A composite resonant heating circuit and frequency control method thereof

Abstract

The present invention discloses a composite resonant heating circuit, which includes a power converter and a resonant heater. An AC voltage with an adjustable frequency f is output between two output ends of the power converter. The heating circuit also includes a resonant heater. The resonant heater includes a proximal capacitor unit, a de-loop cable, and a distal resonant unit. The proximal capacitor unit, the de-loop cable, the distal resonant unit, and the loop cable are connected in sequence. The distal resonant unit has an inherent resonant frequency f. 01 When the resonant heating circuit is working, the circuit maintains f = f 01 The composite resonant circuit can conveniently transmit high-frequency current to the remote end to realize electromagnetic induction heating. The present invention also provides a control method for the composite resonant heating circuit, which adjusts the frequency f of the sinusoidal alternating current output by the power converter according to the working condition of the remote resonant unit, ensures that the remote resonant unit can accurately work at its resonant point, and ensures that the circuit achieves a good resonant heating effect.

Description

Technical Field

[0001] The invention belongs to the technical field of induction heating circuits, and in particular relates to a composite resonant circuit. Background Art

[0002] Induction heating technology is an advanced heating technology. It has advantages that traditional heating methods do not have, such as high heating efficiency, fast speed, good controllability, and easy mechanization and automation. At present, induction heating technology has been widely used in smelting, casting, hot forging, welding, and material surface heat treatment. It has become an indispensable technology in metallurgy, national defense, manufacturing and other industries.

[0003] Since its development, induction heating technology has been widely used in more and more specific industrial scenarios. For example, on the surface of ships, warships, aircraft or other large steel structures, in order to protect the steel structure and achieve waterproof and anti-corrosion effects, operators usually apply paint with corresponding functions on the surface of the steel structure body after it is processed and formed. Once the original paint layer is peeling, damaged or aged, it is necessary to remove the original paint layer as a whole and reapply it. In the process of removing the original paint layer, induction heating technology has become the preferred application technology for operators due to its high heating efficiency, no contact with the workpiece to be processed, and strong controllability.

[0004] In order to better adapt to the processing occasions of paint removal on the surface of large steel structures, the existing induction heating technology generally adopts the series resonance principle to construct the circuit.

[0005] See also Figure 1 When constructing an induction heating circuit using the series resonance principle, from the perspective of circuit principle, the circuit generally includes a power source E, a proximal transformer T1, a series capacitor C, a transmission cable L1, a remote transformer T2, and an inductor L2; the power source E, the proximal transformer T1, the series capacitor C, the transmission cable L1, the remote transformer T2, and the inductor L2 are connected in sequence.

[0006] It can be seen from the above circuit principle that when the circuit is working, the power supply E outputs an alternating voltage of a certain frequency, the series capacitor C and the inductor L2 are connected to form a series resonant structure, and the series resonant structure has its inherent resonant frequency, and the magnitude of the inherent frequency is jointly determined by the capacitive reactance of the series capacitor C1 and the inductive reactance of the inductor L2.

[0007] When the frequency of the alternating voltage output by the power supply is equal to the inherent resonant frequency of the series resonant structure, the series resonant structure resonates. At this time, the inductor L2 will obtain the maximum current in the entire frequency domain, and the alternating magnetic field will be generated by the current, which will further generate an induced electromotive force on the surface of the workpiece to be processed, thereby achieving heating.

