JP4045138B2 - Induction heating cooker - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to control means for an induction heating cooker.
[0002]
[Prior art]
The conventional induction heating cooker is recognized as a heat source that replaces a tabletop gas table and the like because it does not generate an open flame, can control the temperature of the load, and is highly safe. In addition, as an electric cooker incorporated in a system kitchen or the like, conventionally, a resistance source such as a sheathed heater, a plate heater, or a halogen heater is used as a heat source, and one mouth is replaced with induction heating, or two or more are induced. It is being replaced with a cooker.
[0003]
After the induction heating cooker converts the AC power source to the DC power source, it is turned on and off by the switching element to convert it to a high frequency current, and this high frequency current is passed through the heating coil, and is placed in the vicinity due to the electromagnetic induction effect by the generated magnetic flux. A current is generated in a metal load, and the current is heated by self-heating.
[0004]
The operating frequency is about 20 kHz to about 40 kHz, and a current of about 60 A to 80 A flows through the heating coil at the peak. Since such a large current is turned on / off at a high frequency, a noise component induced in the power supply becomes large, which may cause problems such as malfunction of other peripheral devices.
[0005]
In order to cope with this problem, conventionally, consideration has been given to reduce the noise component, for example, by inserting a filter combining a normal mode coil, a common mode coil, and the like with a capacitor into the AC power supply side.
[0006]
Such a conventional example will be described with reference to the drawings. FIG. 6 is a circuit block diagram of a conventional example, FIG. 7 is a diagram showing signal waveforms of main parts in the conventional example, and FIG. 8 is a diagram showing the distribution of noise components in the control method of the conventional example.
[0007]
In FIG. 6, an AC power source 1 is rectified by a rectifier circuit 3 through a filter 2 and converted to a DC power source. The inverter 9 includes a resonance circuit 6 including a heating coil 5 and a resonance capacitor 4, and a switching element 8 including an upper arm switching element 8a and a lower arm switching element 8b. The DC power source is converted into a high frequency current by the switching element 8. Then, it is supplied to the resonance circuit 6 and the load 7 disposed in the vicinity is heated.
[0008]
The control unit 11 sets the duty signal according to the heating power set by the user (not shown) in synchronization with the signal of the power supply synchronization signal generation unit 10 that generates a signal synchronized with the cycle of the AC power supply 1. The signal is output to the means 12 and the oscillation instruction signal is output to the oscillation means 14.
[0009]
The oscillation means 14 outputs an oscillation pulse at a constant frequency in response to the oscillation instruction signal, and the power setting means 12 outputs an on / off duty setting signal corresponding to the duty signal of the control means 11. The drive pulse generator 15 generates and outputs a pulse for driving the switching element 8, that is, the upper and lower arm switching elements 8a and 8b, in accordance with the on / off duty setting signal of the power setting means 12 in synchronization with the oscillation pulse of the oscillation means 14.
[0010]
With the above configuration, the operation of the main part of the circuit will be described with reference to FIG.
[0011]
In FIG. 7, waveform A is the oscillation pulse output from the oscillation means 14, waveform B is the duty signal output from the control means 11, waveform C is the on / off duty setting signal output from the power setting means 12, and waveforms Da and Db are up and down. The output pulse of the drive pulse generation means 15 which drives the arm switching elements 8a and 8b is shown. In FIG. 7, the horizontal axis is all time.
[0012]
The waveform A, that is, the oscillation pulse, always has a constant period, that is, a constant frequency. Waveform B, that is, the duty signal, shows three cases where the set values of the heating power are different, “Low” is a low power set value, “Mid” is a medium power set value, and “High” is a high power set value. Show.
[0013]
The waveform C, that is, the on / off duty setting signal is a signal corresponding to the setting value of the heating power of the duty signal. The drive pulses for the waveforms Da and Db, that is, the upper and lower arm switching elements 8a and 8b, are generated from the waveform A, that is, the oscillation pulse output from the oscillation means 14, and the waveform C, that is, the on / off duty setting signal output from the power setting means 12. The upper and lower arm switching elements 8a and 8b are turned on when the level is high and turned off when the level is low.
