Method for the inductive heating of a cooking vessel and inductive heating device for inductive heating of a cooking vessel

Abstract

The invention relates to a method for the inductive heating of a cooking vessel, having the steps of: - rectifying an AC mains voltage in order to generate an intermediate circuit voltage, - applying a pulse-width modulated control signal to an induction heating coil to generate an alternating magnetic field, wherein the pulse-width modulated control signal is generated from the intermediate circuit voltage by means of an inverter and has an adjustable duty cycle (δ) and / or an adjustable frequency (f), and - setting the duty cycle (δ) and / or the frequency (f) of the pulse-width modulated control signal in such a way that successive time averages (I in ) of an input current (iin) of the inverter, formed over a respective period of the pulse-width modulation, have a predeterminable time characteristic (V).

Description

[0001] Method for inductively heating a cooking vessel and induction heating device for inductively heating a cooking vessel

[0002] CN 1 08 738 179 A discloses a method for inductively heating a cooking vessel, comprising the steps of: rectifying an AC mains voltage to generate an intermediate circuit voltage and applying a pulse-width-modulated control signal to an induction heating coil to generate an alternating magnetic field. The pulse-width-modulated control signal is generated from the intermediate circuit voltage by means of an inverter and has an adjustable duty cycle. The duty cycle of the pulse-width-modulated control signal is varied to form an interference spectrum during a heating interval based on precalculated patterns, wherein an average value of the various pulse widths corresponds to a pulse width that corresponds to a predetermined current intensity and thus to a predetermined heating power.

[0003] The invention is based on the object of providing a method and a device for inductively heating a cooking vessel that cause as little disruption as possible. The invention also relates to an induction heating device for inductively heating a cooking vessel.

[0004] The method according to the invention is used for inductive heating of a cooking vessel.

[0005] The method comprises the following steps: rectifying a particularly single-phase AC mains voltage to generate an intermediate circuit voltage, and applying a pulse-width-modulated control signal to an induction heating coil to generate an alternating magnetic field. The pulse-width-modulated control signal is generated from the intermediate circuit voltage by means of an inverter and has an adjustable duty cycle and / or an adjustable (switching) frequency. The inverter can be any inverter, for example, a so-called single-transistor inverter, a half-bridge inverter, etc. Reference is also made to the relevant specialist literature in this regard.

[0006] According to the invention, the duty cycle and / or the frequency of the pulse-width modulated control signal is / are set such that successive time averages of an input current of the inverter formed over a respective period of the pulse width modulation have a predeterminable time profile.

[0007] The temporal progression is achieved or brought about in particular by adjusting the duty cycle and / or the frequency. The temporal progression can be brought about, for example, by means of a closed-loop control. The controlled variable of the closed-loop control is the time average of the inverter's input current formed over a period of pulse width modulation. The manipulated variable(s) of the closed-loop control is / are the duty cycle and / or the frequency of the pulse-width modulated control signal. The predeterminable time progression forms a time setpoint curve of the closed-loop control. In other words, the successive time averages of the inverter's input current formed over a respective period of pulse width modulation are regulated to the predeterminable time (setpoint) curve, with the duty cycle and / or the frequency of the pulse-width modulated control signal serving as the manipulated variable(s).

[0008] In one embodiment, the predeterminable temporal profile corresponds to a sinusoidal half-oscillation, the duration of which corresponds to the duration of a sinusoidal half-oscillation of the AC mains voltage. The duration can be, for example, 10 ms at a mains frequency of 50 Hz and 8.33 ms at a mains frequency of 60 Hz.

[0009] In one embodiment, a frequency spectrum of the temporal profile at integer multiples of the frequency of the AC mains voltage has values ​​that are below associated predeterminable threshold values. The threshold values ​​can, in particular, correspond to threshold values ​​that comply with a standard regarding permissible interference emissions.

[0010] In one embodiment, the predeterminable temporal profile has a fundamental oscillation amplitude which is dependent on a heating power setpoint.

[0011] In one embodiment, the duty cycle and / or the frequency of the pulse-width modulated control signal is / are changed during a sinusoidal half-oscillation of the AC mains voltage in order to generate the predeterminable time profile.

[0012] The induction heating device according to the invention is used for inductively heating a cooking vessel and comprises: an induction heating coil, a rectifier for rectifying a mains AC voltage to generate an intermediate circuit voltage, an inverter fed from the intermediate circuit voltage for generating a pulse-width modulated control signal, wherein the induction heating coil is supplied with the pulse-width modulated control signal, and a control unit which is designed to control the operation of the induction heating device in such a way that a method according to one of the preceding claims is carried out.

[0013] The invention is described in detail below with reference to the drawings.

