Electrode for a plasma display panel

Abstract

The application provides a current conversion device and method for load induction heating, and belongs to the technical field of electronic power, and solves the problems of complex structure and high cost of the existing three-phase high-frequency AC-AC converter. The device comprises: a filter circuit connected with a three-phase low-frequency AC power supply, and a resonant circuit electrically connected with a power switch circuit; the power switch circuit is alternately turned on in a switching cycle in a manner that the phase difference is a preset phase value according to the resonant parameters of the resonant circuit and a plurality of driving pulse signals, directly converts the AC power in a first frequency range after filtering into AC power in a second frequency range, and generates an alternating magnetic field in the second frequency range through an induction heating coil to heat the load; wherein the minimum value of the second frequency range is greater than the maximum value of the first frequency range. The scheme reduces the device cost, realizes full-cycle soft switching operation, and improves the efficiency and robustness.

Description

Technical Field

[0001] This invention relates to the field of electronic power technology, and in particular to a current conversion device and method for load induction heating. Background Technology

[0002] In high-frequency induction heating applications, three-phase AC power needs to be converted into high-frequency AC power by a power electronic converter to drive the induction heating load. Traditional solutions generally adopt a two-stage AC-DC-AC power conversion architecture, that is, the three-phase power frequency AC power is first converted into DC power through a rectifier stage, and then the DC power is inverted into AC power of the required frequency through a high-frequency inverter stage.

[0003] In this architecture, the rectifier stage typically consists of a three-phase diode bridge or a fully controlled device bridge, followed by a large-capacity DC bus capacitor to maintain DC link voltage stability. The inverter stage usually adopts a full-bridge or half-bridge topology. To achieve power factor correction or reduce input current harmonics, some solutions also incorporate a power factor correction circuit into the rectifier stage.

[0004] At the control level, this type of AC-DC-AC converter needs to detect the phase of the input three-phase voltage in real time and modulate the switching devices accordingly based on the voltage polarity and amplitude information to ensure the correct flow of energy and the quality of the output waveform.

[0005] Furthermore, some improved schemes introduce the concept of an equivalent diode bridge, attempting to achieve specific freewheeling or current-directing functions through the body diodes of the switching devices themselves or external diode bridges. While these traditional schemes can achieve power conversion, they are characterized by a large number of devices and complex circuit topologies, resulting in high system hardware costs and cumbersome control circuit design. These architectural features of existing three-phase high-frequency AC-AC converters present a series of problems that urgently require improvement.

[0006] First, the main power circuit includes multiple power processing stages such as rectifier stage, equivalent diode bridge and DC link filter capacitor. The large number of power switching devices and passive components directly increases material costs and equipment size, which is not conducive to the miniaturization design of the device.

[0007] Secondly, the complex circuit structure requires the configuration of corresponding input phase detection circuits and complex control logic. The control system needs to obtain the phase information of the input voltage in real time to determine the conduction time of each switching device. This not only increases the hardware overhead of the control circuit, but also increases the difficulty of implementing the software algorithm.

[0008] Furthermore, this type of control method, which is sensitive to the phase of the input voltage, makes the system susceptible to electromagnetic interference and external noise. Phase detection errors or control signal delays can cause switching timing disorders, thereby affecting the stability and reliability of system operation. In addition, the cascaded structure of multiple power conversion stages also increases the overall system losses, limiting further efficiency improvements. Summary of the Invention

[0009] This invention provides a current conversion device and method for load induction heating, which solves the problems of complex structure and high cost of existing three-phase high-frequency AC-AC converters.

[0010] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows: This invention provides a current conversion device for load induction heating, comprising: A filter circuit connected to a three-phase low-frequency AC power supply is used to filter the three-phase AC power output by the three-phase low-frequency AC power supply and output filtered AC power in the first frequency range. A power switching circuit electrically connected to the output of the filter circuit; and A resonant circuit electrically connected to the power switching circuit, wherein the induction heating coil of the resonant circuit is close to or surrounds the load to be heated; The power switching circuit alternately conducts the power signals in a switching cycle with a preset phase difference based on the resonance parameters of the resonant circuit and the multi-channel drive pulse signals. It performs a direct AC-AC conversion on the filtered AC power in the first frequency range to obtain AC power in the second frequency range. The AC power in the second frequency range generates an alternating magnetic field in the second frequency range through the induction heating coil to heat the load. The multi-channel drive pulse signals are generated based on preset switching cycle parameters and duty cycle parameters. The value of the duty cycle parameter is less than or greater than a preset value, and the minimum value of the second frequency range is greater than the maximum value of the first frequency range.

[0011] Optionally, the filtering circuit includes: First inductor, second inductor, third inductor, first filter capacitor, second filter capacitor, and third filter capacitor; Wherein, the input terminal of the first inductor is connected to the output terminal of the first power supply in the three-phase low-frequency AC power supply, the input terminal of the second inductor is connected to the output terminal of the second power supply in the three-phase low-frequency AC power supply, the input terminal of the third inductor is connected to the output terminal of the third power supply in the three-phase low-frequency AC power supply, the first filter capacitor is connected between the output terminals of the first inductor and the second inductor, the second filter capacitor is connected between the output terminals of the second inductor and the third inductor, and the third filter capacitor is connected between the output terminals of the third inductor and the first inductor.

[0012] Optionally, the power switching circuit includes: The first transistor, the second transistor, and the third transistor; In this configuration, the drain of the first transistor is connected to the output terminal of the first inductor, the source of the first transistor is connected to the first resonant branch of the resonant circuit, and the gate of the first transistor receives a first driving pulse signal. The drain of the second transistor is connected to the output terminal of the second inductor, the source of the second transistor is connected to the second resonant branch of the resonant circuit, and the gate of the second transistor receives a second driving pulse signal. The drain of the third transistor is connected to the output terminal of the third inductor, the source of the third transistor is connected to the third resonant branch of the resonant circuit, and the gate of the third transistor receives a third driving pulse signal.

[0013] Optionally, the first drive pulse signal, the second drive pulse signal, and the third drive pulse signal are 120° out of phase with each other within one switching cycle, and the duty cycle of each drive pulse signal is less than or greater than 12 / 3.

[0014] Optionally, each of the first transistor, the second transistor, and the third transistor has a body diode inside, with the anode of the body diode connected to the source of the power switching device and the cathode of the body diode connected to the drain of the power switching device.

[0015] Optionally, during a switching cycle, when the drive pulse signal input to the gate of the power switching device changes from a high level to a low level, the power switching device is turned off; After the power switching device is turned off, the node voltage of the source of the power switching device decreases resonantly under the action of the resonant circuit. When the node voltage is below zero, and the difference between the source and drain of the power switching device meets a preset condition, the body diode of the power switching device is turned on and establishes a freewheeling path, clamping the voltage between the drain and source of the power switching device to zero.

