A single-stage high-frequency isolated DC / AC inverter

Abstract

The present invention belongs to the technical field of inverter control, and provides a single-stage high-frequency isolation DC / AC inverter, which includes a primary-side bridge inverter circuit, a high-frequency transformer, a series resonance cavity, and a secondary-side AC circuit connected in sequence. The inverter is modulated by a control circuit, and an inverter modulation method is set in the control circuit, which includes the steps of: determining the internal phase shift angle between the primary-side bridge arms, the external phase shift angle between the primary-side bridge arms and the secondary-side bridge arms, and the switching frequencies of the primary-side switching tubes and the secondary-side switching tubes based on the fundamental wave approximation algorithm to modulate the control quantities of the inverter, and the control quantities include power transmission characteristics and soft-switching characteristics; calculating the exact current value at the switching moment of the primary-side switching tubes to compensate the control quantities according to the exact current value. Based on the fundamental wave approximation method, the present invention can directly obtain the numerical expression of the control quantity without looking up a table, and the execution is simple. Only by sampling the voltage signals on both the input and output sides and entering them into a digital processor for a small amount of calculation can the PWM drive signals of each switching tube be output to modulate the inverter. Moreover, on the basis of the fundamental wave approximation, a soft-switching precise compensation strategy based on numerical calculation is added, which can ensure zero-voltage turn-on of all switching tubes and guarantee the efficiency of the converter.

Description

Technical Field

[0001] The present invention relates to the technical field of inverter control, and particularly to a single-stage high-frequency isolation DC / AC inverter. Background Art

[0002] Green energy is becoming the main direction of the current energy transformation. Among them, solar energy, as a clean energy source with rich resources and wide distribution, is one of the main directions of the current new energy development. If solar energy is to be converted into usable electric energy, a DC / AC inverter is required to convert the direct current of the photovoltaic panel into the alternating current of the power grid. Considering equipment and personal safety, a DC / AC inverter usually needs to be electrically isolated, that is, an isolated DC / AC converter.

[0003] Isolated DC / AC converters can be divided into two-stage and single-stage according to the number of topological power conversions. Compared with the two-stage type, the single-stage isolated DC / AC converter can reduce one stage of power conversion and has potential advantages such as high efficiency, high power density, and low cost. Currently, it has received extensive attention and research.

[0004] For isolated DC / AC converters, phase-shift modulation is one of the main modulation strategies for driving their power circuits. Phase-shift modulation transfers power by changing the phase-shift angle of the switch tube bridge arm. For the dual-active-bridge topology, high-frequency rectification / inversion units on both sides of the high-frequency transformer use power switch tubes. By modulating the phase difference between the drive signals of each switch tube, the effect of phase-shift modulation can be achieved. Therefore, this modulation is commonly used and easy to implement in the dual-active-bridge topology. Based on different phase-shift operating conditions, various phase-shift modulation strategies such as single-phase shift, extended phase shift, double-phase shift, and triple-phase shift have emerged.

[0005] For a resonant single-stage DC / AC converter with at least two passive components in the cavity, the high-frequency inductor current in the cavity changes non-linearly. Therefore, its mathematical modeling process is relatively complex, and it is difficult to directly derive and calculate the relationship between the control freedom and state variables through an analytical expression. If the modulation strategy of the converter is to be optimized to ensure that the converter achieves soft switching, a small effective value of the high-frequency current, and a small switch-off current of the switch tube under different loads and voltage gains, and to optimize the converter efficiency, it is necessary to analyze the working modes and corresponding soft-switching conditions of the resonant converter to find the control method most suitable for the resonant single-stage DC / AC converter. Summary of the Invention

