High frequency dielectric heating device

Abstract

To provide a small-sized high frequency induction heating device which is capable of performing short-time thawing and reduces cost.SOLUTION: The present invention relates to a high frequency induction heating device in which output of a power transmission amplifier for amplifying high frequency signals of MHz bands is given to electrodes disposed oppositely in a vertical direction via a variable matching circuit, an electric field is generated between the electrodes, and frozen food material placed between the electrodes is thawed. The high frequency induction heating device comprises a reflection power measuring circuit for measuring reflection power and a control circuit. The reflection power measuring circuit is a voltage detection circuit for detecting a voltage amplitude between the power transmission amplifier and the variable matching circuit. The control circuit attains impedance matching with the electrodes by adjusting impedance of the variable matching circuit in accordance with a detected voltage of the voltage detection circuit.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a high-frequency dielectric heating device, and more particularly to a high-frequency dielectric heating device that applies a high-frequency electric field in the MHz band to frozen food and thaws the frozen food by dielectric heating. [Background technology]

[0002] In food processing factories, etc., frozen ingredients are sometimes thawed to process the food. As one type of thawing machine used in food processing factories, etc., a high-frequency thawing device is known that applies a high-frequency electric field in the MHz range to frozen food placed between opposing electrodes to thaw the frozen food by dielectric heating.

[0003] High-frequency dielectric heating is a technology that applies a high-frequency voltage to a dielectric object to heat it from the inside by self-heating (dielectric loss) caused by the vibration of the polar molecules that make up the object. With dielectric heating using microwaves (GHz band) in a microwave oven, the difference in heat generation between ice and water is large, so the melted parts on the surface of the food generate significant heat, resulting in uneven heating. However, with high-frequency dielectric heating, which uses a lower frequency band than microwaves, the energy penetrates deeper than microwaves and the difference in heat generation between ice and water is small, so it is generally known that this has the advantage of making it less likely to cause uneven heating.

[0004] An example of a dielectric heating device using a high-frequency electric field in the MHz band, realized by a well-known technology, is shown in Figure 21. The high-frequency dielectric heating device in the figure is composed of a signal source 1 that outputs MHz bands such as 13.56 MHz, 27.12 MHz, and 40.68 MHz, which are ISM bands, a power transmission amplifier 2 that amplifies the power, an impedance conversion circuit 3, a control circuit 4, a directional coupler 20, a variable matching circuit 11, an upper electrode 12, and a lower electrode 13. Furthermore, the directional coupler 20 is composed of grounded capacitors 5 and 10, detection diodes 6 and 9, and reference resistors 7 and 8.

[0005] The high frequency signal (RF signal) in the MHz band from the signal source 1 is amplified by the power transmission amplifier 3 and converted to an intermediate impedance such as 50 Ω by the impedance conversion circuit 3. The impedance is then matched by the variable matching circuit 11 to the equivalent impedance of the frozen food material 13 placed between the upper and lower electrodes 12 and 13, and the power from the power transmission amplifier 2 is transmitted efficiently to thaw the frozen food material 13.

[0006] In high-frequency dielectric heating, when thawing, the dielectric constant of the frozen food changes, causing the impedance between the electrodes to fluctuate and the impedance matching to shift, resulting in a deterioration in heating efficiency. In relation to this, Patent Document 1 describes a method in which the incident power and reflected power are detected by a directional coupler 20, and the VSWR (Voltage Standing Wave Ratio), which is found from the ratio between these powers, is calculated in a control circuit 4 to determine the matching state, and the variable matching circuit 11 is adjusted to achieve matching.

[0007] Furthermore, Patent Document 2 describes that by using a plurality of power transmission amplifiers with relatively small power ratings and combining their outputs in a power combining circuit, the combined outputs of these power transmission amplifiers can be used as the thawing power, which increases the thawing power and shortens the time required for thawing. Furthermore, it describes that since it is possible to obtain an input impedance for impedance matching that is lower than the impedance suitable for the power transmission amplifier, the configuration of the impedance matching circuit can be simplified.

[0008] 21, the output impedance of the amplifier decreases due to the parallel operation of multiple power transmission amplifiers 2, so that the intermediate impedance can be set to a value lower than 50 Ω, thereby lowering the input impedance of the variable matching circuit 11. In addition, since the equivalent series resistance between the electrodes is also a low value compared to 50 Ω, the impedance conversion ratio of the variable matching circuit 11 can be reduced, making it possible to simplify the matching circuit and use circuit components with a relatively low withstand voltage. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] JP 2020-102439 A [Patent Document 2] JP 2003-17238 A Summary of the Invention [Problem to be solved by the invention]

[0010] In Patent Document 1, the impedance is converted to an intermediate impedance such as 50 Ω, and then converted to an impedance between electrodes with a small equivalent series resistance via a variable matching circuit, which increases the conversion ratio and therefore increases the voltage amplitude of the variable matching circuit 11. This requires high-voltage components for the variable matching circuit 11, which poses the problem of large-sized and expensive circuit components.

[0011] On the other hand, when, as in Patent Document 2, multiple power transmission amplifiers are combined in parallel to increase output power and reduce output impedance, and the impedance of the variable matching circuit is reduced to aim for size and cost reduction, a problem arises in which the reflected power and incident power required to detect the matching state between electrodes cannot be measured accurately.

[0012] In FIG. 21, directional coupler 20 detects the reflected power and incident power using the resistance value of reference resistor 7 as a reference and detects the reflected power using the resistance value of reference resistor 8 as a reference. Therefore, if the intermediate impedance is 50 Ω, the resistance values ​​of reference resistors 7 and 8 must be 50 Ω.

[0013] However, in Patent Document 2, because the impedance between the electrodes varies depending on the thawing state of the frozen food material due to the attempt to reduce impedance, it is necessary to change the values ​​of the reference resistors 7 and 8 accordingly. Furthermore, when the thawing power is increased to shorten the thawing time, the electric field is concentrated at the edge of the food material, which promotes heating from the center, and the tendency for uneven heating to occur where the temperature of the edge of the food material rises becomes stronger.

