JP2006344469A - Induction heating cooker - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an induction heating cooker wherein a booster circuit is provided between a rectifier circuit and an inverter circuit to supply the raised voltage to the inverter circuit to thereby improve heating efficiency, and wherein the inverter circuit is configured as a full bridge inverter circuit to reduce a load current to thereby improve the heating efficiency more effectively. <P>SOLUTION: The inverter circuit 10 is configured as a full bridge inverter circuit to reduce the load current, and the booster circuit 3 is provided in a front stage of the inverter circuit 10 to improve heating efficiency. Furthermore, a heating coil 21 is formed into a double layer lap winding structure to improve the heating efficiency, and a further improvement in heating efficiency is obtained by adjusting the gap length. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an induction heating cooker that supplies a high-frequency current to a heating coil to induction-heat an object to be heated.

  Conventionally, “the litz wire of the heating coil is bundled with 40 strands at a time and twisted to make a primary twist, and this primary twist is twisted two by two into a secondary twist, and this secondary twist is further bundled into four bundles. The heating coil formed by winding the litz wire is a ratio of the volume of the element wire (including the insulating layer) to the entire dimension of the heating coil, that is, the occupation. There is an induction heating cooker in which when the product ratio is increased, the strands are first tightly coupled to increase the inductance and the electromagnetic coupling with the pan increases (see, for example, Patent Document 1).

  In addition, “the magnetic field generated from the metal body works in a direction to prevent the leakage magnetic field, so it is possible to increase the amount of magnetic field toward the heated object by confining the magnetic field. There is an induction heating cooker that can improve the heating efficiency (see, for example, Patent Document 2).

  Further, “AC supplied from a commercial power source is converted into DC by a rectifier circuit, and a DC voltage VIN is supplied to an input portion of the booster circuit, that is, one end of the coil. The drive signal VG ′ is output to the gate of the boosting switching element to drive the boosting switching element.The boosting switching element is turned on and off because the drive signal VG ′ is a square wave, and the collector is squared. The collector voltage VCE ′ of the step-up switching element is smoothed and boosted when transmitted to the capacitor via the diode, and becomes a DC voltage VC ′ higher than VCE ′. There is an induction heating device (see, for example, Patent Document 3).

Japanese Patent Laid-Open No. 11-126679 (page 4, FIG. 1) JP 2003-151745 A (page 3, FIG. 1) Japanese Patent Laid-Open No. 2002-246161 (page 4, FIG. 1)

  The induction heating cooker described in Patent Document 1 improves the heating efficiency by devising only the structure of the heating coil. However, when the set power is set large, there is a problem that the voltage of the high-frequency current supplied to the heating coil is lowered, and the heating efficiency is also reduced accordingly. That is, depending on the type of the object to be heated placed on the heating coil, it is difficult to significantly improve the heating efficiency, and the heating performance cannot be remarkably improved.

  In addition, the induction heating cooker described in Patent Literature 2 is provided with a metal body to improve the heating efficiency by reducing the magnetic transmission loss. In other words, the heating efficiency is relatively improved by reducing the magnetic transmission loss between the heating coil and the object to be heated. Therefore, the structure and electric circuit of the heating coil are not particularly devised, and the heating efficiency is not positively improved.

  Furthermore, the induction heating apparatus described in Patent Document 3 is provided with a booster circuit that boosts a DC voltage between the rectifier circuit and the inverter unit, thereby improving the heating efficiency. However, the configuration of the inverter unit is specialized for the monolithic voltage resonance circuit, and the circuit configuration of the inverter unit is not particularly devised. That is, it is clear that the operating frequency of the heating coil decreases as the set power increases, and only a booster circuit is provided to avoid this.

  The present invention has been made to solve the above-described problems, and an object thereof is to provide an induction heating cooker that can effectively improve heating efficiency.

  An induction heating cooker according to the present invention includes a rectifier circuit that full-wave rectifies an AC power supply, a booster circuit that boosts a DC voltage that has been full-wave rectified by the rectifier circuit, and a DC voltage boosted by the booster circuit at a high frequency. Two switching elements, each of which includes an inverter circuit that converts the AC voltage into an AC voltage, and a resonance circuit that is connected to an output stage of the inverter circuit and includes a heating coil and a resonance capacitor, the inverter circuit being connected in series It is characterized by comprising as a full bridge inverter circuit formed by.

