CN216649524U

CN216649524U - Symmetrical double-bridge CLLLC resonant converter

Info

Publication number: CN216649524U
Application number: CN202122515000.3U
Authority: CN
Inventors: 李胜文; 刘翼肇; 任勇; 杨洋; 杨道远; 王翌琛; 韩彭伟
Original assignee: State Grid Electric Power Research Institute Of Sepc; State Grid Shanxi Electric Power Co Ltd; Xian Jiaotong University
Current assignee: State Grid Electric Power Research Institute Of Sepc; State Grid Shanxi Electric Power Co Ltd; Xian Jiaotong University
Priority date: 2021-10-19
Filing date: 2021-10-19
Publication date: 2022-05-31
Anticipated expiration: 2031-10-19

Abstract

The utility model discloses a symmetrical double-bridge CLLLC resonant converter, which comprises two CLLLC resonant circuits, wherein the two CLLLC resonant circuits are respectively connected in series to the primary side of a corresponding high-frequency transformer and the secondary side of the corresponding high-frequency transformer; the input side and the output side of each CLLLC resonant circuit are respectively connected with the middle point of a bridge arm of a six-switch full-bridge circuit, and the six-switch full-bridge circuit on the input side is connected with an input capacitor C in parallel_iThe six-switch full-bridge circuit on the output side is connected in parallel with an output capacitor C_o. The CLLLC resonant converter has good performanceThe method is suitable for the AC/DC hybrid micro-grid.

Description

Symmetrical double-bridge CLLLC resonant converter

Technical Field

The utility model belongs to the technical field of direct current power electronic converters, and particularly relates to a symmetrical double-bridge CLLLC resonant converter.

Background

Renewable energy sources such as wind power, photovoltaic and the like are developed vigorously in the future, and the new energy sources are well utilized after being connected to a microgrid. Meanwhile, with the development of the digital society, direct-current electric equipment such as electric automobiles and novel household appliances can be more and more, and can coexist with various current alternating-current equipment. In application, a pure alternating current or direct current micro-grid needs multiple AC/DC or DC/AC conversion, more loss and harmonic waves are brought, and the control difficulty of a system is improved, so that the alternating current and direct current hybrid micro-grid is widely concerned and researched at home and abroad. The AC-DC hybrid micro-grid has the remarkable characteristics that:

(1) the system comprises an alternating current subsystem, a direct current subsystem and a bidirectional AC/DC converter between an alternating current bus and a direct current bus;

(2) the system can directly supply power to alternating current and direct current loads, and loss caused by an extra converter is reduced;

(3) power between the AC subsystem and the DC subsystem can flow in two directions, each subsystem can be controlled independently or coordinately, and can be switched between a grid-connected mode and an island mode.

The characteristics show that the alternating current and direct current hybrid micro-grid can efficiently integrate a distributed power supply, an energy storage device and various alternating current and direct current loads into a power distribution network, the transformation degree of the existing power grid is small, and the investment cost is reduced.

In order to ensure the stability of the micro-grid and improve the transmission capability, the system needs to adopt a proper DC-DC converter. There are many types of DC-DC converters, and conventional converters either do not have electrical isolation capability, or have poor soft switching performance, or have poor energy bi-directional flow capability. Among them, resonant converters have been widely studied for their excellent soft switching performance. At present, electric automobiles, renewable energy power generation and storage batteries develop faster, higher requirements are put forward on the energy bidirectional flow capacity of a DC-DC converter after the devices are connected to a microgrid, the research on the relatively wide LLC resonant converter has poor energy bidirectional flow capacity due to the limitation of the topological structure of the LLC resonant converter, and the CLLC resonant converter is more applied on the aspect. For the analysis of resonant converters, a fundamental approximation (FHA) is usually used to obtain the voltage gain, since only the fundamental component of the voltage and current of the resonant tank can provide power, while the input impedance of the resonant tank is the lowest. Further, the controller may regulate the output voltage and power by using Pulse Frequency Modulation (PFM). PFM is a basic resonant converter modulation strategy, but the disadvantage of small voltage regulation range is the limitation of using this strategy. Therefore, the method has important significance for the research of a novel DC-DC converter and a control mode used in an alternating current and direct current hybrid micro-grid.