[0016] Optionally, after the body diode is turned on and a freewheeling path is established, the branch current flowing through the power switching device changes according to the transient response law and enters a preset current range. When the branch current is within a preset current range, the drive pulse signal input to the gate of the power switch changes from low level to high level, enabling the power switch to turn on under zero voltage conditions.

[0017] Optionally, the first end of the first resonant branch is connected to the second end of the second resonant branch, the first end of the second resonant branch is connected to the second end of the third resonant branch, and the first end of the third resonant branch is connected to the second end of the first resonant branch. The first resonant branch includes a first induction heating coil and a first resonant capacitor connected in parallel. The second resonant branch includes a second induction heating coil and a second resonant capacitor connected in parallel. The third resonant branch includes a third induction heating coil and a third resonant capacitor connected in parallel.

[0018] Optionally, the resonant parameters of the resonant circuit include the equivalent inductance and equivalent capacitance values, which are determined by the following formula: ; in, The equivalent inductance value is determined by the self-inductance of the induction heating coil and the mutual inductance of the load equivalent to the primary side. This is the equivalent capacitance value; The resonant frequency of the resonant circuit is given by [the given frequency]. Greater than 20kHz.

[0019] This invention also provides a current conversion method for load induction heating, applied to the aforementioned current conversion device for load induction heating, comprising: The three-phase AC power output from the three-phase low-frequency AC power supply is filtered by a filter circuit, and the filtered AC signal in the first frequency range is output. The power switching circuit alternately conducts the filtered AC signal in the first frequency range with a preset phase difference within one switching cycle according to the resonant parameters of the resonant circuit and the multi-channel drive pulse signals. This performs a direct AC-AC conversion to obtain an AC signal in the second frequency range, and outputs the AC signal in the second frequency range to the load through the resonant circuit. The multi-channel drive pulse signals are generated according to preset switching cycle parameters and duty cycle parameters. The duty cycle parameter of the multi-channel drive pulse signals is less than or greater than a preset value, and the minimum value of the second frequency range is greater than the maximum value of the first frequency range.

[0020] The current conversion device for load induction heating of the present invention includes: a filter circuit connected to a three-phase low-frequency AC power supply for filtering the three-phase AC power output from the three-phase low-frequency AC power supply and outputting filtered AC power in a first frequency range; a power switching circuit electrically connected to the output terminal of the filter circuit; and a resonant circuit electrically connected to the power switching circuit, wherein the induction heating coil of the resonant circuit is close to or surrounds the load to be heated; the power switching circuit alternately conducts according to the resonant parameters of the resonant circuit and multiple driving pulse signals with a preset phase difference within one switching cycle, performing direct AC-AC conversion on the filtered AC power in the first frequency range to obtain AC power in a second frequency range, and the AC power in the second frequency range generates an alternating magnetic field in the second frequency range through the induction heating coil to heat the load; wherein the multiple driving pulse signals are generated according to preset switching cycle parameters and duty cycle parameters, the value of the duty cycle parameter is less than or greater than a preset value, and the minimum value of the second frequency range is greater than the maximum value of the first frequency range; this scheme reduces device costs, achieves full-cycle soft-switching operation, and improves efficiency and robustness. Attached Figure Description

[0021] Figure 1 This is a structural diagram of a current conversion device for load induction heating provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the mode conversion of a single-phase equivalent circuit of a current conversion device for load induction heating provided in an embodiment of the present invention; Figure 3 This is a diagram showing the driving pulse signal and the voltage and current waveforms of the resonant branch of the current conversion device for load induction heating provided in an embodiment of the present invention. Figure 4 This is a flowchart of a current conversion method for load induction heating provided in an embodiment of the present invention; Among them, 11 is the filter circuit; 12 is the power switch circuit; and 13 is the resonant circuit. Detailed Implementation

[0022] Exemplary embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the invention are shown in the drawings, it should be understood that the invention may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0023] like Figure 1 As shown, an embodiment of the present invention provides a current conversion device for load induction heating, comprising: The filter circuit 11, which is connected to the three-phase low-frequency AC power supply, is used to filter the three-phase AC power output by the three-phase low-frequency AC power supply and output the filtered AC power in the first frequency range. The power switching circuit 12 is electrically connected to the output terminal of the filter circuit 11; and A resonant circuit 13 electrically connected to the power switch circuit 12, wherein the induction heating coil of the resonant circuit 13 is close to or surrounds the load to be heated; The power switching circuit 12 alternately conducts according to the resonance parameters of the resonant circuit 13 and the multi-channel drive pulse signals in a switching cycle with a preset phase difference, performing direct AC-AC conversion on the filtered first frequency range AC to obtain the second frequency range AC. The second frequency range AC generates an alternating magnetic field of the second frequency range through the induction heating coil, heating the load. The multi-channel drive pulse signals are generated according to preset switching cycle parameters and duty cycle parameters. The value of the duty cycle parameter is smaller than or greater than a preset value, and the minimum value of the second frequency range is greater than the maximum value of the first frequency range.

[0024] In this embodiment, the input terminal of the filter circuit 11 is connected to a three-phase low-frequency AC power supply to receive the three-phase low-frequency AC power. The filter circuit 11 suppresses and attenuates the high-frequency noise components in the three-phase AC power, and outputs filtered AC power within a first frequency range. The first frequency range corresponds to the fundamental frequency range of the three-phase low-frequency AC power supply, preferably the power frequency range. The filtered AC power within the first frequency range is transmitted to the input terminal of the power switching circuit 12.

[0025] The resonant circuit 13 includes an induction heating coil and a resonant capacitor, which are connected in series or parallel to form a resonant circuit. The induction heating coil is placed near or around the load to be heated, which is typically a conductive metal workpiece. The resonant circuit 13 has a natural resonant frequency, which is determined by the equivalent inductance of the induction heating coil and the equivalent capacitance of the resonant capacitor. The equivalent inductance is determined by the self-inductance of the induction heating coil and the mutual inductance of the load to the primary side. When the induction heating coil is near or around the load, the load generates eddy currents under electromagnetic induction. The magnetic field generated by these eddy currents acts back on the induction heating coil, manifesting as a reflected impedance. The inductive component of this reflected impedance is superimposed with the self-inductance of the induction heating coil to form the equivalent inductance. By selecting the parameter values ​​of the induction heating coil and the capacitor, the natural resonant frequency of the resonant circuit 13 is greater than 20kHz, which falls within the second frequency range.

