Three-degree-of-freedom control method and system for fuel cell-oriented ibbb-llc converter

Abstract

The application provides a three-freedom control method and system for an IBBB-LLC converter of a fuel cell, and belongs to the technical field of power electronics. The application is used to solve the problem of voltage surge of the fuel cell under light load, which leads to the failure of voltage regulation of a DC-DC converter and the decrease of efficiency. The method comprises the following steps: S1, determining a control mode under a current working condition; S2, constructing a voltage gain model based on three-freedom decoupling of a duty ratio, a phase shift angle and a switching frequency according to the control mode; S3, solving an optimal combination of the duty ratio, the phase shift angle and the switching frequency which minimizes the total loss of the system under the condition of meeting the output voltage constraint and the safe operation boundary according to the voltage gain model and a system total loss model; and S4, generating an asymmetric driving time sequence to control the operation of the IBBB-LLC converter according to the optimal combination. The system comprises a front-stage interleaved parallel Buck-Boost circuit, a rear-stage full-bridge LLC resonant circuit and a three-freedom control unit.

Description

Technical Field

[0001] This invention relates to a three-degree-of-freedom control method and system for IBBB-LLC converters for fuel cells, belonging to the field of power electronics technology. Background Technology

[0002] Currently, human space activities are shifting from near-Earth orbit exploration to deep space stays and resource development. With the implementation of my country's manned lunar landing program, the International Lunar Research Station, and the Mars exploration program, the goal is to build comprehensive extraterrestrial research bases capable of supporting long-term, in-situ, and sustainable scientific exploration. As a key infrastructure of extraterrestrial research stations, the performance of the energy system directly determines the base's lifespan and the success or failure of missions. Extraterrestrial research stations are evolving from traditional single-power-source architectures to hybrid energy systems that incorporate multiple energy flows, including photovoltaic power generation, radioisotope thermoelectric power generation, fuel cells, and in-situ resource utilization. Among these, fuel cells, due to their advantages of high energy density, high efficiency, and in-situ resource availability, have become the core energy source for deep space exploration missions.

[0003] However, in extreme extraterrestrial environments, fuel cells, as the input source of energy routing systems, exhibit soft characteristics: their terminal voltage is normal under rated load, but under light load conditions such as standby, low power consumption, or no-load operation at the research station, the fuel cell terminal voltage can spike dramatically. This necessitates that the downstream DC-DC converter possess strong wide voltage regulation capabilities. The interleaved parallel Buck-Boost integrated full-bridge LLC resonant converter (IBBB-LLC) combines the buck-boost characteristics of a Buck-Boost circuit with the isolation, voltage regulation, and soft-switching characteristics of an LLC resonant converter, and is considered an ideal downstream topology for achieving lightweight, efficient, and highly reliable operation of energy systems in extraterrestrial research stations.

[0004] In the existing technology, the mainstream control method for IBBB-LLC converters usually adopts a two-degree-of-freedom modulation strategy that combines duty cycle modulation and frequency regulation. However, in the complex energy routing environment of extraterrestrial research stations, the traditional strategy has the following technical defects: (1) The LLC gain curve is flat under light load, the voltage regulation fails, and the risk of output runaway occurs. (2) The extremely high switching frequency passively maintained in order to barely stabilize the voltage will cause the transformer iron loss and the switching tube drive loss to increase exponentially, resulting in a significant drop in the system efficiency under light load. Summary of the Invention

[0005] This invention addresses the problem of voltage spikes in DC-DC converters under light load conditions, leading to voltage regulation failure and efficiency reduction. It proposes a three-degree-of-freedom control method and system for IBBB-LLC converters in fuel cells.

[0006] The technical solution adopted by this invention to solve the above problems is: the three-degree-of-freedom control method for IBBB-LLC converters for fuel cells proposed in this invention, comprising: Step 1: Determine the control mode under the current operating conditions based on the input voltage of the converter and the load power; Step 2: Based on the control mode, construct a voltage gain model based on the decoupling of three degrees of freedom: duty cycle, phase shift angle, and switching frequency; Step 3: Based on the voltage gain model and the total system loss model, under the conditions of satisfying the output voltage constraint and safe operation boundary, solve for the optimal combination of duty cycle, phase shift angle and switching frequency that minimizes the total system loss; Step 4: Generate asymmetric drive timing based on the optimal combination to control the operation of the IBBB-LLC converter.

[0007] Furthermore, step 1 specifically includes: Based on the open-circuit voltage and equivalent internal resistance model of the fuel cell, and combined with the current load power, the input voltage is calculated in real time. Based on the comparison between the input voltage and the preset threshold, a three-degree-of-freedom asymmetric control mode including duty cycle, phase shift angle and switching frequency is determined.

