Modulation method of series resonance dual-active bridge DC-DC converter

Abstract

The invention relates to a modulation method for a series resonance dual-active bridge DC-DC converter. The modulation method comprises the following steps: acquiring operating parameters of the DC-DC converter and a set value of an output current; determining an operation quadrant and a switching frequency based on the operation parameters and the set value of the output current; the switching frequency is compared with the variable frequency modulation range, if the switching frequency is within the variable frequency modulation range, a variable frequency modulation method is adopted for modulation, if the switching frequency is larger than the maximum value of the modulation range, a pulse width modulation method is adopted for modulation, and if the switching frequency is smaller than the minimum value of the modulation range, a pulse width modulation method is adopted for modulation. If yes, a double-phase-shift control modulation method is adopted for modulation; and controlling voltages at two ends of the resonant cavity according to the modulation process to realize resonant current adjustment, and modulating the output current to a set value based on the resonant current to complete the modulation process of the converter. Compared with the prior art, four-quadrant operation can be achieved, the output power range is expanded, the control method is simple, and calculation is convenient.

Description

Technical Field

[0001] This invention relates to the field of electrical engineering, and in particular to a modulation method for a series resonant dual active bridge DC-DC converter. Background Technology

[0002] To effectively alleviate the increasingly serious environmental problems and energy crisis, the application and development of new energy sources are receiving more and more widespread attention. As a crucial component in the efficient conversion and transmission of energy, the DC-DC converter determines the stability and flexibility of renewable energy systems.

[0003] Currently, the main DC-DC topologies used include flyback converters, full-bridge boost converters, and dual active bridge converters, such as... Figure 2 As shown.

[0004] (a) The advantages of flyback converters are their simple structure and low cost. However, their disadvantages include leakage inductance in the high-frequency transformer and poor thermal stability. As the temperature rises, saturation may occur, leading to power loss and reduced efficiency.

[0005] (b) The full-bridge BOOST converter also has a simple structure, while possessing boost capability and high reliability. However, its disadvantage is that due to its duty cycle being greater than 0.5, it requires additional auxiliary circuitry, making it more suitable for medium to high power applications.

[0006] (c) Due to its advantages such as electrical isolation, bidirectional power flow, high power density and high conversion efficiency, dual active bridge converters have been widely used in electric vehicles, DC microgrids, energy storage systems and solid-state transformers.

[0007] The Series Resonant Dual Active Bridge (SRDAB) is an improved topology of the traditional Linear Dual Active Bridge (LDAB). When the converter's operating frequency is much higher than the resonant frequency, the SRDAB can be considered as an LDAB. However, when the operating frequency approaches the resonant frequency, the output current waveform is no longer linear but changes in a sinusoidal shape. With the same average output current, compared to the LDAB, the SRDAB has a smaller effective current value and a smaller peak current, which reduces conduction and switching losses, thereby further improving efficiency. The mathematical model of the SRDAB is relatively complex. Since the inductor current and capacitor voltage have initial values ​​at each stage, the calculations are quite cumbersome.

[0008] During stable operation, the energy loss of a circuit is varied. As an energy conversion device, in addition to input and output energy, the energy conversion during operation also includes other forms of energy such as heat energy, magnetic energy in magnetic components, potential energy in capacitors, and electromagnetic radiation.

[0009] During steady-state operation, based on ampere-second and volt-second balance, the changes in magnetic energy and potential energy are also zero, thus having no impact on operating efficiency. For SRDABs, the proportion of other forms of energy, such as electromagnetic radiation, is very small and can usually be ignored. Therefore, during operation, the main factor affecting energy conversion efficiency is heat. Heat generated on components and circuits diffuses into the air, reducing the efficiency of electrical energy conversion. Furthermore, the generated heat also causes electronic components to heat up. To prevent components from being damaged or having their lifespan shortened due to excessive temperature, overheat protection or heat dissipation devices need to be installed on components that generate significant heat, resulting in unnecessary cost losses. The causes of heat generation are: firstly, conduction losses due to current flowing through components with internal resistance; and secondly, switching losses caused by the short-term overlap of current and voltage waveforms during the switching process of the switching transistor.

[0010] Patent application CN111835204A discloses a zero-return-current power soft-switching modulation method for a resonant dual active bridge converter. This implementation uses a resonant dual active bridge converter with a full-bridge structure on the primary side and a half-bridge structure on the secondary side. By employing discontinuous conduction mode (DCM) and boundary conduction mode (BCM), the goal of completely eliminating return-current power can be achieved, reducing both switching and conduction losses, and significantly improving transmission efficiency. However, this method is suitable for low-power applications. To increase output power, the value of the resonant inductor must be further reduced, which increases the peak current, thereby increasing current stress and conduction losses.

[0011] In summary, the main drawbacks of the existing technology are as follows:

[0012] 1) It cannot achieve four-quadrant operation, and can only achieve the goal of bidirectional energy flow or boost / buck voltage. However, it cannot achieve both simultaneously; 2) The applicable output power range is limited; 3) The control method is too complex. Summary of the Invention

[0013] The purpose of this invention is to provide a modulation method for a series resonant dual active bridge DC-DC converter that enables four-quadrant operation.

[0014] The objective of this invention can be achieved through the following technical solutions:

[0015] A modulation method for a series resonant dual active bridge DC-DC converter includes the following steps:

[0016] Obtain the operating parameters and output current settings of the DC-DC converter;

[0017] Based on the set values ​​of the operating parameters and output current, the operating quadrant is determined and the switching frequency is calculated;

[0018] The switching frequency is compared with the variable frequency modulation range. If the switching frequency is within the variable frequency modulation range, the variable frequency modulation method is used for modulation. If the switching frequency is greater than the maximum value of the variable frequency modulation range, the pulse width modulation method is used for modulation. If the switching frequency is less than the minimum value of the variable frequency modulation range, the dual phase shift control modulation method is used for modulation.

[0019] The voltage across the resonant cavity in the operating quadrant is controlled according to the modulation process to achieve resonant current regulation, and the output current is modulated to the set value based on the resonant current, thus completing the modulation process of the converter.

[0020] Furthermore, the converter operating parameters include the resonant cavity input voltage, the resonant cavity output voltage, the transformer turns ratio, and the values ​​of the resonant inductance and capacitance.

[0021] Furthermore, the calculation expression for the switching frequency is as follows:

[0022]

[0023] In the formula, f sw U is the switching frequency. A U B These are the voltages for segment A and segment B, respectively. ref For the required output current, L r It is a resonant inductor.

