Start-up control method and power conversion device

Abstract

The application discloses a starting control method and a power conversion device. The method comprises the following steps: acquiring an input DC voltage of a primary side circuit and an output DC voltage of a secondary side circuit; calculating a switching duty cycle of the primary side circuit and / or the secondary side circuit according to a preset target voltage excitation value, the input DC voltage, the output DC voltage and a voltage conversion ratio of a power transmission unit; and generating a switching control signal according to the switching duty cycle to control the starting of the power conversion device. The switching duty cycle is calculated in real time, the voltage excitation of the power transmission unit in the starting process is limited in the target value range, the starting surge current can be effectively inhibited, and smooth starting can be realized.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, and in particular to a start-up control method and a power conversion device. Background Technology

[0002] Against the backdrop of the current vigorous development of renewable energy, photovoltaic power generation, as an important component of clean energy, is being increasingly widely used. To improve the power generation efficiency, safety, and flexibility of photovoltaic systems, module-level power electronics technology has emerged. Among them, the micro-inverter, as its core device, can perform independent DC-AC conversion and maximum power point tracking (MPPT) for single or multiple photovoltaic modules, thereby effectively overcoming the shortcomings and shading problems faced by traditional string inverters and significantly improving the overall power generation of the system.

[0003] Microinverters can achieve module-level Maximum Power Point Tracking (MPPT) capability. On one hand, the voltage level of a single photovoltaic panel is often less than 60 volts (V); on the other hand, the rated voltage of the grid voltage on the inverter side is approximately 220V. To achieve control on the inverter side, the DC bus voltage level corresponding to the inverter is often greater than 320V. Therefore, the voltage level of a single photovoltaic panel is several times higher than the voltage level of the high-voltage bus on the inverter side, and conventional boost converter solutions suffer from low efficiency when directly boosting the voltage.

[0004] DC-DC topologies that include high-frequency transformers, such as dual active bridges (DABs), often function as high-ratio DC transformers. DAB converters exhibit high engineering practicality due to their simple control characteristics. However, during startup, a significant voltage difference often exists between the primary and secondary circuits of a DAB converter. If it starts operating directly at the rated duty cycle (e.g., 50%), a large inrush current will be generated in the resonant circuit; this startup process is also known as hard start. A large inrush current can damage power devices, cause resonant capacitor overvoltage, or transformer saturation, severely impacting system reliability.

[0005] Therefore, it is necessary to provide improved technical solutions to overcome the above-mentioned technical problems existing in the prior art. Summary of the Invention

[0006] The purpose of this application is to provide a startup control method and a power conversion device, which aims to solve the technical problem of excessive impulse current during the startup process of the power conversion device.

[0007] To achieve the above objectives: In a first aspect, embodiments of this application provide a startup control method applied to a power conversion device, the power conversion device including a primary circuit, a secondary circuit, and a power transmission unit connected between the primary circuit and the secondary circuit, the method comprising: Obtain the input DC voltage of the primary circuit and the output DC voltage of the secondary circuit; The switching duty cycle of the primary circuit and / or the secondary circuit is calculated based on the preset target voltage excitation value, the input DC voltage, the output DC voltage, and the voltage conversion ratio of the power transmission unit. A switch control signal is generated based on the switch duty cycle to control the power conversion device to start.

[0008] In one embodiment, calculating the switching duty cycle based on a preset target voltage excitation value, the input DC voltage, the output DC voltage, and the voltage conversion ratio of the power transmission unit includes: The input DC voltage is equivalently transformed according to the voltage transformation ratio to obtain an equivalent voltage; Calculate the absolute value of the voltage difference between the equivalent voltage and the output DC voltage; The switch duty cycle is determined based on the ratio of the target voltage excitation value to the absolute value of the voltage difference.