[0008] However, it can be known from the resonance characteristics of the series resonant circuit that when the above circuit reaches its resonance point, a high-frequency, large-amplitude alternating current will flow into both the series capacitor C1 and the inductor L2. At this time, for the series capacitor C1, due to the defects of the existing capacitor manufacturing technology, the series capacitor C1 will inevitably have a dielectric loss, and the dielectric loss will further cause power loss in the series capacitor C1, which not only increases the reactive power in the circuit, but also generates a large amount of heat on the series capacitor C1, seriously affecting the stability of the circuit itself; and for the inductor L2, since the inductor L2 itself is a coil wound with multiple turns of wire, when a high-frequency large current flows into the inductor L2, it will inevitably cause a local overcurrent phenomenon due to the skin effect and proximity effect of the conductor, and further generate heat in the overcurrent area, and in severe cases, the inductor L2 will be burned. And because the circuit itself is used in a high voltage environment, it can be seen from the resonant characteristics of the series resonant circuit that when the series resonant circuit is in a resonant state, both the series capacitor C1 and the inductor L2 will have an overvoltage of Q times the source voltage. It can be seen that when the series resonant circuit is in a resonant state, not only will a high-frequency large current flow into the series capacitor C1 and the inductor L2, but an overvoltage will also be generated. Therefore, the circuit has very strict requirements on the current and voltage resistance characteristics of the series capacitor C1 and the inductor L2.

[0009] When the induction heating technology is applied using the series resonance principle, from the perspective of the device structure principle, the power supply E, the proximal transformer T1, and the series capacitor C are usually set together at the proximal end close to the external three-phase grid voltage port, and the distal transformer T2 and the inductor L2 are set together at the distal end away from the external three-phase grid voltage port to form a movable distal heater. The proximal and distal ends are connected by a transmission cable L1, which makes it convenient for operators to move the distal heater at will to heat and remove paint from different areas on the surface of large steel structures.

[0010] From the structural principle of the above device, it can be seen that due to the distributed inductance on the transmission cable L1, the current will lose a certain amount of energy on the transmission cable during the current transmission process, so the length of the transmission cable L1 will be very limited, which greatly limits the operating radius of the operator. Furthermore, due to the excessively high power density of the remote transformer T2, the remote heater itself generates severe heat, so a water cooling circulation system must be set in the remote heater at the same time, which results in the remote heater being large in size, heavy in weight, and inconvenient to move. The entire induction heating device is very heavy and cannot achieve a good heating effect. Summary of the invention

[0011] To solve the above problems, the purpose of the present invention is to provide a composite resonant heating circuit, which makes full use of the parasitic distributed inductance on the loop cable, combines the series resonant structure with the parallel resonant structure, realizes the transmission of high-frequency current over any distance, and realizes resonance at the remote resonant unit. The remote resonant unit obtains high-frequency alternating current, thereby generating a high-frequency alternating magnetic field to realize induction heating.

[0012] Another object of the present invention is to provide a composite resonant heating circuit with simple structure, good stability, convenient control, light weight and simplicity, and good user experience.

[0013] Another object of the present invention is to provide a control method for a composite resonant heating circuit. The method is based on the composite resonant heating circuit, and uses the current signal and voltage signal obtained in the heating main circuit to judge the working condition of the remote resonant unit, and further adjusts the frequency f of the sinusoidal alternating current output by the power converter according to its working condition, to ensure that the remote resonant unit can accurately operate at its resonant point, and to ensure that the circuit achieves a good resonant heating effect.

[0014] To achieve the above object, the technical solution of the present invention is as follows:

[0015] The invention provides a composite resonant heating circuit, which comprises a power converter and a resonant heater.

[0016] Among them, the power converter includes a rectifier module, a filter module, a voltage regulator module, an inverter module and a transformer. The input end of the rectifier module is connected to the external three-phase grid voltage. The rectifier module, the filter module, the voltage regulator module, the inverter module and the transformer are connected in sequence. An AC voltage with an adjustable frequency f is output between the two output ends of the secondary side of the transformer. When the circuit is working, the current on the primary side of the transformer is taken as the current signal, and the voltage across the parallel capacitor is taken as the voltage signal.

