Single-stage high-frequency isolated DC / ac inverter

Abstract

The present invention provides a single-stage high-frequency isolated DC / AC inverter, comprising a primary-side bridge inverter circuit, a high-frequency transformer, a series resonant cavity and a secondary-side AC circuit which are connected in sequence. The inverter is modulated by a control circuit, and an inverter modulation method is set in the control circuit and comprises the steps of: determining an inner phase shift angle between primary-side bridge arms, an outer phase shift angle between each primary-side bridge arm and a secondary-side bridge arm, and switching frequencies of a primary-side switching transistor and a secondary-side switching transistor on the basis of a fundamental harmonic approximation algorithm, so as to modulate control variables of the inverter, wherein the control variables include a power transmission characteristic and a soft switching characteristic; and calculating precise current values at switching instants of the primary-side switching transistor to compensate the control variables on the basis of the precise current values.

Description

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

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

[0002] Green energy is becoming a major trend in the current energy transition. Solar energy, as a clean energy source with abundant resources and widespread distribution, is a key driver of new energy development. Converting solar energy into usable electricity requires a DC / AC inverter, which converts the direct current (DC) power from photovoltaic panels into alternating current (AC) for the power grid. For both equipment and personnel safety, DC / AC inverters typically require electrical isolation, resulting in isolated DC / AC converters.

[0003] Isolated DC / AC converters can be divided into two-stage and single-stage types according to the number of power conversion times in the topology. Compared with the two-stage type, the single-stage isolated DC / AC converter can reduce one power conversion stage and has potential advantages such as high efficiency, high power density and low cost. It has currently received widespread attention and research.

[0004] Phase-shift modulation is one of the primary modulation strategies for driving the power circuits of isolated DC / AC converters. Phase-shift modulation transfers power by varying the phase-shift angle of the switching tube bridge arms. In a dual-active bridge topology, the high-frequency rectifier / inverter units on both sides of the high-frequency transformer use power switching tubes. Phase-shift modulation is achieved by modulating the phase difference between the drive signals of each switching tube. Therefore, this modulation is commonly used and easy to implement in dual-active bridge topologies. Based on different phase-shift operating conditions, various phase-shift modulation strategies have emerged, including single-phase, extended-phase, double-phase, and triple-phase.

[0005] For resonant single-stage DC / AC converters with at least two passive components within the cavity, the high-frequency inductor current within the cavity varies nonlinearly, making the mathematical modeling process complex. The relationship between the control degrees of freedom and the state variables is difficult to directly derive and calculate using analytical expressions. To optimize the converter's modulation strategy, ensuring soft switching, low high-frequency current RMS values, and switch-off current under varying loads and voltage gains, and optimizing converter efficiency, it is necessary to analyze the resonant converter's operating modes and corresponding soft-switching conditions to identify the most suitable control method for resonant single-stage DC / AC converters. Technical issues

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

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] A modulation method for a single-stage high-frequency isolated DC / AC inverter.

[0009] Determine the internal phase shift angle of the primary-side bridge inverter circuit, the external phase shift angle between the primary-side bridge arm and the secondary-side bridge arm, and the switching frequencies of the primary-side switch tube and the secondary-side switch tube based on the fundamental wave approximation method;

[0010] Based on the inner phase shift angle, outer phase shift angle and switching frequency determined above, the digital processor outputs a driving signal for driving the switch tube to perform power modulation on the inverter.

[0011] Furthermore, the bridge arm on the primary side close to the transformer is defined as the leading bridge arm, and the bridge arm on the primary side close to the DC power supply is defined as the lagging bridge arm;

[0012] The inner phase shift angle is the phase angle at which the leading bridge arm on the DC side drives ahead of the lagging bridge arm, and the outer phase shift angle is the phase angle at which the AC side voltage leads the DC side voltage;

[0013] Based on the fundamental wave analysis method, an equivalent analysis is performed on the inverter main circuit topology. The DC power supply voltage expression on the primary side and the AC power supply voltage expression on the secondary side are decomposed by Fourier, and then the fundamental component expressions of the DC power supply voltage on the primary side and the AC power supply voltage on the secondary side are obtained respectively.

[0014] Furthermore, the phasor corresponding to the resonant current is divided into active current phasor and reactive current phasor according to the phase. Based on the voltage phasor on the DC side and the voltage phasor on the AC side, the expressions of the modulus length of the active current phasor and the modulus length of the reactive current phasor are obtained respectively.

[0015] Furthermore, the fundamental component expression of the AC power supply voltage on the secondary side is combined with the modulus length expression of the active current phasor to obtain the modulus length expression of the active current phasor based on the inner phase shift angle and the outer phase shift angle as variables.

[0016] Furthermore, the phase of the resonant current is ensured to be in phase with the AC side voltage and the dead time of the AC side switch tube is increased; and the modulus length of the reactive current phasor is ensured to be 0.

[0017] Furthermore, it is ensured that the direction of the resonant current is positive before the switch S1 connected to the positive electrode of the DC power supply on the leading bridge arm is turned on, and the output capacitor C isoss Fully charge and discharge;

[0018] An expression for the phase shift angle based on the inner phase shift angle and the outer phase shift angle as variables is defined. Based on the transformer's turns ratio, active current module length, and phase shift angle, an expression for the turn-on current value required by the switch S1 is obtained.

