A control method of a bidirectional power converter, an electronic device, and a storage medium

Abstract

The application discloses a control method of a bidirectional power converter, the bidirectional power converter comprising a transformer, a first directional switch unit, a second directional switch unit, a third MOS tube, a fourth MOS tube, a first capacitor, a second capacitor, a third capacitor and a fourth capacitor. The control method comprises: in response to determining the energy flow direction, obtaining a given demand power; obtaining a zero crossing point through grid phase locking on the alternating current side to generate a gate signal of the first directional switch unit and the second directional switch unit according to the voltage polarity; obtaining a direct current bus voltage and an alternating current bus voltage, judging a current conduction mode based on the number of turns of the transformer, and determining a duty cycle of the third MOS tube and the fourth MOS tube according to the demand power. The application can realize high power density and low complexity on the basis of the single machine converter half-bridge circuit structure, and is easy to realize modulation while controlling the energy flow direction.

Description

Technical Field

[0001] The present application relates to the field of circuit control technology, and in particular to a control method, an electronic device, and a storage medium for a bidirectional power converter. Background Art

[0002] In recent years, with the increasing adoption of smart grids and renewable energy, and the increasing emphasis on power system efficiency and stability, demand for power conversion systems in renewable energy power systems has continued to grow for power supply and demand control and power quality improvement. Typically, the output and capacity of power conversion systems vary depending on their intended use. For example, when energy storage systems are added, power conversion circuits also need to implement bidirectional conversion capabilities.

[0003] In the process of conceiving and forming this application, the applicant discovered at least the following problems: the current power converter circuits are complex, most of which contain a large number of switches and gate drivers, and the control scheme is not easy to achieve stable power output quality and it is difficult to achieve a high power density. Summary of the Invention

[0004] The main purpose of the present application is to provide a control method for a bidirectional power converter, wherein the bidirectional power converter includes a transformer, a first directional switch unit, a second directional switch unit, a third MOS transistor, a fourth MOS transistor, a first capacitor, a second capacitor, a third capacitor, and a fourth capacitor:

[0005] The first end of the first directional switch unit is connected to the live end of the AC bus, the second end of the first directional switch unit is connected to the second end of the second directional switch unit and the first end of the AC side of the transformer, and the first end of the second directional switch unit is connected to the neutral end of the AC bus;

[0006] The drain of the third MOS transistor is connected to the positive terminal of the DC bus, and the source of the third MOS transistor is connected to the first end of the fourth MOS transistor and the negative terminal of the DC bus;

[0007] The first end of the first capacitor is connected to the drain of the third MOS transistor, the second end of the first capacitor is connected to the second end of the second capacitor, the first end of the second capacitor is connected to the source of the fourth MOS transistor, and the second end of the DC side of the transformer is connected to the common end of the first capacitor and the second capacitor;

[0008] The first end of the third capacitor is connected to the first end of the first directional switch unit, the second end of the third capacitor is connected to the second end of the fourth capacitor, the first end of the fourth capacitor is connected to the first end of the second directional switch unit, and the second end of the AC side of the transformer is connected to the common end of the first capacitor and the second capacitor;

[0009] The first directional switch unit and the second directional switch unit are used for directional conduction under different half-cycle currents of the AC bus;

[0010] The control method of the bidirectional power converter includes:

[0011] In response to determining the energy flow direction, obtaining a given required power;

[0012] On the AC side, a zero-crossing point is obtained by phase-locking the power grid to generate gate control signals for the first directional switch unit and the second directional switch unit according to the voltage polarity;

[0013] A DC bus voltage and an AC bus voltage are obtained, and a current conduction mode is determined based on the number of turns of the transformer, so as to determine duty cycles of the third MOS transistor and the fourth MOS transistor according to the required power.

[0014] Optionally, the step of obtaining a given required power in response to determining the energy flow direction includes:

[0015] Obtaining a primary voltage and a secondary voltage of the transformer, and determining an energy flow direction based on a phase relationship between the primary voltage and the secondary voltage;

[0016] The required power is determined according to the input current or the load power.

[0017] Optionally, the first directional switch unit includes a first A MOS transistor, a second B MOS transistor, a first diode and a second diode;

[0018] The drain of the first A MOS transistor is connected to the first end of the first directional switch unit, the source of the first A MOS transistor is connected to the anode of the first diode, and the cathode of the first diode is connected to the second end of the first directional switch unit;

[0019] The drain of the second B MOS transistor is connected to the second end of the first directional switch unit, the source of the second B MOS transistor is connected to the anode of the second diode, and the cathode of the second diode is connected to the first end of the first directional switch unit;

[0020] Or, the second directional switch unit includes a first B MOS transistor, a second A MOS transistor, a third diode and a fourth diode;

[0021] The drain of the first B MOS transistor is connected to the first end of the second directional switch unit, the source of the first B MOS transistor is connected to the anode of the third diode, and the cathode of the third diode is connected to the second end of the second directional switch unit;

[0022] The drain of the second A MOS transistor is connected to the second end of the second directional switch unit, the source of the second A MOS transistor is connected to the anode of the fourth diode, and the cathode of the fourth diode is connected to the first end of the second directional switch unit;

[0023] Alternatively, the first directional switch unit includes a first A MOS transistor and a second B MOS transistor, the drain of the first A MOS transistor is connected to the first end of the first directional switch unit, the source of the first A MOS transistor is connected to the source of the second B MOS transistor, and the drain of the second B MOS transistor is connected to the second end of the first directional switch unit;

[0024] Alternatively, the second directional switch unit includes a second A MOS transistor and a first B MOS transistor, the source of the second A MOS transistor is connected to the first end of the second directional switch unit, the drain of the second A MOS transistor is connected to the drain of the first B MOS transistor, and the source of the first B MOS transistor is connected to the second end of the second directional switch unit;

[0025] The step of obtaining a zero-crossing point by grid phase locking on the AC side to generate a gating signal of the first directional switch unit and the second directional switch unit according to the voltage polarity includes:

[0026] In the positive half cycle of the AC side bus voltage, the first MOS transistor A and the first MOS transistor B are controlled to respectively perform complementary high-frequency switching with a duty cycle of 50%, and the second MOS transistor A and the second MOS transistor B are made normally open;

[0027] In the negative half cycle of the AC side bus voltage, the second MOS transistor A and the second MOS transistor B are controlled to be complementary high-frequency switches with a duty cycle of 50%, and the first MOS transistor A and the first MOS transistor B are kept normally open.

