Method and system for controlling a qr inverter in an induction cooking appliance

Abstract

A method for controlling a quasi-resonant inverter (13) in an induction cooking appliance (1) provided with an induction heating coil (4) is disclosed. The quasi-resonant inverter (13) comprises a switching device (21) electrically connected to the induction heating coil (4) through a node (20) having a first voltage (VC(t)) indicative of a voltage across the power switching device (21). The method comprises the steps of: providing an enabling signal (K1) comprising a plurality of pulses to the switching device (21) in order to turn on and off said switching device (21) within a turn-on period (tON) and a turn-off period (tOFF); determining a second voltage (VCmin) indicative of a minimum value of the first voltage (VC(t)) during the turn-off period (tOFF); adjusting the turn-off period (tOFF) based on the second voltage (VCmin); and adjusting the enabling signal (K1) based on the adjusted turn-off period (tOFF).

Description

BACKGROUND TECHNICAL FIELD

[0002] The present invention relates generally to the field of induction cooking appliances. More specifically, the present invention relates to controlling a quasi-resonant inverter (hereinafter QR inverter) in an induction cooking appliance. BACKGROUND

[0004] Induction cooking appliances for preparing food are well-known household devices that are more convenient and efficient than traditional gas or electric cooking appliances, since on the one hand they are accurate in controlling the cooking temperature and on the other hand they allow a uniform cooking of the food.

[0005] An induction cooking appliance typically comprises a cooking surface provided with one or more cooking zones designed to heat a load (i.e. a cookware containing food) placed on the cooking surface, and a plurality of induction heating coils associated with the cooking zones and generating a time-varying magnetic field that induces eddy currents in the load. The internal electrical resistance of the load causes the induced eddy currents to generate heat in the load itself.

[0006] Generally, the induction coils are selectively operable to be fed by an alternating current (hereinafter indicated with AC current) provided by an electronic control system. The electronic control system typically comprises an inverter circuit that provides the AC current to the corresponding induction coil, and an electronic control unit configured to control the inverter circuit to vary the frequency of the AC current flowing in the induction coil in order to regulate the power delivered from the inverter circuit to the load (i.e. the cookware containing food) based on a target cooking temperature.

[0007] A certain kind of inverter circuit has a quasi-resonant topology / architecture, i.e. a QR inverter circuit, in which the switching section comprises a single power switching device, such as an IGBT switch (abbreviation of Insulated Gate Bipolar Transistor), which receives a pulsed enable signal from a driver unit, which in turn is controlled by the electronic control unit.

[0008] Induction cooking appliances provided with a QR inverter circuit are particularly affected by the losses of the IGBT switch, which can reduce the lifetime of the inverter.

[0009] US 20180176998 discloses a method for evaluating a necessary turn-on duration for zero voltage switching of a quasi-resonant inverter in an induction cooktop. The method comprises: providing pulses to switch a switching element associated with the quasi-resonant inverter; determining a peak voltage, i.e. a maximum voltage, across the switching element during a turn-off duration; determining whether the peak voltage across the switching element is greater than a threshold value within the turn-off duration; and finally determining that the turn-on duration is sufficient for zero voltage switching of the switching element when the peak voltage across the switching element is greater than the threshold value. SUMMARY

[0010] The Applicant has found that it would be advantageous to provide an induction cooking appliance with a control unit configured to control the switching devices of a QR inverter circuit in order to reduce the switching losses of the switching devices, in order to prolong the lifetime of the switching devices and thus of the inverter circuit.

[0011] It is an object of the present invention to provide a method and a system for controlling the switching devices of a QR inverter circuit of an induction cooking appliance, as defined in the claims.

[0012] According to one aspect, the present invention relates to a method for controlling a quasi-resonant inverter in an induction cooking appliance, the induction cooking appliance being provided with at least one induction heating coil, the quasi-resonant inverter comprising a switching device electrically connected to the induction heating coil by a node having a first voltage indicative of a voltage across the power switching device, the method comprising the steps of: providing an enable signal comprising a plurality of pulses to the switching device in order to turn on and turn off the switching device within a turn-on period and a turn-off period; determining a second voltage indicative of a minimum value of the first voltage during the turn-off period; adjusting the turn-off period based on the second voltage; and adjusting the enable signal based on the adjusted turn-off period.

[0013] Preferably, the method further comprises the steps of: determining a third voltage indicative of a value of the first voltage at about the end of the turn-off period; and increasing the turn-off period if the third voltage is greater than the second voltage and the first voltage is decreasing.

[0014] Preferably, the method further comprises the steps of: decreasing the turn-off period if the third voltage is greater than the second voltage and the first voltage is increasing.

