The present invention relates to a high frequency heating apparatus using a magnetron such as a microwave oven. More specifically, the present invention is directed to a control system for suppressing higher harmonic distortion of commercial power supply currents which are supplied to a magnetron driving power supply.
Since conventional power supplies mounted on high frequency heating apparatus are heavy and bulky, there are needs of power supplies made compact in light weight. To this end, various positive ideas capable of constructing such low-cost and compact switching type power supplies in light weight have been proposed in present various fields. In high frequency heating apparatus for cooking food by utilizing microwaves generated from magnetrons, compact and light-weight power supplies for driving the magnetrons are required which could be realized by switching type inverter circuits.
More specifically, among these switching type inverter circuits, high frequency inverter circuits which constitute a subject inverter circuit of the present invention correspond to resonant type circuit systems using switching elements in which arms of a bridge circuit are arranged by two switching elements (refer to, for example, patent publication 1).
When the above-explained magnetron driving power supplies are arranged in the switching type high frequency inverter circuits, the below-mentioned problem is still left in conjunction with such a fact that magnetrons constitute non-linear loads. That is, current waveforms of commercial power supplies, which are supplied to the magnetron driving power supplies, contain a large amount of higher harmonic components.
On the other hand, an absolute value of the above-described higher harmonic components is increased in connection with an increase of power consumption of the magnetron driving power supplies in order to satisfy requirements for shortening cooking time of microwave ovens. This may conduct that higher harmonic currents of power supplies can be more hardly suppressed.
Various sorts of control systems for suppressing higher harmonic currents have been proposed (refer to, for example, patent publication 2).
The DC power supply 1 rectifies an AC voltage of a commercial power supply in a full wave rectification manner to obtain a DC voltage “VDC”, and then, applies the DC voltage “VDC” to a series circuit constituted by the second capacitor 6 and a primary winding 8 of the leakage transformer 2. The first semiconductor switching element 3 has been series-connected to the second semiconductor switching element 4, and the series circuit constituted by the primary winding 8 of the leakage transformer 2 and the second capacitor 6 has been parallel-connected to the second semiconductor switching element 4.
The first capacitor 5 is parallel-connected to the second semiconductor switching element 4, and owns a so-called “snubber role” capable of suppressing a rush current (rush voltage) which is produced when a switching operation is performed. An AC high voltage generated in a secondary winding 9 of the leakage transformer 2 is converted into a DC high voltage by the full wave voltage doubler rectifying circuit 11, and then, the DC high voltage has been applied between an anode and a cathode of the magnetron 12. A third winding 10 of the leakage transformer 2 has supplied a current to the cathode of the magnetron 12.
Both the first semiconductor switching element 3 and the second semiconductor switching element 4 have been constituted by IGBTs and flywheel diodes connected parallel to the IGBTs. As apparent from the foregoing descriptions, the first and second semiconductor switching elements 3 and 4 are not limited only to the above-explained element sort. Alternatively, a thyristor, a GTO switching element, and the like may be employed.
The driving unit 13 contains therein an oscillating circuit (not shown) which is employed so as to produce drive signals for the first semiconductor switching element 3 and the second semiconductor switching element 4. The oscillating circuit produces a rectangular wave having a predetermined frequency, and supplies “DRIVE” signals to the first semiconductor switching element 3 and the second semiconductor switching element 4. Immediately after one of the first semiconductor switching element 3 and the second semiconductor switching element 4 is turned OFF, a voltage between both terminals of the other semiconductor switching element 3, or 4 is high. As a result, if the other semiconductor switching element 3, or 4 is turned OFF at this time instant, then an excessively large current having a spike shape may flow, so that unwanted loss and unnecessary noise may occur. However, since a dead time is conducted, turning-OFF operation is delayed until this voltage between the terminals of the other semiconductor switching element is decreased to approximately 0 V. As a consequence, the unwanted loss and the unnecessary noise can be prevented. Apparently, when these first and second semiconductor switching elements 3 and 4 are switched in a reverse manner, similar operations are carried out.
Since explanations of detailed operations as to the DRIVE signals applied from the driving circuit 13 and the respective operation modes of both the first and second semiconductor switching elements 3 and 4 are described in the above-described patent publication 1, detailed explanations thereof are omitted.
As a feature of the circuit arrangement shown in
Next,
An impedance of the series resonant circuit becomes minimum at a resonant frequency “f0”, and this impedance is increased, as a frequency is separated from the resonant frequency “f0”. As a result, as indicated in this drawing, the current “I1” becomes maximum when the frequency becomes the resonant frequency “f0.” The current “I1” is decreased, as the frequency range is increased to “f1” up to “f3.”
It should be understood that in an actual inverter operation, such a frequency range (indicated by solid line portion “I1”) from “f1” to “f3” is used which is higher than this resonant frequency “f0.”