[0019] Furthermore, a turn-on current reference value of the switch tube S1 is obtained based on the equivalent value of the output junction capacitance of the DC side switch tube, the dead time set for the DC side switch tube, and the DC power supply voltage on the DC side.

[0020] Furthermore, according to the following conditions:

[0021] Active current phasor module length expression based on inner phase shift angle and outer phase shift angle as variables;

[0022] The modulus length of the reactive current phasor is 0;

[0023] An expression for the phase shift angle based on the inner phase shift angle and the outer phase shift angle as variables;

[0024] The reference value of the turn-on current of the switch tube S1;

[0025] The expression of the turn-on current value of the switch tube S1 based on the transformer's turns ratio, active current module length and phase shift angle;

[0026] By combining conditions (1)-(5), we can obtain the expressions of the inner phase shift angle and the outer phase shift angle respectively, and calculate the values ​​of the inner phase shift angle and the outer phase shift angle respectively.

[0027] Furthermore, based on the expression of the inner phase shift angle, the expression of the outer phase shift angle, and the expression of the active current phasor modulus based on the inner phase shift angle and the outer phase shift angle as variables, the expression of the resonant cavity impedance is obtained, and based on the expression of the resonant inductance, the resonant capacitance and the resonant cavity impedance, the expression of the switching frequency of the primary side switching tube is obtained.

[0028] Furthermore, the fundamental component of the DC power supply voltage on the primary side is expressed as:

[0029] The fundamental component of the AC power supply voltage on the secondary side is expressed as:

[0030] The expression of the active current phasor modulus is:

[0031] The expression of reactive current phasor modulus is:

[0032] The modulus length expression of the active current phasor based on the inner phase shift angle and the outer phase shift angle as variables is obtained as follows:

[0033] Based on the expression of the phase shift angle with the inner phase shift angle and the outer phase shift angle as variables, and based on the transformer's turns ratio, active current module length, and phase shift angle, the expression of the turn-on current value required by the switch tube S1 is: I dc_switch =n·I rp sin(α);

[0034] The expression of the internal phase shift angle is:

[0035] The expression of the outward shift phase angle is:

[0036] The expression of the resonant cavity impedance is:

[0037] The expression of the switching frequency of the primary side switch is:

[0038] Where: n is the high-frequency transformer ratio, the instantaneous value of the AC side power supply voltage v ac , DC side power supply voltage V dc , 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 the leading and lagging bridge arm driving, and the external phase shift angle of the AC side voltage leading the DC side voltage Define v ab1,N (ω s The phasor modulus of t) is V p , nv cd1,N (ω s The phasor modulus of t) is V s , Z r is the resonant cavity impedance; define the phase shift angle α, which is divided by the inner phase shift angle θ and the outer phase shift angle Indicated: i ac is the AC side power supply current value; L r Expressed as the resonant inductance value of the series resonant cavity, C r Expressed as the resonant capacitance value in the series resonant cavity.

[0039] Furthermore, the precise current value of the primary-side switch at the switching moment is solved based on the differential equations of the resonant inductor current and the resonant capacitor voltage under different working modes;

[0040] Determine the turn-on current error value based on the turn-on current value of the primary side switch tube and the precise current value;

[0041] According to the error value, a new switching frequency of the primary side switch tube is obtained based on PI regulation, and according to the new switching frequency of the primary side switch tube, a new inner phase shift angle and an outer phase shift angle are obtained in combination with an expression of the resonant cavity impedance, an expression of the inner phase shift angle and an expression of the outer phase shift angle;

[0042] Based on the new inner phase shift angle, outer phase shift angle and switching frequency determined above, the digital processor outputs a driving signal for driving the switch tube to perform power modulation on the inverter.

[0043] Furthermore, based on the differential equations of the resonant inductor current and the resonant capacitor voltage under different operating modes, the expression of the exact current value is:

[0044] Where Z is the characteristic impedance of the resonant cavity, F is the ratio of the switching angular frequency to the resonant angular frequency, and the AC / DC voltage gain ratio M=v ac / nV dc .

[0045] A second aspect of the present invention provides a single-stage high-frequency isolated DC / AC inverter, which adopts the above-mentioned modulation method and includes a primary-side bridge inverter circuit, a high-frequency transformer, a series resonant 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, and the series resonant cavity contains a resonant inductor and a resonant capacitor. The secondary-side AC circuit includes at least two groups of switching tubes 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 resonant cavity.

[0046] Furthermore, in the two bridge arms of the first bridge circuit, two groups of switch tubes form the first bridge arm, and two groups of switch tubes or two capacitors form the second bridge arm; the midpoint of the first bridge arm and the midpoint of the second bridge arm are both connected to the series resonant cavity.

[0047] Furthermore, in the switch tube groups of the first bridge circuit, when each switch tube group is configured as a switch tube, the secondary AC circuit also includes a second bridge circuit, which is connected in parallel to the first bridge circuit and is configured to output AC power.

