JP2019075217A - Grid control circuit and dielectric heating device - Google Patents

Abstract

To provide a grid control circuit and dielectric heating device, capable of improving power coefficient even when an oscillation circuit with a vacuum tube is used.SOLUTION: The grid control circuit is a grid control circuit for intermittently applying a voltage to a grid 22b of a vacuum tube 22 provided in an oscillation circuit 2 of a dielectric heating device 1. If an LC resonance frequency of the oscillation circuit is taken as f and a positive integer is taken as N, the grid control circuit intermittently applies a voltage to a grid so that a switching frequency concerning voltage application becomes f/N.SELECTED DRAWING: Figure 1

Description

  Embodiments of the present invention relate to grid control circuits and dielectric heating devices.

  For example, there is a high frequency dielectric heating device (hereinafter referred to as a dielectric heating device) having two electrodes. The dielectric heating device inverts the polarity of the voltage applied to the electrodes at a high frequency, and generates heat by vibrating the dipole (Dipole) of the molecule of the work including the dielectric provided between the electrodes. Dielectric heating devices are used in various industrial fields related to resin, wood, fiber, food, plasma generation and the like.

  The calorific value of the work to be heated is determined by the loss coefficient resulting from the physical properties of the dielectric, the applied electric field strength, the frequency of the applied voltage, the electrode structure, and the like. Generally, the calorific value is proportional to the loss factor and the frequency. Therefore, if the loss factor is increased or the frequency is increased, the calorific value is increased.

  Also, the power half depth is inversely proportional to the loss factor and the frequency. The half power depth is the depth at which the energy of the high frequency voltage applied to the surface of the work is halved by attenuation. The half power depth is used to indicate how deep the heating is possible from the surface of the work. Therefore, if the loss factor is increased or the frequency is increased, the depth at which heating can be performed becomes shallow. Here, the microwave heating apparatus, which is a heating apparatus for a workpiece, uses microwaves of about 2.4 GHz. On the other hand, in the dielectric heating device, generally, a high frequency of about 20 MHz to 40 MHz is used. Therefore, if the dielectric heating device whose frequency is lower than that of the microwave heating device is used, the half power depth can be made deeper. As a result, even when the thickness of the workpiece is thick, relatively uniform heating is possible.

  There are various types of such dielectric heating devices, from small ones with an output of 1 kW or less to large ones with an output of about several tens of kW. A high frequency oscillation circuit (hereinafter referred to as an oscillation circuit) having a semiconductor element (switching element) is often used for a small-sized dielectric heating device whose output is 1 kW or less. On the other hand, when the output exceeds 1 kW, switching can be performed by connecting a plurality of semiconductor elements in parallel or in series. When a plurality of semiconductor elements are used for the oscillation circuit, the manufacturing cost may be significantly increased. Therefore, an oscillation circuit having a vacuum tube (triode) is often used for a dielectric heating device whose output exceeds 1 kW. Further, since the dielectric heating device has a large load fluctuation, abnormal oscillation is likely to occur. Therefore, if the oscillation circuit has a vacuum tube, the overvoltage protection can be simplified.

In addition, with the progress of power electronics technology, a wide band gap, such as a GaN device, a high frequency semiconductor element having a high electron mobility has been developed. However, at present, it is difficult to use an oscillation circuit having a semiconductor element for a dielectric heating device with an output of several kW to several tens of kW.
Therefore, it is preferable to use an oscillation circuit having a vacuum tube for a dielectric heating device whose output exceeds 1 kW.

However, if an oscillation circuit having a vacuum tube is used, reactive power not contributing to dielectric heating is increased due to grid loss and plate loss, resulting in a new problem that power efficiency is reduced.
Therefore, it has been desired to develop a technology that can improve power efficiency even when using an oscillation circuit having a vacuum tube.

JP, 2015-140015, A

  The problem to be solved by the present invention is to provide a grid control circuit and an dielectric heating device capable of improving power efficiency even when using an oscillator circuit having a vacuum tube.