[0008] Furthermore, step 2 specifically includes: Based on duty cycle D Determine the intermediate DC bus voltage of the preceding Buck-Boost circuit. V bus ; Based on duty cycle D With phase angle f Determine the effective value of the fundamental voltage entering the LLC resonant cavity of the subsequent stage. V ab1 ; The gain of the subsequent LLC resonant cavity is determined based on the switching frequency. G LLC ; Based on the intermediate DC bus voltage V bus The effective value of the fundamental voltage entering the LLC resonant cavity of the subsequent stage. V ab1 Gain of LLC resonant cavity G LLC Determine the total voltage gain of the system. M ; Intermediate DC bus voltage V bus The calculation formula is: (1); In formula (1), The input voltage of the LLC resonant cavity; RMS value of the fundamental voltage entering the LLC resonant cavity of the subsequent stage V ab1 The calculation formula is: (2); Gain of the LLC resonator G LLC The calculation formula is: (3); System total voltage gain M The calculation formula is: (4); In formula (4), n Let be the turns ratio of the transformer's secondary side to its primary side, sin(π) D ) / (1- D ) represents the preceding stage based on duty cycle D The voltage regulation capability is determined.

[0009] Furthermore, based on the duty cycle D With phase angle f Determine the effective value of the fundamental voltage entering the LLC resonant cavity of the subsequent stage. V ab1 ,include: Based on duty cycle D Fourier decomposition is performed on the voltage at the midpoint of the primary side bridge arm to obtain the fundamental component; According to the phase shift angle f Phase cancellation is performed on the fundamental component to obtain the effective value of the fundamental voltage entering the subsequent LLC resonant cavity. V ab1 .

[0010] Furthermore, the construction of the total system loss model includes: Calculate the turn-off loss of the main switch transistor based on the switching frequency, duty cycle, and phase shift angle. Primary power transistor conduction loss Secondary rectifier tube conduction loss Copper loss of pre-amplifier interleaved inductor Copper loss of resonant inductor High-frequency iron loss of transformer and switching transistor drive losses Based on the superposition of various losses, a total system loss model is constructed. ; System total loss model The expression is: (5); Main switch turn-off loss The calculation formula is: (6); In formula (6), t f The switching time of the switching diode, the turn-off loss of the main switch and f s Proportional to the bus voltage V bus by D Adjustment, phase shift angle f It will change the turn-off current of the switching transistors in different bridge arms. I off Transient values, thereby enabling the redistribution of losses; Primary power transistor conduction loss The calculation formula is: (7); In formula (7), I rms These are the effective values ​​of the current in each tube. R ds The on-resistance of the primary power transistor and the phase shift angle are given. f The introduction of this will disrupt the current symmetry of the left and right bridge arms, generating additional reactive circulating current, leading to... I rms change; Secondary rectifier tube conduction loss The calculation formula is: (8); In formula (8), I Lr_rms This is the effective value of the resonant current. R SR The on-resistance of the secondary rectifier diode. I Lr_rms Subject to fundamental voltage V ab1 The decision was made through the duty cycle. D With phase angle f The combined cutting directly affects the magnitude of the current transmitted to the secondary side, thereby affecting the conduction loss of the secondary rectifier tube; Preamplifier interleaved inductor copper loss The calculation formula is: (9); In formula (9), the inductor current ripple is affected by the duty cycle. D and switching frequency f s The high-frequency AC resistance is jointly determined by factors including the skin effect. R L_ac Follow f s It increases in a linear fashion rather than a quantitative one. resonant inductor copper loss The calculation formula is: (10); In formula (10), the effective value of the resonant current I Lr_rms Duty cycle D With phase angle f The combined effects of AC resistance R Lr_ac Subject to switching frequency f s The skin-tonic effect; Transformer high-frequency iron loss The calculation formula is: (11); In formula (11), through sin( f / 2) Reduce the magnetic flux density amplitude, thereby reducing the switching frequency. f s Reduce high-frequency iron losses in transformers; Switch drive losses The calculation formula is: (12); In formula (12), the switching transistor drive loss and f s The loss is linearly proportional to the loss, and this loss can be directly reduced through a phase-shifting frequency reduction mechanism.

[0011] Furthermore, step 3 involves finding the optimal combination of duty cycle, phase shift angle, and switching frequency that minimizes the total system loss, including: Based on the physical limits and thermal design parameters of the converter power devices, safe operating boundaries for duty cycle and switching frequency are set; based on the output voltage closed-loop feedback, output voltage stability constraints are set.

