This application claims priority to European Patent Application No. 22159840.2, filed on 2 Mar. 2022, entitled “METHOD OF CONTROLLING A SWITCHING CONVERTER AND RELATED INDUCTION COOKTOP,” the disclosure of which is hereby incorporated herein by reference in its entirety.
The present disclosure relates to voltage converters and more in particular to methods of controlling a switching converter usable for realizing induction cooktops and for induction heating items of cookware placed above a heating coil powered by the switching converter.
Some cooking appliances, in particular induction cooking appliances, have at least one main switching converter to supply induction heating elements with a supply voltage through actuation of a main energy supply. However, in some instances, (i) a ticking acoustic noise from the cookware is produced, and (ii) a suboptimal amount of dissipated power could be produced.
At least part of these drawbacks are overcome according to this disclosure by purposely not charging a direct current (DC)-bus capacitor during an OFF interval of the switching converter such that the DC-bus capacitor is substantially discharged when a new ON interval of the switching converter is started. For simplicity of language, this disclosure refers to a “DC-bus capacitor.” However, it should be understood that multiple DC-bus capacitors could be utilized instead of a single DC-bus capacitor.
According to an aspect, a switching converter of this disclosure may have a rectifier stage (Rect) comprising silicon controlled rectifiers (SCRs) and may have a control line that senses zero-crossing conditions of an alternating current (AC) input voltage and turns on the SCRs of the rectifying stage only when power is to be supplied to the load.
The present disclosure sets forth two technical advantages: (i) it eliminates the occurrences of the ticking noise, thus resulting in a more pleasing experience for the user; and (ii) it reduces the power dissipation on a controlled output switch S.
The disclosed methods may be used for controlling any kind of switching converters suitable for powering a heating coil of an induction cooktop and are not univocally destined for controlling quasi-resonant (QR) converters.
Switching converters adapted for induction cooktops and a related induction cooktop are also disclosed.
In the Drawings:
Referring to
The switching converters 10 and 10A illustrated in
(A) a rectifier stage Rect having input AC terminals 12 for receiving an AC voltage 14 to be rectified, and a DC-bus having a high-side line for making available a DC voltage on the DC-bus between the high-side line and the low-side line, wherein the DC voltage is generated as a rectified replica of the AC voltage 14 received at the input AC terminals 12;
(B) an output stage 16 connected between the high-side line and the low-side line configured to be supplied with the DC voltage on the DC-bus, comprising:
The QR converter of
Like the QR converters (e.g., switching converters 10 and 10A), other switching converters (e.g., switching converters 10B-10D) suitable for being used in induction cooktops 20 include the rectifier stage Rect, the output stage 16 with at least one inductor L (optionally the output stage 16 may include also a tank capacitor C connected to the inductor to form a L-C resonant pair), and at least one controlled output switch S connected between the intermediate node and either the low-side line or the high-side line, configured to connect or disconnect the inductor L to the DC-bus for being charged by the switching converter or for being discharged.
A drawback of this kind of converter lies in the range of output power being achievable in the Soft-Switching regime. In particular, when the output power being regulated falls below a given limit, the QR converter fails to operate in soft switching mode, leading to an increase in thermal losses (hard switching) and electromagnetic interference. Those limitations can limit the regulation range, which is defined as a ratio between maximum achievable power (limited by maximum voltage across the controlled output switch S) and the minimum achievable power (limited by the losses for hard switching at turn on). This situation can be an issue when the user wants to supply low power to cookware 22.
One mode for overcoming the aforementioned limitation for switching converters 10-10D is to operate the inverter in the so-called burst-mode or ON-OFF mode (as shown in
For example, if a user wants to supply 100 W and the minimum power to have soft switching is 700 W, the system can operate in the above described ON/OFF mode, in which the bursts at 700 W are supplied for a short time interval T1, and for the remainder of the time interval T2 the switching converter is kept off, to obtain an average power delivery of 100 W. However, as alluded to above, this can cause the cookware 22 to produce the ticking noise, and the output switch S to dissipate power. For example, this can happen when the DC-bus capacitance is charged at the peak of the rectified mains line voltage (e.g., 325 V for nominal RMS line voltage of 230 V), and the controlled output switch S is turned on for the first time after a long period of being kept off. For example, if the switching converter is kept inactive for at least 10 ms, the first time it is turned on again after that period has elapsed this situation will occur. In those same conditions, there is also a large amount of power dissipation occurring in the output switch S, the so-called hard-switching condition. Obviously, this problem also occurs every time the output switch S is turned ON for the first time, after the main relay closure.
