Induction Cooking Power Supply Technical Ideas

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Induction Cooking Power Supply Technical Ideas

Summary

Induction heating is a clean, efficient and hygienic heating technology, which is widely used in many industrial fields. The most typical representative in the civil field is induction cooker. The induction cooking power supply used in the market today are mainly developed based on Class D inverters, and are supplemented by two types of power regulation methods: Frequency Modulation Power Modulation (PFM) and Pulse Density Modulation (PDM). The above two power adjustment methods have their application scope and application characteristics. In order to make the induction heating power supply more effective, this article introduces a duplex mode induction cooking power supply.

Introduction

Induction cooking power supply has many applications, taking induction cooker as an example; it is a typical representative of induction heating power supply for personal use. The PFM and PDM adjustment methods used in the power adjustment of the induction heating power supply have their own advantages and disadvantages. The PFM power regulation method has a large loss in the low power section. When the induction heating power supply is designed for power regulation, PDM should be used in the low power section and PFM in the high power section. The combination of the two can effectively improve the power regulation efficiency. In this article, on the basis of analyzing the power loss of PFM, the power regulation mode of induction heating power supply is optimized, so that the induction heating power supply has good working characteristics in duplex mode.

Induction Heating Power Supply

Characteristics: Induction heating power supply is an efficient and clean heating method, which can be used in many different occasions by adjusting the power control. The induction heating power supply mainly changes the working efficiency of the induction heating power supply by means of power regulation, but when the inverter is greatly increased due to the loss, the working efficiency of the induction heating power supply will be seriously affected. Applying the duplex regulation mode to the induction heating power supply and combining PDM and PFM will help reduce the energy loss of the induction heating power supply and improve the working efficiency of the induction cooking power supply.

The induction heating power supply mainly relies on the alternating magnetic field to generate eddy current in the object to generate heat, and uses the change of the alternating current to generate a changing magnetic field. The power control of the induction heating power supply relies on digital chips (DSP, etc.) to achieve precise control of the power frequency, thereby controlling the subtle changes of the magnetic field. Induction heating power supply heating has extremely high safety. It is non-contact heating and relies on eddy current to generate heat. The heating energy is concentrated in the object to be heated to achieve efficient use of electric energy.

The inverters used in the induction heating power supply are mainly divided into single tube, half. There are several types of bridges and full-bridges. The single-tube inverter is mainly used in the frequency band around 1kW due to the limitation of switching stress, while the full-bridge inverter is mainly used in the range above 5kW. In the application of induction heating power supply, the D-type inverter is mostly used, because compared with the DE-type inverter, the D-type inverter has a more flexible adjustment method, is convenient to implement, and has a lower price. When the PFM power regulation scheme is applied in the power regulation of the induction heating power supply, the low power stage will be affected by the larger switching frequency, which will affect the power regulation efficiency.

However, in the high power output stage, the operating frequency of the induction heating power supply should be that the load frequency is close, so that the inverter of the induction heating power supply has a higher operating frequency. It should be noted that the application of the PDM power adjustment method will bring about the problem of voltage flicker, and the PDM power adjustment range will have a positive correlation with the power adjustment period during adjustment. The adjustment has good design characteristics, and can effectively limit the increase of the power frequency of the induction heating power supply. In view of the above characteristics of the induction heating power supply, the power regulation strategy adopted in this paper to solve this problem solves the problem caused by the asymmetry of the upper and lower bridge arms, and the switch tube of the power regulation strategy adopted by the induction heating power supply is always in the ZVS condition. It shows relatively good soft switching characteristics.

In the power regulation circuit design of the induction heating power supply and in the low power PDM power regulation stage, in order to improve the regulation efficiency and reduce the regulation loss, the circuit design based on the class D inverter should be changed, because the class D inverter works in the low power stage. Now, its operating frequency needs to be higher than the resonant frequency of the main circuit, thereby reducing the output power of the inverter. In order to achieve this goal, the switching loss on the main circuit and the on-state loss of the anti-parallel diode are large, which is not conducive to the induction heating power supply.

In order to change this problem, the class DE inverter can be used to replace the class D inverter in the main circuit design of the power regulation of the induction heating power supply stage. Since the maximum output power of the class DE inverter is smaller than that of the class D inverter, the load current on the main circuit will flow through the switch tube and the parallel capacitor to make the on-state of the anti-parallel diode. Thus, losses are minimized.

In addition, in this state, the parallel capacitance of the class DE inverter is larger than that of the class D inverter, thereby reducing the off-voltage slope of the switch tube of the class DE inverter and reducing the turn-off loss.

