High power factor induction heating power supply solution

　　1 Introduction

　　At present, induction heating power supplies have been widely used in heat processing and heat treatment industries such as metal smelting, heat penetration, welding, pipe bending, surface quenching, etc. However, the rectification and transformation of traditional induction heating power supplies generally adopts thyristor phase-controlled rectification or diode uncontrolled rectification. In order to obtain a more stable DC voltage, large capacitors are often used for energy storage and filtering after rectification, resulting in very low power factor on the input side of the power grid, current distortion, and harmonic pollution to the power grid; in addition, it also produces serious electromagnetic interference to the signals of the surrounding and its own system, reducing system efficiency. In order to reduce harmonic current and improve power factor, it is necessary to adopt power factor correction technology (APFC).

　　2 Traditional induction heating power supply and improvement

　　The main circuit structure of the traditional induction heating power supply is shown in Figure 1, which includes four parts: uncontrolled rectification, large capacitor energy storage filtering, inverter circuit and resonant load. In the figure, AC is converted into DC through uncontrolled rectification, and then converted into relatively stable DC as the power supply for the inverter circuit through large capacitor filtering, and the inverter output and power regulation of the system are realized on the inverter side.

　　The whole system is controlled by DSP. The voltage and current detection device detects the voltage and current values ​​of the DC bus and transmits them to DSP to realize power feedback. Load detection includes temperature detection and frequency tracking. The temperature value detected by the infrared sensor is transmitted to DSP to realize temperature feedback; the resonant current and voltage signal of the load are detected and fed back to DSP to realize frequency tracking. The feedback signals such as voltage and current are A/D converted and maintained inside DSP. The actual output power is calculated by digital multiplication and compared with the digital given power. The deviation is digitally PID controlled to realize closed-loop control of power supply output power and DPLL frequency tracking. The fault detection and protection circuit monitors water shortage, overheating, overvoltage, overcurrent and other faults in real time. After comparison and judgment by the DSP fault processing subroutine, various faults are handled in interrupt mode and alarm display is performed.

　　This traditional induction heating power supply uses large capacitor passive filtering, which causes input current distortion, harmonic pollution to the power grid, reduces input power factor, and is not conducive to saving electricity costs. In order to improve energy utilization and reduce the pollution of induction heating devices to the power grid, active power factor correction technology must be used.

　　Since the system has adopted DSP as the main controller, the use of dedicated PFC chip will increase the system hardware cost, reduce the system integration, and make debugging inconvenient and even more unfavorable for system upgrades. Therefore, this paper studies the use of DSP to achieve power factor correction based on the original system.

　　A Boost circuit is added to the rectification and inversion part of the original main circuit, as shown in Figure 2. The Boost circuit is a DC/DC converter used to improve the grid-side current waveform and increase the power factor of the power supply. On the DC bus side, the input voltage, inductor current and output voltage of the Boost circuit are detected, and the on and off of the Boost switch tube is controlled through the DSP software control algorithm to achieve the purpose of power factor correction.

　　3 APFC Implementation Based on DSP

　　Figure 3 shows the APFC control principle diagram based on DSP-TMS320F2812. The TMS320F2812 chip is a 32-bit fixed-point digital signal processor launched by TI. It has powerful control and signal processing capabilities and is a cost-effective DSP chip for digital power electronic conversion and control.

　　The APFC control principle is as follows: the output voltage of the boost circuit, that is, the DC bus voltage V0, is sampled and isolated by the sensor and sent to the ADCIN2 port of the DSP, and converted into a digital signal. It is compared with the reference digital voltage Vref, and its deviation value is sent to the voltage controller Gv. Through the deviation correction control, V0 is made equal to Vref. Gv adopts digital PI control, and has:

　　In this way, a pulse width modulated wave is output, which is isolated and amplified by the driver to drive the switch tube to turn on/off at high frequency, so as to achieve real-time tracking of the inductor current Iin by Iref.

　　When implementing equations (2) and (4), in order to prevent Uv(n) and Uc(n) from being too large and causing the system to lose control, they must be limited to an appropriate range. To this end, discrete control can be achieved using the following method.

　　Similarly for the current loop, when the switch tube operates at a very high frequency (for example, f=100 kHz), the output of the voltage loop regulator Gv remains basically unchanged, so the Iref output by the multiplier is basically a waveform proportional to the input voltage, which can achieve real-time tracking of the input current to the input voltage and keep the two in phase, so that the input power factor is close to 1.

　　4 Experimental studies

　　According to the above theory, a supersonic frequency induction heating power prototype with single-phase input of 220 V, power of 4 kW and resonant frequency of 30 kHz is designed, and the grid-side voltage and current before and after the addition of APFC circuit are compared and analyzed. The experimental results are shown in Figure 4 and Figure 5 respectively. Figure 4 shows the voltage and current waveforms of the grid side of the traditional induction heating power supply. It can be seen from the figure that although the voltage is a sine wave, due to the presence of the large energy storage capacitor in the middle of the DC side, the current conduction angle is only 90°, and the grid-side current waveform is seriously distorted, presenting a series of intermittent spike pulses. Under the same power conditions, the peak value of the current is doubled, the harmonic component is increased, and the power factor of the power supply is reduced (cosφ△0.7). Figure 5 shows the voltage and current waveforms of the grid side of the induction heating power supply after the introduction of APFC. It can be seen from the figure that after the introduction of APFC technology, the current waveform and the voltage waveform are sine waves with the same phase. The induction heating power supply has an input power factor close to 1 and a very low total current distortion rate, which reduces the pollution to the power grid.

　　5 Conclusion

　　The experimental results show that the introduction of APFC technology makes the input power factor of the power supply close to unity power factor, reduces the pollution of harmonics to the AC power grid, and significantly improves the power of the induction heating power supply.