As will be explained later, in a microwave oven using a magnetron which corresponds to a nonlinear load, in the case that a power supply voltage to be inputted is an AC voltage of a commercial power supply, a switching frequency is changed in response to a phase of the power supply voltage.
While the resonance curve of
For instance, in the case that the microwave oven is operated in 200 W, the switching frequency becomes a frequency near “f3”; in the case that the microwave oven is operated in 500 W, the switching frequency becomes a frequency lower than “f3”; and in the case that the microwave oven is operated in 1,000 W, the switching frequency becomes a frequency which is further lower than “f3.”
As apparent from the foregoing description, since either the input power or the input current is controlled, this frequency is changed in response to variations as to voltages of the commercial power supply, temperatures of the magnetron, and the like.
Also, in phases near 0 degree and 180 degrees at which the instantaneous voltage of the commercial power supply becomes the lowest voltage, in correspondence with such a magnetron characteristic that if a high voltage is not applied, then the magnetron cannot be oscillated in a high frequency, the switching frequency is lowered to a frequency in the vicinity of the resonant frequency “f0” so as to increase the step-up ratio of the magnetron applied voltage with respect to the commercial power supply voltage. Thus, it is so set that the phase width of the commercial power supply is widened where electromagnetic waves are generated from the magnetron.
It should be noted that when the switching frequency is extremely approximated to the resonant frequency “f0”, an unstable operation such as abnormal resonance is induced. As a result, a minimum frequency limiting circuit capable of limiting the switching frequency to the frequency “f1” so as to prevent the above-explained phenomenon is required.
As previously explained, since the inverter operating frequency is changed for every power supply phase, such current waveforms which contain a large amount of basic wave (commercial power supply frequency) components, and a small amount of higher harmonic components can be realized.
[Patent Publication 1]
[Patent Publication 2]
However, in the above-explained arrangement, the below-mentioned problem is revealed. That is, due to such a reason that the switching frequency is rapidly changed in the vicinity of a boundary between a time period during which the switching frequency is limited to the frequency “f1” by the minimum frequency limiting circuit, and another time period during which the limitation of the switching frequency is released, higher order distortion is produced in the currents of the commercial power supply.
The present invention has been made to reduce the above-described higher order distortion, and therefore, has an object to provide an inverter circuit capable of reducing not only lower order distortion, but also higher order distortion.
To solve the above-explained problem, a high frequency heating apparatus, according to the present invention, is featured by such a magnetron driving-purpose high frequency heating apparatus comprising: a DC voltage power supply obtained by rectifying a commercial power supply voltage; a series circuit constituted by two semiconductor switching elements; a resonant circuit formed by connecting a primary winding of a leakage transformer to a capacitor, the series circuit being connected parallel to the DC voltage power supply, and the resonant circuit is connected parallel to one of the semiconductor switching elements; drive means for driving the respective semiconductor switching elements; frequency-modulated signal producing means for supplying to the drive means, a frequency-modulated signal which changes a switching frequency in response to a phase of the commercial power supply voltage; minimum frequency limiting means for limiting a minimum frequency of the switching frequency; rectifying means connected to a secondary winding of the leakage transformer; and a magnetron connected to the rectifying means; wherein: the minimum frequency limiting means is arranged in such a manner that a switching frequency located in the vicinity of a boundary between a time period during which the switching frequency is limited to the minimum frequency, and a time period during which the limitation of the switching frequency is released is smoothly changed.
With employment of the above-described arrangement, there is no such a sudden change of the switching frequency in the phases in the vicinity of 0 degree and 180 degrees at which the instantaneous voltage of the commercial power supply becomes the lowest voltage. As a result, the power supply higher harmonic distortion of the higher orders can be reduced.
In accordance with the high frequency heating apparatus of the present invention, the minimum frequency limiting circuit is arranged in such a manner that the switching frequency is smoothly changed which is located in the vicinity of the boundary between the time period during which the switching frequency is limited to the minimum switching frequency, and the time period during which the limitation to the minimum switching frequency is released. As a result, since the sudden change of the switching frequency can be deleted, the power supply higher harmonic distortion of the higher orders which occurs due to this sudden change of the switching frequency can be reduced.
First invention is featured by a magnetron driving-purpose high frequency heating apparatus comprising: a DC voltage power supply obtained by rectifying a commercial power supply voltage; a series circuit constituted by two semiconductor switching elements; a resonant circuit formed by connecting a primary winding of a leakage transformer to a capacitor, the series circuit being connected parallel to the DC voltage power supply, and the resonant circuit is connected parallel to one of the semiconductor switching elements; drive means for driving the respective semiconductor switching elements; frequency-modulated signal producing means for supplying to the drive means, a frequency-modulated signal which changes a switching frequency in response to a phase of the commercial power supply voltage; minimum frequency limiting means for limiting a minimum frequency of the switching frequency; rectifying means connected to a secondary winding of the leakage transformer; and a magnetron connected to the rectifying means; in which the minimum frequency limiting means is arranged in such a manner that a switching frequency located in the vicinity of a boundary between a time period during which the switching frequency is limited to the minimum frequency, and a time period during which the limitation of the switching frequency is released is smoothly changed. Since a sudden change of the switching frequency is deleted, power supply higher harmonic distortion of higher orders can be reduced which occurs due to the frequency sudden change.