[0009] In one embodiment, determining the switch duty cycle based on the ratio of the target voltage excitation value to the absolute value of the voltage difference includes: When the ratio is greater than the preset duty cycle upper limit, the switch duty cycle is determined to be the preset duty cycle upper limit. When the ratio is less than the preset duty cycle lower limit, the switch duty cycle is determined to be the preset duty cycle lower limit. When the ratio is between the preset duty cycle lower limit and the preset duty cycle upper limit, the switch duty cycle is determined as the ratio.

[0010] In one embodiment, generating the switch control signal based on the switch duty cycle includes: A first switch signal and a second switch signal are generated based on the switch duty cycle. The first switch signal and the second switch signal are 180 degrees out of phase, and the duty cycle of the first switch signal and the second switch signal are both the switch duty cycle.

[0011] In one embodiment, the method further includes: During the startup process of the power conversion device, the output DC voltage is monitored in real time. When the output DC voltage reaches the steady-state operating voltage, the switch duty cycle is switched to the rated duty cycle.

[0012] In one embodiment, the power conversion device is a bidirectional active bridge converter, the primary-side circuit includes a primary-side full bridge or a primary-side half bridge, and the secondary-side circuit includes a secondary-side full bridge or a secondary-side half bridge.

[0013] Secondly, embodiments of this application provide a power conversion device, comprising: The primary-side circuit is used to connect to the input DC power supply; The secondary circuit is used to connect the output load; A power transmission unit is connected between the primary circuit and the secondary circuit; A control unit, connected to the primary-side circuit and / or the secondary-side circuit, is configured to perform the startup control method as described above.

[0014] In one embodiment, the power transmission unit includes an isolation coupling unit, which includes a transformer for achieving electrical isolation and voltage transformation between the primary circuit and the secondary circuit.

[0015] In one embodiment, the power transmission unit further includes an energy transfer unit connected between the transformer and the primary circuit and / or the secondary circuit, for cooperating with the transformer to form a resonant circuit.

[0016] In one embodiment, the energy transfer unit includes a resonant inductor and a resonant capacitor.

[0017] The startup control method and power conversion device provided in this application obtain the input DC voltage of the primary circuit and the output DC voltage of the secondary circuit. Based on the target voltage excitation value, the input DC voltage, the output DC voltage, and the voltage conversion ratio of the power transmission unit, the switching duty cycle used to generate the switching control signal is calculated in real time. This ensures that the voltage excitation of the power transmission unit is always limited to the target value range during startup, thereby limiting the current amplitude in the energy transmission path to a safe range. In this way, the surge current during startup can be effectively suppressed, achieving smooth startup. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of a power conversion device provided in an embodiment of this application.

[0020] Figure 2 This is a flowchart illustrating a startup control method provided in an embodiment of this application.

[0021] Figure 3 This is a flowchart illustrating the duty cycle calculation method provided in an embodiment of this application.

[0022] Figure 4a and 4b The diagram shows the current waveforms for hard start and soft start provided in the embodiments of this application. Detailed Implementation

[0023] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numbers in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this application. Rather, they are merely examples of apparatuses and methods consistent with some aspects of this application as detailed in the appended claims.

[0024] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element. Furthermore, components, features, and elements with the same names in different embodiments of this application may have the same meaning or different meanings, the specific meaning of which must be determined by its interpretation in that specific embodiment or further in conjunction with the context of that specific embodiment.

[0025] It should be understood that although the terms first, second, third, etc., may be used herein to describe various information, such information should not be limited to these terms. These terms are used only to distinguish information of the same type from one another. For example, without departing from the scope of this document, first information may also be referred to as second information, and similarly, second information may also be referred to as first information. Depending on the context, the word "if," as used herein, can be interpreted as "when," "when," or "in response to determination." Furthermore, as used herein, the singular forms "a," "an," and "the" are intended to also include the plural forms unless the context indicates otherwise. It should be further understood that the terms "comprising," "including," indicate the presence of the stated feature, step, operation, element, component, item, kind, and / or group, but do not exclude the presence, occurrence, or addition of one or more other features, steps, operations, elements, components, items, kinds, and / or groups. The terms "or" and "and / or" as used herein are to be interpreted as inclusive, or mean any one or any combination thereof. Therefore, "A, B, or C" or "A, B, and / or C" means "any one of the following: A; B; C; A and B; A and C; B and C; A, B, and C". Exceptions to this definition will only occur if the combination of elements, functions, steps, or operations is inherently mutually exclusive in some way.

