Full-soft switching isolation bidirectional DC-DC converter with high voltage gain

Abstract

A full-soft switching isolation bidirectional DC-DC converter with high voltage gain belongs to the technical field of power supplies, and is characterized in that one end of a switch S1p is connected with an inductor L1, a switch S2p and a capacitor C2p; one side of the switch S2p is connected with the inductor L1, the switch S1p and the capacitor C2p, and one end of the switch S2p is connected with the capacitor C1p; one end of the switch S1s is connected with the switch S2s and a secondary winding of the transformer; one end of the S2s is connected with the high-voltage side VH and the capacitor C2s, and the other end is connected with the switch S1s and the secondary winding of the transformer; one end of a primary winding of the transformer is connected with a capacitor C2p, one end of a secondary winding of the transformer is connected with a switch S1s and a switch S2s, and the other end is connected with C1s and C2s. One end of the inductor L1 is connected with the low-voltage side VL, and the other end of the inductor L1 is connected with the switches S1p and S2p and the capacitor C2p. One end of the capacitor C1p is connected with the switch S2p; one end of the capacitor C2p is connected with the inductor L1, the switch S1p and the switch S2p; one end of the capacitor C1s is connected with the capacitor C2s and the secondary winding of the transformer, and the other end is grounded; and one end of the capacitor C2s is connected with the high-voltage side VH and one end of the switch S2s is connected with the capacitor C1s and the secondary winding of the transformer. The topological structure of a traditional isolation DAB bidirectional DC-DC converter is improved, and the overall efficiency of the converter is improved.

Description

Technical Field

[0001] The invention relates to a full soft switch isolated bidirectional DC-DC converter with high voltage gain, belonging to the technical field of power supply. Background Art

[0002] Bidirectional DC-DC converters are widely used in renewable energy systems, power storage systems, and spacecraft power supply and distribution. In renewable energy systems, bidirectional DC-DC converters can convert intermittent renewable energy into DC power and store it; in power storage systems, bidirectional DC-DC converters can store and release electric energy, improve the stability and reliability of the power system, and in spacecraft power supply and distribution systems, bidirectional DC-DC converters can store and release electric energy. Bidirectional DC-DC converters have the advantage of bidirectional power flow from the high voltage side to the low voltage side and from the low voltage side to the high voltage side. As an isolated bidirectional DC-DC converter with high reliability, high energy density, and high efficiency, the isolated DAB structure bidirectional DC-DC converter has attracted widespread attention.

[0003] The output voltage transfer function of the traditional isolated DAB structure bidirectional DC-DC converter includes circuit parameters such as output resistance value, inductance value, switching frequency, transformer turns ratio, and phase shift between switches. In order to achieve the required voltage gain, many parameters need to be reasonably set, so it is complicated to achieve the required output voltage; the traditional isolated DAB structure bidirectional DC-DC converter has different voltage transfer functions in different regions D of phase shift between switches, and its ZVS operation under light load conditions has complexity problems, which also leads to complex control systems; the traditional isolated DAB structure bidirectional DC-DC converter reaches its peak value when the phase shift D=0.5 when the circuit components and switching frequency are determined, which limits the voltage gain, so it has limitations under application conditions that require higher voltage gain. Therefore, the main disadvantages of the existing technology are three points:

[0004] 1. The output voltage transfer function of the isolated DAB bidirectional DC-DC converter is complex and depends on the load size. To achieve the pre-selected output voltage, many parameters need to be correctly selected, and the voltage gain reaches its peak when the phase shift D = 0.5, which limits the voltage gain.

[0005] 2. Under light load conditions, the isolated DAB bidirectional DC-DC converter will reduce the phase shift D while maintaining the same voltage gain without changing other conditions, thus entering the non-ZVS region, and therefore cannot achieve ZVS operation under the full load range.

