A frequency-doubled power three-port high-frequency resonant inverter power supply system and its working method

Abstract

The present invention discloses a three-port high-frequency resonant inverter power supply system for frequency doubling and power doubling. In order to achieve power doubling and frequency doubling output, a three-port LC high-frequency resonant inverter power supply system and a working method thereof are disclosed, which is composed of a three-bridge arm resonant inverter and a three-port LC high-frequency resonant cavity, and the LC high-frequency resonant cavity is powered by the three-bridge arm resonant inverter. The three-port LC resonant cavity is connected by three inductors and three capacitors in a triangular symmetrical structure, and the three inductors heat the same workpiece together. Two heating coils work for the same workpiece at the same time in each resonant cycle, and the output of twice the power and twice the frequency can be achieved in each resonant cycle.

Description

Technical Field

[0001] The invention belongs to the field of high-power high-frequency induction heating, and in particular relates to a three-port high-frequency resonant inverter power supply system and a working method thereof. Background Art

[0002] As a new heating technology, electromagnetic induction heating has developed rapidly with the advantages of energy saving and environmental protection, high heating efficiency, fast speed and long service life. It has been widely used in many fields such as industry, agriculture and daily life. In the past, traditional heating methods mainly included combustion heat heating and resistive heating. Both methods are heated by heat conduction, also called contact heating. However, problems such as large energy loss, slow speed and insecurity are also obvious disadvantages of traditional heating methods. On the contrary, induction heating is a new heating method that uses electromagnetic coupling for heating. Although it also uses electrical energy, the method and principle are completely different from traditional contact heating. The heating efficiency of induction heating can be as high as more than 90%. Under the same conditions, the preheating time is reduced by more than 60% compared with traditional electric heating, and the production efficiency is significantly improved.

[0003] Semiconductor devices such as MOSFET and IGBT have promoted the development of induction heating power supplies towards high frequencies. However, the power of solid-state induction heating power supplies is generally large. When the power frequency moves towards high frequencies, higher requirements are placed on power device losses, circuit design, and wiring layout. The more widely induction heating power supplies are used in human life, the higher the requirements for various indicators. It is required not only to develop towards large capacity, but also towards high frequencies. The heating frequency is related to the heating process. The higher the frequency, the more power is concentrated on the surface of the workpiece for heating the workpiece. In recent years, various highly integrated intelligent chips (such as DSP, FPGA, etc.) have been widely used in induction heating power supplies, making the power supply gradually tend to develop towards intelligence. These intelligent chips have strong controllability, high reliability, and powerful computing functions. They can improve the reliability and stability of the power supply, and can also control the power supply more flexibly, and the requirements for the induction heating system are also increased.

[0004] Figure 1It is a typical high-power resonant inverter structure. Resonant inverters generally adopt a full-bridge structure and are divided into voltage-type resonant inverters and current-type resonant inverters. The DC side of the voltage-type resonant inverter topology is a voltage source, the output voltage is a square wave, and the subsequent load is generally a series resonant structure; while the DC side of the current-type resonant inverter topology is a current source, the output current is a square wave, and the subsequent load is generally a parallel resonant structure. The voltage-type resonant inverter can work in both self-excitation and external excitation states. Usually, the startup design is external excitation to self-excitation, which can easily solve the startup problem. For occasions that need to be started frequently, the series resonant inverter is very practical. The series resonant inverter does not have high requirements for the wiring conditions in the load circuit. If the distributed wire in the load circuit is too long, it will only affect the size of the equivalent inductance value of the load circuit, and will not have much impact on the efficiency and output power of the inverter circuit. The load matching is relatively simple, which reduces the requirements for power devices, and the circuit operation is more stable. It has high practical value, but it cannot achieve power and frequency doubling, and the heating efficiency is low.

[0005] Figure 2 It is a traditional three-bridge high-frequency resonant inverter structure. This topology consists of three bridge arms. The main bridge arm 1 and the auxiliary bridge arm form a full bridge. The load resonant cavity is composed of L1, C1 and R1 in series. The main bridge arm 2 and the auxiliary bridge arm form another full-bridge inverter. The load resonant cavity is composed of L2, C2 and R2 in series. The working principle is as follows: The input voltage U of the load tank circuit d is a rectangular wave, the input current I d is a sine wave. The load equivalent circuit diagram is as follows Figure 2 As shown, L and R are the equivalent inductance and resistance of the coil, and C is the series capacitor. The main bridge arm 1 and the main bridge arm 2 share the secondary bridge arm. L1 and L2 can heat the same workpiece together to achieve power doubling, but cannot achieve frequency doubling. Summary of the invention

[0006] The present invention focuses on the problems existing in the above-mentioned prior art and provides a frequency-doubled and power-doubled three-port high-frequency resonant inverter power supply system and a working method thereof.