SUMMERY OF THE UTILITY MODEL

The technical problem to be solved by the utility model is to provide a symmetrical double-bridge CLLLC resonant converter aiming at the defects in the prior art, reduce the voltage stress of each switching device, facilitate the improvement of the energy transmission capacity and the voltage level of an AC/DC hybrid microgrid, enhance the power transmission capability of the converter, and be suitable for the AC/DC hybrid microgrid.

The utility model adopts the following technical scheme:

furthermore, the utility model is characterized in that:

a symmetrical double-bridge CLLLC resonant converter comprises two CLLLC resonant circuits respectively connected in series in a pairA primary side of the high frequency transformer and a secondary side of the high frequency transformer; the input side and the output side of each CLLLC resonant circuit are respectively connected with the middle point of a bridge arm of a six-switch full-bridge circuit, and the six-switch full-bridge circuit on the input side is connected with an input capacitor C in parallel_iThe six-switch full-bridge circuit on the output side is connected in parallel with an output capacitor C_o。

Specifically, the input side and the output side of each CLLLC resonant circuit are connected through a high-frequency transformer, the input side of each CLLLC resonant circuit is a series circuit formed by a resonant capacitor, an equivalent excitation inductor and a resonant inductor, and the output side of each CLLLC resonant circuit is a series circuit formed by a resonant capacitor and a resonant inductor.

Furthermore, the input side of each CLLLC resonant circuit is connected with an electric field effect tube S in a six-switch full-bridge circuit₁And S₃Midpoint A of the arms in series₁And electric power field effect transistor S₂And S₄Midpoint B of series-connected bridge arms₁The output side of the six-switch full-bridge circuit is connected with an electric field effect tube Q in the other six-switch full-bridge circuit₁And Q₃Midpoint C of series bridge arm₁And electric power field effect transistor Q₂And Q₄Midpoint D of the arms in series₁(ii) a The input side of the other CLLLC resonant circuit is connected with an electric field effect tube S in a six-switch full-bridge circuit₄And S₆Midpoint A of the series arm₂And an electric field effect tube S in a six-switch full-bridge circuit₃And S₅Midpoint B of series-connected bridge arms₂The output side of the six-switch full-bridge circuit is connected with an electric field effect tube Q in the other six-switch full-bridge circuit₄And Q₆Midpoint C of series bridge arm₂And electric field effect tube Q in six-switch full-bridge circuit₃And Q₅Midpoint D of the arms in series₂。

Further, a high frequency transformer T₁Primary side parallel connection equivalent excitation inductance L_m1High frequency transformer T₁One end of the primary side is connected to the resonant inductor L_r1Connecting midpoint A₁The other end of the resonant capacitor C_r1Connecting midpoint B₁High frequency transformer T₁One end of the secondary sideResonant inductor L_r2Connecting midpoint C₁The other end of the resonant capacitor C_r2Connecting midpoint D₁。

Further, a high frequency transformer T₂Primary side parallel connection equivalent excitation inductance L_m2High frequency transformer T₁One end of the primary side is connected to the resonant inductor L_r3Connecting midpoint A₂The other end of the resonant capacitor C_r3Connecting midpoint B₂High frequency transformer T₂One end of the secondary side passes through a resonant inductor L_r4Connecting midpoint C₂The other end of the resonant capacitor C_r4Connecting midpoint D₂。

Specifically, the resonance parameters of the two CLLLC resonant tanks are the same.

Specifically, the primary-secondary side transformation ratios n of the two high-frequency transformers are the same, and n is a positive number.

