Low-harmonic three-phase-single-phase direct ac / ac high-frequency induction heating power supply and minimum phase clamping pulse density modulation method thereof

Abstract

The application discloses a low-harmonic three-phase-single-phase direct AC / AC high-frequency induction heating power supply and a minimum phase clamping pulse density modulation method thereof. The heating power supply comprises a grid-side filter, a three-phase-single-phase alternating current conversion unit, a CLLC type high-frequency resonance network, an induction heating load and a control unit. The control unit generates a three-phase reference current signal according to a three-phase grid-side voltage phase and a power regulation instruction, and obtains a reference vector in an alpha-beta stationary coordinate system through Clarke conversion; error accumulation is carried out on the reference vector in the alpha-beta domain, the alpha-beta domain error cost after the action of a candidate vector in a current working sector is calculated, and the candidate vector with the minimum error cost is selected as an actual output vector in a current high-frequency period. The application can realize large output power and high operation efficiency while keeping a simple structure, and is suitable for induction heating power supplies with high requirements on output power capacity, input current quality and operation reliability.

Description

Technical Field

[0001] This invention belongs to the field of high-frequency induction heating power supply control technology, specifically relating to a low-harmonic three-phase to single-phase direct AC / AC high-frequency induction heating power supply and its minimum phase clamping pulse density modulation method. Background Technology

[0002] Existing induction heating power supplies mostly adopt a two-stage or multi-stage AC-DC-A power conversion structure. This means that the three-phase AC input is converted to DC by a rectifier unit, then powered by a DC bus capacitor or DC current source to a high-frequency inverter unit, and finally output as high-frequency AC to the induction coil and equivalent heating load via a resonant compensation network. While this type of structure is technically mature, it typically requires a large-capacity DC bus energy storage element. For induction heating power supplies with higher output power, a large-capacity DC bus capacitor increases system size and cost, reduces power density, and may become a weak point affecting power supply lifespan and reliability. Furthermore, if the traditional rectifier front-end lacks effective power factor correction or input current shaping measures, it can easily lead to high harmonic content and a low power factor in the grid-side current, which is detrimental to the high-quality grid-connected operation of high-power induction heating power supplies.

[0003] To reduce DC bus energy storage, increase power density, and improve reliability, three-phase to single-phase direct AC / AC conversion technology has gained increasing attention. This technology can directly convert three-phase industrial frequency AC power into single-phase high-frequency AC power, which is then supplied to the induction heating load via a resonant network, thus eliminating the need for the large-capacity DC bus capacitor in traditional AC-DC-AC power supplies. Compared to the traditional two-stage structure, three-phase to single-phase direct AC / AC high-frequency induction heating power supplies have advantages such as compact structure, fewer energy conversion stages, high potential efficiency, and high reliability. However, existing three-phase to single-phase direct AC / AC converters still have shortcomings in high-frequency induction heating applications: if the switching modulation method only selects the maximum line voltage or fixed phase voltage for energy injection, it is easy to cause uneven distribution of the three-phase input current pulses, resulting in large low-order harmonics in the grid-side current; if full shutdown or additional bypass switch freewheeling is used in the non-injection state, it is easy to cause abrupt changes in the resonant current path, triggering current spikes, voltage spikes, and high-frequency oscillations, thereby affecting the safety of power devices and the stability of the thermal field.

[0004] Pulse density modulation (PDM) is a power regulation method suitable for high-frequency resonant power supplies. It adjusts the output power by changing the number of effective energy injection pulses per unit time. Compared with pulse width modulation (PWM) and pulse frequency modulation (PWM), PDM is beneficial for enabling the resonant power supply to operate close to its resonant frequency, facilitating soft switching of power devices, and reducing switching losses and electromagnetic interference. However, traditional PDM primarily focuses on output power regulation and does not adequately consider the pulse distribution characteristics of the three-phase input current. When directly applied to a three-phase-to-single-phase AC / AC high-frequency induction heating system, if the energy injection pulse density of each phase cannot match the corresponding phase voltage amplitude, it will lead to increased distortion of the three-phase input current and a higher total harmonic distortion rate of the grid-side current, making it difficult to meet the requirements of high-output-power induction heating power supplies for low harmonics, high power factor, and stable power supply. Summary of the Invention