[0028] Optionally, the step of obtaining the DC bus voltage and the AC bus voltage, judging the current conduction mode based on the number of turns of the transformer, and determining the duty cycles of the third MOS transistor and the fourth MOS transistor according to the required power further includes:

[0029] determining a mode switching boundary condition based on a target duty cycle in a current conduction mode based on a DC bus voltage and an AC bus voltage;

[0030] When the target duty cycle reaches the mode switching boundary condition, the system operates at a constant frequency until the mode switching is completed.

[0031] Optionally, in the step of determining a mode switching boundary condition based on a target duty cycle in the current conduction mode based on the DC bus voltage and the AC bus voltage, a duty cycle of the fourth MOS transistor increasing from 0 to a maximum current is taken as the target duty cycle, and the target duty cycle is determined according to the following expression:

[0032]

[0033]

[0034] Where d1 is the target duty cycle, and 0 <d1<1;T1为第四MOS管的电流从0增加到最大电流的时间,Ts为工作周期,d2为第三MOS管从最大电流减少到0的占空比,n为变压器匝数,Vac为交流母线电压,Vdc为直流母线电压。

[0035] Optionally, the transformer includes an excitation inductor; and when the target duty cycle reaches the mode switching boundary condition, the step of operating at a constant frequency until the mode switching is completed includes:

[0036] The required current is determined based on the required power. When the target duty cycle is less than the mode switching boundary condition, the mode is switched to the discontinuous conduction mode, and the target duty cycle is calculated according to the following expression:

[0037]

[0038] Among them, Ls is the inductance of the excitation inductor, fs is the operating frequency, and IL is the required current.

[0039] Optionally, when the target duty cycle reaches the mode switching boundary condition, the step of operating at a constant frequency until the mode switching is completed includes:

[0040] The required current is determined based on the required power. When the target duty cycle is greater than the mode switching boundary condition, the mode is switched to the continuous conduction mode, and the target duty cycle is calculated according to the following expression:

[0041]

[0042] Among them, Ls is the inductance of the excitation inductor, fs is the operating frequency, and IL is the required current.

[0043] Optionally, the control method of the bidirectional power converter further includes:

[0044] When operating in the discontinuous conduction mode, the switches of the first directional switch unit, the second directional switch unit, and the third MOS transistor and the fourth MOS transistor operate in a zero voltage switching manner when turned on.

[0045] Optionally, when operating in the discontinuous conduction mode, the switches of the first directional switch unit, the second directional switch unit, and the third MOS transistor operate in a zero-current switching manner when turned off.

[0046] Optionally, when operating in the discontinuous conduction mode, the switches of the first directional switch unit, the second directional switch unit, and the third MOS transistor and the fourth MOS transistor operate in a zero voltage switching manner when turned on.

[0047] The present application also provides an electronic device, comprising a processor and a memory;

[0048] The memory stores a computer program, and when the computer program is executed by the processor, the steps of the control method of the bidirectional power converter as described above are implemented.

[0049] The present application also provides a storage medium having a computer program stored thereon. When the computer program is executed by a processor, the steps of the control method of the bidirectional power converter as described above are implemented.

[0050] The control method, electronic device, and storage medium of the bidirectional power converter provided in the present application are based on the bidirectional power converter circuit provided in the present application, and obtain a given required power by responding to the determined direction of energy flow; obtain a zero crossing point on the AC side through grid phase locking to generate gating signals for the first directional switch unit and the second directional switch unit according to the voltage polarity; obtain the DC bus voltage and the AC bus voltage, and judge the current conduction mode based on the number of turns of the transformer to determine the duty cycle of the third MOS tube and the fourth MOS tube according to the required power; based on the half-bridge circuit structure of a stand-alone converter, combined with the control switching function, it can achieve higher power density and lower complexity, and while controlling the direction of energy flow, an easy-to-implement modulation scheme can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The accompanying drawings herein are incorporated into and constitute a part of the specification, illustrate embodiments consistent with the present application, and together with the specification, are used to explain the principles of the present application. In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for describing the embodiments. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without inventive work.

[0052] Figure 1 A schematic diagram of a bidirectional power converter circuit according to an embodiment of the present application is shown in FIG. Figure 1 .

[0053] Figure 2A schematic diagram of a bidirectional power converter circuit according to an embodiment of the present application is shown in FIG. Figure 2 .

[0054] Figure 3 Schematic diagram of the connection between the first directional switch unit and the second directional switch unit in one embodiment of the present application Figure 1 .

[0055] Figure 4 Schematic diagram of the connection between the first directional switch unit and the second directional switch unit in one embodiment of the present application Figure 2 .

[0056] Figure 5 This is a flow chart of a control method for a bidirectional power converter according to an embodiment of the present application.

[0057] Figure 6 FIG. 1 is a timing diagram of a DC to AC conversion process according to an embodiment of the present application.

[0058] The purpose of this application, its features, and advantages will be further described in conjunction with the embodiments and with reference to the accompanying drawings. The accompanying drawings illustrate specific embodiments of this application, which will be described in more detail below. These drawings and the accompanying text are not intended to limit the scope of the present application in any way, but rather to illustrate the concepts of this application to those skilled in the art by reference to specific embodiments. DETAILED DESCRIPTION

[0059] It should be understood that the specific embodiments described herein are only used to explain the present application and are not intended to limit the present application.

[0060] Now, various embodiments of the present application will be described with reference to the accompanying drawings. In the following description, suffixes such as "module," "component," or "unit" used to represent elements are only used to facilitate the description of the application and have no specific meaning.

[0061] First embodiment

[0062] This application first provides a bidirectional power converter, Figure 1 A schematic diagram of a bidirectional power converter circuit according to an embodiment of the present application is shown in FIG. Figure 1 .

[0063] like Figure 1 As shown, the bidirectional power converter includes a transformer T, a first directional switch unit K1, a second directional switch unit K2, a third MOS transistor S3, a fourth MOS transistor S4, a first capacitor Cp1, a second capacitor Cp2, a third capacitor Cs3 and a fourth capacitor Cs4.

[0064] A power converter is an electronic device that converts one type of current into another, whether it's direct current (DC) or alternating current (AC). For example, a bidirectional AC / DC converter, a power electronic device capable of bidirectional conversion between alternating current (AC) and direct current (DC), operates on multiple key components and requires complex control strategies.