[0017] The resonant heater includes a proximal capacitor unit, a de-loop cable and a distal resonant unit. The distal resonant unit includes a heating inductor and a parallel capacitor. The heating inductor and the parallel capacitor are connected in parallel. One end of the proximal capacitor unit is connected to one of the output ends of the power supply. The other end of the proximal capacitor unit is connected to one of the common ends of the heating inductor and the parallel capacitor through a de-loop cable. The other common end of the heating inductor and the parallel capacitor is connected to the other output end of the power supply through another de-loop cable. The distal resonant unit has an inherent resonant frequency f 01 When the resonant heating circuit is working, the circuit maintains f = f 01 .

[0018] The power converter obtains an AC voltage with adjustable voltage amplitude and frequency on the secondary side of the transformer after rectifying, filtering, regulating, inverting and transforming the external three-phase grid voltage.

[0019] In the resonant heating circuit provided by the present invention, once the heating inductor and the parallel capacitor are set, the inductive reactance of the heating inductor and the capacitive reactance of the parallel capacitor can be known, and f can be further inferred. 01 The size of the quality factor Q of the remote resonant unit is determined jointly by the inductive reactance of the heating inductor and the capacitive reactance of the parallel capacitor.

[0020] Adjust the frequency f of the AC voltage, when f=f 01 When , the remote resonant unit reaches its resonance point. From the resonance characteristics of the parallel resonant circuit, it can be seen that at this time, the remote resonant unit is resistive as a whole, and the voltage across it reaches the maximum value in the full frequency domain. The heating inductor will obtain an alternating current that is always Q times the source current. Therefore, when designing the circuit parameters, the inductive reactance of the heating inductor and the capacitive reactance of the parallel capacitor are reasonably selected to obtain a suitable Q value, and then an alternating current with a large amplitude is obtained on the heating inductor. This large-amplitude alternating current generates an alternating magnetic field with a large intensity, and further generates a large induced electromotive force on the surface of the workpiece to be processed, thereby achieving a good heating effect.

[0021] Furthermore, the proximal capacitor unit includes at least one series capacitor and at least one selection switch; the series capacitors and the selection switches are arranged in a one-to-one correspondence, and one end of each series capacitor is connected to one of the output ends of the power converter; the other end of each series capacitor is respectively connected to one end of the corresponding selection switch, and the other end of each selection switch is connected to the return cable; when the selection switch is turned on, the corresponding series capacitor is connected to the circuit, and when the selection switch is turned off, the corresponding series capacitor is disconnected from the circuit; the capacitive reactance of each series capacitor is unequal.

[0022] From the perspective of circuit structure, the near-end capacitor unit and the far-end resonant unit will be connected through a return cable. Since the return cable will inevitably have distributed inductance, when f=f 01 When the far-end resonant unit is resistive as a whole, the circuit formed by the near-end capacitor unit, the parasitic distributed inductance on the return cable and the far-end resonant unit will be equivalent to an LCR series resonant circuit. It can be seen from the analysis that the equivalent LCR series resonant circuit will also have a corresponding natural resonant frequency f 02 , the natural resonant frequency f 02 The size of will be determined by the capacitive reactance of the proximal capacitor unit, the inductive reactance of the parasitic distributed inductance on the loop cable, the mutual inductance between the two loop cables, and the equivalent impedance of the remote resonant unit.

[0023] After the circuit is built, the circuit will have the natural resonant frequency f of the remote resonant unit at the same time. 01The natural resonant frequency f of the equivalent LCR series resonant circuit 02 When designing the parameters of each electronic component in the circuit, f 02 With f 01 The values ​​are as close as possible, so that f = f in the circuit 01 When the LCR series resonant circuit is connected to the inverter module, it will also be close to its resonant operating point. In this way, it can not only ensure that most of the electric energy provided by the external three-phase grid voltage will be consumed by the resistive remote resonant unit and transmitted to the external workpiece in the form of an alternating magnetic field to achieve a good heating effect, but also ensure that each switching device in the inverter module operates in a weakly inductive or resistive state, reducing its switching stress and switching loss.

