

The circuit arrangement according to the application can be used in low power systems, for example in the range of about 100W, which will generate such a pulse width signal at the switching element of the resonant circuit that there may be only some periods of the rectified mains signal when the induction coil is operated. Between these active periods, switching may not be performed by the switching element and no load is placed on the induction coil, or cooker detection may be performed.

Advantageously, acoustic noise can thus be reduced or completely prevented, and also the lifetime of the electronic components can be prolonged and EMC emissions can be reduced.

By using a separate discharge circuit instead of the igbt switch (switching element of the resonant circuit), a high peak current can be prevented or reduced when the intermediate circuit capacitance is discharged compared to when the intermediate circuit capacitance is discharged with igbt via the sensing region. In addition, such peak currents may occur in the resonant capacitor or circuit by discharging through igbt of the resonant circuit, wherein these peak currents may also be reduced or prevented by discharging (via the mains signal) on a separate discharge circuit, as is done by the present application. By "separate" it is meant that the discharge circuit itself may be a circuit, not belonging to a resonant circuit or igbt switch, and may be connected to an intermediate circuit capacitance at a corresponding connection or joint/terminal.

According to a further embodiment of the circuit arrangement for an induction hob, 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 igbt.

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

The use of only one discharge circuit for several intermediate circuit capacitances for several sensing regions can save components and costs and simplify circuit layout.

According to a further embodiment of the circuit arrangement for an induction hob, the at least one intermediate circuit capacitance has a capacitance between approximately 3 and 20 μf, and/or wherein the separate discharge circuit forms a loop between the input and the output of the rectifier.

The mentioned small-valued capacitors have the advantage that the capacitors as well as the further other components of the discharge circuit can be robust and very small in terms of their characteristic values compared to known discharge switches.

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

The discharge during the positive period is more stable for other components than the negative period.

According to a further embodiment of the circuit arrangement for an induction hob, the switch controller is configured to identify whether the intermediate circuit capacitance is charged at least to a local maximum voltage and thereafter to operate the switch to connect the intermediate circuit capacitance for discharging to the rectified mains voltage when the rectified mains voltage has a maximum.

The charged capacitance with at least a local maximum may be connected to the mains and then the discharge may pull all the energy that has been or may be stored in the capacitor from the capacitor. The discharge may then follow the mains behavior. Thus, the reduction of the voltage in the circuit may correspond to the energy removed from the circuit by the mains (power supply) with little or no energy being dissipated on the electronic components of the circuit when the intermediate circuit capacitance is discharged.

The separate discharge circuit may be implemented completely separate from the resonant circuit and its switching elements and thus independent of the type of application of the switching elements, e.g. igbt, so that the separate discharge circuit and its elements may be independent of variations in the manufacturer's process and may choose to implement different types of transistor switches without changing the parameters of the discharge circuit.

Since there is little power consumption due to the discharge, the discharge circuit can be used for cooker detection in addition, so that the frequency of the pot detection can be increased to have a corresponding pot detection mode.

The total circuit dissipation (energy consumption) per discharge cycle can be kept at about 10mJ, regardless of whether cookware is placed on the sensing area. In this sense, the energy dissipation, more precisely the removal of energy from the circuit, is when the voltage signal from the mains and the voltage associated therewith and at the intermediate circuit capacitance drops. Thus, when the mains signal drops, a major part of the energy from the intermediate circuit capacitance is removed by the voltage source and only a very small part is consumed by the components of the circuit arrangement.

According to the application, an induction hob comprises a circuit arrangement according to the application for an induction hob.

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

The switch may be operated such that the intermediate circuit capacitance may be charged and/or discharged at predetermined times, which represents a control of the capacitance. Thus, the switch may be in a conducting or non-conducting mode.

According to another 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 switches of the separate discharge circuit are operated to discharge the intermediate circuit capacitance during the positive period.

The coincidence may prove that the discharge is performed in a positive cycle and immediately before the switching at the resonant circuit or the cooker detection is intended to be performed.

According to a further embodiment of the method, the switch controller monitors the voltage at the intermediate circuit capacitance, in particular at or after the moment at which the zero crossing of the rectified mains signal occurs. In this case it can be checked whether the residual voltage at the intermediate circuit capacitance after discharge is lower than or equal to a predetermined threshold value, for example a threshold value of about 10V or 20V. By this check it can be demonstrated that the discharge is successful and subsequently, after the intermediate circuit capacitance has been discharged to or below the threshold value, the operation of the cookware detection or sensing area (cooking) can be performed by operating igb transistor.