[0006] The purpose of the present invention is to provide a matrix-switching single-stage isolated DC / AC converter, which adopts a modulation strategy combining extended phase shift and frequency modulation, and on this basis, a precise soft-switching compensation method based on numerical calculation is added to solve the above problems.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A single-stage high-frequency isolation DC / AC inverter, which includes a primary-side bridge inverter circuit, a high-frequency transformer, a series resonance cavity, and a secondary-side AC circuit connected in sequence. The primary-side bridge inverter circuit includes four switching tubes to form an H-bridge. The series resonance cavity contains a resonance inductor and a resonance capacitor. The secondary-side AC circuit contains at least two groups of switching tube groups to form a first bridge circuit. The first bridge circuit includes two bridge arms, and the midpoints of the bridge arms are electrically connected to the series resonance cavity;

[0009] The inverter is modulated by a control circuit, and an inverter modulation method is set in the control circuit. The method includes the steps:

[0010] Based on the fundamental wave approximation algorithm, determine the internal phase shift angle between the primary-side bridge arms, the external phase shift angle between the primary-side bridge arms and the secondary-side bridge arms, and the switching frequencies of the primary-side switching tubes and the secondary-side switching tubes to modulate the control quantities of the inverter. The control quantities include power transfer characteristics and soft-switching characteristics;

[0011] The power transfer characteristics determine the waveform quality of the secondary-side alternating current, and the soft-switching characteristics determine the power loss generated when the switching tube is turned off or on.

[0012] Further, the power transfer characteristics are determined by the average value of the resonance current transferred to the AC-side output terminal in each switching period in the series resonance cavity; the soft-switching characteristics include that the current direction of the secondary-side switching tube in the dead time before its turn-on is the same as the direction of its body diode, and the charging and discharging of the output junction capacitance on both sides of the drain-source of the primary-side switching tube are completed during the dead time of the bridge arm switching tube.

[0013] Further, according to the phase of the resonance current phasor, it is divided into an active current phasor and a reactive current phasor, and the power transfer characteristics of the inverter are controlled by the modulus of the active current phasor.

[0014] Further, the modulus of the active current phasor is used to control the current direction of the secondary-side switching tube in the dead time before its turn-on to be the same as the direction of its body diode.

[0015] Further, the charging and discharging of the output junction capacitance on both sides of the drain-source of the primary-side switching tube are controlled by the resonance current, the turns ratio of the high-frequency transformer, and the phase shift angle. The phase shift angle is determined by the internal phase shift angle and the external phase shift angle.

[0016] Further, the method further includes the step: calculating the accurate current value at the switching moment of the primary-side switching tube to compensate the control quantity according to the accurate current value.

[0017] Further, the step of compensating the control quantity according to the precise current value includes:

[0018] Solving the precise current value at the switching moment of the primary-side switching tube based on the differential equations of the resonant inductor current and the resonant capacitor voltage under different working modes;

[0019] Determining the error value of the turn-on current according to the turn-on current value of the primary-side switching tube and the precise current value;

[0020] Based on the error value, a new switching frequency of the primary-side switching tube is obtained through PI regulation, and the control quantity of the inverter is modulated according to the new switching frequency of the primary-side switching tube.

[0021] Further, in the two arms of the first bridge circuit, two sets of switching tube groups form the first arm, and the second arm is formed by two sets of switching tube groups or two capacitors; the midpoint of the first arm and the midpoint of the second arm are both connected to the series resonant cavity.

[0022] Further, when each switching tube group in the switching tube group of the first bridge circuit is configured as a switching tube, the secondary-side AC circuit further includes a second bridge circuit, which is connected in parallel with the first bridge circuit and is configured to output alternating current.

[0023] Further, the second bridge circuit includes four sets of switching tube groups, and each switching tube group only includes one switching tube.