[0014]
When the user (not shown) sets the heating power and instructs the start of heating, the control means 11 synchronizes with the signal of the power supply synchronization signal generating means 10 and the duty signal (waveform) corresponding to the set heating power. B) is output to the power setting means 12 and the oscillation instruction signal is output to the oscillation means 14. In response to the oscillation instruction signal, the oscillation means 14 outputs an oscillation pulse (waveform A) at a constant frequency.
[0015]
The power setting means 12 outputs an on / off duty setting signal (waveform C) corresponding to the duty signal (waveform B). The drive pulse generation means 15 synchronizes with the oscillation pulse (waveform A) of the oscillation means 14 and pulses (waveform Da) for driving the upper and lower arm switching elements 8a and 8b in accordance with the on / off duty setting signal (waveform C) of the power setting means 12. , Db).
[0016]
The upper and lower arm switching elements 8a and 8b are turned on and off by this drive pulse (waveforms Da and Db), and the alternating current power source 1 converts the rectified and converted direct current power source into a high frequency current and supplies it to the resonance circuit 6 and is disposed in the vicinity. The load 7 is heated.
[0017]
In each case where the set value of the heating power is “Low”, “Mid”, and “High”, the on / off duty setting signal (waveform C) and the drive pulses for the upper and lower arm switching elements 8a and 8b (waveforms Da and Db) are shown in FIG. As shown, the signal waveform corresponds to each set value of the heating power. That is, as the set value of the heating power is changed from the low set value to the high set value, the ON time of the drive pulse for the upper arm switching element 8a (waveform Da) becomes longer, and the drive pulse for the lower arm switching element 8b (waveform Db). The on-time becomes shorter.
[0018]
At this time, the frequency of the drive pulses (waveforms Da and Db) for the upper and lower arm switching elements 8 a and 8 b is the same constant frequency as the oscillation frequency of the oscillation means 14. If this frequency is the fundamental frequency f0, the fundamental frequency f0 and harmonic noise components are induced in the power supply. This pattern is shown in FIG.
[0019]
In FIG. 8, the vertical axis represents noise component energy, and the horizontal axis represents frequency. As shown in the figure, since the ON / OFF frequency of the switching element 8 is always constant, the noise components are the fundamental frequency f0 and the harmonic 2f0 (a frequency twice the frequency of f0, the same applies hereinafter), 3f0, 4f0. In the figure, the energy after 5f0 is omitted). In order to reduce such a noise component, a filter 2 as shown in FIG. 6 is inserted on the power supply side.
[0020]
As conventional examples using filters, there are JP-A-6-275372 and JP-A-5-89955.
[0021]
[Problems to be solved by the invention]
As described above, the conventional induction heating cooker takes measures such as inserting the filter 2 on the power supply side in order to reduce the noise component induced in the power supply.
[0022]
However, as the output of the induction heating cooker increases, the current flowing through the filter 2 increases, and it is necessary to increase the number of turns of the coil constituting the filter 2 in order to attenuate the noise component. It is necessary to increase the capacitor capacity. Therefore, problems such as an increase in the mounting volume of the filter 2 on the product and an increase in manufacturing cost are incurred.
[0023]
Furthermore, since not only the fundamental frequency component of the operating frequency but also the noise of the harmonic component has high energy, there is a limit to the performance improvement of the filter 2 and the operation of the product itself becomes unstable, There is also a problem in operation reliability, such as adversely affecting the operation of other devices.
[0024]
The present invention solves the above-described problems, and aims to reduce the size and cost of a filter and further improve the operational reliability of the product itself and surrounding devices.
[0025]
It is another object of the present invention to realize a software configuration and a circuit configuration with a simple configuration.
[0026]
[Means for Solving the Problems]
In order to solve the above-described problems, the present invention provides a rectifier circuit that rectifies alternating current to convert it into direct current, converts the direct current into a high-frequency current by an upper arm switching element and a lower arm switching element, and a heating coil and a resonant capacitor. An inverter for heating a load disposed in the vicinity of the inverter, a power setting means for changing heating power by setting on / off duty of the upper arm switching element and the lower arm switching element, and the upper Oscillating means for determining the on / off frequency of the arm switching element and the lower arm switching element, and frequency changing means for changing and controlling the oscillation frequency of the oscillating means, the frequency changing means converts the oscillation frequency of the oscillating means to the fundamental frequency ( Approximate period in the range of frequency (f0 ± Δf) slightly changed up and down around f0) The power setting means corrects the on-time of the upper arm switching element to be longer than the on-time at the fundamental frequency f0 at the frequency (f0−Δf), The frequency (f0 + Δf) is corrected so as to be shorter than the on-time at the basic frequency f0 so that the heating power is substantially the same as that at the basic frequency f0 .