[0015] Preferably, the method further comprises the steps of: increasing the turn-on period if the second voltage is greater than a first voltage threshold; and adjusting the turn-on period of the enable signal based on the increased turn-on period.

[0016] Preferably, the method further comprises the steps of: determining a fourth voltage indicative of a maximum value of the first voltage during the off period; and stopping the operation of the quasi-resonant inverter if the fourth voltage is greater than a second voltage threshold.

[0017] The present application further relates to an electronic control system for controlling at least one heating coil of an induction cooking appliance, the electronic control system comprising: a quasi-resonant inverter provided with: a switching device electrically connected to the induction heating coil by a node having a first voltage indicative of a voltage across the switching device; and a driver unit configured to provide an enable signal comprising a plurality of pulses to the switching device in order to turn on and off the switching device within an on period and an off period, the electronic control system comprising a control device configured to: control the driver unit in order to adjust the enable signal; determine a second voltage indicative of a minimum value of the first voltage during the off period; adjust the off period based on the second voltage; and adjust the enable signal based on the adjusted off period.

[0018] Preferably, the control device is further configured to: determine a third voltage indicative of a value of the first voltage at about the end of the off period; and increase the off period if the third voltage is greater than the second voltage and the first voltage is decreasing.

[0019] Preferably, the control device is further configured to: decrease the off period if the third voltage is greater than the second voltage and the first voltage is increasing.

[0020] Preferably, the control device is further configured to: increase the on period if the second voltage is greater than a first voltage threshold; and adjust the on period of the enable signal (K1) based on the increased on period.

[0021] Preferably, the control device is further configured to: determine a fourth voltage indicative of a maximum value of the first voltage during the off period; and stop the operation of the quasi-resonant inverter if the fourth voltage is greater than a second voltage threshold.

[0022] The present invention further relates to an inductive cooking appliance comprising at least one inductive heating coil, a quasi-resonant inverter provided with a switching device electrically connected to the inductive heating coil by a node having a first voltage indicative of a voltage across the switching device, and a driver unit configured to provide an enable signal comprising a plurality of pulses to the switching device in order to turn on and turn off the switching device within a turn-on period and a turn-off period, the present invention further relates to an electronic control system comprising control means configured to control the driver unit in order to adjust the enable signal, the control means being configured to: determine a second voltage indicative of a minimum of the first voltage during the turn-off period; adjust the turn-off period based on the second voltage; and adjust the enable signal based on the adjusted turn-off period.

[0023] Preferably, the control means are further configured to: determine a third voltage indicative of a value of the first voltage at about the end of the turn-off period; increase the turn-off period if the third voltage is greater than the second voltage and the first voltage is decreasing.

[0024] Preferably, the control means are further configured to: decrease the turn-off period if the third voltage is greater than the second voltage and the first voltage is increasing at about the end of the turn-off period.

[0025] Preferably, the control means are further configured to: increase the turn-on period if the second voltage is greater than a first voltage threshold; and adjust the turn-on period of the enable signal based on the increased turn-on period.

[0026] Preferably, the control means are further configured to: determine a fourth voltage indicative of a maximum of the first voltage during the turn-off period; and stop operation of the quasi-resonant inverter if the fourth voltage is greater than a second voltage threshold. BRIEF DESCRIPTION OF DRAWINGS

[0027] These and other characteristics and advantages of the solution according to the present invention will become better understood with the reading of the following detailed description of some embodiments of the invention, provided by way of non-limiting example only, to be read in conjunction with the accompanying drawings, in which:

[0028] Figure 1 is a schematic view showing an example inductive cooking appliance according to example embodiments of the present disclosure,

[0029] Figure 2 is a block diagram schematically showing an example inductive electronic control system according to example embodiments of the present disclosure,

[0030] Figure 3 An example circuit diagram of a QR inverter circuit and a control unit of an inductive electronic control system according to an example of this disclosure is shown schematically.

[0031] Figure 4 A flowchart of an example method for controlling an inductive electronic control system according to the present invention is shown.

[0032] Figure 5A An example of a pulse enable signal provided to the switching device when performing the method proposed in this paper is shown.

[0033] Figure 5B It was shown through Figure 5A The example shown illustrates the change in current flowing through a switching device over time when controlled by a pulse enable signal.

[0034] Figure 5C An example is shown showing how the voltage curve across the switching device changes over time during the resonant period.