[0014] In view of the above, an object of the present invention is to provide a small, low-cost high-frequency dielectric heating device that is capable of thawing in a short time. [Means for solving the problem]

[0015] In view of the above, the present invention is a high-frequency dielectric heating device which provides the output of a power transmission amplifier, which amplifies high-frequency signals in the MHz band, via a variable matching circuit to electrodes arranged opposite each other above and below, generating an electric field between the electrodes to thaw frozen food placed between the electrodes, and which is equipped with a reflected power measuring circuit and a control circuit which measures reflected power, wherein the reflected power measuring circuit is a voltage detection circuit which detects the voltage amplitude between the power transmission amplifier and the variable matching circuit, and the control circuit adjusts the impedance of the variable matching circuit based on the detection voltage of the voltage detection circuit to achieve impedance matching with the electrodes. Effect of the Invention

[0016] The high-frequency dielectric heating device of the present invention provides a small-sized, low-cost high-frequency dielectric heating device capable of defrosting in a short time. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic block diagram showing an example of a circuit configuration of a high-frequency dielectric heating device according to a first embodiment of the present invention. [Diagram 2] FIG. 2 is a circuit diagram showing an example of a power transmission amplifier having a class AB push-pull configuration. [Diagram 3] FIG. 11 is a circuit diagram showing another example of a power transmission amplifier having a class E push-pull configuration. [Figure 4a] FIG. 13 is a diagram showing an example of a case where a constant voltage source is connected to a constant current source conversion circuit. [Figure 4b] FIG. 4b shows an equivalent circuit of FIG. 4a. [Diagram 5] FIG. 1 is a circuit diagram showing an example of a constant current source circuit configured with a T-type low-pass filter. [Figure 6] FIG. 1 is a circuit diagram showing an example of a constant current source circuit configured with a third-order π filter. [Figure 7]FIG. 1 is a circuit diagram showing an example of a variable matching circuit having a variable capacitance configuration using capacitance switching. [Figure 8a] FIG. 13 is a diagram showing an equivalent circuit having a configuration in which a current source is converted into a current source by a current source conversion circuit and the imaginary part of the electrode is cancelled by a variable matching circuit. [Figure 8b] 13 is a diagram showing the results of calculating, by circuit simulation, the voltage value at the variable matching input terminal when the capacitance value of the variable matching circuit is changed. FIG. [Figure 9a] FIG. [Figure 9b] FIG. 4 is a perspective view showing the shape of a convex electrode. [Figure 10a] FIG. 13 is a diagram showing the results of the present invention, in which the heat density distribution of food material during thawing is calculated by electromagnetic field analysis. [Figure 10b] FIG. 13 shows the conventional results of calculating the heat density distribution of food during thawing using electromagnetic field analysis. [Figure 11] FIG. 4 is a characteristic diagram showing an example of real dielectric constant versus temperature when a food material is thawed. [Figure 12] FIG. 4 is a characteristic diagram showing an example of imaginary dielectric constant versus temperature when a food material is thawed. [Figure 13] FIG. 11 is a characteristic diagram showing an example of the results of calculating the equivalent series resistance between electrodes of a frozen food material. [Figure 14] FIG. 11 is a characteristic diagram showing an example of the results of calculating the equivalent series capacitance value between electrodes of a frozen food material. [Figure 15] 4 is a flowchart showing a flow from the start to the end of dielectric heating in the high-frequency dielectric heating device according to the first embodiment of the present invention. [Figure 16] FIG. 5 is a schematic block diagram showing an example of a circuit configuration of a high-frequency dielectric heating device according to a second embodiment of the present invention. [Figure 17] FIG. 11 is a schematic cross-sectional view showing a configuration example of a high-frequency dielectric heating device according to Example 3 of the present invention. [Figure 18] FIG. 11 is a schematic perspective view showing a configuration example of the surface and interior of a high-frequency dielectric heating device according to Example 3 of the present invention. [Figure 19] FIG. 11 is a diagram showing an example of the configuration of the rear side of a high-frequency dielectric heating device according to Example 3 of the present invention. [Figure 20]FIG. 11 is a schematic cross-sectional view showing a structural example of a high-frequency dielectric heating device according to Example 4 of the present invention. [Figure 21] FIG. 1 is a block diagram showing an example of a circuit configuration of a high-frequency dielectric heating device according to the prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0019] Although high-frequency dielectric heating devices can be widely used in heating objects, when they are used specifically for thawing, they are sometimes called high-frequency thawing devices. In the following examples, a high-frequency thawing device will be described.

[0020] In addition, in an embodiment of the present invention, it is assumed 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 frozen foods such as frozen boxed lunches and side dishes stored in a certain amount according to the number of customers entering the store are thawed and then displayed as refrigerated items on display shelves in the store, or the thawed foods are provided to customers.

[0021] However, the following explanations are intended to provide specific examples of the contents of the present invention, and the present invention is not limited to these explanations. Various changes and modifications are possible by those skilled in the art within the scope of the technical ideas disclosed in this specification. EXAMPLES

[0022] 1 is a block diagram showing a schematic circuit configuration of a high-frequency dielectric heating device 100 according to Example 1 of the present invention. The high-frequency dielectric heating device 100 includes an RF signal source 1 that outputs an ISM band in the MHz band, such as 13.56 MHz, 27.12 MHz, or 40.68 MHz, an attenuator 101, a power transmission amplifier 110, a current source conversion circuit 120 also serving as a low-pass filter, a voltage detection circuit 130, a variable matching circuit 140, an upper electrode 12, a lower electrode 13, a temperature sensor 104, a power supply circuit 102, and a control circuit 103, and thaws a frozen food material 14.

[0023] In the above overall configuration, the power transmission amplifier 110 in FIG. 1 is composed of a distribution circuit 115, an amplification element 116, and a combining circuit 117, and an RF signal input from an RF signal input terminal 111 is input to each amplification element 116 by the distribution circuit 115, and the amplified RF signal is combined by the combining circuit 117 and output between RF signal output terminals 113 and 114.