[0012] Calculate the baseline loss state under traditional symmetrical drive based on the current load conditions; Based on the degree of input voltage overvoltage, an initial disturbance amount deviating from the symmetry point is applied to the phase shift angle; Based on the total system loss model, within the output voltage constraint and safe operation boundary, the total loss corresponding to different parameter combinations is calculated iteratively. Based on the iterative calculation results, the optimal duty cycle, phase shift angle, and switching frequency that minimize the total system loss are output.

[0013] Furthermore, this invention also proposes a three-degree-of-freedom control system for IBBB-LLC converters for fuel cells, comprising: The front-end interleaved parallel Buck-Boost circuit includes a first interleaved inductor, a second interleaved inductor, a first power switch group, and a second power switch group. The first and second power switch groups control the charging and discharging states of the first and second interleaved inductors, respectively, to adjust the intermediate DC bus voltage according to the duty cycle. The first power switch group includes switching transistors. S 1 and switching transistor S 4. The second power switch group includes switching transistors. S 2 and switching transistor S 3. Switching transistor S 1 and switching transistor S 4 is a complementary drive, switching transistor S 2 and switching transistor S 3 is a complementary driving force; The subsequent full-bridge LLC resonant circuit includes a primary-side full-bridge inverter unit, a resonant cavity unit, and a secondary-side rectifier unit. The primary-side full-bridge inverter unit includes a left bridge arm and a right bridge arm. There is a phase shift angle between the drive signals of the left bridge arm and the right bridge arm, which makes the input voltage of the resonant cavity unit present an asymmetrical stepped waveform. The three-degree-of-freedom control unit determines the optimal combination of duty cycle, phase shift angle and switching frequency based on the input voltage and load power of the fuel cell, and generates corresponding asymmetric drive timing sequences, which are output to the front-end interleaved parallel Buck-Boost circuit and the rear-end full-bridge LLC resonant circuit respectively.

[0014] Furthermore, the three-degree-of-freedom control unit includes: The operating condition monitoring module calculates the input voltage in real time based on the open-circuit voltage and equivalent internal resistance model of the fuel cell, combined with the current load power. The control mode determination module determines whether to adopt a two-degree-of-freedom symmetrical control mode that includes duty cycle and switching frequency, or a three-degree-of-freedom asymmetrical control mode that includes duty cycle, phase shift angle and switching frequency, based on the comparison result between the input voltage and the preset threshold. The gain model construction module constructs a voltage gain model based on the control mode determined by the control mode determination module, which is decoupled from the duty cycle, phase shift angle and switching frequency in three degrees of freedom. The intermediate DC bus voltage is determined based on the duty cycle, the fundamental voltage amplitude entering the resonant cavity unit is determined based on the duty cycle and phase shift angle using Fourier decomposition, the resonant cavity gain of the subsequent full-bridge LLC resonant circuit is determined based on the switching frequency, and the total voltage gain of the system is determined based on the intermediate DC bus voltage, the fundamental voltage amplitude and the resonant cavity gain. The optimization calculation module, based on the voltage gain model and the total system loss model, solves for the optimal combination of duty cycle, phase shift angle and switching frequency that minimizes the total system loss, under the conditions of satisfying the output voltage constraint and safe operation boundary. The drive timing generation module generates an asymmetric drive timing based on the optimal combination, wherein there is a phase shift angle between the drive signals of the left bridge arm and the drive signals of the right bridge arm.

[0015] Furthermore, the optimization calculation module includes: The reference loss calculation unit calculates the reference loss state under traditional symmetrical drive based on the current load conditions. The initial disturbance application unit applies an initial disturbance amount that deviates from the symmetry point to the phase shift angle according to the degree of input voltage overvoltage; The iterative calculation unit, based on the total system loss model, iteratively calculates the total loss corresponding to different parameter combinations within the output voltage constraint and safe operation boundary. The total system loss model calculates the main switch turn-off loss, primary power transistor conduction loss, secondary rectifier conduction loss, front-stage interleaved inductor copper loss, resonant inductor copper loss, transformer high-frequency iron loss, and switch drive loss based on the switching frequency, duty cycle, and phase shift angle. The optimal parameter output unit outputs the optimal duty cycle, phase shift angle, and switching frequency that minimize the total system loss based on the iterative calculation results.