[0024] Furthermore, the specific steps for determining the operating quadrant include:

[0025] If the input voltage of the resonant cavity is greater than the output voltage of the resonant cavity, and the set value of the output current is greater than 0, then the operating quadrant is the buck-positive quadrant.

[0026] If the input voltage of the resonant cavity is greater than the output voltage of the resonant cavity, and the set value of the output current is less than 0, then the operating quadrant is the buck-reverse quadrant.

[0027] If the voltage on the input side of the resonant cavity is less than the voltage on the output side of the resonant cavity, and the set value of the output current is greater than 0, then the operating quadrant is the boost-positive quadrant.

[0028] If the input voltage of the resonant cavity is less than the output voltage of the resonant cavity, and the set value of the output current is less than 0, then the operating quadrant is the boost-reverse quadrant.

[0029] Furthermore, the process of achieving resonant current regulation in the step-down-positive quadrant using a variable frequency modulation method is as follows:

[0030] Phase 1: During the resonant cavity charging phase t∈(0,t1) in the positive half-cycle, according to the buck-positive quadrant operation mode, the voltage across the resonant cavity is obtained as U1-U2 through the input voltage U1 and the output voltage U2 of the resonant cavity. The voltage U1-U2 across the resonant cavity acts on the resonant cavity, and the resonant current starts to increase from 0. At time t1, the resonant current increases to the positive peak value.

[0031] Phase 2: During the resonant cavity discharge phase t∈(t1,T / 2) of the positive half-cycle, the input voltage U1 of the resonant cavity is 0, and the output voltage of the resonant cavity is -U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is -U2, and the resonant current starts to decay from the positive peak value. At time T / 2, the resonant current decays to 0.

[0032] Phase 3: During the resonant cavity charging phase t∈(T / 2,T / 2+t1) in the negative half-cycle, the resonant cavity input voltage is -U1 and the resonant cavity output voltage is -U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is -U1+U2. The resonant current starts to decrease from 0 and decreases to the negative peak value at time T / 2+t1.

[0033] Phase 4: During the resonant cavity discharge phase t∈(T / 2+t1,T) in the negative half-cycle, the input voltage U1 of the resonant cavity is 0, and the output voltage of the resonant cavity is U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is U2, and the resonant current starts to increase from the negative peak value. At time T, the resonant current increases to 0.

[0034] The duty cycle is adjusted by controlling the resonant current to regulate the output current, and then the next cycle begins, repeating the four-stage modulation process.

[0035] Furthermore, when the variable frequency modulation method achieves resonant current regulation in the buck-positive quadrant, the expression for the resonant current is:

[0036]

[0037] U A_Buck_Forward =U1-U2

[0038] U B_Buck_Forward =U2

[0039] In the formula, I L For the resonant current, Cr For resonant capacitor, L r For resonant inductance, U1 and U2 are the input and output voltages of the resonant cavity, respectively. A_Buck_Forward U B_Buck_Forward U represents the voltage across the resonant cavity, specifically the voltage in segment A during the forward energy transmission of the stepped-down energy and the voltage in segment B during the forward energy transmission of the stepped-down energy. cmax For the peak value of the resonant capacitance, ω r t1 is the resonant angular frequency, t2 is the resonant cavity charging time, t2 is the resonant cavity discharging time, and T is the period.

[0040] Furthermore, the process of achieving resonant current regulation in the buck-positive quadrant using pulse width modulation is as follows:

[0041] Phase 1: During the resonant cavity charging phase t∈(0,t1) in the positive half-cycle, according to the buck-positive quadrant operation mode, the voltage across the resonant cavity is obtained as U1-U2 through the input voltage U1 and the output voltage U2 of the resonant cavity. The voltage U1-U2 across the resonant cavity acts on the resonant cavity, and the resonant current starts to increase from 0. At time t1, the resonant current increases to the positive peak value.

[0042] Phase 2: During the resonant cavity discharge phase t∈(t1,t2) of the positive half-cycle, the input voltage U1 of the resonant cavity is 0, and the output voltage of the resonant cavity is -U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is -U2, and the resonant current starts to decay from the positive peak value. At time t2, the resonant current decays to 0.

[0043] Phase 3: During the positive half-cycle phase t∈(t2,T / 2), the input voltage U1 of the resonant cavity is 0, the output voltage of the resonant cavity is 0, and according to the buck-positive quadrant operation mode, the voltage across the resonant cavity is 0, and the resonant current remains unchanged at 0 until the negative half-cycle begins.

[0044] Phase 4: During the resonant cavity charging phase t∈(T / 2,T / 2+t1) in the negative half-cycle, the resonant cavity input voltage is -U1 and the resonant cavity output voltage is -U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is -U1+U2. The resonant current starts to decrease from 0 and decreases to the negative peak value at time T / 2+t1.

[0045] Phase 5: During the resonant cavity discharge phase t∈(T / 2+t1,T / 2+t2) of the negative half-cycle, the input voltage U1 of the resonant cavity is 0, and the output voltage of the resonant cavity is -U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is U2, and the resonant current starts to increase from the negative peak value. At time T / 2+t2, the resonant current increases to 0.

[0046] Phase 6: During the resonant cavity discharge phase t∈(T / 2+t2,T) of the negative half-cycle, the input voltage U1 of the resonant cavity is 0, the output voltage of the resonant cavity is 0, and according to the buck-positive quadrant operation mode, the voltage across the resonant cavity is 0, and the resonant current remains unchanged at 0 until the end of this cycle.

[0047] Then proceed to the next cycle and repeat the modulation process of the above six stages.

[0048] Furthermore, when the pulse width modulation method achieves resonant current regulation in the buck-positive quadrant, the expression for the relationship between resonant current, charging time, and discharging time is as follows:

[0049]

[0050]

[0051]

[0052] U A_Buck_Forward =U1-U2

[0053] U B_Buck_Forward =U2

[0054] In the formula, I L For the resonant current, C r For resonant capacitor, L r For resonant inductance, U1 and U2 are the input and output voltages of the resonant cavity, respectively. A_Buck_Forward U B_Buck_Forward U represents the voltage across the resonant cavity, specifically the voltage in segment A during the forward energy transmission of the stepped-down energy and the voltage in segment B during the forward energy transmission of the stepped-down energy. cmax For the peak value of the resonant capacitance, ω r t1 is the resonant angular frequency, t2 is the resonant cavity charging time, t2 is the resonant cavity discharging time, T is the period, f is the switching frequency of the switching transistor, and I is the resonant angular frequency. out This is the output current.