  The grid control circuit according to the embodiment is a grid control circuit that intermittently applies a voltage to a grid of a vacuum tube provided in an oscillation circuit of a dielectric heating device. When the LC resonant frequency of the oscillation circuit is f and the positive integer is N, the grid control circuit intermittently applies the voltage to the grid such that the switching frequency related to the application of the voltage is f / N. Apply.

It is a circuit diagram for illustrating a dielectric heating device concerning this embodiment. It is a circuit diagram for illustrating a dielectric heating device concerning a comparative example. FIG. 7 is a circuit diagram for illustrating a dielectric heating device according to another embodiment. It is a circuit diagram for illustrating a power supply. FIG. 6 is a circuit diagram for illustrating a power supply according to another embodiment.

Hereinafter, embodiments will be illustrated with reference to the drawings. In the drawings, the same components are denoted by the same reference numerals and the detailed description will be appropriately omitted.
FIG. 1 is a circuit diagram for illustrating a dielectric heating device 1 according to the present embodiment.
As shown in FIG. 1, the dielectric heating device 1 is provided with an oscillation circuit 2, a DC power supply 3, and a grid control circuit 4.
The oscillation circuit 2 has a vacuum tube 22 and supplies high frequency power to the electrode 20.
The high frequency power applied to the electrode 20 can be 1 kW or more.
The oscillation circuit 2 includes an electrode 20, a coil 21, a vacuum tube 22, a capacitor 23, a resistor 24, and a coil 25.

The electrode 20 can be formed of a conductive material such as metal. The electrodes 20 have, for example, a flat plate shape, and can be provided in pairs so as to be parallel to each other. However, the electrode 20 is not limited to a so-called parallel plate type electrode as long as it can apply high frequency power to the work 100. The electrode 20 may be, for example, a grid, a coil, a cylinder, a roller, or the like. The electrodes 20 may be fixed or move like a conveyor.
When the electrode 20 is a parallel plate type electrode, the work 100 is placed between the electrodes 20. The work 100 is not particularly limited as long as it contains a dielectric.

  The coil 21 can be connected in parallel with the pair of electrodes 20. In this case, the pair of electrodes 20 functions as a capacitor. Therefore, the coil 21 and the pair of electrodes 20 constitute an LC circuit. When the work 100 is placed between the electrodes 20, the pair of electrodes 20 and the work 100 function as a capacitor.

Moreover, the capacitor 20a and the coil 21a can also be provided as needed. A series resonance circuit or a parallel resonance circuit can be configured by the connection form of the coil and the capacitor.
If the electrode 20 is not a parallel plate type electrode, a capacitor connected in parallel or in series with the coil 21 may be provided.
In addition, an impedance conversion circuit such as a transformer or a π circuit may be further provided between the vacuum tube 22 and the electrode 20 (work 100).

The vacuum tube 22 can be a large triode called a transmitter tube. If the vacuum tube 22 is a transmission tube, it can correspond to a dielectric heating device whose output is several kW to several tens of kW.
The vacuum tube 22 which is a triode has a filament 22a, a grid 22b, and a plate 22c. The filament 22 a is connected to one terminal of the DC power supply 3, and the plate 22 c is connected to the other terminal of the DC power supply 3. The grid 22 b is connected to the filament 22 a via the resistor 24 and the coil 25. The resistor 24 is for utilizing the current generated when the thermal electrons emitted from the filament 22a jump into the grid 22b. Therefore, in place of the resistor 24, it is also possible to provide a direct current negative power supply with an output of about 100V to 2000V.

The voltage E1 and the voltage E2 at both ends of the coil 21 are in opposite phase. Self-sustained oscillation can be achieved by feeding back the voltage E2 in reverse phase to the voltage E1 on the anode side to the grid 22b via the grid control circuit 4. The oscillation circuit 2 illustrated in FIG. 1 is a plate-tuned oscillation circuit. The capacitor 23 is a coupling capacitor that cuts a direct current component in the voltage E2 to be fed back to the grid 22b.
The DC power supply 3 is a DC power supply that outputs a DC voltage of about 100 V to 20 kV.