[0024] Compared with the prior art, the present invention has at least the following beneficial effects:

[0025] (1) The single-stage structure adopted by the present invention has only one-stage power conversion, and has advantages such as high efficiency, high power density and low cost;

[0026] (2) The present invention first performs fundamental wave equivalence on the main circuit based on the fundamental wave approximation method, analyzes the conditions required for the modulation strategy from two perspectives of power transmission characteristics and soft-switching characteristics, and on this basis, proposes a modulation strategy combining extended phase shift and frequency modulation, which can theoretically achieve soft switching of all switching tubes within the full voltage range, effectively reduce the effective value of the current in the resonant cavity, and improve the efficiency of the converter;

[0027] (3) Based on the fundamental wave approximation method, the numerical expression of the control quantity can be directly obtained without looking up a table, the execution is simple, and only the voltage signals on both sides of the input and output need to be sampled and enter the digital processor for a small amount of calculation to output the PWM drive signals of each switching tube to modulate the power circuit;

[0028] (4) To ensure that the converters can all operate in the soft-switching state, the present invention adds a precise soft-switching compensation method based on numerical calculation on the basis of fundamental wave approximation, which can ensure zero-voltage turn-on of all switching tubes and guarantee the efficiency of the converter. Description of the Drawings

[0029] Figure 1 is a schematic diagram of the photovoltaic DC-to-AC topology in the embodiment provided by the present invention;

[0030] Figure 2 is the circuit diagram of the main circuit topology of the inverter in Embodiment 1 provided by the present invention;

[0031] Figure 3 is the flowchart of the steps of the inverter modulation method in the embodiment provided by the present invention;

[0032] Figure 4 is the waveform diagram of the square wave of the voltage at the midpoints A and B of the AC-side bridge arm (when the AC voltage is positive) in the embodiment provided by the present invention;

[0033] Figure 5 is the waveform diagram of the square wave of the voltage at the midpoints A and B of the AC-side bridge arm (when the AC voltage is negative) in the embodiment provided by the present invention;

[0034] Figure 6 is the waveform diagram of the square wave of the voltage at the midpoints C and D of the DC-side bridge arm in the embodiment provided by the present invention;

[0035] Figure 7 is the equivalent circuit diagram of the series resonant cavity in the embodiment provided by the present invention;

[0036] Figure 8 is the phase shift angle relationship diagram of the alternating square wave voltage in the embodiment provided by the present invention;

[0037] Figure 9 is the phasor diagram of the high-frequency voltage and current under fundamental wave approximation in the embodiment provided by the present invention;

[0038] Figure 10 is the time-domain steady-state waveform and the corresponding phasor diagram in the series resonant cavity in the embodiment provided by the present invention;

[0039] Figure 11 is the phasor diagram of each voltage and current signal and some drive signals under extended phase shift modulation in the embodiment provided by the present invention;

[0040] Figure 12 is the schematic diagram of compensating soft switching based on numerical calculation in the embodiment provided by the present invention;

[0041] Figure 13 is the circuit diagram of the main circuit topology of the inverter in Embodiment 2 provided by the present invention;

[0042] Figure 14 is the circuit diagram of the main circuit topology of the inverter in the third embodiment provided by the present invention;

[0043] Figure 15 is the circuit diagram of the main circuit topology of the inverter in the fourth embodiment provided by the present invention. Detailed implementation manners

[0044] It should be noted that all directional indications (such as up, down, left, right, front, back...) in the embodiments of the present invention are only used to explain the relative positional relationship, movement conditions, etc. between components in a specific posture (as shown in the drawings). If the specific posture changes, the directional indications will also change accordingly.

[0045] In addition, in the present invention, descriptions such as "first", "second", "one", etc. are only for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the quantity of the indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include at least one such feature. In the description of the present invention, "a plurality" means at least two, such as two, three, etc., unless otherwise specifically defined.

[0046] In the present invention, unless otherwise clearly specified and limited, terms such as "connection" and "fixation" should be understood in a broad sense. For example, "fixation" can be a fixed connection, a detachable connection, or integrated; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and can be the internal connection of two components or the interaction relationship between two components, unless otherwise clearly limited. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0047] In addition, the technical solutions between various embodiments of the present invention can be combined with each other, but it must be based on the ability of those of ordinary skill in the art to implement. When the combination of technical solutions results in contradictions or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the protection scope required by the present invention.

