An isolated bidirectional DC / DC converter and a control method thereof

Abstract

The present invention relates to an isolated bidirectional DC / DC converter and a control method thereof. By adjusting the resonant inductance on the primary side of a CLLC circuit to the secondary side and providing an auxiliary unit in the circuit, the converter can achieve both step-up and step-down conversion in both forward and reverse operation. Furthermore, the present invention analyzes gain characteristic curves under different loads and powers during forward and reverse power flow, and proposes a control strategy applicable to the full operating range. Combining the isolated bidirectional DC / DC converter with the full operating range control strategy can meet a wide range of voltage output applications and effectively achieve soft switching of the device even under light loads.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics, and in particular to an isolated bidirectional DC / DC converter and a control method thereof. Background Art

[0002] With the continuous development of the electric vehicle industry, power electronics technology has also advanced rapidly. Isolated bidirectional DC / DC converters are becoming increasingly popular in applications such as photovoltaic power generation systems, electric vehicle charging systems, and uninterruptible power supply (UPS) systems, and their impact on the power grid is becoming increasingly significant. Vehicle-to-grid (V2G) technology refers to the transmission of power from electric vehicles to the power grid. Its core concept is to utilize the energy storage of large numbers of electric vehicles as a buffer between the power grid and renewable energy sources. The implementation of V2G technology will not only alleviate the pressure on the power grid caused by electric vehicle charging but also bring additional benefits to the power grid and electric vehicle users. Although high-power isolated bidirectional DC / DC converters are in great demand in future smart grid systems, the number of topologies that are truly suitable for such converters is limited. Therefore, it is of great significance to find a circuit topology that can achieve buck-boost in both forward and reverse operation, output a wide voltage range, and exhibit good soft-switching characteristics even under light loads.

[0003] Among current isolated bidirectional topologies, the bidirectional LLC resonant converter is the most representative. Its structure is relatively simple, with full-bridge configurations on both the primary and secondary sides, and it has a wide range of applications. Based on series resonance, the LLC resonant converter utilizes the transformer's magnetizing inductance to participate in circuit resonance. This allows for zero-voltage switching (ZVS) of the primary-side switching transistor and zero-current switching (ZCS) of the secondary-side rectifier across a wide voltage and load range, reducing switching losses. Its control strategy is simple, with output frequency modulation, making it easy to implement. However, during reverse operation, the magnetizing inductance is directly clamped by the input voltage, resulting in an equivalent LC series resonant circuit. This restricts the circuit to buck mode, limiting its voltage gain range.

[0004] The schematic diagram of the CLLLC bidirectional resonant converter circuit structure in the prior art is as follows: Figure 1 As shown in the figure. By adding a resonant inductor and a resonant capacitor to the secondary side of the transformer, the parameters are consistent with those on the primary side. In many designs, the transformer turns ratio is even maintained at 1. This ensures complete converter symmetry. Whether operating in forward or reverse direction, the circuit topology is symmetrical, and the forward and reverse characteristics of the circuit are completely consistent, enabling both buck-boost and soft switching. However, the CLLLC resonant converter adds auxiliary inductors and capacitors, increasing the circuit cost and size, limiting its application in applications requiring high power density.

[0005] The circuit structure diagram of the CLLC bidirectional resonant converter is as follows: Figure 2 As shown. The CLLC structure is developed based on the LLC structure. The CLLC bidirectional resonant converter can achieve bidirectional DC isolation while maintaining the soft switching of the LLC resonant circuit, avoiding the problem of transformer bias saturation due to the asymmetry of the voltage square wave. At the same time, compared with the CLLLC symmetrical circuit, it eliminates an inductor, reduces the circuit volume, reduces the circuit cost, and releases a degree of design freedom, making the circuit design more flexible. However, for the CLLC topology, the secondary side resonant capacitor participates in the resonance process as an important factor that cannot be ignored. Its DC gain at the second resonant frequency point in the forward direction during forward operation is less than 1, and its DC gain at the second resonant frequency point in the reverse direction during reverse operation is greater than 1, making it more suitable for application in forward buck and reverse boost working environments, and its use occasions are limited. Summary of the Invention

[0006] Based on the above situation of the prior art, the purpose of the present invention is to provide an isolated bidirectional DC / DC converter and a control method thereof to solve the problems of the CLLLC topology in the prior art in applications where energy flows in both directions, such as multiple resonance parameters, large cost and large volume.