In particular, the switching frequency f of a six-switch full-bridge circuit_sEqual to the resonant frequency f of the CLLLC resonant tank_r。

Compared with the prior art, the utility model has at least the following beneficial effects:

the utility model relates to a symmetrical double-bridge CLLLC resonant converter which is applied to a mixed AC/DC micro-grid, six improved switch structures on the primary side and the secondary side of the converter can reduce the voltage stress of each switch device, and are favorable for improving the energy transmission capacity and the voltage level of the AC/DC mixed micro-grid. Due to the symmetrical structure of the topology, energy can flow in both directions. Meanwhile, the CLLLC converter can realize soft switching on two sides.

Furthermore, the input side and the output side of each CLLLC resonant tank can be respectively connected with different direct current voltages, and the voltage control of the output side can be realized through a control strategy. Meanwhile, the energy flow direction of the CLLLC resonant tank is bidirectional, and the energy can flow from the input side to the output side or from the output side to the input side.

Furthermore, the input side and the output side of each CLLLC resonant circuit are connected with the midpoint of the six-switch full-bridge circuit, so that the input voltages of the two resonant circuits are equal.

Furthermore, the primary side of the high-frequency transformer T1 is connected in parallel with an equivalent excitation inductance Lm1, one end of the primary side of the high-frequency transformer T1 is connected to a midpoint a1 through a resonant inductor Lr1, the other end is connected to a midpoint B1 through a resonant capacitor Cr1, one end of the secondary side of the high-frequency transformer T1 is connected to a midpoint C1 through the resonant inductor Lr2, and the other end is connected to a midpoint D1 through a resonant capacitor Cr 2. The connection mode widens the soft switching range of the converter and reduces the voltage and current stress of the switching device.

Furthermore, the primary side of the high-frequency transformer T2 is connected in parallel with an equivalent excitation inductance Lm2, one end of the primary side of the high-frequency transformer T1 is connected to a midpoint a2 through a resonant inductor Lr3, the other end is connected to a midpoint B2 through a resonant capacitor Cr3, one end of the secondary side of the high-frequency transformer T2 is connected to a midpoint C2 through the resonant inductor Lr4, and the other end is connected to a midpoint D2 through a resonant capacitor Cr 4. The connection mode ensures that the two resonant circuits have the same structure, and is favorable for balancing the output power of the resonant circuits.

Furthermore, in order to make the power transmission characteristics of the two resonant circuits consistent, the resonant parameters of the two CLLLC resonant circuits should be the same, the transformation ratio of the high-frequency transformer connecting the two circuits should also be the same, the resonant frequencies of the two circuits should be kept consistent, and the bidirectional flow of energy on the input side and the output side should be realized.

Furthermore, the switching frequency of the six-switch full-bridge circuit is equal to the resonant frequency of the CLLLC resonant circuit, and the working mode ensures that the highest working efficiency of the converter is achieved.

In conclusion, the CLLLC resonant converter has good performance and is suitable for an alternating current-direct current hybrid microgrid.

The technical solution of the present invention is further described in detail by the accompanying drawings and embodiments.

Drawings

FIG. 1 is a CLLLC converter topology of the present invention;

FIG. 2 is a steady state operating waveform diagram of the CLLLC converter of the utility model;

FIG. 3 is an equivalent topological diagram of the CLLLC converter average model of the utility model;

FIG. 4 is a simplified mean model equivalent topology diagram of the CLLLC converter of the utility model;

FIG. 5 is a control strategy diagram of the CLLLC converter of the utility model;

FIG. 6 is a simulated waveform diagram of the input voltage of the resonant tank of the CLLLC converter of the utility model;

FIG. 7 is a Bode plot of the voltage transfer function of the CLLLC converter of the utility model;

FIG. 8 is a simulated waveform diagram of the output voltage of the CLLLC converter under soft start and hard start respectively;

FIG. 9 is a graph of the output voltage waveform of the CLLLC converter of the utility model at different switching frequencies;

fig. 10 is a waveform diagram showing the ZVS and ZCS characteristics of the CLLLC converter of the present invention, wherein (a) is the ZVS characteristic of the switching tube S1, and (b) is the ZCS characteristic of the switching tube Q1.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly and may be, for example, fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.