[0035] Figure 5D It demonstrates the first module based on Figure 5C The example of voltage values / curves determined by the voltage curve shown in the image, and

[0036] Figure 5E The second module is based on Figure 5C The example shown is the minimum voltage curve provided by the voltage changing over time. Detailed Implementation

[0037] The configurations shown in the embodiments listed in this specification and accompanying drawings are merely exemplary embodiments of this disclosure, and it should be understood that various modifications may have been made to replace the embodiments described in this specification and accompanying drawings at the time of filing this application. Throughout the following description, similar reference numerals are used to denote similar elements, parts, objects, or features, where applicable.

[0038] Figure 1 An induction cooking appliance 1, according to an example embodiment of this disclosure, is shown. Figure 1 The induction cooking appliance 1 shown corresponds to an induction cooktop. The induction cooking appliance 1 includes a preferably horizontal cooking surface 2, i.e., a conventional cooktop, which has multiple cooking zones 3.

[0039] The induction cooking appliance 1 further comprises a plurality of induction heating coils 4 associated with respective cooking zones 3. For example, the induction heating coils 4 can be placed below the cooking surface 2, adjacent to the respective cooking zones 3. It can be appreciated that the induction cooking appliance 1 can comprise a plurality of induction heating coils 4 or a single induction heating coil 4. It can be further appreciated that the electromagnetic hob is provided by way of example only. Indeed, the present disclosure can be used / applied with other kinds of induction cooking appliances, for example, such as an induction stove, an induction oven or any other similar induction cooking appliance.

[0040] The induction cooking appliance 1 can further comprise a user interface 5 designed to receive user inputs and / or to provide information to the user. The user interface 5 can be placed within a portion of the cooking surface 2, as shown. The user interface 5 can be configured to receive heating values / data indicative of predetermined heating parameters (i.e. cooking power / temperature) selected by the user. The user interface 5 can be further configured to provide graphical cooking information to the user.

[0041] According to the present disclosure, the induction heating coils 4 are selectively operable to be fed by respective AC currents. The AC currents are provided to the induction heating coils 4 by the electronic control system 10.

[0042] With reference to Figure 2 The electronic control system 10 can comprise a power supply 11, a rectifier circuit 12, an inverter circuit 13 and a control unit 14. The control unit 14 is configured to control the inverter circuit 13 so as to regulate the power supplied to the induction heating coils 4 by implementing the phases of the control method disclosed in detail hereinafter.

[0043] The power supply 11 can be configured to supply power to the induction cooking appliance 1. For example, the power supply 11 can conveniently be a two-phase 220-volt alternating current (AC) power supply. For example, the power supply can be provided to a residential property from an energy production source, such as a power utility. It should be appreciated that additionally and / or alternatively, any other power source can be used, such as, for example, a single-phase 110V power supply or a three-phase 380V power supply and / or any other DC power source.

[0044] The rectifier unit 12 can be electrically connected between the power supply 11 and the inverter circuit 13. The rectifier unit 12 can be configured to convert the AC power signal provided by the power supply 11 into a rectified signal, i.e. a DC power signal, to be provided to the inverter circuit 13. The rectifier unit 12 can comprise a diode full-bridge for full-wave rectification or a synchronous rectifier with a plurality of switching elements for active rectification, and / or any similar rectification circuit.

[0045] The inverter circuit 13 is designed to feed the induction coils 4 of the induction cooking appliance 1 with AC currents. According to the present disclosure, the inverter circuit 13 is selectively operable to be fed by the rectified signal provided by the rectifier unit 12.Figure 2 and Figure 3 The inverter circuit 13 is electrically coupled to the rectifier circuit 12 to receive the rectified signal in the input terminal, according to the preferred exemplary embodiment shown in

[0046] The inverter circuit 13 is configured to convert the rectified signal provided by the rectifier circuit 12 into a high-frequency AC high-current signal to the induction coil 4 to generate a time-varying magnetic field for inductively heating a load (not shown) placed on the associated cooking zone 3 of the cooking appliance 1.

[0047] According to Figure 3 The inverter circuit 13 can comprise a DC link section 7, a resonant tank section 15 and a power switch section 16, according to the preferred exemplary embodiment shown in

[0048] Since the DC link 7 section is of known type, it will not be further described except for specifying that it is electrically connected to the rectifier circuit 12 to receive the rectified signal and comprises DC link terminals 7a providing a DC voltage.

[0049] According to Figure 3 The resonant tank section 15 and the power switch section 16 constitute a quasi-resonant (so-called QR) electrical topology / architecture, according to the preferred exemplary embodiment shown in

[0050] The resonant tank section 15 can comprise a resonant capacitor 18 and the induction heating coil 4 associated with the cooking zone 3. According to the exemplary embodiment shown in Figure 3 The resonant capacitor 18 is connected in parallel with the induction heating coil 4 between the DC link terminals 7a and a circuit node 20, according to the exemplary embodiment shown in

[0051] The power switch section 16 comprises a driver unit 8 and a switching device. The switching device can be a power switching device 21 connected in series between the circuit node 20 and a neutral terminal 22. Preferably, the neutral terminal 22 can be associated with a ground potential. The ground potential can be provided by the power supply 11.