Here, generally, the grid of the vacuum tube is always biased to a negative voltage during the oscillation operation. Therefore, the voltage does not become positive even in the state of the highest voltage. However, if the vacuum tube is a transmitter tube, more plate current will be secured. Therefore, when the switch is turned on, the grid voltage is higher than the filament voltage. However, if the grid voltage is higher than the filament voltage, more electrons will be injected into the grid, thereby increasing grid loss.
In addition, since the electrons emitted from the filaments collide with the plate, a loss (hereinafter referred to as plate loss) occurs.

  The surface treatment and the anode structure are made different from general vacuum tubes so that the transmission tubes do not damage the grids and plates even when oscillating. Therefore, the heat-resistant current and the cooling efficiency of the transmission tube are higher than that of a general vacuum tube. However, the grid loss and the anode loss of the transmission tube are considerably larger than the grid loss and the anode loss of a general vacuum tube.

  Grid loss and plate loss are reactive powers that do not contribute to dielectric heating. Therefore, the power efficiency of the dielectric heating device can be improved if at least one of grid loss and plate loss can be suppressed.

FIG. 2 is a circuit diagram for illustrating a dielectric heating device 11 according to a comparative example.
As shown in FIG. 2, the dielectric heating device 11 is provided with an oscillation circuit 2 and a DC power supply 3. That is, the grid control circuit 4 is not provided in the dielectric heating device 11. In the dielectric heating device 11, self-oscillation is caused by causing the voltage E2 in reverse phase to the voltage E1 on the anode side to be fed back to the grid 22b via the wiring 26. In this case, it becomes a self-excited tuning oscillation circuit that switches the vacuum tube at a timing synchronized with the LC resonance frequency.

  With the tuned oscillation circuit, the configuration of the oscillation circuit can be simplified. However, in the case of a tuned oscillation circuit, in principle, the vacuum tube is switched at a timing synchronized with the LC resonance frequency. In the case of the tuned oscillation circuit, plate loss and anode loss occur due to switching of the vacuum tube, and it is difficult to improve the power efficiency of the dielectric heating device 11 if the tuned oscillation circuit increases the number of times of switching. Become.

Therefore, the grid control circuit 4 is provided in the dielectric heating device 1 according to the present embodiment. The grid control circuit 4 intermittently applies a voltage to the grid 22 b of the vacuum tube 22 provided in the oscillation circuit 2 of the dielectric heating device 1. Further, as described later, when the LC resonance frequency of the oscillation circuit 2 is f and the positive integer is N, the grid control circuit 4 sets the grid 22 b so that the switching frequency related to the application of voltage becomes f / N. Apply voltage intermittently.
The grid control circuit 4 has a power supply 40, a switching element 41, and a gate drive circuit 42.
The power supply 40 can be a DC power supply. In this case, the power source 40 may convert AC power from a commercial power source or the like into DC power. Further, the power supply 40 may be provided outside the dielectric heating device 1 and supply DC power to the grid control circuit 4.

  The switching element 41 switches the power supplied from the power supply 40 to the grid 22b. The switching element 41 can be, for example, a semiconductor element such as a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), a HEMT (High Electron Mobility Transistor), a JFET (Junction Field-Effect Transistor), or a junction type transistor.

  The gate drive circuit 42 switches the ON state and the OFF state of the switching element 41. For example, the gate drive circuit 42 turns on the switching element 41 by applying a predetermined voltage to the gate electrode of the switching element 41. When the switching element 41 is turned on, a DC voltage is applied from the power source 40 to the grid 22 b via the switching element 41.