It will be understood that the terms "comprises" and/or "comprising," when used in this specification and the appended claims, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

It is also to be understood that the terminology used in the description of the utility model herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the utility model. As used in the specification of the present invention and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

It should be further understood that the term "and/or" as used in this specification and the appended claims refers to and includes any and all possible combinations of one or more of the associated listed items.

Various structural schematics according to the disclosed embodiments of the utility model are shown in the drawings. The figures are not drawn to scale, wherein certain details are exaggerated and possibly omitted for clarity of presentation. The shapes of the various regions, layers and their relative sizes, positional relationships are shown in the drawings as examples only, and in practice deviations due to manufacturing tolerances or technical limitations are possible, and a person skilled in the art may additionally design regions/layers with different shapes, sizes, relative positions, according to the actual needs.

The utility model provides a symmetrical double-bridge CLLLC resonant converter, six improved switch structures on the primary side and the secondary side can reduce the voltage stress of each switch device, and are beneficial to improving the energy transmission capacity and the voltage level of an alternating current-direct current hybrid micro-grid. Two identical CLLLC resonant circuits are contained in the topology, so that the power transmission capacity of the converter is enhanced, and energy can flow in two directions. The converter is also capable of soft switching on both sides, reducing overshoot with a soft start scheme that reduces duty cycle and increases switching frequency during start-up, controlled with a hybrid pulse Frequency Modulation and Phase Shift (FMPS) control strategy. The CLLLC resonant converter provided by the utility model has good performance and is suitable for an alternating current and direct current hybrid microgrid.

Referring to fig. 1, the symmetrical dual-bridge CLLLC resonant converter of the present invention includes two six-switch full-bridge circuits and two CLLLC resonant circuits, where the two CLLLC resonant circuits are respectively connected in series to a primary side of a corresponding high-frequency transformer and a secondary side of the corresponding high-frequency transformer; the input side and the output side of each CLLLC resonant circuit are respectively connected with the middle point of a bridge arm of a six-switch full-bridge circuit, and the six-switch full-bridge circuit on the input side is connected with an input capacitor C in parallel_iOutput side six-switch full-bridge circuit parallel output capacitor C_oThe primary-secondary side transformation ratio of the high-frequency transformer is n, and the symmetrical double-bridge type is two six-switch full-bridge circuits with the same topology structure at the input side and the output side.

Each CLLLC resonant circuit comprises two resonant inductors, two resonant capacitors, an equivalent excitation inductance and a High Frequency Transformer (HFT); the input side of the CLLLC resonant circuit is a series circuit formed from a resonant capacitor, an equivalent excitation inductor and a resonant inductor, the output side of the CLLLC resonant circuit is a series circuit formed from a resonant capacitor and a resonant inductor, the input side and the output side of the CLLLC resonant circuit are connected by means of a high-frequency transformer, and the resonant frequencies f of the input side and the output side of the CLLLC resonant circuit are_rKeeping consistency; the input side of a CLLLC resonant circuit is connected with an electric field effect tube S₁And S₃Midpoint A of the series arm₁And electric power field effect transistor S₂And S₄Midpoint B of series-connected bridge arms₁The input side of the other CLLLC resonant circuit is connected with an electric field effect tube S₄And S₆Midpoint A of the arms in series₂And electric power field effect transistor S₃And S₅Midpoint B of series-connected bridge arms₂Output side of a CLLLC resonant circuitConnecting power field effect transistor Q₁And Q₃Midpoint C of series bridge arm₁And electric power field effect transistor Q₂And Q₄Midpoint D of the arms in series₁(ii) a The output side of the other CLLLC resonant circuit is connected with a power field effect transistor Q₄And Q₆Midpoint C of series bridge arm₂And electric power field effect transistor Q₃And Q₅Midpoint D of the arms in series₂。

