A multi-mode switching control method for grid-connected isolated inverters

Abstract

The present invention discloses a multi-mode switching control method for a grid-connected isolated inverter. In each half of the grid voltage cycle, the instantaneous grid voltage phase θ output by the phase-locked loop inside the isolated inverter is used to control the grid voltage. g , where the phase θ g In [0,π], the phase of the grid voltage is given by θ g1 and θ g2 Comparing them, the two satisfy θ g1 ≤θ g2 , when 0<θ g <θ g1 and π‑θ g2 <θ g In the range of θ<π, the variable frequency discontinuous conduction mode, namely the variable frequency DCM mode, is used to control the main switch tube of the flyback converter in the isolation inverter to be turned on when the period is greater than the given switching period and the drain-source voltage is at the valley value. g1 <θ g <π‑θ g2 Within this range, the critical conduction mode (BCM) is used to control the flyback converter's main switch to turn on at the drain-source voltage valley. By adopting different operating modes at different output voltages, the present invention reduces the switching losses of the isolated inverter over the full load range, significantly improving the isolated inverter's energy conversion efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronic converter control, and in particular to a multi-mode switching control method suitable for a grid-connected isolation inverter. Background Art

[0002] Isolated converters play a crucial role in the grid-connected integration of renewable energy and energy storage power stations. A flyback converter combined with a DC / AC polarity conversion circuit is currently one of the mainstream isolated inverter topologies. The high-frequency flyback converter circuit primarily implements electrical isolation and closed-loop control of the grid-connected current. It is stable, reliable, and simple to control, reducing power losses and costs. The polarity conversion circuit converts the power-frequency sinusoidal half-wave current into a full-wave current, eliminating high-frequency switching losses. The filter circuit then absorbs harmonics to produce a grid-connected current that meets grid-connected requirements. MOSFET switching losses account for a significant portion of the losses in single-stage isolated inverters. Traditional flyback inverters operate in a hard-switching state, resulting in significant voltage overshoot when the switch is turned off, resulting in low energy conversion efficiency. Therefore, improving efficiency is a current research hotspot for isolated flyback inverters. Summary of the Invention

[0003] The purpose of the present invention is to overcome the shortcomings and deficiencies of the prior art and propose a multi-mode switching control method suitable for grid-connected isolated inverters. The method has simple overall control, can reduce the voltage stress of the switching tube, can realize quasi-resonant soft switching, and adopts variable frequency discontinuous conduction mode control near the zero crossing point of the grid voltage to improve energy conversion efficiency. The switching efficiency is improved by realizing quasi-resonant soft switching and DCM variable frequency control.

[0004] To achieve the above object, the present invention provides a technical solution: a multi-mode switching control method for a grid-connected isolated inverter, wherein the method is to control the instantaneous voltage phase θ of the grid output by the phase-locked loop inside the isolated inverter in each half grid voltage cycle. g , where the phase θ g In [0, π], the phase of the grid voltage is given by θ g1 and θ g2 Comparing them, the two satisfy θ g1 ≤θ g2 , when 0<θ g <θ g1 and π-θ g2 <θ e In the range of <π, the variable frequency discontinuous conduction mode, namely the variable frequency DCM mode, is used to control the main switch tube of the flyback converter in the isolation inverter to be turned on when the period is greater than the given switching period and the drain-source voltage is at the valley value, and in θ g1 <θ g <π-θ g2Within the range, the critical conduction mode, namely BCM mode, is adopted to control the main switch tube of the flyback converter to turn on at the drain-source voltage valley.

[0005] Furthermore, the grid voltage phase θ at the switching moment g1 and θ g2 Equal or unequal, and satisfy:

[0006]

[0007] Where N T is the transformer ratio of the flyback converter, V in is the input voltage of the isolated inverter, V gm is the output sinusoidal voltage amplitude.