[0006] 3. The isolated DAB bidirectional DC-DC converter is based on phase shift control and requires different control systems under different operating parameters, which makes the control system relatively complex. Summary of the invention

[0007] The technical problem solved by the present invention is: to overcome the deficiencies of the prior art, to provide a fully soft-switching isolated bidirectional DC-DC converter with high voltage gain, to improve the topology of the traditional isolated DAB bidirectional DC-DC converter, and to improve the overall efficiency of the converter.

[0008] The technical solution of the present invention is: a fully soft-switched isolated bidirectional DC-DC converter with high voltage gain, including switch S1p, switch S2p, switch S1s, switch S2s, capacitor C1p, capacitor C2p, capacitor C1s, capacitor C2s, inductor L1, transformer T, transformer excitation inductance Lm, transformer leakage inductance Llk:

[0009] One end of the switch S1p is connected to one end of the inductor L1, one end of the switch S2p and one end of the capacitor C2p, and the other end is connected to the negative electrode of the power supply VL; the other end of the inductor L1 is connected to the positive electrode of the power supply VL; the other end of the switch S2p is connected to one end of the capacitor C1p, and the other end of the capacitor C1p is the negative electrode of the power supply VL; the other end of the capacitor C2p is connected to one end of the transformer excitation inductance Lm and one end of the primary winding of the transformer T, the other end of the transformer excitation inductance Lm and the other end of the primary winding of the transformer T are connected to one end of the transformer leakage inductance Llk, and the other end of the transformer leakage inductance Llk is connected to the negative electrode of the power supply VL;

[0010] One end of the switch S1s is connected to one end of the switch S2s and one end of the secondary winding of the transformer T, and the other end is connected to one end of the capacitor C1s and the negative electrode of the power supply VH; the other end of the capacitor C1s is connected to the other end of the secondary winding of the transformer T and one end of the capacitor C2s; the other end of the capacitor C2s is connected to the other end of the switch S2s and the positive electrode of the power supply VH.

[0011] Furthermore, the bidirectional DC-DC converter has two switching states, including a first switching cycle, in which capacitors S1p and S1s are turned on, capacitors S2p ​​and S2s are turned off, and inductor L1 stores energy; and a second switching cycle, in which capacitors S2p ​​and S2s are turned on, capacitors S1p and S1s are turned off, and inductor L1 releases energy; all switches are turned off during the dead time.

[0012] Furthermore, the bidirectional DC-DC converter operates in a cycle of first switching cycle-dead zone-second switching cycle-dead zone as a whole cycle, completing power flow transfer and maintaining a steady state.

[0013] Furthermore, during the first switching cycle, the voltage on the primary winding of the transformer T is -VC2p, the forward current flowing therethrough gradually decreases, and reversely increases to -Imb at a certain intermediate moment, and the current polarity on the secondary winding of the transformer T changes accordingly, charging and discharging the capacitors C1s and C2s; the voltage on the inductor L1 is VL, and the current flowing therethrough gradually increases, eventually reaching a peak value ILa; the voltage across the capacitors S1p and S1s is 0, and the voltage across the capacitors S2p ​​and S2s is VH / n and VH, respectively; n is the transformer turns ratio n, VC2p is the voltage on the primary winding of the transformer T, Imb is the forward current flowing through the primary winding of the transformer T, and ILa is the peak current flowing through the inductor L1.

[0014] Furthermore, during the second switching cycle, the voltage on the primary winding of the transformer T is VC1p-VC2p, the current flowing therethrough gradually decreases in the reverse direction, and at a certain intermediate moment, increases in the forward direction to reach Ima, and the current polarity on the secondary winding of the transformer T changes accordingly, charging and discharging the capacitors C1s and C2s; the voltage on the inductor L1 is VC1p-VL, the current flowing therethrough gradually decreases, and finally reaches the valley value ILb; the voltage across the capacitors S2p ​​and S2s is 0, and the voltage across the capacitors S1p and S1s is VH / n and VH, respectively; Ima is the current flowing through the primary winding of the transformer T, and ILb is the current flowing through the inductor L1.