[0065] The first end of the first directional switch unit K1 is connected to the live terminal of the AC busbar, the second end of the first directional switch unit K1 is connected to the second end of the second directional switch unit K2 and the first end of the AC side of the transformer T, and the first end of the second directional switch unit K2 is connected to the neutral terminal of the AC busbar. The first and second directional switch units K1 and K2 are configured to conduct directional currents under different half-cycle currents of the AC busbar.

[0066] A transformer is a device that uses the principle of electromagnetic induction to change AC voltage. Depending on the number of turns on the source side and the secondary side, the transformer can convert the AC waveform on one side into an AC waveform on the other side for output. Transformers can be used to achieve functions such as voltage conversion, current conversion, impedance conversion, isolation, and voltage stabilization. Please refer to Figure 1 The transformer uses magnetic integration technology, with leakage inductance acting as the energy storage inductor Ls. Using magnetic integration technology, the leakage inductance formed by the air gap in the transformer core acts as the magnetizing inductor to transfer energy. The transformer's magnetizing inductance is used to achieve zero voltage switching (ZVS) operation.

[0067] For example, the first directional switch can conduct current toward the AC source during the positive half-cycle of the AC bus and toward the transformer during the negative half-cycle of the AC bus. The second directional switch can conduct current toward the AC source during the negative half-cycle of the AC bus and toward the transformer during the positive half-cycle of the AC bus. This allows circuit control to flexibly control the direction of current in different situations.

[0068] Please continue to refer to Figure 2 The first end of the third capacitor Cs3 is connected to the first end of the first directional switch unit K1, the second end of the third capacitor Cs3 is connected to the second end of the fourth capacitor Cs4, the first end of the fourth capacitor Cs4 is connected to the first end of the second directional switch unit K2, and the second end of the AC side of the transformer T is connected to the common end of the first capacitor Cp1 and the second capacitor Cp2.

[0069] Exemplarily, the third capacitor and the fourth capacitor can store or release electric quantity in different oscillation periods, so as to form electromagnetic oscillation with the inductance component in the circuit. Exemplarily, there is leakage inductance due to the magnetic flux characteristic in the transformer adopting the magnetic integration technology, and the leakage inductance can act as the energy storage inductance. By controlling the first directional switch unit and the second directional switch unit to be directed on under different half-cycle currents, a suitable alternating current waveform can be provided to the transformer in the process of alternating current to direct current, or a suitable alternating current waveform can be output in the process of direct current to alternating current.

[0070] The drain of the third MOS tube S3 is connected with the positive terminal of the direct current bus, and the source of the third MOS tube S3 is connected with the first terminal of the fourth MOS tube S4 and the negative terminal of the direct current bus.

[0071] Exemplarily, the MOSFET has three pins, which are generally divided into gate, source and drain. By adding a control signal between the gate and the source, the conduction and the cut-off between the drain and the source can be changed.

[0072] The first terminal of the first capacitor Cp1 is connected with the drain of the third MOS tube S3, the second terminal of the first capacitor Cp1 is connected with the second terminal of the second capacitor Cp2, the first terminal of the second capacitor Cp2 is connected with the source of the fourth MOS tube S4, and the second terminal of the direct current side of the transformer T is connected with the common terminal of the first capacitor and the second capacitor Cp2.

[0073] Exemplarily, the first capacitor and the second capacitor can store or release energy in different oscillation periods, so as to form electromagnetic oscillation with the inductance component in the circuit. Exemplarily, there is leakage inductance due to the magnetic flux characteristic in the transformer adopting the magnetic integration technology, and the leakage inductance can act as the energy storage inductance. By turning on the third MOS tube and the fourth MOS tube under different half-cycle currents, a suitable direct current voltage can be output in the process of alternating current to direct current, or a suitable alternating current waveform can be generated for the transformer in the process of direct current to alternating current.

[0074] Optionally, the third MOS tube S3 and the fourth MOS tube S4 are N-type tubes. Optionally, the third MOS tube S3 and the fourth MOS tube S4 have body diodes.

[0075] For example, a MOS transistor body diode, also known as a parasitic diode or built-in diode, refers to a parasitic element present in a metal-oxide-semiconductor field-effect transistor (MOSFET). It is formed by the PN junction between the substrate (B terminal) and drain (D terminal) of the MOS transistor. Due to the short circuit between the source (S terminal) and the substrate (B terminal), a body diode is formed between the source and drain. In this embodiment, by utilizing electromagnetic oscillations on the primary and secondary sides of the transformer and the body diodes of the third and fourth MOS transistors, zero voltage or zero current switching (ZCS) can be achieved in the DC side circuit of the transformer, effectively reducing circuit losses. Compared to a full-bridge structure, this eliminates the need for two MOS transistors on the low-voltage side, reducing control complexity. The simple circuit structure of a single-stage half-bridge converter achieves both zero voltage switching (ZVS) and zero current switching (ZCS), improving power conversion reliability. Lower circuit losses and reduced heat generation also increase the power density of the circuit.

[0076] Figure 2 A schematic diagram of a bidirectional power converter circuit according to an embodiment of the present application is shown in FIG. Figure 2 .

[0077] Please refer to Figure 2 Optionally, the bidirectional power converter circuit further includes a filter capacitor EC3, the positive electrode of the filter capacitor EC3 is connected to the positive terminal of the DC bus, and the negative electrode of the filter capacitor EC3 is connected to the negative terminal of the DC bus.

[0078] Filter capacitors have multiple functions in electronic circuits, mainly including filtering noise, stabilizing voltage, and storing energy. Through its energy storage characteristics, filter capacitors can filter out the AC component in the rectifier circuit, making the output DC voltage smoother and more stable. Specifically, when the rectifier voltage is higher than the capacitor voltage, the capacitor charges; when the rectifier voltage is lower than the capacitor voltage, the capacitor discharges. During this process, the output voltage remains basically stable. As an energy storage device, the filter capacitor can provide a stable DC power supply when needed. In all circuits that need to convert AC power to DC power, the installation of filter capacitors will make the operating performance of the electronic circuit more stable, while also reducing the interference of alternating pulsating ripples on the electronic circuit.