[0048] Furthermore, the second bridge circuit includes four switch tube groups, and each switch tube group includes only one switch tube.

[0049] A third aspect of the present invention provides a single-stage high-frequency isolated DC / AC inverter, comprising a primary-side bridge inverter circuit, a high-frequency transformer, a series resonant cavity, and a secondary-side AC circuit connected in sequence. The primary-side bridge inverter circuit includes four switching transistors to form an H-bridge, the series resonant cavity contains a resonant inductor and a resonant capacitor, and the secondary-side AC circuit includes at least two groups of switching transistors 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 resonant cavity.

[0050] The inverter is modulated by a control circuit, and the control circuit is provided with an inverter modulation method, which includes the steps of:

[0051] Based on the fundamental wave approximation algorithm, the inner phase shift angle between the primary-side bridge arms, the outer phase shift angle between the primary-side bridge arms and the secondary-side bridge arms, and the switching frequencies of the primary-side and secondary-side switching tubes are determined to modulate the control variables of the inverter, including power transfer characteristics and soft switching characteristics.

[0052] The power transfer characteristics determine the waveform quality of the AC current on the secondary side, and the soft switching characteristics determine the power loss generated when the switch tube is turned off or on.

[0053] Furthermore, the power transmission characteristics are determined by the average value of the resonant current transmitted to the AC side output end in each switching cycle in the series resonant cavity; the soft switching characteristics include the current direction of the secondary side switch tube in the dead time before it is turned on being 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 and source of the primary side switch tube being completed within the dead time of the bridge arm switch tube switching.

[0054] Furthermore, the corresponding phasor of the resonant current is divided into an active current phasor and a reactive current phasor according to the phase, and the power transmission characteristics of the inverter are controlled by the module length of the active current phasor.

[0055] Furthermore, the modulus length of the reactive current phasor is used to control the current direction of the secondary-side switch tube in the dead time before the secondary-side switch tube is turned on to be the same as the direction of its body diode.

[0056] Furthermore, the charging and discharging of the output junction capacitance on both sides of the drain and source of the primary side switch tube is controlled by the resonant current, the high-frequency transformer ratio and the phase shift angle, and the phase shift angle is determined by the inner phase shift angle and the outer phase shift angle.

[0057] Furthermore, the method further includes the step of calculating the precise current value at the switching moment of the primary-side switch tube, so as to compensate the control variable according to the precise current value. Beneficial effects

[0058] Compared with the prior art, the present invention has at least the following beneficial effects:

[0059] (1) The single-stage structure adopted by the present invention has only one stage of power conversion, which has the advantages of high efficiency, high power density and low cost;

[0060] (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 the perspectives of power transmission characteristics and soft switching characteristics, and proposes a modulation strategy that combines extended phase shifting with frequency modulation. Theoretically, it can achieve soft switching of all switches within the full voltage range, effectively reduce the effective value of the current in the resonant cavity, and improve the efficiency of the converter.

[0061] (3) Based on the fundamental wave approximation method, the numerical expression of the control quantity can be directly obtained without table lookup. The execution is simple. It only needs to sample the voltage signals on both sides of the input and output and enter the digital processor for a small amount of calculation to output the PWM drive signal of each switch tube to modulate the power circuit.

[0062] (4) In order to ensure that all converters can operate in a soft switching state, the present invention adds a soft switching precision compensation method based on numerical calculation on the basis of fundamental wave approximation, which can ensure that all switching tubes are turned on at zero voltage and guarantee the efficiency of the converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] FIG1 is a schematic diagram of a photovoltaic DC-to-AC topology according to an embodiment of the present invention;

[0064] FIG2 is a circuit diagram of the inverter main circuit topology in Example 1 provided by the present invention;

[0065] FIG3 is a flowchart of the steps of the inverter modulation method according to an embodiment of the present invention;

[0066] FIG4 is a waveform diagram of the square wave voltage (AC voltage is positive) at the midpoint AB of the AC side bridge arm in an embodiment provided by the present invention;

[0067] 5 is a waveform diagram of the square wave voltage (AC voltage is negative) at the midpoint AB of the AC side bridge arm in an embodiment provided by the present invention;

[0068] 6 is a waveform diagram of the square wave voltage at the midpoint CD of the DC side bridge arm in an embodiment provided by the present invention;

[0069] FIG7 is an equivalent circuit diagram of a series resonant cavity according to an embodiment of the present invention;

[0070] FIG8 is a diagram showing the phase shift angle relationship of the alternating square wave voltage in the embodiment provided by the present invention;

[0071] FIG9 is a high-frequency voltage and current phasor diagram under fundamental wave approximation in an embodiment provided by the present invention;

[0072] FIG10 is a time domain steady-state waveform and corresponding phasor diagram in the series resonant cavity according to an embodiment of the present invention;

[0073] 11 is a phasor diagram of voltage and current signals and part of the drive signal under extended phase-shift modulation in an embodiment provided by the present invention;

[0074] FIG12 is a schematic diagram of a soft switch compensation based on numerical calculation in an embodiment of the present invention;

[0075] FIG13 is a circuit diagram of the inverter main circuit topology in the second embodiment provided by the present invention;