[0052] According to the preferred embodiment shown in Figure 3 The power switching device 21 can comprise an insulated gate bipolar transistor (IGBT), according to the preferred embodiment shown in. The IGBT can comprise a collector terminal connected to the circuit node 20, a gate terminal for receiving an enable signal K1 (to be disclosed in detail below) and an emitter terminal connected to the neutral terminal 22. It should be understood that the present application is not limited to insulated gate bipolar transistors (IGBT) but that a power switching device 21 comprising any other similar switch (e.g. such as MOS, SIC MOS) can be envisaged.

[0053] According to Figure 3 In accordance with the preferred exemplary embodiments shown in the figures, the power switching device 21 can further comprise a diode connected in anti-parallel configuration with the IGBT. The power switching device 21 can operate to control the operation of the QR inverter circuit 13 such that the current through the induction heating coil 4 is controlled to have different shapes at different frequencies and at different magnitudes.

[0054] The driver unit 8 is configured to receive a control command SC from the control unit 14 and to provide an enable signal K1 to the power switching device 21 based on the received control command SC.

[0055] In accordance with the preferred embodiments of the present application, the enable signal 1 is a pulsed signal, i.e. a signal comprising pulses, the driver unit 8 is configured to vary / modulate the frequency and width of the pulses based on the control signal SC. It should be appreciated that the frequency and / or width of the pulses of the enable signal K1 is controlled by the control unit 14 by means of the driver unit 8.

[0056] The enable signal K1 is designed to turn the switching device 21 on or off during one or more time periods such that the induction heating coil 4 generates a requested amount of output power. The control unit 14 controls the QR inverter circuit 13 such that it operates in a plurality of charging phases and in a plurality of resonance phases alternately.

[0057] During the charging phase, the induction heating coil 4 stores energy. During the resonance phase, the energy stored during the previous charging phase oscillates between the induction heating coil 4 and the resonance capacitor 18 to generate an alternating voltage signal. The charging phase can correspond substantially to the time period in which the power switching device 21 is turned on. It should be appreciated that the power switching device 21 is turned on for a time period tON when the gate of the IGBT receives a pulse of the enable signal K1. It should be further appreciated that the power switching device 21 is turned off for a time period tOFF when the gate of the IGBT does not receive a pulse of the enable signal K1. Figure 5A An example of the enable signal K1 with pulses and corresponding time periods tON is shown as well as the tOFF period between two consecutive pulses.

[0058] During the charging phase of the inverter circuit 13, the power switching device 21 is turned on during a time period tON to allow the inductive coil 8 to charge to a sufficient level. Vice versa, the power switching device 21 is turned off during a time period tOFF to allow the energy stored in the inductive coil 8 during the time period tON to oscillate between the inductive coil 4 and the resonant capacitor 18, so that an alternating current signal is generated. More specifically, during the time period tOFF, the energy oscillation causes an oscillating voltage on the circuit node 20, hereinafter indicated with VC(t), which substantially corresponds to the voltage across the power switching device 21 (i.e. the voltage drop over the switching device 21).

[0059] During the tON time period, the voltage VC(t) has a low value LV of approximately zero volts, because the power switching device 21 is turned on and VC(t) approximately corresponds to the ground potential Figure 5C It will be appreciated that, during the tON time period, the IGBT current approximately changes as shown in Figure 5B

[0060] During the time period tOFF, the voltage VC(t) approximately oscillates as schematically shown in Figure 5C During the time period tOFF, the voltage VC(t) changes so as to approximately behave as a sinusoidal oscillation. The sinusoidal oscillation comprises a first half wave, in which initially the voltage VC(t) increases from a low value LV (approximately zero volts) (immediately after the end of tON) to a peak value VCMax, which is the maximum oscillation value during the time period tOFF, and later the voltage VC(t) decreases from the peak value VCMax to a minimum value VCmin and then tends to increase again (see the dashed line BL extending within tON in Figure 5C

[0061] In order to reduce the losses of the power switching device 21, the instant tl at which the power switching device 21 is turned on should be synchronized with the instant tm at which the oscillating voltage VC(t) reaches its minimum value VCmin during the time period tOFF. Therefore, in a preferred embodiment of the present application, when the instant tl is different from the instant tm (i.e. the turning on of the power switching device 21 is performed in advance or delayed compared to the instant tm corresponding to the condition of minimum value VCmin of VC(t)), the control unit 14 is configured to modify the time period tOFF.

