METHOD FOR CONTROLLING THE PROVISION OF ELECTRIC POWER TO AN INDUCTION COIL

Abstract

The invention relates to a method for controlling the provision of electric power to an inductive element, in particular an induction coil (L), of an induction cooking appliance (1), the induction cooking appliance (1) comprising a circuitry (10) with an input (11), at least one switching element (S) for providing pulsed electric power to the inductive element, in particular the induction coil (L) and a capacitive element, in particular a capacitor (C) being connected in parallel, in particular in series, to the switching element (S), the method comprising the steps of: —receiving rectified AC-voltage (V.sub.in) at the input (11) of the circuitry (10); —discharging, in particular during a first section/phase (P1), the capacitive element, in particular the capacitor (C) in order to reduce the voltage provided to the switching element (S), in particular during a first period of rectified AC-voltage (V.sub.in); —after at least partially discharging the capacitive element, in particular the capacitor (C), starting, in particular as a second section/phase (P2), a switching operation of the switching element (S), in particular during a second period of rectified AC-voltage (V.sub.in); —stopping the switching operation after a switching operation time, in particular during the second period of rectified AC-voltage (V.sub.in); —iterating the steps of discharging of capacitor (C), starting of switching operation and stopping of switching operation, in particular immediately subsequently, in subsequent periods of rectified AC-voltage (V.sub.in).

Claims

1. Method for controlling the provision of electric power to an induction coil of an induction cooking appliance, the induction cooking appliance comprising the induction coil, a circuitry, at least one circuit switching element configured to control a pulsed electric power to the induction coil, and a capacitor connected in parallel with the circuit switching element, the method comprising the steps of: receiving a rectified AC-voltage at an input of the circuitry; discharging the capacitor, thereby reducing a voltage across the circuit switching element; after at least partially discharging the capacitor, starting a switching operation of the circuit switching element; stopping the switching operation after a switching operation time; and iterating the steps of discharging the capacitor, starting the switching operation, and stopping the switching operation in subsequent periods of the rectified AC-voltage.

2. Method according to claim 1, the method further comprising the step of: during and/or after the switching operation, charging the capacitor to its maximum voltage, wherein the pulsed electric power is below a predetermined range.

3. Method according to claim 1, wherein said discharging is started during a period of time in which the slope of the rectified AC-voltage is falling, and wherein said discharging is performed by a single switching operation, during which a discharging circuit and/or the circuit switching element is gradually changed from a closed state to an opened state.

4. Method according to claim 1, wherein the capacitor is discharged to a voltage of 50V or lower.

5. Method according to claim 1, wherein said discharging is stopped at or close to a time when the rectified AC-voltage is at a zero point.

6. Method according to claim 1, wherein the switching operation is started at or close to a time when the rectified AC-voltage is at a zero point, and/or when the capacitor is discharged to a voltage of 50V or lower.

7. Method according to claim 1, wherein the switching operation is performed with a switching frequency determined by a first zero crossing detection circuit, and/or wherein the switching operation is performed with a Ton time determined by a voltage of the induction coil and/or a voltage of the capacitor, so that thermal losses of or within the circuit switching element are kept below a predetermined range.

8. Method according to claim 1, wherein said switching operation is stopped before the rectified AC-voltage reaches its minimum value.

9. Method according to claim 1, wherein said switching operation is stopped before the rectified AC-voltage reaches its maximum value.

10. Method according to claim 1, wherein an averaged power loss of the circuit switching element is below a predetermined maximum power loss, and/or wherein the switching operation time determines the electric power provided to the induction coil and the capacitor, and/or wherein the switching operation time is determined in response to a power level of a heating zone, so that an electric power corresponding to the power level is provided to the induction coil and the capacitor.

11. Method according to claim 1, wherein discharging of the capacitor (C) is not performed simultaneously with the switching operation.