[0026] The power switching circuit 12 acquires preset switching cycle parameters and duty cycle parameters, with the duty cycle parameter value being less than or greater than a preset value. The switching cycle parameter is set based on the inherent resonant frequency of the resonant circuit 12, ensuring that the switching frequency of the power switching device matches the resonant frequency. The power switching circuit 12 generates multiple drive pulse signals based on the aforementioned switching cycle and duty cycle parameters, with each drive pulse signal differing from the others by a preset phase value within one switching cycle. The generation process of the drive pulse signals does not depend on the real-time phase information of the three-phase low-frequency AC input voltage; therefore, there is no need to set up an input voltage phase detection circuit.

[0027] Under the control of multiple drive pulse signals, the power switching circuit 12 uses multiple power switching devices that alternately turn on and off within a switching cycle with a preset phase difference, performing direct AC-AC conversion on the filtered AC power in the first frequency range. Direct AC-AC conversion means converting AC power in the first frequency range to AC power in the second frequency range directly through the alternating conduction of the power switching devices, without going through the intermediate stage of rectifying AC power to DC power. The minimum value of the second frequency range is greater than the maximum value of the first frequency range, meaning the overall frequency corresponding to the second frequency range is higher than the frequency corresponding to the first frequency range. After obtaining the AC power in the second frequency range, this AC power passes through an induction heating coil, generating an alternating current with the same frequency as the second frequency range within the coil. This alternating current generates an alternating magnetic field in the second frequency range around the induction heating coil. The load located in the alternating magnetic field generates an induced electromotive force and induced eddy currents with the same frequency as the alternating magnetic field within it. The induced eddy currents generate Joule heating on the load resistance, thereby heating the load. Since the minimum value of the second frequency range is greater than the maximum value of the first frequency range, the high-frequency alternating magnetic field generated by the induction heating coil can induce a stronger eddy current effect in the load, thus meeting the high-frequency electrical energy requirements of induction heating.

[0028] The technical solution described in this embodiment uses multiple drive pulse signals with a preset phase difference and a duty cycle smaller than or greater than the preset value to control the power switching circuit to work alternately within one switching cycle. This, combined with a resonant circuit, achieves zero-voltage turn-on of the switching devices. Only a filter circuit, a power switching circuit, and a resonant circuit are needed to directly convert three-phase low-frequency AC to high-frequency AC. Since the main circuit does not include a rectifier stage or an equivalent diode bridge, real-time detection of the input voltage phase is unnecessary. The overall number of components is small, and the circuit structure is compact, which helps reduce hardware costs, improves system anti-interference capability and operational reliability, and is suitable for induction heating applications with high requirements for size and reliability.

[0029] In an optional embodiment of the present invention, the filter circuit 11 includes: First Inductor L 1. Second inductorL 2. Third inductor L 3. First filter capacitor C 1. Second filter capacitor C 2 and the third filter capacitor C 3; Wherein, the first inductor L The input terminal of 1 is connected to the output terminal of the first power supply in the three-phase low-frequency AC power supply, and the second inductor L The input terminal of 2 is connected to the output terminal of the second power supply in the three-phase low-frequency AC power supply, and the third inductor L The input terminal of 3 is connected to the output terminal of the third power supply in the three-phase low-frequency AC power supply, and the first filter capacitor C 1 connected to the first inductor L 1 and second inductor L Between the output terminals of 2, the second filter capacitor C 2 connected to the second inductor L 2 and the third inductor L Between the output terminals of 3, the third filter capacitor C 3 connected to the third inductor L 3 and the first inductor L Between the output terminals of 1.

[0030] In this embodiment, the filter circuit 11 consists of three inductor elements and three filter capacitor elements, forming a three-phase LC filter network. The first inductor... L The input terminal of inductor 1 is connected to the output terminal of the first power supply in the three-phase low-frequency AC power supply to receive the first phase low-frequency AC voltage. The second inductor... L The input terminal of inductor 2 is connected to the output terminal of the second power supply in the three-phase low-frequency AC power supply to receive the second-phase low-frequency AC voltage. The third inductor... L The input terminal of 3 is connected to the output terminal of the third power supply in the three-phase low-frequency AC power supply, and is used to receive the third-phase low-frequency AC voltage. The three-phase low-frequency AC voltage is input to the filter circuit 11 through the inductors of each phase. The inductors exhibit inductive reactance characteristics to AC current, and the inductive reactance increases with the frequency. Therefore, they have a greater impedance to high-frequency current components, while having a smaller impedance to low-frequency current components.

[0031] Three filter capacitors are connected across the output terminals of the three inductors, specifically the first filter capacitor... C 1 connected across the first inductor L The output terminal of 1 and the second inductor L Between the output terminals of 2, the second filter capacitor C 2 connected across the second inductor L 2's output terminal and the third inductor L Between the output terminals of 3, the third filter capacitorC 3 connected across the third inductor L 3's output terminal and the first inductor L Between the output terminals of phase 1. The above connection method ensures that a filter capacitor is connected in parallel between any two phases, forming a three-phase capacitor delta connection structure. Capacitors exhibit capacitive reactance characteristics to alternating current, and the capacitive reactance value decreases as the frequency increases. Therefore, they present a lower impedance path for high-frequency current components and a higher impedance for low-frequency current components.

[0032] When the AC output of a three-phase low-frequency AC power supply contains high-frequency noise components, these components are significantly impeded by inductive reactance as they flow through the inductors of each phase, making it difficult for them to be conducted to the output. Simultaneously, because the filter capacitors provide a low-impedance, high-frequency path between the output terminals of any two phase inductors, the high-frequency noise components that have passed through or been induced to the output are bypassed by the filter capacitors, forming a high-frequency circulating loop among the three filter capacitors. The high-frequency noise energy is consumed or limited during the charging and discharging process of the capacitors, thus achieving effective attenuation of the high-frequency noise.

[0033] For the fundamental component of three-phase low-frequency AC, the inductive reactance of the inductor is relatively small, allowing low-frequency current to be transmitted to the output terminal with minimal loss through each phase inductor. The capacitor exhibits high capacitive reactance to low-frequency components, ensuring that low-frequency current is almost entirely bypassed by the filter capacitor, thus guaranteeing efficient low-frequency energy transmission. By appropriately selecting the inductance values ​​of each inductor and the capacitance values ​​of each filter capacitor, the filter circuit can exhibit low-impedance transmission characteristics for low-frequency AC in the first frequency range and high-impedance attenuation characteristics for high-frequency noise components above the first frequency range, outputting filtered AC in the first frequency range.

[0034] The symmetrical connection of the three inductors and three filter capacitors ensures consistent three-phase filtering characteristics and symmetrical circuit parameters in each phase. This guarantees the amplitude and phase symmetry of the filtered three-phase AC power, providing a smoother and more stable three-phase AC voltage condition for the subsequent power switching circuit 12. Furthermore, the connection method of the filter capacitors across the inductor output terminals provides a low-impedance loop for the transient current generated by the switching action during power switching, reducing the impact of switching transients on the three-phase low-frequency AC power supply.

