## 27 Aug series resonant inverter for induction cooking

Abstract Based on the practical application, the article uses a series resonance method for the circuit topology of the commercial induction cooker, analyzes its different working status, and obtained the optimal switching process of the inverter power devices, theoretically calculated the relation between frequency and phase angle. The power regulation is achieved by a combination of “pulse width modulation (PWM) mode” and “phase-locked loop control”. Based on this, a series resonant inverter test prototype is built and verified the conclusion by experiments. It has a strong application background to

solve practical problems.

Keywords：inverter; induction heating; series resonant; phase-locked control; commercial induction cooker

Commercial induction cooker has many advantages such as high efficiency, environmental protection and energy saving points, especially suitable for places that restrict the use of open fire, especially with the user awareness of its advantages is increasing, The induction heating can not only directly heat the metal materials, but also can be used for the production of the induction heating.

Induction heating can be used not only for direct heating of metallic materials, but also for

indirect heating of non-metallic materials [2]. High-frequency induction

heating technology, the commercial induction furnace is designed with a high heating speed fast, easy to control temperature, high heating efficiency and small footprint, etc.

are widely used in people’s daily life [3]. However, in the actual application process, many problems are encountered. Commercial electric the inherent resonant frequency of a commercial magnetic furnace varies with temperature, load distance, and load material.

The resonant frequencies inherent in commercial electromagnets vary with temperature, load distance, and load material, which make the design difficult. In order to keep the inverter in a quasi-resonant state, the design must ensure that the load frequency is to make the inverter work in quasi-resonant condition, the design must ensure that the load frequency can be automatically tracked and the phase angle locked well.In order to keep the inverter in a quasi-resonant state, the design must ensure that the load frequency can be automatically tracked and the phase angle locked well to achieve the function of changing with the load.

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**1****.****Operating principle and design**

The circuit designed in this paper operates in resonant, inductive

state and capacitive state [4]. The current flowing through the load when the inverter is operating will all flow through the insulated gate bipolar transistor (IGBT).

bipolar transistor (IGBT), i.e., the IGBT is connected in series with the capacitor and the load [5]. The IGBT is connected in series with the capacitor and the load [5].

1.1 Full-bridge series resonant inverter State 1: The system is operating in the capacitive state, as shown in Figure 1.The current is already ahead of the voltage electrical angle, so during the current change, the current is positive, but the voltage is still negative, and the bridge arm tends to be in the renewed diode tube reverse recovery phenomenon [6], which increases the IGBT loss, resulting in large inrush current, and may even damage the IGBT. Therefore, it is necessary to avoid it.

Figure 1 Tolerant state

State 2: The system is operating in the inductive state, as shown in Figure 2.

The voltage has exceeded the current electrical angle. Therefore, the voltage is already positive when the current is negative, making it possible to achieve zero-current conduction of the IGBT in inductive state [4-6] is already positive, so that it achieves zero-current conduction of the IGBT in the inductive state [4-6].

The full-bridge circuit topology is shown in Figure 4. In this paper, the load characteristics are first analyzed.The output voltage Uo is expanded by the Fourier series. The output voltage Uo is expanded by the Fourier series, i.e., Ud is the bus DC voltage; Uo is the load output voltage.

Figure 4. Full-bridge circuit topology

From equation (1), the fundamental amplitude and rms value can be obtained respectively, i.e.

At this point, if the load resonates, the equivalent impedance is R .From equation (2)-equation (4), we get the amplitude of load current and RMS value.

Combining equation (3) and equation (6), the load is obtained from the literature [7 output power.

Similarly, the quality factor of the inverter is obtained from the literature [7]

**1.2 Half-bridge series resonant inverter**

1) Class I half-bridge

The circuit topology of the Class I half-bridge [8] is shown in Figure 5. From the analysis of Fig. 5 It can be seen from the analysis of Fig. 5 that the current flowing through the load will all flow through the IGBT, the first half of the cycle.The current flowing through the load will all flow through the IGBT, and the first half cycle will supply power to the load on the DC side, and the second half cycle will continue through the lower bridge arm.

The current flowing through the load will all flow through the IGBT, with the DC side supplying the load in the first half-cycle and the second half-cycle continuing through the lower bridge arm [9]. Therefore, from equation (1), we can derive the inverter output voltage The output voltage Uo1 , which is 1/2 of the full bridge for the class I half bridge, is obtained from Eq.The output voltage RMS value is obtained from equation (3)

Figure 5 Class I half-bridge circuit topology

2) Class II Half Bridge

As can be seen from the circuit topology shown in Figure 6, the load circuit is connected in parallel and then in series with the bridge arm capacitors are first connected in parallel and then in series, and this topology is generally applied to power relatively large area.

Figure 6 Class II half-bridge circuit topology

Load current RMS

**1.3 Design of control system**

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1) Power regulation method

Among the many control strategies, this paper adopts the pulse width modulation (PWM) method with phase-locked loop and phase control.

(pulse width modulation, PWM) method with phase-locked loop and

The control strategy of pulse width modulation (PWM) combined with phase-locked loop and frequency modulation control is as follows.

2）Frequency regulation method

The industry on the commercial induction cooker power regulation method mainly uses the following

3 kinds, namely pulse width modulation, pulse density modulation and frequency modulation [11]. In this paper focuses on the PWM method, which is commonly used and popular in the industry.

From equation (4) and the previous section for class I half-bridge (9), equation (10), respectively and class II half-bridge type (11) and equation (12) respectively, it can be mathematically deduced that further conclude that the FM power regulation is achieved by adjusting the switching frequency of the inverter IGBT of the inverter to achieve the purpose of regulating the output power. The derivation is as follows.

The fundamental impedance and phase angle are

According to the relationship between impedance and frequency in equations (13) and (14), the impedance and spectrum relationship is obtained as shown in Fig. 7, where to is the inherent resonant frequency and f is the operating frequency.

In this case, o f f , the load undergoes resonance. From equation and Eq. (15) we have

By using Masthead calculation for equation (17).The 2D curves of output power versus frequency for different quality factors Q are shown in Figure 8 shows.

Figure 8 Output power and spectrum relationship

3) FM phase-locked control strategy

When the frequency is tuned to the resonant frequency of the load, the system works in the soft-switching state, when its output reaches the maximum power [2]. However, the

in practical applications often the load will be influenced by external environmental factors into capacitive operation, generating large spike currents that impinge on the IGBT

switch [12], which eventually leads to its failure. Therefore, this paper uses a phase-locked loop control technique to lock the load operating frequency at a slightly higher resonant frequency of the inductive operating state [11].

**1.4 Software design of the control system**

In this paper, the software system is divided into different modules, and according to their functions they are the software system is divided into different modules, which are divided into pot detection program, potless processing program, heating control program, phase locked loop control frequency locking program, phase locked loop control frequency locking program, PWM frequency modulation output and current signal detection program.The main program flow is shown in Figure 9.

**3 Conclusion**

The design, analysis and tests in this paper show that the parameters of resonant inductor and resonant capacitor derived from this article. The parameters of resonant inductor and resonant capacitor calculated in this paper as well as the application of the commercial induction cooker designed by the control method of this paper meets the intended design requirements.At the same time, this design prototype can detect the pots and pans accurately and protect them quickly in the actual commissioning. High accuracy and fast protection, and also achieve the dual purpose of reducing induction pollution and power waste, in line with the national energy-saving and emission reduction policy.

It is in line with the national energy-saving and emission reduction policy, and has a certain guiding role in the design, production and promotion of commercial induction cookers.It has a guiding role in the design, production and promotion of commercial induction cookers.

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