Also, the gate drive circuit 42 is connected to the coil 21. Therefore, the gate drive circuit 42 can detect the LC resonance frequency from the voltage E2 or the current.
In addition, the gate drive circuit 42 can include a counter circuit, a charge circuit, a phase control circuit, and the like. Therefore, the gate drive circuit 42 can determine the switching timing of the switching element 41 based on the LC resonance frequency. For example, the gate drive circuit 42 can cause the switching element 41 to perform switching once when the vacuum tube 22 performs switching a predetermined number of times. That is, when the LC resonant frequency is f and the positive integer is N, the switching frequency of the gate drive circuit 42 (switching element 41) can be f / N.

Preferably, the dielectric heating device 1 does not generate radio interference to the surroundings. In this case, as the ISM frequency band, 6.765 MHz to 6.795 MHz, 13.553 MHz to 13.567 MHz, 26.957 MHz to 27.283 MHz, and 40.66 MHz to 40.70 MHz are used. be able to. The bands for ISM frequencies are set to be two and three times the other.
Therefore, at least one of the LC resonance frequency f and the switching frequency f / N is 6.765 MHz to 6.795 MHz, 13.553 MHz to 13.567 MHz, 26.957 MHz to 27.283 MHz, and 40. It is preferable to be included in any band of 66 MHz or more and 40.70 MHz or less.

Here, the LC resonant circuit is an ideal circuit neglecting the consumption of energy by electrical resistance, and in fact, the energy is attenuated by the consumption of energy by electrical resistance.
In order to maintain resonance (oscillation) without attenuation, it is necessary to replenish energy from the outside each time at a timing synchronized with the resonance frequency.
For example, in the case of the dielectric heating device 11 according to the comparative example illustrated in FIG. 2, by applying the voltage E2 to the grid 22b via the wiring 26, the energy in the LC resonant circuit is synchronized with the LC resonant frequency. Is replenished.
However, in this case, the plate loss and the anode loss occur each time at the same frequency as the resonance frequency, so that the power efficiency of the dielectric heating device 11 is reduced.

  On the other hand, since the grid control circuit 4 is provided in the dielectric heating device 1 according to the present embodiment, a voltage can be intermittently applied to the grid 22b. Therefore, it is possible to suppress the plate loss and the anode loss and to improve the power efficiency of the dielectric heating device 1.

In addition, if voltage application to the grid 22b is performed intermittently, the amplitude of the LC resonance frequency when the voltage is applied becomes the largest, and while the voltage application is not performed, the amplitude of the LC resonance frequency is attenuated.
Therefore, the output from the oscillation circuit 2 is an AC output (AC current or AC voltage) in which two frequencies of the LC resonance frequency and the switching frequency f / N are superimposed.
The calorific value of the work 100 is affected by the material of the work 100. For example, depending on the material of the work 100, high frequency AC output may be easily absorbed, and low frequency AC output may be difficult to be absorbed. Since two frequencies overlap on the AC output of the dielectric heating device 1, it is possible to improve the absorption efficiency to the work 100 and, consequently, the heat generation efficiency of the work 100.

Since the switching element 41 and the gate drive circuit 42 provided in the grid control circuit 4 can be configured by semiconductor elements, miniaturization is easy. Further, as shown in FIG. 1, the grid control circuit 4 can be easily connected to the oscillation circuit 2. That is, the grid control circuit 4 can be detachably connected to the oscillation circuit 2.
Therefore, the grid control circuit 4 can be easily attached to the existing dielectric heating device.

FIG. 3 is a circuit diagram for illustrating a dielectric heating device 1a according to another embodiment.
As shown in FIG. 3, the dielectric heating device 1a is provided with an oscillation circuit 2, a DC power supply 3, and a grid control circuit 4a.
The grid control circuit 4a includes a power supply 40a, a switching element 41, and a gate drive circuit 42.
The power supply 40 illustrated in FIG. 1 is an independent power supply, but the power supply 40 a is a power supply using an AC voltage from the oscillation circuit 2.