[0055] Furthermore, the process of achieving resonant current regulation in the step-down-positive quadrant using the dual-phase-shift control modulation method is as follows:

[0056] Phase 1: During the resonant cavity charging phase t∈(0,t1) of the positive half-cycle, according to the buck-positive quadrant operation mode, the voltage across the resonant cavity is obtained as U1-U2 through the input voltage U1 and the output voltage U2 of the resonant cavity. The voltage U1-U2 across the resonant cavity acts on the resonant cavity, and the resonant current begins to increase. At time t1, the resonant current increases to the positive peak value.

[0057] Phase 2: During the resonant cavity discharge phase t∈(t1,t2) of the positive half-cycle, the input voltage U1 of the resonant cavity is 0, and the output voltage of the resonant cavity is -U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is -U2, and the resonant current begins to decay from the positive peak value.

[0058] Phase 3: During the positive half-cycle phase t∈(t2,T / 2), the voltage on the input side of the resonant cavity is -U1, and the voltage on the output side of the resonant cavity is U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is -U1-U2, and the resonant current continues to decay to the negative value of the resonant current at the beginning of the phase.

[0059] Phase 4: During the resonant cavity charging phase t∈(T / 2,T / 2+t1) in the negative half-cycle, the resonant cavity input side voltage is -U1 and the resonant cavity output side voltage is -U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is -U1+U2, and the resonant current begins to decrease. At time T / 2+t1, the resonant current decreases to the negative peak value.

[0060] Phase 5: During the resonant cavity discharge phase t∈(T / 2+t1,T / 2+t2) in the negative half-cycle, the voltage on the input side of the resonant cavity is 0, and the voltage on the output side of the resonant cavity is -U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is U2, and the resonant current starts to increase from the negative peak value.

[0061] Phase 6: During the resonant cavity discharge phase t∈(T / 2+t2,T) of the negative half-cycle, the voltage on the input side of the resonant cavity is U1, and the voltage on the output side of the resonant cavity is -U2. According to the operating mode of the step-down positive quadrant, the voltage across the resonant cavity is U1+U2. The resonant current continues to increase to the initial value of the resonant current, and this cycle ends.

[0062] Then proceed to the next cycle and repeat the modulation process of the above six stages.

[0063] Furthermore, when the dual-phase-shift control modulation method achieves resonant current regulation in the step-down-positive quadrant, the expression for the voltage across the resonant cavity is:

[0064]

[0065]

[0066] In the formula, U LC U is the voltage across the resonant cavity. A_Buck_Forward U B_Buck_Forward and U C_Buck_ForwardThe voltages in segments A, B, and C of the resonant cavity during the forward energy transmission of the stepped-down energy are respectively represented. U1 and U2 are the input and output voltages of the resonant cavity, respectively. t1 is the charging time of the resonant cavity, t2 is the discharging time of the resonant cavity, and T is the period.

[0067] Compared with the prior art, the present invention has the following beneficial effects:

[0068] (1) In view of the working characteristics of each quadrant, the present invention proposes a hybrid modulation strategy of PWM, DPS and VFM respectively. According to the circuit load requirements, the corresponding modulation strategy is adopted in each quadrant to realize four-quadrant operation.

[0069] (2) The hybrid modulation strategy of the present invention flexibly adopts the corresponding modulation strategy according to the circuit load requirements, which can be applied to various working occasions, realize seamless switching between working modes, and meet the working requirements of off-grid and on-grid.

[0070] (3) The hybrid modulation strategy of PWM, DPS and VFM of the present invention fully balances the characteristics of conduction loss and switching loss, and achieves high-efficiency DC-DC power conversion in the full load domain. The conversion efficiency is significantly better than other strategies. Attached Figure Description

[0071] Figure 1 This is a schematic diagram of the method flow of the present invention;

[0072] Figure 2 The diagram shows a common DC-DC converter topology of the present invention, wherein (a) is a flyback converter, (b) is a full-bridge Boost converter, and (c) is a dual active bridge converter.

[0073] Figure 3 This is a topology diagram of a series resonant dual active bridge converter according to an embodiment of the present invention;

[0074] Figure 4 This is an equivalent circuit diagram of the series resonant dual active bridge converter topology according to an embodiment of the present invention;

[0075] Figure 5 This is a schematic diagram of the pulse width modulation current and voltage waveforms according to an embodiment of the present invention;

[0076] Figure 6 This is a schematic diagram of the dual-phase-shift control modulation current and voltage waveforms according to an embodiment of the present invention;

[0077] Figure 7 This is a schematic diagram of the variable frequency modulation current and voltage waveforms according to an embodiment of the present invention;

[0078] Figure 8These are schematic diagrams of current and voltage waveforms in various modes according to embodiments of the present invention, wherein (a) is a schematic diagram of PWM using the buck-positive quadrant, (b) is a schematic diagram of PWM using the buck-reverse quadrant, (c) is a schematic diagram of VFM using the buck-positive quadrant, (d) is a schematic diagram of VFM using the buck-reverse quadrant, (e) is a schematic diagram of DPS using the buck-positive quadrant, (f) is a schematic diagram of DPS using the buck-reverse quadrant, (g) is a schematic diagram of PWM using the boost-positive quadrant, (h) is a schematic diagram of PWM using the boost-reverse quadrant, (i) is a schematic diagram of VFM using the boost-positive quadrant, (j) is a schematic diagram of VFM using the boost-reverse quadrant, (k) is a schematic diagram of DPS using the boost-positive quadrant, and (l) is a schematic diagram of DPS using the boost-reverse quadrant.

[0079] Figure 9 The following are simulation waveforms for each mode in the embodiments of the present invention, wherein (a) is the simulation waveform under pulse width modulation, (b) is the simulation waveform under dual phase shift control modulation, and (c) is the simulation waveform under variable frequency modulation. Detailed Implementation

[0080] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0081] This embodiment provides a modulation method for a series resonant dual active bridge DC-DC converter, such as... Figure 1 As shown, the method includes the following steps:

[0082] S1. Obtain the operating parameters of the DC-DC converter.