[0016] The beneficial effects of this invention are: This invention introduces a phase shift angle as a third degree of freedom for control, fundamentally transforming the input voltage of the resonant cavity from a symmetrical square wave to an asymmetrical stepped wave, thereby achieving direct intervention in the energy injection of the resonant cavity without altering the switching frequency. Secondly, a voltage gain model based on three-degree-of-freedom decoupling is constructed, utilizing phase cancellation to reduce the fundamental voltage amplitude, thus resolving the frequency modulation limitations and voltage regulation runaway issues under light-load, high-voltage conditions. Finally, this invention introduces a nonlinear optimization control algorithm based on minimizing total system losses, accurately calculating and balancing the total system losses under harsh conditions. This achieves automatic solution for the optimal combination of duty cycle, phase shift angle, and switching frequency, improving the efficiency and overall performance of the system's steady-state operation across the entire load range. Attached Figure Description

[0017] Figure 1 A flowchart of a three-degree-of-freedom control method for an IBB-LLC converter for fuel cells; Figure 2 This is a structural diagram of a three-degree-of-freedom control system for an IBBB-LLC converter designed for fuel cells. Figure 3 The waveforms of the main voltage and current under three-degree-of-freedom joint modulation are shown. Figure 4 The diagram shows the operating modes under three-degree-of-freedom joint modulation. Figure 5 This is a comparison chart of the theoretical efficiency of the present invention and traditional two-degree-of-freedom control. Detailed Implementation Specific Implementation Method 1 like Figure 2 As shown, the structure of the three-degree-of-freedom control system for the IBBB-LLC converter for fuel cells described in this embodiment includes: The front-end interleaved parallel Buck-Boost circuit consists of a full-bridge structure composed of four power switches, among which... S 1 and S 4 is a complementary driving force. S 2 and S 3 is a complementary drive. The controller adjusts two sets of switches. S 2. S 4% duty cycle D To control the two interleaved inductors in the front stage L 1. L The charging and discharging of 2 completes the initial regulation of the bus voltage. Meanwhile, the core of the controller lies in enabling the right bridge arm ( S 3. S 4) The drive signal relative to the left axle arm ( S 1. S 2) Lagging by one phase shift angle f Under this asymmetric drive, a complete switching cycle of the IBB-LLC converter with three-degree-of-freedom joint modulation is divided into six independent operating stages. The main voltage and current waveforms are shown below. Figure 3 As shown.

[0019] The circuit is divided into three key operating modes within a positive half-cycle, such as... Figure 4 As shown. In t 0- t Phase 1, S 1 and S 4 is turned on, at which time the inductor is turned on. L 1. Discharge, L 2. During charging, the midpoint of the bridge arm outputs a high level, and the resonant cavity receives the bus voltage and normally transfers energy to the secondary side. t 1- t In stage 2, the circuit enters the cutting zone. At this time, the primary side no longer supplies current to the secondary side, and the diode current on the secondary side naturally returns to zero to achieve ZCS. The resonant cavity is in a circulating current state with no power transfer. t 2- t 3 is the continuous flow phase, at which time... S 4. Turn off, inductor current extraction S The junction capacitance charge of 3 makes S The body diode of 3 conducts first, thus enabling the subsequent... S The ZVS of 3 created the conditions. The working principle of the second half cycle is the same as that of the first half cycle. Specific Implementation Method Two This invention introduces an asymmetric phase shift angle as a third control dimension to readjust the amplitude of the fundamental voltage entering the resonant cavity, thereby adjusting the system gain. Simultaneously, combined with a system loss minimization optimization algorithm, it achieves improved energy efficiency across the entire load range. Figure 1 As shown, the steps of the three-degree-of-freedom control method for IBBB-LLC converters for fuel cells described in this embodiment include: S1: Determine the control mode under the current operating conditions; In terms of driving timing, the control system enables the switching transistors of the primary-side cross-arms to conduct complementaryly, forming a set duty cycle. This duty cycle is used to control the charging and discharging state of the preceding interleaved inductors, achieving pre-voltage regulation of the system. Simultaneously, the control system also lags the driving signal of one arm behind the other arm by a set asymmetric phase shift angle. This asymmetric driving timing causes a phase shift in the midpoint voltage of the primary-side arm, changing the input voltage of the subsequent LLC resonant cavity from a traditional symmetrical square wave to an asymmetric stepped wave, thus intervening in the energy injection of the resonant cavity without changing the switching frequency.

[0021] S2: Based on the control mode, construct a voltage gain model based on the decoupling of three degrees of freedom: duty cycle, phase shift angle, and switching frequency; Based on the aforementioned asymmetric driving timing, this invention constructs a voltage gain model based on three-degree-of-freedom decoupling. Under traditional two-degree-of-freedom control, the amplitude of the input fundamental voltage of the LLC resonant cavity is only affected by the duty cycle. However, under the modulation strategy of this invention, the actual fundamental input voltage of the resonant cavity is simultaneously affected by both the duty cycle and the phase shift angle. This gain model theoretically demonstrates that when the fuel cell is under light load and the terminal voltage spikes, the system does not need to rely on extremely high switching frequencies to reduce the resonant cavity gain. Instead, it can actively expand the phase shift angle and introduce a fundamental phase cancellation effect to reduce the amplitude of the fundamental voltage entering the resonant cavity, thus solving the frequency modulation limitation and voltage regulation runaway problems under light-load, high-voltage operating conditions.

