Patent Description:
As it is known, an induction cooktop may comprise at least one pair of high frequency current generators, sharing common mains line, rectifier and DC link and configured to energize respective induction heaters (also referred to as "pancake coils"). One major issue involved in driving induction cooktops operated with individual inverters resides in determining the power vs. switching frequency (or switching period) characteristics when the induction heaters are coupled to specific cooking vessels. These characteristics, in fact, form the basis for independent control of the high frequency current generators that meets user's demand of power and, at the same time, avoids audible noise caused by frequency intermodulation which normally occur when two high frequency current generators are operated at different frequencies.

The power vs. switching frequency or period characteristics, which will be hereinafter generally referred to as power characteristics for the sake of simplicity, obviously need to be determined at the start of a cooking process for each induction heaters in use. According to known solutions, described e.g. in <CIT> and in <CIT>, the power characteristics may be preliminarily determined by measuring or estimating power delivered to the cookware during a frequency scan through a plurality of discrete frequency steps.

However, several variable external factors heavily affect the power characteristics during the cooking process and the initial estimation requires to be quite frequently updated. Factors that have influence on power transfer and heating include, for example, cookware temperature, cookware position with respect to the induction heaters, mains voltage and coil temperature. In known control devices, deviations from the currently used power characteristics are monitored and a new frequency scan is carried out every time refresh or new acquisition is required. The problem of changes in power characteristics is thus mitigated, but limitations still remain. Mainly, each frequency scan disrupts power delivery and interruptions in the control sequence may cause power fluctuations and possibly increase of flicker emissions. On the other hand, the power characteristics cannot be updated once the control sequence has been started unless a new frequency scan is performed.

As a result, refresh of the power characteristics is not as frequent as it would be desirable and power delivery is impaired. The document <CIT> discloses an induction cooktop according to the preamble of claim <NUM>.

It is an aim of the present invention to provide an induction cooktop and a method for controlling an induction cooktop that allow the above limitations to be overcome or at least reduced.

According to the present invention there are provided an induction cooktop and a method of controlling an induction cooktop as defined in claims <NUM> and <NUM>, respectively.

The present invention will now be described with reference to the accompanying drawings, which show a number of non-limitative embodiments thereof, in which:.

With reference to <FIG>, an induction cooktop is designated as a whole by number <NUM> and comprises a glass-ceramic plate <NUM>, at least a pair of induction heaters including a first induction heater <NUM> and a second induction heater <NUM> at respective cooking zones below the plate <NUM>, and a converter <NUM>, configured to couple to a supply line (mains) <NUM> through a coupling interface <NUM> to receive an AC supply voltage VAC and to independently energize the induction heaters <NUM>, <NUM>. The coupling interface <NUM> allows connection to the supply line <NUM> and may include a terminal block and EMI (Electro-Magnetic Interference) suppression filters (not shown). In embodiments not shown, an induction cooktop may include a plurality of pairs of induction heaters, each pair of induction heaters being supplied by one respective common mains phase. A user interface <NUM> allows users to select average power levels to be delivered to the induction heaters <NUM>, <NUM>.

In use, induction cooking vessels <NUM>, <NUM> are arranged at the cooking zones in positions corresponding to respective induction heaters <NUM>, <NUM>. When the induction heaters <NUM>, <NUM> are energized, Eddy currents are induced in the cooking vessels <NUM>, <NUM>, which are thus heated.