[0026] It should be understood that although the steps in the flowcharts of this application's embodiments are shown sequentially according to the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some of the steps in the figures may include multiple sub-steps or multiple stages. These sub-steps or stages are not necessarily completed at the same time, but can be executed at different times, and their execution order is not necessarily sequential, but can be performed alternately or in turn with other steps or at least a portion of the sub-steps or stages of other steps.

[0027] It should be noted that step designations such as S110 and S120 are used in this document for the purpose of more clearly and concisely describing the corresponding content, and do not constitute a substantial limitation on the order. In specific implementation, those skilled in the art may execute S120 first and then S110, etc., but these should all be within the protection scope of this application.

[0028] It should be understood that the specific embodiments described herein are merely illustrative of this application and are not intended to limit this application.

[0029] In the following description, the use of suffixes such as "module," "part," or "unit" to denote elements is solely for the purpose of illustrative purposes and has no specific meaning in itself. Therefore, "module," "part," or "unit" may be used interchangeably.

[0030] Figure 1 This is a schematic diagram of a power conversion device provided in an embodiment of this application. Figure 1 As shown, the power conversion device 100 includes a primary circuit 110, a secondary circuit 120, a power transmission unit 130, and a control unit (not shown).

[0031] Specifically, the primary-side circuit 110 is used to connect the input DC power supply (not shown). The primary-side circuit 110 includes a primary-side full-bridge or a primary-side half-bridge, composed of primary-side switching transistors, and is used to switch the input DC voltage u. i Inverted to primary side AC voltage u p .

[0032] The secondary-side circuit 120 is used to connect the output load (not shown). The secondary-side circuit 120 includes a secondary-side full-bridge or secondary-side half-bridge, composed of secondary-side switching transistors, and is used to switch the secondary-side AC voltage u... s Rectified to output DC voltage u o .

[0033] The power transfer unit 130 is connected between the primary circuit 110 and the secondary circuit 120 to realize energy transfer between the primary circuit 110 and the secondary circuit 120. The power transfer unit 130 has a voltage conversion ratio N, which represents the conversion relationship between the voltage on the primary circuit 110 side and the voltage on the secondary circuit 120 side.

[0034] The control unit is connected to the primary circuit 110 and the secondary circuit 120 and is used to generate switch control signals to control the conduction and cutoff of the primary and secondary switching transistors.

[0035] In one embodiment, the switching control signal includes a first switching signal SH and a second switching signal SL. The primary-side circuit 110 and the secondary-side circuit 120 use the same switching control signal. In one application scenario, the first switching signal SH and the second switching signal SL are complementary switching signals with a 50% duty cycle. Therefore, the primary-side AC voltage and the secondary-side AC voltage are in complete phase, differing only in amplitude. (Primary-side AC voltage u) p The amplitude depends on the input DC voltage u on the primary side of circuit 110. i Secondary AC voltage u s The amplitude depends on the output DC voltage u on the secondary side of circuit 120. o Primary AC voltage u p AC voltage u on the secondary side sThe voltage difference between the two voltage levels acts on the power transfer unit 130, forming a resonant current in the energy transfer path. Since the power transfer unit 130 often has a relatively large current / voltage gain, only a small voltage difference is needed to generate a large-amplitude resonant current. This large resonant current, combined with the primary and secondary AC voltages, enables rapid energy transfer. The final steady-state voltage satisfies: Nu i ≈u o .

[0036] In one embodiment, the power transmission unit 130 includes an isolation coupling unit. The isolation coupling unit includes a transformer, which is used to achieve electrical isolation and voltage transformation between the primary circuit 110 and the secondary circuit 120. The transformer's turns ratio is the aforementioned voltage transformation ratio N.