[0035] In an optional embodiment of the present invention, the power switching circuit 12 includes: First transistor Q1, second transistor Q2, and third transistor Q3; Wherein, the drain of the first transistor Q1 is connected to the first inductor LThe output terminal of transistor 1 is connected, the source of the first transistor Q1 is connected to the first resonant branch of the resonant circuit 13, the gate of the first transistor Q1 receives the first driving pulse signal, and the drain of the second transistor Q2 is connected to the second inductor. L The output terminal of transistor 2 is connected, the source of transistor Q2 is connected to the second resonant branch of resonant circuit 13, the gate of transistor Q2 receives the second drive pulse signal, and the drain of transistor Q3 is connected to the third inductor. L The output terminal of 3 is connected, the source of the third transistor Q3 is connected to the third resonant branch of the resonant circuit 13, and the gate of the third transistor Q3 receives the third driving pulse signal. The first drive pulse signal, the second drive pulse signal, and the third drive pulse signal are 120° out of phase with each other within one switching cycle, and the duty cycle of each drive pulse signal is less than or greater than 12 / 3. Each of the first transistor Q1, the second transistor Q2, and the third transistor Q3 has a body diode inside. The anode of the body diode is connected to the source of the power switching device, and the cathode of the body diode is connected to the drain of the power switching device.

[0036] In this embodiment, the power switching circuit 12 consists of a first transistor Q1, a second transistor Q2, and a third transistor Q3, with each transistor corresponding to a phase of the three-phase AC power supply. The drain of the first transistor Q1 is connected to the first inductor. L The output terminal of transistor Q1 is connected to receive the filtered first-phase low-frequency AC voltage, and its drain serves as the power input terminal of the device. The source of the first transistor Q1 is connected to the first resonant branch of the resonant circuit 13, and its source serves as the power output terminal of the device, transferring the switched electrical energy to the first resonant branch. The gate of the first transistor Q1 receives the first drive pulse signal, and the gate serves as the control terminal to receive the drive pulse signal. The voltage level of the drive pulse signal controls the conduction and cutoff of the transistor's conductive channel.

[0037] The drain of the second transistor Q2 and the second inductor L The output terminal of transistor 2 is connected, and its source is connected to the second resonant branch of resonant circuit 13. The gate receives the second drive pulse signal. The drain of the third transistor Q3 is connected to the third inductor. L The output terminal of transistor 3 is connected, and its source is connected to the third resonant branch of resonant circuit 13. The gate input is the third drive pulse signal. The three transistors are arranged symmetrically in three phases in the circuit structure. The connection methods of the input terminals, output terminals and control terminals of each phase device are completely consistent, ensuring the symmetry of the three-phase circuit parameters.

[0038] Within one switching cycle, the three drive pulse signals are 120° out of phase with each other. Specifically, taking the first drive pulse signal as a reference, the second drive pulse signal is delayed by 120° phase angle relative to the first drive pulse signal, and the third drive pulse signal is delayed by 120° phase angle relative to the second drive pulse signal, thus forming a three-phase staggered drive timing sequence. The duty cycle of each drive pulse signal is less than or greater than 12 / 3. The duty cycle is defined as the ratio of the high-level duration of the drive pulse signal within one switching cycle to the total switching cycle time. Since the duty cycle is less than 1 / 3, the high-level phases of the three drive pulse signals will not overlap in time within one switching cycle, ensuring that at most one transistor's gate receives a high-level drive signal at any given time, thus ensuring that at most one transistor is in the conducting state. This avoids the phase-to-phase shoot-through short circuit problem that may be caused by multiple transistors conducting simultaneously.

[0039] Each of the three power switching devices—the first transistor Q1, the second transistor Q2, and the third transistor Q3—integrates a body diode. The body diode is a parasitic diode formed by the PN junction between the P-type and N-type semiconductor regions within the transistor. Its anode is connected to the source of the power switching device, and its cathode is connected to the drain. During normal transistor operation, the drain voltage is higher than the source voltage, and the body diode is reverse-biased and does not participate in conduction. When the transistor is turned off, if the resonant circuit 13 causes the source voltage to rise above the drain voltage, the body diode experiences a forward bias voltage, transitioning from the off state to the on state. Current can flow from the source through the body diode to the drain, providing a conduction path for the energy freewheeling in the resonant circuit 13. After the body diode is turned on, it clamps the voltage between the transistor's drain and source to near zero, providing the voltage condition for the transistor's next zero-voltage turn-on. The three transistors alternately complete freewheeling and zero-voltage turn-on in their respective operating ranges through their respective body diodes. With the help of the three-phase interleaved drive timing and resonant circuit parameters, the direct AC-AC conversion from three-phase low-frequency AC to high-frequency AC is realized.

[0040] In an optional embodiment of the present invention, during one switching cycle, when the drive pulse signal input to the gate of the power switching device changes from a high level to a low level, the power switching device is turned off; After the power switching device is turned off, the node voltage of the source of the power switching device decreases under the action of the resonant circuit 13. When the node voltage is lower than zero, and the difference between the source and drain of the power switching device meets the preset condition, the body diode of the power switching device is turned on and establishes a freewheeling path, and clamps the voltage between the drain and source of the power switching device to zero. After the body diode is turned on and a freewheeling path is established, the branch current flowing through the power switching device changes according to the transient response law and enters the preset current range. When the branch current is within a preset current range, the drive pulse signal input to the gate of the power switch changes from low level to high level, enabling the power switch to turn on under zero voltage conditions.

[0041] In this embodiment, within one switching cycle, the gate input of the power switching device is a drive pulse signal generated by the control unit. This drive pulse signal is a voltage signal that periodically changes between high and low levels. When the drive pulse signal input to the gate changes from high to low, that is, when the drive pulse signal enters a falling edge and transitions to a low-level maintenance state, the voltage between the gate and source of the power switching device rapidly decreases to below the device's turn-on threshold voltage. The conductive channel in the internal semiconductor structure of the device disappears, and the power switching device switches from the on state to the off state. Since the three drive pulse signals are 120° out of phase with each other within one switching cycle and each has a duty cycle less than or greater than 21 / 3, the first transistor Q1, the second transistor Q2, and the third transistor Q3 sequentially undergo the above-mentioned turn-off process within one switching cycle, and at most only one device is in the on state at any given time.