  An induction heating cooker according to the present invention includes a rectifier circuit that full-wave rectifies an AC power supply, a booster circuit that boosts a DC voltage that has been full-wave rectified by the rectifier circuit, and a DC voltage boosted by the booster circuit at a high frequency. Two switching elements, each of which includes an inverter circuit that converts the AC voltage into an AC voltage, and a resonance circuit that is connected to an output stage of the inverter circuit and includes a heating coil and a resonance capacitor, the inverter circuit being connected in series Therefore, the inverter circuit loss can be reduced and the heating efficiency can be improved. Moreover, since the inverter circuit loss is reduced, it is possible to easily realize cooling of the circuits constituting the induction heating cooker.

Hereinafter, an induction heating cooker according to an embodiment of the present invention will be described with reference to the drawings.
[Embodiment 1]
In the first embodiment, a case will be described as an example where the inverter circuit 10 is configured by a full-bridge inverter circuit, and the booster circuit 3 is provided in the previous stage of the inverter circuit 10 to improve the heating efficiency of the heating coil 21.

  FIG. 1 is a schematic configuration diagram showing a circuit configuration of induction heating cooker 50 according to the first embodiment. The induction heating cooker 50 includes a rectifier circuit 2, a booster circuit 3, an inverter circuit 10, a resonance circuit 20, and a snubber capacitor 8. The induction heating cooker 50 is connected to the commercial power source 1, and the AC voltage supplied from the commercial power source 1 is converted into a DC voltage by the rectifier circuit 2.

  The rectifier circuit 2 is composed of a diode bridge that performs full-wave rectification, and rectifies an AC voltage to convert it into a DC voltage. The converted DC voltage is supplied to the booster circuit 3. The booster circuit 3 boosts the DC voltage converted by the rectifier circuit 2. The booster circuit 3 preferably includes a reactor, a smoothing capacitor, etc. (not shown). The DC voltage boosted here is supplied to the inverter circuit 10.

  The inverter circuit 10 includes two sets of switching elements (switching element 11 and switching element 12, switching element 13 and switching element 14) connected in series between the positive and negative buses 4 of the rectifier circuit 2. That is, the inverter circuit 10 is configured as a full bridge inverter circuit formed by two sets of two switching elements connected in series. Here, a case where the four switching elements are IGBTs (insulated gate bipolar transistors) is shown as an example. Each switching element may be provided with a diode (not shown) connected in antiparallel to each switching element.

  A resonance circuit 20 including a heating coil 21 and a resonance capacitor 22 is connected between output points of the switching element 11 and the switching element 12 and the switching element 13 and the switching element 14 in the inverter circuit 10. Although the heating coil 21 and the resonance capacitor 22 form a series resonance circuit and have a resonance frequency, the inverter circuit 10 is driven at a frequency higher than the resonance frequency, so that the resonance circuit 20 has induction characteristics. It has become.

  An object to be heated (a pot, a kettle, a frying pan, etc.) 7 is placed on the heating coil 21, and heating is performed based on an instruction from the user. Further, in order to reduce noise and loss of the switching element 12 and the switching element 14, a snubber capacitor 8 is connected in parallel with each switching element. Note that a snubber circuit may be configured by connecting the snubber capacitor 8 and a resistor in series to reduce noise and loss of the switching element 12 and the switching element 14. Each switching element constituting the inverter circuit 10 is controlled by a control circuit (not shown).

  FIG. 2 is an explanatory diagram showing experimental results of the heating efficiency with respect to the bus voltage of the heating coil 21. In this figure, the horizontal axis indicates the bus voltage (V) and the vertical axis indicates the heating efficiency (%), respectively, and the experimental results of six heating coils 21 (A coil to F coil) are shown. The six heating coils 21 are obtained by changing the inductance (μH) according to the winding specifications (the number of turns and the number of twists). That is, the inductance is set so as to increase in order from the A coil (see FIG. 3). Here, the heating efficiency when rated power is supplied to each heating coil 21 is shown. The bus voltage indicates a voltage output from the booster circuit 3.