[0062] In order to perform this task, the control unit 14 is configured to: determine the minimum value VCmin of the voltage VC(t) during the time period tOFF; adjust the time period tOFF based on said voltage VCmin by means of the driver unit 8; and adjust the enable signal Kl based on the determined time period tOFF so that the instant tl is approximately the instant tm.

[0063] ​​Applicant has found that it is convenient to adjust the period tOFF so as to turn on the power switching device 21 when the oscillating voltage VC(t) reaches its minimum value VCmin. In fact, applicant has found that the losses of the QR inverter circuit 13 depend on the oscillating voltage VC(t) during the period tOFF and that these losses can be greatly reduced if the power switching device 21 is turned on when the oscillating voltage VC(t) has its minimum value VCmin, i.e. when tl = tm.

[0064] According to a preferred embodiment of the convenient electronic topology shown in Figure 3 According to a preferred embodiment of the convenient electronic topology shown in Figure 5D ) provided by the detection circuit 14a.

[0065] The control unit 14 can further comprise a detection circuit 14b configured to provide a signal S2 indicative of the minimum value VCmin of the oscillating voltage VC(t) during the period tOFF.

[0066] The control unit 14 can further comprise a control circuit 14c, e.g. such as a microprocessor, configured to generate a command signal SC to control the driver unit 8 so as to adjust the pulse of the enable signal Kl. The control circuit 14c controls the enable signal Kl provided to the switching device 21, more specifically varies / adjusts the period tOFF, by means of the driver unit 8, so as to adjust the turn-off of the power switching device 21 itself on the basis of the signal S2.

[0067] The control circuit 14c can be further configured to control the enable signal Kl provided to the power switching device 21, more specifically varies / adjusts the period tON, by means of the signal S2 provided to the driver unit 8, so as to control the turn-on of the switching device 21 itself on the basis of the signal S1.

[0068] Preferably, the detection circuit 14a can be electrically connected to the circuit node 20. According to a possible exemplary embodiment, the detection circuit 14a can be configured to sample the oscillating voltage VC(t) on the circuit node 20 during predetermined sampling times tsi, where i is an index of the sample. Preferably, the detection circuit 14a can be configured to provide an electrical signal S1 indicative of the sampled values VC(tsi) of the oscillating voltage VC(t) measured at the sampling times tsi. Figure 5D A schematic example of the oscillating voltage VC(t) within the period tOFF determined by the detection circuit 14a and provided to the control circuit 14c by means of the signal S1 is shown.

[0069] Preferably, the detection circuit 14b can be electrically connected to the circuit node 20. According to a possible exemplary embodiment, the detection circuit 14b can be configured to sample the oscillating voltage VC(t) on the circuit node 20 during a predetermined sampling time tsi and determine a minimum value VCmin of the oscillating voltage VC(t) during the time period tOFF based on the sampled voltage VC(tsi). It should be appreciated that, additionally or alternatively, the detection circuit 14b can receive the sampled voltage VC(ti) and / or the oscillating voltage VC(t) determined during the time period tOFF from the detection circuit 14a. Preferably, the detection circuit 14b can be configured to provide an electrical signal S2 indicative of the minimum value VCmin. Preferably, the electrical signal S2 provided by the detection circuit 14b to the control circuit 14c can be an analog signal in which the minimum value VCmin is maintained for a predetermined time period AtM which will extend until the end of the time period tOFF for sampling VC(t). Figure 5E A schematic example of the signal S2 containing the minimum value VCmin determined during the time period tOFF is shown in which the minimum value VCmin is maintained for a predetermined time period AtM.

[0070] Applicants have found that maintaining the minimum value VCmin contained in the signal S2 for a predetermined time period AtM within the time period tOFF has a technical effect of reducing the computational speed required to perform the tasks disclosed above. It follows that an inexpensive microprocessor can conveniently perform such tasks without thereby affecting the cost of the electronic control system 10.

[0071] The control unit 14 can further comprise a detection circuit 14d configured to receive the signal S1 indicative of the oscillating voltage VC(t) and provide a signal S3 indicative of a peak value VCMax of the oscillating voltage VC(t) to the control circuit 14c. For example, the detection circuit 14d can be configured to elaborate the sample VC(tsi) so as to determine a maximum value sample corresponding to the peak value VCMax during the time period tOFF.

[0072] It should be appreciated that the electronic topology of the control unit 14 is not limited to the embodiment shown in Figure 3 but other embodiments can be envisaged. For example, the control unit 14 can comprise a single microprocessor comprising the detection circuits 14a, 14b, 14d and the control circuit 14c. In other words, according to this embodiment, all the tasks are performed by the microprocessor.