[0022] This invention verifies the voltage regulation capability of a three-degree-of-freedom joint modulation strategy by deriving a system voltage gain model. In this model, the intermediate DC bus voltage... V bus Duty cycle D Regulation, that is: (1); In formula (1), The input voltage of the LLC resonant cavity; RMS value of the fundamental voltage entering the resonant cavity of the subsequent stage V ab1 Then subject to duty cycle D With phase angle f The joint modulation, based on Fourier decomposition, can be expressed as: (2); Finally, the total voltage gain of the system is derived. M The analytical expression is: (3); In formula (3), n Let be the turns ratio of the transformer's secondary side to its primary side, sin(π) D ) / (1- D ) represents the preceding stage based on duty cycle D The voltage regulation capability is determined. G LLC The subsequent stage is determined by the switching frequency. f s The gain of the LLC resonator is determined. G LLC The analytical expression is: (4); When a fuel cell is under extremely light load conditions, causing a severe spike in the input voltage, if a traditional two-degree-of-freedom modulation scheme is used, the system can only increase the frequency indefinitely. f s To reduce G LLC The core of this invention lies in the phase shift angle. f The generated sin( f / 2) Product term, the system adjusts the phase shift angle... f This will make sin( f / 2) The term decreases with a sinusoidal trend, which allows for additional adjustment of the system gain, widens the system's input voltage range, and enables precise voltage regulation under light load and harsh operating conditions.

[0023] S3: Based on the voltage gain model and the total system loss model, under the conditions of satisfying the output voltage constraint and safe operation boundary, solve for the optimal combination of duty cycle, phase shift angle and switching frequency that minimizes the total system loss; To achieve efficient operation across the entire load range, this invention designs an optimization algorithm based on minimizing total system losses. First, this invention establishes a computational model encompassing primary-side conduction losses, switching losses, high-frequency iron losses, and drive losses, and sets safe operating boundaries for the duty cycle and switching frequency. During control, when an excessively high input voltage is detected, the algorithm automatically introduces a phase shift angle to enter a three-degree-of-freedom asymmetric control mode. The core of this algorithm lies in ensuring the closed-loop stability of the output voltage while comprehensively balancing the increased conduction losses due to the introduction of phase-shift circulating current with the reduced core iron losses and switching losses due to avoiding extremely high-frequency operation, thereby accurately calculating the combination of duty cycle, phase shift angle, and switching frequency that minimizes the overall system losses. Through this dynamic loss optimization mechanism, this invention improves the steady-state operating efficiency under all operating conditions.

[0024] At the specific execution level of the algorithm, this invention employs a nonlinear optimization control algorithm based on a system total loss model. First, the controller calculates the input voltage in real time based on the open-circuit voltage and equivalent internal resistance model of the fuel cell, combined with the current load power. Then, the system calculates the total system loss under the current parameters using a built-in loss model. The formula for calculating the total system loss is: (5); The model explicitly quantifies the relationship between various losses and the three degrees of freedom: (1) P off The turn-off loss of the main switch is expressed as: (6); In formula (6), t f Given the descent time of the switching tube, it can be seen from equation (6) that the turn-off loss of the main switch is related to... f s Proportional, and at the same time, bus voltage V bus by D Adjustment, phase shift angle f It will change the turn-off current of the switching transistors in different bridge arms. I off Transient values, thereby enabling the redistribution of losses.

[0025] (2) P cond The primary-side power transistor conduction loss is expressed as: (7); In formula (7), I rms These are the effective values ​​of the current in each tube. R ds The on-resistance of the primary power transistor and the phase shift angle are given. f The introduction of this will disrupt the current symmetry of the left and right bridge arms, generating additional reactive circulating current, leading to the change in equation (7). I rms change.

[0026] (3) P condD The conduction loss of the secondary rectifier diode is expressed as: (8); In formula (8), I Lr_rms This is the effective value of the resonant current. R SR The on-resistance of the secondary rectifier diode. I Lr_rmsSubject to fundamental voltage V ab1 Decision, passed D and f The combined cutting can directly influence the magnitude of the current transmitted to the secondary side, thereby affecting the conduction loss of the secondary rectifier tube.

[0027] (4) P L The copper loss of the preceding interleaved inductor is expressed as: (9); In formula (9), the inductor current ripple is affected by... D and f s The high-frequency AC resistance is jointly determined by factors including the skin effect. R L_ac Follow f s It increases in a linear fashion rather than a linear one.

