A bidirectional DC-DC circuit

Abstract

The application discloses a bidirectional DC-DC circuit, comprising a circuit control part and a circuit power part; the circuit power part comprises low-voltage MOS tubes Q1 and Q2, a high-frequency isolation transformer TX1, a resonant inductor L1, resonant capacitors C1 and C2, high-voltage MOS tubes Q3 and Q4, a high-voltage DC bus capacitor E1 and an output equivalent load Rload; the Q1 and Q2 are located at the low-voltage end of the TX1; the C1 and C2 are connected in parallel with the L1, the Q3, the Q4, the E1 and the output equivalent load Rload at the high-voltage end of the high-frequency isolation transformer TX1; the current return paths of the C1 and C2 are separated; the E1 and the output equivalent load Rload are introduced into a resonant loop; the problem that the ripple current and the ripple voltage of the bus capacitor E1 are large and the temperature is high in the existing circuit scheme can be solved; the efficiency and the reliability are further improved; the design cost is relatively advantageous; and the problem that the current and the heat generation are unbalanced when the C1 and the C2 are directly connected in parallel in the existing circuit can be effectively prevented, and the reliability is high.

Description

Technical Field

[0001] This invention relates to the field of inverter technology, and in particular to a bidirectional DC-DC circuit. Background Technology

[0002] Home photovoltaic inverters, portable inverters, UPS, and other power electronic products have become increasingly common. These devices all require isolating and boosting a lower DC battery voltage (e.g., 24Vdc) to a higher DC bus voltage (e.g., 380Vdc), and then converting the bus voltage to output AC mains voltage. For smaller power devices (e.g., below 3kW), the pre-amplifier DC-DC boost circuit mostly uses a low-cost push-pull boost DC-DC circuit. With technological advancements and circuit evolution, push-pull voltage doubler resonant DC-DC converters have become the mainstream circuit. This circuit utilizes the resonant effect of the inductor and capacitor on the secondary side, causing the primary and secondary switching transistors to be in a soft-switching state. This increases the switching frequency, reduces the size of magnetic components, and improves overall efficiency. Although this DC-DC circuit is widely recognized and favored in the industry, it has the following shortcomings / problems:

[0003] 1. Only half of each switching cycle is used for charging the BUS electrolytic capacitor and supplying the load: as shown in the attached document. Figure 1 and Figure 2 As shown, this represents one operating cycle of a push-pull voltage doubler resonant DC-DC circuit, which is currently the mainstream circuit in the industry. The dashed line represents the main power current path of this circuit. Figure 1 In the first half-cycle, MOSFET Q1 is on and Q2 is off, transferring energy through transformer TX1 to charge capacitors C1 and C2. Simultaneously, during this half-cycle, the energy consumed by the load is supplied by bus capacitor E1 (the energy transferred from transformer TX1 is stored in return path ① and is not supplied to the load). Figure 2 In the second half of the cycle, MOSFET Q1 is turned off and Q2 is turned on, and energy is transferred to inductor L1 through transformer TX1 for freewheeling energy storage. At the same time, capacitors C1 and C2 discharge, together charging bus capacitor E1 and supplying power to the load. The energy return path is ⑥+⑦.

[0004] As can be seen from the working process of the previous switching cycle, only half of the cycle in each switching cycle is used to charge the bus capacitor with the energy transferred from the battery pack. Under a certain load, the ripple current charging the bus capacitor E1 is large, and the ripple voltage of the bus capacitor E1 is also large. This will lead to increased losses and severe heat generation of the bus capacitor, resulting in low efficiency and a shortened estimated lifespan. At the same time, the large ripple voltage is not conducive to the control effect of the downstream inverter. It is necessary to select a bus capacitor with low ESR, good heat dissipation and large size, which will result in higher cost.

[0005] 2. The resonant capacitor suffers from current shunting and uneven heating: (The rest of the text appears to be a fragment and requires further context for accurate translation.) Figure 1As can be seen, multiple resonant capacitors are usually connected in parallel to match the appropriate capacitance (as shown in the figure, C1 and C2 are connected in parallel). Due to batch or individual differences in capacitors, there will always be a certain capacitance deviation. Capacitors with larger capacitance values ​​have lower impedance. Under parallel conditions, the current flowing through them is larger and the heat generated is greater. As the individual temperature rises, the capacitance of the resonant capacitor will increase, and the current flowing through the capacitor will be even greater. This will increase the current difference between C1 and C2, accelerate the aging of the large-capacity resonant capacitor, and shorten the estimated service life of the product. Summary of the Invention

[0006] To overcome the shortcomings of the prior art, this invention provides a bidirectional DC-DC circuit that solves the problems of large ripple current, ripple voltage, and high temperature of the bus capacitor E1 in the existing circuit scheme. The efficiency and reliability are further improved, and the design cost is more advantageous. At the same time, it effectively prevents the problem of uneven current and heat generation when the capacitance values ​​of C1 and C2 are directly connected in parallel in the existing circuit, which leads to the problem of uneven current and heat generation. The reliability is high.