In accordance with a non-limiting embodiment of the present invention illustrated in <FIG>, the converter <NUM> comprises a rectifier <NUM>, a DC link capacitor <NUM>, a control unit <NUM>, a first power switch <NUM>, a second power switch <NUM> and a power detector <NUM>, that in turn includes a voltage sensing network 20a and current sensors 20b, 20c. The first induction heater <NUM> and the second induction heater <NUM> with respective resonant capacitors <NUM>, <NUM> form a first resonant circuit <NUM> and a second resonant circuit <NUM>, respectively driven by the first power switch <NUM> and the second power switch <NUM>, which are operated as switching current generators by the control unit <NUM>. In the embodiment of <FIG>, the converter <NUM> (more specifically the control unit <NUM>, the first power switch <NUM> and the second power switch <NUM>) is in single-ended quasi resonant configuration with the first resonant circuit <NUM> and a second resonant circuit <NUM>. The first power switch <NUM> and the second power switch <NUM> may be any suitable kind of device, such as IGBTs or power MOSFETs. It is also understood that the converter is not limited to the quasi resonant configuration and other configuration may be exploited as well, such as a half-bridge configuration as explained in detail later on.

The rectifier <NUM> and the DC link capacitor <NUM> supply a rectified voltage to rails <NUM>, <NUM> and the control unit <NUM> controls the power switches <NUM>, <NUM> to energize the induction heaters <NUM>, <NUM> and deliver power to the cooking vessels <NUM>, <NUM> in accordance with user's requests.

The power detector <NUM> is configured to continuously sense an active power individually delivered by each of the induction heaters <NUM>, <NUM> to the cooking vessels <NUM>, <NUM> and, in the non-limiting embodiment of <FIG>, includes the voltage sensing network 20a and the current sensors 20b, 20c, as already mentioned. The voltage sensing network 20a may include a voltage divider connected between the rails <NUM>, <NUM> and having an intermediate node coupled to a voltage sense input 15a of the control unit <NUM>. The current sensors 20b, 20c may include resistors in series to conduction terminals of respective power switches <NUM>, <NUM> and are coupled to respective current sense input 15b, 15c of the control unit <NUM>. It is however understood that any suitable power detector may be used in place of the power detector <NUM> of <FIG>, including power detectors with common current sensors for the power switches <NUM>, <NUM>. The power detector <NUM> supplies power sense signals, based on which the control unit <NUM> determines the active power delivered by the power switches <NUM>, <NUM>. In the non-limiting embodiment of <FIG>, power sense signals include a voltage sense signal Ssv supplied by the voltage sensing network 20a and current sense signals SSC1, SSC2 supplied the current sensors 20b, 20c, respectively.

The control unit <NUM> has control outputs 15d, 15e coupled to control terminals of respective power switches <NUM>, <NUM> and is configured to operate the power switches <NUM>, <NUM> on the basis of a control procedure and of power measurements received from or based on the power sense signals Ssv, SSC1, SSC2 provided by the power detector <NUM>, so as to energize the induction heaters <NUM>, <NUM> and deliver power to the cooking vessels <NUM>, <NUM> in accordance with user's requests. Specifically, the power switches <NUM>, <NUM> are operated on control cycles having a control period I<NUM> of duration T, one of which is shown in <FIG>. Each control period I<NUM> includes a plurality of control intervals, in which the first power switch <NUM> and the second power switch <NUM> are operated by the control unit <NUM> as switching current generators at controlled switching frequencies through a first control signal SSW1 and a second control signal SSW2, respectively. The control signals SSW1, SSW2 are provided on the control outputs 15d, 15e of the control unit <NUM> and applied to the control terminals of the respective power switches <NUM>, <NUM>.

Specifically, in a first control interval I<NUM>, having a first duration T<NUM>, the control unit <NUM> activates both the first induction heater <NUM> and the second heater <NUM> simultaneously by operating both the first power switch <NUM> and the second power switch <NUM> with a first switching frequency fSW1.

In a second control interval I<NUM>, having a second duration T<NUM>, the control unit <NUM> activates only one of the induction heaters <NUM>, <NUM>, which has the most demanding task in terms of power to be delivered, by operating the respective power switch. In the example of <FIG>, the first induction heater <NUM> is energized by operating the first power switch <NUM> with a second switching frequency fSW2.

In a third control interval I<NUM>, having a third duration T<NUM>, the control unit <NUM> activates only the induction heater that had been already activated during the second control interval I<NUM>, i.e. the first induction heater <NUM>, by operating the first power switch <NUM> with a third switching frequency fSW3. The third switching frequency fSW3 is different from and preferably greater than the second switching frequency fSW2.