  As shown in the figure, the bus voltage applied to each heating coil 21 is different when rated power is applied to each heating coil 21. That is, when rated power is applied to each heating coil 21, the bus voltage applied to the A coil is about 140V, the bus voltage applied to the B coil is about 200V, the bus voltage applied to the C coil is about 260V, The bus voltage applied to the D coil is about 350V, the bus voltage applied to the E coil is about 360V, and the bus voltage applied to the F coil is about 390V.

  It can also be seen that the heating efficiency of each heating coil 21 is different. That is, the heating efficiency of each heating coil 21 increases so as to be proportional to the bus voltage until the bus voltage reaches about 300 V (A coil to C coil), but becomes slow when the bus voltage exceeds about 300 V. It can be seen that (D coil to F coil). This is because the electromagnetic coupling between the heating coil 21 and the object 7 to be heated deteriorates due to the increase in the number of layers accompanying the increase in the number of turns of the heating coil 21 (that is, the leakage loss increases), and the heating coil loss increases. It is known that it is caused by

FIG. 3 is an explanatory diagram showing the inductance of each heating coil 21 in a no-load state. Here, the resistance value (hereinafter referred to as no-load resistance R _Sn ) and the inductance (L _Sn ) in each heating coil 21 in a state where the heated object 7 is not placed on each heating coil 21 were measured. Each one is shown. It is assumed that the frequency of the rated power to be input is unified at 21 kHz.

As shown in the drawing, the no-load resistance R _Sn of each heating coil 21 is different. That is, the no-load resistance R _Sn increases from the A coil in order. This indicates that the bus voltage generated from the rated power input to each heating coil 21 is also different (see FIG. 2). In other words, it is necessary to change the no-load resistance R _Sn of each heating coil 21 in order for the rated power to be input even if the bus voltage is varied.

  FIG. 4 is an explanatory diagram showing experimental results of the heating efficiency with respect to the load current of the heating coil 21. In this figure, the horizontal axis indicates the load current (A) and the vertical axis indicates the heating efficiency (%), respectively, and shows the experimental results of six heating coils 21 (A coil to F coil). As shown in the figure, the load current when the rated power is input to each heating coil 21 is different. That is, when rated power is applied to each heating coil 21, the load current in the A coil is about 29A, the load current in the B coil is about 21A, the load current in the C coil is about 16A, the load current in the D coil is about 12A, The load current in the E coil is about 12A, and the load current in the F coil is about 11A.

  The heating efficiency of each heating coil 21 shown here corresponds to the heating efficiency shown in FIG. That is, it can be seen that there is no significant change in the heating efficiency until the load current in the heating coil 21 reaches about 13A, and that the heating efficiency decreases in proportion to the load current when the load current exceeds about 15A. This is because the load current is reduced due to the rise in the bus voltage, and the inverter loss (loss of the inverter circuit 10) and the heating coil loss (copper loss) are reduced.

  FIG. 5 is an explanatory diagram showing experimental results of loss in the heating coil 21. In this figure, leakage loss (W), heating coil loss (W), and inverter loss (W) in three heating coils 21 of A coil, C coil, and E coil are shown. In each graph, the upper part shows leakage loss, the interruption shows heating coil loss, and the lower part shows inverter loss. As shown in the figure, the total loss in the A coil is the largest. This is because a two-layer lap winding structure is adopted for the A coil as will be described later.

From the above, in order to improve the heating efficiency in the induction heating cooker 50 with high efficiency, it is desirable to set the bus voltage to be about 300 V or higher. That is, the no-load resistance R _Sn of the heating coil 21 may be set so that the rated power as such a bus voltage is input. In consideration of the withstand voltage of a smoothing capacitor (not shown) provided in the output stage of the booster circuit 3, it is desirable that the bus voltage be 400 V or less. From the above, the induction heating cooker 50 includes the heating coil 21 that has the no-load resistance R _Sn that can obtain such a bus voltage and can withstand the bus voltage. It turns out to be desirable.

Next, the operation of the induction heating cooker 50 will be described.
When an AC voltage is supplied from the commercial power source 1 to the induction heating cooker 50, the AC voltage is full-wave rectified by the rectifier circuit 2 and converted to a DC voltage. The converted DC voltage shows a pulsating voltage waveform and is input to the booster circuit 3. The booster circuit 3 converts this DC voltage into a DC voltage having substantially no ripple. The experimental results shown in FIG. 2 show the heating efficiency of each heating coil 21 with respect to this DC voltage (bus voltage).