[0007] To solve the above technical problems, the present invention provides the following technical solution: a bidirectional DC-DC circuit, including a circuit control section and a circuit power section; the circuit power section includes low-voltage MOSFETs Q1 and Q2, a high-frequency isolation transformer TX1, a resonant inductor L1, resonant capacitors C1 and C2, high-voltage MOSFETs Q3 and Q4, a high-voltage DC bus capacitor E1, and an output equivalent load Rload. Q1 and Q2 are located at the low-voltage end of TX1, and C1 and C2 are connected in parallel with L1, Q3, Q4, E1, and the output equivalent load Rload are located at the high-voltage end of the high-frequency isolation transformer TX1. The current return paths of C1 and C2 are separate, and E1 and the output equivalent load Rload are introduced into the resonant circuit.

[0008] As a preferred embodiment of the present invention, the circuit control section includes a DSP control chip, a drive circuit 2 and a drive circuit 1, an isolation drive circuit 3 and an isolation drive circuit 4. The drive circuit 2 and the drive circuit 1 amplify the power of the complementary PWM control signals emitted by the DSP chips OUTA and OUTB to drive Q1 and Q2. The isolation drive circuit 3 and the isolation drive circuit 4 isolate and amplify the power of the complementary PWM control signals emitted by the DSP chips OUTC and OUTD to drive Q3 and Q4.

[0009] As a preferred embodiment of the present invention, the circuit control section and the circuit power section constitute a push-pull voltage doubler resonant circuit. The push-pull voltage doubler resonant circuit boosts the 24Vdc battery voltage to a DC bus voltage of 380Vdc. The push-pull voltage doubler resonant circuit reverses to form a half-bridge LLC resonant circuit. The half-bridge LLC resonant circuit reduces the DC bus voltage to 24Vdc to supply battery charging. The push-pull voltage doubler resonant circuit and the half-bridge LLC resonant circuit are inverse circuits of each other.

[0010] As a preferred embodiment of the present invention, the main current path of the push-pull voltage doubler resonant circuit in the first half-cycle includes paths ①, ②, ③, and ④, and their combination relationships. When Q1 and Q4 are turned on and Q2 and Q3 are turned off, the winding N4 of the high-frequency isolation transformer TX1 is connected in series with the winding N1. It charges C1 through current path ①, charges C2 through current path ④ while supplying energy to the equivalent load Rload, and charges capacitor E1 through current path ③. Finally, the total current path is ① + ③ + ④, which returns to transformer Pin4.

[0011] As a preferred embodiment of the present invention, in the main current path of the first half cycle of the push-pull voltage doubler resonant circuit, E1 simultaneously supplies power to the load Rload through path ②.

[0012] As a preferred embodiment of the present invention, the main current path of the push-pull voltage doubler resonant circuit in the second half-cycle includes paths ⑤, ⑥, ⑦, ⑧ and their combinations. When Q2 and Q3 are turned on and Q1 and Q4 are turned off, the polarity of the N4 and N1 series windings of TX1 is reversed. The voltage between Pin4 and Pin6 of TX1 is superimposed on the charging voltage of C1 in the first half-cycle of the push-pull voltage doubler resonant circuit, and charges the bus capacitor E1 and supplies power to the load through the resonant inductor L1. The resonant return paths involving C1 are ⑥ and ⑦, and the return path involving C2 is ⑧.

[0013] As a preferred embodiment of the present invention, in the main current path of the second half-cycle of the push-pull voltage doubler resonant circuit, E1 supplies power to the load Rload through path ⑤.

[0014] As a preferred embodiment of the present invention, in the first half-cycle of the push-pull voltage doubler resonant circuit, C1 and E1 are connected in series and then in parallel with C2. In the first half-cycle of the push-pull voltage doubler resonant circuit, E1 is first connected in series with capacitor C2 and then in parallel with C1.

[0015] As a preferred technical solution of the present invention, when Q2 and Q3 are turned on and Q1 and Q4 are disconnected, the main return path of the half-bridge LLC resonant circuit in the first half cycle is ⑨ and ⑩. C2 and E1 discharge together to charge and store energy for capacitor C1, and transfer energy to charge the battery through transformer TX1.

[0016] As a preferred embodiment of the present invention, when Q1 and Q4 are turned on and Q2 and Q3 are turned off, the main return path of the half-bridge LLC resonant circuit in the second half-cycle is as follows: The C1 capacitor is located via the path The battery is charged via L1 and Q4 discharge, and energy is transferred to the battery through transformer TX1. Capacitor E1 is charged via the following path. Charge C2.