In a fourth control interval I<NUM>, having a fourth duration T<NUM>, only the induction heater that was inactive in the second control interval I<NUM> and in the third control interval I<NUM>, i.e. the second inducting heater <NUM>, is energized. For this purpose, the control unit <NUM> operates the second power switch <NUM> at a fourth switching frequency fSW4.

In a fifth control interval I<NUM>, having a fifth duration T<NUM>, the control unit <NUM> activates only the induction heater that had been already activated during the fourth control interval I<NUM>, i.e. the second induction heater <NUM>, by operating the second power switch <NUM> with a fifth switching frequency fsws. The fifth switching frequency fsws is different from and preferably greater than the fourth switching frequency fSW4.

During each of the control intervals I<NUM>-I<NUM>, the control unit <NUM> measures respective values of delivered power on the basis of the power sense signals Ssv, SSC1, SSC2 continuously received from the power detector <NUM>. In particular, the control unit <NUM> uses the values of overall delivered power measured in control intervals I<NUM>-I<NUM> to determine a first power characteristic PC<NUM> of the first inductive heater <NUM> with the cooking vessels <NUM> coupled thereto and a second power characteristic PC<NUM> of the second inductive heater <NUM> with the cooking vessels <NUM> coupled thereto, as illustrated in <FIG> (here, the power characteristic are represented as switching period τSW vs. delivered power P, where τSW = <NUM>/fsw, obviously). The power characteristics PC<NUM>, PC<NUM> may be determined e.g. by linear interpolation, due to the fact that each of the inductive heaters <NUM>, <NUM> is individually energized at two different switching frequencies during control intervals I<NUM>-I<NUM>. Other methods of interpolation may be used as well, in accordance with design preferences. More precisely, the first inductive heater <NUM> is operated (alone, with the second inductive heater <NUM> inactive) at the second switching frequency fSW2 in the second control interval I<NUM> and at the third switching frequency fSW3 in the third control interval I<NUM>. The control units acquires and stores two power measures P<NUM>(fSW2), P<NUM>(fSW3) associated with operation of the first inductive heater <NUM> alone and defines two reference characteristic points (τSW2; P<NUM>(fSW2)), (TSW3; P<NUM>(fSW3) ). Likewise, the second inductive heater <NUM> is operated (alone, with the first inductive heater <NUM> inactive) at the fourth switching frequency fSW4 in the fourth control interval I<NUM> and at the fifth switching frequency fsws in the fifth control interval I<NUM>. The control units acquires and stores two power measures P<NUM>(fSW4), P<NUM>(fSW5) associated with operation of the second inductive heater <NUM> alone and defines two reference characteristic points (τSW4; P<NUM>(fSW4)), (τSW5; P<NUM>(fSW5)). Advantageously, the third switching frequency fSW3 and the fifth control interval I<NUM> are selected as far away as allowed by operative limits of the power switches <NUM>, <NUM> from the second switching frequency fSW2 and from the fourth switching frequency fSW4, respectively. Thus, the power characteristics PC<NUM>, PC<NUM> may be continuously updated at every control period I<NUM> without discontinuities in delivering power to the cooking vessels.

As a general rule, most of the required power for the control period I<NUM> is delivered in the first control interval I<NUM>, in which both the inductive heaters <NUM>, <NUM> are energized through the respective power switches <NUM>, <NUM>, and in the second control interval I<NUM>, in which only the inductive heater <NUM>, <NUM> expected to deliver the highest power is energized (in the example of <FIG>, the first inductive heater <NUM> through the first power switch <NUM>). The durations of the first control interval I<NUM> and of the second control interval I<NUM> and the first switching frequency fswi and second switching frequency fSW2 are selected to approximate overall power delivery requirements.

The remaining control intervals I<NUM>-I<NUM> should be selected as short as possible, yet long enough to accurately and consistently determine measurements of overall delivered power. The switching frequencies fSW3-fSW5 are selected to complete the power delivery tasks of the induction heaters <NUM>, <NUM>.