[0007] To achieve the above object, according to one aspect of the present invention, an isolated bidirectional DC / DC converter is provided, comprising:

[0008] DC power supply unit;

[0009] The DC power supply unit is connected to one end of a primary-side bridge arm unit, and the primary-side bridge arm unit includes one or more bridge arms;

[0010] The other end of the primary side bridge arm unit is connected to one end of the secondary side bridge arm unit through one or more resonance units, and the secondary side bridge arm unit includes one or more bridge arms;

[0011] The other end of the secondary side bridge arm unit is connected to a load;

[0012] The invention also includes an auxiliary unit for selectively connecting the resonance unit between each bridge arm in the bridge arm unit and the DC midpoint or between each bridge arm and the grounding point.

[0013] Furthermore, the resonance unit includes a primary-side resonant capacitor, a primary-side excitation inductor, a transformer, a secondary-side excitation inductor, and a secondary-side excitation capacitor.

[0014] Furthermore, the bridge arm is a two-level bridge arm, a three-level bridge arm with a clamping diode, or a three-level bridge arm with a flying capacitor and a clamping diode.

[0015] Furthermore, the auxiliary unit includes a primary side auxiliary unit and a secondary side auxiliary unit, the primary side auxiliary unit is connected between the primary side bridge arm unit and the resonance unit, and the secondary side auxiliary unit is connected between the resonance unit and the secondary side bridge arm unit.

[0016] Furthermore, the primary-side auxiliary unit includes a first single-pole double-throw switch, and the secondary-side auxiliary unit includes a second single-pole double-throw switch.

[0017] Furthermore, the DC power supply unit includes an adjustable DC voltage source, and the load includes a battery.

[0018] According to another aspect of the present invention, there is provided a control method for the isolated bidirectional DC / DC converter according to the first aspect of the present invention, comprising the steps of:

[0019] When power flows from the DC power supply unit to the load side,

[0020] When the output voltage V out <K*V inmin / N ps When the converter steps down the voltage, if the switching frequency is greater than the upper frequency limit, the phase-shift PSM modulation in the bridge arm is adopted; if the switching frequency is less than or equal to the upper frequency limit, the PFM modulation is adopted;

[0021] When the output voltage V out >K*V inmax / N ps When , the converter boosts the voltage and adopts PFM modulation;

[0022] When K*V inmax / N ps ≥V out ≥K*V inmax / N ps When , the gain G is kept constant, the input voltage is adjusted, and PFM is used for power regulation;

[0023] When power flows from the load to the DC power supply unit,

[0024] When the output voltage V out <K*V inmin / N ps When , the converter boosts the voltage and adopts phase shift control;

[0025] When the output voltage V out >K*V inmax / N ps When , the converter steps down the voltage and adopts PFM modulation;

[0026] When K*V inmax / N ps ≥V out≥K*V inmax / N ps When , the gain G is kept constant, the input voltage is adjusted, and PFM is used for power regulation;

[0027] Among them, V inmin is the lower limit of the input voltage, V inmax is the upper limit of the input voltage, K is the adjustment coefficient, N ps is the primary-to-secondary transformation ratio of the transformer.

[0028] Furthermore, the phase shift control includes performing dps phase shift control at a fixed switching frequency, wherein dps is a phase shift angle between the primary side and the secondary side of the converter.

[0029] Furthermore, the phase shift control includes fixing dps to a small angle capable of maintaining zero voltage switching and adjusting the switching frequency, wherein dps is the phase shift angle between the primary and secondary sides of the converter.