[0048] The following are specific embodiments of the present invention, and the technical solutions of the present invention will be further described in conjunction with the drawings, but the present invention is not limited to these embodiments.

[0049] As Figure 1 shown, a single-stage isolated DC / AC inverter, its main circuit consists of three parts: a primary-side bridge inverter circuit, a secondary-side AC circuit, and a series resonant cavity and a high-frequency transformer in the middle.

[0050] Embodiment 1

[0051] In this embodiment, as Figure 2 shown, the secondary side (AC side) of the circuit adopts a full-bridge structure, and two bridge arms of the full-bridge are composed of four groups of bidirectional matrix switching tubes S5 to S 12 . The primary side (DC side) adopts an H-bridge composed of switching tubes S1 to S4. The turns ratio of the high-frequency transformer T is n, and the series resonance cavity contains a resonance inductor L r and a resonance capacitor C r .

[0052] This topology is a single-stage dual-active bridge structure, with bidirectional matrix switching tubes as the AC side bridge arms, having a simple structure and no need for an additional rectification link. The duty cycle output of the switching tubes is controlled by the PWM drive signal output by the control circuit, so as to realize the control and output regulation of the inverter.

[0053] As Figure 3 shown, a modulation method of the inverter is set in the control circuit. This embodiment proposes a modulation method combining extended phase shift and frequency modulation, which includes the steps:

[0054] S1. Based on the fundamental wave approximation algorithm, determine the internal phase shift angle between the primary side bridge arms, the external phase shift angle between the primary side bridge arms and the secondary side bridge arms, and the switching frequencies of the primary side switching tubes and the secondary side switching tubes, so as to modulate the control quantities of the inverter. The control quantities include power transmission characteristics and soft switching characteristics;

[0055] S2. Calculate the accurate current values at the switching moments of the primary side switching tubes, so as to compensate the control quantities according to the accurate current values.

[0056] In this modulation method, first, the fundamental wave approximation method is used to analyze the power circuit of the DC / AC inverter. Since the AC input voltage of the DC / AC inverter is time-varying, for the convenience of analysis, the following assumptions are made: (1) The inverter is in a steady-state operation state; (2) within one switching period, it is approximately considered that the AC input voltage remains unchanged.

[0057] According to the characteristics of the matrix switch-type circuit, four of the four groups of bidirectional tubes on the AC side work at the power frequency, and the other four work at high frequency. The drive and bridge arm voltage waveforms on its AC and DC sides are as Figures 4 - 6 shown. When the AC voltage is positive, the switching tubes S7 and S8 are always on, the switching tubes S5 and S6 work at high frequency. At the same time, since no additional internal phase shift is added on the AC side, the drive signals of the control switching tubes S 11 and S 12 are the same as those of S7 and S8, and the drive signals of S9 and S 10 are the same as those of S5 and S6. At this time, a square wave voltage v ab is generated at the midpoint AB of the AC side bridge arm, as Figure 4As shown. Similarly, when the AC voltage is negative, its driving and the voltage waveform at the midpoint of the bridge arm are as Figure 5 . The driving and the voltage waveform at the midpoint of the bridge arm of the DC-side H-bridge are as Figure 6 shown, and the output voltage of its bridge arm is v cd .

[0058] At the same time, define the bridge arm where the switching transistors S1 and S2 are located as the leading bridge arm, and the switching transistors S3 and S4 as the lagging bridge arm

[0059] At this time, the equivalent circuit of the resonant cavity in the converter is as Figure 7 shown, and the voltage expressions on the AC side and the DC side are v ab (ω s t) and nv cd (ω s t), respectively. Then, perform Fourier decomposition on them, and the fundamental component expressions of the two can be obtained as follows:

[0060]

[0061]

[0062] In the formula: the instantaneous value of the AC-side power supply voltage v ac , the DC-side power supply voltage V dc , the switching angular frequency ω s = 2πf s (f s is the switching frequency), the internal phase-shift angle θ of the DC-side leading bridge arm driving leading the lagging bridge arm driving; the external phase-shift angle of the AC-side voltage leading the DC-side voltage