[0005] To address the above problems, this invention proposes a low-harmonic three-phase to single-phase direct AC / AC high-frequency induction heating power supply and its minimum phase clamping pulse density modulation method, specifically employing the following technical solution: On one hand, the present invention provides a low-harmonic three-phase to single-phase direct AC / AC high-frequency induction heating power supply, including a grid-side filter, a three-phase to single-phase AC converter unit, a CLLC-type high-frequency resonant network, an induction heating load, and a control unit. The three-phase input terminals of the grid-side filter are respectively connected to the three-phase output terminals of the three-phase AC power supply, and the three-phase output terminals of the grid-side filter are respectively connected to the three-phase input terminals of the three-phase to single-phase AC converter unit. The grid-side filter is used to filter out the high-frequency current component on the three-phase AC input side. The three-phase to single-phase AC converter unit includes multiple power switching devices. The output terminal of the three-phase to single-phase AC converter unit is connected to the input terminal of the CLLC type high-frequency resonant network. The three-phase to single-phase AC converter unit is used to directly convert the three-phase power frequency AC power filtered by the grid-side filter into single-phase high-frequency AC power by turning on and off the multiple power switching devices. The output terminal of the CLLC-type high-frequency resonant network is connected to the electrical input terminal of the induction heating load. The CLLC-type high-frequency resonant network is used to receive the single-phase high-frequency AC power output by the three-phase to single-phase AC converter unit and transmit high-frequency resonant energy to the induction heating load. The control signal output terminal of the control unit is connected to the control terminal signal of each power switching device in the three-phase to single-phase AC converter unit. The control unit is used to output a switch control signal according to the three-phase pulse density modulation signal to control the conduction and cutoff of each power switching device.

[0006] Furthermore, the three-phase to single-phase AC conversion unit includes three sets of switching units corresponding to phase A, phase B, and phase C, respectively. Each set of switching units includes a positive power switch and a negative power switch. The positive power switch is used to conduct during the high-frequency positive half-cycle, and the negative power switch is used to conduct during the high-frequency negative half-cycle.

[0007] Furthermore, the CLLC-type high-frequency resonant network includes a grid-side resonant capacitor C. ra C rb C rc and series resonant inductor L r Parallel resonant capacitor C P Parallel resonant inductor L P Equivalent load R; the grid-side resonant capacitor C ra C rb C rc With the series resonant inductor L r The parallel resonant capacitor C forms a series resonant branch. P With the parallel resonant inductor L P The CLLC-type high-frequency resonant network forms a parallel resonant branch and is used to improve the impedance characteristics of the resonant cavity and create conditions for soft switching of power switching devices.

[0008] Furthermore, the control unit includes a phase-locking module, a three-phase reference current generation module, a Clarke transformation module, an αβ domain error accumulation module, a sector determination module, an H / L / Z candidate vector selection module, and a drive logic module. The phase-locking module is used to acquire the three-phase grid-side voltage phase. The three-phase reference current generation module is used to generate a three-phase reference current signal based on the three-phase grid-side voltage phase and the pulse density modulation coefficient. The Clarke transformation module is used to transform the three-phase reference current signal into a reference vector in the αβ stationary coordinate system. The αβ domain error accumulation module is used to accumulate the error of the reference vector cycle by cycle. The H / L / Z candidate vector selection module is used to calculate the αβ domain error cost after the candidate vector is applied in the current working sector and select the candidate vector with the smallest error cost as the actual output vector of the current high-frequency cycle. The drive logic module is used to output the drive signals of each power switching device based on the actual output vector.

[0009] Furthermore, the control unit divides one power frequency cycle into six working sectors based on the instantaneous amplitude relationship of the three-phase grid side voltage, and determines the maximum phase voltage, the second largest phase voltage, and the minimum phase voltage in each working sector, thereby laying the foundation for the subsequent selection of H / L / Z candidate vectors.

[0010] Furthermore, the H / L / Z candidate vector selection module constructs a high-energy injection vector H, a low-energy injection vector L, and a zero effective input vector Z within the current working sector; the αβ domain error cost includes a reference vector tracking error term; when the high-energy injection vector H or the low-energy injection vector L is selected, the control unit controls the system to enter the energy injection state; when the actual output vector is Z, the control unit controls the system to enter the non-injection clamping freewheeling state.

[0011] Furthermore, in the non-injection clamp freewheeling state, no additional bypass switch is used for freewheeling. Instead, the positive power switch and the negative power switch corresponding to the minimum phase voltage are alternately turned on to form a freewheeling path.

[0012] Furthermore, the control unit is provided with a dead time; when switching between the energy injection state and the non-injection clamping freewheeling state, the control unit keeps the relevant power switching devices off for a preset dead time.

[0013] Furthermore, no large-capacity DC bus energy storage capacitor is installed between the three-phase to single-phase AC converter unit and the CLLC-type high-frequency resonant network. The three-phase AC power supply directly transmits energy to the CLLC-type high-frequency resonant network through the grid-side filter and the three-phase to single-phase AC converter unit.