[0007] The present invention provides a frequency-multiplying and power-multiplying three-port high-frequency resonant inverter power supply system, comprising a three-bridge-arm inverter and a symmetrical three-port LC resonant cavity, wherein the three-bridge-arm inverter comprises a first main bridge arm, a second main bridge arm and an auxiliary bridge arm. The first main bridge arm comprises a first switch tube and a second switch tube, the second main bridge arm comprises a third switch tube and a fourth switch tube, and the auxiliary bridge arm comprises a fifth switch tube and a sixth switch tube.

[0008] The LC resonant cavity comprises a first coil, a second coil, a third coil, a first capacitor, a second capacitor and a third capacitor. The three groups of coils and the three capacitors in the LC resonant cavity are connected symmetrically in a triangle.

[0009] The first switch tube and the second switch tube are connected in series, the third switch tube and the fourth switch tube are connected in series, and the fifth switch tube and the sixth switch tube are connected in series. The source of the first switch tube, the third switch tube and the fifth switch tube are connected to each other and are commonly connected to the first end of the parallel capacitor, the drain of the second switch tube, the fourth switch tube and the sixth switch tube are connected to each other and are commonly connected to the second end of the parallel capacitor, the drain of the first switch tube is connected to the source of the second switch tube, the drain of the third switch tube is connected to the source of the fourth switch tube, and the drain of the fifth switch tube is connected to the source of the sixth switch tube. The drain of the first switch tube and the source of the second switch tube are both connected to the positive electrode of the first coil, the drain of the third switch tube and the source of the fourth switch tube are both connected to the positive electrode of the second coil, the drain of the fifth switch tube and the source of the sixth switch tube are both connected to the negative electrode of the third coil, the negative electrode of the first coil is connected to the first end of the first capacitor, the negative electrode of the second coil is connected to the second end of the second capacitor and the first end of the second capacitor, the second end of the second capacitor is connected to the positive electrode of the coil and the first end of the third capacitor, and the second end of the third capacitor is connected to the negative electrode of the first coil.

[0010] The power supply system has a first resonant cycle and a second resonant cycle in each working cycle. The inverter has two coils to heat the workpiece at the same time in each resonant cycle. In the first resonant cycle, the first main bridge arm and the auxiliary bridge arm constitute a full-bridge inverter, and the first coil, the third coil and the first to third capacitors together constitute a series resonance. In the second resonant cycle, the second main bridge arm and the auxiliary bridge arm constitute another full-bridge inverter, and the second coil, the third coil and the first to third capacitors together constitute a series resonance. In the symmetrical three-port LC resonant cavity, two coils work in each resonant cycle, and the three coils operate alternately. The two coils in each resonant cycle generate twice the magnetic flux. The symmetrical three-port LC resonant cavity is composed of three groups of coils and three capacitors connected in a triangular symmetric manner, where C1=C2=C3, L1=L2=L3. The switch tube is an IGBT switch tube.

[0011] The total impedance of the power system during the first resonant cycle is:

[0012]

[0013] The total impedance of the power system during the second resonant period is:

[0014]

[0015] The total impedance of the power system is:

[0016]

[0017] When the load resonates, the resonant point is:

[0018]

[0019] Among them, L1 is the inductance value of the first coil, L2 is the inductance value of the second coil, L3 is the inductance value of the third coil, C1 is the capacitance value of the first capacitor, C2 is the capacitance value of the second capacitor, C3 is the capacitance value of the third capacitor, ω is the angular frequency, when the switching angular frequency ω0<ω, the circuit is capacitive, when the switching angular frequency ω0>ω, the circuit is inductive.

