JPH0650672B2 - 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. Generally, in a magnetron, in order to prevent radio wave leakage from the input terminal side, that is, the cathode stem side, a choke coil and a feedthrough capacitor provided in a shield case covering the cathode stem constitute a filter circuit. However, if an inverter power source using a frequency of 20 KHz or more is used as the power source, 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. 4 is a cross-sectional view of a main part of a high-frequency heating device using an inverter power supply, which is disclosed in the publication, and is proposed for the purpose of solving the above problems.
In FIG. 4, 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 has a cathode terminal 2 respectively.
1, 22 are provided. A shield case 23 formed of a non-magnetic conductor that surrounds the cathode stem 20 and is electrically connected to the oscillation tube 14 completely shields the inside and the outside of the shield case 23 at high frequencies. It is configured. The primary winding 6 of the step-up transformer 6 is externally exposed through the wall surface 23a of the shield case 23.
a and a primary-side core 24 wound around this, a high-voltage secondary winding 6b, a low-voltage secondary winding 6c, and a secondary-side core 25 wound around this are arranged to face each other. It was done.
In the case of this reference, the primary core 24 and the secondary core 25 are arranged so as to be electromagnetically coupled by mutual induction to the outside and the inside through the wall surface 23a of the shield case 23, whereby the input stem side of the magnetron is arranged. It is said that it is possible to prevent the leakage of high frequencies from the power supply and to supply high-frequency power to the magnetron at the same time.

Problems to be Solved by the Invention However, although the above-described structure seems to work as described at first glance, it is considered that the structure is in principle inconsistent for the reasons described below. .

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, leakage of high frequency 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 so that electromagnetic energy is not transmitted and electromagnetic coupling by mutual induction occurs. There is no arrangement. 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 will not be generated by the shield case 23.
An induced current, that is, an eddy current, is generated on the wall surface 23a of the booster transformer 6, and its electric energy is consumed as Joule heat due to the eddy current.
Because it is not transmitted to. If this phenomenon is explained from another point of view, the shield cheese 23 made of a conductor is
It is equivalent to adding a short-circuited third secondary winding to the step-up transformer 6, and it may be considered that the electric energy is almost consumed by the short-circuit current flowing in the short-circuited secondary winding. . In any case, it must be said that the configuration of the reference is merely a fantasy configuration in that the shield case 23 has an action that cannot be physically realized by the means understood from the reference. .

The present invention realizes the problems described in the above publication in a physically meaningful form.
The purpose is to prevent leakage of high frequency from the 0 side and to simultaneously supply power to the magnetron by a high frequency power source.

Means for Solving the Problems In order to solve the above problems, the high-frequency heating device of the present invention has a part or all of the wall surface of the shield case that covers the cathode stem of the magnetron permeates at a frequency used in the high-frequency inverter circuit. It is composed of a metal thin film or a metal deposition film having a thickness of 1/10 or less of the depth and a dielectric material that serves as its structural support, and the secondary side of the step-up transformer is placed inside and outside the shield case through this wall surface. And the primary side is divided.

Effect of the Invention With the above 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 step-up transformer 72 secondary side circuit arranged in the shield case and the outside. The arranged primary side circuits operate as transformers to each other, and the power of which the frequency is increased by the inverter can be supplied to the magnetron.

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 is a schematic circuit configuration of the high-frequency heating apparatus according to an embodiment of the present invention including the main part shown in FIG. It is a circuit diagram shown.

In FIG. 2, the electric power supplied from the commercial frequency power source 1 is rectified by the rectifier circuit 3, and then is passed through a low-pass filter composed of an inductance 5 and a capacitor 4 to be a capacitor 8, a diode 9, and a switching element. It is supplied to the inverter circuit including the transistor 10. 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 an anode voltage and a cathode voltage of the magnetron, respectively. And applied to the magnetron 13. In FIG. 1, 14 is an oscillating tube which is the main body of the magnetron, and the oscillating tube 14 has an antenna 19 at one end and a cathode stem 2 at the other end.
0 is set. The cathode stem 20 is provided with cathode terminals 21 and 22, respectively. The shield case 28 covering the cathode stem 20 has a lid 28a, which is a part of its wall surface, a non-magnetic metal thin film or a shield film 28b made of a metal vapor deposition film, and a structure support of the shield film 28b made of a dielectric material. 28c.

The step-up transformer 7 is divided into the inside and 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 7d.
The primary winding 7a is wound around the secondary winding core 7e, and the secondary windings 7b and 7c are wound around the secondary core 7e. Regarding each winding, as already explained, the primary winding is
The voltage converted to high frequency by the inverter circuit is supplied,
The high voltage and the low voltage generated in the secondary windings 7b and 7c are applied through the cathode terminals 20 and 21 as the anode voltage and the cathode voltage of the magnetron, respectively.

Here, the shield effect of the shield case 28 and the operation of the step-up transformer 7 will be described.

In general, a conductor represented 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 l Is the depth of 1 / e of the surface current density la,
The penetration depth is called δ (m), and the specific resistance of the conductor is ρ (Ω
・ M), frequency (Hz), relative permeability μr Becomes

Generally, using this penetration depth δ, the current density 1 in the conductor at the position x from the surface is represented by l = lae ^{−x / δ} , where the current density on the surface is la. The attenuation of the magnetic field is proportional to the attenuation of the current, and its attenuation ratio A (dB) is expressed as follows.