[0076] FIG14 is a circuit diagram of the inverter main circuit topology in the third embodiment provided by the present invention;

[0077] FIG15 is a circuit diagram of the inverter main circuit topology in the fourth embodiment provided by the present invention. Modes for Carrying Out the Invention

[0078] It should be noted that all directional indications in the embodiments of the present invention (such as up, down, left, right, front, back, etc.) are only used to explain the relative position relationship, movement status, etc. between the various components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0079] In addition, in the present invention, descriptions such as "first," "second," and "one" are for descriptive purposes only and should not be understood to indicate or imply their relative importance or implicitly specify the number of the technical features indicated. Therefore, a feature specified as "first" or "second" may explicitly or implicitly include at least one of such features. In the description of the present invention, "plurality" means at least two, such as two, three, etc., unless otherwise specifically defined.

[0080] In the present invention, unless otherwise specified or limited, the terms "connection" and "fixation" should be understood in a broad sense. For example, "fixation" can mean fixed connection, detachable connection, or integration; mechanical connection or electrical connection; direct connection or indirect connection through an intermediate medium; internal communication between two elements or interaction between two elements, unless otherwise specified. Those skilled in the art will be able to understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0081] In addition, the technical solutions between the various embodiments of the present invention can be combined with each other, but it must be based on the fact that ordinary technicians in this field can implement it. When the combination of technical solutions is mutually contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0082] The following are specific embodiments of the present invention, and the technical solutions of the present invention are further described in conjunction with the accompanying drawings, but the present invention is not limited to these embodiments.

[0083] As shown in Figure 1, a single-stage isolated DC / AC inverter has a main circuit consisting of three parts: a primary-side bridge inverter circuit, a secondary-side AC circuit, and a series resonant cavity and high-frequency transformer in the middle.

[0084] Example 1

[0085] In this embodiment, as shown in FIG2 , the secondary side (AC side) of the circuit adopts a full-bridge structure, which consists of four sets of bidirectional matrix switch tubes S5 to S6. 12 The two arms of the full bridge are formed. The primary side (DC side) uses switch tubes S1 to S4 to form an H bridge. The high-frequency transformer T has a transformation ratio of n, and the series resonant cavity contains a resonant inductor L. r and a resonant capacitor C r .

[0086] This topology is a single-stage dual-active bridge structure with bidirectional matrix switches as the AC-side bridge arms. It features a simple structure and requires no additional rectification. The duty cycle of the switches is controlled by a PWM drive signal from the control circuit, thereby achieving inverter control and output regulation.

[0087] As shown in FIG3 , a modulation method of an inverter is provided in a control circuit. This embodiment proposes a modulation method combining extended phase shifting with frequency modulation, which includes the following steps:

[0088] S1. Determine, based on a fundamental wave approximation algorithm, an inner phase shift angle between the primary-side bridge arms, an outer phase shift angle between the primary-side bridge arms and the secondary-side bridge arms, and switching frequencies of the primary-side switching tube and the secondary-side switching tube to modulate a control variable of the inverter, the control variable including power transfer characteristics and soft switching characteristics;

[0089] S2. Calculate the precise current value at the switching moment of the primary-side switch tube to compensate the control quantity according to the precise current value.

[0090] In this modulation method, the DC / AC inverter power circuit is first analyzed using the fundamental wave approximation method. Since the AC input voltage of the DC / AC inverter is time-varying, the following assumptions are made for ease of analysis: (1) the inverter is in steady-state operation; and (2) the AC input voltage is assumed to be constant within a switching cycle.

[0091] According to the characteristics of the matrix switch circuit, four of the four bidirectional transistors on the AC side operate at the power frequency, and the other four operate at high frequency. The AC and DC side drive and bridge arm voltage waveforms are shown in Figures 4 to 6. When the AC voltage is positive, the switch tubes S7 and S8 are always on, and the switch tubes S5 and S6 operate at high frequency. At the same time, since no additional internal phase shift is added on the AC side, the control switch tube S 11 and S 12The driving signal of S7 and S8 is the same as that of S9 and S 10 The driving signal of S5 and S6 is consistent. At this time, the middle point AB of the AC side bridge arm generates a square wave voltage v ab , as shown in Figure 4. Similarly, when the AC voltage is negative, its drive and bridge arm midpoint voltage waveforms are shown in Figure 5. The DC side H-bridge drive and bridge arm midpoint voltage waveforms are shown in Figure 6, and its bridge arm output voltage is v ab .

[0092] At the same time, the bridge arm where the switches S1 and S2 are located is defined as the leading bridge arm, and the switches S3 and S4 are defined as the lagging bridge arm. The equivalent circuit of the resonant cavity in the converter is shown in Figure 7. The voltage expressions on the AC side and the DC side are v ab (ω s t) and nv cd (ω s t), and then perform Fourier decomposition on them respectively, and the expressions of the fundamental components of the two can be obtained as follows:

[0093] Where: Instantaneous value of AC power supply voltage v ac , DC side power supply voltage V dc , 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 the leading and lagging bridge arm driving, and the external phase shift angle of the AC side voltage leading the DC side voltage

[0094] At the same time, for the convenience of analysis, some parameters and variables of the inverter are defined here: AC / DC voltage gain ratio M = v ac / nV dc , resonant frequency f r =1 / (2π×sqrt(L r ×C r ), the resonant cavity impedance Z r =ω s L r –1 / ω s C r ;

[0095] Therefore, the inner phase shift angle θ and the outer phase shift angle The specific situation when working in both directions is shown in Figure 8 below.