[0027]
DETAILED DESCRIPTION OF THE INVENTION
As described above, the present invention provides a rectifier circuit that rectifies alternating current into direct current, and converts the direct current into high-frequency current by an upper arm switching element and a lower arm switching element to form a resonance circuit including a heating coil and a resonant capacitor. An inverter that heats a load disposed in the vicinity of the inverter, power setting means that varies the heating power by setting on / off duty of the upper arm switching element and the lower arm switching element, and the upper arm switching element, Oscillating means for determining the on / off frequency of the lower arm switching element, and frequency changing means for changing and controlling the oscillation frequency of the oscillating means, the frequency changing means centering the oscillation frequency of the oscillating means around the fundamental frequency (f0). The change control is performed almost periodically in the range of the frequency (f0 ± Δf) slightly changed up and down. The power setting means corrects the on-time of the upper arm switching element so as to be longer than the on-time at the fundamental frequency f0 at the frequency (f0−Δf), and the frequency (f0 + Δf). ), The heating power is corrected to be shorter than the on-time at the fundamental frequency f0 so that the heating power is substantially the same as that at the fundamental frequency f0 .
[0031]
【Example】
Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
[0032]
FIG. 1 is a circuit block diagram of an embodiment of the present invention, FIG. 2 is a diagram showing an internal configuration of oscillation means in an embodiment of the present invention, FIG. 3 is a diagram showing a control method of the present invention, and FIG. FIG. 5 is a diagram showing a control method of a switching element according to the present invention.
[0033]
The overall configuration of an embodiment of the present invention will be described with reference to FIG.
[0034]
In the figure, 1 is an AC power source. A filter 2 includes two capacitors and a common mode choke coil. The input terminal is connected to the AC power source 1 to reduce noise induced in the power source. Reference numeral 3 denotes a rectifier circuit, which is composed of a plurality of rectifiers, whose input terminals (terminals in the figure) are connected to the output terminal of the filter 2, rectifies the AC power source 1 through the filter 2 and converts it to a DC power source, and outputs the output terminal. Output from (+ terminal and-terminal in the figure).
[0035]
Reference numeral 4 denotes a resonance capacitor, one end of which is connected to the low-voltage side output terminal (-terminal in the figure) of the rectifier circuit 3. A heating coil 5 has one end connected to the other end of the resonance capacitor 4.
[0036]
Reference numeral 6 denotes a resonance circuit, which includes a heating coil 5 and a resonance capacitor 4. 7 is a load.
[0037]
8 is a switching element comprising an upper arm switching element 8a and a lower arm switching element 8b, these two being connected in series, a connection point being connected to the other end of the heating coil 5, and one end being the high voltage side of the rectifier circuit 3. The output terminal (+ terminal in the figure) is connected, the other end is connected to the low-voltage side output terminal of the rectifier circuit 3, and the DC power is converted into a high-frequency current and supplied to the resonance circuit 6.
[0038]
Reference numeral 9 denotes an inverter which is composed of a resonance circuit 5 and a switching element 8 and heats a load 7 arranged in the vicinity.
[0039]
Reference numeral 10 denotes a power supply synchronization signal generating means which is connected to both ends of the AC power supply 1 and generates and outputs a signal synchronized with the half wave period of the AC power supply 1. Reference numeral 11 denotes a control unit which receives a signal from the power supply synchronization signal generation unit 10 and outputs a duty signal and a frequency change instruction signal corresponding to heating power set by a user (not shown) in synchronization with this signal. .
[0040]
A power setting unit 12 receives the duty signal of the control unit 11 and outputs an on / off duty setting signal for setting the on / off duty of the switching element 8 according to this signal to vary the heating power.
[0041]
Reference numeral 13 denotes frequency changing means, which receives a frequency change instruction signal from the control means 11, outputs a signal obtained by converting the frequency change instruction signal, and changes and controls the oscillation frequency of the oscillating means 14 described later. Specifically, the oscillation frequency of the oscillating means 14 to be described later is changed and controlled almost periodically in the range of a frequency (f0 ± Δf) obtained by slightly changing the oscillation frequency up and down around the basic frequency (f0).