Second invention is featured by a magnetron driving-purpose high frequency heating apparatus comprising: a DC voltage power supply obtained by rectifying a commercial power supply voltage; two sets of series circuits constituted by two semiconductor switching elements respectively; a resonant circuit formed by connecting a primary winding of a leakage transformer to a capacitor, two sets of the series circuit being connected parallel to the DC voltage power supply, respectively, one end of the resonant circuit being connected to a center point of one of the two series circuits, and the other end of the resonant circuit being connected to a center point of the other series circuit; drive means for driving the respective semiconductor switching elements; frequency-modulated signal producing means for supplying to the drive means, a frequency-modulated signal which changes a switching frequency in response to a phase of the commercial power supply voltage; minimum frequency limiting means for limiting a minimum frequency of the switching frequency; rectifying means connected to a secondary winding of the leakage transformer; and a magnetron connected to the rectifying means; in which the minimum frequency limiting means is arranged in such a manner that a switching frequency located in the vicinity of a boundary between a time period during which the switching frequency is limited to the minimum frequency, and a time period during which the limitation of the switching frequency is released is smoothly changed. Since a sudden change of the switching frequency is deleted, power supply higher harmonic distortion of higher orders can be reduced which occurs due to the frequency sudden change.
Third invention is featured by a magnetron driving-purpose high frequency heating apparatus comprising: a DC voltage power supply obtained by rectifying a commercial power supply voltage; a series circuit constituted by two semiconductor switching elements; a resonant circuit formed by connecting a primary winding of a leakage transformer to a capacitor, the series circuit being connected parallel to the DC voltage power supply, in an AC equivalent circuit, one end of the resonant circuit being connected to a center point of the series circuit, and the other end of the resonant circuit being connected to one end of the DC voltage power supply; drive means for driving the respective semiconductor switching elements; frequency-modulated signal producing means for supplying to the drive means, a frequency-modulated signal which changes a switching frequency in response to a phase of the commercial power supply voltage; minimum frequency limiting means for limiting a minimum frequency of the switching frequency; rectifying means connected to a secondary winding of the leakage transformer; and a magnetron connected to the rectifying means; in which the minimum frequency limiting means is arranged in such a manner that a switching frequency located in the vicinity of a boundary between a time period during which the switching frequency is limited to the minimum frequency, and a time period during which the limitation of the switching frequency is released is smoothly changed. Since a sudden change of the switching frequency is deleted, power supply higher harmonic distortion of higher orders can be reduced which occurs due to the frequency sudden change.
Fourth invention is featured by that, more specifically, in the high frequency heating apparatus of any one of the first to third invention, the frequency-modulated signal can be represented by a shape of a frequency-modulated waveform; and the minimum frequency limiting means owns at least one of a first limiting function and a second limiting function for limiting a frequency lower than, or equal to the minimum frequency, the first limiting function limits a change of the frequency-modulated waveform from a frequency higher than the minimum frequency toward a lower frequency thereof by gradually increasing an influence degree.
Fifth invention is featured by that, more specifically, in the high frequency heating apparatus of any one of the first to third invention, the frequency-modulated signal can be represented by a shape of a frequency-modulated waveform; and the minimum frequency limiting means owns at least one of a first limiting function and a second limiting function for limiting a frequency lower than, or equal to the minimum frequency to be changed in a very small manner, the first limiting function limits a change of the frequency-modulated waveform from a frequency higher than the minimum frequency toward a lower frequency thereof by gradually increasing an influence degree.
Sixth invention is featured by that, more specifically, in the high frequency heating apparatus of the fourth invention, or the fifth invention, the first limiting function changes the influence degree by employing a resistance value change of a PN junction which is represented by a voltage-to-current characteristic.
Seventh invention is featured by that, more specifically, in the high frequency heating apparatus of the fourth invention, or the fifth invention, the second limiting function changes the influence degree in a very small manner by employing a resistance value change of a PN junction which is represented by a voltage-to-current characteristic.
Eighth invention is featured by that, more specifically, in the high frequency heating apparatus of the fourth invention, or the fifth invention, the frequency-modulated waveform is formed based upon a rectified waveform of a commercial power supply.
Ninth invention is featured by that, more specifically, in the high frequency heating apparatus of any one of the first to third invention, the minimum frequency does not depend upon the voltage of the commercial power supply, but is set to a fixed value.