[0008] Furthermore, the BCM mode adopts average current control or peak current control, and the control process includes the following steps:

[0009] S1, calculate the primary reference current i ref (θ g ): Let the reference current of the main switch be The parameters A and B in the formula are derived from the following formula:

[0010]

[0011] Where, L tot ≈L m is the primary magnetizing inductance of the flyback converter transformer, C tot ≈C s is the equivalent absorption capacitance of the primary side of the flyback converter, P g is the output power of the isolated inverter, V in is the input voltage of the isolated inverter, V g is the effective value of the grid-connected voltage, N is the primary-to-secondary ratio of the transformer;

[0012] S2. Calculate the delay time. In the equivalent circuit of the isolated inverter, the delay time t of the circuit can be obtained by resonance. d_BCM (θ g ) is equal to the unit delay time t d_T (θ g ):

[0013]

[0014] S3. Calculate the opening time t on (θ g ), the volt-second balance of the primary magnetizing inductance of the flyback converter can be obtained:

[0015]

[0016] S4. Calculate the off time t off (θ g ), the volt-second balance of the primary magnetizing inductance of the flyback converter can be obtained:

[0017]

[0018] S5, let the switching period T sw_BCM (θ g )for:

[0019] T sw_BCM (θ g )=t on (θ g )+t off (θ g )+t d_BCM (θ g )

[0020] The above formula can be combined to obtain the turn-on time, turn-off time and delay time of the main switch tube, so that the main switch tube can be turned on at the valley value of the drain-source voltage, reducing the switching loss in the BCM mode.

[0021] Furthermore, the variable frequency DCM mode is implemented by using average current control or peak current control. In the variable frequency DCM mode, the main switch tube is controlled to turn on at the valley bottom of the drain-source voltage resonance while limiting the switching period to be greater than a certain limit value. The control process includes steps S1 to S4 and the following step S6:

[0022] S6. Obtain the delay time t in variable frequency DCM mode according to the reference current, delay time, on-time and off-time obtained in steps S1 to S4. d_DCM (θ g ) and the switching period T sw_DCM (θ g ):

[0023]

[0024] T sw_DCM (θ g )=t on (θ g )+t off (θ g )+t d_DCM (θ g )

[0025] Where n is a counting variable with an initial value of 1, which is accumulated by 1 in the cycle judgment loop; the switching cycle T is judged. sw_DCM (θ g ) is less than the set minimum period T DCM_min , if it is less than the minimum period TDCM_min , then let: n = n + 1, and repeat the judgment until the period is greater than the minimum period T DCM_min , control the main switch tube to conduct;

[0026] By combining the above formulas to make the switching period meet the requirements, the turn-on time, turn-off time and delay time of the main switch tube under the variable frequency DCM mode can be obtained, thus realizing the variable frequency DCM mode and reducing switching losses.

[0027] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0028] 1. Frequency conversion switch in variable frequency discontinuous conduction mode (i.e. variable frequency DCM mode) at both ends of a half-sine wave.

[0029] The present invention controls the flyback converter to adopt a variable frequency discontinuous conduction mode with a lower switching frequency near the zero-crossing point of the output sinusoidal voltage, so that the main switch tube operates at a lower switching frequency, reducing the voltage stress and switching loss of the main switch tube.

[0030] 2. Achieved zero voltage switching.

[0031] By designing circuit parameters, the present invention derives appropriate main switch on and off times, controls the main switch to turn on at the valley of the drain-source voltage, and achieves zero-voltage turn-on of the main switch near the peak of the output sine wave, thereby greatly reducing the switching loss of the circuit and improving the energy conversion efficiency.

[0032] 3. Simple control method and excellent performance.

[0033] The multi-mode switching control method implemented in the present invention has efficient power conversion capability and a stable output current waveform. The inverter is small in size, light in weight, has good reliability and electrical isolation performance, high power generation efficiency, and has obvious economic benefits. It is suitable for various types of isolated inverter systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 Flowchart of the method of the present invention.

[0035] Figure 2 Schematic diagram of the interval of the method of the present invention.

[0036] Figure 3 Schematic diagram of the drain-source voltage and drain current of the main switch tube in half a grid cycle.

[0037] Figure 4 is the voltage V at the main switch terminal in BCM mode ds and drain current I d Waveform diagram.

[0038] Figure 5is the voltage V across the main switch in variable frequency DCM mode ds and drain current I d Waveform diagram. DETAILED DESCRIPTION

[0039] The present invention will be described in further detail below with reference to the embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0040] like Figure 1 As shown, this embodiment discloses a multi-mode switching control method for a grid-connected isolated inverter. The method is to control the instantaneous voltage phase θ of the grid output by the internal phase-locked loop of the isolated inverter in each half grid voltage cycle. g , where the phase θ g In [0, π], the phase of the grid voltage is given by θ g1 and θ g2 Comparing them, the two satisfy θ g1 ≤θ g2 , when 0<θ g <θ g1 and π-θ g2 <θ g In the range of θ<π, the variable frequency discontinuous conduction mode (ie, variable frequency DCM mode) is used to control the main switch of the flyback converter in the isolated inverter to be turned on when the period is greater than the given switching period and the drain-source voltage is at the valley value. g1 <θ g <π-θ g2 Within the range, the critical conduction mode (i.e., BCM mode) is adopted to control the main switch tube of the flyback converter to turn on at the drain-source voltage valley.