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

Typically, a quarter period before the second zero crossing of the positive period reaches the maximum mains value. Additional actions (e.g., cooker detection or providing heating power through the sensing region) may be performed immediately after the discharge to prevent new charging of the intermediate capacitance before further actions are performed.

According to another embodiment of the method, the predetermined period of time is equal to one quarter of a complete period of the mains voltage, and wherein the switching element of the resonant circuit is operated to generate heating power at the induction coil and/or cooker detection is performed immediately thereafter when the predetermined period of time has ended.

According to another embodiment of the method, energy dissipation from the intermediate circuit capacitance at the resonant circuit is prevented or reduced during discharge of the intermediate circuit capacitance, and the mains voltage is used to remove energy from the intermediate circuit capacitance.

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 subsequently operates the switch at a time when the rectified mains voltage has a maximum value to connect the intermediate circuit capacitor for discharging to the rectified mains voltage.

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

The switch controller and/or the controller of the switching element may be operated by software and provide a trigger signal to turn the switch and/or the switching element on and off. In other words, the start of the discharge of the intermediate capacitor is triggered by the software, the discharge itself and the end of the discharge are done by a hardware configuration, wherein the discharge follows the mains voltage behaviour, and the discharge can be stopped when the rectified mains voltage has zero crossings. The trigger signal may be represented by (at least) one pulse, e.g. 5ms, and the duration may last at least until a zero crossing occurs in the rectified mains signal. The software can thus turn on the pulse for the discharge and the hardware generating the zero crossing can stop the discharge even if the trigger signal is present at and after the zero crossing. The zero crossing can thus ensure a stop of the discharge and can override the trigger signal when operating the switch for the discharge. The controller may identify the zero crossing point and operate the switch to stop the discharge.

The behaviour of the rectified mains is forced to discharge and if the intermediate capacitance is connected to the rectified mains at its (two) maximum, the rectified mains will decrease until the zero crossing of the mains is reached. Because the intermediate capacitance is connected to the rectified mains, the voltage at the intermediate circuit capacitance will now be forced to behave like a rectified mains voltage, for example in a decreasing sinusoidal curve. If the rectified mains voltage drops very gently and does not produce a fast peak behavior, the same applies to the discharge voltage of the intermediate circuit capacitor. Thus, the falling rectified mains voltage will draw energy from the intermediate circuit capacitance to the rectifier and back to the mains without dissipating power anywhere in the circuit, except for a small loss in resistivity of the conductive paths or other electronic components in the circuit. This represents a significant difference from commonly known methods and/or circuits in which the discharge of the intermediate capacitor can be accomplished by dissipating energy at the induction coil and/or igbt. The circuit arrangement according to the application can prevent or at least reduce the energy dissipation at the induction coil to almost zero. The forced discharge may gently follow the rectified mains signal and may prevent excessive stress from being generated at components in the separate discharge circuit and/or the circuit of the rectifier, enabling the use of small electronic components. In this sense the term "small" takes into account the characteristic values of specific components, such as low capacitance, low resistivity of capacitors (capacitors used in addition to intermediate circuit capacitance), switches that only need to maintain currents up to, for example, the 1A range. Advantageously, such components are less expensive to use and the absence or reduction of peak currents can significantly extend the life of the components used. In a separate circuit, the types of circuit elements are easy to change, since they can be independent of high peak currents, independent of high switching frequencies, and these components are hardly dissipated, which makes the separate discharge circuit topology more flexible, more robust and cheaper than the known discharge concepts.

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

Performing more operations after a predetermined period allows the coil to be operated at a low power level.

The method is also characterized by the features and advantages of the circuit arrangement of the induction cooker and vice versa. The same applies to induction cookers.

Drawings

The application will be explained in more detail with reference to exemplary embodiments depicted in the accompanying drawings.

The accompanying drawings are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification. The drawings illustrate comparative embodiments and examples of the application and together with the description serve to explain the principles of the application. Other embodiments of the application and many of the intended advantages of the application 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 application.

Fig. 2 shows voltages and trigger signals for triggering the discharge of the capacitance of the intermediate circuit during a method for operating an induction hob according to embodiments of the present application.

Fig. 3a shows the discharge of the intermediate circuit capacitance and the corresponding energy dissipation in the induction hob according to the comparative example.

Fig. 3b shows the discharge of the intermediate circuit capacitance and the corresponding energy dissipation in the induction hob according to an embodiment of the present application.