[0020] Compared with the prior art, the three-port high-frequency resonant inverter power supply system for doubling frequency and power provided by the present invention discloses a high-frequency resonant inverter composed of a symmetrical three-port LC resonant cavity and three bridge arms, which can achieve frequency doubling while saving one bridge arm, i.e., two switching tubes, greatly reducing the cost and loss of the system; the load output power is increased by 1 times, two coils work in each cycle in the symmetrical three-port LC resonant cavity, and the three coils operate alternately. The two coils in each cycle generate twice the magnetic flux, and positive and negative alternating magnetic flux are generated in one cycle, so the output power of the load can be increased to twice the original; the first main bridge arm and the second main bridge arm of the high-frequency resonant inverter operate alternately, and the three coils of the load resonant cavity operate alternately, so the frequency doubling operation of the system can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a typical high-power resonant inverter structure;

[0022] Figure 2 It is a traditional three-bridge-arm high-frequency resonant inverter structure;

[0023] Figure 3 The high-frequency resonant inverter power supply structure of the three-port load resonant cavity of the present invention;

[0024] Figure 4 The working principle of the high-frequency resonant inverter power supply of the three-port load resonant cavity of the present invention;

[0025] Figure 5 It is the simulation result of the high-frequency resonant inverter power supply of the three-port load resonant cavity of the present invention. DETAILED DESCRIPTION

[0026] The technology described below can be transformed in many ways and can have many embodiments, which are described in detail with reference to the accompanying drawings in specific embodiments. However, this does not mean that the technology described below is limited to specific embodiments. It should be understood that the present invention includes all similar modifications, equivalents and substitutions without departing from the spirit and technical scope of the technology described below.

[0027] Figure 3The invention discloses a high-frequency resonant inverter power supply structure of a three-port load resonant cavity, and in order to achieve frequency doubling and power doubling output, a high-frequency resonant inverter power supply structure of a three-port load resonant cavity is invented, including a three-bridge arm inverter and a symmetrical three-port LC resonant cavity. The three-bridge arm inverter includes a first main bridge arm 1, a second main bridge arm 2 and an auxiliary bridge arm, the first main bridge arm 1 includes a first switch tube S1 and a second switch tube S2, the second main bridge arm 2 includes a third switch tube S3 and a fourth switch tube S4, and the auxiliary bridge arm includes a fifth switch tube S5 and a sixth switch tube S6.

[0028] The LC resonant cavity comprises a first coil L1, a second coil L2, a third coil L3, a first capacitor C1, a second capacitor C2, and a third capacitor C3. The three groups of coils and three capacitors in the LC resonant cavity are symmetrically connected in a triangle.

[0029] The first switch tube S1 and the second switch tube S2 are connected in series, the third switch tube S3 and the fourth switch tube S4 are connected in series, the fifth switch tube S5 and the sixth switch tube S6 are connected in series, and the source electrodes of the first switch tube S1, the third switch tube S3 and the fifth switch tube S5 are connected to each other and are commonly connected to the parallel capacitor C d The drain terminals of the second switch tube S2, the fourth switch tube S4 and the sixth switch tube S6 are connected to each other and are commonly connected to the parallel capacitor C d At the second end of the transistor S1, the drain of the first switch tube S1 is connected to the source of the second switch tube S2, the drain of the third switch tube S3 is connected to the source of the fourth switch tube S4, and the drain of the fifth switch tube S5 is connected to the source of the sixth switch tube S6.

[0030] The drain of the first switch tube S1 and the source of the second switch tube S2 are both connected to the positive electrode of the first coil L1, the drain of the third switch tube S3 and the source of the fourth switch tube S4 are both connected to the positive electrode of the second coil L2, and the drain of the fifth switch tube S5 and the source of the sixth switch tube S6 are both connected to the negative electrode of the third coil L3.

[0031] The negative electrode of the first coil L1 is connected to the first end of the first capacitor C1, the negative electrode of the second coil L2 is connected to the second end of the first capacitor C1 and the first end of the second capacitor C2, the second end of the second capacitor C2 is connected to the positive electrode of the third coil L3 and the first end of the third capacitor C3, and the second end of the third capacitor C3 is connected to the negative electrode of the first coil L1.

[0032] The power supply system has a first resonant cycle and a second resonant cycle in each working cycle. The inverter has two coils to heat the workpiece at the same time in each resonant cycle. In the first resonant cycle, the first main bridge arm 1 and the auxiliary bridge arm form a full-bridge inverter. The first coil L1, the second coil L3 and the first to third capacitors C 1～C3 together form a series resonance. In the second resonance period, the second main bridge arm 2 and the auxiliary bridge arm form another full-bridge inverter. The second coil L2, the third coil L3 and the first to third capacitors C 1～ C3 together form a series resonance. In each resonant cycle of the symmetrical three-port LC resonant cavity, two coils work and the three coils operate alternately. The two coils in each resonant cycle generate twice the magnetic flux. The symmetrical three-port LC resonant cavity consists of three groups of coils and three capacitors connected in a triangle, where C1=C2=C3, L1=L2=L3, and the switch tube is an IGBT switch tube.