[0096] Based on the results obtained from the fundamental wave approximation, each voltage and current fundamental wave can be represented in the form of a phasor diagram, as shown in Figure 9. Define v ab1,N (ω s The phasor of t) is nv cd1,N (ωs The phasor of t) is At this time, the voltage excitation applied to both ends of the resonant cavity is When the switching frequency is higher than the resonant frequency, a resonant current i will be generated which lags behind the voltage excitation by 90°. r (ω s t). For the convenience of analysis, this paper corresponds to the phase of the resonant current Divided into active current phasors according to phase and reactive current phasor The modulus length I of the two current phasors rp and I rq The expression is as follows:

[0097] At this time r (ω s The time domain expression of t) is as follows: r (ω s t)=I rp ·sin(ω s t)+I rq ·cos(ω s t) (5)

[0098] For a DC / AC converter, the first thing to pay attention to is its power transmission characteristics, that is, whether the instantaneous current transmitted can follow the AC voltage to ensure a small current total harmonic distortion (THD) to meet 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 cycle in the high-frequency resonant cavity.

[0099] The second area of ​​focus is the implementation of soft switching for the switch. This requires ensuring that the current flow direction is consistent with the body diode during the dead-time before the switch turns on, and that the output junction capacitance on both sides of the switch's drain and source is fully charged and discharged during the dead-time between arm switch switching.

[0100] For the AC side switch tube, when i r (ω s t) Phase ahead of v ab1,N (ω s t), namely the switch tubes S5 and S 10 When opening r is positive, which can achieve ZVS (Zero Voltage Switch), the same applies to switches S6 and S9; for the DC side switch, nv cd1,N (ω s t) Phase ahead of i r (ωs t), when the switches S1 and S4 are turned on, i r If the voltage is negative, ZVS can also be achieved. The same is true for switches S2 and S3. The waveforms of the voltage and current phasors and the drive signals under the fundamental wave approximation method are shown in Figure 10.

[0101] At this time, the phasor corresponding to the fundamental wave of the square wave driven by the switch tubes S1 and S4 is defined as The phasor diagram at this time can be drawn as shown in Figure 11.

[0102] According to the above analysis, this embodiment adopts a three-degree-of-freedom modulation strategy combining extended phase shift and frequency modulation, and the control quantities are the inner phase shift angle θ, the outer phase shift angle And the switching frequency f s This is matched with the three conditions corresponding to the inverter power transfer characteristics and soft switching characteristics mentioned above, namely:

[0103] (1)Condition 1

[0104] The first condition is the power transfer characteristic, which is determined by adjusting I rp Ensure the quality of AC current waveform. According to the results derived above, substitute equation (2) into equation (3) to obtain:

[0105] (2) Condition 2

[0106] The second condition is to ensure that the AC side switch tube can achieve soft switching. Since the AC side H bridge is a high-frequency rectifier link, the direction of the high-frequency current is naturally consistent with the direction of the body diode of its switch tube. and By keeping the phases in the same state and appropriately increasing the dead time of the AC side switch tube, the zero voltage turn-on of all the AC side switch tubes can be achieved. According to formula (4), at this time: I rq =0 (7)

[0107] (3)Condition three

[0108] The third condition is to ensure that the DC side switch tube realizes soft switching. The DC side H bridge is a high-frequency inverter link. According to Figure 10, 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 satisfies the output capacitance C within the dead time. oss Based on this, the required turn-on current of the switch tube S1 is: I dc_switch =n·I rp sin(α) (8)

[0109] The phase shift angle α is defined as shown in Figure 11. This value can be expressed as the inner phase shift angle θ and the outer phase shift angle express:

[0110] 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.

[0111] Combining equations (6), (7), (9), and (10), we can obtain the inner phase shift angle θ and the outer phase shift angle at this time: for:

[0112] Based on this, by substituting equations (11) and (12) into equation (6), the resonant cavity impedance Z at this time can be obtained. r for:

[0113] Therefore, the switching frequency f s Can be expressed as:

[0114] All control variables for extended phase shifting and variable frequency modulation are now determined. Based on these control variables, the digital processor can generate drive signals for each switch from external devices to modulate the inverter's power. This modulation method directly generates numerical expressions for each control variable, eliminating the need for table lookup and enabling simple algorithmic control of the inverter circuit.

[0115] Because the fundamental wave approximation ignores the square wave voltage excitation outside the resonant cavity and the harmonics of the resonant capacitor voltage and resonant current within the resonant cavity, the actual current at the switching moment differs from the theoretically calculated current, resulting in the loss of soft switching. In the DC / AC inverter of this embodiment, the AC side is a high-frequency rectifier. After switching on, the direction of the resonant current remains the same as that of its body diode. Even if there is an error in the instantaneous current, ZVS can still be achieved by appropriately increasing the dead time (Extended Dead-time).

