Electrodes for dielectric heating, high-frequency dielectric heating apparatus, and method

Abstract

To provide a small-size electrode for dielectric heating excellent in heating efficiency and less uneven heating with a low cost, and a high-frequency dielectric heating device.SOLUTION: The electrode for dielectric heating dielectrically heats an object to be heated by the electric field generated between upper and lower electrodes by applying a high-frequency voltage between the upper and lower electrodes of a parallel plate. In the electrode for dielectric heating, a spiral-shaped electrode is provided on either or both of the lower surface of the upper electrode and the upper surface of the lower electrode, and the spiral-shaped electrode is connected to the parallel plate electrode at the outer peripheral end, and the inner peripheral end side is an open end.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an electrode for dielectric heating, a high-frequency dielectric heating device, and a method, and particularly to an electrode for dielectric heating, a high-frequency dielectric heating device, and a method for thawing frozen food by applying a high-frequency electric field in the MHz band to the frozen food and thawing the frozen food by dielectric heating.

Background Art

[0002] In a food processing factory or the like, there are cases where frozen food ingredients are thawed for food processing. As one type of thawing machine used in food processing factories or the like, a high-frequency dielectric heating device (high-frequency thawing device) is known that applies a high-frequency electric field in the MHz band to frozen food arranged between opposing electrodes and thaws the frozen food by dielectric heating.

[0003] High-frequency dielectric heating is a technology in which a high-frequency voltage is applied to a dielectric that is the object to be heated, and the object to be heated is heated from the inside by self-heating (dielectric loss) caused by the vibration of polar molecules constituting the object to be heated. In dielectric heating by microwaves (GHz band) using a microwave oven, since the heat generation difference between ice and water is large, uneven heating occurs due to significant heat generation in the melted part of the food surface layer. However, it is generally known that high-frequency dielectric heating using a frequency band lower than microwaves has the advantage that the penetration depth of energy is deeper than that of microwaves and the difference in heat generation amount between ice and water is also small, so uneven heating is less likely to occur.

[0004] FIG. 17 shows an example of dielectric heating using a high-frequency electric field in the MHz band realized by, for example, Patent Document 1. The high-frequency thawing device 10 in FIG. 17 is composed of a high-frequency power supply 1 that outputs an MHz band such as 13.56 MHz, 27.12 MHz, 40.68 MHz in the ISM band, a reflected power detection circuit 16, a matching circuit 11, an upper electrode 3, and a lower electrode 6. Further, the matching circuit 11 is composed of variable capacitors C12, C13, and a variable coil L10.

[0005] The MHz-band high-frequency signal (RF signal) from the high-frequency power supply 1 is matched with the equivalent impedance of the object to be heated 7, which is placed between the upper electrode 3 and the lower electrode 6, by the matching circuit 11 via the reflected power detection circuit 16, thereby efficiently transmitting power from the high-frequency power supply 1 and thawing the object to be thawed 7.

[0006] Furthermore, in high-frequency dielectric heating, when the dielectric constant of frozen food changes during thawing, the impedance between electrodes fluctuates, causing impedance matching to be misaligned and degrading heating efficiency. Patent Document 1 describes that in the reflected power detection circuit 16, the incident power to the electrodes and the reflected power are detected, and the matching state is determined by calculating the difference in VSWR (Voltage Standing Wave Ratio) obtained from the ratio of these two values, and the matching circuit 11 is adjusted so that matching is achieved.

[0007] Furthermore, Patent Document 2 states that by using multiple transmission amplifiers with relatively small power ratings and combining their outputs in a power combining circuit, the combined output of these transmission amplifiers can be used as the defrosting power. This results in a larger defrosting power, shortening the time required for defrosting. Moreover, it is possible to obtain an input impedance that is lower than the impedance suitable for the transmission amplifier, thus simplifying the configuration of the impedance matching circuit.

[0008] If we illustrate the description in Patent Document 2 with an example in Figure 17, by configuring multiple power transmission amplifiers built into the high-frequency power supply 1 to operate in parallel, the output impedance of the amplifiers is reduced, allowing the intermediate impedance to be lower than 50Ω, thus lowering the input impedance of the matching circuit 11. Furthermore, since the equivalent series resistance between the electrodes is also lower than 50Ω, the impedance conversion ratio of the matching circuit 11 can be reduced, which allows for simplification of the matching circuit and the use of relatively low-voltage circuit components. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2020-145114 [Patent Document 2] Japanese Patent Publication No. 2003-17238 [Overview of the project] [Problems that the invention aims to solve]

[0010] In Patent Document 1 shown in Figure 17, in dielectric heating using parallel plate electrodes, upper electrode 3 and lower electrode 6, even in dielectric heating in the MHz band, which is said to be less prone to uneven heating when the output power is increased to shorten the thawing time, the electric field concentrates at the edges of the object being thawed, causing thawing to progress more at the tips than in the center, resulting in uneven heating where the temperature at the edges rises more than necessary.

[0011] Furthermore, a resonance phenomenon occurs where the phases of the voltages generated between the variable coil C13 of the matching circuit 11 and the upper electrode 3 and lower electrode 6 are 180° apart and cancel each other out, resulting in a large voltage of several thousand volts or more across these terminals. This causes a large current of several amperes or more to flow through the parasitic capacitance between the electrodes and the housing, which results in losses and degrades the heating efficiency. In addition, the generation of high voltage necessitates the use of high-voltage, high-cost vacuum variable capacitors or high-voltage relays for the variable capacitors C12 and C13, resulting in the circuit components becoming large and expensive.

[0012] On the other hand, as described in Patent Document 2, when attempting to increase output power and lower output impedance by combining multiple power transmission amplifiers in parallel, and simultaneously lowering the impedance of the variable matching circuit to reduce size and cost, a problem arises in that it is not possible to accurately measure the reflected power and incident power necessary to detect the matching state between electrodes.

[0013] In Figure 17, the power detection circuit 16, which detects reflected power and incident power, detects them by comparing the incident and reflected power with a reference resistance. However, in Patent Document 2, due to the reduction in impedance, if the impedance between electrodes also fluctuates depending on the thawing state of the frozen food, the accuracy of detecting reflected power decreases and the heating efficiency deteriorates.

[0014] Based on the above, the present invention aims to provide a dielectric heating electrode, a high-frequency dielectric heating apparatus, and a method that are compact, low-cost, have excellent heating efficiency, and exhibit minimal heating unevenness. [Means for solving the problem]

[0015] Based on the above, the present invention is defined as "a dielectric heating electrode that dielectrically heats an object to be heated by an electric field generated between an upper electrode and a lower electrode of a parallel plate by applying a high-frequency voltage between the upper electrode and the lower electrode, characterized in that a spiral-shaped electrode is provided on either the lower surface of the upper electrode or the upper surface of the lower electrode, or both, and the outer peripheral end of the spiral-shaped electrode is connected to the electrode of the parallel plate, while the inner peripheral end is an open end."