The first CLLLC resonant tank comprises two resonant inductors L_r1And L_r2Two resonant capacitors C_r1And C_r2An equivalent excitation inductance L_m1And a high-frequency transformer T₁(ii) a High-frequency transformer T₁Primary side parallel connection equivalent excitation inductance L_m1High frequency transformer T₁One end of the primary side is connected to the resonant inductor L_r1Connecting the power field effect transistor S₁And S₃Midpoint A of the arms in series₁The other end of the resonant capacitor C_r1Connecting the power field effect transistor S₂And S₄Midpoint B of series-connected bridge arms₁High frequency transformer T₁One end of the secondary side passes through a resonant inductor L_r2Connecting power field effect transistor Q₁And Q₃Midpoint C of series bridge arm₁The other end of the resonant capacitor C_r2Connecting power field effect transistor Q₂And Q₄Midpoint D of the arms in series₁；

The second CLLLC resonant tank comprises two resonant inductors L_r3And L_r4Two resonant capacitors C_r3And C_r4An equivalent excitation inductance L_m2And a high-frequency transformer T₂(ii) a High-frequency transformer T₂Primary side parallel connection equivalent excitation inductance L_m2High frequency transformer T₁One end of the primary side is connected to the resonant inductor L_r3Connecting the power field effect transistor S₄And S₆Midpoint A of the series arm₂The other end of the resonant capacitor C_r3Connecting the power field effect transistor S₃And S₅Midpoint B of series-connected bridge arms₂High frequency transformer T₂One end of the secondary side is connected to the resonant inductor L_r4Connecting power field effect transistor Q₄And Q₆Midpoint C of series bridge arm₂The other end of the resonant capacitor C_r4Connecting power field effect transistor Q₃And Q₅Midpoint D of the arms in series₂。

In order to match the power transfer characteristics of the two resonant circuits, the resonance parameters of the two CLLLC resonant circuits are identical, i.e. L_r1＝L_r3＝L_rp，L_r2＝L_r4＝L_rs，C_r1＝C_r3＝C_rp，C_r2＝C_r4＝C_rs，L_m1＝L_m2＝L_mHigh frequency transformer T₁And T₂The original secondary edge transformation ratio n of (2) is the same. Then CLLLC resonant frequency f_rThe calculation is as follows:

high-frequency transformer T₁And T₂Resonant frequency f of both sides_rThe two-way energy transmission between the input side and the output side needs to be kept consistent; the primary side MOSFET will realize ZVS, and S₁，S₆Will suffer from a hard shutdown; in addition, the secondary side Q₁，Q₆The proposed CLLLC converter is highly efficient due to the natural soft switching capability of the diode implementing ZCS.

And establishing an average model and a simplified average model of the converter by using a fundamental wave approximation method (FHA), and obtaining expressions of the input voltage, the input current, the equivalent resistance, the voltage transfer function and the output power of the resonant loop of the converter. Analyzing to obtain that the larger the excitation inductance is, the higher the output power is, and the lower the effective value of the resonant current is; when the output load is a resistive load, the CLLLC converter works under an inductive condition; output voltage passing duty ratio D and switching frequency f of CLLLC converter_sAnd (5) controlling.

The converter adopts a hybrid pulse modulation and phase shift (FMPS) control strategy, and the output voltage and power of the converter depend on the switching frequency and A of the MOSFET₁And B₂Voltage betweenDuty cycle of (d), when the switching frequency f_sWhen decreasing or the duty cycle D increasing, the output voltage of the converter will increase.

Considering the voltage and current overshoot, a soft start strategy is required, which can be implemented by increasing the switching frequency f during the start phase_sAnd means to reduce the duty cycle D to limit voltage and current overshoot.

The controller is two PI controllers, the feedback signal is the output voltage Vo, and 6 PWM signals are generated to trigger the switching device. The controller compensates the switching frequency of the PWM signal via the voltage feedback signal and establishes a voltage loop to adjust the switching frequency of the primary-side switching device and the S₁、S₂、S₅、S₆The duty cycle of (a) varies the output voltage.