[0024] The current source conversion circuit 120 is configured as a π-type low-pass filter including capacitors 123 and 125 and an inductor 124, and converts the output of the power transmission amplifier connected to a current source conversion terminal 121 into a current source circuit, which is output from a current source conversion terminal 122. Note that the capacitance values ​​of the capacitors 123 and 125 and the inductance value of the inductor 124 are set to constants such that the reactance values ​​are equal at the RF signal frequency and are also equal to the characteristic impedance, thereby enabling current source conversion.

[0025] Variable matching circuit 140 is composed of variable capacitance capacitor 143 and matching inductor 144, and is configured to cancel the imaginary part of the impedance of the electrode connected to variable matching output terminal 142 as viewed from variable matching input terminal 141. For this reason, if the inductance value of matching inductor 144 is determined so that it is inductive when there is no food between the electrodes and the capacitance value is the smallest, the imaginary part can be canceled by using a variable capacitance capacitor connected in series with it.

[0026] The voltage detection circuit 130 is connected near the variable matching circuit input terminal 141 and is composed of resistors 133, 135, a grounded capacitor 132, and a detection diode 134. The voltage amplitude of the voltage amplitude input terminal 131 is detected by the detection diode 134, the resistor 133, and the grounded capacitor 132, and a detection voltage corresponding to the voltage amplitude is output to the control circuit 103.

[0027] The upper electrode 12 and the lower electrode 13 face each other to form a convex electrode, and frozen food 14 placed between them is thawed by dielectric heating. The lower electrode is connected to GND, and a temperature sensor 104 is installed near the convex electrode to enable measurement of the temperature of the frozen food.

[0028] Furthermore, the control circuit 103 in FIG. 1 adjusts the attenuator 101 and the variable matching circuit 140 in response to the output of the temperature sensor 104 .

[0029] In order to obtain a small, low-cost high-frequency dielectric heating device capable of defrosting in a short time, in the high-frequency dielectric heating device 100 roughly configured as described above, the present invention has been devised in various aspects, such as increasing the output of the power transmission amplifier 110, achieving a balance between miniaturization and low cost of circuits such as the variable matching circuit 140 and the temperature sensor 104, the structure of the electrodes 12, 13 which reduces uneven heating that becomes noticeable at high output, the current source conversion circuit 120, and the voltage detection circuit 130.

[0030] In the following, the effects of the configuration based on these ideas will be described for each component. First, the power transmission amplifier 110 configured from the viewpoint of high output to enable short-time thawing will be described with reference to Figs. 2 and 3.

[0031] 2 and 3 show an example of the internal circuit of the power transmission amplifier 110. Fig. 2 shows the configuration of a linear amplifier with a class AB push-pull configuration used in transmitters such as amateur radio, and is made up of a bias circuit 801, grounded capacitors 802 and 814, input matching inductors 804, 805, 806 and 807, and MOS FET amplifier elements 810, 811, 812 and 813. Furthermore, the distribution circuit 115 is made up of a transformer 803 with a center tap, and the synthesis circuit 117 is made up of gain improvement capacitors 815 and 817, and a transformer 816 with a center tap. Moreover, the amplifier elements 810 and 811 are housed in a package 808, and the amplifier elements 812 and 813 are housed in a package 809.

[0032] In FIG. 2, the signal input from RF signal input terminal 111 is converted to a differential signal by distribution circuit 115, and one side of the differential signal is input to the gates of amplifying elements 810 and 812 via input matching inductors 804 and 806, while the other differential signal, which is in opposite phase, is input to the gates of amplifying elements 811 and 813 via input matching inductors 805 and 807.

[0033] The input matching inductor is set so that the gate amplitude voltage applied to the gate of the amplifying element becomes the highest as a value that resonates with the input capacitance of the gate of the amplifying element and the input RF signal frequency. These input RF signals are amplified by signals having opposite phases from the drains of amplifying elements 810 and 812 and the drains of amplifying elements 811 and 813, and are power-combined by combining circuit 117 to be output from RF signal output terminals 113 and 114.

[0034] When using two amplifying elements, each of which has two amplifying elements integrated into one package, if the gate and drain are common to each package, there is a risk that left-right imbalance will occur during push-pull operation due to device variations between the packages, resulting in a deterioration in efficiency. Therefore, by using a cross-connected configuration as shown in Figure 2, it is possible to reduce the decrease in efficiency due to left-right imbalance.

[0035] In addition to combining power, combining circuit 117 supplies power supply voltage from power supply terminal 112 to the drain of each amplifying element via transformer 816 with a center tap, and compensates for gain reduction due to leakage flux when coupling between transformers by gain improvement capacitors 815, 817 connected in parallel on the primary and secondary sides of the transformers. In addition, distribution circuit 115 distributes the input signal, and bias circuit 801 supplies a bias voltage from the center tap of transformer 803 to the gate of each amplifying element.

[0036] With the above configuration, it is possible to configure a push-pull configuration using four amplifying elements instead of the two that was previously required. This not only improves the output power but also reduces the withstand voltage required for the amplifying elements, allowing the use of relatively low-cost amplifying elements.

[0037] Fig. 3 shows the internal circuit of a switching amplifier with a class E push-pull configuration as another example of the power transmitting amplifier 110 shown in Fig. 1. The power transmitting amplifier in the figure is composed of input matching inductors 903, 904, 905, 906, bias resistors 907, 908, 909, 910, MOS FET amplifying elements 911, 912, 913, 914, resonant capacitors 915, 916, 917, 918, and resonant inductors 919, 920, 921, 922. Furthermore, the distribution circuit 115 is composed of a transformer 902 with an input matching capacitor 901 connected in parallel to the primary side, and the synthesis circuit 117 is composed of a transformer 923 configured such that the power from the power supply terminal 112 is supplied from a center tap via a choke coil 924.