[0014] On the other hand, the present invention provides a minimum phase clamping pulse density modulation method for the low harmonic three-phase to single-phase direct AC / AC high-frequency induction heating power supply, comprising the following steps: Step 1: Collect the three-phase grid-side voltage and obtain the phase and instantaneous amplitude relationship of the three-phase grid-side voltage; Step 2: Generate a three-phase reference current signal with a phase difference of 120° based on the phase of the three-phase grid-side voltage, and set the pulse density modulation coefficient m according to the target output power; Step 3: Perform Clarke transform on the three-phase reference current signal to obtain the reference vector in the αβ stationary coordinate system; divide the current working sector according to the phase or instantaneous amplitude relationship of the three-phase grid-side voltage, and determine the maximum phase voltage, intermediate phase voltage and minimum phase voltage in the current working sector; Step 4: Using the resonant frequency of the resonant cavity load obtained by the phase-locked loop as a reference and the trigger signal, input the reference vector in the αβ stationary coordinate system into the αβ domain error accumulator to form the prediction error vector of the current high-frequency cycle; Step 5: Establish a set of candidate vectors for H / L / Z based on the current working sector; the control unit calculates the αβ domain error cost after the action of H, L, and Z respectively, and selects the candidate vector with the smallest error cost as the actual output vector for the current high-frequency cycle; when the actual output vector is H or L, the system enters the energy injection state, and turns on the corresponding positive power switch in the positive half-cycle of the high-frequency cycle and the corresponding negative power switch in the negative half-cycle of the high-frequency cycle according to the selected injection vector, so that the three-phase input current pulses are distributed according to the grid-side voltage vector relationship; Step 6: When the actual output vector is Z, the system enters the non-injection clamp freewheeling state. During the high-frequency positive half-cycle, the positive power switch corresponding to the minimum voltage is turned on, and during the high-frequency negative half-cycle, the negative power switch corresponding to the same minimum voltage is turned on, forming the minimum phase clamp freewheeling path. Step 7: Update the αβ domain error accumulation state according to the actual output vector, and adjust the pulse density modulation coefficient in the three-phase reference current signal through power closed loop to realize the adjustment of induction heating output power and match the three-phase input current pulse distribution with the change of three-phase grid-side voltage vector; In the energy injection state, the control unit selects vector H or vector L according to the current working sector and αβ domain error cost, and maps the selected injection vector to the corresponding positive and negative power switch combinations, so that the input current pulse is distributed among the three phases according to the voltage vector relationship.

[0015] Compared with the prior art, the present invention has at least the following beneficial effects: Firstly, the present invention adopts a three-phase to single-phase direct AC / AC conversion structure, which eliminates the large-capacity DC bus energy storage link in the traditional AC-DC-AC induction heating power supply, which helps to reduce the number of power conversion stages, simplify the system structure, and improve the system power density and reliability.

[0016] Secondly, this invention employs a minimum phase clamping pulse density modulation method based on Clarke transform, which performs reference vector error accumulation and H / L / Z candidate vector selection in the αβ stationary coordinate system. Additional terms such as light load energy penalty, switching state switching penalty, or input current feedback correction can be added to the error cost, so that the three-phase input current pulse distribution changes with the grid-side voltage vector, thereby reducing the grid-side input current THD and improving the grid-side power factor.

[0017] Third, when the zero effective input vector Z is selected, the present invention adopts the minimum phase clamp freewheeling mode, so that the resonant current still has a continuous path in the non-injection state, which can suppress current spikes, voltage spikes and high-frequency parasitic oscillations caused by full shutdown or additional bypass switches.

[0018] Fourth, the present invention uses a CLLC-type high-frequency resonant network, which improves the impedance matching capability of the induction heating load, which is beneficial to expanding the soft switching range and reducing system losses and electromagnetic interference.

[0019] Fifth, this invention achieves high output power and high operating efficiency while maintaining a simple structure, making it suitable for induction heating power supplies with high requirements for output power capability, input current quality, and operational reliability. It is particularly suitable for medium- or high-frequency induction heating of graphite substrates, graphite trays, graphite thermal field components, or equivalent induction heating elements in SiC epitaxial growth equipment, meeting the requirements of such equipment for continuous power regulation, low input current harmonics, resonant current continuity, and soft-switching operation. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the low-harmonic three-phase to single-phase direct AC / AC high-frequency induction heating power supply structure of the present invention. Figure 2 This is a schematic diagram of the voltage sector division on the three-phase grid side of the present invention; Figure 3 This is a control block diagram of the minimum phase clamp pulse density modulation strategy of the present invention; Figure 4 Schematic diagram of different operating modes of a three-phase to single-phase direct AC / AC converter; Figure 5 A schematic diagram of the switching process of the minimum phase clamp pulse density modulation strategy in sector 1; Figure 6 Schematic diagram of output waveforms under different power switching conditions; Figure 7 A schematic diagram comparing the grid-side input current THD of the minimum phase clamping pulse density modulation strategy with that of the existing single-phase modulation method. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the protection scope of the present invention.