[0083] This embodiment uses a series resonant dual active bridge converter as the topology of the DC-DC converter, such as... Figure 3 As shown, this topology consists of a primary-side full-bridge, a secondary-side full-bridge, a series resonant cavity, and a transformer. DC and AC voltages are modulated into high-frequency voltages of the same frequency by switches S1-S4 on the primary side and switches S5-S8 on the secondary side of the transformer. The subtraction of the high-frequency voltages on the primary and secondary sides constitutes the resonant cavity L. r C r The input voltage, L r For resonant inductance, C r This is a resonant capacitor. By controlling the phase, duty cycle, and frequency of the voltage across the resonant cavity, bidirectional energy flow and voltage boosting / debossing can be achieved.

[0084] Figure 4 This is the ideal equivalent circuit of the topology. To establish a large-scale steady-state model and analyze the steady-state operation of the circuit under different operating conditions, the following assumptions are made: 1) The values ​​of the input bus U1 and the output bus U2 do not change within one switching cycle. 2) All metal-oxide-semiconductor field-effect transistors (MOSFETs, insulated-gate enhancement-mode N-MOS) are ideal switches: their resistance is zero when turned on and infinite when turned off. 3) The magnetizing impedance of the high-frequency transformer is infinite. Based on these assumptions, mathematical models for three modulation methods—PWM, dual-phase-shift control, and frequency modulation—will be given.

[0085] Obtain basic circuit operation data: input voltage U1, AC voltage U2, transformer turns ratio n, resonant inductance L r and capacitor C r The value of the required output current I ref .

[0086] For ease of description, the symbols involved in this embodiment are first explained as shown in Table 1.

[0087] Table 1 Explanation of Main Symbols

[0088] symbol name <![CDATA[S1-S8]]> MOSFET name <![CDATA[D1-D8]]> Name of a diode in parallel with a MOSFET <![CDATA[C IN ]]> Input capacitor <![CDATA[C out ]]> Output capacitor <![CDATA[L r ]]> Resonant inductor <![CDATA[C r ]]> Resonant capacitor <![CDATA[U1]]> Input DC voltage <![CDATA[U2]]> Converted to primary side output voltage <![CDATA[U LC ]]> Voltage across the resonant cavity <![CDATA[U cmax ]]> Peak resonant capacitance T Switching cycle of switching transistor f Switching frequency of switching transistor <![CDATA[t1]]> Resonant cavity charging time <![CDATA[t2]]> Resonant cavity discharge time <![CDATA[ω r ]]> Resonant angular frequency <![CDATA[I L ]]> Resonant inductor current <![CDATA[I out ]]> Output current <![CDATA[U A_Buck_Forward ]]> The resonant cavity transmits voltage segment A in the forward direction of the stepped-down energy. <![CDATA[U B_Buck_Forward ]]> The resonant cavity transmits voltage in the B segment in the forward direction of the stepped-down energy. <![CDATA[U C_Buck_Forward ]]> The resonant cavity transmits voltage in the C segment in the forward direction of the stepped-down energy. <![CDATA[U A_Boost_Forward ]]> The resonant cavity transmits voltage segment A in the forward direction of the boosted energy. <![CDATA[U B_Boost_Forward ]]> The resonant cavity transmits voltage segment B in the forward direction of the boosted energy. <![CDATA[U C_Boost_Forward ]]> The resonant cavity transmits voltage in the C segment in the forward direction of the boosted energy. <![CDATA[U A_Buck_Reverse ]]> The resonant cavity reverses the energy transfer of segment A voltage. <![CDATA[U B_Buck_Reverse ]]> The resonant cavity transmits voltage in the reverse direction of the step-down energy in segment B. <![CDATA[U C_Buck_Reverse ]]> The resonant cavity transmits voltage in the reverse direction of the step-down energy in segment C. <![CDATA[U A_Boost_Reverse ]]> The resonant cavity transmits the voltage of segment A in the reverse direction of the boosted energy. <![CDATA[U B_Boost_Reverse ]]> The resonant cavity transmits voltage in the B segment in the reverse direction of the boosted energy. <![CDATA[U C_Boost_Reverse ]]> The resonant cavity transmits voltage in the B segment in the reverse direction of the boosted energy.

[0089] S2. Based on the operating parameters, determine the operating quadrant and calculate the switching frequency.

[0090] Table 2 determines the direction of energy flow and the pressure boosting / depressurization mode, thus determining which quadrant the system is operating in.

[0091] Table 2 Four-Quadrant Operation Mode

[0092]

[0093] After determining the operating quadrant, calculate the switching frequency using the following formula:

[0094]

[0095] S3. Compare the switching frequency with the variable frequency modulation range. If the switching frequency is within the variable frequency modulation range, then use the variable frequency modulation method for modulation. If the switching frequency is greater than the maximum value of the variable frequency modulation range, then use the pulse width modulation method for modulation. If the switching frequency is less than the minimum value of the variable frequency modulation range, then use the dual phase shift control modulation method for modulation.

[0096] If the frequency is within the range of variable frequency modulation, then the frequency modulation (VFM) mode is used. If the frequency is greater than the maximum switching frequency, it indicates a light load condition, and therefore the pulse width modulation (PWM) mode is used. If the frequency is less than the minimum switching frequency, it indicates a heavy load condition, and therefore the phase-shift control modulation (DPS) mode is used.

[0097] The voltage across the resonant cavity varies depending on whether it is a step-down or step-up operation and whether the energy flows in the forward or reverse direction. The expressions for the voltage across the resonant cavity are shown in Table 3.

[0098] Table 3 Expressions for voltages across the resonant cavity in different modes

[0099]

[0100] 1. Pulse Width Modulation (PWM)

[0101] When the circuit is under light load or the required output current is small, it will operate under pulse width modulation. Taking the forward transmission of buck power as an example, the frequency is fixed, and the resonant current is discontinuous in steady state, with six different states within one cycle.

[0102] Phase 1: t∈(0,t1). A new cycle begins at t=0. The current at the end of the previous cycle was 0, therefore the switch is a zero-current switch (ZCS). For the primary side of the transformer, switches S1 and S4 are open, and S2 and S3 are closed, with the input voltage on the left side of the resonant cavity being U1. For the secondary side of the transformer, switches S5 and S8 are open, and S6 and S7 are closed, with the output voltage on the right side of the resonant cavity being U2. According to the calculation expression in Table 3, the voltage across the resonant cavity is U1-U2, and the resonant current starts to increase from 0.