[0038] The power transmission amplifier in Fig. 3 is different from the power transmission amplifier in Fig. 2 in that the gates of the amplifier elements are connected to GND by bias resistors 907-910, forming a zero-bias circuit in which no drain current flows when no signal is present, and furthermore, the drains of the amplifier element outputs form a resonant circuit configuration that resonates at the RF signal frequency by resonant capacitors 915-918 and resonant inductors 919-922. The voltage applied to the power supply terminal 112 is stored in the transformer 923 by the drain current that flows when the amplifier element is in the on state via the choke coil 924. Then, when the drain is turned off, a resonant current flows in the resonant circuit formed by the resonant capacitor and resonant inductor at a voltage twice the power supply voltage due to the power supply voltage and the energy stored in the transformer 923, and this is power-combined by the transformer 923 and output from the RF signal output terminals 113 and 114.

[0039] Since the above-described power transmission amplifier 110 has a resonant circuit at the output, it has a fixed frequency. However, in addition to obtaining the same effect as the AE-class amplifier in FIG. 2, the power supply efficiency is increased by about 10%, so that the power supply efficiency can be improved, and therefore the output can be increased, and the heat dissipation fans used to dissipate heat from the power transmission amplifier can also be made smaller.

[0040] Next, a description will be given of the current source conversion circuit 120. First, the reason for current conversion in the current source conversion circuit 120 will be described with reference to Figs. 4a and 4b.

[0041] 4a shows an example of a case where a constant voltage source is connected to a constant current source converter circuit, in which the current source converter circuit 120 shown in FIG. 1 is connected to a voltage source 501 that outputs an RF signal frequency f and voltage V, and further connected to a load 502. Also, since the reactance values ​​of the capacitors 123 and 125 and the inductor 124 are equal at frequency f, when ωL=1 / ωC=X, the impedance Za seen from the voltage source 501 is Za=X 2 / Z, which is inversely proportional to the load Z.

[0042] If X is made equal to the characteristic impedance Z0, then when the load Z is Z0, Za = Z0, and is equal to the characteristic impedance. Furthermore, if we calculate the current IL flowing through the load Z, we get IL = -jV / X, and since a constant current flows regardless of the load Z, it can be considered as a current source.

[0043] The equivalent circuit of this is the circuit in FIG. 4b, in which load 502 is connected to current source 503, and it can be seen that current source conversion is possible by current source conversion circuit 120 shown in FIG.

[0044] 1 shows a π-type low-pass filter configuration, but as shown in Fig. 5, a T-type low-pass filter in which capacitor 603 is connected between the junction of inductors 601 and 602 and ground can also produce the same effect, and in this case too, it is sufficient to set constants such that the reactance values ​​of inductors 120, 122 and capacitor 603 are equal to the characteristic impedance value. Furthermore, while Fig. 1 shows a π-type third-order filter, the same effect can be obtained by connecting an odd number of third-order π-type filters, and by increasing the order, a steeper filter characteristic can be obtained.

[0045] Fig. 6 shows a seventh-order example in which three third-order π-type filters are connected in three stages. Capacitors 701, 703, 705, and 707 are connected between the connection point of inductors 702, 704, and 706 and the ground, but since adjacent capacitors are combined, capacitors 703 and 705 have double the capacity. Also, when an even number of stages are connected, it is converted into a voltage source rather than a current source.

[0046] Next, we will explain variable matching circuit 140, which has been devised from the viewpoint of miniaturization. Variable capacitor 141 used in variable matching circuit 140 is generally a variable capacitor that increases or decreases the capacitance value by changing the opposing area of ​​a sector-shaped metal plate by mechanically rotating the axis to which the metal plate is fixed, but since a mechanical mechanism for rotating the axis is required, the shape is large and the cost is high, it is desirable to reduce the number of variable elements or use a different variable means.

[0047] FIG. 7 shows a variable capacitance configuration using capacitance switching, which is composed of capacitors 201, 202, 203, switch circuits 204, 205 such as relays, and matching inductor 144. By turning on and off capacitors 202, 203 and switch circuits 204, 205 connected in series with them, the combined capacitance value of these capacitors increases or decreases, making it possible to make stepwise adjustments.

[0048] Next, a description will be given of the newly devised voltage detection circuit 130. Conventionally, the matching state was detected by measuring the reflected power and the incident power using a directional coupler and calculating the VSWR. In contrast, the reason why the matching state can be detected by the voltage detection circuit 130 of the first embodiment only from the voltage amplitude will be described with reference to Figs. 8a and 8b.

[0049] FIG. 8a shows an equivalent circuit in which a current source is converted into a current source by a current source conversion circuit and the imaginary part of the electrode is cancelled by a variable matching circuit, and FIG. 8b shows the results of calculating the equivalent circuit of FIG. 8a by circuit simulation.

[0050] In the equivalent circuit of Figure 8a, when the inductance value of the variable matching circuit is L, the combined capacitance of the equivalent series capacitance between the electrodes and the variable capacitor is C, and the equivalent series resistance between the electrodes is Rrs, the voltage V between the terminals can be calculated using equation (1).

[0051]

number

[0052] In equation (1), when Rrs is constant, the terminal voltage V is minimum when ωL-1 / ωC=0, that is, when the imaginary part of the electrode is cancelled out by the variable matching circuit. Therefore, it is sufficient to use a voltage detection circuit to adjust the value of the variable capacitance so that the voltage at the variable matching input terminal is minimum.

[0053] Figure 8b shows the results of calculations, by circuit simulation, of the voltage value at the variable matching input terminal when the capacitance value of the variable matching circuit is changed in the dielectric heating device shown in Figure 1. The frequency of the power transmission amplifier was 13.56 MHz, the power supply voltage was 65 V, an AB-class push-pull circuit was used, the equivalent series resistance of the electrodes was 20 Ω, and the reactance of the inductor and capacitor of the current source circuit was also 20 Ω for the calculations.

[0054] In the simulation results shown in the figure, the horizontal axis is the ratio of the absolute value of the impedance Z, including the impedance between the variable matching circuit and the electrode, to the equivalent series resistance Rrs of the electrode; when this value is 1, the imaginary part is cancelled out. The vertical axis is the peak voltage at the variable matching input terminal. From the characteristics in the figure, it can be seen that the voltage amplitude is at a minimum when |Z| / Rrs=1, which means that the imaginary part is cancelled out. For this reason, it is possible to detect the matching state by detecting this terminal voltage using a voltage detection circuit.