[0022] like Figure 1As shown, the low-harmonic three-phase to single-phase direct AC / AC high-frequency induction heating power supply provided in this embodiment includes a grid-side filter, a three-phase to single-phase AC converter unit, a CLLC-type high-frequency resonant network, an induction heating load, and a control unit. The grid-side filter is connected to the three-phase AC power supply and is used to filter out the high-frequency current component on the three-phase AC input side. The three-phase to single-phase AC converter unit is connected between the grid-side filter and the CLLC-type high-frequency resonant network and is used to directly convert the three-phase power frequency AC power into single-phase high-frequency AC power. The CLLC-type high-frequency resonant network is connected between the three-phase to single-phase AC converter and the induction heating load. It is used to achieve impedance matching of the induction coil, the heated workpiece, and the equivalent induction heating load, high-frequency resonant energy transmission, and soft switching of the power switching devices. This improves the continuity of the resonant current during pulse density power modulation, supporting higher output power and higher efficiency operation. The induction heating load is the equivalent load formed by the induction coil and the equivalent induction heating load. The control unit is used to realize three-phase grid-side voltage detection, working sector division, three-phase pulse density modulation, switch state allocation, and non-injection clamping freewheeling control. No large-capacity DC bus energy storage capacitor is installed between the three-phase to single-phase AC converter and the CLLC-type high-frequency resonant network; the three-phase AC power supply directly transmits energy to the CLLC-type high-frequency resonant network via the grid-side filter and the three-phase to single-phase AC converter.

[0023] The three-phase to single-phase AC converter unit includes three sets of switching units corresponding to phases A, B, and C, respectively. Each set of switching units includes a positive power switch and a negative power switch. The positive power switch is used to form an energy transmission path or clamp a continuation path during the high-frequency positive half-cycle, and the negative power switch is used to form an energy transmission path or clamp a continuation path during the high-frequency negative half-cycle. Therefore, the three-phase power frequency AC power can directly supply energy to the high-frequency resonant network through the three-phase to single-phase AC converter unit without needing to pass through a large-capacity DC bus energy storage stage.

[0024] The CLLC-type high-frequency resonant network includes a grid-side resonant capacitor C. ra C rb C rc and series resonant inductor L r Parallel resonant capacitor C P Parallel resonant inductor L P Equivalent load R. Wherein, the grid-side resonant capacitance C... ra C rb C rc With series resonant inductor L r This forms a series resonant branch, with a parallel resonant capacitor C. P With parallel resonant inductor L P This forms a parallel resonant branch. By appropriately setting C...ra C rb C rc L r C P L p The parameter relationship between the three-phase to single-phase AC converter and the equivalent load R can enable the three-phase to single-phase AC converter to obtain the equivalent impedance characteristics suitable for soft switching near the target operating frequency, thereby reducing switching losses and electromagnetic interference, and improving the operating efficiency of the induction heating power supply under high output power conditions.

[0025] Furthermore, the control unit includes a phase-locking module, a three-phase reference current generation module, a Clarke transform module, an αβ domain error accumulation module, a sector determination module, an H / L / Z candidate vector selection module, and a drive logic module. Specifically: The phase-locking module is used to acquire the phase of the three-phase grid-side voltage, and the three-phase reference current generation module is used to generate three-phase reference current signals based on the phase of the three-phase grid-side voltage and the pulse density modulation coefficient. The three-phase reference current signals are respectively I... A *=m*cos(θ), I B *=m*cos(θ-2π / 3) and I C =m*cos(θ+2π / 3), where m is the pulse density modulation coefficient and θ is the phase of the three-phase grid voltage.

[0026] The Clarke converter module is used to convert the three-phase reference current signal I... A *、I B *、I C The * is transformed into reference vectors Iα* and Iβ* in the αβ stationary coordinate system, and the number of energy injection pulses per unit time is changed by adjusting the pulse density modulation coefficient m to achieve the adjustment of the induction heating output power.

[0027] The transformation process is as follows: assuming the three-phase reference current and the grid-side three-phase voltage are in phase, their expressions are respectively: I A *=m*cos(θ), I B *=m*cos(θ-2π / 3) and I C Since *=m*cos(θ+2π / 3), the three-phase reference current can be written as follows after Clarke transformation:

[0028] Substituting the three-phase reference currents into the above equation, we get Iα*=m*cosθ, Iβ*=m*sinθ.

[0029] Therefore, the three-phase sinusoidal reference current can be equivalently represented as a rotating reference vector in the αβ stationary coordinate system:

[0030] The sector determination module is used to divide one power frequency cycle into six working sectors based on the instantaneous amplitude relationship of the three-phase grid-side voltage. Then, based on the grid-side voltage phase provided by the phase-locked loop, it determines the current sector in real time and derives the specific maximum phase voltage, intermediate phase voltage, and minimum phase voltage, laying the foundation for subsequent H / L / Z candidate vector selection. Figure 2 As shown, the working sector is used to determine the phase combination of candidate vectors within the current high-frequency cycle and the switching phases participating in energy injection or clamping freewheeling. By dynamically selecting the conducting phases according to the three-phase voltage vector relationship, the three-phase input current pulse distribution can be matched with the three-phase grid-side voltage changes, thereby reducing the harmonic content of the grid-side input current.