[0103] When t∈(0,t1), U1-U2=U A_Buck_Forward When applied to the resonant cavity, the current starts from 0 and increases. In the forward power transfer mode, the current increases in the forward direction; in the reverse power transfer mode, the current increases in the reverse direction.

[0104] Phase Two: t∈(t1,t2). A new phase begins at t=t1. Switch S1 is turned off and switch S2 is turned on. Due to the capacitance within the switching transistors, the switches are zero-voltage switches (ZVS). At this time, on the primary side of the transformer, S2 and S4 are on, while S1 and S3 are off, resulting in an output voltage of 0. On the secondary side of the transformer, S5 and S8 are on, while S6 and S7 are off, resulting in an output voltage of U2. According to the calculation expressions in Table 3, the voltage across the resonant cavity is -U2, and the resonant cavity current begins to decrease from its maximum value.

[0105] When t∈(t1,t2), the current will decay to zero. At the moment t=t1, the current reaches its peak value and then decays to zero.

[0106] Phase 3: t∈(t2,0.5T). Phase 3 begins when the current drops to 0, i.e., t=t2. Since the current is 0, it is a zero-current switch (ZCS). On the primary side, S2 and S4 are open, and S1 and S3 are closed, resulting in a zero output voltage. On the secondary side, S5 and S7 are closed, and S6 and S8 are open, also resulting in a zero output voltage. According to the calculation expressions in Table 3, the voltage across the resonant cavity is 0, thus clamping the resonant current to 0 until the negative half-cycle begins.

[0107] During the time interval t∈(t2,T / 2), both the primary and secondary switches will be turned off, and the resonant current will remain at 0 until the start of the second half-cycle.

[0108] The current change in the second half-cycle is the same as that in the first half-cycle, but the state is opposite.

[0109] Phase 4: t∈(0.5T, 0.5T+t1). The negative half-cycle begins at t=T / 2. The current is 0 at the end of the first half-cycle, therefore the switch is a zero-current switch (ZCS). For the primary side of the transformer, switches S2 and S3 are open, and S1 and S4 are closed, with the input voltage on the left side of the resonant cavity being -U1. For the secondary side of the transformer, switches S6 and S7 are open, and S5 and S8 are closed, with the output voltage on the right side of the resonant cavity being -U2. According to the calculation expression in Table 3, the voltage across the resonant cavity is -U1+U2, and the resonant current decreases from 0.

[0110] Phase 5: t∈(0.5T+t1,0.5T+t2). A new phase begins at t=0.5T+t1. At this time, switch S2 is turned off and switch S1 is turned on. Due to the capacitance within the switching transistors, the switches are zero-voltage switches (ZVS). On the primary side of the transformer, S1 and S3 are on, while S2 and S4 are off, resulting in an output voltage of 0. On the secondary side, S6 and S7 are on, while S5 and S8 are off, resulting in an output voltage of -U2. According to the calculation expression in Table 3, the voltage across the resonant cavity is U2, and the resonant cavity begins to increase from its maximum negative current value.

[0111] Phase Six: t∈(0.5T+t2,T). Phase Six begins when the current drops to 0, i.e., t=0.5T+t2. Since the current is 0, it is a zero-current switch (ZCS). On the primary side, S1 and S3 are open, and S2 and S4 are closed, resulting in a zero output voltage. On the secondary side, S5 and S7 are open, and S6 and S8 are closed, also resulting in a zero output voltage. According to the calculation expressions in Table 3, since the voltage across the resonant cavity is 0, the resonant current is clamped to 0 until the end of the cycle.

[0112] Expressions (2)-(4) give the resonant current I. LThe detailed expression and the relationship between charging time t1 and discharging time t2 are shown. It can be seen that when the charging time t1 is small, the maximum value of the capacitor is also small, and the resonant current is relatively linear. In this case, the circuit can be approximated as a linear dual active bridge converter. However, when t1 increases, the capacitor voltage cannot be ignored, so the resonant circuit must be expressed using trigonometric functions.

[0113]

[0114]

[0115]

[0116] Based on PWM modulation, the waveforms of current and voltage when the circuit operates in the energy-forward buck mode were obtained, as follows: Figure 5 As shown. From Figure 5 It can be observed that in pulse width modulation (PWM), the lagging arm signals on the primary and secondary sides are consistent; the only controllable variable is the phase difference between the DC-side leading and lagging arms. Once the phase difference between the DC-side arms is known, the phase difference between the AC-side arms can also be calculated. Therefore, pulse width modulation can be considered a special case of single-phase shift control. In this mode, since the inductor's initial state is 0, zero return current power and ZCS (zero-current switching) and ZVS (zero-voltage switching) within the operating range can be achieved. However, the disadvantage is weak power output capability, making it suitable only for light load conditions.

[0117] 2. Phase-Shift Controlled Modulation (DPS)

[0118] When the circuit is under heavy load or requires a large output current, it will operate in dual-phase-shift control mode. Taking the forward transfer of buck power as an example, the frequency is fixed. Due to the large required output current, the resonant current cannot operate in discontinuous conduction mode, and the current will enter continuous conduction mode.

[0119] There are six different states within one cycle.

[0120] Phase 1: t∈(0,t1). A new cycle begins at t=0. At the end of the previous cycle, the current is small and the switching transistor has internal capacitance, therefore the switch is a zero-voltage switch (ZVS). For the primary side of the transformer, switches S1 and S4 are open, and S2 and S3 are closed, with the input voltage on the left side of the resonant cavity being U1; for the secondary side of the transformer, switches S5 and S8 are open, and S6 and S7 are closed, with the output voltage on the right side of the resonant cavity being U2. According to the calculation expression in Table 3, the voltage across the resonant cavity is U1-U2, and the resonant current begins to increase.

[0121] As shown in equation (5), when t∈(0,t1), the voltage across the resonant cavity is U.A_Buck_Forward The direction depends on the direction of power flow, and the current will continue to increase at this time.

[0122]

[0123] By balancing the volt-seconds of the inductor and the ampere-seconds of the capacitor, we can obtain expression (5).

[0124] The expression for the average output current can be obtained through (6).

[0125] In continuous conduction mode, it is impossible to control the initial state of the inductor to be zero. In order to further reduce the peak voltage, the two phase shift angles can be optimized by equation (7) to obtain the minimum switching loss. At the same time, the expressions of the two control variables t1 and t2 can be obtained by equation (7).