[0014] Here: Fig. 1 shows a schematic diagram of an induction heating device,

[0015] Fig. 2 schematically shows a time course of a pulse-width modulated control signal of an induction heating coil of the induction heating device shown in Fig. 1,

[0016] Fig. 3 shows a time profile of an input current of an inverter of the induction heating device shown in Fig. 1, a time profile of a duty cycle and a frequency of the pulse-width modulated control signal of Fig. 2, and a time profile of time average values ​​of the input current of the inverter,

[0017] Fig. 4 shows a spectrum of the time course of the time average values ​​of the input current of the inverter, and

[0018] Fig. 5 is a block diagram of a control loop for setting the switching frequency and input current.

[0019] Fig. 1 shows an induction heating device 100, for example as a component of an induction hob, for inductively heating a cooking vessel 200.

[0020] The induction heating device 100 conventionally comprises: an induction heating coil L1, a rectifier 3 for rectifying a mains AC voltage UN for generating an intermediate circuit voltage U in , one from the intermediate circuit voltage U infed inverter 2 for generating a pulse width modulated control signal AS with adjustable duty cycle 5 and adjustable frequency f, see also Fig. 2 and 3, wherein the induction heating coil L1 is supplied with the pulse width modulated control signal AS, and a control unit 4, for example in the form of a microcontroller, which is designed to control the operation of the induction heating device 100 as described below.

[0021] Referring to Fig. 3, the duty cycle 5 of the pulse-width modulated control signal AS is set such that successive time averages I formed over a respective period P of the pulse width modulation in an input current i in of the inverter 2 have a predeterminable temporal profile V which essentially corresponds to a sinusoidal half-oscillation, the duration of which corresponds to a duration of a sinusoidal half-oscillation of the AC mains voltage UN.

[0022] Elements L1 and R15 from Fig. 1 form an equivalent circuit for the induction heating coil and a pot. V3 is a mains voltage source. C11 and C12 are resonant circuit capacitors. S5, D5, and S6 and D6 correspond to an IGBT half-bridge. O2 is an intermediate circuit capacitor. D1-D4 form the mains voltage rectifier 3. L4, R3, O3, and L2 are a mains input filter. The IGBT half-bridge 2 is controlled via a PWM voltage, which conventionally drives a gate driver 5.

[0023] Referring to Fig. 4, a frequency spectrum S of the time profile V at integer multiples of the frequency fO = 50 Hz of the AC mains voltage UN has values ​​that are below the associated predeterminable threshold values ​​SO, S2 or S3.

[0024] The predeterminable time profile V has a fundamental oscillation amplitude which depends on a heating output setpoint Psoll.

[0025] To set the desired power Psoll of the induction heating device 100, the switching frequency and duty cycle of the semiconductor switches S5 and S6 (IGBT half-bridge) of the inverter 2 are specified by the control unit 4 such that the pulse-width modulated control signal AS has the desired duty cycle 5 and the desired frequency f.

[0026] In the prior art, the control unit 4 records the actual power value and adjusts the frequency f and / or duty cycle 5 until the target and actual values ​​match. However, to meet EMC requirements, for example in the form of requirements regarding interference voltages and mains harmonics, the choice of switching frequency must be restricted. Firstly, depending on the power, modulation of the switching frequency is necessary, for example in the form of a frequency wobble or frequency hopping. Furthermore, in some cases, a limitation of the permissible frequency value range may also be necessary; for example, the switching frequency must not be selected higher than 25 kHz if a power threshold is exceeded. In particular, the switching frequency modulation, but also the cookware used, cause an undesirable, increased harmonic spectrum in the mains current. These mains harmonics must not exceed a permissible value range (EN61000-3-2).These constraints result in a lengthy "trial and error" adaptation of the software of the control unit 4 in the current state of the art, which requires a considerable amount of time during the development process. Furthermore, it cannot be guaranteed in every case that the grid harmonic requirements will be met, as this would require testing all available cookware.

[0027] According to the invention, not switching frequency f and duty cycle <5, but switching frequency f and mean value / In of the input current i in or alternatively duty cycle 5 and mean value / £ “of the input current i in specified by the control unit 4. The control unit 4 still records the actual power value and adjusts the switching frequency f (or alternatively the duty cycle 5) and the fundamental oscillation amplitude of the mean value I in of the inverter input current i inuntil the setpoint and actual values ​​match. By specifying the input current waveform V, compliance with the EMC standard for mains harmonics (EN61000-3-2) can be directly ensured. Complex software adaptation to different cookware is no longer necessary.

[0028] For a controlled influence of the network harmonics (EN61000-3-2), the input current i in of the inverter 2. However, since only the harmonics of the mains frequency are to be controlled, not the harmonics of the switching frequency, it is sufficient to set the mean value I in of the input current i in during a switching period P (quasi-static approximation).