12. Method according to claim 1, wherein a period of the rectified AC-voltage comprises a first phase, a second and a third phase, the third phase being between a time of stopping the switching operation and a time of discharging the capacitor, and wherein: during the third phase of the period of the rectified AC-voltage, the capacitor is charged, and/or during the third phase of the period of the rectified AC-voltage, a voltage of the capacitor is at least essentially constant, and/or the third phase of the period of the rectified AC-voltage starts before 40% of the period of the rectified AC-voltage elapses and ends after 60% of the period of the rectified AC-voltage elapses, and/or the third phase of the period of the rectified AC-voltage starts and ends when the rectified AC-voltage is at least 20% below its maximum.

13. Method according to claim 1, wherein said discharging and said switching operation are repeated periodically with the periodicity of the rectified AC-voltage.

14. System for controlling the provision of electric power to an induction coil of an induction cooking appliance, the system comprising: at least one circuit switching element configured to control a pulsed electric power to the induction coil; a capacitor connected in parallel with the circuit switching element; a discharging circuit configured to enable a discharging of said capacitor; and a controller configured to: control discharge of said capacitor by said discharging circuit, and after the capacitor is at least partially discharged, control a start of a switching operation of the circuit switching element such that the pulsed electric power is provided to the induction coil.

15. System according to claim 14, wherein said discharging circuit is connected in parallel with the capacitor.

16. System according to claim 14, wherein the controller is configured to control the switching operation in only a portion of a period of the rectified AC-voltage that is shorter than the entire period of rectified AC-voltage.

17. Induction cooking appliance comprising the system according to claim 14.

18. Method for controlling a supply of electric power to an induction coil of an induction cooking appliance with a circuit comprising a first semiconductor switch and the induction coil connected in series with each other between a first node and a second node, a capacitor connected between the first node and the second node and in parallel with the first semiconductor switch and the induction coil, and a second semiconductor switch connected between the first node and the second node and in parallel with the capacitor, the method comprising: receiving a rectified AC-voltage between the first node and the second node, the rectified AC-voltage being a periodic signal, each period extending between adjacent zero points of the rectified AC-voltage and comprising a first phase in which the rectified AC-voltage is decreasing, a second phase in which the rectified AC-voltage is increasing, and a third phase between the second phase and the first phase in which the rectified AC-voltage reaches a maximum value; during the first phase of each period of the rectified AC-voltage, operating the second semiconductor switch such that the capacitor is discharged to 50V or less, and a voltage across the first semiconductor switch and the induction coil is reduced; during the second phase of each period of the rectified AC-voltage, operating the first semiconductor switch, electric power being supplied to an induction coil in accordance with operation of the first semiconductor switch; during the second phase of each period of the rectified AC-voltage, charging the capacitor to a value less than a maximum voltage; and during the third phase of each period of the rectified AC-voltage, charging the capacitor to the maximum voltage and maintaining the maximum voltage across the capacitor, wherein the third phase of each period of the rectified AC-voltage starts before 40% of the period of the rectified AC-voltage elapses and ends after 60% of the period of the rectified AC-voltage elapses, and the third phase of each period of the rectified AC-voltage starts and ends when the rectified AC-voltage is at least 20% below its maximum, wherein discharging of the capacitor ends when the rectified AC-voltage is at a zero-point, wherein operation of the first semiconductor switch begins when the rectified AC-voltage is at the zero point, and wherein discharging the capacitor is not performed simultaneously with operation of the first semiconductor switch.

19. Method according to claim 18, wherein operation of the first semiconductor switch is performed with a switching frequency determined based on a zero point detection of the rectified AC-voltage, wherein operation of the first semiconductor switch is performed with an ON time of the first semiconductor switch determined by a voltage of the induction coil and/or a voltage of the capacitor, such that thermal losses of or within the circuit switching element are kept below a predetermined range, wherein an average power loss of the first semiconductor switch is less than a predetermined maximum power loss.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0108] The various aspects of the invention, including its particular features and advantages, will be readily understood from the following detailed description and the accompanying drawings, in which:

[0109] FIG. 1 shows an example top view on a cooking appliance comprising multiple heating zones;

[0110] FIG. 2 shows a schematic diagram of a system for controlling the provision of electric power to an induction coil according to an embodiment of the invention; and

[0111] FIG. 3 shows time diagrams of input voltage and capacitor voltage applied to the capacitor.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0112] The present invention will now be described more fully with reference to the accompanying drawings, in which example embodiments are shown. The embodiments in the figures may relate to preferred embodiments, while all elements and features described in connection with embodiments may be used, as far as appropriate, in combination with any other embodiment and feature as discussed herein, in particular related to any other embodiment discussed further above. However, this invention should not be construed as limited to the embodiments set forth herein. Throughout the following description similar reference numerals have been used to denote similar elements, parts, items or features, when applicable.