[0042] After the power switching device is turned off, the source node voltage of the power switching device begins to resonate under the action of the resonant circuit 13. The resonant circuit 13 is composed of an equivalent inductance and an equivalent capacitance connected in series. During the conduction period of the power switching device, current flows through the equivalent inductance and stores magnetic field energy in the equivalent inductance. When the power switching device is turned off, the current path through the equivalent inductance is cut off. According to the principle of electromagnetic induction, the current in the equivalent inductance cannot change abruptly, so an induced voltage is generated across the equivalent inductance, and the stored magnetic field energy begins to seek a release path. At this time, an energy exchange loop is formed between the equivalent inductance and the equivalent capacitance. The magnetic field energy released by the equivalent inductance charges the equivalent capacitance, increasing the electric field energy across the equivalent capacitance and raising the voltage of the equivalent capacitance. Subsequently, the equivalent capacitance discharges into the equivalent inductance, and the electric field energy is converted into magnetic field energy. This cycle repeats to form the resonant process. During resonance, the voltage at the source node of the power switching device gradually decreases from the initial value of turn-off according to the resonance law. The rate and waveform of voltage decrease are determined by the resonant frequency, which is jointly determined by the equivalent inductance and equivalent capacitance, as well as the circuit quality factor.

[0043] When the node voltage difference between the source and drain of the power switching device drops to a level that forward-biases the body diode, the body diode begins to conduct, clamping the drain-source voltage near the forward conduction voltage drop of the body diode. When the node voltage at the source of the power switching device further decreases under resonance and reaches below zero, the body diode of the power switching device is subjected to forward bias. The anode of the body diode is connected to the source, and the cathode is connected to the drain. At this time, the source voltage is below zero, while the drain voltage is the reference voltage, and the anode voltage is higher than the cathode voltage. The body diode transitions from the reverse cutoff state to the forward conduction state. After the body diode conducts, a freewheeling path is established from the source to the drain, and the freewheeling current of the equivalent inductor in the resonant circuit 13 flows along this freewheeling path. When the body diode is forward-conducting, its forward conduction voltage drop is very small, typically below 1 volt. Therefore, the body diode clamps the voltage between the drain and source of the power switching device to a level close to zero. This near-zero voltage clamping state continues until the body diode conducts.

[0044] After the body diode conducts and establishes a freewheeling path, the branch current flowing through the power switching device changes according to a transient response law. This transient response law is described by the second-order circuit differential equation formed by the equivalent inductance, equivalent capacitance, and parasitic resistance of the resonant circuit. In the initial stage of establishing the freewheeling path, the branch current... i Driven by the magnetic field energy stored in the equivalent inductance, the current flows from the source to the drain along the freewheeling path, with the current direction consistent with the forward conduction direction of the body diode. As energy in the equivalent inductance is gradually transferred to the equivalent capacitance, and some energy is consumed as Joule heat in the parasitic resistance, the amplitude of the branch current exhibits a decaying oscillating trend. At a specific moment during the resonance process, the electric field energy stored in the equivalent capacitance begins to drive the current in the equivalent inductance in the reverse direction, changing the direction of the branch current from the freewheeling direction to the opposite direction, i.e., entering the preset current range. This preset current range is the range in which the branch current flows from the drain to the source, i.e., the negative current range.

[0045] When the branch current is within a preset current range, the drive pulse signal input to the gate of the power switch changes from low to high, applying a turn-on drive signal to the gate of the power switch. Since the voltage between the drain and source of the power switch is clamped to near zero by the body diode during the freewheeling phase of the body diode, and the body diode is not yet fully turned off or has just turned off when the branch current is in the negative current range, and the voltage across the drain and source has not yet been established, the voltage difference between the drain and source of the power switch is close to zero at the instant the turn-on drive signal is applied. The power switch completes the switching from the off state to the on state under the condition that the voltage across its terminals is close to zero, i.e., achieving zero-voltage turn-on. Zero-voltage turn-on makes the product of the current flowing through the device and the voltage across the device very small at the instant the power switch is turned on, significantly reducing the instantaneous power loss during the turn-on process, thereby reducing turn-on losses and achieving soft-switching operation.

[0046] like Figure 1 As shown, in an optional embodiment of the present invention, the first end of the first resonant branch is connected to the second end of the second resonant branch, the first end of the second resonant branch is connected to the second end of the third resonant branch, and the first end of the third resonant branch is connected to the second end of the first resonant branch. The first resonant branch includes a first induction heating coil K1 and a first resonant capacitor connected in parallel. C 4. The second resonant branch includes a second induction heating coil K2 and a second resonant capacitor connected in parallel. C 5. The third resonant branch includes a third induction heating coil K3 and a third resonant capacitor connected in parallel. C 6; Wherein, the first induction heating coil K1 is used to induction heat the first load R1, the second induction heating coil K2 is used to induction heat the second load R2, and the third induction heating coil K3 is used to induction heat the third load R3. The resonant parameters of the resonant circuit 13 include the equivalent inductance and equivalent capacitance values, which are determined by the following formula: ; in, The equivalent inductance value is determined by the self-inductance of the induction heating coil and the mutual inductance of the load equivalent to the primary side. This is the equivalent capacitance value; The resonant frequency of the resonant circuit 13 is the inherent resonant frequency, and Greater than 20kHz.

[0047] In this embodiment, the first, second, and third resonant branches of the resonant circuit 13 are connected in a triangular configuration. Specifically, the first end of the first resonant branch is connected to the second end of the second resonant branch, the first end of the second resonant branch is connected to the second end of the third resonant branch, and the first end of the third resonant branch is connected to the second end of the first resonant branch, thus forming a closed triangular resonant network structure. The three resonant branches constitute a mutually coupled overall resonant system through the above connection method. The current in each resonant branch can form a circulating current in the triangular closed loop, realizing the transfer and exchange of three-phase resonant energy between the branches.

[0048] The first resonant branch includes a first induction heating coil K1 and a first resonant capacitor connected in parallel. C 4. The main body of the first induction heating coil K1 is positioned close to or around the load to be heated. When a high-frequency alternating current flows through the first induction heating coil K1, an alternating magnetic field is generated in the surrounding space. First resonant capacitor. C 4 is connected in parallel with the first induction heating coil K1 to form a parallel resonant circuit. The impedance of the parallel resonant circuit reaches its maximum value at the resonant frequency and decreases rapidly in the frequency range deviating from the resonant frequency. The second resonant branch includes the second induction heating coil K2 and the second resonant capacitor connected in parallel. C 5. The third resonant branch includes the third induction heating coil K3 and the third resonant capacitor connected in parallel. C 6. The internal structure of each resonant branch is consistent and the parameters are symmetrical. The first end of each resonant branch is connected to the source of the corresponding power switching device to receive the high-frequency pulse power output by the power switching circuit, and the second end is connected to the first end of the adjacent resonant branch through a delta connection.