[0029] The average value of the input current corresponds to the integral of the input current i in over a switching period P, divided by the switching period duration. Where T s5the duty cycle of the switch S5 below and T the switching period. By appropriately selecting the duty cycle T s5 or duty cycle 5, the mean value I in of the inverter input current i in The publication Claudio Adragna (2022), "LLC Resonant Converters: An Overview of Modeling, Control and Design Methods and Challenges", Foundations and Trends® in Electric Energy Systems: Vol. 5: No. 2-4, pp. 75-491, provides an example of how such an analog current control can be designed.

[0030] The duty cycle is set as shown in Fig. 3 in such a way that the temporally successive mean values ​​I in of the input current i inof the inverter 2 essentially have a temporal profile V of a sinusoidal half-wave, the duration of which corresponds to the duration of a sinusoidal half-wave of the AC mains voltage UN. The fundamental oscillation amplitude or the maximum value of the predeterminable temporal profile V is set as a function of the heating output setpoint Psoll.

[0031] The control of the inverter input current i in allows two degrees of freedom: the switching frequency f and the input current waveform V. These can be obtained in very different ways. Fig. 5 shows a possible design that is cascaded as a higher-level control loop with the inverter input current control described above with a duty cycle of 5 as the manipulated variable.

[0032] The starting point is the setpoint of the input current f n(t). This consists of a spectrum of frequency components that are transformed into the time domain using an inverse Fourier transform (iFFT). The fundamental oscillation of the input current i 50 Hz (50 Hz component) is calculated by multiplying the inverse of the measured input voltage l / u;n and the desired input power P so u. The remaining spectral components in the range above 50 Hz result from the desire for a duty cycle <5(t) that is as constant as possible over time. To achieve this, the temporal profile of the duty cycle <5(t) is decomposed into its spectrum 0(f) (FFT). The frequency components above 50 Hz are controlled to zero by a PI controller using harmonic injection. However, before the necessary harmonic spectrum can be applied to the output, it must be limited by the specifications of EN61000-3-2.

[0033] The switching frequency results from the desired duty cycle 5 of close to 50%. The duty cycle setpoint 8 soU However, it must be slightly below the maximum value of 50% (e.g., 47%). The deviation between the mean value of the duty cycle signal D( / ) |O HZ and the setpoint is compensated using a PI controller by adjusting the switching frequency f SiDC to zero. If the switching frequency is to be modulated additionally, the desired alternating component f SiAC be added to the output of the switching frequency controller.

Claims

Patent claims 1 . Method for inductively heating a cooking vessel (200) comprising the steps of: rectifying an alternating mains voltage (UN) to generate an intermediate circuit voltage (U in ), and Applying a pulse-width-modulated control signal (AS) to an induction heating coil (L1) to generate an alternating magnetic field, wherein the pulse-width-modulated control signal (AS) is generated by means of an inverter (2) from the intermediate circuit voltage (U in ) and has an adjustable duty cycle (5) and / or an adjustable frequency (f), characterized by the steps: Adjusting the duty cycle (5) and / or the frequency (f) of the pulse-width modulated control signal (AS) in such a way that successive time averages (I in ) of an input current (i in) of the inverter (2) have a predeterminable time profile (V).

2. Method according to claim 1, characterized in that the predeterminable time profile (V) corresponds to a sinusoidal half-oscillation, the duration of which corresponds to a duration of a sinusoidal half-oscillation of the mains alternating voltage (UN).

3. Method according to one of the preceding claims, characterized in that a frequency spectrum (S) of the time profile (V) at integer multiples of the frequency (fO) of the AC mains voltage (UN) has values ​​which are below associated predeterminable threshold values ​​(S0, S2, S3).

4. Method according to one of the preceding claims, characterized in that the predeterminable time profile (V) has a fundamental oscillation amplitude which is dependent on a heating power setpoint (Psoll).

5. Method according to one of the preceding claims, characterized in that during a sinusoidal half-oscillation of the AC mains voltage (UN) the duty cycle (5) and / or the frequency (f) of the pulse-width modulated control signal (AS) is / are changed in order to generate the predeterminable time profile.

6. Induction heating device (100) for inductively heating a cooking vessel (200), comprising: an induction heating coil (L1), a rectifier (3) for rectifying an alternating mains voltage (UN) to generate an intermediate circuit voltage (U in ), one from the intermediate circuit voltage (U in) fed inverter (2) for generating a pulse width modulated control signal (AS), wherein the induction heating coil (L1) is supplied with the pulse width modulated control signal (AS), and a control unit (4) which is designed to control the operation of the induction heating device (100) such that a method according to one of the preceding claims is carried out.

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