[0126]

[0127]

[0128] Phase Two: t∈(t1,t2). A new phase begins at t=t1. At this time, switch S1 is turned off and switch S2 is turned on. Due to the capacitance within the switching transistors, the switches are zero-voltage switches (ZVS). On the primary side of the transformer, S2 and S4 are on, while S1 and S3 are off, resulting in an output voltage of 0. On the secondary side, S5 and S8 are on, while S6 and S7 are off, resulting in an output voltage of U2. According to the calculation expression in Table 3, the voltage across the resonant cavity is -U2, and the resonant cavity begins to decrease from its maximum current value.

[0129] At t∈(t1,t2), the voltage across the resonant cavity becomes -U2=U B_Buck_Forward The current began to decrease.

[0130] Phase 3: t∈(t2,0.5T). When t=t2, Phase 3 begins. Because the switching transistors have internal capacitance, the switching is a zero-voltage switch (ZVS). On the primary side, S2 and S3 are on, and S1 and S4 are off, with an output voltage of -U1; on the secondary side, S6 and S7 are off, and S5 and S8 are on, with an output voltage of U2. According to the calculation expression in Table 3, the voltage across the resonant cavity is -U1-U2, therefore the resonant current will decrease rapidly until the negative half-cycle begins.

[0131] At t∈(t2,T / 2), the voltage across the resonant cavity becomes -U1-U2=U c_Buck_Forward It quickly drops to the initial value of the inductor current.

[0132] Figure 6 These are the voltage and current waveforms of DPS in the energy positive flow buck mode; the waveforms in the other quadrants are shown in... Figure 8As shown in Table 3, the expressions for the voltages of segments A, B, and C in different operating quadrants are given. For the pulse width modulation modes in other quadrants, simply replace the voltages of segments A, B, and C with the corresponding voltages.

[0133] Phase 4: t∈(0.5T, 0.5T+t1). The negative half-cycle begins at t=T / 2. At the end of the first half-cycle, the current is small and the switching transistor has internal capacitance, therefore the switch is a zero-voltage switch (ZVS). For the primary side of the transformer, switches S2 and S3 are open, and S1 and S4 are closed, with the input voltage on the left side of the resonant cavity being -U1. For the secondary side of the transformer, switches S6 and S7 are open, and S5 and S8 are closed. According to the calculation expression in Table 3, the output voltage on the right side of the resonant cavity is -U2. The voltage across the resonant cavity is -U1+U2, and the resonant current begins to increase in the reverse direction.

[0134] Phase 5: t∈(0.5T+t1,0.5T+t2). A new phase begins at t=0.5T+t1. At this time, switch S2 is turned off and switch S1 is turned on. Due to the capacitance within the switching transistors, the switches are zero-voltage switches (ZVS). On the primary side of the transformer, S1 and S3 are on, while S2 and S4 are off, resulting in an output voltage of 0. On the secondary side, S6 and S7 are on, while S5 and S8 are off, resulting in an output voltage of -U2. According to the calculation expression in Table 3, the voltage across the resonant cavity is U2, and the resonant cavity begins to increase from the maximum reverse current value.

[0135] Phase Six: t∈(0.5T+t2,T). When t=0.5T+t2, Phase Six begins. Because the current is small and the switching transistors have internal capacitance, the switches are zero-voltage switches (ZVS). On the primary side, S1 and S4 are on, and S2 and S3 are off, with an output voltage of U1; on the secondary side, S5 and S8 are off, and S6 and S7 are on, with an output voltage of -U2. According to the calculation expression in Table 3, the voltage across the resonant cavity is U1+U2, therefore the resonant current will rise rapidly until the start of the next cycle.

[0136] 3. Variable Frequency Modulation (VFM)

[0137] By removing the time periods when the current is zero from the PWM mode, a variable frequency modulation mode can be obtained. In this mode, the frequency is determined by the charging time t1. The resonant current then has four states.

[0138] Phase 1: t∈(0,t1). A new cycle begins at t=0. The current at the end of the previous cycle was 0, therefore the switch is a zero-current switch (ZCS). For the primary side of the transformer, switches S1 and S4 are open, and S2 and S3 are closed, with the input voltage on the left side of the resonant cavity being U1. For the secondary side of the transformer, switches S5 and S8 are open, and S6 and S7 are closed, with the output voltage on the right side of the resonant cavity being U2. According to the calculation expression in Table 3, the voltage across the resonant cavity is U1-U2, and the resonant current starts to increase from 0.

[0139] The resonant cavity is charged at t∈(0,t1).

[0140] Phase Two: t∈(t1,0.5T). A new phase begins at t=t1. Switch S1 is turned off and switch S2 is turned on. Due to the capacitance within the switching transistors, the switches are zero-voltage switches (ZVS). At this time, on the primary side of the transformer, S2 and S4 are on, while S1 and S3 are off, resulting in an output voltage of 0. On the secondary side of the transformer, S5 and S8 are on, while S6 and S7 are off, resulting in an output voltage of U2. According to the expression in Table 3, the voltage across the resonant cavity is -U2, and the resonant cavity current begins to decrease from its maximum value.

[0141] Discharge the resonant cavity at t∈(t1,0.5T).

[0142] Phase 3: t∈(0.5T, 0.5T+t1). When the resonant current is 0, the negative half-cycle begins immediately. The current at the end of the first half-cycle is 0, therefore the switch is a zero-current switch (ZCS). For the primary side of the transformer, switches S2 and S3 are open, and S1 and S4 are closed, with the input voltage on the left side of the resonant cavity being -U1. For the secondary side of the transformer, switches S6 and S7 are open, and S5 and S8 are closed. According to the expression in Table 3, the output voltage on the right side of the resonant cavity is -U2. The voltage across the resonant cavity is -U1+U2, and the resonant current begins to decrease from 0.

[0143] Phase 4: t∈(0.5T+t1,T). A new phase begins when t=0.5T+t1. At this time, switch S2 is turned off and switch S1 is turned on. Due to the capacitance within the switching transistors, the switches are zero-voltage switches (ZVS). On the primary side of the transformer, S1 and S3 are on, while S2 and S4 are off, resulting in an output voltage of 0. On the secondary side, S6 and S7 are on, while S5 and S8 are off, resulting in an output voltage of -U2. According to the expression in Table 3, the voltage across the resonant cavity is U2. The resonant cavity begins to increase from its maximum negative current value until the current reaches 0, at which point it enters the next cycle.