[0033] Figure 4 It is the working mode of the high-frequency resonant inverter power supply of the three-port load resonant cavity, mode 1 corresponds to the first upper half cycle of the first resonant cycle, mode 2 corresponds to the first lower half cycle of the first resonant cycle, mode 3 corresponds to the second upper half cycle of the second resonant cycle, and mode 4 corresponds to the second lower half cycle of the second resonant cycle: the power supply system works periodically, the first resonant cycle and the second resonant cycle both have an upper half cycle and a lower half cycle, in the first resonant cycle, the first coil L1 and the third coil L3 generate twice the magnetic flux to heat the workpiece, and the polarity of the magnetic flux of the coil in the first upper half cycle of the first resonant cycle and the polarity of the magnetic flux of the coil in the first lower half cycle are opposite to each other, in the second resonant cycle, the second coil L2 and the third coil L3 generate twice the magnetic flux to heat the workpiece, and the polarity of the magnetic flux generated by the coil in the second upper half cycle of the second resonant cycle and the polarity of the magnetic flux of the coil in the second lower half cycle are opposite to each other.

[0034] In the first upper half cycle of the first resonant cycle, the first switch tube S1 of the first main bridge arm 1 of the inverter and the sixth switch tube S6 of the auxiliary bridge arm are turned on, and the current flows through the first switch tube S1, the first coil L1, the first to third capacitors C 1～ C3, the third coil L3, and then flows out from the sixth switch tube S6. The first coil L1 and the third coil L3 generate twice the magnetic flux to heat the workpiece. At this time, the current flows into the positive pole of the third coil L3 and the first coil L1, and flows out from the negative pole, and the magnetic flux is positive.

[0035] In the first second half of the first resonant cycle, the fifth switch tube S5 of the auxiliary bridge arm and the second switch tube S2 of the first main bridge arm 1 are turned on, and the current flows through the fifth switch tube S5, the third coil L3, the first to third capacitors C 1～ C3 and the first coil L1, and then flows out from the second switch tube S2. At this time, the current flows into the negative pole of the third coil L3 and the first coil L1 and flows out from the positive pole. The magnetic flux is negative, which is opposite to the polarity of the magnetic flux in the first upper half cycle of the first resonance cycle.

[0036] In the second upper half cycle of the second resonant cycle, the third switch S3 of the second main bridge arm 2 of the inverter and the sixth switch tube S6 of the auxiliary bridge arm are turned on, and the current flows through the third switch tube S3, the second coil L2, the first to third capacitors C 1～ C3, the third coil L3, and then flows out from the sixth switch tube S6. At this time, the current flows into the positive pole of the third coil L2 and the third coil L3, and flows out from the negative pole, generating twice the same magnetic flux to heat the workpiece. At this time, the magnetic flux is positive.

[0037] In the second lower half of the second resonant cycle, the fifth switch tube S5 of the auxiliary bridge arm and the fourth switch tube S4 of the second main bridge arm 2 are turned on, and the current flows through the fifth switch tube S5, the third coil L3, the first to third capacitors C 1～ C3 and the second coil L2, and then flows out from the fourth switch tube S4. At this time, the current flows into the negative pole of the third coil L3 and the second coil L2 and flows out from the positive pole. The magnetic flux is negative, which is opposite to the magnetic flux of the second upper half cycle.

[0038] The total impedance of the power system during the first resonant cycle is:

[0039]

[0040] The total impedance of the power system during the second resonant period is:

[0041]

[0042] The total impedance is:

[0043]

[0044] When the load resonates, the resonant point is:

[0045]

[0046] Among them, L1 is the inductance value of the first coil, L2 is the inductance value of the second coil, L3 is the inductance value of the third coil, C1 is the capacitance value of the first capacitor, C2 is the capacitance value of the second capacitor, C3 is the capacitance value of the third capacitor, ω is the angular frequency, when the switching angular frequency ω0<ω, the circuit is capacitive, when the switching angular frequency ω0>ω, the circuit is inductive.