[0055] Next, the structure of the electrodes 12 and 13 that reduces uneven heating, which becomes noticeable at higher output, will be described.

[0056] 9a and 9b show a front view and a perspective view showing an example of the convex-structure electrode shown in Fig. 1. In these figures, the upper electrode 12 and the lower electrode 13 are both 300 mm x 260 mm in size, the distance between the electrodes is 95 mm, and a convex-structure electrode with a bottom surface of 130 mm x 195 mm, a top surface of 120 mm x 180 mm, and a height of 20 mm faces these electrodes. It is assumed that a frozen food material 14 of a general lunch box size of 170 mm x 230 mm x 35 mm is placed at a height of 10 mm from the lower electrode, and the upper area of ​​the convex-structure electrode is slightly smaller than the assumed size of the frozen food material, so that the electric field in the center of the frozen food material is increased, thereby suppressing uneven heating due to electric field concentration at the edge of the food material.

[0057] Figures 10a and 10b show the results of electromagnetic field analysis of the heat generation distribution of frozen food materials when heated using the electrodes shown in Figure 9. As an example of frozen food materials, Figure 10a shows the state of thawing a piece of lean tuna measuring 170 mm x 230 mm x 35 mm using the electrode structure shown in Figure 9, and for comparison, Figure 10b shows the state of thawing using conventional parallel plates (300 mm x 260 mm).

[0058] In addition, the distance from the frozen food material was set to 5 mm with the conventional parallel plates, but with the convex-structured electrode, the area of ​​the top surface of the convex part is smaller, and the electric field is more concentrated, so the distance between the top surface of the convex part and the frozen food material was set to 10 mm. These calculation results show that with the conventional parallel plates, heat is generated more around the food material than at the center, and thawing begins from the periphery of the food material, which makes it easy for uneven heating to occur. In contrast, with the convex-structured electrode, although there are some areas where heat is generated slightly around the frozen food material, the structure of the electrode makes the electric field in the center of the food material higher, so heat is generated most in the center. In addition, because the center is heated more with the convex-structured electrode, it is possible to grasp the thawing status regardless of the size of the food material by measuring the temperature near the center of the food material.

[0059] FIG. 11 shows the real part εr' of the relative dielectric constant (vertical axis) versus the temperature (horizontal axis) of the red tuna used in the electromagnetic field analysis of FIGS. 10a and 10b, and FIG. 12 shows an example of the imaginary part εr'' of the relative dielectric constant.

[0060] From these figures, we can see that in the temperature range of -20°C to -5°C where thawing actually occurs, the real part of the dielectric constant εr' is approximately 5 to 18, and the imaginary part is approximately 2 to 12, and both increase with increasing temperature.

[0061] Furthermore, Figure 13 shows the equivalent series resistance (vertical axis) between the electrodes as a function of temperature (horizontal axis) when red tuna is placed on the parallel plate electrodes shown in Figure 10b, and Figure 14 shows the results of calculating the equivalent series capacitance (vertical axis) between the electrodes. The frequency is 13.56 MHz. From Figure 13, we can see that in the temperature range of approximately -20°C to -5°C, the equivalent series resistance decreases from 40Ω to approximately 20Ω, and from Figure 14, the equivalent series capacitance increases from 28pF to 38pF.

[0062] In FIG. 13, we want to proceed with thawing at around -20°C, where thawing begins, so we need to increase the amount of heat generated, and decrease it at around -5°C, where thawing ends. However, with a conventional constant voltage source type power transmission amplifier, the amount of heat generated is V 2 / R, which is inversely proportional to the equivalent series resistance; therefore, at around -5°C when thawing is nearly complete, the equivalent series resistance becomes small and heat generation increases, making it necessary to control the output power of the transmission amplifier, for example.

[0063] In contrast, the high-frequency induction device shown in FIG. 1 has a heat generation amount of I 2 R and proportional to the equivalent series resistance, it is maximum at -20°C when heat generation is most required. As the heat generation gradually decreases as thawing progresses, the control of the power transmission amplifier, including the detection means such as temperature detection, can be simplified, leading to cost reduction. Note that since the series equivalent capacitance in Figure 14 increases in capacitance as thawing progresses, it can be seen that the variable capacitance capacitor 143 of the variable control circuit 140 of the high frequency dielectric device shown in Figure 1 can be gradually reduced in capacitance from the start of thawing.

[0064] Fig. 15 is a flow chart showing the flow up to thawing the frozen food material 14 in the high frequency dielectric heating device 100 according to the embodiment 1 shown in Fig. 1, and will be described with reference to Fig. 15 and Fig. 1. This flow chart shows the flow of processing by the control circuit 103 in Fig. 1, which is realized by a computer.

[0065] In this process, first, in process step S101, the control circuit 103 adjusts the attenuator 101 to start transmitting RF signal power output from the transmission amplifier 110 to the frozen food material 14 placed between the upper electrode 12 and the lower electrode 13 at a power lower than the power required for normal thawing.

[0066] Next, in processing step S102, control circuit 103 sets the capacitance value of variable capacitor 143 of variable matching circuit 140 to the largest value, and at this time voltage detection circuit 130 measures the detection voltage corresponding to the voltage amplitude of variable matching circuit input terminal 141, and further, in processing step S103, measures the detection voltage with the capacitance value of variable capacitor 143 set to a value one step smaller.

[0067] In process step S104, control circuit 103 compares the detected voltage results before and after, and if the detected voltage has decreased, returns to process step S103, reduces the capacitance value of variable capacitor 143, and measures the detected voltage at that time.

[0068] If the detection voltage increases, in processing step S105, the imaginary part of the impedance of the electrode is cancelled out by the variable matching circuit 140, and it is considered that the impedance has changed from capacitive to slightly inductive, so the capacitance value of the variable capacitor 143 is increased by one step and normal power transmission is started.