[0031] The αβ domain error accumulation module is used to accumulate the error of the reference vector cycle by cycle; the H / L / Z candidate vector selection module is used to construct a high-energy injection vector H, a low-energy injection vector L, and a zero effective input vector Z in the current working sector, calculate the αβ domain error cost after the candidate vectors are applied, and select the candidate vector with the smallest error cost as the actual output vector of the current high-frequency cycle; the αβ domain error cost includes a reference vector tracking error term, and may further include at least one of the following additional terms: a light load energy penalty term, a switching state switching penalty term, and an input current feedback correction term.

[0032] Specifically, let the candidate vector of the current high-frequency cycle be... Where H is the high-energy injection vector, L is the low-energy injection vector, and Z is the minimum phase clamp freewheeling vector. If the current cycle prediction cumulative error is... Candidate vectors in The expression in the coordinate system is Then the basic cost function can be written as:

[0033] This reflects the deviation between the average vector of the input current and the reference vector after the candidate vector is applied. The controller selects the state that minimizes the error cost by comparing the error costs of the three candidate states (H, L, and Z). The output vector of the domain accumulation error.

[0034] To improve control flexibility over a wide power range, based on the basic error term, a light-load energy penalty term, a switching state transition penalty term, and an input current feedback correction term can be further introduced, resulting in an extended cost function:

[0035] in, This is the per-unit reference value for output power. Indicates the energy level of the candidate vector. This represents the amount of switch state change required to switch from the previous cycle state to the current candidate state. The actual input current feedback error, This is the scaling factor between the candidate vector and the change in input current. , and These are the weighting coefficients for light load energy penalty, switching penalty, and current feedback correction, respectively.

[0036] The drive logic module is used to output drive signals for each power switching device according to the actual output vector.

[0037] like Figure 3 As shown, the minimum phase clamping pulse density modulation method for the low harmonic three-phase to single-phase direct AC / AC high-frequency induction heating power supply of the present invention is as follows: Step 1: Collect the three-phase grid-side voltage U A U B U C The phase and instantaneous amplitude relationship of the three-phase grid side voltage is obtained; the resonant frequency of the resonant cavity is obtained through the port voltage Ui and the output voltage Uo.

[0038] Step 2: Generate a three-phase reference current signal I with a phase difference of 120° based on the phase of the three-phase grid-side voltage. A *、I B *、I C * and set the pulse density modulation coefficient m according to the target output power.

[0039] Step 3: For the three-phase reference current signal I... A *、I B *、I C Perform Clarke transformation to obtain reference vectors Iα* and Iβ* in the αβ stationary coordinate system; divide the current working sector according to the phase or instantaneous amplitude relationship of the three-phase grid side voltage, and determine the maximum phase voltage, intermediate phase voltage and minimum phase voltage in the current working sector.

[0040] Step 4: Using the resonant frequency of the resonant cavity load obtained by the phase-locked loop as a reference and the trigger signal, input the reference vectors Iα* and Iβ* in the αβ stationary coordinate system into the αβ domain error accumulator to form the prediction error vector of the current high-frequency cycle.

[0041] Step 5: Establish a candidate vector set of H / L / Z based on the current working sector, where H is the high-energy injection vector, L is the low-energy injection vector adjacent to H, and Z is the zero effective input vector. The control unit calculates the αβ domain error cost after the effects of H, L, and Z, and selects the candidate vector with the smallest error cost as the actual output vector for the current high-frequency cycle. The αβ domain error cost includes a reference vector tracking error term, and at least one additional term can be superimposed based on the operating conditions, such as a light-load energy penalty term, a switch state switching penalty term, and an input current feedback correction term. When the actual output vector is H or L, the system enters the energy injection state, and conducts the corresponding positive power switch in the positive half-cycle of the high-frequency period and the corresponding negative power switch in the negative half-cycle of the high-frequency period according to the selected injection vector, so that the three-phase input current pulses are distributed according to the grid-side voltage vector relationship. Thus, the average change trend of the three-phase input current can follow the change of the three-phase grid-side voltage, thereby reducing the low-order harmonic content and improving the grid-side power factor.

[0042] Step 6: When the actual output vector is Z, the system enters the non-injection clamp freewheeling state. During the high-frequency positive half-cycle, the positive power switch corresponding to the minimum voltage is turned on, and during the high-frequency negative half-cycle, the negative power switch corresponding to the same minimum voltage is turned on, forming a minimum phase clamp freewheeling path. This method can maintain the continuity of the resonant current path while reducing effective energy injection, avoiding the voltage forced clamping, parasitic LC ringing, and peak stress introduced by the independent bypass freewheeling branch, which is beneficial to improving the freewheeling continuity and system reliability in the non-injection state.

[0043] Step 7: Update the αβ domain error accumulation state according to the actual output vector, and adjust the pulse density modulation coefficient in the three-phase reference current signal through power closed-loop adjustment to realize the induction heating output power regulation, and match the three-phase input current pulse distribution with the three-phase grid-side voltage vector change.