[0049] During the operation of the resonant circuit 13, its resonant parameters include the equivalent inductance and equivalent capacitance. The equivalent inductance and equivalent capacitance together determine the inherent resonant frequency of the resonant circuit 13, and this relationship is determined by the following formula: ; Equivalent inductance value The inductance is determined by both the self-inductance of the induction heating coil and the mutual inductance of the load equivalent to the primary side. The self-inductance of the induction heating coil refers to the inductance generated by the coil itself under current excitation, determined by factors such as the number of turns, geometric dimensions, and magnetic circuit medium. When the induction heating coil is near or around the conductive load to be heated, a high-frequency alternating current flows through the coil, generating an alternating magnetic field. Under the influence of this alternating magnetic field, eddy currents are induced within the load, which in turn generate a magnetic field acting on the induction heating coil, manifesting as reflected impedance. The inductive component within this reflected impedance is the mutual inductance of the load equivalent to the primary side. The total inductance, resulting from the superposition of the self-inductance and the equivalent mutual inductance, constitutes the equivalent inductance value. Equivalent capacitance value This refers to the capacitance value of the resonant capacitor, specifically the capacitance value of the parallel resonant capacitors in each resonant branch. The inherent resonant frequency. The frequency at which the resonant circuit 13 oscillates freely without external excitation is given by the equivalent inductance value. and equivalent capacitance value The product is determined by the number of turns and geometric parameters of the induction heating coil, as well as the capacitance value of the resonant capacitor. and equivalent capacitance value Satisfy resonant frequency The requirement is greater than 20kHz. When the resonant frequency is higher than 20kHz, the induction heating coil can generate an alternating magnetic field with a sufficiently high frequency. The eddy current frequency induced in the load to be heated by this high-frequency alternating magnetic field is correspondingly increased. By utilizing the skin effect generated when the high-frequency current flows in the conductor, the eddy current is concentrated in the surface area of ​​the load, thereby increasing the eddy current density and heating efficiency, and meeting the high-frequency electrical energy requirements of induction heating.

[0050] like Figure 1 and Figure 2 As shown, a specific embodiment of the current conversion device for load induction heating provided in this invention is as follows: The current conversion device for load induction heating comprises a main circuit including a filter unit, three power switching devices, and three sets of LC resonant branches. The filter unit is preferably an LC filter, used to filter and stabilize the three-phase low-frequency AC input. The three power switching devices are preferably MOSFETs, denoted as Q1, Q2, and Q3, serving as the core switching units of the main power stage. The three resonant branches are connected to the corresponding switching devices and the induction heating load to achieve high-frequency resonant energy transfer. The entire main power circuit consists only of an input LC filter, three MOSFETs, and three resonant branches; no additional auxiliary switching branches, rectifier stages, or equivalent diode bridge circuits are included. Figure 1The overall topology of a three-switch three-phase direct AC-AC converter is presented. This topology consists of an input-side LC filter network, three sets of switching devices, and a resonant branch, which realizes the direct conversion of three-phase AC energy without passing through a DC intermediate link.

[0051] Three MOSFETs, or three equivalent MOSFETs (a group of two or more MOSFETs connected in series and parallel and controlled by multiple identical control signals), are controlled by an interleaved drive method with a phase difference of 120° within one switching cycle. The duty cycle of each switching device is less than or greater than 12 / 3. Through this drive method, the three switching devices operate sequentially within the same cycle, achieving direct energy conversion from three-phase low-frequency input to high-frequency output. For any single MOSFET, its switching operation can be equivalently regarded as a single switching unit in a single-phase converter.

[0052] Figure 2 The corresponding single-phase equivalent circuit and its operating modes are divided. Based on the different conduction states of switching devices Q1 and Q2 and the different directions of the resonant branch current, the system can be divided into three typical operating modes: Mode 1, Mode 2, and Mode 3. Among them, Mode 1 and Mode 2 correspond to different energy transmission paths and current directions, which are energy transmission stages; while Mode 3 is a continuous current transition stage, used to achieve smooth switching between modes. This mode is not an independent energy transmission stage, but a transition state in the resonance process, used to ensure current continuity and achieve soft switching conditions.

[0053] The following selects a representative input line voltage condition (UV phase voltage). U UV Phase voltage greater than zero, VW U VW Less than zero, WU phase voltage U WU (Less than zero) to analyze the working process of switching device Q1.

[0054] S1, Q1 is turned off when the gate drive signal of Q1 changes from high level to low level.

[0055] After Q1 is turned off, the voltage of the corresponding switching node S2 begins to resonate under the action of the resonant network and gradually decreases.

[0056] S3, when the voltage at this switching node drops below zero, the body diode of Q1 naturally conducts and current flows through it. i This will establish a sustainable circulation path.

[0057] S4. During this process, the current in the switch branch changes according to the LR transient response law and gradually increases.

[0058] S5, when the branch current enters the negative current range, a conduction signal is applied to Q1 again. Since the voltage across Q1 has been clamped to near zero voltage by the body diode, Q1 can conduct under zero voltage conditions, thus achieving zero voltage turn-on.

[0059] S6, after Q1 completes this conduction, turns off again after a predetermined conduction time, thus completing one switching cycle.

[0060] The waveforms of the drive pulse signal and the voltage and current of the resonant branch within one switching cycle are shown below. Figure 3 As shown, from Figure 3 It can be seen that a complete switching cycle consists of multiple operating modes in sequence, including a MOSFET driving square wave with a duty cycle of less than or greater than 12 / 3 and a continuous resonant branch voltage. u swv and resonant branch current i swv The waveform transitions naturally between modes through a resonance process, thereby achieving stable energy transfer and good dynamic characteristics.

[0061] Because the topology proposed in this embodiment has a strictly three-phase symmetrical structure, the operating states of the remaining voltage ranges can be obtained through cyclic permutation of phase variables, thus the operating mechanism of each sector is essentially the same. Through a symmetrical driving strategy, the switching devices of each phase participate in energy transfer with the same conduction law and time sequence, thereby ensuring a statistically balanced distribution of three-phase power. Simultaneously, the resonant network maintains consistent transient characteristics in each range, ensuring that the evolution laws of the switching node voltage and branch current remain consistent, thus achieving stable energy transfer and zero-voltage turn-on conditions throughout the entire power frequency cycle. Therefore, the above analysis results are universal and can be extended to the complete input voltage cycle, with overall system stability and balanced three-phase energy distribution.

[0062] The individual inductance in the three resonant branches is determined by the self-inductance of the induction coil and the mutual inductance equivalent to the load on the primary side. The equivalent inductance of the three inductors is: L e The equivalent capacitance of the three is C e The resonant frequency is determined according to the following formula, where the resonant frequency is greater than 20kHz: ; The current conversion device for load induction heating described in this embodiment uses only three MOSFETs in the main power stage, eliminating the need for a rectifier stage, an equivalent diode bridge, and an input phase detection circuit. This significantly reduces the number of components, resulting in a simpler and more compact structure. Through three-phase 120° interleaved drive and resonant branch coordination, the switching devices can achieve zero-voltage turn-on when conducting, and the voltage across the devices transitions naturally during turn-off under the action of the resonant network. This ensures soft-switching operation throughout the entire operating range, thereby reducing switching losses and improving system efficiency. Due to the reduction in detection and auxiliary circuits, the system is less sensitive to noise and interference, exhibiting stronger robustness. It is particularly suitable for induction heating high-frequency power conversion applications requiring compact size, low cost, and high reliability.

