JPH0370357B2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a high-frequency heating device using a high-frequency oscillator using a semiconductor element such as a transistor.
This invention relates to improvement of a power supply circuit that guides high frequency waves generated by a high frequency oscillator to a heating device.

Conventional configurations and their problems Conventional microwave ovens, which have been widely used as high-frequency heating devices for consumer use, have mostly used magnetrons, which are microwave vacuum tubes, as high-frequency oscillators, but in recent years transistor technology has improved. As technology advances, studies are underway to construct high-frequency oscillators using semiconductor elements such as transistors to achieve lower voltage, lighter weight, smaller size, and longer life.

However, it is said that the radio wave generation ability of such transistors generally decreases in proportion to the first to the second power of the operating frequency f, and it is difficult to create high-output, high-efficiency transistors in the 2450MHz frequency band used in conventional microwave ovens. The difficulty is great. Therefore, even with the current technology, the current situation is that transistorization requires the use of the low frequency of the 915MHz band, which is another ISM frequency band.

In the case of a high-frequency heating device using a high-frequency oscillator using a solid-state element such as a transistor, as shown in FIG. It is common to use a configuration in which

The reason for using such an expensive component such as the circulator 7 is as follows. First of all
Second, the high-frequency oscillator 1 itself, which uses solid-state elements such as transistors, is vulnerable to reflected power.Secondly, the use of 915MHz radio waves, which is necessary for solid-state production, makes the heating chamber 3 side visible from the antenna 2. Keeping the standing wave ratio (impedance) at a low value regardless of changes in load is even more difficult than when using radio waves in the 2450MHz band. For the above two reasons, the conventional configuration of a transistorized microwave oven is such that all the radio waves reflected from the heating chamber 3 are absorbed into the non-reflection terminal 8 and the radio waves are not returned to the high frequency oscillator 1.

Here, I will briefly explain why it is more difficult to achieve impedance matching when using 915MHz than when using 2450MHz.
In the case of 915MHz, the wavelength is 33cm, which is about three times that of 2450MHz, and in the case of the high-frequency heating device for general consumer use according to the present invention, the wavelength almost corresponds to the size of the heating chamber. Therefore, compared to 2450MHz, the number of modes that can resonate in the heating chamber 3 is greatly reduced, and therefore, when the object to be heated 4 changes, the impedance seen from the antenna 2 toward the heating chamber 3 changes greatly. . For this reason, impedance matching is more difficult than at 2450MHz, especially under light loads and near-empty conditions, and without a circulator, most of the radio waves emitted from oscillator 1 will be lost depending on the load condition. It will reflect and come back.

However, this circulator 7 is not only an expensive component, but also has the problem that the high-power ones used in microwave ovens, such as several hundred W class, are very large and poorly made. This has been one of the major obstacles when using semiconductor high-frequency oscillators in devices.

Purpose of the Invention An object of the present invention is to provide a high-frequency heating device using a high-frequency oscillator using a semiconductor element, in which a circulator, which is conventionally required to protect the high-frequency oscillator from radio waves reflected from a heating chamber, can be omitted. That is.

Structure of the Invention In order to achieve the above object, the high-frequency heating device of the present invention includes a directional coupler that is cheaper than a circulator and has a simpler configuration, a plurality of antennas that each guide the divided radio waves into a heating chamber, and a heating device. It is configured with a non-reflection termination to absorb some of the radio waves reflected from the room.

With this configuration, a part or all of the reflected power from the heating chamber can be absorbed by the non-reflective termination using a simple component called a directional coupler, and reflection to the high frequency oscillator can be reduced.

DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. A typical example of the directional coupler 5 in FIG. Its function is that no output appears. The general properties of such a directional coupler are expressed by the following equation using an S matrix that expresses the relationship between the incident wave and the reflected wave at each terminal.

b ₁ b ₂ b ₃ b ₄ = O jβ α O jβ O O α α O O jβ O α jβ Oa ₁ a ₂ a ₃ a ₄ ...(1) However, α2+β2=1 (α, β, real number) Formula In (1), a ₁ , a ₂ , a ₃ , a ₄ are each terminal 1,
The amplitude and phase of the radio waves incident on 2, 3, and 4 are expressed as complex quantities, and a _i・a _i ^* (i=1, 2, 3,
4) is defined to be the incident power. or
b ₁ , b ₂ , b ₃ , and b ₄ represent the amplitude and phase of the radio waves going out from each terminal 1, 2, 3, and 4, and as with a _i , b _i and b _i ^*
indicates the power of the outgoing radio waves.

This embodiment will be explained according to the function of the above-mentioned directional coupler. Radio waves generated by the high frequency oscillator 1 enter the directional coupler 5 from the terminal 1. If this radio wave is a ₁ , the radio waves coming out to terminals 2, 3, and 4 are b ₂ , b ₃ , b ₄
is obtained by setting a ₂ = a ₃ = a ₄ = 0 in equation (1), and b ₂ = jβa ₁ , b ₃ = αa ₁ , b ₄ = 0...(2) Therefore, (b ₂・b ₂ ^* )/( _a1・_a1 ^* )=β2, ( _b3・_b3 ^* )/( _a1・_a1 ^* )=α2...(3). To simplify the discussion here, α2=β2=
If we consider the case of a 3 dB coupler where α = β = 1/√2 (for simplicity, we will explain the case of a 3 dB coupler where α = β = 1/√2, but α = β
It is easy to understand that the same function works in cases other than = 1/√2) (b ₂・ b ₂ ^* ) / (a ₁・ a ₁ ^* ) = 1/2 (b ₃・ b ₃ ^* ) / (a ₁ · a ₁ ^* ) = 1/2 (b ₄ · b ₄ ^* ) / (a ₁ · a ₁ ^* ) = 0 ... (4) Therefore, the radio wave entering from terminal 1 is divided into two. The signal goes out from terminals 2 and 3, but does not appear at terminal 4.