Fig. 4 shows a flow chart of method steps of a method for operating an induction cooker according to an embodiment of the application.

Detailed Description

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a wide 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 application. In general, this disclosure is intended to cover any adaptations or variations of the specific embodiments discussed herein.

The circuit arrangement 1 in the induction hob 10 comprises a rectifier RT connectable to an alternating mains voltage MS and configured to rectify the mains voltage MS. With respect to the embodiment shown in fig. 1, two induction zones/coils L1 and L2 are shown with a first resonant circuit LC1 and a second resonant circuit LC2, wherein the first resonant circuit LC1 is connected to a first intermediate circuit capacitance Cf1 and the second resonant circuit LC2 is connected to a second intermediate circuit capacitance Cf2, both intermediate circuit capacitances being comprised in the circuit arrangement 1, wherein both the first intermediate circuit capacitance Cf1 and the second intermediate circuit capacitance Cf2 are connected to and can be discharged by a separate discharge circuit SDC. Intermediate circuit capacitances Cf1 and Cf2 are coupled between the outputs of the rectifier RT, wherein the intermediate circuit capacitances Cf1 and Cf2 are configured to buffer the rectified mains voltage for their particular sensing region/coil and to provide a buffered voltage from the rectified mains voltage.

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

The circuit arrangement 1 further comprises a controller CT connected to the switching elements T1 and T2 and 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, which preferably also discharges several intermediate circuit capacitances Cf1 and Cf2 and comprises a switch SW and a switch controller SE, wherein the separate discharge circuit SDC is connected to the intermediate circuit capacitances Cf1 and Cf2 and is separate from the resonant circuits LC1 and LC 2. The switch controller SE is configured to operate the switch SW such that the intermediate circuit capacitances Cf1 and Cf2 are discharged for a predetermined period of time before the switching elements T1 and/or T2 are operated, respectively.

A separate discharge circuit SDC may form a loop between the input and the output of the rectifier RT. Since a separate discharge circuit SDC can be used to discharge the intermediate circuit capacitances Cf1 and Cf2 of the two resonant circuits, it is sufficient to provide only one separate discharge circuit SDC, which contributes to cost reduction compared to providing more than one separate discharge circuit.

The first switching element T1 and the second switching element T2 each include an ibb transistor (insulated gate bipolar transistor), and the switch SW includes a first mosfet M1, a second mosfet M2, and a third mosfet M3 in the embodiment of fig. 1. The switch controller SE is configured to provide a trigger signal for operating switching of the mosfets M1, M2 and M3.

Within the exemplary range of 4.7 μf capacitance of the intermediate circuit capacitance, it can be estimated that the maximum current flowing through the mosfet to discharge the intermediate circuit capacitance can only be within 1A, e.g. for two 4.7 μf capacitors, which after coupling is equal to 10 μf, where 1A is a very small value compared to the known concept, e.g. a linear discharge by igbt on a resonant circuit would require components capable of withstanding higher currents and energy dissipation, which may increase the cost of providing electronic components for higher currents.

Thus, the separate discharge circuits SDC may be configured such that two (or three) cost-effective, e.g. SOT223 (smd 3.7mm x 4.6 mm), packaged transistors/mosfets are realized. mosfets M1, M2 and M3 may be relatively slow because the operation of discharging and generating heating power may occur below the resonant frequency (30 KHz) of the sensing region. In contrast, the individual discharge circuits SDC only need to be turned on during mains frequencies in the millisecond range, so a high switching speed is not considered as a very important parameter for the individual discharge circuits SDC. Furthermore, the switching in the separate discharge circuit SDC may be performed differently from hard switching, in particular when the intermediate circuit capacitance voltage is equal to the mains voltage, the mosfets may be activated to conduct, so that almost no (near zero) switching losses occur in the separate discharge circuit SDC and the discharge occurs very gently, for example following a sinusoidal curve of the falling mains voltage MS. For example, the maximum value of the mains voltage MS may be 325V.

The same advantages and behavior can also be provided for a single resonant circuit and is not limited to descriptions using two or more sensing regions/circuits.

As can be seen from fig. 2, the rectified mains voltage MS-RT shown in the middle diagram may follow a sinusoidal signal consisting of a rectified part of the negative mains signal MS-N and a rectified part of the positive part MS-P of the mains signal. The rectified mains voltage MS-RT may oscillate between a maximum of 325V and a minimum of 0V or another predetermined threshold (e.g. 10V or 20V) in case cooker detection is desired. In case the positive part MS-P of the mains signal and the negative mains signal MS-N are equal to each other, zero crossings ZC occur.