[0042]
Reference numeral 14 denotes an oscillating means, which receives a signal from the frequency changing means 13, determines the on / off frequency of the switching element 8 in accordance with this signal, and outputs an oscillation pulse at this frequency.
[0043]
Reference numeral 15 denotes a drive pulse generating means, which receives the oscillation pulse of the oscillation means 14 and the on / off duty setting signal of the power setting means 12, and is synchronized with the oscillation pulse of the oscillation means 14 in accordance with the on / off duty setting signal of the power setting means 12. A pulse for driving the switching element 8, that is, the upper and lower arm switching elements 8a and 8b is generated.
[0044]
FIG. 2 is a diagram showing two examples of the internal configuration of the oscillation means 14. 2A, 16 is an oscillator formed by a general-purpose analog oscillation IC (555 or the like), 17 is a resistor, 18 is a capacitor circuit comprising a plurality of capacitors 18a, 18b, 18c,... Having different capacities. , 19 is switch means for switching the capacitors 18a, 18b, 18c. In short, the charging / discharging means is formed by the resistor 17 and the capacitor circuit 18, and the oscillator 16 is connected to the charging / discharging means to constitute an oscillation circuit. This operation will be described next.
[0045]
When the capacitor 18a in the capacitor circuit 18 is selected by the switch means 19 and connected to the resistor 17, an oscillation pulse having a frequency corresponding to the capacity of the capacitor 18a and the charge / discharge speed determined by the resistor 17 is generated by the oscillator 16, that is, the oscillation. Output from the means 14. When a frequency change signal is input from the frequency change means 13, the switch means 19 switches the connection with the capacitor circuit 18.
[0046]
For example, when the connection is switched from the capacitor 18a to the capacitor 18b, the frequency of the oscillation pulse output from the oscillation means 14 is changed to a frequency corresponding to the charge / discharge speed determined by the capacitance of the capacitor 18b and the resistor 17. If the number of capacitors constituting the capacitor circuit 18 and the number of switching of the switching means 19 are the same as the number of frequencies to be changed, an arbitrary number of frequencies can be changed.
[0047]
In FIG. 2B, 20 is an oscillator that oscillates at a frequency higher than the frequency output from the oscillating means 14, 21 is a binary counter, and 22 is a counter value setting means. The binary counter 21 counts the high frequency signal from the oscillator 20 input to the CK terminal, and if the counter value matches the counter value set in the counter value setting means 22 by the frequency change signal from the frequency changing means 13. A pulse is output, and at the same time the counter is cleared and counted again. Thereafter, this operation is repeated.
[0048]
With this configuration, the oscillating unit 14 can output an oscillation pulse having a frequency corresponding to the counter value set in the counter value setting unit 22, and the counter value set in the counter value setting unit 22 can be set as a frequency change signal. By switching at, the oscillation means 14 can output an oscillation pulse having an arbitrary frequency. As a similar method, a configuration in which a microcomputer is used without using a counter IC and the internal counter is used is also possible.
[0049]
In the above configuration, the operations of the same basic components as in the conventional case are almost the same as those in the conventional case. Therefore, the description of the operations of the basic components is omitted, and the operation related to the frequency changing means 13 different from the conventional case is shown in FIG. To explain.
[0050]
In FIG. 3, the vertical axis indicates the oscillation frequency of the oscillation means 14, the AC power supply, and the power supply synchronization signal, and the horizontal axis indicates time. The following description will be made mainly with reference to the oscillation frequency diagram of the oscillation means 14.
[0051]
When heating is started, the frequency changing unit 13 outputs a frequency changing signal that sets the oscillation frequency of the oscillating unit 14 to the fundamental frequency (f0). After a substantially fixed time, the frequency changing means 13 outputs a frequency changing signal that makes the oscillation frequency of the oscillating means 14 slightly change above the basic frequency (f0) (f0 + Δf). Further, after a substantially constant time, the frequency changing means 13 outputs a frequency changing signal that makes the oscillation frequency of the oscillating means 14 slightly smaller than the fundamental frequency (f0) (f0−Δf). Further, after a substantially constant time, the changing means 13 outputs a frequency changing signal having the oscillation frequency of the oscillating means 14 as the fundamental frequency (f0). Thereafter, the frequency changing means 13 repeats this change control almost periodically.