[0015] Furthermore, during the dead time from the first switching cycle to the second switching cycle, due to the inductor freewheeling, the voltage across capacitors S2p ​​and S2s changes to 0 before the control signal changes; during the dead time from the second switching cycle to the first switching cycle, the voltage across capacitors S1p and S1s changes to 0 before the control signal changes, thereby achieving a soft switching effect.

[0016] A bidirectional DC-DC conversion method implemented by using the fully soft-switching isolated bidirectional DC-DC converter comprises:

[0017] The whole cycle is cyclically operated with the first switching cycle-dead zone-second switching cycle-dead zone to complete the power flow transfer and maintain a steady state;

[0018] In the first switching cycle, the voltage on the primary winding of the transformer T is -VC2p, the forward current flowing through it gradually decreases, and reversely increases to -Imb at a certain intermediate moment, and the current polarity on the secondary winding of the transformer T changes accordingly, charging and discharging the capacitors C1s and C2s; the voltage on the inductor L1 is VL, and the current flowing through it gradually increases and eventually reaches a peak value ILa; the voltage across the capacitors S1p and S1s is 0, and the voltage across the capacitors S2p ​​and S2s is VH / n and VH respectively; n is the transformer turns ratio n, VC2p is the voltage on the primary winding of the transformer T, Imb is the forward current flowing through the primary winding of the transformer T, and ILa is the peak current flowing through the inductor L1;

[0019] In the second switching cycle, the voltage on the primary winding of the transformer T is VC1p-VC2p, the current flowing through it gradually decreases in the reverse direction, and at a certain intermediate moment it becomes positive and increases to reach Ima, and the current polarity on the secondary winding of the transformer T will change accordingly, charging and discharging the capacitors C1s and C2s; the voltage on the inductor L1 is VC1p-VL, the current flowing through it gradually decreases, and finally reaches the valley value ILb; the voltage across the capacitor S2p and the capacitor S2s is 0, and the voltage across the capacitor S1p and the capacitor S1s is VH / n and VH respectively; Ima is the current flowing through the primary winding of the transformer T, and ILb is the current flowing through the inductor L1.

[0020] Furthermore, during the dead time from the first switching cycle to the second switching cycle, due to the inductor freewheeling, the voltage across capacitors S2p ​​and S2s changes to 0 before the control signal changes; during the dead time from the second switching cycle to the first switching cycle, the voltage across capacitors S1p and S1s changes to 0 before the control signal changes, thereby achieving a soft switching effect.

[0021] The advantages of the present invention compared with the prior art are:

[0022] 1. Compared with the traditional isolated DAB converter, the topology of the present invention can achieve ZVS operation of four switches in the entire duty cycle range, and the overall efficiency of the converter is improved.

[0023] 2. Compared with the traditional isolated DAB converter, the fully soft-switching isolated bidirectional DC-DC converter topology with high voltage gain proposed in the present invention can achieve a positive correlation between the voltage gain and the duty cycle D in the full duty cycle range without adding additional components. Therefore, a higher voltage gain can be achieved, thus having a wider range of application scenarios.

[0024] 3. The fully soft-switching isolated bidirectional DC-DC converter with high voltage gain proposed in the present invention is based on duty cycle control rather than traditional phase shift control. The control method is the same over the full duty cycle range, so only a single control system is required, thereby simplifying the control system and improving system reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present invention. Moreover, the same reference symbols are used throughout the accompanying drawings to represent the same components. In the accompanying drawings:

[0026] Figure 1 It is the circuit topology principle diagram of the present invention;

[0027] Figure 2 This is a steady-state working voltage and current waveform diagram of the present invention;

[0028] Figure 3 This is a schematic diagram of the closed-loop controller structure. DETAILED DESCRIPTION

[0029] In order to better understand the above technical scheme, the technical scheme of the present invention is described in detail below through the accompanying drawings and specific embodiments. It should be understood that the embodiments of the present invention and the specific features in the embodiments are detailed descriptions of the technical scheme of the present invention, rather than limitations on the technical scheme of the present invention. In the absence of conflict, the embodiments of the present invention and the technical features in the embodiments can be combined with each other.