The upper graph shows the buffer voltage VB of the intermediate circuit capacitor that can be discharged a number of times. Between discharge cycles, a maximum value of about 325V may remain almost as constant as the buffer voltage of the rectified mains voltage MS-RT.

The following diagram shows a trigger signal ST for discharging the intermediate discharge capacitance as a pulse signal from the switch controller or the external mcu.

The switch controller monitors the mains voltage parts MS-N and MS-P and identifies a positive period PP when the mains voltage between the rectified mains voltage MS-RT and the first zero crossing ZC1 and the second zero crossing ZC2 (middle graph) of the positive part MS-P of the mains signal is positive. The switches of the separate discharge circuits are then operated to discharge the intermediate circuit discharge capacitance during the positive period PP. The predetermined period TP may partially coincide with the positive period PP and end at the second zero crossing ZC 2.

The predetermined period TP may be equal to one quarter D/4 of the full period D of the mains voltage (or the rectified mains voltage MS-RT) and wherein the switching element in the resonant circuit is then operated to generate heating power at the induction coil and/or to perform a cookware detection immediately after the predetermined period TP has ended and starting from the second zero crossing ZC2 when the voltage VB is zero or at a threshold value.

When the second zero-crossing ZC2 is reached, the intermediate circuit capacitance may be disconnected again from the rectified mains voltage, and the switch controller may be configured to detect the second zero-crossing ZC2 and to drop the switch signal ST for the switches of the individual circuits to zero (or an open state). The closing or opening of the trigger switch may be performed by a signal from an external micro controller unit (mcu) or the switch controller itself. The trigger signal ST may be a pulse having a duration at least until the discharge is to be stopped by the second zero-crossing ZC 2. Checking whether the mains voltage is in a positive period between two zero crossings can also be performed by the external mcu and/or the switch controller itself. The switch controller may be connected to the external mcu.

During discharge of the intermediate circuit capacitance, energy dissipation of the intermediate circuit capacitance at/to the resonant circuit may be prevented or kept within a minimum range, and the mains voltage may be used to remove energy from the intermediate circuit capacitance. It can be seen that during the period TP the falling voltage VB of the intermediate circuit capacitance is almost or completely equal to the falling voltage of the positive part MS-P of the mains signal until at the second zero crossing ZC2 the intermediate circuit capacitance is disconnected again from the rectified mains signal. Thus, the mains signal may force a smooth and gentle discharge behavior, and energy from the intermediate capacitance may be removed from the circuit by the mains signal, and reduce or even prevent high dissipation currents. The switch controller may recognize whether the intermediate circuit capacitance is charged or not, if 325V, for this example, the trigger ST may operate the switch to connect the intermediate circuit capacitance for discharging to the rectified mains voltage at a time when the rectified mains voltage has a maximum, in particular a quarter D/4 of the complete period D before the second zero crossing ZC 2. It can also be seen in fig. 2 that between 0 and 25 milliseconds a longer period of time is shown during which no switching is performed, and then the intermediate circuit capacitance is not discharged in between. Since the operating frequency of the induction coil can be low enough, the intermediate capacitance can be fully charged between the next operations of the coil. Thus, discharge may be required.

Fig. 3a shows the discharge of the intermediate circuit capacitance and the corresponding energy dissipation or consumption in the induction hob according to the comparative embodiment.

The situation shown corresponds to the linear mode of the ibt switch of the resonant circuit. The upper graph shows the voltage at the rectifying branch and the intermediate capacitance Uc of the mains voltage MS-RT over a period of time, wherein the discharge is triggered at the maximum voltage. The following graph shows the dissipated power P in the switching element (ibt in linear mode) circuit during dissipation/discharge, wherein high peaks and high peak currents of the dissipated power P can be immediately identified when ibt is switched. The intermediate circuit capacitance is in this case only discharged to the threshold value. Energy from the discharge may be dissipated and in this sense dissipated at the induction zone/coil.

Fig. 3b shows the discharging of the intermediate circuit capacitance and the corresponding energy dissipation or consumption in the induction hob according to an embodiment of the present application.