[0037] In one embodiment, the power transmission unit 130 further includes an energy transfer unit. The energy transfer unit is connected between the transformer and the primary circuit 110 and / or the secondary circuit 120, and is used to cooperate with the transformer to form a resonant circuit to achieve energy transfer. The energy transfer unit includes a resonant inductor and a resonant capacitor.

[0038] In one embodiment, the power conversion device 100 further includes an input voltage detection unit (not shown) and an output voltage detection unit (not shown). The input voltage detection unit is connected to the primary-side circuit 110 and is used to detect the input DC voltage u. i The output voltage detection unit is connected to the secondary circuit 120 and is used to detect the output DC voltage u. o The control unit is connected to both the input voltage detection unit and the output voltage detection unit to acquire the input DC voltage u. i and output DC voltage u o .

[0039] During startup, due to the input DC voltage u of the primary circuit 110 i With the output DC voltage u of the secondary circuit 120 o There is often a large voltage difference. If the switching control signal is generated directly with the rated duty cycle, the voltage difference on both sides of the power transmission unit 130 will directly act on the energy transmission path, forming a large impulse current.

[0040] To avoid the above problems, this application provides a startup control method. Figure 2 This is a flowchart illustrating a startup control method provided in an embodiment of this application. Figure 2 As shown, the method includes the following steps: Step S210: Obtain the input DC voltage of the primary circuit and the output DC voltage of the secondary circuit.

[0041] Specifically, the control unit can obtain the input DC voltage u of the primary circuit 110 through the input voltage detection unit. i The output DC voltage u of the secondary circuit 120 is obtained through the output voltage detection unit. o .

[0042] Step S220: Calculate the switching duty cycle of the primary circuit and / or secondary circuit based on the preset target voltage excitation value, input DC voltage, output DC voltage, and voltage conversion ratio of the power transmission unit.

[0043] Specifically, the control unit calculates the switching duty cycle based on the preset target voltage excitation value, input DC voltage, output DC voltage, and voltage conversion ratio of the power transmission unit 130.

[0044] Figure 3 This is a flowchart illustrating the duty cycle calculation method provided in an embodiment of this application. Figure 3 As shown, step S220 specifically includes the following sub-steps: Step S221: Perform an equivalent transformation on the input DC voltage according to the voltage transformation ratio to obtain the equivalent voltage.

[0045] Specifically, the input DC voltage u i Multiplying this by the voltage transformation ratio N yields the equivalent voltage. This equivalent voltage characterizes the input DC voltage u. i The voltage value after transformation according to the voltage transformation relationship of the power transmission unit 130.

[0046] Step S222: Calculate the absolute value of the voltage difference between the equivalent voltage and the output DC voltage.

[0047] Specifically, the equivalent voltage and the output DC voltage u are calculated. o The absolute value of the difference is used to obtain the absolute value of the voltage difference |Nu i -u o The absolute value of this voltage difference reflects the degree of voltage mismatch between the primary circuit 110 and the secondary circuit 120.

[0048] Step S223: Determine the switch duty cycle based on the ratio of the target voltage excitation value to the absolute value of the voltage difference.

[0049] Specifically, the target voltage excitation value u is calculated. The ratio of u to the absolute value of the voltage difference / |Nu i -u o | Use this ratio as the initial calculated value for the switch duty cycle.

[0050] In one embodiment, the step of determining the switching duty cycle further includes: when the ratio of the target voltage excitation value to the absolute value of the voltage difference u / |Nu i -u o When the duty cycle exceeds the preset upper limit value dmax, the switching duty cycle is determined to be the preset upper limit value, i.e., d = dmax. In practical applications, the preset upper limit value of the duty cycle can be less than and approximately equal to 50%, determined by the topology dead time and switching characteristics of the power conversion device 100. When the ratio u / |Nu i -u o When the duty cycle is less than the preset duty cycle lower limit dmin, the switching duty cycle is set to the preset duty cycle lower limit, i.e., d = dmin. The preset duty cycle lower limit is determined by the minimum on-time of the power conversion device 100 to ensure reliable switching. When the ratio u / |Nu i -u o When the switch is located between the preset duty cycle lower limit dmin and the preset duty cycle upper limit dmax, the switch duty cycle is set to this ratio, i.e., d = u. / |Nu i -u o |

[0051] Step S230: Generate a switch control signal based on the switch duty cycle to control the power conversion device to start.