[0030] The following is a further detailed description of a fully soft-switching isolated bidirectional DC-DC converter with high voltage gain provided by an embodiment of the present invention in conjunction with the accompanying drawings. Figure 1 , the specific implementation method may include: consisting of switches S1p, S2p, S1s, S2s, capacitors C1p, C2p, C1s, C2s, inductor L1, and transformer T1, Lm and Llk are the transformer excitation inductance and leakage inductance respectively. VL and VH are the low-voltage side and high-voltage side of the bidirectional DC-DC transformer respectively, and the circuit topology has a bidirectional power flow.

[0031] One end of switch S1p is connected to inductor L1, switch S2p and capacitor C2p, and one end is grounded; one side of switch S2p is connected to inductor L1, switch S1p and capacitor C2p, and one end is connected to capacitor C1p; one end of switch S1s is connected to switch S2s and secondary winding of transformer T, and one end is grounded; one end of S2s is connected to high voltage side VH and capacitor C2s, and one end is connected to switch S1s and secondary winding of transformer T; one end of primary winding of transformer T is connected to capacitor C2p, and one end is grounded, one end of secondary winding of transformer is connected to switch S1s and switch S2s, and one end is connected to C1s and C2s. One end of inductor L1 is connected to low voltage side VL, and one end is connected to switch S1p, S2p and capacitor C2p. One end of capacitor C1p is connected to switch S2p, and one end is grounded; one end of capacitor C2p is connected to inductor L1, switch S1p and switch S2p; one end of capacitor C1s is connected to capacitor C2s and the secondary winding of transformer T, and one end is grounded; one end of capacitor C2s is connected to the high voltage side VH and switch S2s, and one end of capacitor C1s is connected to the secondary winding of transformer T.

[0032] The bidirectional DC-DC converter has two switching states: in the first switching cycle, S1p and S1s are turned on, S2p and S2s are turned off, and the inductor L1 stores energy; in the second switching cycle, S2p and S2s are turned on, S1p and S1s are turned off, and the inductor L1 releases energy. All switches are turned off during the dead time. Since the inductor current cannot change suddenly, the switch node is automatically charged and discharged to complete the ZVS operation. The bidirectional DC-DC converter works in a cycle of the first switching cycle-dead zone-second switching cycle-dead zone as the entire cycle Ts, completing the power flow transfer and maintaining a steady state.

[0033] Assuming that the capacitors are large enough, the steady-state operating voltage and current waveforms of key components are shown in Figure 2 .

[0034] In the first switching cycle, the voltage on the primary winding of the transformer is -VC2p, so the forward current flowing through it gradually decreases, and increases in the reverse direction to -Imb at a certain intermediate moment, and the current polarity on the secondary winding will change accordingly, charging and discharging capacitors C1s and C2s; the voltage on L1 is VL, and the current flowing through it gradually increases, eventually reaching the peak value ILa; the voltage across S1p and S1s is 0, and the voltage across S2p ​​and S2s is VH / n and VH, respectively. By analyzing the transformer current, it can be qualitatively analyzed that the voltage on the high-voltage side VH increases first and then decreases at the intermediate moment.

[0035] In the second switching cycle, the voltage on the primary winding of the transformer is VC1p-VC2p, so the current flowing through it gradually decreases in the reverse direction, and at a certain intermediate moment, it becomes positive and increases to reach Ima. The current polarity on the secondary winding will change accordingly, charging and discharging capacitors C1s and C2s; the voltage on L1 is VC1p-VL, and the current flowing through it gradually decreases, eventually reaching the valley value ILb; the voltage across S2p ​​and S2s is 0, and the voltage across S1p and S1s is VH / n and VH, respectively. By analyzing the transformer current, it can be qualitatively analyzed that the voltage on the high-voltage side VH increases first and then decreases at the intermediate moment.