  Since the inverter circuit 10 is configured as a full bridge inverter circuit, the load current can be further reduced as compared with the case where the inverter circuit 10 is configured as a half bridge inverter circuit. Further, the induction heating cooker 50 includes the booster circuit 3 and boosts the DC voltage by the booster circuit 3 and then supplies the DC voltage to the inverter circuit 10, so that the booster circuit 3 is not provided. Furthermore, the heating efficiency is improved.

  The switching element 11 and the switching element 12 constituting the inverter circuit 10 are turned on / off by a drive signal output from a control circuit (not shown) (not shown). Similarly, the switching element 13 and the switching element 14 constituting the inverter circuit 10 are also turned on / off by a drive signal output from a control circuit (not shown).

  The control circuit turns off the switching element 12 while the switching element 11 is turned on, and turns on the switching element 12 while the switching element 11 is turned off. The drive signal for alternately turning on / off 12 is output. Similarly, a drive signal for alternately turning on / off the switching element 13 and the switching element 14 is output.

  Next, the switching of the inverter circuit 10 and the timing of the voltage applied to the resonance circuit 20 will be described. In this inverter circuit 10, the switching element 11 and the switching element 12, the switching element 13 and the switching element 14 are 180 degrees out of phase, and each is driven at a high frequency at a predetermined frequency. That is, the heating output of the heating coil 21 is controlled by adjusting the energization rate (on-time ratio) of the switching element 11 and switching element 13 and the switching element 12 and switching element 14.

  As described above, the booster circuit 3 is provided in the previous stage of the inverter circuit 10 so that the bus voltage is about 300 V to 400 V, and the inverter circuit 10 is configured as a full bridge inverter circuit, so the booster circuit 3 is not provided. It becomes possible to improve heating efficiency than time. Further, since the bus voltage is converted into a DC voltage substantially free of ripples by the booster circuit 3, it is possible to prevent pan vibration caused by the commercial frequency.

[Embodiment 2]
In the second embodiment, a case where the heating efficiency in the induction heating cooker 50 is further improved by the winding structure and winding specifications (the number of turns and the number of twists) of the heating coil 21 will be described as an example. That is, if the induction heating cooker 50 is configured in combination with the first embodiment, it is possible to obtain a higher efficiency of heating efficiency.

First, the winding structure of the heating coil 21 will be described.
FIG. 6 is a coil top view showing the winding structure of the heating coil 21. When winding a winding having a twisted structure on the coil base 9, if the first turn is (a) in the figure, the next second turn (b) is wound so as to enter the diagonally lower stage of (a), The next third turn (c) is wound around the diagonally upper part of (b).

  By doing so, (c) returns to the same plane as (a). This may be repeated a predetermined number of times to achieve the desired number of turns. In addition, although the case where the winding number is 9 turns is shown here as an example, it is not limited to this. Moreover, although it is made to wind in the outer peripheral direction in order from the 1st turn (a), (a) may be made into the outermost periphery, and you may wind in the inner peripheral direction in order.

  FIG. 7 is a cross-sectional view showing a cross section of the heating coil 21. Here, the cross section of the heating coil 21 having a predetermined number of turns as shown in FIG. 6 is shown. The heating coil 21 formed by winding in this way will be referred to as a two-layer overlapping structure. If the heating coil 21 has such a two-layered structure, even if the number of layers of the heating coil 21 is two, the gap (hereinafter referred to as a gap) between the winding of the heating coil 21 and the bottom surface of the top plate is reduced. Because it becomes possible. That is, since sufficient electromagnetic coupling between the heating coil 21 and the heated object 7 can be obtained, leakage loss can be reduced. It is assumed that this two-layer overlapping structure is adopted for the A coil. Therefore, the leakage loss of the A coil is reduced compared to the other heating coils 21 (see FIG. 5).