[0069] Then, in process step S106, it is determined whether the temperature of the temperature sensor 104 has reached the temperature setting value for completing thawing, and if the set temperature has not been reached, heating is continued continuously until the detection voltage becomes high by a loop of process steps S106, S107, S108, and S106, and if the detection voltage becomes high, the imaginary part of the impedance of the electrodes becomes inductive from capacitive in process step S109, so the transmission power is reduced, and processing returns to step S103, and the capacitance value of the variable capacitor 143 is reduced until the imaginary part is cancelled, and power transmission is started at a high power. In process step S110, these steps are repeated until the set temperature is reached, and if the set value is reached, power transmission is stopped and a notice is given that thawing has been completed, and the process ends.

[0070] In this way, the matching state can be detected by measuring the voltage amplitude of the variable matching circuit input, making it possible to thaw frozen food materials with relatively simple control.

[0071] In the high frequency dielectric heating device according to the first embodiment, the power transmission amplifier can be made to have a high output, and the high frequency dielectric heating device is small in size, low in cost, and causes less heating unevenness. EXAMPLES

[0072] FIG. 16 is a block diagram of a schematic circuit configuration of a high-frequency dielectric heating device 100 according to a second embodiment of the present invention, and the configuration and operation of the second embodiment of the present invention will be described with reference to FIG.

[0073] FIG. 16 has a different configuration from the first embodiment shown in FIG. 1 in that a differential signal is applied to the electrodes. The same reference numerals are used for the parts that overlap with those of the high-frequency dielectric heating device according to the first embodiment shown in FIG. 1, and the description thereof will be omitted.

[0074] 16 differs from that of FIG. 1 in that the current source conversion circuit 120 is configured as 1510, the voltage detection circuit 130 is configured as 1530, and the variable matching circuit 140 is configured as 1540, 1550.

[0075] Among these, the current source conversion circuit 1510 is composed of capacitors 1515, 1516, 1519, 1520, and inductors 1517, 1518. The π-type current conversion circuit composed of the capacitor 1515, the inductor 1517, and the capacitor 1519 and the π-type current source conversion circuit composed of the capacitor 1516, the inductor 1518, and the capacitor 1520 are folded back to connect the RF signal output terminals 113, 114 of the power transmission amplifier 110 to the current source conversion terminals 1511, 1512, so that the differential constant voltage source input between the current source conversion terminals 1511, 1512 is converted into a constant current source and output from the current source conversion terminals 1513, 1514. Note that, since it is a differential signal, the reactance value of the capacitor and inductor of the current source conversion circuit may be determined as a constant at half the size of the characteristic impedance.

[0076] Voltage detection circuit 1530 detects and outputs the voltage amplitude across variable matching circuit input terminals 1541 and 1542 via resistors 1537 and 1538 using a bridge circuit made up of detection diodes 1533, 1534, 1535 and 1536, resistor 1512 and grounded capacitor 1531.

[0077] Variable matching circuits 1540 and 1550 are each composed of a matching inductor 1544 and a variable capacitor 1543 in a symmetrical configuration, and a matching inductor 1554 and a variable capacitor 1553 , and are connected to upper electrode 12 and lower electrode 13 via variable matching output terminals 1542 and 1552 .

[0078] The high-frequency dielectric heating device of Example 2 described above has the same effects as the high-frequency dielectric heating device of Example 1. In addition, by adopting a differential configuration, the voltage applied to the current source conversion circuit 1510 and the matching circuits 1540 and 1550 is reduced, allowing the use of small, low-cost circuit components with low voltage resistance, and the electric field between the electrodes and the housing GND is also reduced, allowing the distance between the electrodes and the housing GND to be reduced, making it possible to make the device smaller and less costly. EXAMPLES

[0079] Third Embodiment FIG. 17 is a schematic cross-sectional view showing an example of the configuration of a high-frequency dielectric heating device 100 according to a third embodiment of the present invention. The configuration of the third embodiment of the present invention will be described with reference to FIGS. 17 to 19 and 1.

[0080] Fig. 1 mainly shows the electric circuit configuration of the high-frequency dielectric heating device, and Fig. 17 is a side view showing the structure inside a housing that houses the electric circuit of Fig. 1. Note that parts that overlap with those of the high-frequency dielectric heating device according to Example 1 shown in Fig. 1 are given the same reference numerals and their explanations will be omitted.

[0081] In FIG. 17, within a housing 1601, a cover 1602, a GND shield plate 1603, a mounting board 1604, a heat sink 1605, heat sink fins 1606, an outlet 1607, a fan 1608, an insulating plate 1609, etc. are arranged in the positions shown in the figure.

[0082] In the figure, the distribution circuit 115, amplification element 116, and combination circuit 117 that constitute the power transmitting amplifier 110 are mounted on a mounting board 1604, a heat sink 1605 is attached to the back surface of the mounting board 1604, and heat sink fins 1606 and a fan 1608 are further mounted on the heat sink 1605.

[0083] Furthermore, by mounting the power transmission amplifier 110 and heat sink 1605 so that they are leaned up against the rear of the housing 1601 of the high-frequency dielectric heating device, heat can be easily dissipated from the rear, and they can be connected over a short distance using cables 1610 and 1611 to the upper and lower electrodes 12 and 13 via the outlet 1607 of the GND shield plate 1603, thereby reducing cable loss.

[0084] In addition, an insulating plate 1609 is placed on top of the lower electrode 13 to ensure a distance between the electrode and the frozen food material 14, a temperature sensor 104 is installed near the center of the lower electrode, and a power supply circuit 102 is mounted on the bottom of the power transmission amplifier 110.

[0085] Fig. 18 is a schematic perspective view showing an example of the structure of the surface and interior of the high-frequency dielectric heating device 100 according to the third embodiment of the present invention shown in Fig. 17. In the figure, parts that overlap with those of the high-frequency dielectric heating device according to the third embodiment shown in Fig. 17 are given the same reference numerals and descriptions thereof will be omitted.

