As the name suggests, a microwave oven uses microwaves to heat, and the frequency used is 2450MHz microwaves.
Microwave energy is generated by a microwave generator, which includes a microwave tube and a microwave tube power supply. The function of the microwave tube power supply (referred to as power supply or microwave source) is to convert commonly used AC power into DC power to create conditions for the microwave tube to work. The microwave tube is the core of the microwave generator, which converts DC power into microwave energy.
Microwave tubes are divided into two categories: microwave transistors and microwave electron tubes. Microwave transistors have small output power and are generally used in fields such as measurement and communications. There are many types of microwave tubes, commonly used ones include magnetrons, klystrons, traveling wave tubes, etc. They have different working principles, different structures, and different performances. They are widely used in radar, navigation, communications, electronic countermeasures and heating, scientific research, etc. Magnetrons are particularly suitable for microwave heating and other applications of microwave energy due to their simple structure, high efficiency, low operating voltage, simple power supply and strong ability to adapt to load changes. Magnetrons can be divided into pulse magnetrons and continuous wave magnetrons due to different working conditions. Microwave heating equipment mainly works in the continuous wave state, so continuous wave magnetrons are often used.
A magnetron is an electrical vacuum device used to generate microwave energy. Essentially a diode placed in a constant magnetic field. Under the control of the mutually perpendicular constant magnetic field and constant electric field, the electrons in the tube interact with the high-frequency electromagnetic field and convert the energy obtained from the constant electric field into microwave energy, thereby achieving the purpose of generating microwave energy.
There are many types of magnetrons. Here we mainly introduce multi-cavity continuous wave magnetrons.
A magnetron is composed of a tube core and magnetic steel (or electromagnet). The structure of the tube core includes four parts: anode, cathode, energy output device and magnetic circuit system. A high vacuum is maintained inside the tube. The structure and functions of each part are introduced below.
1 Anode
The anode is one of the main components of the magnetron. Together with the cathode, it forms a space where electrons interact with high-frequency electromagnetic fields. Under the action of a constant magnetic field and a constant electric field, electrons complete the task of energy conversion in this space. In addition to collecting electrons like the anode of an ordinary diode, the anode of the magnetron also plays a decisive role in the oscillation frequency of the high-frequency electromagnetic field.
The anode is made of a metal material with good conductivity (such as oxygen-free copper) and is equipped with multiple resonant cavities. The number of resonant cavities must be an even number. The higher the working frequency of the tube, the more cavities there are. The types of anode resonant cavities are often slot-shaped, sector-shaped and slot-sector. Each small resonant cavity on the anode is equivalent to a parallel 2C oscillation circuit. Taking the slot fan cavity as an example, it can be considered that the slot part of the cavity mainly constitutes the capacitance of the oscillation circuit, and its sector part mainly constitutes the inductance of the oscillation circuit. According to the theory of microwave technology, the resonant frequency of the resonant cavity is inversely proportional to the geometric size of the cavity. The larger the cavity, the lower the operating frequency. Therefore, we can estimate its operating frequency band based on the size of the cavity. The anode of the magnetron is coupled together by many resonant cavities to form a complex resonant system. The resonant cavity frequency of this system is mainly determined by the resonant frequency of each small resonant cavity. We can also estimate the working frequency band of the magnetron based on the size of the small resonant cavity.
In addition to the required electromagnetic oscillation, the anode resonance system of the magnetron can also produce a variety of electromagnetic oscillations with different characteristics. In order to make the magnetron work stably in the required mode, "isolation tape" is often used to isolate the interference mode. The isolation tape connects the anode fins one by one to increase the distance between the working mode and adjacent interference modes. frequency interval between.
In addition, since the electrons after energy exchange still have a certain amount of energy, these electrons hit the anode to increase the temperature of the anode. The more electrons collected by the anode (that is, the greater the current), or the more energy the electrons have. The higher the energy conversion rate (the lower the energy conversion rate), the higher the anode temperature. Therefore, the anode needs to have good heat dissipation capabilities. Generally, power tubes are forced air-cooled, and the anodes are equipped with heat sinks. High-power tubes are often water-cooled, and the anodes are Cooling water jacket.
2 Cathode and its lead
The cathode of the magnetron is the emitter of electrons and an integral part of the interaction space. The performance of the cathode has a great influence on the working characteristics and life of the tube, and is regarded as the heart of the entire tube.
There are many types of cathodes with different properties. The direct-heated cathode is commonly used in continuous wave magnetrons. It is made of tungsten wire or pure tungsten wire wound into a spiral shape. After being heated to a specified temperature by current, it has the ability to emit electrons. This cathode has the advantages of short heating time and strong resistance to electron bombardment, and is widely used in continuous wave magnetrons.
This type of cathode heating current is large, requiring the cathode lead to be short and thick, and the connecting parts to be in good contact. The cathode lead of high-power tubes has a very high temperature when working, and forced air cooling is often used to dissipate heat. When the magnetron is working, the cathode is connected to negative high voltage, so the lead part should have good insulation performance and meet the requirements of vacuum sealing. In order to prevent the anode from overheating due to electron bombardment, the cathode current should be reduced according to regulations after the magnetron is stable to extend its service life.
3 Energy exporter
The energy exporter is a device that delivers the microwave energy generated in the interaction space to the load. The function of the energy output device is to pass the microwave without loss and breakdown, ensuring the vacuum seal of the tube, and at the same time, it must be easily connected to the external system. Most low-power continuous wave magnetrons use coaxial output in the anode resonant cavity where the high-frequency magnetic field is strongest. A coupling ring is placed. When the magnetic flux passing through the ring surface changes, a high-frequency induced current will be generated on the ring, thereby directing high-frequency power to the outside of the ring. The larger the coupling loop area, the stronger the coupling.
High-power continuous wave magnetrons commonly use axial energy output devices, and the output antenna is connected to the anode fin through the pole shoe hole. The antenna is generally made into a strip or round rod or a cone. The entire antenna is sealed by the output window.
Output windows are often made of glass or ceramics with low loss characteristics. It does not need to ensure loss-free passage of microwave energy and good vacuum airtightness. The output window of high-power tubes is often forced air cooling to reduce the heat generated due to dielectric loss.
4 Magnetic circuit system
Normal operation of a magnetron requires a strong constant magnetic field, and its magnetic field induction intensity is generally thousands of Gauss. The higher the operating frequency, the stronger the magnetic field applied. The magnetic circuit system of the magnetron is a device that generates a constant magnetic field. Magnetic circuit systems are divided into two categories: permanent magnet and electromagnetic. The permanent magnet system is generally used for small power tubes. The magnet steel and the tube core are firmly integrated into a so-called package type. High-power tubes often use electromagnets to generate magnetic fields. The tube core and the electromagnet are used together. There are upper and lower pole shoes in the tube core to fix the distance of the magnetic gap. When the magnetron is working, the output power and operating frequency can be easily adjusted by changing the strength of the magnetic field. In addition, the anode current can also be fed into the electromagnetic wire package to improve the stability of the tube operation.