[0063] At the same time, for the convenience of analysis, some parameters and variables of the inverter are defined here: the AC-DC voltage gain ratio M = v ac / nV dc , the resonant frequency f r = 1 / (2π×sqrt(L r ×C r ))), the impedance of the resonant cavity Z r = ω s L r –1 / ω s C r ;

[0064] Thus, the specific situations of the internal phase-shift angle θ and the external phase-shift angle during bidirectional operation are as follows Figure 8 shown.

[0065] Based on the results obtained by fundamental wave approximation, the fundamental waves of each voltage and current can be represented in the form of a phasor diagram, as Figure 9 shown. Define vab1,N (ω s t) has a phasor of nv cd1,N (ω s t) has a phasor of At this time, the voltage excitation applied across the resonator is On the premise that the switching frequency is higher than the resonant frequency, a resonant current i that lags behind the voltage excitation by 90° in phase will be generated r (ω s t). For the convenience of analysis, this article will use the phasor corresponding to the resonant current to be divided into an active current phasor according to the phase and a reactive current phasor The magnitudes I rp and I rq of the two current phasors are expressed as follows:

[0066]

[0067]

[0068] At this time, the time-domain expression of i r (ω s t) is expressed as follows:

[0069] i r (ω s t) = I rp ·sin(ω s t) + I rq ·cos(ω s t) (5)

[0070] For a DC / AC converter, first of all, its power transmission characteristics need to be concerned about, that is, whether the magnitude of the current instantaneously transmitted can follow the AC voltage to ensure a small total harmonic distortion THD (Total Harmonic Distortion) of the current to meet the grid connection requirements. Therefore, it is necessary to control the average value of the resonant current transmitted to the AC side output terminal in each switching period of the high-frequency resonator.

[0071] Secondly, what needs to be concerned about is the realization of soft switching of the switching tubes. To achieve soft switching, it is necessary to ensure that during the dead time before the switching tube is turned on, the current direction is the same as the direction of its body diode, and the output junction capacitance on both sides of the drain-source of the switching tube is charged and discharged during the dead time between the switching of the bridge arm switching tubes.

[0072] For the AC side switching tubes, when i r (ω s t) is ahead of v ab1,N (ω s t) in phase, that is, the switching tubes S5 and S10 When it is turned on, i r is positive, and its ZVS (Zero Voltage Switch) can be achieved. The same applies to switching tubes S6 and S9. For the DC-side switching tubes, nv cd1,N (ω s t) is ahead of i r (ω s t). When switching tubes S1 and S4 are turned on, i r is negative, and its ZVS can also be achieved. The same applies to switching tubes S2 and S3. The waveforms of the voltage and current phasors and the driving signals under the fundamental wave approximation method are as shown in Figure 10 shown below.

[0073] At this time, define the phasor corresponding to the fundamental wave of the driving square wave of switching tubes S1 and S4 as The phasor diagram at this time can be drawn as shown in Figure 11 shown below.

[0074] According to the above analysis, this embodiment adopts a three-degree-of-freedom modulation strategy combining extended phase shift and frequency modulation. The control variables are the internal phase shift angle θ, the external phase shift angle and the switching frequency f s . Matched with it are three conditions corresponding to the inverter power transmission characteristics and soft-switching characteristics mentioned above, namely:

[0075] (1) Condition 1

[0076] The first condition is the power transmission characteristic. By adjusting I rp the quality of the AC current waveform is ensured. According to the results derived above, substituting Equation (2) into Expression (3), we can obtain:

[0077]

[0078] (2) Condition 2

[0079] The second condition is to ensure that the AC-side switching tubes achieve soft switching. Since the AC-side H-bridge is a high-frequency rectification link, the direction of the high-frequency current is naturally consistent with the direction of the body diode of its switching tubes. By ensuring that is in the same phase as and appropriately increasing the dead time of the AC-side switching tubes, zero-voltage turn-on of all the AC-side switching tubes can be achieved. According to Equation (4), at this time, we have:

[0080] I rq = 0 (7)

[0081] (3) Condition 3

[0082] The third condition is to ensure that the AC-side switching tubes achieve soft switching. The DC-side H-bridge is a high-frequency inversion link. According toFigure 10 It can be seen that it is necessary to ensure that before the switch tube S1 is turned on, the resonant current i r The direction is positive and the size is sufficient to satisfy the output capacitance C within the dead time. oss Fully charge and discharge. Based on this, the required turn-on current of the switch tube S1 is:

[0083] I dc_switch =n·I rp sin(α) (8)

[0084] The phase shift angle α is defined as Figure 11 As shown, this value can be obtained by using the inner phase shift angle θ and the outer phase shift angle express:

[0085]

[0086] In order to ensure that the switch tube S1 realizes soft switching at this time, the reference value of the turn-on current I can be obtained at this time. dc_switch_ref As shown in the following formula (10), where C oss_dc is the equivalent value of the output junction capacitance of the DC side switch tube, t d_dc is the dead time set for the DC side switch tube, based on which the phase shift angle α can be calculated.

[0087]

[0088] Combining equations (6), (7), (9), and (10), we can obtain the inner phase angle θ and the outer phase angle for:

[0089]

[0090]

[0091] Based on this, substituting equations (11) and (12) into equation (6), we can obtain the resonant cavity impedance Z at this time. r for:

[0092]

[0093] Therefore, the switching frequency f s It can be expressed as:

[0094]

[0095] At this point, all the control quantities of extended phase shift and frequency conversion modulation can be obtained. Based on these control quantities, the drive signals of each switch tube can be obtained from the peripherals in the digital processor to modulate the power of the inverter. This modulation method directly obtains the numerical expression of each control quantity, and the inverter circuit can be modulated without table lookup, and the algorithm is simple.

[0096] Since the fundamental wave approximation method ignores the square wave voltage excitation outside the resonant cavity and the harmonics of the resonant capacitor voltage and resonant current in the resonant cavity, there is a certain difference between the actual current and the theoretically calculated current at the switching moment, and the situation of missing soft switching exists. In the DC / AC inverter of this embodiment, the AC side is a high-frequency rectification link. After being turned on, the direction of the resonant current is still the same as that of the body diode. Even if there is an error in the instantaneous current, the realization of ZVS can still be ensured by appropriately increasing the dead time (Extended Dead-time).

[0097] In order to ensure the soft switching of the switching tubes on the DC side, it is necessary to adjust the phase shift angle α to ensure that the turn-on current I of the switching tube S1 dc_switch meets the requirements of soft switching. If the adjustment of α is small, it may lead to insufficient soft switching or even hard turn-on of the switching tube; if the adjustment of α is large, it may increase the effective value of the resonant current and increase the conduction loss.

[0098] Therefore, in this embodiment, a soft switching precise compensation method based on numerical calculation is added on the basis of the fundamental wave approximation method, which specifically includes the following steps:

[0099] T1. Solve the exact current value at the switching moment of the primary side switching tube according to the differential equations of the resonant inductor current and resonant capacitor voltage under different operating modes;

[0100] T2. Determine the error value of the turn-on current according to the turn-on current value and the exact current value of the primary side switching tube;

[0101] T3. Based on the PI regulation according to the error value, obtain a new switching frequency of the primary side switching tube, and modulate the control quantity of the inverter according to the new switching frequency of the primary side switching tube.

