JPH0665147B2 - High frequency heating device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-frequency heating device that boosts high-frequency power with a step-up transformer and supplies it to a magnetron.

2. Description of the Related Art In recent years, in a high-frequency heating device, a power supply system in which a commercial frequency is once increased by an inverter circuit and then boosted and applied to the magnetron is being studied in order to reduce the size, weight and cost of the magnetron power supply. , This is called the inverter power supply. In general, a magnetron forms a filter circuit by a choke coil or a feedthrough capacitor provided in a shield case that covers the cathode stem of the magnetron in order to prevent power leakage from the input terminal side, that is, the cathode stem side. However, when an inverter power supply using a frequency of 20 kHz or more is used as the power supply, there arises a problem that the high frequency power itself is suppressed by the filter circuit and is difficult to be supplied to the magnetron. FIG. 5 is a cross-sectional view of a main part of a high-frequency heating device using an inverter power source, which is seen in a conventional publication.
It is proposed for the purpose of solving the above problems. In FIG. 5, reference numeral 14 denotes an oscillating tube. The oscillating tube 14 is provided with an antenna 19 at one end and a cathode stem 20 at the other end, and the cathode stem 20 is provided with cathode terminals 21 and 22, respectively. The cathode stem 20
And a shield case 23 formed of a non-magnetic conductor that is electrically connected to the oscillation tube 14
The inside and outside of the shield case 23 are completely cut off at high frequencies. This shield case 2
The primary side winding 6a of the step-up transformer 6 and the primary side core 24 wound around the primary side winding 6a and the high voltage secondary side winding 6b and the low voltage secondary side winding 6c are provided on the inside through the wall surfaces 23a of No. And the secondary side core 25 wound with this is arranged to face each other. In the case of this reference, the primary core 24 and the secondary core 2
5 are arranged so as to be electromagnetically coupled by mutual induction to the outside and the inside via the wall surface 23a of the shield case 23,
As a result, it is said that it is possible to prevent leakage of high frequencies from the input stem side of the magnetron and to simultaneously supply high-frequency power to the magnetron.

Problems to be Solved by the Invention However, although the above-described configuration seems to work as described at first glance, the configuration is in principle inconsistent due to the reason described below, and therefore, the description of the reference is omitted. It is just a virtual configuration that cannot be realized as it is.

That is, according to the description of the reference, the shield case 23 surrounding the cathode stem 20 provided at one end of the oscillation tube 14 is formed of a non-magnetic conductor, and is shielded via the wall surface 23 a of the shield case 23. The primary side and the secondary side of the step-up transformer are arranged inside the. In this case, the high frequency leakage from the cathode stem 20 can be surely prevented, but the primary side and the secondary side of the step-up transformer 6 are also shielded at the same time, and the electromagnetic energy is not transmitted. There is no such thing. Because the wall surface 23a of the shield case 23 is made of a conductor, the magnetic field generated by the high-frequency current flowing through the primary winding 6a of the step-up transformer 6 causes an induced current to the wall surface 23a of the shield case 23. It produces an electric current whose electrical energy is This is because it is consumed as Joule heat due to the electric current and is not transmitted to the secondary windings 6b and 6c of the step-up transformer 6 or is greatly attenuated. To explain this phenomenon from another point of view, the shield case 2 made of a conductor is used.
3 is equivalent to adding a short-circuited third secondary winding to the step-up transformer 6, and it is considered that the electric energy is almost consumed by the short-circuit current flowing in the short-circuited secondary winding. May be. In any case, it must be said that the configuration of the conventional example is merely a fantasy configuration in that the shield case 23 has a function that cannot be physically realized by the means understood from the description of the conventional example. Absent.

The present invention realizes the problems that the conventional example has attempted to solve in a physically meaningful manner. It prevents leakage of high frequencies from the side of the cathode stem 20 of the magnetron and supplies power to the magnetron with a high-frequency power source. It is intended to realize both of these simultaneously.

Means for Solving the Problems In order to solve the above problems, the high-frequency heating device of the present invention is
A part or all of the wall surface of the shield case that covers the cathode stem 20 of the magnetron is made of a honeycomb made of a metal foil, and the secondary side and the primary side of the step-up transformer are provided inside and outside the shield case through the wall surface portion. It is configured to be divided.

Effect of the Invention With the above-described configuration, the present invention can confine microwaves and high-frequency radio noise leaking from the cathode stem side of the magnetron into the shield case, and at the same time, the secondary side circuit of the step-up transformer arranged inside the shield case and the external circuit. The primary side circuits arranged in the above are operated as a transformer with each other, and the high frequency electric power can be supplied to the magnetron by the inverter.