[0028] (5) P Lr The copper loss of the resonant inductor is expressed as: (10); Similar to rectifier tube losses, I Lr_rms by D and f The combined effects of AC resistance R Lr_ac by f s The skin effect.

[0029] (6) P core For the high-frequency iron loss of the transformer, the expression is: (11); This part is the core advantage of the present invention, under traditional control at light load frequencies. f s Extremely high levels lead to this dramatic increase, and the introduction of f Then, it can be done through sin( f / 2) Reduce the amplitude of magnetic flux density, thereby reducing f s This reduces high-frequency iron losses in transformers.

[0030] (7) P drive The driving loss of the switching transistor is expressed as: (12); From formula (12), it can be seen that the switching transistor drive loss is related to... fs The loss is linearly proportional to the loss. This invention can directly reduce this loss through a phase-shifting frequency reduction mechanism.

[0031] When executing the optimization algorithm, the controller first calculates the reference state of the traditional symmetrical drive waveform. Then, based on the current input overvoltage situation, the controller actively applies an initial disturbance amount deviating from the symmetrical point to the phase shift angle to perform asymmetrical optimization. Under the premise of ensuring that the output voltage meets the gain equation constraint and that each parameter does not exceed the physical safety boundary, the algorithm performs real-time comparison calculation. From equations (5) to (12), it can be seen that the total system loss is P It is about f s , f , D For nonlinear multivariate functions, the optimization algorithm of this invention automatically solves and outputs the solution that minimizes the total system loss by balancing various indicators. P The optimal control combination that achieves the minimum value yields the optimal operating point. f s , f , D Value selection. The flowchart of the condition judgment and parameter optimization execution logic of the above algorithm is as follows: Figure 1 As shown.

[0032] S4: Generate asymmetric drive timing based on the optimal combination to control the operation of the IBBB-LLC converter.

[0033] To verify the effectiveness of the proposed three-degree-of-freedom optimal modulation strategy in improving energy efficiency across the entire load range, this invention, based on the aforementioned optimization algorithm, uses a nonlinear modeling and algorithm verification platform to compare the theoretical operating efficiency of the IBB-LLC converter under different load conditions. The boundary conditions for the calculation are: output voltage stabilized at 48V, load power varying from a light load of 200W to a rated power of 1000W, and duty cycle and switching frequency within physically safe ranges. The total system loss and efficiency under traditional two-degree-of-freedom symmetrical control and the three-degree-of-freedom optimal control of this invention are solved separately. The theoretical efficiency comparison curves are shown below. Figure 5 As shown.

[0034] Calculation data shows that near the rated load condition, the theoretical efficiency of traditional two-degree-of-freedom control and the three-degree-of-freedom optimization control of this invention is basically the same, with both achieving a peak system efficiency of around 95.7%. However, when the load drops below 700W, the efficiency difference between the two becomes significant, with the three-degree-of-freedom modulation proposed in this invention achieving an efficiency improvement of more than 0.4% compared to the traditional modulation strategy. Furthermore, when the load is further reduced to the extremely light load range below 200W, the system under the traditional control strategy cannot stabilize the output voltage at 48V, exhibiting severe voltage regulation instability, while the solution of this invention maintains an efficiency of 88.46% while ensuring voltage stability.

[0035] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent substitutions, and improvements made to the above embodiments without departing from the scope of the present invention, based on the technical essence of the present invention and within the spirit and principles of the present invention, shall still fall within the protection scope of the present invention.

Claims

1. A three-degree-of-freedom control method for IBBB-LLC converters for fuel cells, characterized in that, include: Step 1: Determine the control mode under the current operating conditions based on the input voltage of the converter and the load power; Step 2: Based on the control mode, construct a voltage gain model based on the decoupling of three degrees of freedom: duty cycle, phase shift angle, and switching frequency; Step 3: Based on the voltage gain model and the total system loss model, under the conditions of satisfying the output voltage constraint and safe operation boundary, solve for the optimal combination of duty cycle, phase shift angle and switching frequency that minimizes the total system loss; Step 4: Generate asymmetric drive timing based on the optimal combination to control the operation of the IBBB-LLC converter.

2. The three-degree-of-freedom control method for IBBB-LLC converters for fuel cells according to claim 1, characterized in that, Step 1 specifically includes: Based on the open-circuit voltage and equivalent internal resistance model of the fuel cell, and combined with the current load power, the input voltage is calculated in real time. Based on the comparison between the input voltage and the preset threshold, a three-degree-of-freedom asymmetric control mode including duty cycle, phase shift angle and switching frequency is determined.