[0016] Furthermore, the present invention provides a high-frequency dielectric heating device that applies a high-frequency voltage from a high-frequency power supply between the upper and lower electrodes of a parallel plate via a matching circuit that variably adjusts the impedance, thereby dielectrically heating an object to be heated by the electric field generated between the upper and lower electrodes, characterized in that spiral electrodes are provided on either the lower surface of the upper electrode or the upper surface of the lower electrode, or both, and the outer peripheral end of the spiral electrode is connected to the electrode of the parallel plate, while the inner peripheral end is an open end.

[0017] In the present invention, there is provided a high-frequency dielectric heating method in which a high-frequency voltage from a high-frequency power source is applied between an upper electrode and a lower electrode of a parallel plate through a matching circuit that variably adjusts impedance, and a dielectric object is dielectrically heated by an electric field generated between the upper electrode and the lower electrode. In this method, a spiral electrode is provided on either one or both of the lower surface of the upper electrode and the upper surface of the lower electrode. The outer peripheral end side of the spiral electrode is connected to the electrode of the parallel plate, and the inner peripheral end side thereof is an open end. Further, the outer peripheral end side of the spiral electrode can be connected to the electrode of the parallel plate through a plurality of switch circuits at a plurality of locations. The method is characterized in that it detects a high-frequency voltage or high-frequency power from a high-frequency power source and controls either one or both of the impedance of the matching circuit and a plurality of switch circuits according to the temperature of the object to be heated.

Advantages of the Invention

[0018] With the high-frequency dielectric heating device of the present invention, a high-frequency dielectric heating device with small size, low cost, excellent heating efficiency, and less heating unevenness can be obtained.

Brief Description of the Drawings

[0019] [Figure 1] A diagram showing a configuration example of a high-frequency dielectric heating device according to Embodiment 1 of the present invention. [Figure 2a] A configuration diagram showing the standing wave and electric field distribution generated by the spiral of the dielectric heating electrode of the present invention. [Figure 2b] A diagram showing another example of the spiral arrangement of a high-frequency dielectric heating device according to Embodiment 1 of the present invention. [Figure 3a] A cross-sectional configuration diagram showing the configuration of the dielectric heating electrode of the present invention. [Figure 3b] An equivalent circuit diagram showing the configuration of the dielectric heating electrode of the present invention. [Figure 4a] An impedance characteristic diagram showing the electromagnetic field analysis result of upper and lower spiral electrodes configured by the prior art. [Figure 4b] An impedance characteristic diagram showing the electromagnetic field analysis result of the dielectric heating electrode according to Embodiment 1 of the present invention. [Figure 5a] Figure showing the results of a conventional parallel plate electrode in which the heat generation density distribution of food materials during thawing was calculated by electromagnetic field analysis. [Figure 5b] Figure showing the results of the present invention in which the heat generation density distribution of food materials during thawing was calculated by electromagnetic field analysis. [Figure 6] Figure showing a configuration example of a high-frequency dielectric heating device according to Example 2 of the present invention. [Figure 7] Circuit diagram showing an example of a MOS FET switch circuit for switching the length of a spiral electrode according to Example 2 of the present invention. [Figure 8] Figure showing a configuration example of a dielectric heating device according to Example 3 of the present invention. [Figure 9a] Figure showing the results of a conventional parallel plate electrode in which the electric field distribution of a flat plate electrode during thawing was calculated by electromagnetic field analysis. [Figure 9b] Figure showing the results of the present invention in which the electric field distribution of a flat plate electrode during thawing was calculated by electromagnetic field analysis. [Figure 10] Figure showing a configuration example of a high-frequency dielectric heating device according to Example 4 of the present invention. [Figure 11] Circuit diagram showing an example of a capacitance switching circuit of a high-frequency dielectric heating device according to Example 4 of the present invention. [Figure 12a] Figure showing an equivalent circuit configured to convert a current source by a current source conversion circuit according to Example 4 of the present invention and cancel the imaginary part of an electrode by a variable matching circuit. [Figure 12b] Figure showing the result of calculating by circuit simulation the value of the voltage at a variable matching input terminal when the capacitance value of a variable matching circuit according to Example 4 of the present invention is changed. [Figure 13] Flow chart showing the process from the start to the end of dielectric heating of a high-frequency dielectric heating device according to Example 4 of the present invention. [Figure 14] Schematic cross-sectional view showing a configuration example of a high-frequency dielectric heating device according to Example 5 of the present invention. [Figure 15] Figure showing a configuration example of the back side of a high-frequency dielectric heating device according to Example 5 of the present invention. [Figure 16] Schematic perspective view showing configuration examples of the surface and the interior of a high-frequency dielectric heating device according to Example 5 of the present invention. [Figure 17] A block diagram showing an example of a circuit configuration for a conventional high-frequency dielectric heating device. [Modes for carrying out the invention]

[0020] The embodiments of the present invention will be described below with reference to the drawings.

[0021] High-frequency dielectric heating devices can be widely applied to heating objects, but when used specifically for thawing, they are sometimes called high-frequency thawing devices, and the following examples will describe a high-frequency thawing device.

[0022] Furthermore, in the embodiments of the present invention, it is envisioned that a high-frequency dielectric heating device is installed in the back room of a retail store such as a convenience store or supermarket, and that frozen foods such as bento boxes and prepared foods stored in a frozen state are thawed according to the number of customers entering the store, and then displayed on shelves in the store as chilled products, or that the thawed food is provided to customers.

[0023] However, the following description is merely a concrete example of the content of the present invention, and the present invention is not limited to this description. Various modifications and changes are possible by those skilled in the art within the scope of the technical ideas disclosed herein. [Examples]

[0024] Figure 1 shows a schematic configuration example of a high-frequency thawing device according to Embodiment 1 of the present invention. The high-frequency thawing device 10 consists of a high-frequency power supply 1 that outputs MHz bands such as 13.56 MHz, 27.12 MHz, and 40.68 MHz, which are ISM bands, a housing 2, an upper flat plate electrode 3, a lower flat plate electrode 6, an upper spiral 4, and a lower spiral 5, and thaws frozen food 7. The length (total length) of the upper spiral 4 and the lower spiral 5 is approximately λ / 4 of the output frequency of the high-frequency power supply 1. The outer circumference end of the upper spiral 4 is connected to the upper flat plate electrode 3, and the inner circumference end is open. Similarly, the outer circumference end of the lower spiral 5 is connected to the lower flat plate electrode 6, and the inner circumference end is open.

