Introduction Induction heating directly heats the workpiece, offering advantages such as high efficiency, favorable operating conditions, easy temperature control, minimal metal loss, and no need for preheating. Traditional induction heating equipment utilizes electron tubes and fast thyristors as power electronic devices. Electron tubes suffer from high voltage, poor stability, strong radiation, and low efficiency, and are nearing obsolescence; however, their high frequency and power output still give them a place in the market. Fast thyristors are currently the mainstay, boasting high voltage withstand, high current capacity, and strong overcurrent and overvoltage resistance. However, their operation is limited to below 10000Hz, restricting their application range. IGBTs are composite power devices that combine the advantages of bipolar transistors and power MOSFETs. They feature voltage-type control, high input impedance, low drive power, simple control circuitry, low switching losses, fast switching speed, high operating frequency, and large component capacity. They not only achieve frequencies unattainable by thyristors (above 60kHz) but are also gradually replacing fast thyristors. IGBTs are widely used in induction heating from 1kHz to 80kHz abroad, which is the development direction of induction heating power supplies. Figure 1 shows the application of various power devices abroad. 2 IGBT Power Supply Structure and Working Principle 2.1 The main circuit adopts a parallel resonant inverter, as shown in Figure 2. The current source parallel resonant inverter has the advantages of strong load adaptability and strong resistance to load short circuits. The waveform of this device is better, which is conducive to improving the efficiency and reliability of the device. The main circuit is a three-phase full-wave uncontrolled rectification plus filtering, and then chopping before input to the inverter. Due to the high chopping frequency of IGBTs (about 20kHz), the output waveform is better, and the reactor size can be reduced to 1/3 of the original. The rectifier bridge of this device uses ordinary rectifier diodes, the filter capacitor is an electrolytic capacitor of 450V/1500μF, the chopping IGBT and diodes are products of Fuji Electric, the smoothing reactor is self-made, the inverter IGBT is also a product of Fuji Electric, the resonant circuit capacitor is a specially made ultrasonic capacitor, and the power output transformer is designed and manufactured in-house. 2.2 Chopper Control The chopper control uses the SG3525 pulse width modulation controller. The SG3525 is an integrated PWM controller with comprehensive control functions, making it very suitable for chopping. It has a full push-pull output configuration with a peak output of ±500mA. The power supply voltage is (8～35)V. It has an internal undervoltage stop circuit, which cuts off the output stage when the voltage is too low. It features a 5.1V reference regulated power supply with a temperature coefficient of ±1%, an error amplifier, a sawtooth wave oscillator with an oscillator frequency of 100Hz～400kHz (the value is determined by the external resistor Rt and capacitor Ct), a soft-start circuit, a synchronization circuit, a shutdown circuit, a pulse width modulation comparator, an RS register, and protection circuits. The block diagram of the SG3525 is shown in Figure 3. 2.3 Inverter Control The IGBT is a self-turn-off device, capable of operating in both capacitive and inductive modes. However, operating in either capacitive or inductive mode will cause voltage or current glitches. Therefore, a zero-locking circuit is used to ensure that the power supply operates essentially in a resonant state. In this scenario, both the sinusoidal voltage and square current (on the resonant circuit) are preferable. This is significant not only for reducing switching losses and increasing device lifespan but also for alleviating the burden on the resistor-capacitor absorption circuit. The common externally excited to self-excited circuit is not used here. Externally excited to self-excited circuits use an externally excited signal at low voltages, automatically switching to a self-excited signal as the voltage increases. This has a drawback: when the inductor is replaced and the externally excited and self-excited frequencies differ, overcurrent can occur during voltage rise. Our equipment modifies the externally excited fixed-frequency generator into a variable-frequency generator, gradually changing from 100kHz to 10kHz while simultaneously detecting the resonant voltage. At the resonant point, it switches to self-excited operation and triggers at zero crossing, ensuring the equipment operates at zero degrees. The inverter circuit control block diagram is shown in Figure 4. This inverter control method prevents inverter failure during the externally excited to self-excited process and also prevents the circuit from failing to find a self-excited frequency under small signal conditions. 