[0030] Furthermore, when the power is relatively low so that dps=0, the phase shift angle d1 is adjusted to perform power regulation, where d1 is the phase shift angle within the bridge arm.

[0031] In summary, the present invention provides an isolated bidirectional DC / DC converter and a control method thereof. By adjusting the resonant inductance on the primary side of the CLLC circuit to the secondary side and providing an auxiliary unit in the circuit, the converter can achieve step-up and step-down conversion in both forward and reverse operation. At the same time, the present invention also analyzes the gain characteristic curves under different loads and powers during forward and reverse power flow, and proposes a control strategy applicable to the full operating range. Combining the isolated bidirectional DC / DC converter with the full operating range control strategy can meet a wide range of voltage output occasions and can also well achieve soft switching of the device under light load.

[0032] The technical solution provided by the present invention has the following beneficial technical effects:

[0033] (1) An isolated bidirectional DC / DC converter topology is proposed. Through the design of the circuit structure, the converter can achieve step-up and step-down conversion in both forward and reverse operation, expanding the application of bidirectional DC / DC converters. It is especially suitable for situations where electric vehicles are feeding power back to the power grid. In addition, the resonant component parameters are small, which reduces the cost.

[0034] (2) By setting up auxiliary units in the circuit, the bridge arm topology of the converter can be transformed between multiple topologies, and the circuit structure is more flexible.

[0035] (3) The gain characteristic curves under different loads and powers when the power flows in the forward direction and the power flows in the reverse direction are analyzed, and a control strategy applicable to the full operating range is proposed. This effectively solves the working conditions where the frequency modulation cannot meet the requirements when the forward light load and the reverse low voltage output are in place. It can show good soft switching characteristics at both full load and light load, reducing the loss of the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a schematic diagram of the circuit structure of a CLLLC bidirectional resonant converter in the prior art;

[0037] Figure 2 It is a circuit structure diagram of a CLLC bidirectional resonant converter in the prior art;

[0038] Figure 3 Schematic diagram of the circuit topology of the isolated bidirectional DC / DC converter of the present invention;

[0039] Figure 4 This is a circuit topology suitable for the converter bridge arm of this embodiment;

[0040] Figure 4-1 It is a two-level bridge arm structure;

[0041] Figure 4-2 It is a three-level bridge arm with a clamping diode;

[0042] Figure 4-3 It is a three-level bridge arm with flying capacitor and clamping diode;

[0043] Figure 5 This is a specific implementation circuit diagram of the isolated bidirectional DC / DC converter of the present invention;

[0044] Figure 6 This is a gain characteristic diagram of the circuit of the present invention, in which power flows forward, the output voltage is large, and the power is full;

[0045] Figure 7 This is a voltage gain characteristic diagram of the present invention when the circuit power flows forward, the output voltage is 800V, and the load changes.

[0046] Figure 8 This is a gain characteristic diagram of the circuit of the present invention, in which power flows forward, the output voltage is small, and the power is full;

[0047] Figure 9 This is a gain characteristic diagram of the circuit power flow in the forward direction, 200V output, and load change of the present invention;

[0048] Figure 10 This is a graph showing the gain characteristics of the circuit for the present invention, which shows reverse power flow, input voltage variation, and full power.

[0049] Figure 11 This is a gain characteristic diagram of the circuit power reverse flow, 650V input, and full power of the present invention;

[0050] Figure 12 This is a gain characteristic diagram of the circuit power reverse flow, 650V input, and load variation of the present invention;

[0051] Figure 13 This is a control strategy distribution diagram for the full working range when the circuit power flows in the forward direction specifically implemented by the present invention;

[0052] Figure 14 The present invention specifically implements the control strategy allocation of the full working range when the circuit power flows in the reverse direction. DETAILED DESCRIPTION

[0053] To make the objectives, technical solutions, and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings. It should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessary confusion of the concepts of the present invention.