[0025] In the example shown in Figure 1, spiral electrodes are provided on both the lower surface of the upper electrode and the upper surface of the lower electrode. However, these may be provided on either the lower surface of the upper electrode or the upper surface of the lower electrode. Furthermore, the length (total length) of spirals 4 and 5 is best set to λ / 4 or an integer multiple of λ / 4 of the output frequency of the high-frequency power supply 1. However, since the effects of the present invention can be achieved even with a length close to λ / 4, in the following description of the present invention, lengths close to λ / 4 will be treated as λ / 4.

[0026] In the overall configuration outlined above, high-frequency power in the MHz band from the high-frequency power supply 1 shown in Figure 1 is applied between the upper flat plate electrode 3 and the lower flat plate electrode 4, generating an electric field within the flat plate electrode. This electric field excites the upper spiral 4 due to the potential difference with the lower flat plate electrode 6, and the lower spiral 5 due to the potential difference with the upper flat plate electrode 3, generating standing waves. As a result, the electric field concentrates in the center of the electrodes, which is the open end, rather than in the outer edges of spirals 4 and 5. This allows for an electrode configuration in which the center of the frozen food 7 is heated more thoroughly, making it less likely for uneven heating to occur at the edges.

[0027] In this invention, the combination of the upper flat electrode 3, the lower flat electrode 6, the upper spiral 4, and the lower spiral 5 is collectively referred to as a dielectric heating electrode. The dielectric heating electrode is an electrode that includes a flat electrode and a spiral.

[0028] Figure 2a shows the distribution of standing waves and electric fields generated on a λ / 4 spiral in the high-frequency defrosting apparatus shown in Figure 1.

[0029] The upper part of Figure 2a shows an example of the lower spiral 5, one of the upper and lower spirals 4 and 5. In the upper part of Figure 2a, the length l of the lower spiral 5 is made to be one-quarter of the wavelength λ, and the outer end is connected (short-circuited) to the lower flat plate electrode 6, while the inner end is left open. As a result, a standing wave of open-short with a potential difference of λ / 4 is excited in the lower spiral 5 due to the potential difference with the upper flat plate electrode 3. The potential of the open inner end is higher than that of the outer end, which is connected to the lower flat plate electrode 6 and at the same potential as the lower flat plate electrode 6, by the amplitude of the standing wave, so the electric field E in the central part becomes higher.

[0030] The middle section of Figure 2a shows the relationship between the distance r from the inner end to the outer end of the spiral 5 and the electric field E near the spiral electrode, indicating that the electric field E is higher in the central part. The lower section of Figure 2a shows the position when the spiral is extended and the relationship between the voltage V and current I at the spiral electrode at that position. It shows a tendency for the voltage to increase and the current to decrease as the position farther from the outer end increases.

[0031] In Figure 2a, the lower spiral 5 is connected to the lower flat electrode 6, but the same configuration is used for the upper flat electrode 3. Furthermore, by connecting spirals 4 and 5 to both the upper and lower flat electrodes 3 and 6, an even greater electric field concentration effect can be obtained.

[0032] In this case, the magnetic flux generated by the current flowing in the winding directions of the upper and lower spirals 4 and 5 has an equivalent length of λ / 4 that differs depending on whether it is in the same direction or opposite directions. In Figure 1, when the winding directions of the upper and lower spirals 4 and 5 are opposite, the generated magnetic fluxes are in the same direction and reinforce each other, so the upper and lower spirals are shortened slightly by the amount of mutual coupling. On the other hand, if the winding directions of the upper and lower spirals are the same but the magnetic fluxes are in opposite directions, the length of the spirals needs to be increased by the amount of mutual coupling.

[0033] Figure 2b also shows another example of spiral arrangement, which includes lower spirals 5a and 5b. The lower spirals 5a and 5b are connected at their outer ends to the lower flat plate electrode 6, respectively, and their inner ends are open. Multiple frozen food items 7a and 7b are placed on these spirals. As shown in Figure 2b, the same effect can be obtained by arranging multiple spirals, making it possible to thaw multiple food items or to accelerate heating in specific areas. Furthermore, even if the spiral length is shorter than λ / 4, the electric field concentration effect will be smaller, but it will still be possible to reduce uneven heating.

[0034] Figure 3a is a cross-sectional diagram showing the dielectric heating electrodes of the high-frequency defrosting apparatus in Figure 1, and Figure 3b shows its equivalent circuit. Parts that overlap with Figure 1 are given the same numbers and their explanations are omitted. These figures will be used to explain the impedance characteristics between the electrodes and the matching method.

[0035] In Figure 3a, capacitance C1 is generated between the upper flat electrode 3 and the frozen food 7, capacitance C2 between the lower flat electrode 6 and the frozen food 7, capacitance C3 between the upper spiral 4 and the frozen food 7, and capacitance C4 between the lower spiral 5 and the frozen food 7. Furthermore, since the lower flat electrode 6 is connected to the housing which is connected to GND, a parasitic capacitance C5 is generated between the upper flat electrode 3 and the housing 2.

[0036] Furthermore, Figure 3b shows the equivalent circuit of the configuration shown in Figure 3a, where the frozen food item 7 is represented by an equivalent series capacitance C6 and an equivalent series resistance R6. In Figure 3b, capacitances C1 and C2 are generated at both ends of the frozen food item 7, and a parasitic capacitance C5 is generated on the upper spiral 5 side due to contact with the casing. In addition, a series resonant circuit is formed by the inductor L1 and capacitance C3, equivalent capacitance C6, equivalent series resistance R6, capacitance C4 of the upper spiral 5, and the inductor L2 of the lower spiral 6.

[0037] Therefore, when the resonant frequency is close to the transmission frequency, the impedance between the electrodes decreases due to series resonance, making matching easier. Furthermore, because the impedance is lower, the voltage applied between the flat electrodes can also be reduced, so the current flowing through the parasitic capacitance C5 between the housing and the electrodes also decreases, reducing losses due to coupling between the housing and electrodes and improving heating efficiency.

[0038] Furthermore, as the distance between the spirals increases, the capacitances C3 and C4 decrease. Therefore, the Q value, which indicates the loss state of the resonant circuit, increases. This can be calculated using the formula Q = ωL / R = 1 / (ωCR), where L is the inductance, C is the capacitance, and R is the resistance. As a result, the loss of the resonant circuit effectively decreases. Consequently, increasing the distance between the spirals increases the Q value of the resonant circuit, which reduces the decrease in heating efficiency when the distance increases.