[0024] In order to adapt to different processing occasions, the de-loop cable is usually selected as needed. Once the de-loop cable is selected, the inductive reactance of the parasitic distributed inductance on it and the mutual inductance between the two de-loop cables are also determined. At this time, by changing the on-off status of different selection switches and connecting series capacitors with different capacitive reactances to the circuit, the inherent resonant frequency f of the LCR series resonant circuit can be adjusted. 02 , to ensure that f 02 With f 01 Keep the values ​​as close as possible.

[0025] Furthermore, the resonant heater also includes a series inductor, which is arranged between the selection switch and the return cable, one end of the series inductor is connected to the selection switch, and the other end of the series inductor is connected to the corresponding return cable. Connecting a series inductor in the heating circuit can, on the one hand, minimize the influence of the inductive reactance of the heating inductor on the total inductive reactance in the heating circuit, so that the resonant heater can accurately operate at its required resonant frequency; on the other hand, it can also make the current signal and voltage signal sampled in the circuit accurately reflect the resonant condition of the remote resonant unit.

[0026] Furthermore, the resonant heating circuit also includes an inverter controller, and the inverter controller includes:

[0027] A voltage zero-crossing detection unit for detecting a zero-crossing point of a voltage signal;

[0028] A voltage zero-crossing pulse generating unit for generating a corresponding pulse at a voltage signal zero-crossing point;

[0029] A current zero-crossing detection unit for detecting a zero-crossing point of a current signal;

[0030] A current zero-crossing pulse generating unit for generating a corresponding pulse at a zero-crossing point of a current signal;

[0031] A phase comparison unit for comparing the phases between two input pulses;

[0032] A phase difference amplification unit for amplifying the phase difference;

[0033] A PFM regulating unit for regulating the output frequency of the inverter module;

[0034] and, an inverter drive unit for generating control pulses for the inverter module;

[0035] The current on the primary side of the transformer is taken as the current signal, and the voltage across the parallel capacitor is taken as the voltage signal; the current signal is connected to the current zero-crossing detection unit, and the output end of the current zero-crossing detection unit is connected to the current zero-crossing pulse generating unit; the current zero-crossing detection unit detects the zero-crossing point of the current signal, and the voltage signal is connected to the voltage zero-crossing detection unit, and the output end of the voltage zero-crossing detection unit is connected to the voltage zero-crossing pulse generating unit; the voltage zero-crossing detection unit detects the zero-crossing point of the voltage signal;

[0036] The output ends of the current zero-crossing pulse generating unit and the voltage zero-crossing pulse generating unit are connected to the phase comparison unit to compare the phase difference between the two.

[0037] The output end of the phase comparison unit is connected to the phase difference amplification unit, which amplifies the phase difference. The output end of the phase difference amplification unit is connected to the PFM adjustment unit; the PFM adjustment unit outputs a PFM modulation signal according to the phase difference, and the output end of the PFM adjustment unit is connected to the inverter drive unit; the inverter drive unit generates an inverter drive pulse according to the PFM modulation signal, and the inverter drive pulse is connected to the inverter module to change the working condition of the inverter module accordingly.

[0038] The current signal is obtained on the primary side of the transformer, and the voltage signal is obtained at both ends of the parallel capacitor. The phase difference between the two is compared. If there is a phase difference between the two, it means that the remote resonant unit has not reached its resonant operating point at this time. At this time, the phase difference is used for PFM modulation, and the working condition of the inverter module is further adjusted. The frequency f of the sinusoidal alternating voltage output between the two output ends of the power converter at the next moment can be adjusted, so that the frequency f is further close to the inherent resonant frequency f 01 , after continuous testing and adjustment, finally f = f 01 .

[0039] Compared with the prior art, the composite resonant heating circuit provided by the present invention utilizes the resonant characteristics of parallel resonance to easily obtain high-frequency and large current on the heating inductor, thereby achieving a good induction heating effect. The circuit structure is simple and stable, and the electrical performance is excellent. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a schematic diagram of the induction heating technology realized by applying the series resonance principle in the prior art.

