The core circuit of electromagnetic heater is a frequency conversion circuit

However, the core circuit of the induction cooker is a frequency conversion circuit, that is, the low-frequency mains power of 50 or 60Hz is converted into high-frequency power of 20kHz, which is supplied to the induction coil to generate a high-frequency magnetic field, so that the metal cooker generates eddy current and generates heat under the action of the magnetic field. At present, there are three main forms of induction cooker frequency conversion circuit: single tube circuit, half bridge circuit and full bridge circuit.

Although the popularity of induction cooker in Europe is lower than that in Asia, the invention of induction cooker originated in Germany. The domestic induction cooker produced in Europe basically adopts half bridge circuit. The full bridge circuit is generally used in commercial high-power induction cooker or industrial high-power electromagnetic heating equipment. Chinese household induction cooker comes from Japan and Taiwan, and basically adopts single tube circuit. Although they can be heated and cooked in the end, there are still great differences in their working characteristics, reliability and cost. The following is a brief introduction to single tube circuit, half bridge circuit and full bridge circuit.

1、 Single tube circuit:

Single transistor circuit means that an IGBT is responsible for opening and closing. When the IGBT is opened, the current is quickly charged through the coil. When the IGBT is turned off, the coil current charges the resonant capacitor to produce a very high back voltage. When it exceeds the withstand voltage of the IGBT, the IGBT will break down instantly (commonly known as blasting machine). Generally, during normal operation, the voltage is about 1000-1100v. When the circuit is affected by interference or other unexpected factors, the voltage will rise and must be protected in time. Single transistor circuit also has a great fatal congenital defect. When the power is low, it will affect the working state of IGBT, resulting in serious heating of IGBT, shortening the service life of IGBT, and even IGBT breakdown (blasting), or this situation will occur when the coil is close to the heated object. Therefore, the continuous heating power of a single tube induction cooker is generally greater than 50% of the high power, that is, a large 2000W induction cooker is heated intermittently below 1000W. For example, 200W power is realized by heating 1000W for 2 seconds and stopping for 8 seconds, which is inappropriate in many cooking occasions. At present, domestic induction cooker is basically a single tube circuit, and the circuit composition is basically the same. The product quality and grade are different, which mainly depends on the selection of parts and components and the control of technology and quality. The cost of this single tube circuit structure is relatively cheap and can only be applied to household induction cooker with low requirements. Many manufacturers of electromagnetic heaters also supply power to users under the banner of energy saving. The price is very low. In fact, it is a single tube circuit circuit of household induction cooker. Some manufacturers also use two IGBTs. In fact, this circuit is only used in parallel. Please take it seriously and don't be cheated.

2、 Half bridge circuit:

Half bridge induction cooker uses two IGBTs to work in turn. After one is closed and the other is opened, there is no problem of high reverse voltage. There is no hidden danger of overvoltage damaging IGBT, and the reliability of products is greatly improved. Therefore, half bridge scheme is adopted for foreign high-grade induction cooker (especially multi head furnace for kitchen). Another advantage of half bridge circuit is that continuous power can be very low. There is no problem that low power affects the working state of IGBT. Theoretically, all power can be heated continuously, but limited by the working frequency of the device, it can generally achieve 20% of the high power. In particular, it is designed to achieve 10% of the high power, that is, 2000W induction cooker and 200W induction cooker can also be heated continuously, realizing that all fire gears are heated continuously, which greatly improves the practicability of induction cooker. The small and medium-sized electromagnetic heater (2.5 ~ 15kw) of our company (Xiamen Xinrui) is designed with half bridge circuit. Generally, the high design power of half bridge circuit is about 20-30kw, which is far from being achieved by single transistor circuit. Half bridge circuit design is different from single tube. Different manufacturers have different circuit designs. There are many different ways from PWM generation, power control, phase control, current detection to IGBT drive. Therefore, the performance and reliability of products vary greatly. Reasonable design has become the key link. Xiamen Xinrui adopts full digital control based on AVR. Compared with the traditional half bridge circuit, it has the characteristics of stable performance, easy installation, easy installation, simple debugging and low failure rate. It can be widely used in plastic machinery (such as energy-saving transformation projects of injection molding machine, wire drawing machine, granulator and extruder), food machinery, crude oil transportation and other similar heating industries.

3、 Full bridge circuit:

The two bridge arm resonant capacitors of the half bridge circuit are changed into two IGBTs, and the coil is connected in series with a wiping capacitor to form a full bridge circuit. IGBT1 and igbt4, igbt2 and igbt3 work alternately, so as to obtain relatively high power. The cost of full bridge circuit is much higher than that of single transistor and half bridge circuit. The general control methods include frequency modulation and phase shift. The control circuit is basically similar to the half sample circuit. Xiamen Xinrui # adopts full digital control based on AVR. It is mainly used in high-power electromagnetic heating controller above 20kW. It is a high-power general control board. Please click the figure for details.

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