[0113] The features of the present invention disclosed in the specification, the claims, examples and/or the figures may both separately and in any combination thereof be material for realizing the invention in various forms thereof.

[0114] FIG. 1 illustrates a schematic diagram of an induction cooking appliance 1, in the present example an electric induction hob.

[0115] The induction cooking appliance 1 comprises multiple heating zones 2. Each heating zone 2 may be, for example, associated with one or more heating power transferring elements, specifically, one or more induction coils. The induction cooking appliance 1 may be configured to combine two or more heating zones 2 in order to form larger-sized cooking zones.

[0116] In addition, the induction cooking appliance 1 comprises a user interface 3, based on which a user may control the induction cooking appliance 1. For example, based on the user interface 3, the user may control the power level of the heating zones 2. The power level may be chosen between a minimum power level P.sub.min and a maximum power level P.sub.max.

[0117] FIG. 2 shows a schematic diagram of a system for controlling the provision of electric power to an induction coil L, specifically for decreasing switching losses arising at a switching element when operating the heating zone 2 at low power level.

[0118] The system comprises a circuitry 10 and a control entity 12 for controlling the operation of the circuitry 10.

[0119] The circuitry 10 comprises an input 11 for receiving an input voltage V.sub.in. The input voltage V.sub.in may be a rectified AC-voltage. For example, the rectified AC-voltage may be derived by rectifying a sinusoidal AC-voltage with a certain frequency. The rectified AC-voltage may comprise the double frequency of the sinusoidal AC-voltage. So, for example, if the sinusoidal AC-voltage has a frequency of 50 Hz, the rectified AC-voltage may have a frequency of 100 Hz.

[0120] In the embodiments, the AC-voltage can be a sinusoidal AC-voltage having a frequency of 50 Hz, 60 Hz or between 50 Hz and 60 Hz. The rectified AC-voltage correspondingly can be a sequence of essentially positive half sinus waves and/or can have a frequency of 100 Hz to 120 Hz. The rectified AC-voltage can in particular be a sinusoidal AC-voltage which has been rectified by a rectifier, in particular by a bridge rectifier, more in particular by a diode bridge rectifier.

[0121] The circuitry 10 further comprises a switching element S. The switching element S is electrically coupled with the induction coil L in order to provide electric power to the induction coil L. The induction coil L is further electrically coupled with a capacitor C.sub.R for creating a resonance circuit. The capacitor C.sub.R may be connected in parallel to the induction coil L. Furthermore, the switching element S may be electrically connected in series to said resonance circuit.

[0122] The switching element S is electrically connected with the control entity 12 for receiving a switching signal SW. Based on the switching signal SW, a switching operation of the switching element S can be performed. During said switching operation, pulsed electric power may be provided to the induction coil L which causes an induction heating process. Depending on the electrical dimensioning of the resonance circuit, the switching frequency during switching operation may be in the range of 20 kHz to 30 kHz, specifically 25 kHz.

[0123] The circuit 10 further comprises a capacitor C. Said capacitor C is connected in parallel to the switching element S. More in detail, said capacitor C may be connected in parallel to the serial connection of switching element S and resonance circuit. Said capacitor C may be dimensioned such that the voltage V.sub.c across the switching element S can be stabilized at least in a certain time span during the period of input voltage V.sub.in.

[0124] Furthermore, circuit 10 comprises a discharging entity D. Said discharging entity D may be connected in parallel to the capacitor C. Said discharging entity D is configured to time-selectively discharge the capacitor C in order to reduce hard-switching situations and thereby ensuring the operation of the switching element S below a maximum operation temperature.