[0052] Specifically, the control unit generates a switching control signal based on the determined switching duty cycle d, and controls the switching transistors in the primary circuit 110 and the secondary circuit 120 to turn on and off, so that the power conversion device 100 starts with a limited current.

[0053] In this embodiment, the voltage excitation of the power transmission unit 130 is proportional to the switching duty cycle and the voltage difference between the primary circuit 110 and the secondary circuit 120. By controlling the switching duty cycle, the voltage excitation of the power transmission unit 130 can be controlled, thereby controlling the current amplitude in the energy transmission path. The target voltage excitation value needs to be determined based on the upper limit of the current allowed by the power conversion device 100. The selection of the target voltage excitation value must ensure that, during startup, the current amplitude in the energy transmission path does not exceed the safe operating range of the power device.

[0054] In the initial startup phase, the output DC voltage is approximately zero, and the absolute value of the voltage difference is approximately equal to the equivalent voltage, reaching its maximum value. At this time, the voltage excitation of the power transmission unit 130 is limited by the target voltage excitation value, and the calculated switching duty cycle is small, which can avoid generating a large inrush current. As the startup process progresses, the output DC voltage gradually builds up, the absolute value of the voltage difference gradually decreases, and the switching duty cycle gradually increases. When the output DC voltage approaches the steady-state operating voltage, the absolute value of the voltage difference approaches zero, the switching duty cycle reaches the preset upper limit value, and the power conversion device 100 enters the normal operating state.

[0055] In this embodiment, the control unit generates a first switch signal and a second switch signal. The first switch signal and the second switch signal are 180 degrees out of phase, and the duty cycles of both the first switch signal and the second switch signal are the switch duty cycles determined in the above embodiments. The first switch signal is used to control the upper bridge arm switch in the primary side circuit 110 and / or the secondary side circuit 120, and the second switch signal is used to control the lower bridge arm switch in the primary side circuit 110 and / or the secondary side circuit 120.

[0056] In one embodiment, the output DC voltage u can be monitored in real time during the startup process of the power conversion device 100. o When the output DC voltage u o When the steady-state operating voltage is reached, the switching duty cycle is switched to the rated duty cycle, and the power conversion device 100 enters steady-state operation.

[0057] Figure 4a and 4b The diagram shows the current waveforms for hard start and soft start provided in the embodiments of this application. Figure 4a The waveform shows the current during hard start, with a very large current spike appearing at the moment of startup. Figure 4b The current waveform when using the startup control method (i.e., soft start) of the embodiment of this application is shown. The current gradually increases from zero, and the startup process is smooth and without shock.

[0058] In one embodiment, the power conversion device 100 can be a bidirectional active bridge converter. The primary-side circuit 110 includes a primary-side full bridge or a primary-side half bridge, and the secondary-side circuit 120 includes a secondary-side full bridge or a secondary-side half bridge. The startup control method provided in this application embodiment is applicable to the following topologies: primary-side full bridge-secondary-side full bridge structure, primary-side half bridge-secondary half bridge structure, primary-side full bridge-secondary half bridge structure, and primary-side half bridge-secondary full bridge structure.

[0059] In this embodiment, the power conversion device 100 employs a resonant bidirectional active bridge converter, meaning the power transmission unit 130 includes a resonant inductor and a resonant capacitor. The startup control method provided in this embodiment is also applicable to non-resonant bidirectional active bridge converters, i.e., topologies that rely solely on transformer leakage inductance as the energy transfer element.

[0060] In one application scenario, the power conversion device 100 is a micro-inverter, with a photovoltaic module as the input DC power source and the inverter-side bus as the output load. The output voltage of the photovoltaic module is typically less than 60V, while the inverter-side bus voltage is typically greater than 320V. Through the startup control method provided in this application embodiment, the power conversion device 100 effectively suppresses impulse current during startup, protects power devices, and improves system reliability.