More specifically, the parameters to meet user's request of power delivery may be determined based on the following procedure.

A first power target P<NUM>' for the first induction heater <NUM> and a second power target P<NUM>' for the second induction heater <NUM> are set by a user and indicate the average power to be delivered over each control period I<NUM>.

The first power target P<NUM>' for the first induction heater <NUM> and the second power target P<NUM>' are related to the durations T<NUM>-T<NUM> and to the switching frequencies fSW1-fSW5 as follows <MAT> <MAT> with the constraint <MAT>.

Additional constraints allow to determine the durations T<NUM>-T<NUM> and the switching frequencies fSW1-fSW5.

First, the durations T<NUM>-T<NUM> are selected to be as short as possible, provided that accurate measurement of the delivered power can be carried out. For example, the durations T<NUM>-T<NUM> may be from one up to <NUM> mains half-cycles and are all equal in one embodiment, e.g. <NUM>, corresponding to <NUM> mains half cycles in a <NUM> mains line.

Then, in order to cope with symmetry requirements on mains current drained by household appliances, the control period I<NUM> may be set to an odd number of half-cycles of the AC supply voltage VAC received from the voltage supply line <NUM>. The current symmetry is thus re-established every two control cycle durations in the worst case. Moreover, the control period I<NUM> is selected to be smaller than a thermal time constant of the cooking vessels <NUM>, <NUM>, whereby power delivery is smooth. In one embodiment, the control period I<NUM> may be <NUM>.

The third switching frequency fSW3 (higher than the second switching frequency fSW2) and fifth third switching frequency fSW5 (higher than the fourth switching frequency fSW4) are selected in a respective upper operative frequency ranges, which are delimited by respective lower limit frequencies and by respective upper operative limits. The upper operative limits define the highest switching frequencies at which the power switches <NUM>, <NUM> may be safely and correctly operated. In the upper operative frequency ranges, minimum power is delivered to the inductive heaters <NUM>, <NUM> through the power switches <NUM>, <NUM>.

The fourth switching frequency fSW4 is selected in a lower operative frequency range, which is delimited by an upper limit frequency, lower than the lower limit frequency of the upper operative frequency range of the second inductive heater <NUM>, and by a lower operative limit. The lower operative limit defines the lowest switching frequency at which the power switches <NUM>, <NUM> may be safely operated, without incurring in failure e.g. because of switch voltage breakdown or thermal runaway. In the lower operative frequency range, maximum power is delivered to the inductive heaters <NUM>, <NUM> through the power switches <NUM>, <NUM>.

Solutions for the remaining parameters (first duration T<NUM>, first switching frequency fSW1, second switching frequency fSW2; the second duration T<NUM> is immediately determined from equation (<NUM>) once a value for the first duration T<NUM> has been selected) may be determined with a view of optimizing operation of the induction cooktop <NUM> in other aspects, e.g. flickering, power loss, component stress and the like. For example, as the first duration T<NUM> and the second duration T<NUM> are bound by equation (<NUM>) once the third duration T<NUM>, the fourth duration T<NUM> and the fifth duration T<NUM> have been set. A pair of values of the first switching frequency fSW1 and of the second switching frequency fSW2 that best fits an optimization criteria (e.g. minimization of flickering) is selected and the corresponding first duration T<NUM> is determined. The second duration T<NUM> is determined from equation (<NUM>).

The above procedure is carried out at least when both the induction heaters <NUM>, <NUM> are in use and a power target is above a programmed minimum power threshold. The power target is set by the user through the user interface <NUM> and possibly adjusted based on the actual coupling of the vessels <NUM>, <NUM> with the respective induction heaters <NUM>, <NUM>. When the power target is below the minimum power threshold, normally relatively low, a different control procedure may be used. For example, in the first control interval I<NUM> only one or none of the induction heaters <NUM>, <NUM> may be activated and the programmed first duration T<NUM> and delivered power may be determined from stored rated data. However, any suitable control procedure may be used.