Embodiment An embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a sectional view of a main part of a high-frequency heating apparatus according to an embodiment of the present invention, and FIG. 2 (a) is a schematic view of a high-frequency heating apparatus according to an embodiment of the present invention including the main part shown in FIG. It is a circuit diagram which shows a circuit structure.

In FIG. 2 (a), the electric power supplied from the commercial frequency power source 1 is rectified by the rectifier circuit 3 and then the inductance 5
It is supplied to an inverter circuit including a capacitor 8, a diode 9 and a transistor 10 which is a switching element through a low-pass filter including a capacitor 4 and a capacitor 4. The transistor 10 is on / off controlled by a high frequency pulse output from the control circuit 11.

After being converted into a high frequency of about 20 kHz by the inverter circuit, the voltage supplied to the primary winding 7a of the step-up transformer 7 is induced in the secondary windings 7b and 7c as the anode voltage and the cathode voltage of the magnetron, respectively. , Is applied to the magnetron 13. In FIG. 1, reference numeral 14 is an oscillating tube which is a main body of a magnetron, and the oscillating tube 14 has an antenna 19 at one end and a cathode stem 20 at the other end. The cathode stem 20 is provided with cathode terminals 21 and 22, respectively. In the shield case 28 that covers the cathode stem 20, a lid 28a that is a part of the wall surface is composed of a honeycomb core 28b made of aluminum foil and a structural support 28c of the honeycomb core 28b made of a dielectric material.

The step-up transformer 7 is divided into the inside and the outside of the shield case 28 by a lid 28a whose core is a part of the wall surface of the shield case 23, and the outside primary core 7p.
A primary winding 7a is wound around the secondary winding, and a secondary winding 7b, 7c is wound around a secondary core 7s therein. Here, as the material of the core, it is general to use a ferrite core in terms of frequency, and regarding each winding, as already described, the primary winding has a voltage converted to a high frequency by the inverter circuit. The high voltage and the low voltage generated in the secondary windings 7b and 7c are applied as cathode voltage and cathode voltage of the magnetron through the cathode terminals 20 and 21, respectively.

FIG. 3 is for explaining the details of the structure of the aluminum honeycomb core 28b, and FIG. 3 (a) is a perspective view of a part of the aluminum honeycomb core 28b cut out into a substantially rectangular shape,
FIG. 3 (b) is an enlarged view of one cell (unit structure) forming the aluminum honeycomb core 28b. Honeycomb core is third
As can be understood from the enlarged view of FIG. (B), the aluminum foil 32 is laminated to form a hexagonal cell, which is continuously formed in a large number. Despite being extremely lightweight, it can be given a considerable amount of mechanical strength, so it can be used as a structure for aircraft, and it can also shield electromagnetic waves while maintaining breathability. It is widely used for ventilation holes in rooms. In the case of this embodiment, the foil thickness S of the aluminum foil used for the honeycomb core 28b is
For example, when 20kHz is used for the inverter frequency,
We use aluminum with a penetration depth of less than 1/10 of 0.6 mm for this frequency. The cell size D and the core height T of the honeycomb core 28b are selected on the basis of the maximum frequency of unnecessary radiation to be shielded. In the case of this embodiment, the core height T is 1.5 mm and the cell size D is The value is about 3 mm.

Next, the operation of the above configuration will be described. First, the function of the step-up transformer will be described with reference to FIG. Fig. 4
(a) shows a honeycomb core 28b made of aluminum foil 32 forming the wall surface of the lid 28a of the shield case 28, and a step-up transformer 7 and a step-up transformer 7 which are separately arranged inside and outside the shield case 28 by the honeycomb core 28b. The relationship with the magnetic flux lines 31 of the generated high-frequency magnetic field of 20 kHz is shown. FIG. 4 (b) is an enlarged view showing the relationship between the honeycomb core 28b and the magnetic flux lines 31. The magnetic flux lines 31 of the high frequency magnetic field correspond to the tube axis direction of each cell (unit structure) constituting the honeycomb core 28b. It is almost parallel.

In general, a conductor typified by a metal generates an induced current when an alternating magnetic field is applied regardless of whether it is a magnetic substance or a non-magnetic substance, but the depth of penetration of the induced current from the surface is finite, and the current density I Is a depth of 1 / e of the surface current density I _O ,
The penetration depth is called δ (m), and the specific resistance of the conductor is ρ
(Ωm), frequency (Hz), relative permeability μr, Becomes Using this penetration depth δ, the current density I in the conductor at the position x from the surface is expressed by I = I _O e ^{-x / δ} (1) where I _O is the current density on the surface. Be done. For example, if the conductor is aluminum, 20kHz
The penetration depth is about 0.6 mm. The direction of the induced current is perpendicular to the direction of the magnetic field (the magnetic force lines 31).