[0047] When the first main bridge arm 1 and the auxiliary bridge arm are working in the first resonant cycle, the first coil L1, the third coil L3 and the three capacitors resonate in series. When the second main bridge arm 2 and the auxiliary bridge arm are working in the second resonant cycle, the second coil L2, the third coil L3 and the three capacitors C 1～ When series resonance occurs, the impedance is minimum and the inverter output current is maximum, that is, the current flowing through the power device is also maximum.

[0048] Figure 5 The simulation results of the high-frequency resonant inverter power supply of the three-port load resonant cavity of the present invention are as follows: A simulation platform was built, and the resonant frequency of the LC resonant cavity was set to f = 5627Hz; inductance L1 = L2 = L3 = 5uH, C1 = C2 = C3 = 13.34uF. The simulation results are as follows:

[0049] Figure 5 (a) is the waveform diagram of the turn-on signals of different switch tubes, where the first and second rows are the driving signals of the first switch S1 and the second switch S2, respectively, with a switching frequency of f=5627Hz and a duty cycle of 0.25. The third and fourth rows are the driving signals of the third switch S3 and the fourth switch S4, respectively, with a switching frequency of f=5627Hz and a duty cycle of 0.25, and S1-S4 are turned on in sequence with a lag of one quarter cycle. The fourth and fifth rows are the driving signals of the fifth switch S5 and the sixth switch S6 of the auxiliary bridge arm, respectively, and the two main bridge arms and the auxiliary bridge arm are turned on alternately in this way. Figure 5 (b) are the currents flowing through the first coil L1, the second coil L2 and the third coil L3 respectively. The first row is the current of the coil L1, the second row is the current of the second coil L2, and the third row is the current of the third coil L3. Figure 5 (b) shows that the first coil L1, the second coil L2 and the third coil L3 are alternately turned on. Figure 5 It can be concluded that the frequency of the load is f = 11254Hz, so it can be seen that the frequency of the main bridge arm is f = 5627Hz, and the frequency doubling of the load is finally achieved. In addition, two coils work simultaneously in each cycle, so the power output is increased to twice the original.

[0050] Although the present invention has been described in detail above with general description and specific embodiments, some modifications or improvements can be made on the basis of the present invention. The above description is only a preferred embodiment of the present invention and is not limited to the scope of the present invention. Other changes and modifications made by those skilled in the art without departing from the spirit and scope of protection of the present invention are still included in the scope of protection of the present invention.

Claims

1. A frequency-doubled and power-doubled three-port high-frequency resonant inverter power supply system, comprising a three-bridge-arm inverter and a symmetrical three-port LC resonant cavity; It is characterized in that The three-bridge-arm inverter comprises a first main bridge arm, a second main bridge arm and an auxiliary bridge arm; the first main bridge arm comprises a first switch tube and a second switch tube; the second main bridge arm comprises a third switch tube and a fourth switch tube; the auxiliary bridge arm comprises a fifth switch tube and a sixth switch tube; The LC resonant cavity comprises a first coil, a second coil, a third coil, a first capacitor, a second capacitor, and a third capacitor; the three groups of coils and three capacitors in the LC resonant cavity are connected symmetrically in a triangle; The first switch tube and the second switch tube are connected in series, the third switch tube and the fourth switch tube are connected in series, and the fifth switch tube and the sixth switch tube are connected in series; The drains of the first switch tube, the third switch tube and the fifth switch tube are connected to each other and are commonly connected to the first end of the parallel capacitor, the sources of the second switch tube, the fourth switch tube and the sixth switch tube are connected to each other and are commonly connected to the second end of the parallel capacitor, the source of the first switch tube is connected to the drain of the second switch tube, the source of the third switch tube is connected to the drain of the fourth switch tube, and the source of the fifth switch tube is connected to the drain of the sixth switch tube; The source of the first switch tube and the drain of the second switch tube are both connected to the positive electrode of the first coil, the source of the third switch tube and the drain of the fourth switch tube are both connected to the positive electrode of the second coil, and the source of the fifth switch tube and the drain of the sixth switch tube are both connected to the negative electrode of the third coil; The negative electrode of the first coil is connected to the first end of the first capacitor, the negative electrode of the second coil is connected to the second end of the first capacitor and the first end of the second capacitor, the second end of the second capacitor is connected to the positive electrode of the third coil and the first end of the third capacitor, and the second end of the third capacitor is connected to the negative electrode of the first coil; The power supply system has a first resonant cycle and a second resonant cycle in each working cycle. In the first resonant cycle, the first main bridge arm and the auxiliary bridge arm constitute a full-bridge inverter, and the first coil, the third coil and the first to third capacitors together constitute a series resonance; in the second resonant cycle, the second main bridge arm and the auxiliary bridge arm constitute another full-bridge inverter, and the second coil, the third coil and the first to third capacitors together constitute a series resonance. In each resonant cycle, two coils in the symmetrical three-port LC resonant cavity work, and the three coils operate alternately.