[0041] Figure 2: is a circuit schematic diagram of a composite resonant heating circuit implemented in a specific embodiment of the present invention, in which:

[0042] M1 is a rectifier module, M2 is a filter module, M3 is a voltage regulator module, and M4 is an inverter module;

[0043] C2 is a series capacitor, K2 is a selection switch, L21 is a series inductor, L22 is the total equivalent result of the parasitic distributed inductance on the loop cable and the mutual inductance generated between the two loop cables, C1 is a parallel capacitor, and L1 is a heating inductor.

[0044] Figure 3 It is a structural schematic diagram of the composite resonant heating device implemented in a specific embodiment of the present invention, in which E is a power converter, L21 is a series inductor, L22 is a de-loop cable, C1 is a parallel capacitor, and L1 is a heating inductor. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0046] To achieve the above object, the technical solution of the present invention is as follows:

[0047] See also Figure 2 .

[0048] In this specific embodiment, a composite resonant heating circuit is provided.

[0049] The resonant heating circuit includes a heating loop, and the heating loop includes a power converter and a resonant heater.

[0050] Among them, the power converter includes a rectifier module M1, a filter module M2, a voltage regulator module M3, an inverter module M4 and a transformer T. The input end of the rectifier module M1 is connected to the external three-phase grid voltage. The rectifier module M1, the filter module M2, the voltage regulator module M3, the inverter module M4 and the transformer T are connected in sequence. An AC voltage with an adjustable frequency f is output between the two output ends of the secondary side of the transformer T. When the circuit is working, the current on the primary side of the transformer is taken as the current signal, and the voltage across the parallel capacitor is taken as the voltage signal.

[0051] The resonant heater includes a proximal capacitor unit, a de-loop cable and a distal resonant unit; the distal resonant unit includes a heating inductor L1 and a parallel capacitor C1, and the heating inductor L1 is connected in parallel with the parallel capacitor C1; one end of the proximal capacitor unit is connected to one of the output ends of the power converter, and the other end of the proximal capacitor unit is connected to one of the common ends of the heating inductor L1 and the parallel capacitor C1 through the de-loop cable, and the other common end of the heating inductor L1 and the parallel capacitor C1 is connected to the other output end of the power converter through the de-loop cable, and distributed inductance is generated on each de-loop cable, and mutual inductance is also generated between the two de-loop cables, and the total equivalent result is L22.

[0052] In this specific embodiment, the proximal capacitor unit includes at least one series capacitor C2 and at least one selection switch K2; the series capacitors C2 and the selection switches K2 are arranged in a one-to-one correspondence, and one end of each series capacitor C2 is connected to one of the output ends of the inverter module M3; the other end of each series capacitor C2 is respectively connected to one end of the corresponding selection switch K2, and the other end of each selection switch K2 is connected to one end of the series inductor L21, and the other end of the series inductor L21 is connected to the remote resonant unit through a return cable; when the selection switch K2 is turned on, the corresponding series capacitor C2 is connected to the circuit, and when the selection switch K2 is turned off, the corresponding series capacitor C2 is disconnected from the circuit; the capacitive reactance of each series capacitor C2 is unequal.

[0053] Furthermore, the resonant heating circuit also includes an inverter controller, and the inverter controller includes:

[0054] A voltage zero-crossing detection unit for detecting a zero-crossing point of a voltage signal;

[0055] A voltage zero-crossing pulse generating unit for generating a corresponding pulse at a voltage signal zero-crossing point;

[0056] A current zero-crossing detection unit for detecting a zero-crossing point of a current signal;

[0057] A current zero-crossing pulse generating unit for generating a corresponding pulse at a zero-crossing point of a current signal;

[0058] A phase comparison unit for comparing the phases between two input pulses;

[0059] A phase difference amplification unit for amplifying the phase difference;