[0044] In the energy injection state, the control unit selects a high energy injection vector H or a low energy injection vector L based on the current operating sector and the αβ domain error cost, and maps the selected injection vector to the corresponding positive and negative power switch combinations, so that the input current pulses are distributed among the three phases according to the voltage vector relationship.

[0045] Figure 4 This diagram illustrates the different operating modes of a three-phase to single-phase direct AC / AC converter, including three forward conduction modes (①, ②, ③) and three negative conduction modes (④, ⑤, ⑥). ① represents the forward conduction transmission of phase A, ② represents the forward conduction transmission of phase B, ③ represents the forward conduction transmission of phase C, ④ represents the negative conduction transmission of phase A, ⑤ represents the negative conduction transmission of phase B, and ⑥ represents the negative conduction transmission of phase C. A complete cycle consists of one forward conduction mode and one negative conduction mode. Figure 4As shown, in the energy injection state, the control unit maps the H or L candidate vectors selected in the αβ domain to the switching states of the three-phase to single-phase AC converter unit. The H or L corresponds to different positive and negative conduction mode combinations according to the current operating sector. Therefore, the input current pulses can be distributed among the three phases according to the voltage vector relationship, so that the average variation trend of the three-phase input current follows the variation of the three-phase grid-side voltage, thereby reducing low-order harmonic content and improving the grid-side power factor.

[0046] Continue to refer to Figure 4 In the non-injection clamping freewheeling state, the control unit does not employ a full shutdown method, nor does it use an additional bypass switch for hard clamping of the freewheeling. Instead, it utilizes the positive and negative conduction modes corresponding to the minimum phase voltage to alternately conduct during the high-frequency positive and negative half-cycles, forming a minimum phase clamping freewheeling path. This freewheeling path can maintain the continuity of the resonant current and reduce the current surge during the switching between the energy injection state and the non-injection state.

[0047] like Figure 5 As shown, during the switching process of sector 1, the control unit determines the corresponding H / L / Z switch combination based on the current αβ domain error state and the relevant candidate vector set of the sector, and coordinates with the bidirectional switch parallel capacitor C. ra C rb C rc The charging and discharging process enables clamping and freewheeling, thereby creating conditions for zero-voltage turn-off and zero-current turn-on of power switching devices.

[0048] Taking sector [0, π / 6] as an example, at this time... U A The maximum phase voltage, U B This is the intermediate phase voltage. U C This is the minimum phase voltage. The control unit selects the H / L / Z switch combination based on the current αβ domain error state and the candidate vector set. If the high-energy injection vector H is selected, the system conducts during the positive half-cycle. S ap Negative half-cycle conduction S cn ,form A + C - Injection state; if a low-energy injection vector L is selected, then the positive half-cycle is on. S bp The negative half-cycle is still conducting. S cn ,form B + C -Injection states. Both H and L are effective energy injection modes, with H having a larger equivalent voltage difference and higher single-cycle injection energy; L has a smaller equivalent voltage difference and lower single-cycle injection energy. By appropriately allocating the action ratio of H and L, basic switching states can be provided for input current shaping and output power regulation.

[0049] The picture shows S ap and S cn Taking alternating operation as an example, we can illustrate the soft-switching commutation process within one high-frequency cycle. In Mode 1, S ap When the resonant cavity is turned on, the input current is... I p through S ap , L r , C p And the load branch flows, transferring energy to the induction heating load. After entering mode 2 and mode 3, S ap Shutdown, due to L r The current in the medium cannot change abruptly. I p Continue by using a parallel buffer capacitor connected to a switch. C sa , C sb , C sc and input-side return capacitor C ra , C rb , C rc Forming a switching path, enabling S ap The voltage across the terminals gradually builds up from zero, thus achieving near-zero voltage turn-off.

[0050] In mode 4, the resonant current approaches zero and begins to reverse. S cn The circuit is switched on under near-zero current conditions, achieving zero-current turn-on. Mode 5 then represents the stable energy transfer phase during the negative half-cycle. I p through S cn The flow is in the opposite direction to the resonant network. In modes 6 and 7, S cn The circuit is turned off, and the node voltage commutation is completed by the buffer capacitor, allowing the voltage across it to establish a finite slope, achieving zero-voltage turn-off. In mode 8, the current again approaches zero. Sap The system restarts under near-zero current conditions and enters the next high-frequency cycle. Overall, the natural zero-crossing of the resonant current and the charging and discharging process of the buffer capacitor together ensure the soft commutation of the positive and negative switches and provide a continuous and reliable working basis for high-frequency energy injection under H / L pulse distribution.

[0051] Where H corresponds to A + C - L corresponds to B + C - The soft-switching process can be summarized as follows: during shutdown, utilizing... I p ZVS is achieved by charging and discharging the buffer capacitor, and during turn-on, it utilizes... I p Natural zero crossings are used to achieve ZCS.