3. The three-degree-of-freedom control method for IBBB-LLC converters for fuel cells according to claim 1, characterized in that, Step 2 specifically includes: Based on duty cycle D Determine the intermediate DC bus voltage of the preceding Buck-Boost circuit. V bus ; Based on duty cycle D With phase angle φ Determine the effective value of the fundamental voltage entering the LLC resonant cavity of the subsequent stage. V ab1 ; The gain of the subsequent LLC resonant cavity is determined based on the switching frequency. G LLC ; Based on the intermediate DC bus voltage V bus The effective value of the fundamental voltage entering the LLC resonant cavity of the subsequent stage. V ab1 Gain of LLC resonant cavity G LLC Determine the total voltage gain of the system. M ; Intermediate DC bus voltage V bus The calculation formula is: （1）； In formula (1), The input voltage of the LLC resonant cavity; RMS value of the fundamental voltage entering the LLC resonant cavity of the subsequent stage V ab1 The calculation formula is: （2）； Gain of the LLC resonator G LLC The calculation formula is: （3）； System total voltage gain M The calculation formula is: （4）； In formula (4), n Let be the turns ratio of the transformer's secondary side to its primary side, sin(π) D ) / (1- D ) represents the preceding stage based on duty cycle D The voltage regulation capability is determined.

4. The three-degree-of-freedom control method for IBBB-LLC converters for fuel cells according to claim 3, characterized in that, Based on duty cycle D With phase angle φ Determine the effective value of the fundamental voltage entering the LLC resonant cavity of the subsequent stage. V ab1 ,include: Based on duty cycle D Fourier decomposition is performed on the voltage at the midpoint of the primary side bridge arm to obtain the fundamental component; According to the phase shift angle φ Phase cancellation is performed on the fundamental component to obtain the effective value of the fundamental voltage entering the subsequent LLC resonant cavity. V ab1 .

5. The three-degree-of-freedom control method for IBBB-LLC converters for fuel cells according to claim 1, characterized in that, The construction of the total system loss model includes: Calculate the turn-off loss of the main switch transistor based on the switching frequency, duty cycle, and phase shift angle. Primary power transistor conduction loss Secondary rectifier tube conduction loss Copper loss of pre-amplifier interleaved inductor Copper loss of resonant inductor High-frequency iron loss of transformer and switching transistor drive losses Based on the superposition of various losses, a total system loss model is constructed. ; System total loss model The expression is: （5）； Main switch turn-off loss The calculation formula is: （6）； In formula (6), t f The switching time of the switching diode, the turn-off loss of the main switch and f s Proportional to the bus voltage V bus by D Adjustment, phase shift angle φ It will change the turn-off current of the switching transistors in different bridge arms. I off Transient values, thereby enabling the redistribution of losses; Primary power transistor conduction loss The calculation formula is: （7）； In formula (7), I rms These are the effective values ​​of the current in each tube. R ds The on-resistance of the primary power transistor and the phase shift angle are given. φ The introduction of this will disrupt the current symmetry of the left and right bridge arms, generating additional reactive circulating current, leading to... I rms change; Secondary rectifier tube conduction loss The calculation formula is: （8）； In formula (8), I Lr_rms This is the effective value of the resonant current. R SR The on-resistance of the secondary rectifier diode. I Lr_rms Subject to fundamental voltage V ab1 The decision was made through the duty cycle. D With phase angle φ The combined cutting directly affects the magnitude of the current transmitted to the secondary side, thereby affecting the conduction loss of the secondary rectifier tube; Preamplifier interleaved inductor copper loss The calculation formula is: （9）； In formula (9), the inductor current ripple is affected by the duty cycle. D and switching frequency f s The high-frequency AC resistance is jointly determined by factors including the skin effect. R L_ac Follow f s It increases in a linear fashion rather than a quantitative one. resonant inductor copper loss The calculation formula is: （10）； In formula (10), the effective value of the resonant current I Lr_rms Duty cycle D With phase angle φ The combined effects of AC resistance R Lr_ac Subject to switching frequency f s The skin-tonic effect; Transformer high-frequency iron loss The calculation formula is: （11）； In formula (11), through sin( φ / 2) Reduce the magnetic flux density amplitude, thereby reducing the switching frequency. f s Reduce high-frequency iron losses in transformers; Switch drive losses The calculation formula is: （12）； In formula (12), the switching transistor drive loss and f s The loss is linearly proportional to the loss, and this loss can be directly reduced through a phase-shifting frequency reduction mechanism.