[0054] The technical solution of the present invention is described in detail below with reference to the accompanying drawings. According to one embodiment of the present invention, an isolated bidirectional DC / DC converter is provided. The circuit topology diagram of the converter is shown in FIG. Figure 3 As shown, it includes: a DC power supply unit, for example, an adjustable DC voltage source, for providing an input voltage V in. The DC power supply unit is connected to one end of the primary-side bridge arm unit, and the primary-side bridge arm unit may include, for example, bridge arm A1, bridge arm A2, ... bridge arm An, and the number of bridge arms may be set to one or more according to actual needs, and is not limited here. The specific structure of the bridge arm will be introduced below. The other end of the primary-side bridge arm unit is connected to one end of the secondary-side bridge arm unit through one or more resonance units, and the secondary-side bridge arm unit may include, for example, bridge arm B1, bridge arm B2, ... bridge arm Bn, and the number of bridge arms may be set to one or more according to actual needs, and is not limited here. The specific structure of the bridge arm will be introduced below. The resonance unit is connected between the primary-side bridge arm unit and the secondary-side bridge arm unit, and the number may also be one or more. The resonant unit may include a primary-side resonant capacitor 004, a primary-side magnetizing inductor 005, a transformer 006, a secondary-side magnetizing inductor 007, and a secondary-side magnetizing capacitor 008. The resonant unit adjusts the resonant inductor, which is located on the primary side in the existing CLLC topology, to the secondary side, thereby overcoming its limited application scenarios and enabling voltage step-up and step-down in both forward and reverse operation. The other end of the secondary-side bridge arm unit is connected to a load, which may be, for example, a battery. The converter also includes an auxiliary unit for selectively connecting the resonant unit between each bridge arm in the bridge arm unit and the DC midpoint or between each bridge arm and the ground point. The auxiliary unit includes a primary-side auxiliary unit 010 and a secondary-side auxiliary unit 011. The auxiliary unit only functions when only one of the primary-side or secondary-side bridge arm units is operating. The presence of the auxiliary unit can make the circuit topology composed of the operating bridge arms a half-bridge circuit. The auxiliary unit can be put into operation when the circuits in the primary and secondary-side bridge arm units are consistent, and the operations of the primary-side auxiliary unit 010 and the secondary-side auxiliary unit 011 are also corresponding. The primary side auxiliary unit is connected between the primary side bridge arm unit and the resonance unit, and the secondary side auxiliary unit is connected between the resonance unit and the secondary side bridge arm unit. The primary side auxiliary unit and the secondary side auxiliary unit can be realized by two switching tubes connected at one end. Figure 3 As shown, the primary side auxiliary unit 010 includes a switch S1, a first contact O1 and a second contact N1, and the secondary side auxiliary unit 011 includes a switch S2, a third contact O2 and a fourth contact N2. Figure 4-1 As shown in , when the switch S1 of the primary-side auxiliary unit 010 is connected to the first contact O1, one end of the primary side of the resonant unit is connected to the bridge arm, and the other end is connected to the midpoint of the primary-side DC bus capacitor. At this time, the circuit topology is a two-level half-bridge circuit; when the circuit in the bridge arm unit is Figure 4-2Or as shown in Figure 4-3, when switch S1 of primary-side auxiliary unit 010 is connected to second contact N1, one end of the primary side of the resonant unit is connected to the bridge arm, and the other end is connected to the primary-side ground terminal. In this case, the circuit topology is a two-level half-bridge circuit. The situation of secondary-side auxiliary unit 011 is similar to that of primary-side auxiliary unit 010. By providing auxiliary units in the circuit, the converter operates in a half-bridge circuit topology under light load, improving system efficiency. At the same time, the operating bridge arm of the converter can be controlled to switch between various bridge arms, providing a high degree of circuit structure flexibility.