2. The frequency-doubled and power-doubled three-port high-frequency resonant inverter power supply system according to claim 1, characterized in that: The capacitance values ​​of the first capacitor, the second capacitor and the third capacitor are the same, and the inductance values ​​of the first coil, the second coil and the third coil are the same.

3. The frequency-doubled and power-doubled three-port high-frequency resonant inverter power supply system according to claim 1, characterized in that: The switch tube is an IGBT switch tube.

4. The frequency-doubled and power-doubled three-port high-frequency resonant inverter power supply system according to claim 3, characterized in that: The switch tube is of N type or P type.

5. The frequency-doubled and power-doubled three-port high-frequency resonant inverter power supply system according to claim 1, characterized in that: The total impedance of the power system during the first resonant period is: The total impedance of the power system during the second resonant period is: Among them, L1 is the inductance value of the first coil, L2 is the inductance value of the second coil, L3 is the inductance value of the third coil, C1 is the capacitance value of the first capacitor, C2 is the capacitance value of the second capacitor, C3 is the capacitance value of the third capacitor, ω is the angular frequency, when the switching angular frequency ω0<ω, the circuit is capacitive, when the switching angular frequency ω0>ω, the circuit is inductive.

6. A working method of a frequency-doubler and power-doubler three-port high-frequency resonant inverter power supply system based on any one of claims 1 to 5, characterized in that: The power supply system performs periodic operation, the first resonance period includes a first upper half period and a first lower half period, and the second resonance period includes a second upper half period and a second lower half period; In the first resonance period, the first coil and the third coil generate twice the magnetic flux to heat the workpiece, and the polarities of the magnetic flux of the coil in the first upper half period and the magnetic flux of the coil in the first lower half period of the first resonance period are opposite to each other; During the second resonance period, the second coil and the third coil generate twice the magnetic flux to heat the workpiece, and the polarities of the magnetic flux generated by the coils during the second first half period and the magnetic flux generated by the coils during the second second half period of the second resonance period are opposite to each other.

7. The working method according to claim 6, characterized in that: In the first upper half cycle of the first resonant cycle, the first switch tube of the first main bridge arm and the sixth switch tube of the auxiliary bridge arm of the inverter are turned on, and the current flows through the first switch tube, the first coil, the first to third capacitors, the third coil, and then flows out from the sixth switch tube. The first coil and the third coil generate twice the magnetic flux to heat the workpiece. At this time, the current flows into the positive pole of the first coil and the third coil and flows out from the negative pole, and the magnetic flux is positive.

8. The working method according to claim 7, characterized in that: In the first second half cycle of the first resonant cycle, the fifth switch tube of the auxiliary bridge arm and the second switch tube of the first main bridge arm are turned on, and the current flows through the fifth switch tube, the third coil, the first to third capacitors and the first coil, and then flows out from the second switch tube. At this time, the current flows into the negative pole of the third coil and the first coil and flows out from the positive pole, and the magnetic flux is negative, which is opposite to the polarity of the magnetic flux in the first first half cycle of the first resonant cycle.

9. The working method according to claim 6, characterized in that: In the second upper half cycle of the second resonant cycle, the third switch tube of the second main bridge arm and the sixth switch tube of the auxiliary bridge arm of the inverter are turned on, and the current flows through the third switch tube, the second coil, the first to third capacitors, the third coil, and then flows out from the sixth switch tube. At this time, the current flows into the positive pole of the second coil and the third coil and flows out from the negative pole, generating twice the same magnetic flux to heat the workpiece. At this time, the magnetic flux is positive.

10. The working method according to claim 9, characterized in that: In the second second half cycle of the second resonant period, the fifth switch tube of the auxiliary bridge arm and the fourth switch tube of the second main bridge arm are turned on, and the current flows through the fifth switch tube, the third coil, the first to third capacitors and the second coil, and then flows out from the fourth switch tube. At this time, the current flows into the negative pole of the third coil and the second coil and flows out from the positive pole, and the magnetic flux is negative, which is opposite to the polarity of the magnetic flux in the second first half cycle.

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