Another situation where ticking noise occurs is during the so-called “pan detection” operation, that is when the presence of cookware 22 on an induction heating cooktop is detected. The detection of the cookware 22 can be accomplished by feeding power to the induction heating coils and by assessing at least an electrical parameter of the QR converters of the induction heating cooktop 20 which is modified when the cookware 22 is placed on one or more induction coils. For example, a cookware 22 detection operation is to stimulate the induction coils with short PWM pulses and record the value of the electrical parameter of the converter. If the cookware 22 detection is operated when the DC-bus capacitor is charged, this could cause the noise to be produced.
Tests carried out by the Applicant have shown that the same drawback occurs also in induction cooktops 20 with different types of switching converters, other than QR converters. Therefore, it is believed that the above issues are not tied to the topology of QR converters but it affects any type of switching converter.
A switching converter 10-10D, that may be for example the QR converters depicted in
The switching converters 10A, 10C of
In the shown embodiments, there is also an optional capacitor C connected to the inductor L in order to form a L-C resonant pair. In general, the capacitor C may be omitted in all those topologies of switching converters 10A, 10C that do not require a L-C resonant pair.
According to an aspect, the start of each ON time interval T1 is set to approximately coincide with the zero crossing of the mains line voltage, as detected by a zero-cross detection block ZC DETECTION. A microcontroller MICRO CONTROLLER, which controls also the driver S DRIVER of the controlled output switch S (or SH, SL), receives a zero-crossing signal from the ZC DETECTION block and commands a driver of the rectifier stage RECT DRIVER, which drives the controlled rectifier stage Rect in order to turn it on, so that the DC-bus capacitor is charged. When an OFF time interval T2 begins, the controlled rectifier stage RECT DRIVER is turned off by the driver RECT DRIVER in order to disconnect the DC-bus from the supply line, preventing the DC-bus capacitor from being charged and thus keeping null or negligible the DC voltage available thereon.
Preventing the capacitance of the DC-bus from being charged before powering the load is an efficient technique for avoiding the generation of the ticking noise when the switching converter 10-10D is operated by periodically alternating ON time intervals T1, during which the load coupled with the inductor L is powered by the switching converter 10-10D to OFF time intervals T2, during which the load is not powered by the switching converter 10-10D and the inductor L is left discharging. The ticking noise is typically generated when a switching converter is operated to deliver an average power smaller than the minimum power that can be delivered to the load while maintaining the controlled output switch S, SH, SL in soft-switching operation.
In these situations, shown for example in
According to an exemplary aspect of this disclosure, the switching converter 10B, 10D may be realized as shown in
The other components are as in
Such a controlled rectifier stage Rect can be realized using controlled components such as, for example, SCRs, gate turn-off thyristors (GTOs), solid state transistors, relays, insulated gate bipolar transistors (IGBTs), and other components. Most advantageously, the controlled rectifier stage Rect is implemented using two solid state diodes and two SCRs, as shown in
The start of each ON time interval is set to approximately coincide with the zero crossing of the mains line AC voltage 14, as detected by the ZC DETECTION block. At this time, the system will activate the SCR DRIVER circuit in order to turn on the SCRs SCR1 and SCR2, so that the voltage rectifier Rect behaves as a traditional diode bridge rectifier, and the DC-bus is charged. This operation can be repeated for every zero-crossing event occurring during the ON time interval.
Due to the normal operation of the switching converter 10-10D, after the peak of the mains line AC voltage 14, the DC-bus capacitor will naturally be discharged, in a manner well known in the art, until the voltage across the capacitance Cbus of the DC-bus reaches almost zero in the vicinity of the subsequent zero crossing of the mains line AC voltage 14.
The beginning of each OFF time interval is also set to approximately coincide with the zero crossing of the mains line AC voltage 14, as detected by the ZC DETECTION block.
Starting from the beginning of an OFF time interval, and for the whole duration of said
OFF time interval, the SCRs of the rectifier stage Rect are kept off and thus the DC-bus capacitor cannot charge: when the subsequent ON time interval begins, the DC voltage on the DC-bus—and thus on the current terminals of the controlled output switch S, SH, SL—is practically null and a soft-switching or a “low hard-switching” is performed. This avoids charging the DC-bus capacitance CBus in the time intervals where the system does not intend to deliver power to the load (e.g., the cooking implement). In fact, what is described in this aspect of the present invention is not a method for discharging the DC-bus capacitance Cbus but it is a method for not charging it at all when not needed. However, this method cannot be used for silent cookware detection (because the energy of the cookware detection pulse is not enough to discharge the capacitance Cbus) but only at the beginning and during the power delivery. The rectifier stage in this case may be formed by two discrete low side diodes D1, D2 and two discrete high side controlled switches, for example the SCRs labeled SCR1, SCR2, that are turned on by the respective gate trigger signals G1, G2. Referring to the exemplary time graphs of
The switching converters 10 presented above may be used to realize an induction cooktop 20, for heating an item of cookware 22, by using an induction heating coil as the inductive component L depicted in the
Number | Date | Country | Kind |
---|---|---|---|
22159840.2 | Mar 2022 | EP | regional |