[0060] A PFM regulating unit for regulating the output frequency of the inverter module;

[0061] and, an inverter drive unit for generating control pulses for the inverter module;

[0062] The current on the primary side of the transformer T is taken as the current signal, and the voltage across the parallel capacitor C1 is taken as the voltage signal; the current signal is connected to the current zero-crossing detection unit, and the output end of the current zero-crossing detection unit is connected to the current zero-crossing pulse generating unit; the current zero-crossing detection unit detects the zero-crossing point of the current signal, and the voltage signal is connected to the voltage zero-crossing detection unit, and the output end of the voltage zero-crossing detection unit is connected to the voltage zero-crossing pulse generating unit; the voltage zero-crossing detection unit detects the zero-crossing point of the voltage signal;

[0063] The output ends of the current zero-crossing pulse generating unit and the voltage zero-crossing pulse generating unit are connected to the phase comparison unit to compare the phase difference between the two.

[0064] The output end of the phase comparison unit is connected to the phase difference amplification unit, which amplifies the phase difference. The output end of the phase difference amplification unit is connected to the PFM adjustment unit; the PFM adjustment unit outputs a PFM modulation signal according to the phase difference, and the output end of the PFM adjustment unit is connected to the inverter drive unit;

[0065] See also Figure 3 :

[0066] In this specific embodiment, a device structure based on a composite resonant heating circuit is also provided.

[0067] This structure sets the power converter E and the series capacitor C2 at the proximal end close to the external three-phase grid voltage port, and sets the parallel capacitor C1 and the heating inductor L1 together at the distal end away from the external three-phase grid voltage port. The proximal end and the distal end are connected by a return cable L22.

[0068] From the above structural principle, it can be seen that the device using the composite resonant heating circuit has a simple structure. As long as the parallel capacitor C1 and the heating inductor L1 with large flow rate and strong voltage resistance are reasonably selected, the remote resonant unit can work normally. The overall size of the device is small, the heating problem of the device is not serious, the maintenance and operation are convenient, and the user experience is good.

[0069] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A composite resonant heating circuit, the resonant heating circuit comprising a power converter, the input end of the power converter is connected to an external three-phase grid voltage, and an AC voltage with an adjustable frequency f is output between two output ends of the power converter; It is characterized in that The resonant heating circuit also includes a resonant heater, which includes a proximal capacitor unit, a de-loop cable and a distal resonant unit, one end of the proximal capacitor unit is connected to one of the output ends of the power supply, the other end of the proximal capacitor unit is connected to one end of the distal resonant unit via a de-loop cable, and the other end of the distal resonant unit is connected to the other output end of the power supply via another de-loop cable; The remote resonant unit has a natural resonant frequency When the resonant heating circuit is working, the circuit maintains f= ; The remote resonant unit includes a heating inductor and a parallel capacitor, two ends of the parallel capacitor are respectively connected to two loop cables, and the heating inductor is connected in parallel with the parallel capacitor; The proximal capacitor unit includes at least one series capacitor and at least one gate switch; the series capacitors and the gate switches are arranged in a one-to-one correspondence, and one end of each series capacitor is connected to one of the output ends of the power converter; the other end of each series capacitor is respectively connected to one end of the corresponding gate switch, and the other end of each gate switch is connected to the parallel capacitor through a return cable; When the gating switch is turned on, the corresponding series capacitor is connected to the circuit, and when the gating switch is turned off, the corresponding series capacitor is disconnected from the circuit; The capacitive reactances of each of the series capacitors are not equal to each other; The power converter includes a rectifier module, a filter module, a voltage regulator module, an inverter module and a transformer, wherein the rectifier module, the filter module, the voltage regulator module, the inverter module and the transformer are connected in sequence, the input end of the rectifier module is connected to an external three-phase grid voltage, and an AC voltage with an adjustable frequency f is output between two output ends of the secondary side of the transformer; The current on the primary side of the transformer is taken as the current signal, and the voltage across the parallel capacitor is taken as the voltage signal.