[0036] During the dead time from the first switching cycle to the second switching cycle, due to the inductor freewheeling, the voltage across S2p ​​and S2s will quickly change to 0 before the control signal changes; during the dead time from the second switching cycle to the first switching cycle, due to the inductor freewheeling, the voltage across S1p and S1s will quickly change to 0 before the control signal changes, thereby achieving a soft switching effect. This ZVS operation is not limited by the relationship between phase shift and voltage gain, and does not require changing the switching frequency, so it is easy to implement.

[0037] Under the condition of Lm>>Llk, the voltage conversion ratio expression can be obtained according to the principles of volt-second balance and charge balance, which is expressed by formula (2). This voltage conversion ratio is only related to the coil turns ratio n and the duty cycle D, and is independent of the load, switching frequency, and phase shift between switches. This voltage conversion ratio is positively correlated with the duty cycle D. Ideally, a very high voltage gain can be achieved, breaking through the limitation of the traditional DAB structure that the voltage gain reaches the extreme value when D=0.5.

[0038] At the same time, it can be obtained that in steady state, the voltage on C1p is VL / (1-D), the voltage on C2p is VL, the voltage on C1s is (1-D)VH, and the voltage on C2s is DVH. Considering the voltage stress on S2s and S2p during the first switching cycle, it can be obtained as VH / n, and considering the voltage stress on S1s and S1p during the second switching cycle, it can be obtained as VH. The average current stress IS1s=IS2s=IH, IS2p=0, IS1p=IL. The minimum and maximum currents on inductor L1 are ILa=IL-(VL / 2L1)DTs and ILb=IL+(VL / 2L1)DTs.

[0039]

[0040] The closed-loop controller structure is shown in Figure 3The switching control of the bidirectional DC-DC converter is based on duty cycle rather than phase shift, so the control system needs to adjust the duty cycle according to the output voltage. The output voltage VH is compared with the reference voltage VH-ref, and then amplified by gain and compared with the triangle wave to obtain a pair of dead zone control signals for use in S1p, S1s, S2p, and S2s. When starting, the difference between the output voltage and the reference voltage is large, and the input to the positive input of the comparator is small, so the generated PWM signal A small duty cycle is conducive to the increase of VH voltage; the increase of VH causes the duty cycle of the PWM signal to change, and finally stabilizes when VH≈VH_ref. This control system is simple and stable and easy to implement. The duty cycle D of the output pair of PWM signals with dead zone is only controlled by the difference between the output voltage and the reference voltage.

[0041] In summary, energy exchange between buses of different voltage levels is required in the energy storage system. As a converter with bidirectional energy mobility, the bidirectional DC / DC converter plays an important role in energy exchange in this application field. In the field of power supply and distribution of spacecraft, high reliability requirements prompt the bidirectional DC-DC converter to use a safer isolation transformer so that the input and output ends are electrically isolated. In addition, zero voltage switching (ZVS) can significantly reduce the conduction loss of the switch tube and improve the overall efficiency of the converter. In order to make the isolated bidirectional DC-DC converter have zero voltage switching (ZVS), high voltage gain, simple switching mode and control system, the present invention proposes a fully soft-switched isolated bidirectional DC-DC converter with high voltage gain by improving the traditional isolated DAB bidirectional DC-DC converter topology.

[0042] Based on the same inventive concept, the present invention also provides a bidirectional DC-DC conversion method implemented by using the fully soft-switching isolated bidirectional DC-DC converter, comprising:

[0043] The whole cycle is cyclically operated with the first switching cycle-dead zone-second switching cycle-dead zone to complete the power flow transfer and maintain a steady state;