[0079] Please continue to refer to Figure 3 Optionally, the bidirectional power conversion circuit further includes a first inductor Ldc, which is connected between the drain of the third MOS transistor S3 and the positive electrode of the filter capacitor EC3, or the first inductor Ldc is connected between the source of the fourth MOS transistor S4 and the negative electrode of the filter capacitor EC3.

[0080] Please continue to refer to Figure 1Optionally, the bidirectional power conversion circuit further includes a second inductor Lac. The second inductor Lac is connected between the first end of the first directional switch unit K1 and the live terminal of the AC bus, or the second inductor Lac is connected between the first end of the second directional switch unit K2 and the neutral terminal of the AC bus.

[0081] For example, the first inductor and the second inductor, combined with the capacitor, can achieve an oscillation effect and a filtering effect. The inductor plays the role of energy storage in the circuit. When the switch tube is turned on, the inductor current rises linearly, and the inductor stores energy. In the circuit at both ends of the transformer, the inductor's functions mainly include filtering, oscillation, and energy storage. The inductor not only affects the oscillation frequency, but can also be used to adjust circuit performance parameters, filter, isolate noise, etc., to improve the stability and reliability of the circuit. Through switch control, the inductor current can be switched between DCM (discontinuous conduction mode) and CCM (continuous conduction mode), where it can work in DCM mode at low power and in CCM mode at high power, which is easier to implement in algorithm control.

[0082] Figure 3 Schematic diagram of the connection between the first directional switch unit and the second directional switch unit in one embodiment of the present application Figure 4 .

[0083] like Figure 2 As shown, optionally, the first directional switch unit K1 includes a first A MOS transistor S1a and a second B MOS transistor S2a, the drain of the first A MOS transistor S1a is connected to the first end of the first directional switch unit K1, the source of the first A MOS transistor S1a is connected to the source of the second B MOS transistor S2b, and the drain of the second B MOS transistor S2b is connected to the second end of the first directional switch unit K1.

[0084] Optionally, the second directional switch unit K2 includes a second A MOS transistor S2a and a first B MOS transistor S1b, the source of the second A MOS transistor S2a is connected to the first end of the second directional switch unit K2, the drain of the second A MOS transistor S2a is connected to the drain of the first B MOS transistor S1b, and the source of the first B MOS transistor S1b is connected to the second end of the second directional switch unit K2.

[0085] For example, the first and second directional switch units are connected in reverse order via two MOS transistors, and one of the MOS transistors can be controlled to be normally on via a control terminal. When the other MOS transistor is controlled to be off, the unidirectional conduction characteristics of the MOS transistor's body diode can be utilized to achieve zero-voltage or zero-current switching of the circuit, thereby further reducing circuit losses.

[0086] Figure 4Schematic diagram of the connection between the first directional switch unit and the second directional switch unit in one embodiment of the present application Figure 4 .

[0087] like Figure 5 As shown, optionally, the first directional switch unit K1 includes a first A MOS transistor S1a, a second B MOS transistor S2b, a first diode D1 and a second diode D2.

[0088] For example, a diode is an electronic device made of semiconductor materials (silicon, selenium, germanium, etc.) [1]. A diode has two electrodes: a positive electrode, also called an anode; and a negative electrode, also called a cathode. When a forward voltage is applied between the two electrodes of the diode, the diode is turned on. When a reverse voltage is applied, the diode is turned off. The diode has unidirectional conductivity. When it is turned on, the current flows from the anode through the tube to the cathode. The conduction and cutoff of the diode are equivalent to the on and off of a switch.

[0089] Furthermore, the drain of the first A MOS transistor S1a is connected to the first end of the first directional switch unit K1, the source of the first A MOS transistor S1a is connected to the anode of the first diode D1, and the cathode of the first diode D1 is connected to the second end of the first directional switch unit K1.

[0090] By connecting a diode and a MOS transistor in series, it is possible to ensure that current can flow in one direction in a predetermined branch. For example, the series connection of the first MOS transistor A and the first diode can limit the current in the branch to flow from the AC bus toward the transformer.

[0091] Furthermore, the drain of the second MOS transistor S2b is connected to the second end of the first directional switch unit K1, the source of the second MOS transistor S2b is connected to the anode of the second diode D2, and the cathode of the second diode D2 is connected to the first end of the first directional switch unit K1.

[0092] By connecting a diode and a MOS transistor in series, it is possible to ensure that current can flow in one direction in a predetermined branch. For example, the series connection of a second MOS transistor and a second diode can limit the current in the branch to flow from the transformer toward the AC bus.

[0093] Please continue to refer to Figure 5 Optionally, the second directional switch unit K2 includes a first B MOS transistor S1b, a second A MOS transistor S2a, a third diode D3 and a fourth diode D4.

[0094] The drain of the first B MOS transistor S1b is connected to the first end of the second directional switch unit K2, the source of the first B MOS transistor S1b is connected to the anode of the third diode D3, and the cathode of the third diode D3 is connected to the second end of the second directional switch unit K2.

[0095] The drain of the second A MOS transistor S2a is connected to the second end of the second directional switch unit K2, the source of the second A MOS transistor S2a is connected to the anode of the fourth diode D4, and the cathode of the fourth diode D4 is connected to the first end of the second directional switch unit K2.

[0096] By connecting a diode and a MOS transistor in series, it is possible to ensure that current can flow in a single direction in a predetermined branch. For example, by connecting a first B MOS transistor and a third diode in series, the current in that branch can be limited to flowing from the AC bus toward the transformer. For example, by connecting a second A MOS transistor and a fourth diode in series, the current in that branch can be limited to flowing from the transformer toward the AC bus.

[0097] This embodiment utilizes a single-stage converter half-bridge DAB structure, where both the AC and DC sides are half-bridges, enabling bidirectional switching. This provides a half-bridge direct matrix AC / DC converter capable of single-stage power conversion and bidirectional power transmission. By utilizing a half-bridge structure on both the AC and DC sides, this embodiment utilizes fewer switches and gate drivers than a full-bridge converter, simplifying the circuit board structure and reducing the complexity of the AC / DC converter, thereby improving reliability and power density. High power density requires more efficient heat dissipation or suppression, making ZCS (zero current switching) and ZVS (zero voltage switching) particularly important.