[0052]
As a result, the noise component induced in the power supply is as shown in FIG.
[0053]
FIG. 4 shows the energy of noise components on the vertical axis and the frequency on the horizontal axis, and the conventional case is indicated by a broken line for reference. As shown in the figure, in the vicinity of the fundamental frequency (f0), not only the fundamental frequency (f0) but also a frequency (f0 + Δf) slightly changed above the fundamental frequency (f0) and a small change below the fundamental frequency (f0). Energy is also generated at the frequency (f0−Δf), but since each energy is distributed to these three frequencies, the energy is lower than in the conventional case of a single frequency. Further, the noise components of the harmonics of 2 times, 3 times, 4 times,... Are dispersed and become energy lower than in the conventional case. In the figure, the fifth harmonic (5f0) and thereafter are omitted.
[0054]
As a result, the filter 2 can be reduced in size and cost, and the operational reliability of the product itself and surrounding devices can be improved.
[0055]
However, as described above, the power control method when the oscillation frequency is periodically changed and controlled, that is, the control method of the switching element 8 needs to be changed and corrected as compared with the conventional case.
[0056]
FIG. 5 shows drive pulse waveforms of the upper and lower arm switching elements 8a and 8b in the states E, F, F ′, G, and G ′, which will be described in detail below.
[0057]
In the state E, the oscillation frequency is the fundamental frequency (f0), and the period Te at this time has a relationship of “Te = 1 / f0” with the frequency (f0), the ON time of the upper arm switching element 8a is THe, and the lower arm The ON time of the switching element 8b is TLe. This state is a basic state.
[0058]
The state F is a case where the oscillation frequency is (f0−Δf) and the ON time of the upper arm switching element 8a is the same as that in the state E. Since the period Tf at this time has a relationship of the frequency (f0−Δf) and “Tf = 1 / (f0−Δf)”, the period Tf is longer than the period Te in the state E. At this time, when the ON time of the upper arm switching element 8a is set to the same THe as in the state E, the ON time of the lower arm switching element 8b is a time TLf obtained by removing THe and a blank time from one cycle Tf, and TLf is TLe. Become bigger. In this case, since the ON ratio of the upper arm switching element 8a per cycle is lower than that in the state E, there arises a problem that the current flowing from the power source side to the load side decreases, that is, the heating power decreases. Therefore, in order to correct this, it is necessary to lengthen the ON time of the upper arm switching element 8a.
[0059]
A state F ′ shows a pattern in which this correction is performed. That is, the ON time of the upper arm switching element 8a is set to THf ′ longer than THe, and the ON time of the lower arm switching element 8b is set to TLf ′ shorter than TLf. By changing the correction in this way, even if the oscillation frequency is changed to (f0−Δf), the heating power is the same as that in the state E, that is, the fundamental frequency (f0).
[0060]
The state G is a case where the oscillation frequency is (f0 + Δf) and the ON time of the upper arm switching element 8a is the same as that in the state E. Since the period Tg at this time is in a relationship of the frequency (f0 + Δf) and “Tg = 1 / (f0 + Δf)”, the period Tg is smaller than the period Te in the state E. At this time, if the ON time of the upper arm switching element 8a is set to the same THe as in the state E, the ON time of the lower arm switching element 8b is a time TLg obtained by removing THe and the blank time from one cycle Tg, and TLg is TLe. Smaller. In this case, since the ON ratio of the upper arm switching element 8a per cycle is higher than that in the state E, there arises a problem that the current flowing from the power source side to the load side increases, that is, the heating power increases. Therefore, in order to correct this, it is necessary to shorten the ON time of the upper arm switching element 8a.
[0061]
A state G ′ shows a pattern in which this correction is performed. That is, the ON time of the upper arm switching element 8a is set to THg ′ shorter than THe, and the ON time of the lower arm switching element 8b is set to TLg ′ longer than TLg. By changing the correction in this way, even if the oscillation frequency is changed to (f0 + Δf), the heating power is the same as in the state E, that is, the fundamental frequency (f0).