[0052] Figure 6 The diagrams show the output waveforms under different power switching conditions. (a) shows the waveform when the pulse density modulation coefficient m switches from 1 to 0.6; (b) shows a magnified view of the waveform when the pulse density modulation coefficient m=1; and (c) shows a magnified view of the waveform when the pulse density modulation coefficient m=0.6. It can be seen that when the pulse density modulation coefficient m switches from 1 to 0.6, the system can change the occurrence density of the energy injection pulse according to different power commands. When the pulse density is higher, the number of high-frequency cycles entering the energy injection state per unit time increases, and the induction heating output power increases. When the pulse density is lower, the number of high-frequency cycles entering the non-injection clamping freewheeling state per unit time increases, and the output power decreases. Since the non-injection clamping freewheeling state still provides a continuous path for the resonant current, the high-frequency resonant current under different pulse densities can maintain good continuity, and the output power adjustment process is smoother.

[0053] like Figure 7 As shown, compared with traditional fixed phase selection, maximum line voltage injection, or single-phase pulse density modulation methods, the minimum phase clamping pulse density modulation method based on Clarke transform described in this invention can make the three-phase input current pulse distribution closer to the three-phase grid-side voltage variation law, thereby reducing the total harmonic distortion rate of the grid-side input current, improving the input power factor, and improving the grid-connected current quality of high-power induction heating power supplies.

[0054] Although the present invention has been described in detail above with general descriptions and specific embodiments, some modifications or improvements can be made to it. The above descriptions are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Other changes and modifications made by those skilled in the art without departing from the spirit and scope of the present invention are still included within the scope of protection of the present invention.

Claims

1. A low-harmonic three-phase to single-phase direct AC / AC high-frequency induction heating power supply, comprising a grid-side filter, a three-phase to single-phase AC converter unit, a CLLC-type high-frequency resonant network, an induction heating load, and a control unit, characterized in that: The three-phase input terminals of the grid-side filter are respectively connected to the three-phase output terminals of the three-phase AC power supply, and the three-phase output terminals of the grid-side filter are respectively connected to the three-phase input terminals of the three-phase to single-phase AC converter unit. The grid-side filter is used to filter out the high-frequency current component on the three-phase AC input side. The three-phase to single-phase AC converter unit includes multiple power switching devices. The output terminal of the three-phase to single-phase AC converter unit is connected to the input terminal of the CLLC type high-frequency resonant network. The three-phase to single-phase AC converter unit is used to directly convert the three-phase power frequency AC power filtered by the grid-side filter into single-phase high-frequency AC power by turning on and off the multiple power switching devices. The output terminal of the CLLC-type high-frequency resonant network is connected to the electrical input terminal of the induction heating load. The CLLC-type high-frequency resonant network is used to receive the single-phase high-frequency AC power output by the three-phase to single-phase AC converter unit and transmit high-frequency resonant energy to the induction heating load. The control signal output terminal of the control unit is connected to the control terminal signal of each power switching device in the three-phase to single-phase AC converter unit. The control unit is used to output a switch control signal according to the three-phase pulse density modulation signal to control the conduction and cutoff of each power switching device.

2. The low harmonic three-phase to single-phase direct AC / AC high-frequency induction heating power supply according to claim 1, characterized in that, The three-phase to single-phase AC converter unit includes three sets of switching units corresponding to phases A, B, and C, respectively. Each set of switching units includes a positive power switch and a negative power switch. The positive power switch is used to conduct during the positive half-cycle of high frequency, and the negative power switch is used to conduct during the negative half-cycle of high frequency.

3. The low harmonic three-phase to single-phase direct AC / AC high-frequency induction heating power supply according to claim 1, characterized in that, The CLLC-type high-frequency resonant network includes a grid-side resonant capacitor C. ra C rb C rc and series resonant inductor L r Parallel resonant capacitor C P Parallel resonant inductor L P Equivalent load R; the grid-side resonant capacitor C ra C rb C rc With the series resonant inductor L r The parallel resonant capacitor C forms a series resonant branch. P With the parallel resonant inductor L P The CLLC-type high-frequency resonant network forms a parallel resonant branch and is used to improve the impedance characteristics of the resonant cavity and create conditions for soft switching of power switching devices.

4. The low harmonic three-phase to single-phase direct AC / AC high-frequency induction heating power supply according to claim 1, characterized in that, The control unit includes a phase-locking module, a three-phase reference current generation module, a Clarke transform module, an αβ domain error accumulation module, a sector determination module, an H / L / Z candidate vector selection module, and a drive logic module. The phase-locking module acquires the phase of the three-phase grid-side voltage. The three-phase reference current generation module generates a three-phase reference current signal based on the phase of the three-phase grid-side voltage and the pulse density modulation coefficient. The Clarke transform module transforms the three-phase reference current signal into a reference vector in the αβ stationary coordinate system. The αβ domain error accumulation module accumulates the error of the reference vector cycle by cycle. The H / L / Z candidate vector selection module calculates the αβ domain error cost after the candidate vector is applied within the current working sector and selects the candidate vector with the smallest error cost as the actual output vector for the current high-frequency cycle. The drive logic module is used to output drive signals for each power switching device according to the actual output vector.