[0063] like Figure 4 As shown, this embodiment of the invention also provides a current conversion method for load induction heating, applied to the above-mentioned current conversion device for load induction heating, comprising: Step 41: The three-phase AC power output from the three-phase low-frequency AC power supply is filtered by the filter circuit 11, and the filtered AC signal in the first frequency range is output. Step 42: The power switch circuit 12 alternately conducts the filtered AC signal in the first frequency range according to the resonance parameters of the resonant circuit 13 and the multi-channel drive pulse signal with a preset phase difference within one switching cycle, and performs a direct AC-AC conversion to obtain an AC signal in the second frequency range. The AC signal in the second frequency range is then output to the load through the resonant circuit 13. The multi-channel drive pulse signal is generated according to preset switching cycle parameters and duty cycle parameters. The value of the duty cycle parameter of the multi-channel drive pulse signal is less than or greater than a preset value, and the minimum value of the second frequency range is greater than the maximum value of the first frequency range.

[0064] In step 41 of this embodiment, the three-phase AC power output from the three-phase low-frequency AC power supply is filtered by the filter circuit 11. The three-phase AC power output from the three-phase low-frequency AC power supply contains a fundamental frequency component and possibly superimposed high-frequency noise components, and the fundamental frequency of the AC power is located within a first frequency range. The filter circuit 11 consists of a first inductor... L 1. Second inductor L 2. Third inductor L 2 and the first filter capacitor C 1. Second filter capacitor C 2 and the third filter capacitor C 3. This constitutes a three-phase LC filter network. The first inductor... L The input terminal of 1 is connected to the first phase output terminal of the three-phase low-frequency AC power supply, and the second inductor... L The input terminal of 2 is connected to the second phase output terminal of the three-phase low-frequency AC power supply, and the third inductor... LThe input terminal of 3 is connected to the third phase output terminal of the three-phase low-frequency AC power supply. The three-phase AC power is input through the inductors of each phase. Inductors exhibit inductive reactance characteristics to AC current, and the reactance value is proportional to the frequency. Therefore, inductors significantly impede high-frequency current components but have a relatively small impeding effect on low-frequency fundamental components within the first frequency range. The first filter capacitor... C 1 connected across the first inductor L The output terminal of 1 and the second inductor L Between the output terminals of 2, the second filter capacitor C 2 connected across the second inductor L 2's output terminal and the third inductor L Between the output terminals of 3, the third filter capacitor C 3 connected across the third inductor L 3's output terminal and the first inductor L Between the output terminals of phase 1. Capacitors exhibit capacitive reactance characteristics to alternating current, with the reactance value inversely proportional to frequency. Therefore, capacitors present lower impedance to high-frequency current components and higher impedance to low-frequency fundamental components. High-frequency noise components in the three-phase AC current are significantly attenuated by inductive reactance as they flow through the inductors of each phase. Simultaneously, the capacitors provide a low-impedance bypass path for high-frequency components between the output terminals of any two inductors, further suppressing the circulating current formed by residual high-frequency components through the capacitors. Meanwhile, the low-frequency fundamental components within the first frequency range are transmitted through the inductors with minimal loss, and due to the high impedance characteristic of capacitors for low frequencies, they are almost non-shunt. After processing by the filtering circuit, the filtered AC signal within the first frequency range is output. The three-phase voltage waveform of this signal is smoother and more stable, with a significant reduction in high-frequency noise content.

[0065] In step 42, the filtered AC signal in the first frequency range is directly AC-AC converted using the power switching circuit 12. The power switching circuit 12 includes a first transistor Q1, a second transistor Q2, and a third transistor Q3. The drain of each device is connected to the output terminal of the corresponding phase inductor, and the source is connected to the corresponding resonant branch of the resonant circuit 13. The gates of each device receive the first, second, and third drive pulse signals, respectively. The multiple drive pulse signals are generated according to preset switching cycle parameters and duty cycle parameters. The duty cycle parameter is less than or equal to 12 / 3, and the generation of the drive pulse signals does not depend on the real-time phase information of the three-phase low-frequency AC input voltage. The switching cycle parameters are determined based on the resonant parameters of the resonant circuit 13, which include the equivalent inductance and equivalent capacitance values. The switching cycle parameters correspond to the resonant frequency, ensuring that the resonant frequency is greater than 20kHz. The first, second, and third drive pulse signals are 120° out of phase with each other within one switching cycle. The duty cycle of each drive pulse signal is less than or equal to 12 / 3, ensuring that at any given time, at most one transistor is in the on state. Under the control of the interleaved drive pulse signals, the three transistors alternately turn on and off, performing a direct AC-AC conversion on the AC signal in the first frequency range. This means that without the intermediate step of rectifying the AC to DC, the AC signal in the first frequency range is directly converted to the AC signal in the second frequency range through the alternating switching of the switching devices. The minimum value of the second frequency range is greater than the maximum value of the first frequency range. During the switching process, after each transistor turns off, its source node voltage resonates and decreases under the action of the corresponding resonant branch. When the node voltage drops below zero, the body diode conducts to establish a freewheeling path and clamps the voltage across the device to near zero. When the branch current enters the negative current range, the next turn-on drive signal is applied, achieving zero-voltage turn-on. The alternating current signal in the second frequency range obtained by the transformation is output to the load through each resonant branch of the resonant circuit 13. The induction heating coil in each resonant branch is set close to or around the load. The alternating current signal in the second frequency range generates an alternating magnetic field in the second frequency range in the induction heating coil. The alternating magnetic field induces eddy currents inside the load. The eddy currents generate Joule heat on the load resistance, thereby realizing induction heating of the load.

[0066] The technical solution described in this embodiment generates multiple drive pulse signals by acquiring preset switching cycle parameters and duty cycle parameters. The generation process does not rely on the real-time phase information of the three-phase low-frequency AC input voltage, thus eliminating the need for an input voltage phase detection circuit. This simplifies the hardware structure of the control system, reduces control complexity and implementation cost, and avoids timing errors caused by phase detection errors or external interference, improving system robustness. Within one switching cycle, the multiple drive pulse signals are phase-separated by a preset phase value and have a duty cycle smaller than or greater than a preset value, causing multiple power switching devices to conduct alternately. This prevents phase-to-phase shoot-through in terms of timing and achieves a statistically balanced distribution of the three-phase input power, improving output power quality. During the alternating operation of the power switching devices, a resonant network enables zero-voltage turn-on, effectively reducing switching losses and improving system conversion efficiency. It also helps reduce electromagnetic interference levels, enabling the device to meet the requirements of high-frequency induction heating applications with high demands for size, cost, and reliability.