[0061] In summary, the startup control method and power conversion device provided in this application calculate the switching duty cycle in real time based on the target voltage excitation value, the input DC voltage, the output DC voltage, and the voltage conversion ratio of the power transmission unit. This ensures that the voltage excitation of the power transmission unit is always limited to the target value range during startup, thereby limiting the current amplitude in the energy transmission path to a safe range. This effectively suppresses the surge current during startup and achieves smooth startup.

[0062] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the 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 specification.

[0063] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A start-up control method, characterized in that, The method is applied to a power conversion device, the power conversion device including a primary circuit, a secondary circuit, and a power transmission unit connected between the primary circuit and the secondary circuit, the method comprising: Obtain the input DC voltage of the primary circuit and the output DC voltage of the secondary circuit; The switching duty cycle of the primary circuit and / or the secondary circuit is calculated based on the preset target voltage excitation value, the input DC voltage, the output DC voltage, and the voltage conversion ratio of the power transmission unit. A switch control signal is generated based on the switch duty cycle to control the power conversion device to start.

2. The start-up control method according to claim 1, characterized in that, The step of calculating the switch duty cycle based on the preset target voltage excitation value, the input DC voltage, the output DC voltage, and the voltage conversion ratio of the power transmission unit includes: The input DC voltage is equivalently transformed according to the voltage transformation ratio to obtain an equivalent voltage; Calculate the absolute value of the voltage difference between the equivalent voltage and the output DC voltage; The switch duty cycle is determined based on the ratio of the target voltage excitation value to the absolute value of the voltage difference.

3. The start-up control method according to claim 2, characterized in that, Determining the switch duty cycle based on the ratio of the target voltage excitation value to the absolute value of the voltage difference includes: When the ratio is greater than the preset duty cycle upper limit, the switch duty cycle is determined to be the preset duty cycle upper limit. When the ratio is less than the preset duty cycle lower limit, the switch duty cycle is determined to be the preset duty cycle lower limit. When the ratio is between the preset duty cycle lower limit and the preset duty cycle upper limit, the switch duty cycle is determined as the ratio.

4. The start-up control method according to claim 1 or 3, characterized in that, The step of generating a switch control signal based on the switch duty cycle includes: A first switch signal and a second switch signal are generated based on the switch duty cycle. The first switch signal and the second switch signal are 180 degrees out of phase, and the duty cycle of the first switch signal and the second switch signal are both the switch duty cycle.

5. The start-up control method according to claim 4, characterized in that, Also includes: During the startup process of the power conversion device, the output DC voltage is monitored in real time. When the output DC voltage reaches the steady-state operating voltage, the switch duty cycle is switched to the rated duty cycle.

6. The start-up control method according to claim 5, characterized in that, The power conversion device is a bidirectional active bridge converter, the primary side circuit includes a primary side full bridge or a primary side half bridge, and the secondary side circuit includes a secondary side full bridge or a secondary side half bridge.

7. A power conversion device, characterized in that, include: The primary-side circuit is used to connect to the input DC power supply; The secondary circuit is used to connect the output load; A power transmission unit is connected between the primary circuit and the secondary circuit; A control unit, connected to the primary side circuit and / or the secondary side circuit, is configured to perform the startup control method as described in any one of claims 1 to 6.

8. The power conversion device according to claim 7, characterized in that, The power transmission unit includes an isolation coupling unit, which includes a transformer. The transformer is used to achieve electrical isolation and voltage transformation between the primary circuit and the secondary circuit.

9. The power conversion device according to claim 8, characterized in that, The power transmission unit further includes an energy transfer unit, which is connected between the transformer and the primary circuit and / or the secondary circuit, and is used to cooperate with the transformer to form a resonant circuit.

10. The power conversion device according to claim 9, characterized in that, The energy transfer unit includes a resonant inductor and a resonant capacitor.

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