[0102] According to the numerical model of the differential equations of the resonant inductor current and resonant capacitor voltage under different operating modes, the expression of the exact current value I dc_switch_TDA is:

[0103]

[0104] where, ω r is defined as the resonant angular frequency; Z is the characteristic impedance of the resonant cavity; F is the ratio of the switching angular frequency to the resonant angular frequency (to ensure that the resonant impedance is inductive, so the frequency ratio F is always greater than 1):

[0105]

[0106]

[0107]

[0108] Substitute \(f\) in equations (11), (12), and (14) s , and \(\theta\) into equation (15) to obtain \(I\) at this time dc_switch_TDA . Subtract it from the theoretical reference value \(I\) dc_switch_ref of the on - current of the high - frequency switching tube on the AC side, and the error value \(I\) dc_switch_Er of the on - current at this time can be obtained as shown in the following equation:

[0109]

[0110] Derive the modulation principle of the phase - shift angle \(\alpha\) in the inverter operation. According to equation (8), it can be known that there is a monotonic relationship between \(I\) dc_switch_TDA and the phase - shift angle \(\alpha\). Therefore, the switching frequency \(f\) s can be adjusted in a closed - loop manner to dc_switch_TDA realize the adjustment of \(I\). The specific control block diagram is as shown in Figure 12 . The switching frequency \(f\) s at this time consists of two parts. The first part is the switching frequency \(f\) s_FHA obtained from equation (14); the second part is the closed - loop output obtained by feeding the error value \(I\) dc_switch_Er into the PI regulator. Finally, substitute the obtained new \(f\) s into equations (11) and (12) to re - solve the external phase - shift angle and the internal phase - shift angle \(\theta\). Thus, a new external phase - shift angle , internal phase - shift angle \(\theta\) and switching frequency \(f\) s are formed to realize the modulation of the inverter circuit.

[0111] The above - mentioned soft - switching precise compensation strategy compensates for the errors existing in the fundamental - wave approximation method, ensuring that the inverter can operate in the soft - switching state within the full - voltage range and guaranteeing the efficiency of the inverter.

[0112] Embodiment 2

[0113] As shown in Figure 13 , the difference from Embodiment 1 is that in this embodiment, the secondary AC circuit adopts a half - bridge structure. A bridge arm is composed of two groups of bidirectional matrix switching tubes S5 - S8. The mid - point A of these two groups of switching tubes in this bridge arm is connected to the series resonant cavity. In the secondary AC circuit, there are also two capacitors C1 and C2, which form another bridge arm. The mid - point B of these two capacitors in this bridge arm is connected to the series resonant cavity.

[0114] During the modulation process of the inverter, the same modulation method as in Embodiment 1 is adopted.

[0115] Embodiment 3

[0116] As shown in Figure 14As shown, the difference from the first embodiment is that in this embodiment, the secondary AC circuit includes a first bridge circuit and a second bridge circuit. The first bridge circuit is a full-bridge structure and is configured to perform full-wave rectification of the AC voltage at the industrial frequency based on the unfolded bridge, while the second bridge circuit simultaneously realizes the power quality and soft switching of the AC current.

[0117] In the first bridge circuit, it includes four groups of switch tube groups, and each switch tube group only contains one switch tube S5 - S8. In the second bridge circuit, it also includes four groups of switch tube groups, and each switch tube group only contains one switch tube S9 - S 12 . During the inverter modulation process, the same modulation method as in the first embodiment is adopted.

[0118] Embodiment 4

[0119] As Figure 15 shown, the difference from the third embodiment is that in the first bridge circuit of this embodiment, a half-bridge structure is adopted. A bridge arm is composed of switch tubes S5 - S6, and the midpoint A of the two groups of switch tube groups of this bridge arm is connected to the series resonant cavity. The other bridge arm is composed of capacitors C ac1 and C ac2 , and the midpoint B of this bridge arm is connected to the series resonant cavity. At the same time, the capacitor bridge arm of the half-bridge simultaneously undertakes the function of capacitor C ac in the third embodiment. C ac1 and C ac2 are both thin-film capacitors in the uF level. During the inverter modulation process, the same modulation method as in the first embodiment is adopted.