[0073] Figure 4 A flowchart of a control method 100 according to the present disclosure is shown. The method 100 can be performed by the electronic control system 10.

[0074] A pulse of the enable signal K1 can be provided Figure 5AThe first pulse P1 on the left enables (block 100) the power switching device 21 of the QR inverter circuit 13 in the induction cooking appliance 1 for a time period tON and a time period tOFF. Initially, the time periods tON and tOFF can have predetermined values. For example, the time periods tON and tOFF can be determined based on a user command provided by the user interface 5. For example, the time periods tON and tOFF can depend on the cooking temperature and / or the load.

[0075] During the tON period, the pulse of the enable signal K1 (first pulse P1 in Fig. 5a) turns on the power switching device 21 and the voltage VC(t) on the circuit node 20 has a low value LV Figure 5C ).

[0076] The method waits for the time period tOFF (block 110) (first tOFF after the first pulse P1 in Fig. 5b). Figure 5A

[0077] During the time period tOFF, the switching device 21 is turned off and the voltage VC(t) oscillates Figure 5C ). During the time period tOFF, the method determines the voltage VC(t) (block 120). Preferably, in this stage, the control circuit 14c receives from the detection circuit 14a a signal S1 Figure 5D ) indicating the oscillating voltage VC(t). The method further determines the voltage VC(te) in the circuit node 20 at about the end of the time period tOFF (i.e. at the last instant te of the time period tOFF, followed by the next time period tON, i.e. immediately after the instant tl at which the IGBT is turned on).

[0078] The method 100 further determines the minimum value VCmin of the oscillating voltage VC(t) during the time period tOFF. Preferably, the control circuit 14c can receive a signal S2 (block 120) indicating the minimum value VCmin from the detection circuit 14b.

[0079] Preferably, the method can further determine the maximum value VCMax of the voltage VC(t) during the time period tOFF. In this step, if the maximum value VCMax is greater than the voltage threshold TH1 (block 130), the method determines a critical resonant voltage, i.e. VCMax >= TH1, and stops the operation of the inverter circuit 13 (YES output from block 130).

[0080] ​If the method 100 does not determine a critical resonant voltage, i.e. VCMax < TH1 (NO output from block 130), the method 100 compares the minimum value VCmin with a voltage threshold TH2 (block 140). The voltage threshold TH2 can be determined based on the electrical characteristics of the switching device 21. Preferably, the voltage threshold TH2 can correspond to a voltage that can cause the power switching device 21 to be damaged.

[0081] If the minimum value VCmin is greater than the voltage threshold TH2 (YES output from block 140), the method 100 increases the period tON for the next pulse P2 of the enable signal K1 (block 145). The period tON can be incremented based on a number of procedures. For example, the period tON can be incremented based on the difference between the voltage VCmin and the threshold TH2 or based on a predetermined value. Figure 5A

[0082] If the minimum value VCmin is lower than or equal to the threshold TH2 (NO output from block 140), the method 100 controls whether the voltage VC(te) on the circuit node 20 at the end of the period tOFF, i.e. at a time te before tl, is greater than or equal to the minimum value VCmin (block 150). The stage of block 150 is further executed if the minimum value VCmin is not greater than the voltage threshold TH2 (NO output from block 140).

[0083] If the voltage VC(te) of the circuit node 20 at the end of the period tOFF, i.e. at a time te, is lower than or equal to the minimum value VCmin, i.e. VC(te) <= VCmin (NO output from block 150) and the voltage Vc(t) is decreasing, the method 100 determines that the period tOFF is correct and executes again the stages disclosed above in blocks 110, 120, 130, 140, 150. In this case, the method 100 provides a new pulse P2 of the enable signal K1 (block 220) by maintaining the period tOFF unchanged. Figure 5A

[0084] If the voltage VC(te) on the circuit node 20 at the end of the period tOFF is greater than the minimum value VCmin, i.e. (VC(te) > VCmin) (YES output from block 150) and the voltage Vc(t) is decreasing, the method determines that the power switching device 21 has been switched before a time tm, i.e. tl < tm. In this case, the method 100 increases the period tOFF of the enable signal K1 and starts a second pulse P2 of the enable signal K1 (block 160). The increase of the period tOFF can be performed by a number of procedures. For example, the period tOFF can be incremented based on the difference between VC(te) and VCmin or based on a predetermined value. Figure 5A

[0085] ​​​After the phases contained in the execution block 160, the method 100 performs a phase of waiting for the next period tOFF following the last generated pulse (in Figure 5A the period tOFF following the pulse P2).