Analyzing the steady-state working condition of the converter, describing four working modes in steady-state operation in detail, and obtaining the characteristic that the converter has natural soft switching capacity, and the efficiency is high; analyzing the CLLLC converter by a fundamental wave approximation method (FHA) to obtain an average model of the CLLLC converter and obtain an expression of the output voltage of the resonant tank; simplifying the average model to obtain an expression of a voltage transfer function and transmission power; a mixed pulse frequency modulation and phase shift control (FMPS) scheme is adopted, and the switching frequency fs and the duty ratio D of a switching device are adjusted to realize the control of output voltage; a soft start strategy that increases the switching frequency and decreases the duty cycle is employed during start-up to reduce voltage and current overshoot.

The CLLLC converter provided by the utility model is explained in three aspects of steady-state working condition, characteristic analysis and control strategy.

The steady-state resonance waveform and the gate signal of the CLLLC converter are shown in FIG. 2, wherein Ts ═ 1/fs is the switching period; the CLLLC converter has eight operating modes within one switching cycle. The first four modes of operation are similar to the last four modes of operation. The mode of operation of the first half cycle is described in detail below.

When power is transferred from the input side to the output side, the primary MOSFET operates in a reverse state and the secondary MOSFET operates in an uncontrolled rectifying state.

Mode 1[ t ]₀～t₁]: this mode is from S₁、S₄And S₅The start of (1); primary current ip₁And ip₂By S₁、S₄And S₅Three anti-parallel diodes. At the same time, Q₁、Q₄And Q₅The anti-parallel diode is conducted, and the resonant current increases in a sine trend. Voltage VA of each resonant tank₁B₁And VA₂B₂Increasing from 0 to Vin/2. Thus, the primary current ip₁Ip2 and field current im₁、im₂The decrease is started.

Mode 2[ t ]₁～t₂]: primary current ip₁And ip₂Will become positive, which causes Q to be positive₁、Q₄And Q₅Operating under ZVS conditions; during mode 2, power will be transferred to the secondary rectifier through the CLLLC resonant tank. The voltage across each excitation inductor is kept constant, so that im₁And im₂Increasing linearly. Thus, L_m1And L_m2Do not participate in resonance. Primary current ip₁And ip₂Varying with a sinusoidal trend with frequency fr.

Mode 3[ t ]₂～t₃]: with S₁And S₅Off, VA₁B₁And VA₂B₂Changes to 0 during this period; s₃The body diode of (a) begins conducting a primary current; primary current ip₁、ip₂And secondary current is₁、is₂And drops sharply. Due to is₁And is₂Not equal to 0, im₁And im₂And still not participate in resonance.

Mode 4[ t ]₃～t₄]: this situation is from Q₁、Q₄And Q₅Zero current turn off of (c) begins. Since ip is at time t3₁＝ip₂＝im₁＝im₂，is₁And is₂All fall to 0. Height ofFrequency transformer T₁And T₂The primary side input current of (2) is 0. L is_m1And L_m2Where the mode will participate in resonance.

It can be seen that the primary side MOSFET will achieve ZVS, while S₁、S₆Will suffer from a hard shut down. In addition, the secondary side Q₁、Q₆The diodes of (a) may also implement ZCS. Due to the natural soft switching capability, the CLLLC converter provided by the utility model has higher efficiency.

And (3) characteristic analysis:

the average model of the CLLLC converter can be obtained by FHA, assuming the switching frequency f of all MOSFET gate signals_sIs f_rThe equivalent topology of a CLLLC converter is shown in fig. 3. Where Vi is the input voltage, Ii is the average input current of the CLLLC resonant tank, R_eqIs an equivalent load and VAB is A₁And B₂The fundamental component of the two-terminal voltage. Furthermore, the resonant inductance and capacitance of the secondary side need to be translated to the primary side. The parameters of the resonance circuit after conversion are expressed as

L_r2'＝L_r4'＝n²L_r2＝L_rp

Due to the switching frequency f of the MOSFET gate signal_sIs f_rEnergy is transmitted primarily through the voltage and current fundamental components. Thus, the input voltage of the resonant tank is

Equivalent resistance R_eqCalculated by the following formula

To simplify the averaging model shown in fig. 3, two CLLLC resonant tanks connected in series would be equivalent to the circuit shown in fig. 4, the voltage transfer function being obtained from fig. 4 as:

L_eq1＝L_rp+2L_m

L_eq2＝L_rp+L_m

the input current ip of the CLLLC resonant tank shown in fig. 4 is represented as

Where Zi is the input impedance of the averaging model.