Tenth invention is featured by that, more specifically, in the high frequency heating apparatus of any one of the first to third invention, the minimum frequency is changed, depending upon the voltage of the commercial power supply.
Referring now to drawings, various embodiments of the present invention will be described. It should be understood that the present invention is not limited by the embodiments.
In the driving circuit 14 used to drive the first and second semiconductor switching elements 3 and 4, first of all, a frequency-modulated waveform is formed by a frequency-modulated signal producing circuit 15 by employing a waveform which has been divided by resistors based upon a voltage of a commercial power supply. Also, the frequency-modulated signal producing circuit 15 receives a signal supplied from a power control circuit 19, and then, controls the received signal to become desirable high frequency power (200 W, 600 W etc.), as previously explained.
Next, based upon the frequency-modulated waveform produced by the frequency-modulated signal producing circuit 15, the oscillating circuit 16 oscillates a switching frequency signal, while a desirable dead time is determined by a dead time producing circuit 17 based upon the switching frequency signal. Then, rectangular wave signals are produced by a switching element driving circuit 18 in response to both the switching frequency signal and the desirable dead time signal, and then, these rectangular wave signals are applied to a gate of the first semiconductor switching element 3 and a gate of the second semiconductor switching element 4.
Since the oscillating circuit 16 is arranged in the above-described circuit arrangement, the potential of the capacitor 163 becomes a triangular wave, and then, this triangular wave signal is transferred to the switching element drive circuit 18.
Also, the charge current I16 and the discharge current I17 with respect to the capacitor 163 are determined by a parallel-combined resistance between a resistor 161 and a resistor 162 based upon the frequency-modulated signal produced from the frequency-modulated signal producing circuit 15. These resistors 161 and 162 are connected to an MOD terminal of
Also, a first limiting function may function at the same time as the second limiting function (Claims 4 and 5). The first limiting function limits a change in the above-described frequency-modulated waveforms from such a frequency (in
It should also be noted that although such a frequency-modulated waveform of a portion, which is higher than the fixed voltage V2 and different from the voltage-divided waveform obtained by rectifying the commercial power supply voltage may contribute to reduce the distortion of the commercial power supply current waveform of the lower order, since this distortion reduction is different from the major object of the present invention, detailed explanations thereof are omitted.
In accordance with this third embodiment, a resistance value change of a PN junction which is indicated by a voltage-to-current characteristic may be employed as a first limiting function by the transistor 159 (Claim 6).
When a potential difference of the PN junction is increased, a resistance value of this PN junction is decreased. As a result, as the potential of the voltage-divided waveform obtained by rectifying the commercial power supply voltage becomes lower than the voltage V1 and is separated from this voltage V1, an influence degree of the first limiting function is increased, so that the waveform lower than, or equal to V1 is smoothly changed.
Also, while plural sets of the first limiting functions are provided, set potentials and limiting degrees thereof are changed. As a result, such a frequency-modulated waveform whose frequency change becomes more smoothly may be obtained.
In a portion that a potential of a frequency-modulated waveform becomes lower than the fixed voltage V2, since a second limiting function is added in combined with a first limiting function, an influence degree thereof is changed in a very small manner, and further, a frequency change becomes very small.
Also, since a minimum frequency limitation is not fixed, but is variable with respect to a voltage variation, the minimum frequency limitation may be increased/decreased based upon commercial power supply voltage information (Claim 10).
Since this circuit arrangement is employed, even in the respective power supply voltages, the formation of optimum frequency-modulated waveform capable of suppressing the generation of the higher harmonic component can be realized.
In the first embodiment, as shown in
Since the magnetron driving power supply circuits of the fifth embodiment and the sixth embodiment are arranged as same as that of the first embodiment except for the driving circuit, the minimum frequency limiting means is constituted as explained in the second embodiment to the fourth embodiment, so that a similar effect to the effect of the first embodiment may be achieved (Claims 4 to 9).
While the present invention has been described in detail with reference to the specific embodiments, it is apparent for ordinarily skilled engineers to modify and change the inventive ideas in various manners without departing from the technical scope and spirit of the present invention.
The present invention has been made based upon Japanese Patent Application No. 2005-009849 filed on Jan. 18, 2005, the contents of which has been incorporated herein by reference.
As previously explained, in the high frequency heating apparatus according to the present invention, there is no such a sudden change of the switching frequency in the phases in the vicinity of 0 degree and 180 degrees at which the instantaneous voltage of the commercial power supply becomes the lowest voltage. As a result, the power supply higher harmonic distortion of the higher orders can be reduced, so that the high frequency heating apparatus can be applied to various sorts of inverter circuits.
Number | Date | Country | Kind |
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2005-009849 | Jan 2005 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP06/00635 | 1/18/2006 | WO | 00 | 7/5/2007 |