[0017] Compared with the prior art, the beneficial effects that this invention can achieve are:

[0018] 1. During DC-DC push-pull boost discharge, the circuit of this invention can introduce the DC high-voltage bus capacitor and the load into the resonant circuit, so that the energy transferred from TX1 can be supplied to charge the load and the bus capacitor throughout the entire resonant cycle, without changing the working state of the resonant circuit. It has the soft-switching effect of existing circuits and can perfectly solve the problems of large ripple current, ripple voltage and high temperature of the bus capacitor E1 in existing circuit schemes. The efficiency and reliability are further improved, and the design cost is more advantageous.

[0019] 2. In this invention, resonant capacitors C2 and C1 with the same capacitance are connected in series with the bus capacitor in the first and second half of a switching cycle, respectively. In the first half of the cycle, the current flowing through C1 is slightly larger, and in the second half of the cycle, the current flowing through C2 is slightly larger. By alternately bearing the larger current, the heating of capacitors C1 and C2 is balanced, which effectively prevents the problem of uneven current and heating when C1 and C2 are directly connected in parallel in existing circuits. This invention has high reliability.

[0020] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This describes the current path in the existing circuit design and the first half of the current cycle.

[0023] Figure 2 This describes the current path in the existing circuit design and the second half of the cycle.

[0024] Figure 3 This is a diagram showing the push-pull voltage doubler resonant circuit of the present invention and the current path in the first half-cycle.

[0025] Figure 4 This is a diagram showing the push-pull voltage doubler resonant circuit of the present invention and the current path in the second half-cycle.

[0026] Figure 5 This is the equivalent circuit diagram of the main circuit before the improvement of the push-pull voltage doubler resonant circuit of the present invention;

[0027] Figure 6This is the equivalent circuit diagram of the improved push-pull voltage doubler resonant circuit of the present invention.

[0028] Figure 7 This is a diagram showing the current path of the half-bridge LLC resonant circuit and the first half-cycle of the present invention.

[0029] Figure 8 This is a diagram showing the current path of the half-bridge LLC resonant circuit and the second half-cycle of the present invention.

[0030] Figure 9 The diagram shows the test data for the existing circuit design.

[0031] Figure 10 This is a test data diagram of the circuit scheme of the present invention. Detailed Implementation

[0032] To make the technical means, creative features, and achieved objectives and effects of this invention easier to understand, the invention is further described below with reference to specific embodiments. However, the following embodiments are merely preferred embodiments of this invention and not all of them. Other embodiments obtained by those skilled in the art based on the embodiments described herein without creative effort are all within the protection scope of this invention. Unless otherwise specified, the experimental methods in the following embodiments are conventional methods, and the materials and reagents used in the following embodiments are commercially available unless otherwise specified.

[0033] See Figures 3-10 As shown, the present invention provides a bidirectional DC-DC circuit.

[0034] A bidirectional DC-DC circuit includes a control section and a power section. The power section includes low-voltage MOSFETs Q1 and Q2, a high-frequency isolation transformer TX1, a resonant inductor L1, resonant capacitors C1 and C2, high-voltage MOSFETs Q3 and Q4, a high-voltage DC bus capacitor E1, and an output equivalent load Rload. Q1 and Q2 are located at the low-voltage end of TX1, and C1 and C2 are connected in parallel with L1, Q3, Q4, E1, and the output equivalent load Rload is located at the high-voltage end of the high-frequency isolation transformer TX1. The current return paths of C1 and C2 are separated, and the current return paths of E1 and the output equivalent load Rload are also separated. The effective load Rload is introduced into the resonant circuit; the circuit control part includes a DSP control chip (such as STM32F103 or TMS28034), drive circuit 2 and drive circuit 1, isolation drive circuit 3 and isolation drive circuit 4. Drive circuit 2 and drive circuit 1 amplify the power of the complementary PWM control signals emitted by DSP chips OUTA and OUTB to drive Q1 and Q2. Isolation drive circuit 3 and isolation drive circuit 4 isolate and amplify the power of the complementary PWM control signals emitted by DSP chips OUTC and OUTD to drive Q3 and Q4.

[0035] Preferably, the circuit control section and the circuit power section form a push-pull voltage doubler resonant circuit. The push-pull voltage doubler resonant circuit boosts the 24Vdc battery voltage to a DC bus voltage of 380Vdc. The push-pull voltage doubler resonant circuit reverses to form a half-bridge LLC resonant circuit. The half-bridge LLC resonant circuit reduces the DC bus voltage to 24Vdc to supply battery charging. The push-pull voltage doubler resonant circuit and the half-bridge LLC resonant circuit are inverse circuits of each other.