The above formula shows that a larger excitation inductance can improve the output power and reduce the effective value of the resonant current. Thus, L_mShould be much larger than L_rsAnd L_rpThis may improve power conversion efficiency and reduce conduction losses of the CLLLC converter. Furthermore, when the output load is a resistive load, the CLLLC converter operates in an inductive condition. The output voltage of the CLLLC converter can be controlled by means of the duty cycles D and f_sAnd (5) controlling.

And (3) control strategy:

the proposed symmetric dual-bridge CLLLC resonant converter employs a hybrid pulse Frequency Modulation and Phase Shift (FMPS) control strategy. The output voltage and power of the converter depend on the switching frequency of the MOSFETs and A₁And B₂The duty cycle of the voltage in between. When f is_sWhen decreasing or the duty cycle D increasing, the output voltage of the converter will increase. In addition, the controller design should take voltage and current overshoot into account, and therefore a soft start strategy needs to be employed. During the starting phase, f can be increased_sAnd reducing duty cycle D to limit voltage and current overshoot. A detailed control diagram of the CLLLC converter is shown in fig. 5. Wherein

f_i＝f_r(1+e^-tT)

D_i＝D_s(1-e^-tT)

Di and fi are steady-state values controlling the duty cycle and frequency, respectively, of the output PWM signal, D_sIs the duty cycle at steady state. T is a soft start time constant, and is equal to 0.01-0.02. The FMPS controller consists of two PI controllers, the feedback signal is the output voltage Vo, and 6 PWM signals can be generated to trigger the switching device. The controller compensates the switching frequency of the PWM signal by outputting a voltage feedback signal and establishes a voltage loop to adjust the switching frequency and S of the primary side switching device₁、S₅And S₆The duty cycle of (a) changes the output voltage.

The simulation results are shown in fig. 6 to 10. As can be seen from FIG. 6, the input voltage VA is identical due to the identical parameters of the two CLLLC resonant tanks₁B₁And VA₂B₂Always equal. Figure 7 shows a bode plot of the average model voltage transfer function. To compare the difference between hard and soft start, the CLLLC converter is operated in open loop mode. As shown in fig. 8, the proposed soft start strategy can effectively reduce the voltage overshoot during the start phase. To mitigate open loop oscillations and improve the reliability of the controller, an FMPS control strategy is used in the CLLLC converter. FIG. 9 shows f_sThe output voltage curves at 20kHz, 25kHz and 30kHz respectively. The result shows that the CLLLC converter can output stable direct current voltage under the regulation of FMPS controller. When working at f_rAt frequency, the output voltage of the CLLLC converter follows f_sIs increased and decreased. S₁And Q₁The voltage and current waveforms of fig. 10 illustrate that the proposed CLLLC converter can achieve soft switching, improving overall efficiency.

The analysis and simulation results show that the CLLLC converter and the control strategy thereof provided by the utility model have good performance and are suitable for an alternating current-direct current hybrid micro-grid.

In summary, according to the symmetrical double-bridge CLLLC resonant converter, the six improved switch structures on the primary side and the secondary side of the converter can reduce the voltage stress of each switch device, and are beneficial to improving the energy transmission capacity and the voltage level of the ac-dc hybrid microgrid. The topology includes two identical CLLLC resonant tanks to enhance the power transfer capability of the converter. Due to the symmetrical structure of the topology, energy can flow in both directions. The CLLLC converter can also implement soft switching on both sides. A soft start scheme is used to reduce the overshoot by reducing the duty cycle and increasing the switching frequency during start-up. And performing simulation verification on the CLLLC converter adopting the FMPS control strategy in the PLECS. Simulation results show that the CLLLC converter adopting the proposed control scheme has good performance and is suitable for an alternating current-direct current hybrid micro-grid.