[0041] The isolated inverter samples the grid voltage and sends it to the phase-locked loop, which determines the working mode to be adopted based on the sampled grid phase.

[0042] The grid voltage phase θ at the switching moment of the multi-mode switching control method is g1 and θ g2 Can be equal or unequal, and satisfy:

[0043]

[0044] Where N T is the transformer ratio of the flyback converter, V in is the input voltage of the isolated inverter, V gm is the output sinusoidal voltage amplitude, Figure 2 Schematic diagram of the phase switching interval.

[0045] The BCM mode is used at the peak of the sine wave of the grid cycle. If the switching period T sw_BCM (θg )for:

[0046] T sw_BCM (θ g )=t on (θ g )+t off (θ g )+t d_BCM (θ g )

[0047] According to the volt-second balance principle of the primary magnetizing inductance of the flyback converter, it can be obtained that:

[0048]

[0049] Where V in is the output voltage of the isolated inverter, V g is the effective value of the grid voltage, N is the transformer ratio in the flyback converter, then the delay time t d_BCM (θ g ) is equal to the unit delay time t d_T (θ g ):

[0050]

[0051] Where, L tot ≈L m is the primary magnetizing inductance of the flyback converter transformer, C tot ≈C s is the equivalent absorption capacitance of the primary side of the flyback converter circuit. The following equations are combined to obtain the turn-on time t on (θ g ) and the turn-off time t off (θ g )for:

[0052]

[0053] The reference current is The parameters A and B in the formula can be obtained as follows:

[0054]

[0055] Where, P g is the output power of the isolated inverter, V in is the input voltage of the isolated inverter, V g is the effective value of the grid voltage.

[0056] Combining the above formula, we can get the turn-on time, turn-off time and delay time, and the switching period T can be obtained by comprehensive arrangement. sw_BCM (θ g )for:

[0057]

[0058] The main switch tube is controlled to turn on at the drain-source voltage valley. At this time, the voltage across the main switch tube is zero, thereby achieving quasi-resonant soft switching, reducing switching losses in BCM mode, and greatly improving energy conversion efficiency.

[0059] At both ends of the sinusoidal half-wave, the output power of the circuit is small. Controlling the main switch tube to operate in the variable frequency DCM mode can enable the main switch tube to be turned on at the bottom of the drain-source voltage resonance. At the same time, the switching period is limited to be greater than a certain threshold, thereby reducing the switching frequency, reducing losses, and improving the efficiency of the isolated inverter.

[0060] Let the switching period T sw_DCM (θ g )for:

[0061] T sw_DCM (θ g )=t on (θ g )+t off (θ g )+t d_DCM (θ g )

[0062] The reference current, turn-on time, and turn-off time for this mode are the same as those for the BCM mode. However, an appropriate delay time can make the switching period greater than the threshold while driving the main switch to turn on at the drain-source voltage valley. This can reduce circuit losses and improve efficiency under quasi-resonant soft switching. The delay time t for low-frequency switching of the main switch is d_DCM (θ g )for:

[0063]

[0064] Where n is a counting variable with an initial value of 1 and is incremented by 1 in the cycle judgment loop. Then the switching cycle T sw_DCM (θ g )for:

[0065]

[0066] When the switching period is greater than the set minimum period T DCM_min That is, when the switching frequency is lower than a given threshold, the main switch tube is controlled to turn on at the drain-source voltage valley, which can reduce the switching frequency and reduce the loss.

[0067] The turn-on time and turn-off time can be known, and the main switch can be controlled to turn on at the drain-source voltage valley, reducing losses and improving efficiency.