[0067] The above are preferred embodiments of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A current conversion device for induction heating of a load, characterized in that, include: The filter circuit (11) connected to the three-phase low-frequency AC power supply is used to filter the three-phase AC power output by the three-phase low-frequency AC power supply and output the filtered AC power in the first frequency range. The power switch circuit (12) is electrically connected to the output terminal of the filter circuit (11); and A resonant circuit (13) electrically connected to the power switch circuit (12), wherein the induction heating coil of the resonant circuit (13) is close to or surrounds the load to be heated; The power switch circuit (12) alternately conducts the AC power in the first frequency range according to the resonance parameters of the resonant circuit (13) and the multi-channel drive pulse signal in a switching cycle with a phase difference of a preset value. It performs direct AC-AC conversion on the filtered AC power in the first frequency range to obtain AC power in the second frequency range. The AC power in the second frequency range generates an alternating magnetic field in the second frequency range through the induction heating coil to heat the load. The multi-channel drive pulse signal is generated according to the preset switching cycle parameters and duty cycle parameters. The value of the duty cycle parameter is less than or greater than the preset value, and the minimum value of the second frequency range is greater than the maximum value of the first frequency range.

2. The current conversion device for load induction heating according to claim 1, characterized in that, The filter circuit (11) includes: First inductor ( L 1) Second inductor ( L 2) Third inductor ( L 3) First filter capacitor ( C 1) Second filter capacitor ( C 2) and the third filter capacitor ( C 3); Among them, the first inductor ( L 1) The input terminal is connected to the output terminal of the first power supply in the three-phase low-frequency AC power supply, and the second inductor ( L 2) The input terminal is connected to the output terminal of the second power supply in the three-phase low-frequency AC power supply, and the third inductor ( L 3) The input terminal is connected to the output terminal of the third power supply in the three-phase low-frequency AC power supply, and the first filter capacitor ( C 1) Connected to the first inductor ( L 1) and the second inductor ( L 2) between the output terminals, the second filter capacitor ( C 2) Connected to the second inductor ( L 2) and the third inductor ( L Between the output terminals of 3), the third filter capacitor ( C 3) Connected to the third inductor ( L 3) and the first inductor ( L 1) Between the output terminals.

3. The current conversion device for load induction heating according to claim 2, characterized in that, The power switching circuit (12) includes: The first transistor (Q1), the second transistor (Q2), and the third transistor (Q3); Wherein, the drain of the first transistor (Q1) is connected to the first inductor (Q1). L The output terminal of 1) is connected, the source of the first transistor (Q1) is connected to the first resonant branch of the resonant circuit (13), the gate of the first transistor (Q1) receives the first driving pulse signal, and the drain of the second transistor (Q2) is connected to the second inductor ( L The output terminal of the second transistor (Q2) is connected, the source of the second transistor (Q2) is connected to the second resonant branch of the resonant circuit (13), the gate of the second transistor (Q2) receives the second driving pulse signal, and the drain of the third transistor (Q3) is connected to the third inductor (Q2). L The output terminal of the third transistor (Q3) is connected, the source of the third transistor (Q3) is connected to the third resonant branch of the resonant circuit (13), and the gate of the third transistor (Q3) is input with a third driving pulse signal.

4. The current conversion device for load induction heating according to claim 3, characterized in that, The first drive pulse signal, the second drive pulse signal, and the third drive pulse signal are 120° out of phase with each other within one switching cycle, and the duty cycle of each drive pulse signal is less than or greater than 12 / 3.

5. The current conversion device for load induction heating according to claim 3, characterized in that, Each of the first transistor (Q1), the second transistor (Q2), and the third transistor (Q3) has a body diode inside. The anode of the body diode is connected to the source of the power switching device, and the cathode of the body diode is connected to the drain of the power switching device.

6. The current conversion device for load induction heating according to claim 5, characterized in that, During one switching cycle, when the drive pulse signal input to the gate of the power switching device changes from a high level to a low level, the power switching device is turned off; After the power switching device is turned off, the node voltage of the source of the power switching device decreases under the action of the resonant circuit (13). When the node voltage is below zero, and the difference between the source and drain of the power switching device meets a preset condition, the body diode of the power switching device is turned on and establishes a freewheeling path, clamping the voltage between the drain and source of the power switching device to zero.

7. The current conversion device for load induction heating according to claim 6, characterized in that, After the body diode is turned on and a freewheeling path is established, the branch current flowing through the power switching device changes according to the transient response law and enters the preset current range; When the branch current is within a preset current range, the drive pulse signal input to the gate of the power switch changes from low level to high level, enabling the power switch to turn on under zero voltage conditions.

8. The current conversion device for load induction heating according to claim 6, characterized in that, The first end of the first resonant branch is connected to the second end of the second resonant branch, the first end of the second resonant branch is connected to the second end of the third resonant branch, and the first end of the third resonant branch is connected to the second end of the first resonant branch. The first resonant branch includes a first induction heating coil (K1) and a first resonant capacitor connected in parallel. C 4), the second resonant branch includes a second induction heating coil (K2) and a second resonant capacitor connected in parallel. C 5), the third resonant branch includes a third induction heating coil (K3) and a third resonant capacitor connected in parallel. C 6).

9. The current conversion device for load induction heating according to claim 8, characterized in that, The resonant parameters of the resonant circuit (13) include the equivalent inductance and equivalent capacitance values, which are determined by the following formula: ； in, The equivalent inductance value is determined by the self-inductance of the induction heating coil and the mutual inductance of the load equivalent to the primary side. This is the equivalent capacitance value; The resonant frequency of the resonant circuit (13) is the inherent resonant frequency, and Greater than 20kHz.

10. A current conversion method for load induction heating, applied to the current conversion device for load induction heating as described in any one of claims 1 to 9, characterized in that, include: The three-phase AC power output from the three-phase low-frequency AC power supply is filtered by the filter circuit (11) to output the filtered AC signal in the first frequency range. The power switch circuit (12) alternately conducts the AC signal in the first frequency range with a phase difference of a preset value within one switching cycle according to the resonance parameters of the resonant circuit (13) and the multi-channel drive pulse signal. It performs direct AC-AC conversion on the filtered AC signal in the first frequency range to obtain the AC signal in the second frequency range. The AC signal in the second frequency range is then output to the load through the resonant circuit (13). The multi-channel drive pulse signal is generated according to the preset switching cycle parameters and duty cycle parameters. The value of the duty cycle parameter of the multi-channel drive pulse signal is less than or greater than the preset value. The minimum value of the second frequency range is greater than the maximum value of the first frequency range.

Images