[0116] In order to ensure the soft switching of the DC side switch tube, it is necessary to adjust the phase shift angle α to ensure the turn-on current I dc_switch Meet the requirements of soft switching. If α is adjusted too small, the soft switching of the switch tube may be insufficient or even hard-turned on; if α is adjusted too large, the effective value of the resonant current may be increased, increasing the conduction loss.

[0117] Therefore, this embodiment adds a soft switching precise compensation method based on numerical calculation on the basis of the fundamental wave approximation method, which specifically includes the following steps:

[0118] T1. Solve the precise current value of the primary side switch at the switching moment based on the differential equations of the resonant inductor current and the resonant capacitor voltage under different working modes;

[0119] T2. Determine the turn-on current error value based on the turn-on current value of the primary-side switch tube and the precise current value;

[0120] T3. Obtain a new switching frequency of the primary-side switch tube based on the PI adjustment according to the error value, and modulate the control quantity of the inverter according to the new switching frequency of the primary-side switch tube.

[0121] The precise current value expression I is obtained based on the numerical model of differential equations based on the resonant inductor current and resonant capacitor voltage under different working modes. dc_switch_TDA for:

[0122] Where, define ω r is 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, the frequency ratio F is always greater than 1):

[0123] In formulas (11), (12), and (14), s 、 Substituting and θ into equation (15), we can get I dc_switch_TDA Compare it with the theoretical reference value of the AC side high frequency switch current I dc_switch_ref By making a difference, we can get the error value of the turn-on current I dc_switch_Er As shown in the following formula:

[0124] Derived the modulation principle of the inverter working phase shift angle α, according to formula (8) we can know I dc_switch_TDA There is a monotonic relationship between the switching frequency f and the phase shift angle α, so the switching frequency f can be adjusted in a closed loop s Realize I dc_switch_TDA The specific control block diagram is shown in Figure 12. At this time, the switching frequency f s It consists of two parts. The first part is the switching frequency f obtained by formula (14). s_FHA ; The second part is to convert the error value I dc_switch_Er Enter the closed-loop output of the PI regulator. Finally, the new f s Substitute into equations (11) and (12) to solve the external phase angle again And the inner phase angle θ. Thus forming a new outer phase angle Internal phase shift angle θ and switching frequency f s To achieve modulation of the inverter circuit.

[0125] The above-mentioned soft switching precise compensation strategy compensates for the error existing in the fundamental wave approximation method, ensuring that the inverter can operate in a soft switching state within the full voltage range, thereby ensuring the efficiency of the inverter.

[0126] Example 2

[0127] As shown in Figure 13, this embodiment differs from the first embodiment in that the secondary AC circuit employs a half-bridge structure, with two groups of bidirectional matrix switches S5-S8 forming a bridge arm. The midpoint A of this bridge arm, connecting the two switch groups, is connected to the series resonant cavity. The secondary AC circuit also includes two capacitors C1 and C2 as the other bridge arm, with the midpoint B of this bridge arm connected to the series resonant cavity.

[0128] During the inverter modulation process, the same modulation method as in the first embodiment is adopted.

[0129] Example 3

[0130] As shown in FIG14 , 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, wherein the first bridge circuit is a full-bridge structure and is configured to perform industrial frequency full-wave rectification on the AC voltage based on an unfolding bridge, while the second bridge circuit simultaneously realizes power quality and soft switching of the AC current.

[0131] In the first bridge circuit, it includes four switch tube groups, each of which includes only one switch tube S5 to S8. In the second bridge circuit, it also includes four switch tube groups, each of which includes only one switch tube S9 to S10. 12 During the inverter modulation process, the same modulation method as that in the first embodiment is adopted.

[0132] Example 4

[0133] As shown in FIG15 , the difference from the third embodiment is that the first bridge circuit in this embodiment adopts a half-bridge structure, with the switches S5 and S6 forming a bridge arm, and the midpoint A of the bridge arm of the two switch tube groups is connected to the series resonant cavity. The other bridge arm is composed of capacitor C ac1 and C ac2 The midpoint B of the bridge arm is connected to the series resonant cavity. At the same time, the capacitance bridge arm of the half bridge also bears the capacitance C in the third embodiment. ac The role of C ac1 and C ac2 The film capacitors are all uF level. During the inverter modulation process, the same modulation method as in the first embodiment is adopted.

[0134] The specific embodiments described herein are merely illustrative of the spirit of the present invention. Persons skilled in the art may make various modifications, additions, or substitutions to the described specific embodiments without departing from the spirit of the present invention or exceeding the scope of the appended claims.

Claims

1. A modulation method for a single-stage high-frequency isolated DC / AC inverter, characterized in that: Determine the internal phase shift angle of the primary-side bridge inverter circuit, the external phase shift angle between the primary-side bridge arm and the secondary-side bridge arm, and the switching frequencies of the primary-side switch tube and the secondary-side switch tube based on the fundamental wave approximation method; Based on the inner phase shift angle, outer phase shift angle and switching frequency determined above, the digital processor outputs a driving signal for driving the switch tube to perform power modulation on the inverter.