5. The low harmonic three-phase to single-phase direct AC / AC high-frequency induction heating power supply according to claim 4, characterized in that, The control unit divides one power frequency cycle into six working sectors based on the instantaneous amplitude relationship of the three-phase grid side voltage, and determines the maximum phase voltage, the second largest phase voltage, and the minimum phase voltage in each working sector, thereby laying the foundation for the subsequent selection of H / L / Z candidate vectors.

6. The low harmonic three-phase to single-phase direct AC / AC high-frequency induction heating power supply according to claim 4, characterized in that, The H / L / Z candidate vector selection module constructs a high-energy injection vector H, a low-energy injection vector L, and a zero effective input vector Z within the current working sector; the αβ domain error cost includes a reference vector tracking error term; when the high-energy injection vector H or the low-energy injection vector L is selected, the control unit controls the system to enter the energy injection state; When the actual output vector is Z, the control unit controls the system to enter a non-injection clamping freewheeling state.

7. The low harmonic three-phase to single-phase direct AC / AC high-frequency induction heating power supply according to claim 6, characterized in that, The non-injection clamp freewheeling state does not use an additional bypass switch for freewheeling. Instead, it uses the alternating conduction of the positive power switch and the negative power switch corresponding to the minimum phase voltage to form a freewheeling path.

8. The low harmonic three-phase to single-phase direct AC / AC high-frequency induction heating power supply according to claim 6, characterized in that, The control unit is equipped with a dead time; when switching between the energy injection state and the non-injection clamp freewheeling state, the control unit keeps the relevant power switching devices off for a preset dead time.

9. The low harmonic three-phase to single-phase direct AC / AC high-frequency induction heating power supply according to claim 1, characterized in that, No large-capacity DC bus energy storage capacitor is installed between the three-phase to single-phase AC converter unit and the CLLC type high-frequency resonant network. The three-phase AC power supply directly transmits energy to the CLLC type high-frequency resonant network through the grid-side filter and the three-phase to single-phase AC converter unit.

10. A minimum phase clamping pulse density modulation method applied to the low harmonic three-phase to single-phase direct AC / AC high-frequency induction heating power supply according to any one of claims 1 to 9, characterized in that, Includes the following steps: Step 1: Collect the three-phase grid-side voltage and obtain the phase and instantaneous amplitude relationship of the three-phase grid-side voltage; Step 2: Generate a three-phase reference current signal with a phase difference of 120° based on the phase of the three-phase grid-side voltage, and set the pulse density modulation coefficient m according to the target output power; Step 3: Perform Clarke transform on the three-phase reference current signal to obtain the reference vector in the αβ stationary coordinate system; divide the current working sector according to the phase or instantaneous amplitude relationship of the three-phase grid-side voltage, and determine the maximum phase voltage, intermediate phase voltage and minimum phase voltage in the current working sector; Step 4: Using the resonant frequency of the resonant cavity load obtained by the phase-locked loop as a reference and the trigger signal, input the reference vector in the αβ stationary coordinate system into the αβ domain error accumulator to form the prediction error vector of the current high-frequency cycle; Step 5: Establish a set of candidate vectors for H / L / Z based on the current working sector; the control unit calculates the αβ domain error cost after the action of H, L, and Z respectively, and selects the candidate vector with the smallest error cost as the actual output vector of the current high-frequency cycle; When the actual output vector is H or L, the system enters the energy injection state and turns on the corresponding positive power switch in the high-frequency positive half-cycle and the corresponding negative power switch in the high-frequency negative half-cycle according to the selected injection vector, so that the three-phase input current pulses are distributed according to the grid-side voltage vector relationship. Step 6: When the actual output vector is Z, the system enters the non-injection clamp freewheeling state. During the high-frequency positive half-cycle, the positive power switch corresponding to the minimum voltage is turned on, and during the high-frequency negative half-cycle, the negative power switch corresponding to the same minimum voltage is turned on, forming the minimum phase clamp freewheeling path. Step 7: Update the αβ domain error accumulation state according to the actual output vector, and adjust the pulse density modulation coefficient in the three-phase reference current signal through power closed loop to realize the adjustment of induction heating output power and match the three-phase input current pulse distribution with the change of three-phase grid-side voltage vector; In the energy injection state, the control unit selects vector H or vector L according to the current working sector and αβ domain error cost, and maps the selected injection vector to the corresponding positive and negative power switch combinations, so that the input current pulse is distributed among the three phases according to the voltage vector relationship.

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