[0055] Figure 5 Figure 2 shows a specific implementation circuit of an isolated bidirectional DC / DC converter according to the present invention. In this circuit, the primary-side bridge arm unit includes three three-level bridge arms with flying capacitors and clamping diodes, forming a three-phase, three-level circuit. Three resonant units form a resonant circuit. The secondary-side bridge arm unit includes three three-level bridge arms with flying capacitors and clamping diodes, forming a three-phase, three-level circuit. This circuit has an input voltage of 650V-850V and can achieve full power output within an output voltage range of 200-800V.

[0056] The gain characteristic curves of the converter under different loads and powers when power flows in the forward direction and the reverse direction are analyzed in this embodiment to formulate a targeted control strategy. Figure 6 The gain characteristic diagram of the converter of this specific implementation circuit is shown when the power flows forward, the output voltage is large, and the power is full. The lower limit of the resonant frequency f1 = 85kHz, the upper limit of the resonant frequency f2 = 230kHz, and the forward power is 850V input. The required maximum gain is M.gv_max (output 800V), and M.gv_max decreases as the output voltage decreases. Figure 6-Figure 12 In the figure, the horizontal axis is the frequency fR, and the vertical axis is the ratio of the output to input voltage as the frequency changes under different loads R.

[0057] Figure 7 The gain characteristic diagram of the converter power of the specific implementation circuit is shown as follows: when the input is 850V, the output is 800V, and the load changes; Figure 8 The gain characteristic diagram of the converter power of the specific implementation circuit is shown as the forward flow, full load 20kW, input 650V, and the output voltage changes from 200V to 433V;

[0058] Figure 9The gain characteristics of this specific implementation circuit are shown for the converter's forward power flow, 200V output, and varying loads. For the forward power, at a 650V input voltage, the minimum required gain is g4 (200V output), and g4 increases with increasing output voltage. The figure shows that as the load becomes lighter, even when the frequency reaches the upper limit, the voltage gain requirement cannot be met. Therefore, when the system output is low voltage and lightly loaded, if the switching frequency rises to the upper limit, intra-phase shift control can be used.

[0059] Figure 10 The gain characteristics of this specific implementation circuit are shown for reverse power flow, 800V output, varying input voltage, and load. For reverse power, with an input voltage of 850V, the minimum required gain is g1 (800V output), which increases as the output voltage decreases. A higher gain requires a lower switching frequency. If a significant drop in switching frequency is undesirable, the input voltage can be reduced to maintain PFM.

[0060] Figure 11 and Figure 12 The inverter power of this specific implementation circuit is shown to flow in reverse, with a 650V input negative. Figure 11 is the gain characteristic diagram of full power, Figure 12 The gain characteristic diagram of load change is shown in Figure 2. Figure 11 and 12 As can be seen, with reverse power, at a 650V input and low output voltage, it's difficult to achieve sufficient high-frequency gain characteristics in the mid- and high-frequency bands due to the heavy load. Excessive use of PWM or phase-shift control will increase switching losses, ultimately increasing overall switching device losses across the entire operating range.

[0061] Based on the above analysis of the gain characteristic curve, according to another embodiment of the present invention, a control method for an isolated bidirectional DC / DC converter as described in the first embodiment of the present invention is provided. This method adopts different control methods for different situations of forward and reverse power flow, as well as when the output voltage is in different ranges. This effectively solves the operating conditions where frequency modulation cannot meet the requirements during forward light load and reverse low voltage output. It exhibits good soft switching characteristics under both full load and light load conditions, thereby reducing device losses.

[0062] Figure 13 The control strategy distribution of the full working range of the converter provided by the present invention when power flows in the forward direction is shown: Since the input voltage can be adjusted, the upper and lower boundaries of the input voltage V inmax and V inmax (In this embodiment, for example, 850V and 650V respectively) multiply the coefficient K / N respectively ps , N psFor example, a ratio of 3 / 2 can be used as the boundary for controlling the output voltage, dividing the output voltage into three regions: upper, middle, and lower. For boosting in the upper region, only PFM modulation is required, which makes it difficult to reach the upper frequency limit of 230kHz. In the middle region, input voltage can be adjusted while maintaining the gain constant, using PFM to adjust power. In the lower region, PFM modulation is used for step-down, with switching frequencies below 230kHz. Phase-shifted PSM modulation is added above this region.