Selection of parameters may be carried out quickly and the selected parameters are readily available. In one embodiment, all the selected parameters are kept constant through subsequent cycles, until power transfer conditions change (e.g. because the user changes cooking settings or a cooking vessel is moved with respect to induction heaters <NUM>, <NUM>) and the power characteristics PC<NUM>, PC<NUM> are updated.

In another embodiment, the control unit <NUM> adjusts the switching frequencies fSW2-fSW5 in subsequent control periods I<NUM>. Specifically, the third switching frequency fSW3 and the fifth switching frequency fSW5 are initially set at respective safe values in the upper operative frequency range, relatively far away from the upper operative limit, and then the control unit <NUM> adjusts the selected values in accordance with actual operating conditions. For example, the third switching frequency fSW3 and the fifth switching frequency fSW5 may be increased (by decreasing the turn-on time) until the onset of hard-switching conditions or decreased (by increasing the turn-on time) if the voltage on conduction terminals of the power switches <NUM>, <NUM> is zero at turn-on, thus corresponding to a perfect soft switching condition.

In a specular manner, the second switching frequency fSW2 and the fourth switching frequency fSW4 are initially set at respective safe values in the lower operative frequency range, relatively far away from the lower operative limit, and then the control unit <NUM> adjusts the selected values in accordance with actual operating conditions. Thereafter, the second switching frequency fSW2 and/or the fourth switching frequency fSW4 may be decreased if the operative limits of the power switches <NUM>, <NUM> are sufficiently distant or otherwise increased if the operative limits are being approached. For example, in the quasi resonant converter configuration of <FIG>, the maximum power the power switches <NUM>, <NUM> may deliver is limited by the breakdown voltage VBD of the power switches <NUM>, <NUM> themselves. Considering a derating margin of <NUM>, the control unit <NUM> may increase the second switching frequency fSW2 and/or the fourth switching frequency fSW4 if the measured voltage across the conduction terminals of the active power switches <NUM>, <NUM> becomes greater than <NUM>*VBD. On the other hand, if the measured voltage across the conduction terminals of the active power switches <NUM>, <NUM> goes below <NUM>*VBD, the control unit <NUM> may decrease the second switching frequency fSW2 and/or the fourth switching frequency fSW4.

The adjustment of the switching frequencies fSW2-fSW5 may be carried out either during control intervals I<NUM>-I<NUM> of each control period I<NUM> or between subsequent control periods I<NUM>.

The induction cooktop and the method described above has several advantages. Besides avoiding audible noise, because the inductive heaters are never simultaneously energized with different switching frequencies, the cooktop is operated in conditions that allow to define power characteristics at every control period without discontinuing power supply to cooking vessels coupled to the inductive heaters. In fact, in each control cycle both the inductive heaters are separately and individually operated with two respective different frequencies in distinct control intervals. This allows to determine the power characteristics of the converter by measuring the overall active power in each of the control intervals in which only one of the power switches is activated and by simply interpolating the measured power values. Thus, the power characteristics may be frequently updated, possibly even at every control period, without the need to perform a frequency scan. On the one side, therefore, frequent updates do not affect power delivery and, on the other side, consistency of the power characteristics may be accurately maintained, to the advantage of efficiency and quality of the cooking process.

Although other solutions are available within the scope of the invention, the overall power delivered by the converter may be measured using extremely simple and cheap sensors. Even a single resistor is perfectly fit to fulfil the task of providing continuous monitoring of power delivery and measurement for the purpose of determining the power characteristics.

The quasi resonant configuration of the converter is particularly advantageous. Quasi resonant converters are widely used as high frequency power supply for induction cooktops and proved to be particularly attractive as being structurally simple and inexpensive, because a single solid state power switch (typically an IGBT) and a single resonant capacitor are required for each induction coil. Quasi resonant converters are also very well suited to the above described control because of fairly linear relationship between delivered power and switching period. In fact, interpolation is simple and accurate, which is a favorable property to achieve good and efficient power control.