2. The modulation method for a single-stage high-frequency isolated DC / AC inverter according to claim 1, characterized in that: The bridge arm on the primary side close to the transformer is defined as the leading bridge arm, and the bridge arm on the primary side close to the DC power supply is defined as the lagging bridge arm; The inner phase shift angle is the phase angle at which the leading bridge arm on the DC side drives ahead of the lagging bridge arm, and the outer phase shift angle is the phase angle at which the AC side voltage leads the DC side voltage; Based on the fundamental wave analysis method, an equivalent analysis is performed on the inverter main circuit topology. The DC power supply voltage expression on the primary side and the AC power supply voltage expression on the secondary side are decomposed by Fourier, and then the fundamental component expressions of the DC power supply voltage on the primary side and the AC power supply voltage on the secondary side are obtained respectively.

3. The modulation method for a single-stage high-frequency isolated DC / AC inverter according to claim 2, characterized in that: The phasor corresponding to the resonant current is divided into active current phasor and reactive current phasor according to the phase. Based on the voltage phasor on the DC side and the voltage phasor on the AC side, the expressions of the modulus length of the active current phasor and the modulus length of the reactive current phasor are obtained respectively.

4. The modulation method for a single-stage high-frequency isolated DC / AC inverter according to claim 3, characterized in that: The expression of the fundamental component of the AC power supply voltage on the secondary side is combined with the expression of the modulus length of the active current phasor to obtain the expression of the modulus length of the active current phasor based on the inner phase shift angle and the outer phase shift angle as variables.

5. The modulation method for a single-stage high-frequency isolated DC / AC inverter according to claim 4, characterized in that: Ensure that the phase of the resonant current is in phase with the AC side voltage and increase the dead time of the AC side switch tube; ensure that the reactive current phasor modulus is 0.

6. The modulation method for a single-stage high-frequency isolated DC / AC inverter according to claim 5, characterized in that: Ensure that the direction of the resonant current is positive before the switch S1 connected to the positive pole of the DC power supply on the leading bridge arm is turned on, and the output capacitor C is oss Fully charge and discharge; An expression for the phase shift angle based on the inner phase shift angle and the outer phase shift angle as variables is defined. Based on the transformer's turns ratio, active current module length, and phase shift angle, an expression for the turn-on current value required by the switch S1 is obtained.

7. The modulation method for a single-stage high-frequency isolated DC / AC inverter according to claim 6, characterized in that: The turn-on current reference value of the switch tube S1 is obtained based on the equivalent value of the output junction capacitance of the DC side switch tube, the dead time set by the DC side switch tube, and the DC power supply voltage on the DC side.

8. The modulation method for a single-stage high-frequency isolated DC / AC inverter according to claim 7, characterized in that: Based on the following conditions: Active current phasor module length expression based on inner phase shift angle and outer phase shift angle as variables; The modulus length of the reactive current phasor is 0; An expression for the phase shift angle based on the inner phase shift angle and the outer phase shift angle as variables; The reference value of the turn-on current of the switch tube S1; The expression of the turn-on current value of the switch tube S1 based on the transformer's turns ratio, active current module length and phase shift angle; By combining conditions (1)-(5), we can obtain the expressions of the inner phase shift angle and the outer phase shift angle respectively, and calculate the values ​​of the inner phase shift angle and the outer phase shift angle respectively.

9. The modulation method for a single-stage high-frequency isolated DC / AC inverter according to claim 8, characterized in that: Based on the expression of the inner phase shift angle, the expression of the outer phase shift angle and the expression of the active current phasor modulus based on the inner phase shift angle and the outer phase shift angle as variables, the expression of the resonant cavity impedance is obtained, and based on the expression of the resonant inductance, the resonant capacitance and the resonant cavity impedance, the expression of the switching frequency of the primary side switch tube is obtained.

10. The modulation method for a single-stage high-frequency isolated DC / AC inverter according to claim 9, characterized in that: The fundamental component of the DC power supply voltage on the primary side is expressed as: The fundamental component of the AC power supply voltage on the secondary side is expressed as: The expression of the active current phasor modulus is: The expression of reactive current phasor modulus is: The modulus length expression of the active current phasor based on the inner phase shift angle and the outer phase shift angle as variables is obtained as follows: Based on the expression of the phase shift angle with the inner phase shift angle and the outer phase shift angle as variables, and based on the transformer's turns ratio, active current module length, and phase shift angle, the expression of the turn-on current value required by the switch tube S1 is: I dc_switch =n·I rp sin(α); The expression of the internal phase shift angle is: The expression of the outward shift phase angle is: The expression of the resonant cavity impedance is: The expression of the switching frequency of the primary side switch is: Where: n is the high-frequency transformer ratio, the instantaneous value of the AC side power supply voltage v ac , DC side power supply voltage V dc , 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 the leading and lagging bridge arm driving, and the external phase shift angle of the AC side voltage leading the DC side voltage Define v ab1,N (ω s The phasor modulus of t) is V p , nv cd1,N (ω s The phasor modulus of t) is V s , Z r is the resonant cavity impedance; define the phase shift angle α, which is divided by the inner phase shift angle θ and the outer phase shift angle Indicated: i ac is the AC side power supply current value; L r Expressed as the resonant inductance value of the series resonant cavity, C r Expressed as the resonant capacitance value in the series resonant cavity.