[0039] Figure 4a shows the electromagnetic field analysis results for the impedance of electrodes in a configuration where two spiral electrodes with a length of λ / 4 are formed on the GND plane and face each other, the outer ends of these spirals are connected, one end of the inner circumference is left open, and a high-frequency power supply is applied from the other end to defrost frozen food placed between the spirals, as an example of known technology. Figure 4b shows the electromagnetic field analysis results for the impedance of the electrode configuration of the high-frequency defroster of Example 1 shown in Figure 1.

[0040] Both analyses were performed with spiral distances of 40mm and 80mm. The spiral size was 250mm x 185mm with 3.5 turns, a pattern width of 15mm, a pattern thickness of 0.3mm, and a frequency of 40.68MHz. Additionally, a simulated food item measuring 200mm x 100mm with a height of 25mm, frozen to -20°C, was used as the analysis model.

[0041] In the known technique shown in Figure 4a, connecting two λ / 4 spirals results in a length of λ / 2, causing the reflected waves to be in phase and the impedance to become very high near the transmission frequency. However, at a distance of 40 mm, the impedance is effectively reduced by the amount consumed in thawing the frozen food, making matching using a matching circuit possible. However, at 80 mm, the heating efficiency for the frozen food deteriorates and the electrode impedance becomes very high, making matching difficult.

[0042] In contrast, the electrode structure of the present invention shown in Figure 4b operates in series resonance, resulting in a low impedance. Furthermore, the Q value of the resonant circuit increases even when the distance between spirals increases, so the change in impedance is small. This makes impedance matching with the 50Ω commonly used in high-frequency power supplies easier to achieve even at a distance of 80mm compared to conventional technology.

[0043] Figure 5a shows the heat distribution inside the food when the simulated food is heated using the known technique shown in Figure 17, and Figure 5b shows the heat distribution obtained by electromagnetic field analysis when heated using the dielectric heating electrode of Example 1 shown in Figure 1.

[0044] In the known technique shown in Figure 5a, a 300mm x 260mm flat electrode with a 40mm distance between electrodes was used as the analytical model, along with a 200mm x 150mm and 25mm high simulated food item that had been frozen to -20°C.

[0045] Furthermore, the electrode structure of Example 1 shown in Figure 5b consisted of a flat electrode measuring 300 mm × 260 mm and a spiral electrode measuring 250 mm × 185 mm with 3.5 turns, a pattern width of 15 mm, and a pattern thickness of 0.3 mm. The frozen food used was the same shape as in known technology. Both analyses were performed at a frequency of 40.68 MHz.

[0046] Comparing these, while the known parallel plate electrode promotes heating at the edges of the simulated food, the configuration with a spiral connected to the plate promotes heating more at the center of the food, thus reducing uneven heating at the edges.

[0047] In the configuration of Example 1 described above, the electric field is concentrated in the center, which reduces uneven heating. In addition, losses due to coupling between the flat plate electrode and the housing are reduced, and dielectric heating electrodes that are easy to match even when the distance between electrodes is large can be obtained. Furthermore, by suppressing uneven heating, it becomes possible to shorten the thawing time by increasing the power output. [Examples]

[0048] Figure 6 is a schematic diagram showing the configuration of a high-frequency defrosting apparatus according to Embodiment 2 of the present invention. The high-frequency defrosting apparatus 10 consists of a matching circuit 11, a variable capacitor C12, a control circuit 13, switch circuits SW1 to SW3, and a temperature sensor 17. Other parts that overlap with Embodiment 1 of the high-frequency defrosting apparatus shown in Figure 1 are given the same numbers and their descriptions are omitted.

[0049] Figure 6 differs from the high-frequency defrosting device 10 of Embodiment 1 shown in Figure 1 in that a matching circuit 11 consisting of a variable capacitor C12 is connected between the high-frequency power supply 1 and the electrodes, and multiple switch circuits SW1 to SW3 are connected between the outer peripheral end of the lower spiral 5 and the lower flat plate electrode 6 connected to the housing 2 which is connected to GND, and the spiral length of the lower spiral 6 can be adjusted by turning the switch circuits SW1 to SW3 on and off using the control circuit 13.

[0050] With this configuration, for example, the temperature sensor 17 detects the thawing state of the frozen food 7, and the control circuit 13 switches between switch circuits SW1 to SW3, thereby matching the impedance fluctuations between electrodes due to changes in dielectric constant caused by thawing. Furthermore, if matching cannot be achieved by switching between switch circuits SW1 to SW3, adjustment can also be made using the variable capacitor C12 of the matching circuit 11.

[0051] With the above configuration, the same effects as in Embodiment 1 shown in Figure 1 can be obtained. Furthermore, because the switching is performed on the portion of the lower spiral 5 that is close to the GND potential at the outer edge, it is not necessary to use a high-voltage relay with a voltage rating of several thousand volts or more. In addition, since the voltage amplitude of the upper flat electrode 3 is only about several hundred volts, it is not necessary to use a high-voltage vacuum variable capacitor or the like for the variable capacitance C12, thus reducing the cost of the matching circuit 11.

[0052] Figure 7 shows the circuit configuration when MOS FETs are used instead of the switch circuits SW1 to SW3 shown in Figure 6. In Figure 7, the switch circuit SW has the sources of MOS FETs 75 and 76 connected in common, and both drain terminals 72 and 73 are connected between the outer edge of the lower spiral 5 and the lower flat plate electrode 6. In addition, each gate is connected to the control circuit 13 from the control terminal 74 via gate protection resistors R7 and R8. By using the MOS FETs shown in Figure 7 for the switch circuits SW1 to SW3, it is possible to make the circuit even smaller and lower in cost. [Examples]

[0053] Figure 8 is a schematic diagram showing the configuration of a high-frequency defrosting device according to Embodiment 3 of the present invention. The high-frequency defrosting device 10 consists of an RF signal source 80 that outputs MHz bands such as 13.56 MHz, 27.12 MHz, and 40.68 MHz, an attenuator 82, a power transmission amplifier 81, a low-pass filter circuit 85, a reflected power detection circuit 16, a variable matching circuit 11, a power supply circuit 14, and a control circuit 13. Other parts that overlap with Embodiment 1 of the high-frequency defrosting device shown in Figure 1 are given the same numbers and their explanations are omitted.

[0054] In the high-frequency defrosting device shown in Figure 8, the high-frequency power supply 1 is composed of an RF signal source 80, an attenuator 82, a power transmission amplifier 81, and a low-pass filter 85. In addition, a variable matching circuit is used as the matching circuit 11, and a reflected power detection circuit 16 is used as the power detection circuit, which is an additional feature compared to Figure 1.