[0125] The discharging entity D may be electrically coupled with said control entity 12 in order to provide a discharging control signal DCS to the discharging entity D. Based on said discharging control signal DCS, a time-selective activation of the discharging entity D can be obtained.

[0126] The discharging entity D may, for example, comprise a resistor and a switching element which receives discharging control signal DCS for time-selective activation of said discharging entity D. The switching element may be, for example, a semiconductor switching element, e.g. a transistor etc. According to an embodiment, the switching element S may be used also for activating/deactivating the discharging entity D.

[0127] FIG. 3 shows diagrams of the input voltage V.sub.in and the capacitor voltage V.sub.c vs time t. As mentioned before, the input voltage V.sub.in may be a rectified sinusoidal AC-voltage. The input voltage V.sub.in may comprise a period time T. For controlling the power provision to the induction coil L, three phases may occur during a certain period T of input voltage V.sub.in.

[0128] Before starting the first phase P1, the capacitor voltage V.sub.c may be at a certain value, specifically at a maximum value. In other words, the capacitor C is fully charged. At the beginning of the first phase P1, discharging entity D is activated in order to discharge capacitor C. Said activation may be obtained by a change of voltage level of discharging control signal DCS. For example, said discharging control signal DCS may be switched from low to high or vice versa. The first phase P1 may start during a descending slope of input voltage V.sub.in. During said discharging of capacitor C, no switching operation may be performed by the switching element S.

[0129] After at least partially discharging the capacitor C (e.g. to a voltage of 50V or lower), phase P1 stops and phase P2 is started. Stopping of phase P1 may refer to a change of voltage level of discharging control signal DCS such that discharging obtained by discharging entity D is deactivated. It is worth mentioning that, in preferred embodiments, phases P1 and P2 do not overlap. In phase P2, switching operation of switching element S may be started. The start of phase P2 may coincide or may be close to zero point of input voltage V.sub.in, wherein “close to zero point” may be defined as an area around zero point which has a time duration of 10% or less, specifically 5% or less than the period T of input voltage V.sub.in.

[0130] During phase P2, a high-frequency switching signal SW is profs to the switching element S. Thereby, oscillations within the resonance circuit are excited which provides electric power to the induction coil L. Phase P2 may be stopped before input voltage V.sub.in get zero, specifically at or before input voltage V.sub.in reaches its peak value, i.e. during the rising edge of input voltage V.sub.in.

[0131] At the end of phase P2, the provision of high-frequency switching signal SW to the switching element S is stopped. Thereby, the capacitor C can be loaded to its maximum voltage value in phase P3 in which discharging entity D is also deactivated. Afterwards, upper-mentioned cycle including phases P1 to P3 can be performed once again.

[0132] The cycle including phases P1 to P3 may be repeated periodically with the periodicity of input voltage V.sub.in.

[0133] In the embodiments, the sequence constituted by a first phase or section P1, a second phase or section P2 and a third phase or section P3 is in particular equal in terms of time to and has the same duration like a single, more in particular phase shifted, period of rectified AC-voltage.

[0134] Due to activating the switching element only during a certain portion of the period of input voltage V.sub.in, it is possible to have, in average, a lower power transfer to the induction coil which reduces to minimum power level. In addition, upper-mentioned discharging of capacitor C before starting switching operation reduces switching losses because due to said discharging of capacitor C, voltage applied to switching element S is reduced.

[0135] It should be noted that the description and drawings merely illustrate the principles of the proposed invention. Those skilled in the art will be able to implement various arrangements that, although not explicitly described or shown herein, embody the principles of the invention.

LIST OF REFERENCE NUMERALS

[0136] 1 induction cooking appliance [0137] 2 heating zone [0138] 3 user interface [0139] 10 circuitry [0140] 11 input [0141] 12 control entity [0142] C capacitor [0143] C.sub.R resonance capacitor [0144] D discharging entity [0145] DCS discharging control signal [0146] L induction coil [0147] P1 first phase [0148] P2 second phase [0149] P3 third phase [0150] S switching element [0151] SW switching signal [0152] V.sub.c capacitor voltage [0153] V.sub.in input voltage