[0098] This embodiment provides a current sensorless modulation scheme. The MOS switches involved achieve both zero-current switching (ZCS) and zero-voltage switching (ZVS) across the entire AC voltage range and a wide load range. The performance of the proposed AC / DC converter has been verified using a 2 kW silicon carbide (SiC)-based prototype, achieving a high efficiency of 96.8% under rated conditions, a current total distortion (THD) of less than 4%, and a power density of 1.8 kW / dm³.

[0099] Second embodiment

[0100] On the basis of the first embodiment, the power converter circuit provided by the present application can operate at a constant switching frequency and can implement soft switching in the entire operating range through a simple modulation scheme.

[0101] Figure 6This is a flow chart of a control method for a bidirectional power converter according to an embodiment of the present application.

[0102] like Figure 6 As shown, the control method of the bidirectional power converter includes:

[0103] S10: In response to determining the energy flow direction, obtaining a given required power;

[0104] S20: obtaining a zero-crossing point by grid phase locking on the AC side to generate gate control signals for the first directional switch unit and the second directional switch unit according to voltage polarity;

[0105] S30: Obtaining a DC bus voltage and an AC bus voltage, and determining a current conduction mode based on the number of turns of the transformer, so as to determine duty cycles of the third MOS transistor and the fourth MOS transistor according to the required power.

[0106] For example, the energy direction is determined first: AC (alternating current) to DC (direct current) or DC to AC. The power demand is then determined. For example, DC to AC is generally determined by the input current, ensuring that the incoming energy is inverted. AC to DC, on the other hand, can be determined by the load power. After grid phase locking, the zero-crossing point is determined, and a control signal is generated based on the voltage polarity, thereby controlling the duty cycle of the DC-side switch according to the corresponding conduction mode.

[0107] Optionally, the control method of the bidirectional power converter further includes:

[0108] When operating in the discontinuous conduction mode, the switches of the first directional switch unit, the second directional switch unit, and the third MOS transistor and the fourth MOS transistor operate in a zero voltage switching manner when turned on.

[0109] Optionally, when operating in the discontinuous conduction mode, the switches of the first directional switch unit, the second directional switch unit, and the third MOS transistor operate in a zero-current switching manner when turned off.

[0110] Optionally, when operating in the discontinuous conduction mode, the switches of the first directional switch unit, the second directional switch unit, and the third MOS transistor and the fourth MOS transistor operate in a zero voltage switching manner when turned on.

[0111] This embodiment determines the switch gating signal on the AC side by measuring the zero-crossing point on the AC side. Based on the voltage ratio and conduction mode requirements on the DC side, the duty cycle of the MOS transistor is controlled on the DC side while taking into account the gating timing of the third MOS transistor and the fourth MOS transistor. In the full range of AC voltage and a wide load range, the MOS transistor switches involved achieve zero voltage switching and zero current switching over the entire AC voltage range, thereby improving power conversion efficiency.

[0112] Optionally, the step of obtaining a given required power in response to determining the energy flow direction includes:

[0113] Obtaining a primary voltage and a secondary voltage of the transformer, and determining an energy flow direction based on a phase relationship between the primary voltage and the secondary voltage;

[0114] The required power is determined according to the input current or the load power.

[0115] Figure 6 The following is a timing diagram of DC to AC conversion according to an embodiment of the present application. (a) is a timing diagram of discontinuous conduction mode (DCM), and (b) is a timing diagram of continuous conduction mode (CCM).

[0116] For example, in the process of determining whether the energy direction is AC (alternating current) to DC (direct current) or DC to AC, the voltage curves of the AC side and the DC side of the transformer can be collected and determined based on the voltage phase relationship on both sides of the transformer. For example, in terms of phase relationship, the side with the leading phase is the input side, which serves as the primary side of the transformer, and the side with the lagging phase is the output side, which serves as the secondary side of the transformer. Figure 2 In the embodiment shown, the DC side voltage phase leads, and the AC side voltage phase lags. Therefore, this embodiment is described in detail using the DC to AC control logic.

[0117] Optionally, the step of obtaining a zero-crossing point by grid phase locking on the AC side to generate gating signals for the first directional switch unit and the second directional switch unit according to voltage polarity includes:

[0118] In the positive half cycle of the AC side bus voltage, the first MOS transistor A and the first MOS transistor B are controlled to respectively perform complementary high-frequency switching with a 50% duty cycle, and the second MOS transistor A and the second MOS transistor B are made normally open;

[0119] In the negative half cycle of the AC side bus voltage, the second MOS transistor A and the second MOS transistor B are controlled to be complementary high-frequency switches with a duty cycle of 50%, and the first MOS transistor A and the first MOS transistor B are kept normally open.

[0120] Please continue to refer to Figure 3 And also refer to Figure 4 、 Figure 6 and Figure 2 For example, in the positive half cycle of the AC, the first MOS transistor S1a and the first MOS transistor S1b each have a 50% duty cycle, which is set as the gate signal of the complementary high-frequency switch to control the second MOS transistor S1a and the second MOS transistor S1b to be normally open.

[0121] Please continue to refer to Figure 3 And also refer to Figure 4、 Figure 2 and Figure 6 Exemplarily, in the negative half cycle of the alternating current, the second MOS tube S1a and the second MOS tube S1b are set as complementary high-frequency switch control signals with a duty cycle of 50%, and the first MOS tube S1a and the first MOS tube S1b are always open.

[0122] By controlling different currents in different half cycles of the alternating current side, the first directional switching unit K1 and the second directional switching unit K2 are directionally turned on under different half cycle currents of the alternating current side, so as to realize DC to AC conversion, achieve zero voltage switching and zero current switching in the entire alternating voltage range, and improve power conversion efficiency.

[0123] Optionally, the step of obtaining the direct current bus voltage and the alternating current bus voltage, determining the current conduction mode based on the number of turns of the transformer, and determining the duty cycle of the third MOS tube and the fourth MOS tube according to the demand power further comprises:

[0124] determining a mode switching boundary condition based on a target duty cycle in the current conduction mode based on the direct current bus voltage and the alternating current bus voltage;

[0125] when the target duty cycle reaches the mode switching boundary condition, working at a constant frequency until the mode switching is completed.

[0126] Exemplarily, in the switching process of the discontinuous conduction mode (DCM) and the continuous conduction mode (CCM), switching needs to be performed according to the boundary condition. Exemplarily, in the calculation process of the switching boundary condition of the discontinuous conduction mode (DCM) and the continuous conduction mode (CCM) when switching from alternating current to direct current, when the fourth MOS tube is turned on, the duty cycle of the energy storage current increasing from 0 to the maximum current can be a reference factor, and when the third MOS tube is turned on, the duty cycle of the energy storage current decreasing from the maximum current to 0 can also be a reference factor.