[0055] The power transmission amplifier 81 in Figure 8 consists of a distribution circuit 815, an amplification element 816, a combining circuit 817, and a power supply terminal 812. It amplifies the RF signal input from the RF signal input terminal 811 and outputs it to the RF signal output terminals 813 and 814. The low-pass filter 85 is configured to attenuate harmonics generated by the power transmission amplifier 81.

[0056] The reflected power detection circuit 16 detects the amount of reflected power generated by the matching state with the electrodes and outputs it to the control circuit 13.

[0057] The variable matching circuit 11 consists of a variable capacitor C12 connected to GND and a variable capacitor C13 connected in series. The upper and lower spirals 4 and 5 are made slightly longer than λ / 4 to account for impedance changes due to the thawing state of the frozen food 7, making the inter-electrode impedance inductive and allowing matching with the variable matching circuit 11, which consists only of variable capacitors.

[0058] In the above configuration, the RF signal amplified by the power transmission amplifier 81 is used to thaw the frozen food 7 placed between the upper and lower spirals 4 and 5 by dielectric heating. The reflected power detection circuit 16 detects the matching state of the inter-electrode impedance, which changes depending on the type and size of the frozen food and the thawing state, and the control circuit 13 adjusts the variable capacitors C12 and C13 of the variable matching circuit 11 to achieve matching, thereby enabling efficient thawing. The completion of thawing is determined by measuring the temperature of the frozen food 7 with the temperature sensor 17.

[0059] The present invention provides a high-frequency defrosting device 10 with the general configuration described above, and in order to obtain a high-frequency defrosting device 10 that is small, low-cost, has excellent heating efficiency and less heating unevenness, various improvements have been made, including high output and low cost of the power transmission amplifier 81, miniaturization and low cost of circuits such as the variable matching circuit 11, and connection of spirals 4 and 5 to the flat plate electrodes 3 and 6 to reduce heating unevenness which becomes more pronounced with high output.

[0060] The following explanation will describe the effects of these design features for each component. First, the power transmission amplifier 81, which is configured with the aim of enabling high output for quick defrosting, receives the RF signal from the RF signal source 80 via the attenuator 82 to the RF signal input terminal 811. The input RF signal is then fed to each amplification element 816 by the distribution circuit 815, and the amplified RF signal is combined by the combining circuit 817 and output between the RF signal output terminals 813 and 814.

[0061] In the above configuration, by using a push-pull amplifier circuit with four amplification elements, the output power can be improved, and the voltage resistance required for the amplification elements can be lowered, thus reducing the cost of the amplification elements.

[0062] The variable matching circuit 11 makes the length of the electrode spirals 4 and 5 longer than λ / 4, thereby making the impedance between the electrodes inductive and allowing matching to be achieved solely by the capacitive components of the variable capacitors C12 and C13. As a result, there is no voltage rise between the upper plate electrode 3 and the lower plate electrode 6 due to the matching inductor, and thus the losses due to capacitive coupling between the upper plate electrode 3 and the housing 2 are reduced, improving heating efficiency.

[0063] Figure 9a shows the results of electromagnetic field analysis of the voltage distribution of the upper flat electrode 3 of the known technology shown in Figure 17, and Figure 9b shows the results of the voltage distribution analysis of the upper flat electrode 3 of Example 3 shown in Figure 8.

[0064] The analysis was similar to the heat generation analysis of the simulated food shown in Figures 5a and 5b. In Figure 9a, a 300mm x 260mm flat electrode with a distance of 40mm between electrodes was used, and a 200mm x 150mm, 25mm high simulated food sample was frozen to -20°C. In Figure 9b, a 300mm x 260mm flat electrode was used, and a 250mm x 185mm spiral electrode with 3.5 turns, a pattern width of 15mm, and a pattern thickness of 0.3mm was used. The frequency for both was 40.68MHz.

[0065] In the known technology shown in Figure 9a, an inductor is used in the matching circuit 11, resulting in a relatively high voltage across the upper flat electrode 3. In contrast, in the embodiment 3 shown in Figure 8, where a spiral is connected to the parallel flat electrode in Figure 9b, the matching circuit 11 does not use an inductor, resulting in a lower voltage across the upper flat electrode 3.

[0066] With the above configuration, the same effects as in Example 1 can be obtained, and in addition, the power transmission amplifier can be made more powerful and less expensive. Furthermore, the use of high-voltage variable capacitors C12 and C13 in the matching circuit 11 is not required, and general air variable capacitors or capacitance switching using capacitors and relays can be used, thus making it possible to reduce the cost of the matching circuit.

[0067] Furthermore, the same effect can be obtained by using electrodes with a configuration in which the changeover switch circuit SW of Embodiment 2 shown in Figure 6 is added to the lower spiral 5. In this case, since the equivalent series-connected inductance value is adjusted, it becomes possible to omit the series-connected variable capacitor C13. [Examples]

[0068] Figure 10 shows a schematic configuration example of a high-frequency defrosting device according to Embodiment 4 of the invention. The high-frequency defrosting device 10 has a current source conversion circuit 85A that also serves as a low-pass filter, a variable matching circuit 11A, and a voltage detection circuit 16A. Other parts that overlap with Embodiment 2 shown in Figure 6 and Embodiment 3 of the high-frequency defrosting device shown in Figure 8 are given the same numbers and their descriptions are omitted.

[0069] Comparing the high-frequency defrosting device in Figure 10 with the high-frequency defrosting device in Figure 8, the differences from Figure 8 are that the low-pass filter 85 is configured as a current source conversion circuit 85A which also functions as a low-pass filter, the reflected power detection circuit 16 is configured as a voltage detection circuit 16A, and the variable matching circuit 11 is configured as a variable matching circuit 11A, and furthermore, switch circuits SW1 to SW3 are added to the lower spiral 5.

[0070] The current source circuit 85A, which also functions as a low-pass filter in Figure 10, is composed of a π-type filter using capacitors C21 and C22 and inductor L21. If the capacitance values ​​C of capacitors C21 and C22 are equal, inductor L21 is L, the transmission frequency is f (ω=2πf), the output voltage of the transmission amplifier 81 is V, and the reactance at the transmission frequency is X, then setting the values ​​of capacitor C and inductor L such that the relationship X=jωL=-j1 / (ωC) holds true will result in the current I output from the current source output terminal 1012 being I=-jV / X, thus creating a constant current source determined by the output voltage V and reactance X. In this case, if the value of X is equal to the characteristic impedance Z0 (generally 50Ω) of the transmission amplifier 81, then under load conditions equal to the characteristic impedance, voltage source driving and current source driving become equivalent.