The converter need not be in quasi resonant configuration, however. In the embodiment of <FIG>, for example, where parts already described are indicated by the same reference numbers, an induction cooktop <NUM> the first induction heater <NUM>, the second induction heater <NUM> and a converter <NUM>, configured to couple to the supply line <NUM> through the coupling interface <NUM> and to independently energize the induction heaters <NUM>, <NUM>. The converter <NUM> comprises the rectifier <NUM>, the DC link capacitor <NUM>, a control unit <NUM>, a first switching current generator <NUM>, a second switching current generator <NUM> and a power detector <NUM>. The first switching current generator <NUM> and the second switching current generator <NUM> comprises two first power switches 117a, 117b and the second switching current generator <NUM> comprises two second power switches 118a, 118b in half-bridge configuration. Specifically, the first induction heater <NUM> forms a first resonant circuit <NUM> driven by the first switching current generator <NUM> with respective first resonant capacitors 122a, 122b and the second induction heater <NUM> forms a second resonant circuit <NUM> driven by the second switching current generator <NUM> with respective second resonant capacitors 123a, 123b.

The power detector <NUM> comprises a voltage sensing network <NUM> and current sensors 120b, 120c and supplies power sense signals, based on which the control unit <NUM> determines the active power delivered by the switching current generators <NUM>, <NUM>. The voltage sensing network 120a may include a voltage divider connected between the rails <NUM>, <NUM> and having an intermediate node coupled to a voltage sense input of the control unit <NUM> to provide a voltage sense signal Ssv. The current sensors 120b, 120c are configured to sense currents supplied by the switching current generators <NUM>, <NUM>, respectively, and to provide corresponding current sense signals SSC1, SSC2 to current sense inputs of the control unit <NUM>. The power sense signals supplied by the power detector <NUM> include the voltage sense signal Ssv and the current sense signals SSC1, SSC2.

The first switching current generator <NUM> and the second switching current generator <NUM> are operated by the control unit <NUM> at the switching frequencies fSW1-fSW5 in the control intervals I<NUM>-I<NUM> of each control period I<NUM>. For this purpose, the control unit <NUM> supplies first control signals SSW1', SSW1" to control terminals of the power switches 117a, 117b of the first switching current generator <NUM> and second control signals SSW2', SSW2" to control terminals of the second switching current generator <NUM>.

Finally, it is clear that modifications and variants can be made to the cooktop and to the method described herein without departing from the scope of the present invention, as defined in the appended claims.

Claim 1:
An induction cooktop comprising:
a first induction heater (<NUM>) and a second induction heater (<NUM>);
a control unit (<NUM>; <NUM>);
a first switching current generator (<NUM>; <NUM>) and a second switching current generator (<NUM>; <NUM>), operable by the control unit (<NUM>) in subsequent control periods (I<NUM>) to energize the first induction heater (<NUM>) and the second induction heater (<NUM>), respectively;
wherein the control unit (<NUM>; <NUM>) is configured to:
at least when a power target to be delivered is above a programmed minimum power threshold, operate both the first switching current generator (<NUM>; <NUM>) and the second switching current generator (<NUM>) with a first switching frequency (fSW1) in a first control interval (I<NUM>) of each control period (I<NUM>) characterised in that the control unit is further configured to:
operate only the first switching current generator (<NUM>; <NUM>) with at least two respective different switching frequencies (fSW2, fSW3) in a second control interval (I<NUM>) and in a third control interval (I<NUM>) of each control period (I<NUM>), while the second switching current generator (<NUM>; <NUM>) is inactive;
operate only the second switching current generator (<NUM>; <NUM>) with at least two respective different switching frequencies (fSW4, fSW5) in a fourth control interval (I<NUM>) and in a fifth control interval (I<NUM>) of each control period (I<NUM>), while the first switching current generator (<NUM>; <NUM>) is inactive.