[0144] Compared to pulse width modulation, it eliminates the time period when the current is 0, which can further reduce the effective value of the output current and reduce conduction losses.

[0145] The expressions for the inductor current and frequency are shown in equations (8) and (9).

[0146]

[0147]

[0148] In variable frequency mode, the controlled variables are the charging time t1 and the switching frequency. However, the switching frequency is also determined by t1. Therefore, variable frequency modulation is still a special single-phase shift modulation method.

[0149] Based on the modulation process within one cycle described above, the voltage and current waveforms of the buck-down VFM are obtained, as shown below. Figure 7 As shown.

[0150] Based on the modulation strategy described above, schematic diagrams of the waveforms for each mode in each quadrant can be obtained, as shown below. Figure 8 As shown.

[0151] S4. Control the voltage across the resonant cavity in the operating quadrant according to the modulation process to achieve resonant current regulation, and modulate the output current to the set value based on the resonant current to complete the modulation process of the converter.

[0152] The above modulation method regulates the output current by controlling the resonant current to meet the load requirements.

[0153] This embodiment also performed simulation analysis in the Simulink simulation software of MATLAB, and the relevant experimental parameters are listed in Table 4.

[0154] Waveforms for three modulation modes were obtained through simulation, such as Figure 9 As shown, Figure 9 (a) shows the simulated waveform under pulse width modulation, (b) shows the simulated waveform under dual phase shift control modulation, and (c) shows the simulated waveform under variable frequency modulation. The results obtained from the figures are consistent with the established mathematical model, which verifies the rationality and feasibility of the present invention.

[0155] Table 4 Simulation Analysis Parameters

[0156] <![CDATA[Resonant inductor L r > 15μH <![CDATA[Resonant capacitor C r > 4.22μF <![CDATA[Output capacitor C out > 15μF Input voltage 55V Minimum operating frequency 25kHz Maximum operating frequency 80kHz

[0157] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the invention.

[0158] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A modulation method for a series resonant dual active bridge DC-DC converter, characterized in that, Includes the following steps: Obtain the operating parameters and output current settings of the DC-DC converter; Based on the set values ​​of the operating parameters and output current, the operating quadrant is determined and the switching frequency is calculated; The switching frequency is compared with the variable frequency modulation range. If the switching frequency is within the variable frequency modulation range, the variable frequency modulation method is used for modulation. If the switching frequency is greater than the maximum value of the variable frequency modulation range, the pulse width modulation method is used for modulation. If the switching frequency is less than the minimum value of the variable frequency modulation range, the dual phase shift control modulation method is used for modulation. The voltage across the resonant cavity in the operating quadrant is controlled according to the modulation process to achieve resonant current regulation, and the output current is modulated to the set value based on the resonant current, thus completing the modulation process of the converter.

2. The modulation method for a series resonant dual active bridge DC-DC converter according to claim 1, characterized in that, The converter operating parameters include the resonant cavity input voltage, resonant cavity output voltage, transformer turns ratio, and the values ​​of resonant inductance and capacitance.

3. The modulation method for a series resonant dual active bridge DC-DC converter according to claim 1, characterized in that, The expression for calculating the switching frequency is: In the formula, f sw U is the switching frequency. A U B These are the voltages for segment A and segment B, respectively. ref For the required output current, L r It is a resonant inductor.

4. The modulation method for a series resonant dual active bridge DC-DC converter according to claim 1, characterized in that, The specific steps for determining the operating quadrant include: If the input voltage of the resonant cavity is greater than the output voltage of the resonant cavity, and the set value of the output current is greater than 0, then the operating quadrant is the buck-positive quadrant. If the input voltage of the resonant cavity is greater than the output voltage of the resonant cavity, and the set value of the output current is less than 0, then the operating quadrant is the buck-reverse quadrant. If the voltage on the input side of the resonant cavity is less than the voltage on the output side of the resonant cavity, and the set value of the output current is greater than 0, then the operating quadrant is the boost-positive quadrant. If the input voltage of the resonant cavity is less than the output voltage of the resonant cavity, and the set value of the output current is less than 0, then the operating quadrant is the boost-reverse quadrant.

5. The modulation method for a series resonant dual active bridge DC-DC converter according to claim 4, characterized in that, The process of achieving resonant current regulation in the step-down positive quadrant using variable frequency modulation is as follows: Phase 1: During the resonant cavity charging phase t∈(0,t1) in the positive half-cycle, according to the buck-positive quadrant operation mode, the voltage across the resonant cavity is obtained as U1-U2 through the input voltage U1 and the output voltage U2 of the resonant cavity. The voltage U1-U2 across the resonant cavity acts on the resonant cavity, and the resonant current starts to increase from 0. At time t1, the resonant current increases to the positive peak value. Phase 2: During the resonant cavity discharge phase t∈(t1,T / 2) of the positive half-cycle, the input voltage U1 of the resonant cavity is 0, and the output voltage of the resonant cavity is -U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is -U2, and the resonant current starts to decay from the positive peak value. At time T / 2, the resonant current decays to 0. Phase 3: During the resonant cavity charging phase t∈(T / 2,T / 2+t1) in the negative half-cycle, the resonant cavity input voltage is -U1 and the resonant cavity output voltage is -U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is -U1+U2. The resonant current starts to decrease from 0 and decreases to the negative peak value at time T / 2+t1. Phase 4: During the resonant cavity discharge phase t∈(T / 2+t1,T) in the negative half-cycle, the input voltage U1 of the resonant cavity is 0, and the output voltage of the resonant cavity is U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is U2, and the resonant current starts to increase from the negative peak value. At time T, the resonant current increases to 0. The duty cycle is adjusted by controlling the resonant current to regulate the output current, and then the next cycle begins, repeating the four-stage modulation process.

6. The modulation method for a series resonant dual active bridge DC-DC converter according to claim 5, characterized in that, When the variable frequency modulation method achieves resonant current regulation in the step-down positive quadrant, the expression for the resonant current is: U A_Buck_Forward =U1-U2 U B_Buck_Forward =U2 In the formula, I L For the resonant current, C r For resonant capacitor, L r For resonant inductance, U1 and U2 are the input and output voltages of the resonant cavity, respectively. A_Buck_Forward U B_Buck_Forward U represents the voltage across the resonant cavity, specifically the voltage in segment A during the forward energy transmission of the stepped-down energy and the voltage in segment B during the forward energy transmission of the stepped-down energy. cmax For the peak value of the resonant capacitance, ω r t1 is the resonant angular frequency, t2 is the resonant cavity charging time, t2 is the resonant cavity discharging time, and T is the period.