Next, the winding specification of the heating coil 21 will be described.
As described above, in order to increase the bus voltage when applying the rated power, it is necessary to increase the load impedance by increasing the no-load resistance R _Sn . If the number of turns of the heating coil 21 is simply increased in order to increase the no-load resistance R _Sn , the number of layers will increase and leakage loss will increase as described above. In order to increase the no-load resistance R _Sn without increasing the leakage loss and without increasing the number of layers of the heating coil 21, it is necessary to reduce the coil cross-sectional area of the heating coil 21 (formula ( 1)). However, this will in turn increase the heating coil loss.

Ｌ（ｍ）は、コイルの巻数（つまり、層数）に相当するコイルの長さを、Ｓ（ｍ² ）は、コイルの撚り数（つまり、コイル断面積）に相当するコイル断面積を、ρ（Ω・ｍ）は、コイル線の材質による抵抗率をそれぞれ示している。したがって、加熱コイル損失をも低減するような巻線仕様の加熱コイル２１を誘導加熱調理器５０に備えるようにするのが望ましいことがわかる。

L (m) is the coil length corresponding to the number of turns of the coil (that is, the number of layers), and S (m ² ) is the coil cross-sectional area corresponding to the number of twists of the coil (that is, the coil cross-sectional area), ρ (Ω · m) indicates the resistivity depending on the material of the coil wire. Therefore, it can be seen that it is desirable to provide the induction heating cooker 50 with the heating coil 21 having a winding specification that also reduces the heating coil loss.

  FIG. 8 is an explanatory diagram showing experimental results of loss in the heating coil 21. In this figure, leakage loss (W), heating coil loss (W), and inverter loss (W) in three heating coils 21 of C coil, D coil, and F coil are shown. In each graph, the upper part shows leakage loss, the interruption shows heating coil loss, and the lower part shows inverter loss. The F coil is assumed to have a smaller coil cross-sectional area than the C coil and the D coil. Therefore, it can be seen that the heating coil loss of the F coil is increased compared to the other heating coils 21.

From the above, in order to reduce the loss in the induction heating cooker 50 and improve the heating efficiency, the leakage loss of the heating coil 21 is not increased (that is, the number of layers is not increased), and the heating coil loss is reduced. It can be seen that it is required to obtain a heating coil winding specification that does not increase. This heating coil loss (P _C ) is calculated from the no-load resistance R _Sn and the load current I _R of the heating coil 21 (formula (2)).

FIG. 9 is an explanatory diagram showing experimental results of the load current I _{R, the} no-load resistance R _Sn , and the heating coil loss P _C of the heating coil 21. Here, four heating coils 21 of B coil, C coil, D coil, and F coil are shown. Those heating coils 21 are selected based on the heating efficiency. That is, the heating coil 21 (C coil, D coil, F coil) with high heating efficiency is compared with the heating coil 21 (B coil) with relatively low heating efficiency (see FIG. 2).

When no load resistor R _Sn as B coil is small, the load current I _R for obtaining the rated power becomes remarkably large, the heating coil loss P _C it can be seen that significantly increased accordingly. Conversely, it can be seen that when the no-load resistance R _Sn is large as in the F coil, the load current I _R becomes small, but the heating coil loss P _C tends to increase. From this, it was possible to obtain the result that the heating coil loss P _C becomes the minimum value by setting the resistance at no load R _Sn ≈0.3Ω to 0.7Ω. Therefore, it can be seen that in order to reduce the heating coil loss P and to obtain a highly efficient induction heating cooker 50, it is desirable to set the resistance at no load R _Sn ≈0.3Ω to 0.7Ω.

As described above, in addition to the first embodiment, the heating coil 21 has a two-layer winding structure, and the no-load resistance R _{Sn of} the heating coil 21 is 0.3Ω to 0.7Ω. by winding specifications, it is possible to reduce the heating coil losses P _C and leakage losses, is to be able to be improved further heating efficiency than in the first embodiment. Furthermore, by reducing the heating coil loss P _C, also it can be easily realized cooling of the heating coil 21.

[Embodiment 3]
In the third embodiment, a case in which the heating efficiency of the heating coil 21 is further improved by limiting the gap (the gap between the winding of the heating coil 21 and the bottom surface of the top plate) will be described as an example. That is, if the induction heating cooker 50 is configured by combining the first embodiment and the second embodiment, it is possible to obtain a higher efficiency of heating efficiency.