11. The modulation method for a single-stage high-frequency isolated DC / AC inverter according to any one of claims 6 to 10, characterized in that: Based on the differential equations of the resonant inductor current and resonant capacitor voltage under different working modes, the precise current value of the primary side switch tube at the switching moment is solved; Determining an error value of the turn-on current according to the turn-on current value of the primary-side switch tube and the precise current value; Obtain a new switching frequency of the primary-side switch tube based on the error value based on PI regulation, and obtain new inner and outer phase shift angles based on the new switching frequency of the primary-side switch tube in combination with an expression for the resonant cavity impedance, an expression for the inner phase shift angle, and an expression for the outer phase shift angle; Based on the new inner phase shift angle, outer phase shift angle and switching frequency determined above, the digital processor outputs a driving signal for driving the switch tube to perform power modulation on the inverter.

12. The modulation method for a single-stage high-frequency isolated DC / AC inverter according to claim 11, characterized in that: Based on the differential equations of the resonant inductor current and the resonant capacitor voltage under different working modes, the expression of the precise current value is: Where Z is the characteristic impedance of the resonant cavity, F is the ratio of the switching angular frequency to the resonant angular frequency, and the AC / DC voltage gain ratio M=v ac / nV dc .

13. A single-stage high-frequency isolated DC / AC inverter, characterized in that: Adopting the modulation method according to any one of claims 1 to 12, The invention comprises a primary-side bridge inverter circuit, a high-frequency transformer, a series resonant cavity, and a secondary-side AC circuit connected in sequence. The primary-side bridge inverter circuit comprises four switching tubes to form an H-bridge. The series resonant cavity contains a resonant inductor and a resonant capacitor. The secondary-side AC circuit comprises at least two groups of switching tubes to form a first bridge circuit. The first bridge circuit comprises two bridge arms, and the midpoints of the bridge arms are electrically connected to the series resonant cavity.

14. The single-stage high-frequency isolated DC / AC inverter according to claim 13, characterized in that: In the two bridge arms of the first bridge circuit, two groups of switch tubes form a first bridge arm, and two groups of switch tubes or two capacitors form a second bridge arm; the midpoint of the first bridge arm and the midpoint of the second bridge arm are both connected to the series resonant cavity.

15. The single-stage high-frequency isolated DC / AC inverter according to claim 14, characterized in that: In the switch tube groups of the first bridge circuit, when each switch tube group is configured as a switch tube, the secondary AC circuit further includes a second bridge circuit, which is connected in parallel to the first bridge circuit and is configured to output AC power.

16. The single-stage high-frequency isolated DC / AC inverter according to claim 15, characterized in that: The second bridge circuit includes four switch tube groups, and each switch tube group includes only one switch tube.

17. A single-stage high-frequency isolated DC / AC inverter, characterized in that: It includes a primary-side bridge inverter circuit, a high-frequency transformer, a series resonant 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 resonant cavity contains a resonant inductor and a resonant capacitor. The secondary-side AC circuit includes at least two groups of switching tubes 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 resonant cavity. The inverter is modulated by a control circuit, and the control circuit is provided with an inverter modulation method, the method comprising the steps of: Determining, based on a fundamental wave approximation algorithm, an inner phase shift angle between primary-side bridge arms, an outer phase shift angle between the primary-side bridge arms and the secondary-side bridge arms, and switching frequencies of the primary-side switching tube and the secondary-side switching tube to modulate a control variable of the inverter, wherein the control variable includes power transfer characteristics and soft switching characteristics; The power transmission characteristics determine the waveform quality of the secondary-side AC current, and the soft switching characteristics determine the power loss generated when the switch tube is turned off or on.

18. The single-stage high-frequency isolated DC / AC inverter according to claim 17, characterized in that: The power transfer characteristic is determined by the average value of the resonant current transmitted to the AC side output terminal in each switching cycle in the series resonant cavity; the soft switching characteristic includes the current direction of the secondary side switch tube being the same as the direction of its body diode during the dead time before it is turned on, and the charging and discharging of the output junction capacitance on both sides of the drain and source of the primary side switch tube being completed during the dead time of the bridge arm switch tube switching; 19. The single-stage high-frequency isolated DC / AC inverter according to claim 18, characterized in that: According to the phase corresponding to the resonant current, it is divided into an active current phasor and a reactive current phasor, and the modulus length of the active current phasor is used to control the power transmission characteristics of the inverter; the modulus length of the reactive current phasor is used to control the current direction of the secondary side switch tube in the dead time before it is turned on to be the same as the direction of its body diode; the charging and discharging of the output junction capacitance on both sides of the drain and source of the primary side switch tube is controlled by the resonant current, the high-frequency transformer ratio and the phase shift angle, and the phase shift angle is determined by the inner phase shift angle and the outer phase shift angle.

20. The single-stage high-frequency isolated DC / AC inverter according to claim 17, characterized in that: The method further comprises the steps of: calculating the precise current value at the switching moment of the primary-side switch tube, and compensating the control variable according to the precise current value.

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