6. The three-degree-of-freedom control method for IBBB-LLC converters for fuel cells according to claim 5, characterized in that, Step 3 involves finding the optimal combination of duty cycle, phase shift angle, and switching frequency that minimizes the total system loss, including: Based on the physical limits and thermal design parameters of the converter power devices, safe operating boundaries for duty cycle and switching frequency are set; based on the output voltage closed-loop feedback, output voltage stability constraints are set. Calculate the baseline loss state under traditional symmetrical drive based on the current load conditions; Based on the degree of input voltage overvoltage, an initial disturbance amount deviating from the symmetry point is applied to the phase shift angle; Based on the total system loss model, within the output voltage constraint and the safe operation boundary, the total loss corresponding to different parameter combinations is calculated iteratively. Based on the iterative calculation results, the optimal duty cycle, phase shift angle, and switching frequency that minimize the total system loss are output.

7. A three-degree-of-freedom control system for an IBB-LLC converter for fuel cells, applied to the three-degree-of-freedom control method for an IBB-LLC converter for fuel cells as described in any one of claims 1-6, characterized in that, include: The front-end interleaved parallel Buck-Boost circuit includes a first interleaved inductor, a second interleaved inductor, a first power switch group, and a second power switch group. The first and second power switch groups control the charging and discharging states of the first and second interleaved inductors, respectively, to adjust the intermediate DC bus voltage according to the duty cycle. The first power switch group includes switching transistors. S 1 and switching transistor S 4. The second power switch group includes switching transistors. S 2 and switching transistor S 3. Switching transistor S 1 and switching transistor S 4 is a complementary drive, switching transistor S 2 and switching transistor S 3 is a complementary driving force; The subsequent full-bridge LLC resonant circuit includes a primary-side full-bridge inverter unit, a resonant cavity unit, and a secondary-side rectifier unit. The primary-side full-bridge inverter unit includes a left bridge arm and a right bridge arm. There is a phase shift angle between the drive signals of the left bridge arm and the right bridge arm, which makes the input voltage of the resonant cavity unit present an asymmetrical stepped waveform. The three-degree-of-freedom control unit determines the optimal combination of duty cycle, phase shift angle and switching frequency based on the input voltage and load power of the fuel cell, and generates corresponding asymmetric drive timing sequences, which are output to the front-end interleaved parallel Buck-Boost circuit and the rear-end full-bridge LLC resonant circuit respectively.

8. The three-degree-of-freedom control system for an IBBB-LLC converter for fuel cells according to claim 7, characterized in that, The three-degree-of-freedom control unit includes: The operating condition monitoring module calculates the input voltage in real time based on the open-circuit voltage and equivalent internal resistance model of the fuel cell, combined with the current load power. The control mode determination module determines whether to adopt a two-degree-of-freedom symmetrical control mode that includes duty cycle and switching frequency, or a three-degree-of-freedom asymmetrical control mode that includes duty cycle, phase shift angle and switching frequency, based on the comparison result between the input voltage and the preset threshold. The gain model construction module constructs a voltage gain model based on the control mode determined by the control mode determination module, which is decoupled from the duty cycle, phase shift angle and switching frequency in three degrees of freedom. The intermediate DC bus voltage is determined based on the duty cycle, the fundamental voltage amplitude entering the resonant cavity unit is determined based on the duty cycle and phase shift angle using Fourier decomposition, the resonant cavity gain of the subsequent full-bridge LLC resonant circuit is determined based on the switching frequency, and the total voltage gain of the system is determined based on the intermediate DC bus voltage, the fundamental voltage amplitude and the resonant cavity gain. The optimization calculation module, based on the voltage gain model and the total system loss model, solves for the optimal combination of duty cycle, phase shift angle and switching frequency that minimizes the total system loss, under the conditions of satisfying the output voltage constraint and safe operation boundary. The drive timing generation module generates an asymmetric drive timing based on the optimal combination, wherein there is a phase shift angle between the drive signals of the left bridge arm and the drive signals of the right bridge arm.

9. The three-degree-of-freedom control system for an IBBB-LLC converter for fuel cells according to claim 7, characterized in that, The optimization calculation module includes: The reference loss calculation unit calculates the reference loss state under traditional symmetrical drive based on the current load conditions. The initial disturbance application unit applies an initial disturbance amount that deviates from the symmetry point to the phase shift angle according to the degree of input voltage overvoltage; The iterative calculation unit, based on the total system loss model, iteratively calculates the total loss corresponding to different parameter combinations within the output voltage constraint and safe operation boundary. The total system loss model calculates the main switch turn-off loss, primary power transistor conduction loss, secondary rectifier conduction loss, front-stage interleaved inductor copper loss, resonant inductor copper loss, transformer high-frequency iron loss, and switch drive loss based on the switching frequency, duty cycle, and phase shift angle. The optimal parameter output unit outputs the optimal duty cycle, phase shift angle, and switching frequency that minimize the total system loss based on the iterative calculation results.

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