[0127] Optionally, in the step of determining a mode switching boundary condition based on a target duty cycle in the current conduction mode based on the direct current bus voltage and the alternating current bus voltage, the duty cycle of the fourth MOS tube current increasing from 0 to the maximum current is the target duty cycle, and the target duty cycle is determined according to the following expression:

[0128]

[0129]

[0130] Where d1 is the target duty cycle, and 0 <d1<1;T1为第四MOS管的电流从0增加到最大电流的时间,Ts为工作周期,d2为第三MOS管从最大电流减少到0的占空比,n为变压器匝数,Vac为交流母线电压,Vdc为直流母线电压。

[0131] Please also refer to Figure 6 For example, the circuit topology of this embodiment supports bidirectional energy flow, that is, it can achieve inversion. For example, DC input and AC output can achieve PFC, that is, the direction of energy flow of AC input and DC output is determined by the boundary duty cycle db mentioned below, which affects the duty cycle of the third MOS transistor S3 and the fourth MOS transistor S4. b is the abbreviation of boundary, which means boundary.

[0132] For example, the boundary conditions for switching between AC and DC operating modes (DCM, CCM) are:

[0133]

[0134] At this time, d1 is the duty cycle of the energy storage current (inductor current) increasing from 0 to the maximum current when the fourth MOS transistor S4 is turned on. t1 is the time, and the current t1 value can be obtained by looking up the table based on the current control frequency. d2 is the duty cycle of the energy storage current (inductor current) decreasing from the maximum current to 0 when the third MOS transistor S3 is turned on.

[0135] Optionally, the transformer includes an excitation inductor; and when the target duty cycle reaches the mode switching boundary condition, the step of operating at a constant frequency until the mode switching is completed includes:

[0136] The required current is determined based on the required power. When the target duty cycle is less than the mode switching boundary condition, the mode is switched to the discontinuous conduction mode, and the target duty cycle is calculated according to the following expression:

[0137]

[0138] Among them, Ls is the inductance of the excitation inductor, fs is the operating frequency, and IL is the required current.

[0139] Exemplarily, when the target duty cycle d1 is less than the boundary duty cycle db, the control system operates in the discontinuous conduction mode.

[0140] Optionally, when the target duty cycle reaches the mode switching boundary condition, the step of operating at a constant frequency until the mode switching is completed includes:

[0141] The required current is determined based on the required power. When the target duty cycle is greater than the mode switching boundary condition, the mode is switched to the continuous conduction mode, and the target duty cycle is calculated according to the following expression:

[0142]

[0143] Among them, Ls is the inductance of the excitation inductor, fs is the operating frequency, and IL is the required current.

[0144] For example, when the target duty cycle d1 is greater than the boundary duty cycle db, the control system operates in continuous conduction mode. According to the calculation formula for the target duty cycle d1, once the target duty cycle d1 is determined, the duty cycle d2 at which the third MOS transistor decreases from maximum current to zero can also be determined. Once all the quantities in the above block diagram are known, topology control can be achieved.

[0145] For example, a gap, called dead time, is left between turning on the third MOS transistor and turning off the fourth MOS transistor. This prevents the entire load from short-circuiting when the third and fourth MOS transistors are turned on simultaneously. The duty cycle Ts and operating frequency fs are determined based on the performance of the hardware controller; the higher the better.

[0146] Please refer to Figure 2 For example, referring to the single phase-shift control strategy of the DAB topology, when switching from DCM to CCM mode, at the critical switching point, the proportion of t1 will gradually equal t2. At a certain point, t1 of the third MOS tube S3 will be equal to t2. t1 is the on-time, t2 is the off-time, and the duty cycle is 0.5. From this point to the complete CCM mode, constant frequency operation will be maintained to achieve constant frequency switching control. From this moment on, in the entire CCM mode, since there is a linear relationship between power and phase shift angle, the only variable is the phase shift angle between Vp and Vs. It can be seen from the above formula that in the DCM and CCM control process, it is only necessary to detect the zero crossing point, and there is no need to detect the current. This greatly simplifies the control difficulty and becomes very easy to control. There is no need to design complex algorithms to achieve the target state.

[0147] Please continue to refer to Figure 6 , depending on whether the secondary current drops to zero during Ts / 2, it can be divided into discontinuous conduction mode (DCM) or continuous conduction mode (CCM)

[0148] Figure a shows the DCM mode, where the inductor current reaches 0 in advance within Ts / 2.

[0149] b shows the CCM mode, where the inductor current has not reached zero at the end of Ts / 2.

[0150] For example, a MOS tube that can freewheel during operation can clamp the voltage across the diode through its own body diode. At this time, current can flow when the switch is turned on. As can be seen from P=IV, I is relatively large at this time, but V is only the diode voltage drop of 0.7V (the default voltage is zero), which can greatly reduce the conduction loss of the diode, thus achieving ZVS and thus realizing ZVS logic.

[0151] Please also refer to Figure 6 and Figure 6 For example, ZCS stands for Zero Current Shutdown, meaning that in DCM mode, the transformer current can drop to zero. A transformer current zero-crossing comparator can be designed using a comparator. Once the current crosses zero, the target MOSFET is turned off, ensuring that I in P=IV is 0, significantly reducing MOSFET shutdown losses. For the complementary transistors S3 and S4, the third MOSFET S3 is turned off when the current reaches zero. After a dead time, the fourth MOSFET S4 is turned on at 0.7V, achieving ZVS.

[0152] For example, the inductor current returns to zero during a half-cycle, allowing the AC-side MOSFET to achieve ZCS soft switching. For the complementary switches S1 and S2, one switches off at zero current, and after a dead time, the other switches on at 0.7V, achieving ZVS.

[0153] like Figure 6 As shown in Figure 2, the switching loss of S4 on the DC side is higher than that of S3 because S4 is turned off at high current, while S3 is zero current shutdown (ZCS). ​ (a). This means that in DCM mode, the control angle has two dimensions: one is the duty cycle of S3 and S4, and the other is the phase shift angle between the DC side and the AC side, that is, the delay time of the S3 and S4 wave transmission.