The power supply 40a can use at least one of an alternating current voltage of LC resonant frequency and an alternating current voltage of switching frequency f / N.
For example, an AC voltage at the LC resonance frequency may be reflected toward the work 100 by an LC filter or the like, and only an AC voltage at the switching frequency f / N may be used.
Although an alternating voltage is input to the power supply 40a, a DC voltage is output from the power supply 40a and applied to the grid 22b. Therefore, it is preferable that the power supply 40a include an AC-DC converter circuit 40b.

Here, in order to drive the grid 22b of the vacuum tube 22 which is a heavy load with high efficiency, the AC energy input while the switching element 41 is not switched is stored, and the stored AC energy is stored. Is preferably used when the next switching is performed.
That is, it is preferable that the grid control circuit 4a further include an energy storage circuit 40c. The energy storage circuit 40c can be provided integrally with the AC / DC converter circuit 40b, or can be provided separately from the AC / DC converter circuit 40b.

Below, the case where the power supply 40a has an alternating current direct current conversion function and an energy storage function is demonstrated.
FIG. 4 is a circuit diagram for illustrating the power supply 40a.
As shown in FIG. 4, the power supply 40a can be a Cockcroft-Walton circuit. And a capacitor can be used as an energy storage means.

The switching element 41 is connected to the terminal 40a1 of the power supply 40a, and the oscillation circuit 2 is connected to the terminal 40a2 and the terminal 40a3.
The power supply 40a includes a plurality of circuits 40a6 configured of two diodes 40a4 and two capacitors 40a5. The plurality of circuits 40a6 are connected in series. The circuit 40a6 rectifies and charges one cycle of the input AC voltage.

  For example, when switching the switching element 41 at the switching frequency f / N, N circuits 40a6 can be connected in series. In addition, what was illustrated in FIG. 4 is a case where the switching element 41 is switched at the frequency of "f / 3".

By connecting the plurality of circuits 40a6 in series, the voltage applied to one capacitor 40a5 and the diode 40a4 can be reduced. In addition, the amount of heat generation in one capacitor 40a5 and the diode 40a4 can be reduced.
Further, since the circuit 40a6 is composed of the diode 40a4 and the capacitor 40a5, it becomes easy to miniaturize the power supply 40a.
Furthermore, if energy is stored using the capacitor 40a5, there is also an advantage that the phase of switching can be controlled by comparing the stored charge amount (voltage) with the threshold voltage.
Note that a voltage doubler rectifier circuit, a half rectifier circuit, a bridge rectifier circuit, or the like, which is a circuit including a capacitor and having an AC / DC conversion function and an energy storage function, can also be used.

FIG. 5 is a circuit diagram for illustrating a power supply 40a according to another embodiment.
As shown in FIG. 5, the power supply 40a can be a flyback circuit. The flyback circuit stores energy as a current flowing through the coil (flyback transformer 40a7).
When the flyback transformer 40a7 is used, the switching element 40a8 having a lower withstand voltage can be used as compared to the energy storage using the above-described capacitor. Therefore, a semiconductor element using silicon or the like with a small band gap can be used.

The switching element 40a8 can be, for example, a semiconductor element such as a MOSFET, a HEMT, a JFET, or a junction transistor.
Further, as shown in FIG. 5, the gate drive circuit 42 can be built in the power supply 40a.
In this case, the terminal 40a1 can be connected to the gate electrode of the switching element 41.
The terminal 40a1 can also be connected to the capacitor 23. In this case, the DC voltage is directly applied to the grid 22b by the gate drive circuit 42 built in the power supply 40a.

As described above, the grid control circuit 4a has the power supply 40a, and an AC current or AC voltage of the LC resonance frequency f or the switching frequency f / N is input to the power supply 40a.
The power supply 40a may include one of a Cockcroft-Walton circuit, a voltage doubler rectifier circuit, a half rectifier circuit, a bridge rectifier circuit, and a flyback circuit.