The situation shown corresponds to a discharge according to the application through a separate discharge circuit. The upper graph shows the voltage of the rectifying branch and the intermediate capacitor Uc of the mains voltage MS-RT over time t, wherein the discharge is triggered at the maximum voltage. The following graph shows the power P dissipated or consumed during dissipation/discharge (in a separate discharge circuit and/or in general), wherein the energy contained in the circuit varies with the change of the mains signal over time (p=u current). The energy generated by the discharge is returned to the mains (power supply) and therefore there is little energy consumption in the circuit, except for some small losses in the circuit. In the average value of one oscillation period it can be seen that the average power in the circuit is close to zero, and in addition the local peak at switching can be estimated to be 100-1000 times smaller than in the linear igbt mode in fig. 3 a. When a discharge is triggered, dissipation peaks in the circuit that occur as energy consumption may occur, which are caused by the local resistivity in the circuit, and can be much lower than in the case of fig. 3 a. Thus, the energy discharged (consumed) in the circuit and further to the coil can be 1/1000 to 1/100 of that compared to the discharge of the linear igbt system (fig. 3 a). The simulation of fig. 3b has been performed using an intermediate capacitance of 4.7 muf, a charge and discharge duration of 10 milliseconds and a cooker resistance of 1 ohm. Finally, this dissipated energy in the circuit returns to 6mJ. The case of the same size component in fig. 3a results in a dissipated (consumed in the circuit) energy of 665mJ, which is a much larger load for the component and coil.

Thus, a lower and continuous power of about 100W can be (better) achieved by using a separate discharge circuit compared to the case of fig. 3a, which can lead to good energy efficiency, since the discharge does not account for energy waste and can also reduce the noise of the cookware.

The method of operating an induction cooker comprises the steps of: providing an S1 alternating current mains voltage at a rectifier, and rectifying the mains voltage by the rectifier S2; buffering S3 the rectified mains voltage and providing a buffered voltage from the rectified mains voltage by means of at least one intermediate circuit capacitor coupled between the output terminals of the rectifier; the resonant circuit of the induction coil of the S4 induction cooker is operated with a snubber voltage, wherein a switching element for the resonant circuit is operated by a pulse switching signal, and/or the S4a resonant circuit is operated to detect the cooker. The method is further characterized in that the at least one intermediate circuit capacitance is controlled S5 by a separate discharge circuit connected to the at least one intermediate circuit capacitance and separated from the resonance circuit, wherein the switching of the separate discharge circuit is operated S5a by the switch controller such that the at least one intermediate circuit capacitance is discharged a predetermined period of time before the switching element in the resonance circuit is operated to generate S5b heating power at the induction coil and/or before the cooker detection S4a is performed. With respect to cooker detection, various steps and methods may be applied, as is also known to those skilled in the art. The feature that the intermediate circuit capacitance is controlled by a separate discharge circuit means that a discharge can be performed. In this sense, the control may also include the step of charging the intermediate capacitance.

In the foregoing detailed description, various features are grouped together in one or more examples for 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

1. A circuit arrangement (1) for an induction hob (10) comprises:

-a Rectifier (RT) connectable to an alternating mains voltage (MS) and configured to rectify the alternating mains voltage (MS);

at least one intermediate circuit capacitance (Cf) coupled between the output terminals of the Rectifier (RT), wherein the at least one intermediate circuit capacitance (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 capacitance (Cr);

a switching element (T1) connected to the resonant circuit (LC), wherein in an activated state of the switching element (T1), the resonant circuit (LC) is configured to operate the induction coil (L) by buffering the voltage to provide heating power and/or to perform cookware detection;

a Controller (CT) connected to the switching element (T1) and configured to operate the switching element (T1) by providing a pulse 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 capacitance (Cf) and is separated from the resonance circuit (LC), wherein the switch controller (SE) is configured to operate the Switch (SW) such that the at least one intermediate circuit capacitance (Cf) is discharged for a predetermined period of Time (TP) before the switching element (T1) is operated.

2. 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.

3. Circuit arrangement (1) according to claim 1 or 2, wherein the circuit arrangement (1) comprises a first resonant circuit (LC 1) connected to a first intermediate circuit capacitance (Cf 1) and a second resonant circuit (LC 2) connected to a second intermediate circuit capacitance (Cf 2), wherein both the first intermediate circuit capacitance (Cf 1) and the second intermediate circuit capacitance (Cf 2) are connected to and can be discharged by a Separate Discharge Circuit (SDC).

4. A circuit arrangement (1) according to any one of claims 1 to 3, wherein the at least one intermediate circuit capacitance (Cf) has a capacitance between 3 μf and 20 μf, and/or wherein a Separate Discharge Circuit (SDC) forms a loop between an input and an output of the Rectifier (RT).