[0071] The variable matching circuit 11A is composed of a series-connected variable capacitor C13. The voltage detection circuit 16A is composed of a detection diode D1, a capacitor C23, and resistors R10 and R11, and detects the voltage applied to the input terminal 1021 of the variable matching circuit 11A, and the control circuit 13 detects the matching state.

[0072] In the above configuration, the output voltage of the power transmission amplifier 81 is converted to a constant current source by a current source conversion circuit 85A which also serves as a low-pass filter, and matching is achieved by a variable matching circuit 11A and switch circuits SW1~SW3. The frozen food 7 placed between the upper and lower spirals 4 and 5 is thawed by dielectric heating. The impedance between electrodes changes depending on the thawing state, so the matching state is detected by a voltage detection circuit 16A, and the control circuit 13 adjusts the variable capacitance C13 of the variable matching circuit 11A to achieve matching, thereby efficiently thawing the food.

[0073] The present invention provides a high-frequency dielectric heating device 10 with the general configuration described above, and in order to obtain a high-frequency dielectric heating device that is small, low-cost, has excellent heating efficiency and less heating unevenness, improvements have been made in various aspects, such as simplifying control by adding a current source conversion circuit 85A that also serves as a low-pass filter, and miniaturizing and reducing the cost of the circuit by using a variable matching circuit 11A and a voltage detection circuit 16A.

[0074] The following explanation will describe the effects of these design features for each component requirement.

[0075] First, the current source conversion circuit 85A, which is designed for ease of control, simplifies the output control of the power transmission amplifier 81 in response to changes in impedance between electrodes due to thawing. The impedance between electrodes is relatively low due to the connection of the upper and lower spirals, as shown in Figure 4b.

[0076] On the other hand, the power transmission amplifier 81 also consists of multiple amplification elements connected in parallel, resulting in a low output impedance. Therefore, it is necessary to first raise it to a typical characteristic impedance of 50Ω and then lower it using the matching circuit 11, which makes the matching circuit 11 complex.

[0077] Therefore, the circuit can be simplified by directly driving the electrodes with the low impedance of the power transmission amplifier 81. However, in this case, the equivalent resistance of the frozen food 7 is about 40Ω at -20℃ and about 20Ω at -5℃, and the resistance decreases as thawing progresses. Therefore, when the power transmission amplifier 81 is a voltage source V, the heating power at -20℃ is V 2 / 40, -5℃ is V 2 Because the power consumption is multiplied by 20, the highest power consumption occurs when the temperature reaches -5°C, which is when thawing is almost complete.

[0078] In contrast, if current source I is used, the heating power at -20℃ is I 2 ×40, -5℃ is I 2 Since the multiplier is 20, the power is highest when the most heating power is needed at the start of thawing, and gradually decreases as thawing progresses, which simplifies the output control of the power transmission amplifier 81.

[0079] The variable matching circuit 11A is used to match impedance when adjustment is not possible with the selector switch circuits SW1~SW3 connected to the lower spiral 5 by using a series-connected variable capacitor C13. By matching impedance through series resonance, including the impedance between electrodes, it enables low-impedance driving. Furthermore, as shown in Figure 11, the variable matching circuit 11A can also be matched by installing parallel capacitors C32 and C33 in addition to the normally connected series capacitor C31, and switching the capacitance values ​​using selector switches SW4 and SW5.

[0080] Because the voltage detection circuit 16A is a low-impedance circuit with series resonance compared to the equivalent circuit in the spiral connection shown in Figure 3b, the detection accuracy of the reflected power detection circuit 16, which performs detection based on a fixed impedance, deteriorates.

[0081] Figure 12a shows the results of determining the terminal voltage V in a series resonant circuit when a current source is driven. In the figure, if the impedance between electrodes including the frozen food is the equivalent inductor L, the equivalent capacitance C, and the resistance mainly consumed during the thawing of the frozen food is the equivalent resistance Rrs, then the terminal voltage V is lowest when the impedance becomes equal to Rrs at resonance. Therefore, it can be determined that the point where the voltage is lowest by adding the voltage detection circuit 1030 is matched.

[0082] Figure 12b shows the results of a simulation in which a load equivalent to the inter-electrode impedance, including the power transmission amplifier 81, current source conversion circuit 85A, variable matching circuit 11A, and frozen food 7, is connected, and the variable capacitance of the variable matching circuit 11A is changed. The horizontal axis represents inductive or capacitive impedance, and the vertical axis represents voltage amplitude. From the results in the figure, it can be seen that the terminal voltage is lowest at resonance (matching).

[0083] Figure 13 is a flowchart showing the process of thawing frozen food 7 in the high-frequency dielectric heating device 10 according to Example 4 shown in Figure 10, and will be explained with reference to Figure 10. This flowchart shows the processing flow by the control circuit 13 in Figure 10, which is realized using a computer.

[0084] In this process, the length of the lower spiral 5 is adjusted to achieve alignment. If alignment cannot be achieved by adjusting the spiral length, the variable capacitance C13 is adjusted to achieve alignment.

[0085] First, in processing step S1, the control circuit 13 adjusts the attenuator 82 to begin transmitting RF signal power output from the power transmission amplifier 81 to the frozen food 7 placed between the upper spiral 4 and the lower spiral 5 at a power lower than the power required for normal thawing.

[0086] Next, in processing step S2, the control circuit 13 turns off all of the toggle switches SW1 to SW3 of the lower spiral 5 to maximize the length of the spiral, and sets the capacitance value of the variable capacitance capacitor C13 of the variable matching circuit 11A to its maximum value. At this time, the voltage detection circuit 16A measures the detection voltage corresponding to the voltage amplitude at the input terminal 1021 of the variable matching circuit, and further measures the detection voltage with the toggle switch SW of the lower spiral 5 set to a value one step smaller in processing step S3.

[0087] In processing steps S4 and S5, the control circuit 13 sets the variable capacitor C13 to a value one step smaller if the length of the lower spiral is at its shortest. In processing step S6, it compares the detection voltage results before and after the measurement. If the detection voltage has decreased, it returns to processing step S3, shortens the length of the lower spiral 5 by one step, and measures the detection voltage at that time. If the detection voltage has increased in processing step S6, in processing step S7, if the previous processing shortened the length of the lower spiral 5 by one step, it lengthens the spiral by one step. If the capacitance value of the variable capacitor C13 was set to a value one step smaller, it returns it to a value one step larger and starts normal power transmission.