Therefore, for example, when considering an Al plate, the penetration depth δ at 20 KHz is about 0.6 mm, and it is assumed that the thickness of the thin Al plate is about 0.6 mm.
Even with a plate, the attenuation ratio A is 8.686 dB, and the AC magnetic field applied from one surface of the Al plate is hardly transmitted to the opposite side, and most of it is consumed as ohmic loss due to the induced current in the Al plate.

However, for example, considering an Al foil with a thickness of 20 μm, it is 20 KH.
It is 1/30 of the penetration depth δ0.6mm (600μm) at z, and A
= 0.3dB, which means that most of the energy can be transferred to the other side. FIG. 3 is an example of an actual measurement value showing how the shielding rate of such a metal foil or a paint film against an alternating magnetic field changes with frequency. Fig. 3 and the formula of penetration depth δ As can be seen from, as the frequency increases,
The penetration depth δ becomes smaller, so that the shielding property of the metal foil or the conductive paint film is increased.

Therefore, the non-magnetic stainless steel (SU
S) Considering the case where it is adopted as a metal thin film of 50 μm, it can be said that there is no shielding property for an AC magnetic field having a frequency of 100 KHz or less.
Although the primary side and the secondary side are separated by the lid 28a which is a part of the wall surface of the shield case 28, the function as a step-up transformer is fully exhibited. On the other hand, the noise generated by the magnetron is 2450 which is the fundamental frequency.
Although it extends to a wide range of low frequencies other than the high frequency of MHz, the shield film 28b is a metal foil of non-magnetic stainless steel (SUS) 50 μm at a high frequency of 10 MHz or higher which substantially requires a shield effect. Also has a shield characteristic of 25 dB (magnetic field mode) or more, and the effect becomes greater as the frequency becomes higher. It should be noted that the coupling between the windings of the transformer is due to the AC magnetic field, and the data in FIG. 3 is also measured in the magnetic field mode.
It has been experimentally confirmed that even with the same non-magnetic stainless steel (SUS) metal foil of 50 μm, a shield performance of 40 dB or more including 10 MHz or less can be obtained when measured in an electric field mode. It is considered that an intermediate value between the magnetic field mode and the electric field mode has a substantial shield effect on the electromagnetic wave leaked from the cathode stem 20 which is intended to prevent the leakage now. It has a sufficient effect.

The thickness of the shield film 28b is determined in consideration of characteristics such as the step-up transformer 7 and shield characteristics.
Considering the loss at b, it is considered that one of the targets is 1/10 or less of the penetration depth δ.

As can be understood from the above description, according to the configuration of the above-described embodiment, the shield case 28 has a function of preventing high-frequency leakage from the cathode stem 20 side of the magnetron, and is divided by the wall surface of the shield case 28. The boosted transformer 7 thus fully exerts its function as a transformer.

When a magnetic metal is used for the shield film 28b, the shield film 28b forms a shunt channel of the magnetic circuit of the step-up transformer 6 formed by the core 7, and the magnetic efficiency is reduced. It is desirable to use metal. However, in the case of a non-magnetic material typified by copper (Cu) or aluminum (Al), since the specific resistance ρ (Ω · m) is also small, the penetration depth δ is also small, and it is considered that processing problems occur. The material of the shield film 28b should be decided in consideration of these points.

Further, the wall surfaces of the shield case 28 other than the shield film 28b are also preferably made of a non-magnetic metal in order to reduce magnetic leakage from the step-up transformer 7.

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

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

(1) Microwaves 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 inside this shield case and the primary side circuit arranged outside. Operate as transformers with each other, and the power whose frequency is increased by the inverter can be efficiently supplied to the magnetron.

(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 inductance and capacitance to prevent high-frequency leakage, and therefore the voltage drop and power loss caused by this filter circuit can be eliminated. Also, because the magnetron anode voltage is as high as around 4 KV, it has a high withstand voltage, so it is possible to save expensive capacitance and inductance, thereby greatly reducing costs.

[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, FIG. 2 is a schematic circuit diagram of the same high-frequency heating apparatus, and FIG. 3 is for explaining the principle of the present invention. Characteristic diagram showing the shielding characteristics of the paint film against the AC magnetic field, No. 4
FIG. 1 is a sectional view of a main part of a conventionally proposed high-frequency heating device. 7 ... Step-up transformer, 7p ... Primary core, 7s ... Secondary core, 28 ... Shield case, 28a ... Lid, 28b ... Shield film.

Claims (4)

[Claims] 1. A magnetron, a high-frequency inverter circuit, a primary winding connected to the output of the high-frequency inverter circuit, a high-voltage secondary winding for supplying the anode voltage of the magnetron, and a cathode voltage of the magnetron. A step-up transformer including a core around which the low-voltage secondary winding, the primary winding, and the high-voltage and low-voltage secondary windings are wound, and a shield case that covers the cathode stem portion of the magnetron, and the wall surface of the shield case A part or all of the shield film made of a metal thin film or a metal vapor deposition film having a thickness not more than the penetration depth at the frequency used in the high frequency inverter circuit, and the structure support of the shield film made of a dielectric material. The wall of the step-up transformer, which is divided into the outside and the inside of the shield case, is connected to the primary side coil. , The secondary core, wherein each primary winding and the high-frequency heating apparatus winding a secondary winding. 2. The high frequency heating apparatus according to claim 1, wherein the material of the shield film is a non-magnetic metal. 3. The high frequency heating apparatus according to claim 1, wherein the shield case has a wall surface made of a non-magnetic metal plate except for the shield film and its structural support. 4. A high-frequency heating apparatus according to claim 1, wherein the shield case has a shield film and a structural support thereof for all or only a part of the lid portion of the wall surface of the shield case.