[0154] Please continue to refer to ​ In CCM mode, the current iL is non-zero at each MOSFET switching instant. During continuous conduction mode (CCM) operation, all MOSFET switches achieve zero voltage switching (ZVS). In this case, power can be adjusted by shifting the phase angle. In other words, in DCM mode, ZVS can be achieved when all MOSFETs except S4 are turned on, and ZCS can be achieved when turned off, because the transformer current reaches zero. In CCM mode, all MOSFETs can only achieve ZVS, not ZCS, because the current is non-zero at each instant.

[0155] In this implementation, magnetic integration technology is employed to transfer energy using leakage inductance created by gapping the transformer core. This magnetizing inductance is used to achieve zero-voltage switching (ZVS) operation over a wide load range, which helps reduce switching losses. Furthermore, controllable inductor switching between DCM and CCM is possible to adapt to different operating conditions. Furthermore, the modulation scheme derived from time-domain analysis is also easy to implement, simplifying the design.

[0156] The proposed converter has a minimum number of switches and gate drivers, making the circuit board simpler. Furthermore, the proposed converter operates at a constant switching frequency and can achieve soft switching across the entire operating range using a simple modulation scheme. It is primarily used in solar micro-grid-connected inverters, enabling bidirectional energy flow. Only software modifications are required to control the direction of energy flow, which helps simplify the hardware.

[0157] Third embodiment

[0158] The present application also provides an electronic device, comprising a processor and a memory;

[0159] The memory stores a computer program, and when the computer program is executed by the processor, the steps of the control method of the bidirectional power converter as described above are implemented.

[0160] The present application also provides a storage medium having a computer program stored thereon. When the computer program is executed by a processor, the steps of the control method of the bidirectional power converter as described above are implemented.

[0161] The control method, electronic device, and storage medium of the bidirectional power converter provided in the present application are based on the bidirectional power converter circuit provided in the present application, and obtain a given required power by responding to the determined direction of energy flow; obtain a zero crossing point on the AC side through grid phase locking to generate gating signals for the first directional switch unit and the second directional switch unit according to the voltage polarity; obtain the DC bus voltage and the AC bus voltage, and judge the current conduction mode based on the number of turns of the transformer to determine the duty cycle of the third MOS tube and the fourth MOS tube according to the required power; based on the half-bridge circuit structure of a stand-alone converter, combined with the control switching function, it can achieve higher power density and lower complexity, and while controlling the direction of energy flow, an easy-to-implement modulation scheme can be obtained.

[0162] It should be noted that in this application, step codes such as S10 and S20 are used for the purpose of expressing the corresponding content more clearly and concisely, and do not constitute a substantial restriction on the order. When implementing the step, those skilled in the art may execute S20 first and then S10, etc., but these should all be within the scope of protection of this application.

[0163] In the embodiments of the device and storage medium provided in this application, all technical features of any of the above-mentioned method embodiments may be included. The expanded and explained contents of the specification are basically the same as those of the above-mentioned method embodiments and will not be repeated here.

[0164] An embodiment of the present application further provides a computer program product, which includes computer program code. When the computer program code runs on a computer, the computer executes the methods in the various possible implementation modes described above.

[0165] An embodiment of the present application also provides a chip, including a memory and a processor, wherein the memory is used to store computer programs, and the processor is used to call and run the computer programs from the memory, so that a device equipped with the chip executes the methods in the various possible implementation modes as described above.

[0166] It is understood that the above scenarios are merely examples and do not limit the application scenarios of the technical solutions provided in the embodiments of this application. The technical solutions of this application can also be applied to other scenarios. For example, those skilled in the art will appreciate that with the evolution of device architectures and the emergence of new business scenarios, the technical solutions provided in the embodiments of this application are also applicable to similar technical problems.

[0167] The serial numbers of the above embodiments of the present application are for description only and do not represent the advantages or disadvantages of the embodiments.

[0168] The steps in the method of the embodiment of the present application can be adjusted in order, combined and deleted according to actual needs.

[0169] The units in the device of the embodiment of the present application can be merged, divided and deleted according to actual needs.

[0170] In this application, the same or similar terminology, technical solutions and / or application scenario descriptions are generally only described in detail the first time they appear. When they appear again later, they are generally not repeated for the sake of brevity. When understanding the technical solutions and other contents of this application, for the same or similar terminology, technical solutions and / or application scenario descriptions that are not described in detail later, you can refer to the previous relevant detailed descriptions.

[0171] In this application, the description of each embodiment has its own focus. For parts that are not described or recorded in detail in a certain embodiment, please refer to the relevant description of other embodiments.

[0172] The various technical features of the technical solution of this application can be combined arbitrarily. In order to make the description concise, not all possible combinations of the various technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this application.

[0173] The above are only preferred embodiments of the present application and do not limit the scope of application of the present application. Any equivalent structure or equivalent process transformation made using the contents of the present application description and drawings, or directly or indirectly applied in other related technical fields, are also included in the scope of protection of the present application.

Claims

1. A control method for a bidirectional power converter, characterized in that: The bidirectional power converter includes a transformer, a first directional switch unit, a second directional switch unit, a third MOS transistor, a fourth MOS transistor, a first capacitor, a second capacitor, a third capacitor and a fourth capacitor: The first end of the first directional switch unit is connected to the live end of the AC bus, the second end of the first directional switch unit is connected to the second end of the second directional switch unit and the first end of the AC side of the transformer, and the first end of the second directional switch unit is connected to the neutral end of the AC bus; The drain of the third MOS transistor is connected to the positive terminal of the DC bus, and the source of the third MOS transistor is connected to the first end of the fourth MOS transistor and the negative terminal of the DC bus; The first end of the first capacitor is connected to the drain of the third MOS transistor, the second end of the first capacitor is connected to the second end of the second capacitor, the first end of the second capacitor is connected to the source of the fourth MOS transistor, and the second end of the DC side of the transformer is connected to the common end of the first capacitor and the second capacitor; The first end of the third capacitor is connected to the first end of the first directional switch unit, the second end of the third capacitor is connected to the second end of the fourth capacitor, the first end of the fourth capacitor is connected to the first end of the second directional switch unit, and the second end of the AC side of the transformer is connected to the common end of the first capacitor and the second capacitor; The first directional switch unit and the second directional switch unit are used for directional conduction under different half-cycle currents of the AC bus; The control method of the bidirectional power converter includes: In response to determining the energy flow direction, obtaining a given required power; On the AC side, a zero-crossing point is obtained by phase-locking the power grid to generate gate control signals for the first directional switch unit and the second directional switch unit according to the voltage polarity; A DC bus voltage and an AC bus voltage are obtained, and a current conduction mode is determined based on the number of turns of the transformer, so as to determine duty cycles of the third MOS transistor and the fourth MOS transistor according to the required power.