[0120] The specific embodiments described herein are only illustrative of the spirit of the present invention. Those skilled in the art of the present invention can make various modifications or supplements to the described specific embodiments or use similar ways to replace them, but will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims

1. A single-stage high-frequency isolation DC / AC inverter, characterized in that, It includes a primary - side bridge - type inverter circuit, a high - frequency transformer, a series resonance cavity, and a secondary - side AC circuit connected in sequence. The primary - side bridge - type inverter circuit includes four switching tubes to form an H - bridge. The series resonance cavity contains a resonance inductor and a resonance capacitor. The secondary - side AC circuit includes at least two groups of switching - tube groups to form a first bridge circuit. The first bridge circuit includes two bridge arms, and the mid - points of the bridge arms are electrically connected to the series resonance cavity; The inverter is modulated by a control circuit, and an inverter modulation method is set in the control circuit. The method includes the steps: Based on the fundamental - wave approximation algorithm, determine the internal phase - shift angle between the primary - side bridge arms, the external phase - shift angle between the primary - side bridge arms and the secondary - side bridge arms, and the switching frequencies of the primary - side switching tubes and the secondary - side switching tubes to modulate the control quantities of the inverter. The control quantities include power transfer characteristics and soft - switching characteristics. The power transfer characteristics determine the waveform quality of the secondary - side AC current, and the soft - switching characteristics determine the power loss generated when the switching tubes turn off or on; Ensure the waveform quality of the AC current by the magnitude of the active - current phasor of the resonance current; And, Ensure that the resonance - current phasor is in the same phase as the AC - side voltage phasor so that the AC - side switching tubes achieve soft - switching; And Determine the dead - time of the switching of the bridge - arm switching tubes from the resonance current, the turns ratio of the high - frequency transformer, and the phase - shift angle to ensure that the charging and discharging of the output junction capacitance on both sides of the drain - source of the primary - side switching tubes are completed within the dead - time of the switching of the bridge - arm switching tubes.

2. The single-stage high-frequency isolated DC / AC inverter according to claim 1, wherein Ensure that the resonance - current phasor is in the same phase as the AC - side voltage phasor by the magnitude of the reactive - current phasor of the resonance current.

3. A single-stage high-frequency isolated DC / AC inverter according to claim 1, characterized in that, The phase - shift angle is determined by the internal phase - shift angle and the external phase - shift angle.

4. A single-stage high-frequency isolated DC / AC inverter according to claim 1, characterized in that, The method further includes the step: calculate the accurate current value at the switching moment of the primary - side switching tubes to compensate the control quantity according to the accurate current value.

5. A single-stage high-frequency isolation DC / AC inverter according to claim 4, characterized in that, The step of compensating the control quantity according to the accurate current value includes: Solve the accurate current value at the switching moment of the primary - side switching tubes based on the differential equations of the resonance - inductor current and the resonance - capacitor voltage under different working modes; Determine the error value of the turn - on current according to the turn - on current value of the primary - side switching tubes and the accurate current value; Based on the error value, obtain a new switching frequency of the primary - side switching tubes through PI regulation, and modulate the control quantity of the inverter according to the new switching frequency of the primary - side switching tubes.

6. The single-stage high-frequency isolated DC / AC inverter according to claim 1, characterized in that, In the two bridge arms of the first bridge circuit, two groups of switching - tube groups form the first bridge arm, and the second bridge arm is formed by two groups of switching - tube groups or two capacitors; the mid - points of the first bridge arm and the second bridge arm are both connected to the series resonance cavity.

7. A single-stage high-frequency isolated DC / AC inverter according to claim 6, characterized in that, When each switching - tube group in the switching - tube groups of the first bridge circuit is configured as a switching tube, the secondary - side AC circuit further includes a second bridge circuit. The second bridge circuit is connected in parallel with the first bridge circuit and is configured to output alternating current.

8. A single-stage high-frequency isolated DC / AC inverter according to claim 7, characterized in that, The second bridge circuit includes four groups of switching - tube groups, and each switching - tube group only includes one switching tube.

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