[0063] Figure 14 The control strategy distribution of the full working range when the power flows in the reverse direction according to the converter provided by the present invention is shown; when the power direction is reverse, from the power direction, the upper area is stepped down, and only PFM modulation is required, and the operating frequency is near the upper limit of 230kHz; the middle area can keep the gain unchanged to adjust the input voltage, and PFM adjusts the power; the lower area is stepped up, and phase shift control is used. The phase shift control can fix the switching frequency to perform dps phase shift control (dps is the phase shift angle between the primary and secondary sides), or it can fix dps to a small angle to maintain ZVS and adjust the switching frequency. When fixing dps to adjust the switching frequency, when the power is small, the phase shift angle dps = 0, and the power can be adjusted by adjusting the phase shift angle d1 (d1 is the phase shift within the bridge arm).

[0064] The control method comprises the following steps:

[0065] When power flows from the DC power supply unit to the load side,

[0066] When the output voltage V out <K*V inmin / N ps When the converter steps down, if the switching frequency is greater than the upper frequency limit, the phase-shift PSM modulation in the bridge arm is adopted; if the switching frequency is less than or equal to the upper frequency limit, the PFM modulation is adopted.

[0067] When the output voltage V out >K*V inmax / N ps When , the converter boosts the voltage and adopts PFM modulation;

[0068] When K*V inmax / N ps ≥V out ≥K*V inmax / N ps When , the gain G is kept constant, the input voltage is adjusted, and PFM is used for power regulation;

[0069] When power flows from the load to the DC power supply unit,

[0070] When the output voltage V out <K*V inmin / N ps When , the converter boosts the voltage and adopts phase shift control;

[0071] When the output voltage V out >K*V inmax / N ps When , the converter steps down the voltage and adopts PFM modulation;

[0072] When K*V inmax / N ps ≥V out ≥K*V inmax / N ps When , the gain G is kept constant, the input voltage is adjusted, and PFM is used for power regulation;

[0073] Among them, V inmin is the lower limit of the input voltage, V inmax is the upper limit of the input voltage, K is the adjustment coefficient, by adjusting K, the system can achieve the desired parameter index, N ps is the primary-to-secondary transformation ratio of the transformer.

[0074] In summary, the present invention relates to an isolated bidirectional DC / DC converter and a control method thereof. By adjusting the resonant inductance of the primary side in the CLLC circuit to the secondary side and providing an auxiliary unit in the circuit, the converter can achieve step-up and step-down conversion in both forward and reverse operation. At the same time, the present invention also analyzes the gain characteristic curves under different loads and powers when power flows forward and reverse, and proposes a control strategy applicable to the full operating range. Combining the isolated bidirectional DC / DC converter with the full operating range control strategy can meet a wide range of voltage output occasions and can also well achieve soft switching of the device under light load. The present invention, through the design of the circuit structure, enables the converter to achieve step-up and step-down conversion in both forward and reverse operation, expands the application of bidirectional DC / DC converters, and is particularly suitable for applications where electric vehicles are feeding power back to the power grid. In addition, the resonant component parameters are small, which reduces costs. By providing an auxiliary unit in the circuit, the bridge arm topology of the converter can be converted between multiple topologies, and the circuit structure is highly flexible. By analyzing the gain characteristic curves under different loads and powers when power flows in the forward and reverse directions, a control strategy applicable to the full operating range is proposed. This effectively solves the working conditions where frequency modulation cannot meet the requirements during forward light load and reverse low voltage output. It exhibits good soft switching characteristics at both full load and light load, reducing device losses.