[0088] Then, in processing step S8, it is determined whether the temperature of the temperature sensor 17 has reached the set temperature value for thawing completion. If the set temperature has not been reached, heating continues continuously through the loop of processing steps S11, S9, and S8 until the detection voltage becomes high. If the detection voltage becomes high, in processing step S12, the imaginary part of the electrode impedance has changed from capacitive to inductive, so the power transmission is reduced, and the process returns to step S3. The length of the lower spiral 5 is shortened or the capacitance value of the variable capacitor C13 is reduced until the imaginary part is canceled out, and power transmission is started at a high power level. In processing step S10, these steps are repeated until the set temperature is reached. Once the set value is reached, power transmission is stopped, and the process terminates with a notification that thawing is complete.

[0089] By measuring the voltage amplitude at the input of the variable matching circuit in this way, the matching state can be detected, making it possible to defrost frozen food with relatively simple control.

[0090] With the above configuration, the high-frequency defrosting device according to Example 4 can obtain the same effects as the high-frequency defrosting device according to Example 3 shown in Figure 8. In addition, the control can be simplified by using the current source conversion circuit 85A, and the reflected power detection circuit can be simplified by using the voltage detection circuit. [Examples]

[0091] Figure 14 is a schematic side view showing an example of the configuration of a high-frequency defrosting apparatus according to Embodiment 5 of the present invention, and the configuration of Embodiment 5 of the present invention will be explained with reference to Figure 8.

[0092] Figure 8 shows the main electrical circuit configuration of the high-frequency defrosting device, while Figure 14 is a side view showing the internal structure of the housing that contains the electrical circuit shown in Figure 8. Note that parts that overlap with the high-frequency dielectric heating device according to Embodiment 3 shown in Figure 8 are denoted by the same reference numerals and their descriptions are omitted.

[0093] In Figure 14, the enclosure 2 contains a reflected power detection circuit 16, a power transmission amplifier 81, a cover 142, a ground shield plate 143, a mounting board 144, a heat sink 145, heat sink fins 146, an outlet 147, cables 148 and 149, an insulating plate 141, a fan 1411, and other components arranged in the positions shown.

[0094] In the diagram, the distribution circuit 815, amplification element 816, and combining circuit 817 that constitute the power transmission amplifier 81 are mounted on a mounting board 144. A heat sink 145 is attached to the back surface of the mounting board 144, and heat sink fins 146 and a fan 1411 are mounted on the heat sink 145.

[0095] Furthermore, by mounting the power transmission amplifier 81 and heat sink 145 upright at the back of the housing 2 of the high-frequency defroster, heat can be easily dissipated from the back, and cables 148 and 149 can be used to connect the upper and lower flat electrodes 3 and 4 to the outlet 147 of the GND shield plate 143 over a short distance, thereby reducing cable losses.

[0096] Furthermore, an insulating plate 141 is placed on the upper part of the lower spiral 6 to ensure distance between the electrodes and the frozen food 7, a temperature sensor 17 is installed near the center of the lower electrodes, and a power supply circuit 14 is mounted on the lower part of the power transmission amplifier 81.

[0097] Figure 15 is a rear view showing an example of the rear structure of the high-frequency defrosting apparatus according to Embodiment 5 of the present invention shown in Figure 14. In the figure, parts that overlap with the high-frequency defrosting apparatus according to Embodiment 5 shown in Figure 14 are denoted by the same reference numerals and their descriptions are omitted.

[0098] In Figure 15, 151 is a power outlet, and the rear side of the enclosure 2 has cutouts for mounting the heat sink 145, heat sink fins 146, and fan 1411, improving the heat dissipation of the power transmission amplifier. In addition, the power supply circuit 14 is located below the power transmission amplifier 81, the control circuit 13 is located on the operation panel side (left side), and the variable matching circuit 11 is located behind the GND shield plate 143 on the opposite side, and is connected to the upper flat plate electrode 3 via a cable outlet from the output terminal 1022 of the variable matching circuit.

[0099] Figure 16 is a schematic perspective view showing an example of the surface and interior structure of the high-frequency thawing apparatus according to Embodiment 5 of the present invention shown in Figure 14. In the figure, parts that overlap with the high-frequency thawing apparatus according to Embodiment 3 shown in Figure 8 are denoted by the same reference numerals and their descriptions are omitted.

[0100] According to Figure 16, shelf supports 1601 and 1602, a display panel 1603, push buttons 1604 and 1605, and an adjustment knob 1606 are positioned on the surface and inside the chamber as shown. Furthermore, a GND shield plate 143, provided to reduce the influence on other circuits and the electromagnetic field radiated to the outside due to the voltage applied between the upper and lower flat plate electrodes 3 and 4, has an outlet 147 for pulling out cables 148 and 149 that connect to the electrodes, and resin-made insulated shelf supports 1601 and 1602 (the shelf support on the right is not shown) are attached to both sides of the GND shield plate 143.

[0101] This shelf support structure holds the upper and lower flat electrodes 3 and 6 and the insulating plate 141 (see Figure 14), and the height can be changed by changing the position of the shelf support. The front display panel 1603 can display the current defrosting status and temperature setting, and there are also buttons 1604 and 1605 for starting and stopping heating, and an adjustment knob 1606 for setting the temperature at which defrosting is complete. Behind these control panels is a control circuit 13.

[0102] With the above configuration, a high-frequency thawing device can be obtained that has excellent heat dissipation to cope with the increased heat generated by the higher output of the power transmission amplifier, reduces transmission loss by shortening the length of the cable between the power transmission amplifier and the upper and lower electrodes, and allows the height of the electrodes to be changed, enabling thawing of frozen food items of different heights.

[0103] In summary, through Examples 1 to 5, we have described efforts to reduce uneven heating, increase power output, miniaturize the device, and reduce costs. The main points of these efforts can be summarized and listed as follows:

[0104] Firstly, by forming a spiral of length λ / 4 on parallel plates and connecting the outer ends to the plates, the electric field is concentrated in the center, thus reducing uneven heating at the edges. Furthermore, since the voltage generated on the plate electrodes is reduced, losses due to coupling between the plate electrodes and the housing are reduced, improving heating efficiency.

[0105] The second point is that the configuration achieves impedance matching by switching the length of the spiral using a switch circuit at the outer end of the spiral on the GND-grounded side. This allows the use of low-voltage MOS FETs, resulting in lower costs and a smaller matching circuit.

[0106] The third point concerns the power transmission amplifier, which is configured to have a constant current source conversion circuit that also functions as a low-pass filter (LPF) to suppress harmonics added to its output. This allows for a power transmission amplifier output characteristic where the heat generation is high at the start of thawing and gradually decreases, in contrast to the tendency for the equivalent series resistance of frozen food to decrease with the thawing temperature during thawing. This simplifies the detection control, which involves detecting the thawing state and controlling the output accordingly.