[0044] In the first switching cycle, the voltage on the primary winding of the transformer T is -VC2p, the forward current flowing through it gradually decreases, and reversely increases to -Imb at a certain intermediate moment, and the current polarity on the secondary winding of the transformer T changes accordingly, charging and discharging the capacitors C1s and C2s; the voltage on the inductor L1 is VL, and the current flowing through it gradually increases and eventually reaches a peak value ILa; the voltage across the capacitors S1p and S1s is 0, and the voltage across the capacitors S2p ​​and S2s is VH / n and VH respectively; n is the transformer turns ratio n, VC2p is the voltage on the primary winding of the transformer T, Imb is the forward current flowing through the primary winding of the transformer T, and ILa is the peak current flowing through the inductor L1;

[0045] In the second switching cycle, the voltage on the primary winding of the transformer T is VC1p-VC2p, the current flowing through it gradually decreases in the reverse direction, and at a certain intermediate moment it becomes positive and increases to reach Ima, and the current polarity on the secondary winding of the transformer T will change accordingly, charging and discharging the capacitors C1s and C2s; the voltage on the inductor L1 is VC1p-VL, the current flowing through it gradually decreases, and finally reaches the valley value ILb; the voltage across the capacitor S2p and the capacitor S2s is 0, and the voltage across the capacitor S1p and the capacitor S1s is VH / n and VH respectively; Ima is the current flowing through the primary winding of the transformer T, and ILb is the current flowing through the inductor L1.

[0046] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0047] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

[0048] The contents not described in detail in the specification of the present invention belong to the common knowledge of those skilled in the art.

Claims

1. A fully soft-switching isolated bidirectional DC-DC converter with high voltage gain, characterized in that: Including switch S1p, switch S2p, switch S1s, switch S2s, capacitor C1p, capacitor C2p, capacitor C1s, capacitor C2s, inductor L1, transformer T, transformer excitation inductance Lm, transformer leakage inductance Llk: One end of the switch S1p is connected to one end of the inductor L1, one end of the switch S2p and one end of the capacitor C2p, and the other end is connected to the negative electrode of the power supply VL; the other end of the inductor L1 is connected to the positive electrode of the power supply VL; the other end of the switch S2p is connected to one end of the capacitor C1p, and the other end of the capacitor C1p is the negative electrode of the power supply VL; the other end of the capacitor C2p is connected to one end of the transformer excitation inductance Lm and one end of the primary winding of the transformer T, the other end of the transformer excitation inductance Lm and the other end of the primary winding of the transformer T are connected to one end of the transformer leakage inductance Llk, and the other end of the transformer leakage inductance Llk is connected to the negative electrode of the power supply VL; One end of the switch S1s is connected to one end of the switch S2s and one end of the secondary winding of the transformer T, and the other end is connected to one end of the capacitor C1s and the negative electrode of the power supply VH; the other end of the capacitor C1s is connected to the other end of the secondary winding of the transformer T and one end of the capacitor C2s; the other end of the capacitor C2s is connected to the other end of the switch S2s and the positive electrode of the power supply VH.

2. A fully soft-switching isolated bidirectional DC-DC converter with high voltage gain according to claim 1, characterized in that: The bidirectional DC-DC converter has two switching states, including a first switching cycle, in which the capacitors S1p and S1s are turned on, the capacitors S2p ​​and S2s are turned off, and the inductor L1 stores energy; And in the second switching cycle, the capacitor S2p and the capacitor S2s are turned on, the capacitor S1p and the capacitor S1s are turned off, and the inductor L1 releases energy; all switches are turned off during the dead time.

3. A fully soft-switching isolated bidirectional DC-DC converter with high voltage gain according to claim 2, characterized in that: The bidirectional DC-DC converter operates in a cycle of first switching cycle-dead zone-second switching cycle-dead zone to complete power flow transfer and maintain a steady state.