Now, the radio waves emitted from terminals 2 and 3 are transmitted to antenna 2.
It is radiated into the heating chamber 3 through the antennas d and 2b, but since the impedance seen from each antenna toward the heating chamber 3 changes when the object to be heated 4 changes, it is impossible to always achieve a matching state. Therefore, a part of the radio waves coming out from the terminals 2 and 3 is reflected and returns to the directional coupler 5 again. Now, if we consider only the reflected waves, these reflected waves are incident waves to terminals 2 and 3 when viewed from the directional coupler 5, so we can write them as a ₂ ′ and a ₃ ′. If a ₁ ′=a ₄ ′=0, then It can be written as The reflection from the antennas 2a, 2b varies depending on the design of the heating chamber 3 including the antennas 2a, 2b and the object to be heated 4. Let P ₂ and P _{3 be the reflection coefficients when looking at the antenna side from terminals 2 and 3} , and as one of the simplest cases, P ₂ = P ₃ = Re ^jo (R, o arbitrary real numbers) ...(6) Assuming that, using the relationship of equation (2), Substituting equation (7) into equation (5) gives b ₁ = 0 b ₄ = jbe ^jo a ₁ ...(8), and the reflections from terminals 2 and 3 are in phase as shown in equation (6). and have the same amplitude, all of the reflected waves appear only at the terminal 4 coupled to the non-reflection termination 6, and do not appear at the terminal 1 coupled to the high frequency oscillator 1.

The opposite case to the above example is when P ₂ = −P ₃ = Re ^jo (R, o, arbitrary real numbers) ...(9) where the reflections have the same amplitude but the phase is 180゜Different. From the relationship between equation (9) and equation (2), If equation (10) is substituted into equation (5), b ₁ =−Re ^jo a ₁ b ₄ =0 (11), and all reflected waves appear only at terminal 1.

Although the simplest conditions have been set in the above two cases, it is highly unlikely that such conditions will actually occur, and it is thought that an intermediate condition between the two will actually occur. That is, terminals 2 and 3
Of the radio waves emitted to the antennas 2a and 2b, those reflected due to impedance mismatch are:
A portion is absorbed by the high-frequency oscillator 1 through the terminal 1, and the rest is absorbed by the non-reflection termination 6 through the terminal 4.

Therefore, in the conventional example, if the circulator 7 was not used, all of the radio waves reflected from the antenna 2 would return to the high frequency oscillator 1, whereas in the above configuration, a part of the reflected waves would be transmitted to the non-reflection terminal 6. is absorbed by
Reflection to the high frequency oscillator 1 is greatly reduced.

On the other hand, in order to efficiently supply radio waves from the antenna 2 to the heating chamber 3, it is necessary to achieve impedance matching as much as possible regardless of changes in the object to be heated. As already mentioned, the number of resonant modes is greatly reduced due to this relationship, making impedance matching difficult. Regarding this problem itself, for example, in the above example, 2
Since power is supplied by two antennas 2a and 2b, each antenna has a mode that is easy to excite, and impedance matching over a wide range that cannot be achieved with one antenna 2 can be achieved. reflections are greatly reduced.

In addition, in the above example, we explained a configuration in which the high frequency is divided into two and then fed, but two directional couplers are added to further divide the two divided radio waves into two, and a total of three directional couplers are used for four distributions. It is also possible to obtain a configuration in which the above-mentioned effects are further improved by feeding the generated radio waves using four antennas.

Effects of the Invention As explained above, according to the configuration of the present invention, only a part of the reflected radio waves from the heating chamber returns to the high-frequency oscillator, and the rest is absorbed by the non-reflection termination, so basically The reflection to the high frequency oscillator is reduced. In addition, when the high-frequency oscillator is constructed with semiconductor elements such as transistors, the IMS frequency band is lower than the 2450MHz band commonly used in the past.
The 915MHz band will be used, but in this case the wavelength is approximately three times that of the conventional 2450MHz band, so the number of excitation modes is reduced and impedance matching is difficult. To solve this problem, since power is being supplied by multiple antennas, each antenna has a mode that is easy to excite, which makes impedance matching easier, and therefore reduces the reflected power to the high-frequency oscillator 1. Can be made small over a wide load range. For the above two reasons, even when using the high frequency oscillator 1 made of semiconductor elements such as transistors that are susceptible to reflected power, there is no need to use expensive parts such as circulators, and it is possible to generate inexpensive and reliable high frequency oscillators. The heating device can be configured using a high frequency oscillator made of a semiconductor element such as a transistor.

[Brief explanation of drawings]

Figure 1 is a basic configuration diagram of a conventional high-frequency heating device.
FIG. 2 is a configuration diagram of a high frequency heating device showing an embodiment of the present invention. 1...High frequency oscillator, 2...Antenna, 3...
heating chamber, 4... object to be heated, 5... directional coupler,
6...Reflection-free termination, 7...Circulator, 8...
…Reflection-free termination.

Claims (1)

[Claims] 1 A high-frequency oscillator using a semiconductor element, a directional coupler that splits the output radio waves of this high-frequency oscillator, multiple antennas for guiding each of the split radio waves into the heating chamber, and one of the reflected radio waves from the heating chamber. A high-frequency heating device consisting of a non-reflective termination that absorbs the