[0068] A simulation experiment was conducted in MATLAB / Simulink, with a run time of 0.08s. The simulated isolated inverter input voltage was 37V, the primary-to-secondary turns ratio of the flyback converter transformer was 1:6, the converted total resonant inductance of the primary side was 3uH, and the total resonant capacitance was 151.26uF. The grid voltage amplitude was 311V, the frequency was 50Hz, and the output capacitance was 0.66uF. The control method used was the average current method. The delay time td0 in the BCM mode was 0.672us, which is also half the resonant period of the variable frequency DCM mode. The minimum switching period of the variable frequency DCM mode was set to 10us. The mode switching parameters θ1 were set to 0.15 and θ2 to 2.97.

[0069] The drain-source voltage and drain current of the main switch tube of the isolated inverter in half a grid cycle are measured as follows: Figure 3 As shown, the voltage V of the main switch tube in BCM mode in this example is ds and drain current I d The waveform is as follows Figure 4 As shown, with the increase of transmission power, V ds The peak value of is also increasing. The voltage V ds The waveform of the drain current T d The waveform is as follows Figure 5 As shown in the figure. In the variable frequency DCM mode, when the excitation current drops to zero, the primary circuit of the flyback inverter begins to resonate. Since the minimum period of the given main switch is 10us, the main switch is only turned on when the switching period of the main switch is greater than this threshold and the drain-source voltage is at the resonance valley, which can reduce the switching loss of the switch.

[0070] The above description is merely a specific embodiment of the present application, but the scope of protection of the present application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this application should be included in the scope of protection of this application. Therefore, the scope of protection of this application should be based on the scope of protection of the claims.

Claims

1. A multi-mode switching control method for a grid-connected isolated inverter, characterized in that: This method is to use the grid instantaneous voltage phase output by the phase-locked loop inside the isolation inverter to generate a voltage signal in each half grid voltage cycle. , where the phase exist Within, with the given grid voltage phase and Comparing them, the two meet ,exist as well as The variable frequency discontinuous conduction mode (DCM) is used within the range to control the main switch of the flyback converter in the isolated inverter to be turned on when the period is greater than the given switching period and the drain-source voltage is at the valley value. Within the range, the critical conduction mode (BCM) is used to control the main switch of the flyback converter to turn on at the drain-source voltage valley. The grid voltage phase at the switching moment and Equal or unequal, and satisfy: ； Where, is the transformer primary-to-secondary ratio, is the input voltage of the isolated inverter, is the output sinusoidal voltage amplitude; The BCM mode adopts average current control or peak current control, and the control process includes the following steps: S1. Calculate the primary reference current :Let the reference current of the main switch be , the parameters in the formula and It is derived from the following formula: ； Where, is the primary magnetizing inductance of the flyback converter transformer, is the equivalent absorption capacitance of the primary side of the flyback converter, To isolate the output power of the inverter, is the input voltage of the isolated inverter, is the effective value of the grid voltage; S2. Calculate the delay time. In the equivalent circuit of the isolated inverter, the delay time of the circuit can be obtained by resonance. Equal to the unit delay time : ； S3. Calculate the opening time , the volt-second balance of the primary magnetizing inductance of the flyback converter can be obtained: ； S4. Calculate the shutdown time , the volt-second balance of the primary magnetizing inductance of the flyback converter can be obtained: ； S5, make the switching cycle for: ； The above formula can be combined to obtain the turn-on time, turn-off time and delay time of the main switch tube, so that the main switch tube can be turned on at the valley value of the drain-source voltage, reducing the switching loss in the BCM mode.

2. A multi-mode switching control method applicable to a grid-connected isolated inverter according to claim 1, characterized in that: The variable frequency DCM mode is implemented using average current control or peak current control. In the variable frequency DCM mode, the main switch is controlled to turn on at the valley bottom of the drain-source voltage resonance while limiting the switching period to be greater than a certain limit value. The control process includes steps S1 to S4 and the following step S6: S6. Obtain the delay time in variable frequency DCM mode according to the reference current, delay time, on-time, and off-time obtained in steps S1 to S4. and switching cycles : ； ； Where, It is a counting variable with an initial value of 1. It is incremented by 1 in the cycle judgment loop. Is it less than the set minimum period? , if it is less than the minimum period , then let: , loop judgment until the period is greater than the minimum period , control the main switch tube to be turned on; By combining the above formulas to make the switching period meet the requirements, the turn-on time, turn-off time and delay time of the main switch tube under the variable frequency DCM mode can be obtained, thus realizing the variable frequency DCM mode and reducing switching losses.

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