7. The modulation method for a series resonant dual active bridge DC-DC converter according to claim 4, characterized in that, The process of achieving resonant current regulation in the buck-positive quadrant using pulse width modulation is as follows: Phase 1: During the resonant cavity charging phase t∈(0,t1) in the positive half-cycle, according to the buck-positive quadrant operation mode, the voltage across the resonant cavity is obtained as U1-U2 through the input voltage U1 and the output voltage U2 of the resonant cavity. The voltage U1-U2 across the resonant cavity acts on the resonant cavity, and the resonant current starts to increase from 0. At time t1, the resonant current increases to the positive peak value. Phase 2: During the resonant cavity discharge phase t∈(t1,t2) of the positive half-cycle, the input voltage U1 of the resonant cavity is 0, and the output voltage of the resonant cavity is -U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is -U2, and the resonant current starts to decay from the positive peak value. At time t2, the resonant current decays to 0. Phase 3: During the positive half-cycle phase t∈(t2,T / 2), the input voltage U1 of the resonant cavity is 0, the output voltage of the resonant cavity is 0, and according to the buck-positive quadrant operation mode, the voltage across the resonant cavity is 0, and the resonant current remains unchanged at 0 until the negative half-cycle begins. Phase 4: During the resonant cavity charging phase t∈(T / 2,T / 2+t1) in the negative half-cycle, the resonant cavity input voltage is -U1 and the resonant cavity output voltage is -U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is -U1+U2. The resonant current starts to decrease from 0 and decreases to the negative peak value at time T / 2+t1. Phase 5: During the resonant cavity discharge phase t∈(T / 2+t1,T / 2+t2) of the negative half-cycle, the input voltage U1 of the resonant cavity is 0, and the output voltage of the resonant cavity is -U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is U2, and the resonant current starts to increase from the negative peak value. At time T / 2+t2, the resonant current increases to 0. Phase 6: During the resonant cavity discharge phase t∈(T / 2+t2,T) of the negative half-cycle, the input voltage U1 of the resonant cavity is 0, the output voltage of the resonant cavity is 0, and according to the buck-positive quadrant operation mode, the voltage across the resonant cavity is 0, and the resonant current remains unchanged at 0 until the end of this cycle. Then proceed to the next cycle and repeat the modulation process of the above six stages.

8. The modulation method for a series resonant dual active bridge DC-DC converter according to claim 7, characterized in that, When using pulse width modulation (PWM) to regulate resonant current in the buck-positive quadrant, the relationship between resonant current, charging time, and discharging time is expressed as follows: U A_Buck_Forward =U1-U2 U B_Buck_Forward =U2 In the formula, I L For the resonant current, C r For resonant capacitor, L r For resonant inductance, U1 and U2 are the input and output voltages of the resonant cavity, respectively. A_Buck_Forward U B_Buck_Forward U represents the voltage across the resonant cavity, specifically the voltage in segment A during the forward energy transmission of the stepped-down energy and the voltage in segment B during the forward energy transmission of the stepped-down energy. cmax For the peak value of the resonant capacitance, ω r t1 is the resonant angular frequency, t2 is the resonant cavity charging time, t2 is the resonant cavity discharging time, T is the period, f is the switching frequency of the switching transistor, and I is the resonant angular frequency. out This is the output current.

9. The modulation method for a series resonant dual active bridge DC-DC converter according to claim 1, characterized in that, The process of achieving resonant current regulation in the step-down-positive quadrant using the dual-phase-shift control modulation method is as follows: Phase 1: During the resonant cavity charging phase t∈(0,t1) of the positive half-cycle, according to the buck-positive quadrant operation mode, the voltage across the resonant cavity is obtained as U1-U2 through the input voltage U1 and the output voltage U2 of the resonant cavity. The voltage U1-U2 across the resonant cavity acts on the resonant cavity, and the resonant current begins to increase. At time t1, the resonant current increases to the positive peak value. Phase 2: During the resonant cavity discharge phase t∈(t1,t2) of the positive half-cycle, the input voltage U1 of the resonant cavity is 0, and the output voltage of the resonant cavity is -U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is -U2, and the resonant current begins to decay from the positive peak value. Phase 3: During the positive half-cycle phase t∈(t2,T / 2), the voltage on the input side of the resonant cavity is -U1, and the voltage on the output side of the resonant cavity is U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is -U1-U2, and the resonant current continues to decay to the negative value of the resonant current at the beginning of the phase. Phase 4: During the resonant cavity charging phase t∈(T / 2,T / 2+t1) in the negative half-cycle, the resonant cavity input side voltage is -U1 and the resonant cavity output side voltage is -U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is -U1+U2, and the resonant current begins to decrease. At time T / 2+t1, the resonant current decreases to the negative peak value. Phase 5: During the resonant cavity discharge phase t∈(T / 2+t1,T / 2+t2) in the negative half-cycle, the voltage on the input side of the resonant cavity is 0, and the voltage on the output side of the resonant cavity is -U2. According to the buck-positive quadrant operation mode, the voltage across the resonant cavity is U2, and the resonant current starts to increase from the negative peak value. Phase 6: During the resonant cavity discharge phase t∈(T / 2+t2,T) of the negative half-cycle, the voltage on the input side of the resonant cavity is U1, and the voltage on the output side of the resonant cavity is -U2. According to the operating mode of the step-down positive quadrant, the voltage across the resonant cavity is U1+U2. The resonant current continues to increase to the initial value of the resonant current, and this cycle ends. Then proceed to the next cycle and repeat the modulation process of the above six stages.

10. The modulation method for a series resonant dual active bridge DC-DC converter according to claim 9, characterized in that, When the dual-phase-shift control modulation method achieves resonant current regulation in the step-down-positive quadrant, the expression for the voltage across the resonant cavity is: In the formula, U LC U is the voltage across the resonant cavity. A_Buck_Forward U B_Buck_Forward and U C_Buck_Forward The voltages in segments A, B, and C of the resonant cavity during the forward energy transmission of the stepped-down energy are respectively represented. U1 and U2 are the input and output voltages of the resonant cavity, respectively. t1 is the charging time of the resonant cavity, t2 is the discharging time of the resonant cavity, and T is the period.

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