[0086] The method determines the voltage VC(t) during the period tOFF (block 180). Preferably, in this phase the control circuit 14c can receive from the detection circuit 14a a signal S1 indicative of the voltage VC(t).

[0087] The method further determines the voltage VC(te) in the circuit node 20 at the end of tOFF, i.e. at the instant te, immediately followed by the next tON (block 180).

[0088] The method 100 further determines the minimum value VCmin of the oscillating voltage VC(t) during the period tOFF following the last pulse (P2 in Figure 5A ). Preferably, in this phase the control circuit 14c can receive from the detection circuit 14b a signal S2 indicative of the minimum value VCmin (block 180).

[0089] The method 100 compares the minimum value VCmin with a threshold value TH2 (block 190). If the minimum value VCmin is greater than the threshold value TH2 (yes from block 190), the method 100 increases the period tON of the next pulse (pulse P3 in Figure 5A ) of the enable signal K1 (block 200).

[0090] If the minimum value VCmin is lower than or equal to the threshold value TH2 (no from block 190), the method 100 controls whether the voltage VC(te) of the circuit node 20 at the end of the period tOFF (i.e. at the instant te) following the pulse P2 in Figure 5A is greater than the minimum value VCmin (block 210). The phase of block 210 is further performed after the phase of block 200.

[0091] If the voltage VC(te) of the circuit node 20 at the end of the period tOFF (instant te) is lower than or equal to the minimum value VCmin (VC(te) <= VCmin) (no from block 210), the method 100 performs again the phases disclosed above for blocks 220, 110, 120, 130, 140, 150. In this case, the method provides a new pulse (pulse P3 in Figure 5A ) of the enable signal K1 by maintaining the last determined period tOFF unchanged (block 220).

[0092] If the voltage Vc(t) at the circuit node 20 at the end of the period tOFF (moment te) is greater than the minimum value Vcmin (Vc(te) > Vcmin) (Yes output from block 210), the method 100 checks whether the voltage Vc(t) on the circuit node 20 is increasing (block 230). The increase of the voltage Vc(t) can be determined, for example, on the basis of the signal S1 provided by the detector circuit 14a.

[0093] If the voltage Vc(t) on the circuit node 20 is decreasing (No output from block 230), the method 100 performs again the phases of blocks 160, 170, 180, 190, 200, 210.

[0094] If the voltage Vc(t) on the circuit node 20 is increasing (Yes output from block 230), the method 100 determines that the time at which the power switching device 21 is turned on is too late compared to the moment tm, i.e. ti > tm, in which the voltage Vc(t) has its minimum value Vcmin. In this case, the method reduces the period tOFF and starts a new pulse (block 240). The period tOFF can be reduced, for example, on the basis of a predetermined value or on the basis of the difference between Vc(te) and the minimum value Vcmin.

[0095] The advantages of the present application are the following: the electronic control system can obtain a higher efficiency and a longer life of the switching device by reducing the losses and by using a simple and therefore inexpensive electronic architecture.

[0096] Obviously, changes and variations can be made to the cooking appliance, to the method and to the electronic system, without however departing from the scope of the present application.

Claims

1. A method for controlling a quasi-resonant inverter (13) in an induction cooking appliance (1) provided with at least one induction heating coil (4); The quasi-resonant inverter (13) includes a switching device (21) electrically connected to the induction heating coil (4) via a node (20) having a first voltage (VC(t)) indicating the voltage across the switching device (21). The method includes the following steps: a) Provide an enable signal (K1) comprising multiple pulses to the switching device (21) to turn the switching device (21) on and off during the on-time (tON) and off-time (tOFF) periods. b) Determine a second voltage (VCmin) that indicates the minimum value of the first voltage (VC(t)) during the off-time period (tOFF). c) Adjusting the off-time (tOFF) based on the second voltage (VCmin), and d) Adjust the enable signal (K1) based on the adjusted off-time (tOFF). in: Step b) further includes: b1) Determine a third voltage (VC(te)) that indicates the value of the first voltage (VC(t)) at the end of the off-time period (tOFF) (te), and Step c) includes: c1) If the third voltage (VC(te)) is greater than the second voltage (VCmin) and the first voltage (VC(t)) is decreasing, then increase the off-time (tOFF).

2. The method according to claim 1, wherein: Step c) further includes: c2) If the third voltage (VC(te)) is greater than the second voltage (VCmin) and the first voltage (VC(t)) is increasing, then reduce the off-time (tOFF).

3. The method according to claim 1 or 2, comprising: e) If the second voltage (VCmin) is greater than the first voltage threshold (TH2), then the on-time (tON) is increased. Step d) includes the following steps: adjusting the on-time (tON) of the enable signal (K1) based on the increased on-time (tON).