2. The control method of a bidirectional power converter according to claim 1, wherein: The step of obtaining a given required power in response to determining the energy flow direction includes: Obtaining a primary voltage and a secondary voltage of the transformer, and determining an energy flow direction based on a phase relationship between the primary voltage and the secondary voltage; The required power is determined according to the input current or the load power.

3. The control method of a bidirectional power converter according to claim 2, wherein: The first directional switch unit includes a first MOS transistor A, a second MOS transistor B, a first diode and a second diode; The drain of the first A MOS transistor is connected to the first end of the first directional switch unit, the source of the first A MOS transistor is connected to the anode of the first diode, and the cathode of the first diode is connected to the second end of the first directional switch unit; The drain of the second B MOS transistor is connected to the second end of the first directional switch unit, the source of the second B MOS transistor is connected to the anode of the second diode, and the cathode of the second diode is connected to the first end of the first directional switch unit; Or, the second directional switch unit includes a first B MOS transistor, a second A MOS transistor, a third diode and a fourth diode; The drain of the first PMOS transistor is connected to the first end of the second directional switch unit, the source of the first PMOS transistor is connected to the anode of the third diode, and the cathode of the third diode is connected to the second end of the second directional switch unit; The drain of the second NMOS transistor is connected to the second end of the second directional switch unit, the source of the second NMOS transistor is connected to the anode of the fourth diode, and the cathode of the fourth diode is connected to the first end of the second directional switch unit; Alternatively, the first directional switch unit includes a first NMOS transistor and a second PMOS transistor. The drain of the first NMOS transistor is connected to the first end of the first directional switch unit, the source of the first NMOS transistor is connected to the source of the second PMOS transistor, and the drain of the second PMOS transistor is connected to the second end of the first directional switch unit; Alternatively, the second directional switch unit includes a second NMOS transistor and a first PMOS transistor. The source of the second NMOS transistor is connected to the first end of the second directional switch unit, the drain of the second NMOS transistor is connected to the drain of the first PMOS transistor, and the source of the first PMOS transistor is connected to the second end of the second directional switch unit; The step of obtaining the zero crossing point by grid phase-locking on the AC side to generate the gating signals of the first directional switch unit and the second directional switch unit according to the voltage polarity includes: In the positive half-cycle of the AC side bus voltage, control the first NMOS transistor and the first PMOS transistor as complementary high-frequency switches with a 50% duty cycle respectively, and keep the second NMOS transistor and the second PMOS transistor on; In the negative half-cycle of the AC side bus voltage, control the second NMOS transistor and the second PMOS transistor as complementary high-frequency switches with a 50% duty cycle respectively, and keep the first NMOS transistor and the first PMOS transistor on.

4. A control method for a bidirectional power converter according to any one of claims 1 to 3, characterized in that: The step of obtaining the DC bus voltage and the AC bus voltage, judging the current conduction mode based on the turns of the transformer, and determining the duty cycles of the third MOS transistor and the fourth MOS transistor according to the required power further includes: Determine the mode switching boundary condition based on the target duty cycle in the current conduction mode based on the DC bus voltage and the AC bus voltage; When the target duty cycle reaches the mode switching boundary condition, operate at a constant frequency until the mode switching is completed.

5. The control method of a bidirectional power converter according to claim 4, characterized in that: In the step of determining the mode switching boundary condition based on the target duty cycle in the current conduction mode based on the DC bus voltage and the AC bus voltage, take the duty cycle when the current of the fourth MOS transistor increases from 0 to the maximum current as the target duty cycle, and determine the target duty cycle according to the following expression: Where, d1 is the target duty cycle, and 0 < d1 < 1; T1 is the time when the current of the fourth MOS transistor increases from 0 to the maximum current, Ts is the working cycle, d2 is the duty cycle when the third MOS transistor decreases from the maximum current to 0, n is the number of turns of the transformer, Vac is the AC bus voltage, and Vdc is the DC bus voltage.

6. The control method of a bidirectional power converter according to claim 5, characterized in that: The transformer includes an excitation inductor; and when the target duty cycle reaches the mode switching boundary condition, the step of operating at a constant frequency until the mode switching is completed includes: The required current is determined based on the required power. When the target duty cycle is less than the mode switching boundary condition, the mode is switched to the discontinuous conduction mode, and the target duty cycle is calculated according to the following expression: Among them, Ls is the inductance of the excitation inductor, fs is the operating frequency, and IL is the required current.

7. The control method of a bidirectional power converter according to claim 6, characterized in that: The step of operating at a constant frequency until the mode switching is completed when the target duty cycle reaches the mode switching boundary condition includes: The required current is determined based on the required power. When the target duty cycle is greater than the mode switching boundary condition, the mode is switched to the continuous conduction mode, and the target duty cycle is calculated according to the following expression: Among them, Ls is the inductance of the excitation inductor, fs is the operating frequency, and IL is the required current.

8. The control method of a bidirectional power converter according to claim 7, characterized in that: The control method of the bidirectional power converter further includes: When operating in the discontinuous conduction mode, the switches of the first directional switch unit, the second directional switch unit, and the third MOS transistor and the fourth MOS transistor operate in a zero voltage switching manner when turned on; And / or, when operating in the discontinuous conduction mode, the switches of the first directional switch unit, the second directional switch unit, and the third MOS transistor operate in a zero-current switching manner when turned off; And / or, when operating in the discontinuous conduction mode, the switches of the first directional switch unit, the second directional switch unit, and the third MOS transistor and the fourth MOS transistor operate in a zero voltage switching manner when turned on.

9. An electronic device, characterized in that: The electronic device includes a processor and a memory; The memory stores a computer program, and when the computer program is executed by the processor, the steps of the control method of the bidirectional power converter according to any one of claims 1 to 8 are implemented.

10. A storage medium, characterized in that: The storage medium stores a computer program, which, when executed by a processor, implements the steps of the method for controlling a bidirectional power converter according to any one of claims 1 to 8.

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