4. A fully soft-switching isolated bidirectional DC-DC converter with high voltage gain according to claim 2, characterized in that: In the first switching cycle, the voltage on the primary winding of the transformer T is -VC2p, the forward current flowing through it gradually decreases, and at a certain intermediate moment, it increases in the opposite direction to reach -Imb, and the current polarity on the secondary winding of the transformer T changes accordingly, charging and discharging the capacitors C1s and C2s; the voltage on the inductor L1 is VL, and the current flowing through it gradually increases, eventually reaching a peak value ILa; the voltage across the capacitors S1p and S1s is 0, and the voltage across the capacitors S2p ​​and S2s is VH / n and VH, respectively; n is the transformer turns ratio n, VC2p is the voltage on the primary winding of the transformer T, Imb is the forward current flowing through the primary winding of the transformer T, and ILa is the peak current flowing through the inductor L1.

5. A fully soft-switching isolated bidirectional DC-DC converter with high voltage gain according to claim 4, characterized in that: In the second switching cycle, the voltage on the primary winding of the transformer T is VC1p-VC2p, the current flowing through it gradually decreases in the reverse direction, and at a certain intermediate moment it becomes positive and increases to reach Ima, and the current polarity on the secondary winding of the transformer T will change accordingly, charging and discharging the capacitors C1s and C2s; the voltage on the inductor L1 is VC1p-VL, the current flowing through it gradually decreases, and finally reaches the valley value ILb; the voltage across the capacitor S2p and the capacitor S2s is 0, and the voltage across the capacitor S1p and the capacitor S1s is VH / n and VH respectively; Ima is the current flowing through the primary winding of the transformer T, and ILb is the current flowing through the inductor L1.

6. A fully soft-switching isolated bidirectional DC-DC converter with high voltage gain according to claim 2, characterized in that: During the dead time from the first switching cycle to the second switching cycle, due to the inductor freewheeling, the voltage across capacitors S2p ​​and S2s changes to 0 before the control signal changes; during the dead time from the second switching cycle to the first switching cycle, the voltage across capacitors S1p and S1s changes to 0 before the control signal changes, achieving a soft switching effect.

7. A bidirectional DC-DC conversion method implemented by using the fully soft-switching isolated bidirectional DC-DC converter according to claim 1, characterized in that: include: The whole cycle is cyclically operated with the first switching cycle-dead zone-second switching cycle-dead zone to complete the power flow transfer and maintain a steady state; In the first switching cycle, the voltage on the primary winding of the transformer T is -VC2p, the forward current flowing through it gradually decreases, and reversely increases to -Imb at a certain intermediate moment, and the current polarity on the secondary winding of the transformer T changes accordingly, charging and discharging the capacitors C1s and C2s; the voltage on the inductor L1 is VL, and the current flowing through it gradually increases and eventually reaches a peak value ILa; the voltage across the capacitors S1p and S1s is 0, and the voltage across the capacitors S2p ​​and S2s is VH / n and VH respectively; n is the transformer turns ratio n, VC2p is the voltage on the primary winding of the transformer T, Imb is the forward current flowing through the primary winding of the transformer T, and ILa is the peak current flowing through the inductor L1; In the second switching cycle, the voltage on the primary winding of the transformer T is VC1p-VC2p, the current flowing through it gradually decreases in the reverse direction, and at a certain intermediate moment it becomes positive and increases to reach Ima, and the current polarity on the secondary winding of the transformer T will change accordingly, charging and discharging the capacitors C1s and C2s; the voltage on the inductor L1 is VC1p-VL, the current flowing through it gradually decreases, and finally reaches the valley value ILb; the voltage across the capacitor S2p and the capacitor S2s is 0, and the voltage across the capacitor S1p and the capacitor S1s is VH / n and VH respectively; Ima is the current flowing through the primary winding of the transformer T, and ILb is the current flowing through the inductor L1.

8. The fully soft-switch isolated bidirectional DC-DC conversion method according to claim 7, characterized in that: During the dead time from the first switching cycle to the second switching cycle, due to the inductor freewheeling, the voltage across capacitors S2p ​​and S2s changes to 0 before the control signal changes; during the dead time from the second switching cycle to the first switching cycle, the voltage across capacitors S1p and S1s changes to 0 before the control signal changes, achieving a soft switching effect.

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