  FIG. 10 is a configuration cross-sectional view showing a cross section of induction heating cooker 50 according to the third embodiment. The gap is configured as an air layer between the upper surface of the winding of the heating coil 21 and the lower surface of the top plate 5, and is held by a spacer (not shown) provided on the coil base 9 of the heating coil 21.

  FIG. 11 is an explanatory diagram showing experimental results of the heating efficiency with respect to the gap length of the induction heating cooker 50. In this figure, the horizontal axis indicates the gap (mm), and the vertical axis indicates the heating efficiency (%). As shown in the figure, the heating efficiency varies depending on the length of the gap. That is, if the gap is set to be long, the electromagnetic coupling between the heating coil 21 and the object to be heated 7 is weakened and the heating efficiency is lowered.

The influence of the gap length on the heating efficiency will be described.
FIG. 11 is experimental data showing the heating efficiency of the heating coil 21 when the rated power is input while keeping the bus voltage constant. According to this experimental data, it can be seen that when the gap is about 2.5 mm or less, the difference in leakage loss is relatively small, and there is no significant change in heating efficiency. However, it can be seen that the leakage loss increases and the heating efficiency decreases as the gap becomes about 2.5 mm or more. Therefore, it can be seen that the gap is desirably about 2.5 mm or less in order to obtain high heating efficiency.

  As described above, in addition to the first embodiment and the second embodiment, the gap that is the gap between the winding of the heating coil 21 and the lower surface of the top plate 5 is set to about 2.5 mm or less. Leakage loss can be suppressed, and the induction heating cooker 50 having a higher heating efficiency than that of the first and second embodiments can be obtained.

It is a schematic block diagram which shows the circuit structure of the induction heating cooking appliance which concerns on Embodiment 1. FIG. It is explanatory drawing which shows the experimental result of the heating efficiency with respect to the bus-line voltage of a heating coil. It is explanatory drawing which shows the inductance of each heating coil in a no-load state. It is explanatory drawing which shows the experimental result of the heating efficiency with respect to the load current of a heating coil. It is explanatory drawing which shows the experimental result of the loss in a heating coil. It is a coil top view which shows the winding structure of a heating coil. It is sectional drawing which shows the cross section of a heating coil. It is explanatory drawing which shows the experimental result of the loss in a heating coil. It is explanatory drawing which shows the experimental result of load current, a no-load resistance, and a heating coil loss. It is composition sectional drawing which shows the state of the gap in an induction heating cooking appliance. It is explanatory drawing which shows the experimental result of the heating efficiency with respect to the length of a gap.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Commercial power supply, 2 Rectifier circuit, 3 Booster circuit, 4 Positive / negative bus, 5 Top plate, 7 Heated object, 8 Snubber capacitor, 9 Coil base, 10 Inverter circuit, 11 Switching element, 12 Switching element, 13 Switching element, 14 Switching element, 20 resonance circuit, 21 heating coil, 22 resonance capacitor, 50 induction heating cooker.

Claims (6)

A rectifier circuit for full-wave rectification of the AC power supply;
A booster circuit that boosts a DC voltage that has been full-wave rectified by the rectifier circuit;
An inverter circuit that converts the DC voltage boosted by the booster circuit into a high-frequency AC voltage;
A resonance circuit connected to the output stage of the inverter circuit and composed of a heating coil and a resonance capacitor;
An induction heating cooker characterized in that the inverter circuit is a full-bridge inverter circuit formed by two sets of two switching elements connected in series. In the booster circuit,
The induction heating cooker according to claim 1, wherein the voltage value of the DC voltage to be boosted is set in advance to be a predetermined voltage value. The induction heating cooker according to claim 1 or 2, wherein a predetermined voltage value set in the booster circuit is 300V to 400V. The heating coil;
The induction heating cooker according to claim 1, wherein the induction heating cooker has a two-layer wrapping structure with two windings. The induction heating cooker according to claim 4, wherein a resistance value of the heating coil when the object to be heated is not placed on the heating coil is 0.3Ω to 0.7Ω. The induction heating according to any one of claims 1 to 5, wherein a gap that is a gap between a winding of the heating coil and a top plate disposed on an upper surface of the heating coil is 2.5 mm or less. Cooking device.

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