5. Circuit arrangement (1) according to any one of claims 1 to 4, wherein the switch controller (SE) is configured to monitor the alternating mains voltage (MS) and to identify as a Positive Period (PP) when the alternating mains voltage (MS) is positive between two Zero Crossings (ZC) of the alternating mains voltage (MS), and wherein the Switches (SW) of the Separate Discharge Circuits (SDC) are operated to discharge the intermediate circuit capacitance (Cf) during the Positive Period (PP).

6. Circuit arrangement (1) according to any one of claims 1 to 5, wherein the switch controller (SE) is configured to identify whether an intermediate circuit capacitance (Cf) is charged at least to a local maximum voltage and thereafter to operate the Switch (SW) at a time instant when the rectified mains voltage has a maximum to connect the intermediate circuit capacitance (Cf) for discharging to the rectified mains voltage.

7. An induction hob (10) comprising a circuit arrangement (1) according to any one of the claims 1 to 6.

8. A method of operating an induction cooker (10), comprising the steps of:

providing (S1) an ac mains voltage (MS) at a Rectifier (RT), and rectifying (S2) the ac mains voltage (MS) by means of the Rectifier (RT);

buffering (S3) the rectified mains voltage by means of at least one intermediate circuit capacitor (Cf) coupled between the 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 an induction hob (10) with a snubber voltage, wherein a switching element (T1) for the resonant circuit (LC) is operated by a pulse switching signal (PWM), and/or operating (S4 a) the resonant circuit (LC) to detect a cookware;

it is characterized in that the method comprises the steps of,

the at least one intermediate circuit capacitance (Cf) is controlled (S5) by a Separate Discharge Circuit (SDC) connected to the at least one intermediate circuit capacitance (Cf) and separated from the resonance circuit (LC), wherein the Switches (SW) of the Separate Discharge Circuit (SDC) are operated (S5 a) by a switch controller (SE) such that the at least one intermediate circuit capacitance (Cf) is discharged for a predetermined period of Time (TP) before the switching element (T1) for the resonance circuit (LC) is switched (S5 b) to operate the induction coil (L) to generate heating power and/or before the cooker detection (S4 a) is performed.

9. The method according to claim 8, wherein the switch controller (SE) monitors the alternating mains voltage (MS) and recognizes as a Positive Period (PP) when the alternating mains voltage (MS) is positive between two Zero Crossings (ZC) of the alternating mains voltage (MS), and wherein the Switches (SW) of the Separate Discharge Circuit (SDC) are operated during the Positive Period (PP) to discharge the intermediate circuit capacitance (Cf).

10. Method according to claim 9, wherein the predetermined period of Time (TP) coincides at least partially with a timing period (PP) and ends at a Zero Crossing (ZC) of the alternating mains voltage (MS).

11. Method according to claim 10, wherein the predetermined period of Time (TP) is equal to one quarter (D/4) of a complete period (D) of the alternating mains voltage (MS), and wherein the switching element (T1) for the resonant circuit (LC) is operated to generate (S5 b) heating power at the induction coil (L) and/or to perform cookware detection immediately afterwards when the predetermined period of Time (TP) has ended.

12. The method according to any one of claims 8 to 11, wherein during discharge of the intermediate circuit capacitance (Cf), energy dissipation from the intermediate circuit capacitance (Cf) at the resonant circuit (LC) is prevented or reduced, and the ac mains voltage (MS) is used to remove energy from the intermediate circuit capacitance (Cf).

13. The method according to any one of claims 8 to 12, wherein the switch controller (SE) recognizes whether the intermediate circuit capacitance (Cf) is charged at least to a local maximum voltage and thereafter operates the Switch (SW) at a time instant when the rectified mains voltage has a maximum to connect the intermediate circuit capacitance (Cf) for discharging to the rectified mains voltage.

14. The method according to claim 13, wherein the switch controller (SE) identifies whether the intermediate circuit capacitance (Cf) is charged to a maximum value of the rectified mains voltage and connects the intermediate circuit capacitance (Cf) to the rectified mains voltage at the maximum value of the rectified mains voltage such that the intermediate circuit capacitance (Cf) is discharged according to a subsequent time behaviour of the rectified mains voltage and is disconnected from the rectified mains voltage when discharged to a predetermined voltage value.

15. Method according to any of claims 8 to 14, wherein the resonant circuit (LC) is operated by the pulsed switching signal (PWM) only after a predetermined period of the alternating mains voltage (MS).