[0062]
That is, the ON time control of the upper and lower arm switching elements 8a and 8b is changed from the state E to the state G ′ in accordance with the change control of the oscillation frequency of the oscillation means 14 from f0 → f0 + Δf → f0−Δf → f0 →. → The heating power can be stabilized by performing the state F ′ → the state E →.
[0063]
Further, the frequency changing means 13 is configured so that the change frequency of the oscillation frequency is the shortest half-wave cycle of the AC power supply 1 and the oscillation frequency is changed in synchronization with the zero cross point of the AC power supply 1.
[0064]
As shown in FIG. 3, the power supply synchronization signal generating means 10 outputs a signal for generating a pulse at a timing when the AC power supply 1 becomes approximately zero volts. Then, this power supply synchronizing signal outputs a pulse at a half wave period and zero cross point of the AC power supply 1. Since this signal is transmitted to the frequency changing means 13 via the control means 11, the frequency changing means 13 can be configured as described above.
[0065]
With such a configuration, first, the control means 11 has a microcomputer, and it becomes easy to control the switching element 8 and the like as described above by program processing of the microcomputer, that is, software. . That is, the microcomputer software needs to periodically perform input / output processing, but this processing may be performed in synchronization with the power supply frequency, that is, the output signal of the power supply synchronization signal generating means 10, so that the software configuration is simple. It can be realized by configuration.
[0066]
Secondly, it is necessary to perform current detection (not shown) and power control processing of the inverter 9 in a predetermined phase with reference to the zero cross point of the AC power supply 1, but the power supply without adding a new circuit Since the synchronization signal generating means 10 may be used as it is, it can be realized with the simplest circuit configuration.
[0067]
When the frequency changing process may be processed at a slow cycle, the frequency changing unit 13 may set the change frequency of the oscillation frequency to a multiple of the half wave cycle of the AC power supply 1, and in this case as well, It is feasible with the configuration.
[0068]
However, when it is necessary to perform the frequency changing process at a fast cycle, the frequency changing unit 13 may perform several stages of frequency changing processes from the zero cross point of the AC power supply 1 to the zero cross point.
[0069]
【The invention's effect】
As described above, according to the induction heating cooker of the present invention, it is possible to reduce the size and cost of the filter, further improve the operation reliability with respect to surrounding equipment, and stabilize the heating power. I can .
[Brief description of the drawings]
FIG. 1 is a circuit block diagram of an embodiment of the present invention.
FIG. 2 is a diagram showing an internal configuration of oscillation means in an embodiment of the present invention.
FIG. 3 is a diagram showing a control method of the present invention.
FIG. 4 is a diagram showing a distribution of noise components in the control method of the present invention.
FIG. 5 is a diagram showing a method for controlling a switching element according to the present invention.
FIG. 6 is a circuit block diagram of a conventional example.
FIG. 7 is a diagram showing signal waveforms of main parts in a conventional example.
FIG. 8 is a diagram illustrating a distribution of noise components in a conventional control method.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 AC power supply 3 Rectification circuit 4 Resonance capacitor 5 Heating coil 6 Resonance circuit 7 Load 8 Switching element 9 Inverter 10 Power supply synchronous signal generation means 12 Power setting means 13 Frequency change means 14 Oscillation means

Claims (1)

A rectifier circuitry to convert the direct current by rectifying an AC, the dc more is converted into a high frequency current comprising heating coil Contact and resonant capacitor or et resonant circuits in the upper arm switching element and lower arm switching elements is supplied to the inverter for heating the load placed in the vicinity, the power setting means to vary the heating power by setting the duty of on-off of the upper arm switching element and lower arm switching element, the upper arm an oscillation means to determine the frequency of the on-off switching element and the lower arm switching element, comprising a frequency changing means to change control an oscillation frequency of the oscillating hand stage, the frequency change hands stage oscillation frequency of the oscillation hand stage Is changed and controlled almost periodically in the range of the frequency (f0 ± Δf) that is slightly changed up and down around the fundamental frequency (f0) ,
The power setting means corrects the on-time of the upper arm switching element to be longer than the on-time at the fundamental frequency f0 when the frequency is (f0−Δf), and the frequency (f0 + Δf) In the induction heating cooker, the heating power is corrected to be shorter than the on-time at the basic frequency f0 so that the heating power is substantially the same as that at the basic frequency f0 .

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