2. The composite resonant heating circuit according to claim 1, characterized in that: The resonant heater also includes a series inductor, which is arranged between the selection switch and the de-loop cable. One end of the series inductor is connected to the selection switch, and the other end of the series inductor is connected to the corresponding de-loop cable.

3. The composite resonant heating circuit according to claim 2, characterized in that: The resonant heating circuit also includes an inverter controller, which includes: A voltage zero-crossing detection unit for detecting a zero-crossing point of a voltage signal; A voltage zero-crossing pulse generating unit for generating a corresponding pulse at a voltage signal zero-crossing point; A current zero-crossing detection unit for detecting a zero-crossing point of a current signal; A current zero-crossing pulse generating unit for generating a corresponding pulse at a zero-crossing point of a current signal; A phase comparison unit for comparing the phases between two input pulses; A phase difference amplification unit for amplifying the phase difference; A PFM regulating unit for regulating the output frequency of the inverter module; and, an inverter drive unit for generating control pulses for the inverter module; The current signal is connected to a current zero-crossing detection unit, and the output end of the current zero-crossing detection unit is connected to a current zero-crossing pulse generating unit; the voltage signal is connected to a voltage zero-crossing detection unit, and the output end of the voltage zero-crossing detection unit is connected to a voltage zero-crossing pulse generating unit; The output ends of the current zero-crossing pulse generating unit and the voltage zero-crossing pulse generating unit are connected to the phase comparison unit; The output end of the phase comparison unit is connected to the phase difference amplification unit, and the output end of the phase difference amplification unit is connected to the PFM adjustment unit; the output end of the PFM adjustment unit is connected to the inverter drive unit; The output end of the inverter drive unit is connected to the inverter module, and the inverter drive unit generates an inverter drive pulse to control the operation of the inverter module.

4. A frequency control method applied to the composite resonant heating circuit according to any one of claims 1 to 3, characterized in that: The method comprises the following steps: S1: Get signal: take the current on the primary side of the transformer as the current signal, and take the voltage across the parallel capacitor as the voltage signal; S2: Phase comparison: compare and obtain the phase difference between the current signal and the voltage signal; S3: Frequency adjustment: The power converter is adjusted according to the phase difference between the current signal and the voltage signal, and the frequency f of the AC voltage output between the two output terminals of the power converter is changed so that the current signal and the voltage signal are in phase.

5. The frequency control method of the composite resonant heating circuit according to claim 4, characterized in that: The S2: phase comparison is specifically as follows: S21: the current signal is connected to a current zero-crossing detection unit, and the current zero-crossing detection unit detects the zero-crossing point of the current signal; the voltage signal is connected to a voltage zero-crossing detection unit, and the voltage zero-crossing detection unit detects the zero-crossing point of the voltage signal; S22: the current zero-crossing pulse generating unit generates a current zero-crossing pulse corresponding to the zero-crossing point of the current signal, and the voltage zero-crossing pulse generating unit generates a voltage zero-crossing pulse corresponding to the zero-crossing point of the voltage signal; S23: the current zero-crossing pulse and the voltage zero-crossing pulse are connected to a phase comparison unit to compare the phase difference between the two. S24: The phase difference is connected to a phase difference amplification unit, and the phase difference amplification unit amplifies the phase difference.

6. The frequency control method of the composite resonant heating circuit according to claim 4, characterized in that: The S3: frequency adjustment is specifically as follows: S31: the amplified phase difference is connected to a PFM adjustment unit, and the PFM adjustment unit outputs a PFM modulation signal according to the phase difference; S31: the PFM modulation signal is connected to the inverter drive unit, the inverter drive unit generates an inverter drive pulse according to the PFM modulation signal, and the inverter drive pulse is connected to the inverter module to change the working condition of the inverter module accordingly.

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