[0107] The fourth point concerns the variable matching circuit. Conventionally, variable matching circuits, which match the impedance between electrodes that change during thawing, required two variable elements to match the real and imaginary parts of the electrode impedance. However, by connecting the variable matching circuit in series with the electrodes and canceling only the imaginary part of the electrodes, the number of variable matching elements was reduced to one, resulting in miniaturization and cost reduction. Furthermore, by using a single variable element, the relatively low equivalent resistance of the frozen food between the electrodes becomes directly visible. In a power transmission amplifier, the output impedance is reduced by combining the outputs of multiple amplification elements, making it a suitable configuration for directly driving the low equivalent series resistance between the electrodes.

[0108] The fifth point concerns the measurement of reflected power. Conventionally, to detect the matching state, a reflected electrode detection circuit was used between the power transmission amplifier and the variable matching circuit (input side) to detect the incident power and reflected power, and the VSWR, which is obtained from the voltage amplitude ratio of these, was used as a threshold for determining the matching state. However, in the above-mentioned configuration that cancels only the imaginary part of the inter-electrode impedance, the series equivalent resistance of the frozen food, which is the real part, changes depending on the thawing state. Therefore, the matching state cannot be grasped by VSWR measurement using a directional coupler that measures by comparing it with a reference resistance value. In contrast, in the above-mentioned configuration that cancels only the imaginary part, the voltage amplitude at the input terminal of the variable matching circuit is lowest when the imaginary part of the electrode impedance is canceled out. By adding a detection circuit that detects the voltage amplitude at the input terminal of the variable matching circuit, it is possible to detect the matching state, and the circuit is simplified and costs are reduced by eliminating the need for the directional coupler that was previously required. [Explanation of Symbols]

[0109] 1: High frequency power supply 2: Cabinet 3: Upper plate electrode 4: Upper spiral 5, 5A, 5B: Lower spiral 6: Lower plate electrode 7, 7a, 7b: Frozen ingredients 10: High-frequency defrosting device 11, 11A: Matching circuit (variable matching circuit) 13: Control circuits 14:Power circuit 16: Reflected power detection circuit 17: Temperature sensor 72, 73: Drain terminals 74: Control terminal 75, 76: MOS FET 80:RF signal source 81: Power transmission amplifier 82: Attenuator 85: Low-pass filter 85A: Current source conversion circuit 142: Lid 143: GND Shielding Plate 144: Implemented circuit board 145: Heat sink 146: Finn 147: Drawer opening 148, 149: Cable 141: Insulating board 811: RF signal input terminal 812: Power terminal 813, 814: RF signal output terminals 815: Distribution circuit 816: Amplifying elements 817:Synthesis circuit 821, 1011, 1021, 1031: Input terminals 822, 1012, 1022: Output terminals 16A, 1030: Voltage detection circuit R10, R11: Resistance D1: Detection diode 1411: Fan 151: Electrical outlet 1601, 1602: Shelf brackets 1603: Display Panel 1604, 1605: Push button 1606: Adjustment knob C1, C2, C3, C4, C5: Capacitance C6: Equivalent series capacitance C12, C13: Variable capacity C21, C22, C23: Capacitors L1, L2, L21: Inductors R6: Equivalent series resistance R7, R8: Gate protection resistors SW1~SW5: Switch Circuit

Claims

1. A dielectric heating electrode that applies a high-frequency voltage between the upper and lower electrodes of a parallel plate, thereby dielectrically heating an object to be heated by the electric field generated between the upper and lower electrodes, A dielectric heating electrode characterized in that a spiral-shaped electrode is provided on either the lower surface of the upper electrode or the upper surface of the lower electrode, or both thereof, the outer peripheral end of the spiral-shaped electrode is connected to the parallel plate electrode, and the inner peripheral end is an open end.

2. A dielectric heating electrode according to claim 1, The dielectric heating electrode is characterized in that the length of the spiral electrode is 1 / 4 the length of the wavelength λ of the high-frequency voltage or an integer multiple of λ / 4.

3. A dielectric heating electrode according to claim 1, A dielectric heating electrode characterized by the ability to connect to the parallel plate electrodes via multiple switch circuits at multiple locations on the outer peripheral end of the spiral electrode, thereby allowing the length of the spiral electrode to be adjusted in stages.

4. A dielectric heating electrode according to claim 3, The aforementioned switch circuit is a dielectric heating electrode characterized by using a relay or a MOS FET.

5. A high-frequency dielectric heating device that applies a high-frequency voltage from a high-frequency power supply between the upper and lower electrodes of a parallel plate via a matching circuit that variably adjusts the impedance, thereby dielectrically heating an object to be heated by the electric field generated between the upper and lower electrodes, A high-frequency dielectric heating apparatus characterized in that spiral electrodes are provided on either the lower surface of the upper electrode or the upper surface of the lower electrode, or both thereof, the outer peripheral end of the spiral electrode is connected to the electrode of the parallel plate, and the inner peripheral end is an open end.

6. A high-frequency dielectric heating apparatus according to claim 5, A high-frequency dielectric heating device characterized by the ability to connect to the parallel plate electrodes via multiple switch circuits at multiple locations on the outer peripheral end of the spiral electrode, thereby allowing the length of the spiral electrode to be adjusted in stages.

7. A high-frequency dielectric heating apparatus according to claim 6, A high-frequency dielectric heating apparatus characterized by comprising a control circuit that detects a high-frequency voltage or high-frequency power from the high-frequency power supply and controls either or both of the impedance of the matching circuit and the plurality of switch circuits according to the temperature of the object to be heated.

8. A high-frequency dielectric heating apparatus according to any one of claims 5 to 7, The high-frequency dielectric heating apparatus is characterized in that the high-frequency power supply is a current source.

9. A high-frequency dielectric heating method is used to dielectrically heat an object to be heated by applying a high-frequency voltage from a high-frequency power supply between the upper and lower electrodes of a parallel plate via a matching circuit that variably adjusts the impedance, thereby generating an electric field between the upper and lower electrodes. A spiral electrode is provided on either the lower surface of the upper electrode or the upper surface of the lower electrode, or both thereof, wherein the outer peripheral end of the spiral electrode is connected to the electrode of the parallel plate, and the inner peripheral end is an open end, Multiple switch circuits can be connected to the electrodes of the parallel plate at multiple locations on the outer peripheral end of the spiral electrode. A high-frequency dielectric heating method characterized by detecting a high-frequency voltage or high-frequency power from the high-frequency power supply and controlling either or both of the impedance of the matching circuit and the plurality of switch circuits according to the temperature of the object to be heated.

10. A high-frequency dielectric heating method according to claim 9, A high-frequency dielectric heating method characterized by adjusting the impedance of the matching circuit after adjusting the plurality of switch circuits.

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