Now, FIG. 4 (c) shows an aluminum foil 32 having a thickness S forming a honeycomb core and magnetic field lines 31 of an alternating magnetic field applied in parallel therewith.
And the state of the induced current induced in the aluminum foil 32. The current induced in the aluminum foil exponentially decreases from the surface as shown in the equation (1), but its direction is perpendicular to the magnetic field and the current densities I _{a in the} opposite directions on the front and back sides of the foil. , I _b . By the way, the thickness S of the aluminum foil forming the honeycomb 28b is a value sufficiently smaller than the penetration depth δ = 0.6 mm, for example, if it is selected to be 1/10 of the penetration depth, it is symmetrical in the front and back of the foil and in the opposite direction. The current densities I _a and I _b induced by the two cancel each other out, and the current density I actually flowing becomes the difference between I _a and I _b . The current induced in such an aluminum foil placed parallel to the magnetic field is almost negligible when the thickness is sufficiently small, for example, 0.02 mm, which is 1/10 or less of the penetration depth δ as in the embodiment. Become.

Therefore, the aluminum honeycomb 28b is the same as that of the step-up transformer 7.
Despite being interposed between the secondary core 7p and the secondary core 7s, there is almost no loss with respect to a high-frequency magnetic field of 20 kHz, and therefore, the step-up transformer 7 has a sufficient function. Demonstrate.

Next, the function of the shield case 28 of the magnetron 13 will be described as to the operation of the present invention. Unwanted radiation leaking from the cathode stem 20 of the magnetron 13 extends to a wide range and low frequencies in addition to the fundamental wave, which is a harmonic of 2450 MHz, but the shield case 28 needs a shielding effect from a wide range of 20 MHz. The range is about 20 GHz.

When the penetration depth δ of the aluminum forming the honeycomb core 28b is calculated for the case of 20 MHz, the case of 20 kHz is calculated. 1 or about 0.02 mm, which corresponds to the foil thickness of the honeycomb core 28b in this embodiment. That is, if the frequency is 20 MHz or more, the 0.002 mm-thick aluminum foil forming the honeycomb core 28b also has the penetration depth or more,
Therefore, it shows the characteristics as a normal aluminum plate.
As a result, each cell constituting the honeycomb core 28b is regarded as a waveguide, and each of the waveguides exhibits a generally known shielding characteristic caused by exhibiting a cut-off phenomenon with respect to leakage electromagnetic waves. It will be. On the other hand, regarding the higher frequency, in the case of this embodiment, the cell size D is 3
Since we have selected mm, the maximum frequency of unwanted radiation is 20
Even if it extends to GHz, the cell size D becomes 1/5 of the wavelength λ = 15 mm, which is sufficiently within the cutoff region, and therefore shows a high shielding effect.

As can be understood from the above description, according to the configuration of the present invention, the shield case 28 has a function of preventing leakage of unnecessary radiation from the cathode stem 20 side of the magnetron 13, and is divided by the wall surface of the shield case 28. The step-up transformer 7 also fully functions as a transformer.

Moreover, in the case of a conventional step-up transformer that steps up the commercial frequency as it is, for example, even if it handles a very general electric power of about 1 KW, the weight is 4 Kg and the external dimensions are about 110 × 110 × 7.
It is an iron mass of about 0 mm, and it is very heavy and large compared to a magnetron with a weight of 1 KW and an external dimension of about 100 x 100 x 80 mm, so not only the weight and size of the entire power supply circuit increase, but also the boosting. It was impossible to integrate the transformer with the magnetron shield case. However, in the case of the present invention, since the high frequency inverter circuit converts to a high frequency of several tens of kHz, the magnetic core may change from silicon steel plate to ferrite, and the weight is 400 g, and the external dimensions are 70 × 70 ×.
Since it is lightweight and compact with a size of about 40 mm, the step-up transformer can be integrated with the shield case of the magnetron with the configuration as shown in the above embodiment.

As can be understood from the above description, the thickness of the metal foil forming the honeycomb core 28b depends on the inverter frequency (20 kHz in the embodiment), the lowest frequency of unnecessary radiation to be shielded, its effect, and the boost transformer. It depends on electrical characteristics such as efficiency, but considering the mechanical strength, the penetration depth at the inverter frequency is 1
The optimum thickness is in the range of / 10 to 1/100.