[0086] 18, shelf brackets 1701, 1702, display panel 1703, push buttons 1704, 1705, and adjustment knob 1706 are arranged on the surface and inside the cabinet at the positions shown in the figure. Furthermore, GND shield plate 1603, which is provided to reduce the effect on the circuit of the electric field applied between upper and lower electrodes 12, 13 and the electromagnetic field radiated to the outside, has outlet 1607 for pulling out cables 1610, 1611 connected to the electrodes, and resin insulated shelf brackets 1701, 1702 (the shelf bracket on the right side is not shown) are attached to both sides of GND shield plate 1603.

[0087] This shelf support is structured to support the upper and lower electrodes 12, 13 and the insulating plate 1609, and the height can be changed by changing the position of the shelf support. In addition, a display panel 1703 is provided on the front, which can display the current thawing status and the temperature setting value, and further includes buttons 1704 and 1705 for starting and stopping heating and an adjustment knob 1704 for setting the temperature at the completion of thawing, and the control circuit 103 is mounted behind these operation panels.

[0088] Fig. 19 is a rear view showing an example of the structure of the rear side of the high-frequency dielectric heating device 100 according to Example 3 of the present invention shown in Fig. 17. In the figure, parts that overlap with those of the high-frequency dielectric heating device according to Example 3 shown in Fig. 17 are given the same reference numerals and descriptions thereof will be omitted.

[0089] 19, 1801 is a power outlet, and the rear side of the housing 1601 is cut out to mount the heat sink 1605, heat sink fins 1606, and fan 1608, improving the heat dissipation of the power transmitting amplifier. Also, the power supply circuit 102 is arranged below the power transmitting amplifier, and the control circuit 103 is arranged on the operation panel side. The variable matching circuit 140 is arranged on the opposite side, behind the GND shield plate 1603, and is connected from the variable matching circuit output terminal 142 to the upper electrode 12 via the lead-out port 1607 by the cable 1610.

[0090] With the above configuration, it is possible to obtain a high-frequency dielectric heating device that has excellent heat dissipation properties against the increased heat generated as the power transmission amplifier becomes higher output, and by shortening the length of the cable between the power transmission amplifier and the upper and lower electrodes, transmission loss can be reduced.In addition, the structure allows the height of the electrodes to be changed, making it possible to defrost frozen food materials of different heights. EXAMPLES

[0091] Fourth Embodiment FIG. 20 is a side view showing an example of the configuration of a high-frequency dielectric heating device 100 according to a fourth embodiment of the present invention, and the configuration of the fourth embodiment of the present invention will be described with reference to this figure.

[0092] In comparison with the high frequency dielectric heating device of Example 3 shown in Figure 17, Figure 20 provides an insulating plate 1901 above the upper electrode 12, making it possible to place frozen food material 1902 on top of it. If the parasitic capacitance generated between the upper electrode 12 and the GND shield plate 1603 is large, the high frequency current flows via the parasitic capacitance rather than through the frozen food material 14, reducing the heating efficiency, so it is necessary to provide a certain distance between the upper electrode 12 and the GND shield plate 1603. For this reason, by placing other frozen food materials in this space, it becomes possible to defrost multiple food materials in parallel.

[0093] As described above, the improvements made to increase output and reduce size have been described through the first to fourth embodiments. The main improvements made thereto can be summarized and listed as follows.

[0094] First, the power transmission amplifier uses a distribution circuit that distributes an RF signal in the MHz band to at least two or more amplifier elements, inputs it to each amplifier element, and uses a push-pull amplifier circuit to combine the amplified RF signals using a combiner circuit and output them. This allows the use of low-cost amplifier elements with a relatively low breakdown voltage, resulting in a configuration that suppresses the increase in costs associated with higher output.

[0095] The second point is also related to the power transmission amplifier, and a constant current source conversion circuit that also functions as a low pass filter (LPF) to suppress harmonics in the output is added. This allows the power transmission amplifier output characteristics to be such that the amount of heat generated is large 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, and therefore simplifies the detection control that detects the thawing state and controls the output.

[0096] The third point is about the variable matching circuit. Conventionally, two variable elements were required to match the real and imaginary parts of the electrode impedance in a variable matching circuit that matches the impedance between electrodes that changes during thawing, but by connecting the variable matching circuit in series with the electrodes and canceling out only the imaginary part of the electrode, the variable matching element is reduced to one element, thereby achieving size reduction and cost reduction. Note that by making one element variable, the configuration allows the relatively low equivalent resistance of the frozen food between the electrodes to be directly seen, but the power transmission increase combines the outputs of multiple amplifier elements, lowering the output impedance, making it possible to create a configuration suitable for directly driving the low equivalent series resistance between the electrodes.

[0097] The fourth point is related to the measurement of reflected power. Conventionally, in order to detect the matching state, a directional coupler is used between the power transmission amplifier and the variable matching circuit (input side) to detect the amount of incident power and the amount of reflected power, and the VSWR is calculated from the voltage amplitude ratio of these to determine the threshold value for judging the matching state. However, in the configuration in which only the imaginary part of the impedance between the electrodes is cancelled out, the series equivalent resistance of the frozen food, which is the real part, changes depending on the thawing state, so the matching state cannot be grasped by measuring the VSWR using a directional coupler that measures by comparing it with a reference resistance value. In contrast, in the configuration in which only the imaginary part is cancelled out, the voltage amplitude at the input end of the variable matching circuit is the lowest when the imaginary part of the electrode impedance is cancelled out. Therefore, by adding a detection circuit that detects the voltage amplitude at the input end of the variable matching circuit, it is possible to detect the matching state, and the directional coupler that was previously required is no longer necessary, which simplifies the circuit and reduces costs.