4. The method according to claim 1 or 2, comprising the following steps: f) Determine a fourth voltage (VCMax) that indicates the maximum value of the first voltage (VC(t)) during the off-time (tOFF) period. g) If the fourth voltage (VCMax) is greater than the second voltage threshold (TH1), then stop the operation of the quasi-resonant inverter (13).

5. An electronic control system (10) for controlling an induction heating coil (4) of an induction cooking appliance (1), the electronic control system (10) comprising: A quasi-resonant inverter (13) is provided with a switching device (21) electrically connected to the induction heating coil (4) via a node (20) having a first voltage (VC(t)) indicating the voltage across the switching device (21). and a driver unit (8) configured to provide an enable signal (K1) comprising multiple pulses to the switching device (21) to turn the switching device (21) on and off during an on-time period (tON) and an off-time period (tOFF). The electronic control system (10) includes a control device (14) configured to control the driver unit (8) to adjust the enable signal (K1). The electronic control system (10) is characterized in that the control device (14) is further configured to: - Determine a second voltage (VCmin) that indicates the minimum value of the first voltage (VC(t)) during the off-time period (tOFF). - Adjust the off-time (tOFF) based on the second voltage (VCmin). - The enable signal (K1) is adjusted based on the adjusted off-time (tOFF). The control device (14) is further configured to: - Determine a third voltage (VC(te)) that indicates the value of the first voltage (VC(t)) at the end of the off-time period (tOFF) (te). - If the third voltage (VC(te)) is greater than the second voltage (VCmin) and the first voltage (VC(t)) is decreasing, then the off-time (tOFF) is increased.

6. The electronic control system (10) according to claim 5, wherein, The control device (14) is further configured to reduce the off-time (tOFF) if the third voltage (VC(te)) is greater than the second voltage (VCmin) and the first voltage (VC(t)) is increasing.

7. The electronic control system (10) according to claim 5 or 6, wherein, The control device (14) is further configured to: If the second voltage (VCmin) is greater than the first voltage threshold (TH2), then the on-time (tON) is increased. The on-time (tON) of the enable signal (K1) is adjusted based on the increased on-time (tON).

8. The electronic control system (10) according to claim 5 or 6, wherein, The control device (14) is further configured to: Determine a fourth voltage (VCMax) that indicates the maximum value of the first voltage (VC(t)) during the off-time period (tOFF). If the fourth voltage (VCMax) is greater than the second voltage threshold (TH1), then the operation of the quasi-resonant inverter (13) is stopped.

9. A sensor-activated cooking appliance (1), comprising: At least one induction heating coil (4), A quasi-resonant inverter (13) is provided with a switching device (21) electrically connected to the induction heating coil (4) via a node (20) having a first voltage (VC(t)) indicating the voltage across the switching device (21). and a driver unit (8) configured to provide an enable signal (K1) comprising multiple pulses to the switching device (21) to turn the switching device (21) on and off during an on-time period (tON) and an off-time period (tOFF). An electronic control system (10) comprising a control device (14) configured to control the driver unit (8) to adjust an enable signal (K1). The sensor-activated cooking appliance (1) is characterized in that the control device (14) is further configured to: - Determine a second voltage (VCmin) that indicates the minimum value of the first voltage (VC(t)) during the off-time period (tOFF). - Adjust the off-time (tOFF) based on the second voltage (VCmin). - The enable signal (K1) is adjusted based on the adjusted off-time (tOFF). The control device (14) is further configured to: - Determine a third voltage (VC(te)) that indicates the value of the first voltage (VC(t)) at the end of the off-time period (tOFF) (te). - If the third voltage (VC(te)) is greater than the second voltage (VCmin) and the first voltage (VC(t)) is decreasing, then the off-time (tOFF) is increased.

10. The induction cooking appliance (1) according to claim 9, wherein, The control device (14) is further configured to reduce the off-time (tOFF) if the third voltage (VC(te)) is greater than the second voltage (VCmin) and the first voltage (VC(t)) is increasing.

11. The induction cooking appliance (1) according to claim 9 or 10, wherein, The control device (14) is further configured to: increase the on-time (tON) if the second voltage (VCmin) is greater than the first voltage threshold (TH2); and adjust the on-time (tON) of the enable signal (K1) based on the increased on-time (tON).

12. The induction cooking appliance (1) according to claim 9 or 10, wherein, The control device (14) is further configured to: determine a fourth voltage (VCMax) indicating the maximum value of the first voltage (VC(t)) during the off period (tOFF); and stop the operation of the quasi-resonant inverter (13) if the fourth voltage (VCmax) is greater than a second voltage threshold (TH1).

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