The switching elements 41 and 40a8 provided in the grid control circuits 4 and 4a are preferably semiconductor elements having a band gap of 1.4 eV or more or electron mobility of 1500 cm ² / V · s or more. A semiconductor element having a band gap of 1.4 eV or more, or an electron mobility of 1500 cm ² / V · s or more can include, for example, GaAs, GaN, SiC, InP, or the like.
With such a semiconductor element, low loss can be achieved, the withstand voltage of the switching elements 41 and 40a 8 can be increased, and the switching speed of the vacuum tube 22 can be increased. Moreover, if such a semiconductor element is used, the power efficiency and the reliability of the dielectric heating device 1 can be improved.
The diode 40a4 provided in the power source 40a or the like is also preferably a diode having a band gap of 1.4 eV or more or an electron mobility of 1500 cm ² / V · s or more. The above “band gap is 1.4 eV or more, or electron mobility is 1500 cm ² / V · s or more” is “band gap is 1.4 eV or more” and “electron mobility is 1500 cm ² / V · s or more” It means that at least one should be satisfied.

  While certain embodiments of the present invention have been illustrated, these embodiments have been presented by way of example only, and are not intended to limit the scope of the invention. These novel embodiments can be implemented in other various forms, and various omissions, replacements, changes, and the like can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and the gist of the invention, and are included in the invention described in the claims and the equivalent scope thereof. In addition, the embodiments described above can be implemented in combination with each other.

  DESCRIPTION OF SYMBOLS 1 dielectric heating apparatus, 2 oscillation circuits, 3 DC power supplies, 4 grid control circuits, 20 electrodes, 21 coils, 22 vacuum tubes, 22a filaments, 22b grids, 22c plates, 40 power supplies, 40a power supplies, 40a4 diodes, 40a5 capacitors, 40a6 circuits , 40a7 flyback transformer, 40a8 switching element, 40b AC DC conversion circuit, 40c energy storage circuit, 41 switching element, 42 gate drive circuit, 100 work

Claims (9)

A grid control circuit for intermittently applying a voltage to a grid of a vacuum tube provided in an oscillation circuit of a dielectric heating device, comprising:
When the LC resonant frequency of the oscillation circuit is f and the positive integer is N, the grid control circuit intermittently applies the voltage to the grid such that the switching frequency related to the application of the voltage is f / N. Grid control circuit to apply.   40. at least one of the LC resonant frequency and the switching frequency is 6.765 MHz to 6.795 MHz, 13.553 MHz to 13.567 MHz, 26.957 MHz to 27.283 MHz, and 40.66 MHz to 40. The grid control circuit according to claim 1, wherein the grid control circuit is included in any band of 70 MHz or less.   The grid control circuit according to claim 1 or 2, wherein the grid control circuit has a power supply, and an AC current or an AC voltage of the LC resonance frequency or the switching frequency is input to the power supply.   The grid control circuit according to claim 3, wherein the power supply includes one of a Cockcroft-Walton circuit, a voltage doubler rectifier circuit, a half rectifier circuit, a bridge rectifier circuit, and a flyback circuit. The switching element provided in the grid control circuit is a semiconductor element having a band gap of 1.4 eV or more, or an electron mobility of 1500 cm ² / V · s or more. Grid control circuit. The grid control circuit according to claim 3 or 4, wherein the diode provided in the power supply is a diode having a band gap of 1.4 eV or more or an electron mobility of 1500 cm ² / V · s or more. An electrode,
An oscillator circuit having a vacuum tube and supplying high frequency power to the electrodes;
The grid control circuit according to any one of claims 1 to 6, wherein a voltage is intermittently applied to a grid of the vacuum tube;
Dielectric heating device.   The dielectric heating apparatus according to claim 7, wherein the grid control circuit is detachably connected to the oscillation circuit.   The dielectric heating device according to claim 7 or 8, wherein the high frequency power applied to the electrode is 1 kW or more.

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