[0098] Furthermore, as a fifth point, by using electrodes with a convex structure that are slightly smaller than the surrounding shape of the frozen food material, the electric field strength on the inside of the food material is increased, thereby suppressing uneven heating of the edges of the food material, which becomes more noticeable as the output is increased. [Explanation of symbols]

[0099] 1:RF signal source 2, 110: Power transmission amplifier 3: Impedance conversion circuit 4, 103: Control circuit 5, 10, 132, 802, 814, 925: Ground capacitor 6, 9, 134, 1533, 1534, 1535, 1536: Detector diodes 7, 8: Reference resistor 11, 140, 1540, 1541: Variable matching circuit 12: Upper electrode 13; Lower electrode 14: Frozen ingredients 101: Attenuator 102: Power supply circuit 104: Temperature sensor 111: RF signal input terminal 112: Power terminal 113, 114: RF signal output terminals 116, 810, 811, 812, 813: Amplification elements 117:Synthesis circuit 120: Current source conversion circuit 121, 122, 1511, 1512, 1513, 1514: Current source conversion terminals 123, 125, 603, 701, 703, 705, 707, 1515, 1516, 1519, 1520: Capacitors 124, 201, 202, 203, 601, 602, 702, 704, 706, 1517, 1518: Inductors 130, 1530: Voltage detection circuit 131: Voltage amplitude input terminal 133, 135: Resistance 141, 1541, 1542: Variable matching input terminal 142, 1542, 1552: Variable matching output terminal 143, 1543, 1553: Variable capacitors 144, 1544, 1554: Matching inductors 204, 205; Switch circuit 501: Voltage source 502: Load resistance 503: Current source 801: Bias circuit 803, 816, 902, 923: Trans 804, 805, 806, 807, 903, 904, 905, 906: Input matching inductors 907, 908, 909, 910: Bias resistors 808, 809: Package 815, 817, 901: Input matching capacitors 915, 916, 917, 918: Resonant capacitors 919, 920, 921, 922: Resonant inductors 924: Choke coil 1601: Housing 1602: Lid 1603: GND shield plate 1604: Mounting board 1605: Heat sink 1606: Finn 1607: Drawer opening 1608: Fan 1609, 1610: Cable 1701, 1702: Shelf brackets 1703: Display panel 1704, 1705: Push button 1706: Adjustment knob

Claims

1. A high frequency dielectric heating device comprising: an output of a power transmission amplifier for amplifying a high frequency signal in the MHz band is applied to electrodes arranged opposite to each other above and below via a variable matching circuit; an electric field is generated between the electrodes to thaw a frozen food material placed between the electrodes; and the device is equipped with a reflected power measurement circuit and a control circuit for measuring reflected power, The reflected power measurement circuit is a voltage detection circuit that detects a voltage amplitude between the power transmission amplifier and a variable matching circuit, and the control circuit adjusts the impedance of the variable matching circuit based on a detection voltage of the voltage detection circuit to achieve impedance matching with the electrode, A current source conversion circuit that also serves as a low-pass filter for suppressing harmonics is added to the output side of the power transmission amplifier, The current source conversion circuit is a π-type or T-type third-order low-pass filter, and the reactance of each element of the low-pass filter is equal to each other at the transmission frequency and is equal to the characteristic impedance.

2. 2. The high-frequency dielectric heating device according to claim 1, The high frequency dielectric heating device is characterized in that the power transmission amplifier has a push-pull configuration consisting of at least two amplifying elements having a distribution circuit at the input and a combining circuit at the output.

3. 2. The high-frequency dielectric heating device according to claim 1, The variable matching circuit comprises an inductor and a variable capacitor and is connected in series with the electrodes, and the control circuit varies the variable capacitor of the variable matching circuit to adjust it so as to cancel out the imaginary part of the equivalent impedance between the electrodes.

4. 4. The high-frequency dielectric heating device according to claim 3, The variable capacitor is configured by connecting multiple series-connected elements in parallel, each of which has a capacitor and a switch circuit connected in series, and the capacitance value is varied by turning the switch circuit on and off.

5. 2. The high-frequency dielectric heating device according to claim 1, The high frequency dielectric heating device according to the present invention, wherein the low pass filter is a fifth or seventh order low pass filter formed by connecting third order low pass filters in three or five stages in series.

6. A high-frequency dielectric heating device comprising: an output of a power transmission amplifier that amplifies a high-frequency signal in the MHz band, which is applied via a variable matching circuit to electrodes arranged opposite each other above and below; an electric field is generated between the electrodes to thaw frozen food placed between the electrodes; and the device is equipped with a reflected power measurement circuit and a control circuit that measure reflected power, The reflected power measurement circuit is a voltage detection circuit that detects a voltage amplitude between the power transmission amplifier and a variable matching circuit, and the control circuit adjusts the impedance of the variable matching circuit based on a detection voltage of the voltage detection circuit to achieve impedance matching with the electrode, The power transmission amplifier is a linear amplifier having a class AB push-pull configuration or a class E push-pull power transmission amplifier, The amplifying element of the power transmission amplifier is a class AB amplifier having a push-pull configuration in which first and second field effect transistors are integrated in a single package, and similarly, third and fourth field effect transistors are integrated in a single package, the gates and drains of the first and third field effect transistors are connected to each other, and the gates and drains of the second and fourth field effect transistors are connected to each other.

7. The high-frequency dielectric heating device according to any one of claims 1 to 6, The high-frequency dielectric heating device has a housing having a substantially rectangular shape, A high-frequency dielectric heating device characterized in that a power transmission amplifier module, in which a board on which the power transmission amplifier is mounted is placed on a heat sink, is mounted by leaning it against the back side of the high-frequency dielectric heating device, heat generated by the power transmission amplifier is dissipated from the back side of the high-frequency dielectric heating device by the heat sink, and upper and lower electrodes are mounted on the side opposite the heat sink of the power transmission amplifier.

8. The high-frequency dielectric heating device according to any one of claims 1 to 6, A high-frequency dielectric heating device characterized in that the upper and lower electrodes are shielded by a grounded shield plate, allowing frozen food materials to be heated simultaneously by both the electric field generated between the upper and lower electrodes and the electric field generated between the shield plate and the upper part of the lower electrode.

9. 4. The high-frequency dielectric heating device according to claim 3, This high-frequency dielectric heating device is characterized in that the power transmission amplifier uses an impedance adjustment method in which, when heating begins, the capacitance value of the variable capacitor is maximized to begin power transmission, and the capacitance value of the variable capacitor is gradually reduced so that the detection voltage of the voltage detection circuit is minimized, thereby achieving impedance matching.

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