When a magnetic metal is used as the metal material, the honeycomb core 28b forms a shunt flow path (shunt circuit) of the magnetic circuit of the step-up transformer 7, and the magnetic efficiency is reduced. It is desirable to use a non-magnetic metal. However, copper (Cu) and aluminum (A
In the case of a non-magnetic material represented by l), the specific resistance ρ (Ω ・
Since m) is also small, the penetration depth δ also becomes small, and it is also necessary to consider that processing problems will occur.
The material of should be decided comprehensively considering these points. For example, Monel is a non-magnetic material and has a relatively large specific resistance, so it can be said that it has desirable characteristics as a whole.

The shield wall surface of the shield case 28 other than the honeycomb core 28b is also made of a non-magnetic metal having good electric conductivity, such as aluminum, in order to reduce the influence of leakage magnetism from the step-up transformer. (2
It is desirable that the penetration depth at 0 kHz) be 0.6 mm or more.

Ventilation holes 28d provided on the wall surface of the shield case 28,
Reference numeral 28e is for cooling the secondary side of the step-up transformer 7, and is configured to perform forced air cooling from the outside of the shield case 28 with the cooling fan 2.

Further, in the present embodiment, the secondary side voltage of the step-up transformer is directly applied to the magnetron as the anode voltage, but the diode and the capacitor are double-voltage rectified, or the full-wave rectification is performed by a plurality of diodes and then the anode voltage is applied. It goes without saying that there are various circuit types, such as applying to. Further, in the case of such a circuit configuration, the circuit elements such as the diode and the capacitor are naturally arranged in the shield case. FIG. 2 (b) shows an example of the circuit configuration when such a circuit configuration is adopted.
And the condenser 30 constitute a voltage doubler rectifier circuit.

Effects of the Invention As described above, according to the high frequency heating device of the present invention, the following effects are obtained.

(1) The microwave and high-frequency radio noise leaking from the cathode stem side of the magnetron can be confined in the shield case, while the secondary circuit of the step-up transformer arranged in this shield case and the primary circuit arranged outside. By mutual mutual magnetic coupling and functioning as a transformer,
It becomes possible to efficiently supply the high frequency electric power to the magnetron with the inverter.

(2) As a result, it is possible to eliminate the filter circuit that was conventionally provided in the shield case of the magnetron and that consists of an inductance and a capacitance (feedthrough capacitor) for preventing the leakage of high frequency radio noise. In addition to eliminating voltage drop and power loss, the anode voltage of the magnetron is as high as around 4KV, so it is possible to greatly reduce costs by eliminating expensive capacitance (feed-through capacitor) and inductance that are conventionally high withstand voltage.

[Brief description of drawings]

FIG. 1 is a sectional view of a main part of a high-frequency heating apparatus according to an embodiment of the present invention, FIGS. 2 (a) and 2 (b) are circuit configuration diagrams of the same high-frequency heating apparatus, and FIG. Perspective view of the honeycomb core,
3 (b) is an enlarged plan view of the same, FIGS. 4 (a) to 4 (c) are illustrations of the same action, and FIG. 5 is a sectional view of a conventional example. 7 ... Step-up transformer, 7p ... Primary side core, 7s ... 2
Next core 28, shield case 28a, lid,
28b ... Honeycomb core.

Claims (4)

[Claims] 1. A magnetron having a shield case for covering a cathode stem portion to prevent radio wave leakage, a high frequency inverter circuit for converting power of a commercial frequency into high frequency power, and an output of this high frequency inverter circuit is supplied. A secondary winding, a high-voltage secondary winding for supplying an anode voltage and a cathode voltage of the magnetron, a low-voltage secondary winding, and a step-up transformer having a core formed by winding the primary winding and the secondary winding. The step-up transformer is composed of a honeycomb core made of a metal foil as a part or all of the wall surface that constitutes the shield case of the magnetron, and the wall surface portion made of the honeycomb core divides the wall between the inside and outside of the shield case. The primary core and the secondary core are respectively used as cores, and the primary winding and the secondary winding are wound around each core. Wave heating device. 2. The metal foil constituting the honeycomb core has a thickness of about 1/10 or less of a penetration depth of the metal with respect to a frequency of a high frequency used in an inverter circuit. High frequency heating device. 3. The material of the metal foil constituting the honeycomb core,
The high frequency heating device according to claim 1, wherein the non-magnetic metal is used. 4. The high frequency heating apparatus according to claim 1, wherein the shield case has a metal portion of the wall surface other than the honeycomb core made of a non-magnetic metal.