2.4 IGBT Driving IGBTs can be driven by either active or passive methods. Passive driving is relatively simple, but waveform adjustment is not very convenient. Therefore, the Fuji 841 circuit is used, as shown in Figure 5. Many articles have introduced the 841, and here we only raise two issues: (1) The 841 does not completely block the pulse during protection, which poses a threat to device safety. Therefore, an RS flip-flop is added between the overcurrent output and the drive signal input to completely block the drive pulse when there is an overcurrent output. (2) The 841 overcurrent detection detects the voltage between the CE of the IGBT when the gate is turned on. When it exceeds 6V and is delayed by 10μs, it is judged as an overcurrent. However, in practice, it has been found that many IGBTs are damaged when the voltage between the CE is 6V. Therefore, we connect a 3V Zener diode in series between the collector of the IGBT and the 6th pin of the 841 to reduce the detection value of the 841 from 6V to 3V. Practice has shown that this improvement significantly increases the sensitivity of the 841 to overcurrent detection, so that the circuit can not only drive the device normally, but also protect the device more effectively during overcurrent. 2.5 Overcurrent and Overvoltage Protection (1) Overcurrent IGBTs are relatively weak in overcurrent protection compared to SCRs, so the circuit design must ensure the safety of IGBTs. There are two main methods: one is 841 overcurrent protection, but this method is risky. The other is to connect a current sensor in series between the reactor and the inverter bridge input. When its output value exceeds the predetermined value, it blocks the chopper pulse on the one hand and the inverter pulse on the other. This measure enables the IGBT to pass the load short circuit test. (2) Overvoltage There are two main types of overvoltage in this resonant circuit: 1. As the load current and voltage angle increase, the load voltage will become higher and higher, which will pose a threat to the device. The solution is to lock the inverter control at 0 degrees. In addition, a voltage sensor is added to the load to detect the voltage. When it is too high, it is controlled. 2. Voltage spikes during the commutation process. This phenomenon is mainly absorbed by adding RC. It is worth noting that the diodes in the inverter circuit also need to be absorbed by adding RC, as shown in Figure 6. 3 Application Examples (1) A unit requires welding the liquid storage tank of the air conditioner compressor, as shown in Figure 7. ①③ Copper parts, ② Steel parts, require welding of ①～② and ②～③ connection points, the welded surface is bright and basically does not deform. We successfully solved this problem by using IGBT ultra-high frequency 20kW, 40kHz power supply with nitrogen protection. In order to improve efficiency, we used the technology of matching two transformers with one power supply at the same time to complete the welding tasks of each component in turn. Each welding process takes 4 to 5 seconds, the welded surface is smooth and without deformation. (2) Melting platinum Platinum has a melting point of 1800 and is relatively difficult to melt. In the past, electron tube power supplies were used, but they are large, consume a lot of power and are difficult to control. They are not suitable for many occasions, especially in the laboratory. Foreign countries mostly use solid-state power supplies, which are very expensive (about 5 to 10 times the price of domestic products). We successfully solved this problem by using 30kW/30kHz IGBT ultra-high frequency power supply. It has the advantages of small size, rapid melting, high control accuracy and reliability. (3) The copper tube of the refrigerator compressor shell is usually brazed using high frequency welding. A certain manufacturer now uses the IGBT-20kW-20kHz IGBT ultra-high frequency power supply produced by our factory. Its main advantages are: firstly, the heating efficiency is high, reaching 85%, the vibration is uniform, and the heating is rapid, reaching the level of the T-10 ultra-high frequency power supply of the HFB-303H-1A high frequency brazing equipment produced in Japan, saving the country a lot of foreign exchange. (4) Local quenching (annealing) of roller bearings A certain factory requires local quenching and local annealing of roller bearings, as shown in Figure 8. The induction heating power supply equipment produced by our factory was successfully tested. It is worth mentioning that: the high frequency power supply of electron tube has strong electromagnetic field radiation. If you work in such an environment for a long time, it will be harmful to the health of the operator. However, the IGBT ultra-high frequency power supply developed by our factory has a much lower output voltage and frequency than the high frequency power supply of electron tube. After testing, the intensity of the electromagnetic field radiation around it did not exceed the national standard. The test results are shown in Table 1. The raceway region is the quenching zone, with a temperature of T=850℃. K is the local annealing zone, with a temperature of T=800℃. 4. Conclusion According to statistics, there are currently over 10,000 100kW-class high-frequency power supplies in operation across various production sectors in China, and nearly 1,000 new tube or thyristor high-frequency power supply devices are added annually, resulting in significant energy waste. Adopting IGBT ultra-high frequency power supplies would not only offer significant energy savings and high efficiency but also protect the environment. Its widespread application will undoubtedly generate substantial economic and social benefits.