The above-mentioned contents are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modification made on the basis of the technical idea of the present invention falls within the protection scope of the claims of the present invention.

Claims

1. A symmetrical double-bridge CLLLC resonant converter is characterized by comprising two CLLLC resonant circuits, wherein the two CLLLC resonant circuits are respectively connected in series with the primary side of a corresponding high-frequency transformer and the secondary side of the corresponding high-frequency transformer; the input side and the output side of each CLLLC resonant circuit are respectively connected with the middle point of a bridge arm of a six-switch full-bridge circuit, and the six-switch full-bridge circuit on the input side is connected with an input capacitor C in parallel_iThe six-switch full-bridge circuit on the output side is connected in parallel with an output capacitor C_o。

2. The symmetrical dual-bridge CLLLC resonant converter according to claim 1, wherein the input side and the output side of each CLLLC resonant tank are connected by a high-frequency transformer, the input side of the CLLLC resonant tank is a series circuit formed by a resonant capacitor, an equivalent excitation inductor and a resonant inductor, and the output side of the CLLLC resonant tank is a series circuit formed by a resonant capacitor and a resonant inductor.

3. The symmetrical double-bridge CLLLC resonant converter according to claim 2, wherein the input side of each CLLLC resonant tank is connected with the power FET S in the six-switch full-bridge circuit₁And S₃Midpoint A of the series arm₁And electric power field effect transistor S₂And S₄Midpoint B of series-connected bridge arms₁The output side of the six-switch full-bridge circuit is connected with an electric field effect tube Q in the other six-switch full-bridge circuit₁And Q₃Midpoint C of series bridge arm₁And electric power field effect transistor Q₂And Q₄Midpoint D of the arms in series₁(ii) a The input side of the other CLLLC resonant circuit is connected with an electric field effect tube S in a six-switch full-bridge circuit₄And S₆Midpoint A of the arms in series₂And an electric field effect transistor S in a six-switch full-bridge circuit₃And S₅Midpoint B of series-connected bridge arms₂The output side of the six-switch full-bridge circuit is connected with an electric field effect tube Q in the other six-switch full-bridge circuit₄And Q₆Midpoint C of series-connected bridge arm₂And electric field effect tube Q in six-switch full-bridge circuit₃And Q₅Midpoint D of the arms in series₂。

4. A symmetric dual-bridge CLLLC resonant converter according to claim 3, characterized by a high-frequency transformer T₁Primary side parallel connection equivalent excitation inductance L_m1High frequency transformer T₁One end of the primary side is connected with a resonant inductor L_r1Connecting midpoint A₁The other end of the resonant capacitor C_r1Connecting midpoint B₁High frequency transformer T₁One end of the secondary side passes through a resonant inductor L_r2Connecting midpoint C₁The other end of the resonant capacitor C_r2Connecting midpoint D₁。

5. A symmetric dual-bridge CLLLC resonant converter according to claim 3, characterized by a high-frequency transformer T₂Primary side parallel connection equivalent excitation inductance L_m2High frequency transformer T₁Primary side end resonant inductorL_r3Connecting midpoint A₂The other end of the resonant capacitor C_r3Connecting midpoint B₂High frequency transformer T₂One end of the secondary side passes through a resonant inductor L_r4Connecting midpoint C₂The other end of the resonant capacitor C_r4Connecting midpoint D₂。

6. A symmetric dual bridge CLLLC resonant converter according to claim 1, characterized in that the resonant parameters of the two CLLLC resonant tanks are identical.

7. A symmetric dual-bridge CLLLC resonant converter as claimed in claim 1, characterized in that the primary and secondary side transformation ratios n of the two high-frequency transformers are the same, n being a positive number.

8. The symmetrical dual-bridge CLLLC resonant converter as claimed in claim 1, characterized in that the switching frequency f of the six-switch full-bridge circuit_sEqual to the resonant frequency f of the CLLLC resonant tank_r。