Duplex Mode Induction Heating Power Inverter Solution Principle and Structure

The structure of the duplex mode induction heating power inverter is shown in Figure 1. In Figure 1, the inverter relies on relay1 to realize the switching between the parallel capacitors of the switch tubes, and realizes the switching of the load capacitance by adjusting the relay2. Through the above method, the inverter working mode conversion of the induction heating power supply under different power characteristics is recognized, and the precise adjustment of the relay is mainly realized by the I/O output control of the micro-control chip.

In the design of the control circuit of the induction heating power inverter, if the class D mode inverter is used, it mainly relies on the parallel capacitor at both ends of the inverter to reduce the turn-off loss. It should be noted that the capacitors connected in parallel at both ends of the inverter need to be reasonably selected to avoid being too large or too small.

In the selection of the parallel capacitor, it is necessary to ensure that the charging time of the parallel capacitor is higher than the sum of the turn-off fall and tailing time of the IGBT, so that the turn-off loss can be further reduced. In the DE mode of the inverter, the charging and discharging time is proportional to the capacity of the parallel capacitor. To realize this type of DE inverter, the parallel capacitor is much higher than that of the D inverter. The switch tube of the DE type inverter has a smaller turn-off voltage slope of the switch tube, so that the turn-off loss of the switch tube is less. In the class DE mode, the capacitor connected in parallel with the switch tube of the inverter reduces the turn-on of the switch tube to make the voltage to zero, and avoids the anti-parallel diode while realizing the inverter ZVS, thus making the on state loss of the anti-parallel diode does not exist.

In general, the class DE inverter will effectively avoid the on state loss of the anti-parallel diode and improve the working efficiency of the inverter. In the design of the switch tube of the induction heating power inverter, it is necessary to reasonably determine the number of parallel capacitors used and the working time to realize the duplex design of the main circuit of the induction heating power inverter.

Fig 1 Induction Heating Power Supply

Working characteristics of induction heating power supply in duplex mode

  • PDM and PFM power regulation analysis

PFM power regulation is simple to achieve, and is mostly used in induction heating applications with high load quality factor, and can complete a wide range of power in a small frequency range adjustment. However, when the load quality factor is in a small range, the efficiency of PFM power regulation will drop significantly, making the inverter at a lower working efficiency pass through modeling and analysis of the working characteristics of the induction heating power supply in the process of PFM power regulation, it is found that the PFM power regulation is proportional to the load quality factor of the inverter, that is, the higher the quality factor, the better the power regulation efficiency is. Under the condition of fixed load quality factor, the loss of the inverter will increase first and then decrease with the increase of the power frequency. The existence of the IGBT turn-off loss will make the working efficiency of the inverter continue to decrease. After analyzing the PFM power regulation characteristics in the low power stage, it is found that the PFM power regulation in the low power stage will greatly increase the switching loss of the inverter and greatly reduce the working efficiency of the induction heating power supply.

PDM power regulation mainly adjusts the output power of the inverter by adjusting the driving pulse density of the switch tube, and the operating frequency of the PDM remains basically constant during the power regulation period, so that the PDM power regulation has relatively good soft switching characteristics. It should be noted that the fluctuation of the output power of PDM power regulation will have a certain impact on the voltage quality of the induction heating power supply, resulting in voltage fluctuation and flicker problems. The voltage fluctuation mainly comes from the violent fluctuation of the load. When the flicker standard is met, the PDM power adjustment period is proportional to the PDM power adjustment power range. In order to ensure that the induction heating power supply can achieve good application effect, it is necessary to do a good job in matching the PDM power regulation cycle with the PDM power regulation power.

  • Modeling and simulation of the working characteristics of the induction heating power inverter

Based on the PSIM simulation model parameters (load equivalent resistance 5Ω, load equivalent inductance 74 μ H, switch shunt capacitance D22nF/DE274nF, power frequency 20 kHz -50 kHz) to complete the induction cooking power supply, the simulation of the working characteristics, through the analysis of the simulation results, it is found that the power consumption of the class D inverter is the smallest when the power frequency is 20kHz, and the efficiency is the highest. In the low power stage, when the output power of the inverter changes from 200w to 1500w, the turn-off voltage rise rate of the switch tube DE type inverter is constantly changing, and at 600w, the rise rate is higher than that of D. The turn-off voltage rise rate of the class inverter is much smaller, only about 1/14 of that of the class D inverter. It can be seen from the simulation data that the use of DE-type inverters in the low power stage will effectively improve the working characteristics of the power supply.

Conclusion

Induction cooking power supply has broad application prospects. In the modern period, the development and application of new technologies help improve the industrial application of induction heating power supply. In this piece, based on the analysis of the technical characteristics of the induction cooking power supply, the duplex mode application of the induction heating power supply is projected. On the basis of improving the working characteristics of the induction heating power supply in the low power stage, the working energy consumption of the induction heating power supply is reduced, and the working efficiency is improved. The main control circuit of the duplex mode induction heating power supply is analyzed and introduced.

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