[0075] It should be understood that the above-described specific embodiments of the present invention are merely illustrative or illustrative of the principles of the present invention and do not constitute limitations of the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc. made without departing from the spirit and scope of the present invention should be included within the scope of protection of the present invention. In addition, the appended claims are intended to cover all variations and modifications that fall within the scope and metes and bounds of the appended claims, or equivalents thereof.

Claims

1. A control method for an isolated bidirectional DC / DC converter, characterized in that The isolated bidirectional DC / DC converter includes: a DC power supply unit; The DC power supply unit is connected to one end of a primary-side bridge arm unit, and the primary-side bridge arm unit includes one or more bridge arms; The other end of the primary side bridge arm unit is connected to one end of the secondary side bridge arm unit through one or more resonance units, and the secondary side bridge arm unit includes one or more bridge arms; The other end of the secondary side bridge arm unit is connected to a load; The system further includes an auxiliary unit for selectively connecting the resonant unit between each bridge arm in the bridge arm unit and the DC midpoint or between each bridge arm and the grounding point. The control method includes the steps of: When power flows from the DC power supply unit to the load side, When the output voltage V out <K*V inmin / N ps When the converter steps down the voltage, if the switching frequency is greater than the upper frequency limit, the phase-shift PSM modulation in the bridge arm is adopted; if the switching frequency is less than or equal to the upper frequency limit, the PFM modulation is adopted; When the output voltage V out >K*V inmax / N ps When , the converter boosts the voltage and adopts PFM modulation; When K*V inmax / N ps ≥V out ≥K*V inmax / N ps When , the gain G is kept constant, the input voltage is adjusted, and PFM is used for power regulation; When power flows from the load to the DC power supply unit, When the output voltage V out <K*V inmin / N ps When , the converter boosts the voltage and adopts phase shift control; When the output voltage V out >K*V inmax / N ps When , the converter steps down the voltage and adopts PFM modulation; When K*V inmax / N ps ≥V out ≥K*V inmax / N ps When , the gain G is kept constant, the input voltage is adjusted, and PFM is used for power regulation; Among them, V inmin is the lower limit of the input voltage, V inmax is the upper limit of the input voltage, K is the adjustment coefficient, N ps is the primary-to-secondary transformation ratio of the transformer.

2. The control method according to claim 1, characterized in that: The resonant unit includes a primary-side resonant capacitor, a primary-side excitation inductor, a transformer, a secondary-side excitation inductor, and a secondary-side excitation capacitor.

3. The control method according to claim 1, wherein: The bridge arm is a two-level bridge arm, a three-level bridge arm with a clamping diode, or a three-level bridge arm with a flying capacitor and a clamping diode.

4. The control method according to claim 1, wherein: The auxiliary unit includes a primary side auxiliary unit and a secondary side auxiliary unit. The primary side auxiliary unit is connected between the primary side bridge arm unit and the resonance unit, and the secondary side auxiliary unit is connected between the resonance unit and the secondary side bridge arm unit.

5. The control method according to claim 4, characterized in that: The primary side auxiliary unit includes a first single-pole double-throw switch, and the secondary side auxiliary unit includes a second single-pole double-throw switch.

6. The control method according to claim 1, characterized in that: The DC power supply unit includes an adjustable DC voltage source, and the load includes a battery.

7. The control method according to any one of claims 1 to 6, characterized in that: The phase shift control includes performing DPS phase shift control at a fixed switching frequency, wherein DPS is the phase shift angle between the primary side and the secondary side of the converter.

8. The control method according to any one of claims 1 to 6, characterized in that: The phase shift control includes fixing dps to a small angle that can maintain zero voltage switching and adjusting the switching frequency, wherein dps is the phase shift angle between the primary and secondary sides of the converter.

9. The control method according to claim 8, characterized in that: When the power is low so that dps=0, the phase shift angle d1 is adjusted to perform power regulation, where d1 is the phase shift angle within the bridge arm.

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