[0036] Preferably, the main current path in the first half-cycle of the push-pull voltage doubler resonant circuit includes paths ①, ②, ③, and ④, and their combinations. When Q1 and Q4 are on and Q2 and Q3 are off, the winding N4 of the high-frequency isolation transformer TX1, connected in series with the winding N1, charges C1 through current path ①, charges C2 through current path ④ while simultaneously supplying power to the equivalent load Rload, and charges capacitor E1 through current path ③. The total current is then summarized as ① + ③ + ④, returning to transformer Pin4. Within the main current path of the first half-cycle of the push-pull voltage doubler resonant circuit, E1 simultaneously supplies power to the load Rload through path ②.

[0037] Preferably, the main current path in the second half-cycle of the push-pull voltage doubler resonant circuit includes paths ⑤, ⑥, ⑦, and ⑧, and their combinations. When Q2 and Q3 are on and Q1 and Q4 are off, the polarity of the N4 and N1 series windings of TX1 reverses. The voltage between Pin4 and Pin6 of TX1 is superimposed on the charging voltage of C1 in the first half-cycle of the push-pull voltage doubler resonant circuit, and charges the bus capacitor E1 and supplies power to the load through the resonant inductor L1. Among these, C1 participates in resonant return paths ⑥ and ⑦, and C2 participates in return path ⑧. Within the main current path of the second half-cycle of the push-pull voltage doubler resonant circuit, E1 supplies power to the load Rload through path ⑤.

[0038] Preferably, during the first half-cycle of the push-pull voltage doubler resonant circuit, C1 and E1 are connected in series, and then in parallel with C2. During the first half-cycle of the push-pull voltage doubler resonant circuit, E1 is first connected in series with capacitor C2, and then in parallel with C1.

[0039] Preferably, when Q2 and Q3 are on and Q1 and Q4 are off, the main return paths of the half-bridge LLC resonant circuit in the first half cycle are ⑨ and ⑩. C2 and E1 discharge together to charge and store energy for capacitor C1, and the energy is transferred to the battery through transformer TX1.

[0040] Preferably, when Q1 and Q4 are on and Q2 and Q3 are off, the main return path of the half-bridge LLC resonant circuit in the second half of the cycle is as follows: Capacitor C1 passes through the path The battery is charged via L1 and Q4 discharge, and energy is transferred to the battery through transformer TX1. Capacitor E1 is charged via the following path. Charge C2.

[0041] Implementation Case:

[0042] like Figure 3 As shown, this can be used as the DC-DC boost circuit of this invention (to boost the 24Vdc battery voltage to the 380Vdc bus voltage). The figure shows the working state of the push-pull voltage doubler resonant circuit in the first half of the resonant cycle. When Q1 and Q4 are on (Q2 and Q3 are off), the transformer winding N4 in series with the winding N1 charges capacitor C1 through current path ①, charges capacitor C2 through current path ④, and simultaneously supplies power to the load Rload. At the same time, it charges the bus capacitor E1 through current path ③, and finally the total of ①+③+④ returns to transformer Pin4. Meanwhile, in this first half of the cycle, the bus capacitor E1 supplies power to the load Rload through path ②. Since the resonant capacitors C1 and C2 have small capacitances (usually in the range of a few tenths of an µF), while E1 is a large energy storage capacitor (usually in the range of several hundred µF), combined with... Figure 5 Analysis shows that during this half-cycle, the equivalent series connection of C2 and E1 eventually leads to parallel connection with C1. Since the total capacitance of C2 and E1 after series connection is smaller than that of C2 and close to the original capacitance of C2, the total resonant capacitance after parallel connection with C1 is basically consistent with the resonant parameters of existing mature circuits, and the resonance and control states are basically consistent. Since the capacitance of C1 is greater than (C2 in series with E1), the resonant current flowing through C1 in the first half-cycle will be slightly greater than that of the series connection of C2 and E1. At the end of the half-cycle, the voltage on capacitor C1 has been charged to approximately 1 / VBUS = 190V.

[0043] like Figure 4 As shown, this can be used as the DC-DC boost circuit of the present invention. The figure shows the working state of the push-pull voltage doubler resonant circuit in the second half of the resonant cycle. When Q2 and Q3 are turned on (Q1 and Q4 are turned off), the polarity of the series windings of transformer N4 and N1 is reversed, and the voltage between transformer Pin4 and Pin6 is close to 1 / VBUS = 190V. Superimposed with the voltage of 190V already charged on C1 in the first half of the cycle, the total voltage is about 380V. The bus capacitor E1 is charged and the load is powered through the resonant inductor L1. Among them, C1 participates in the resonant return path ⑥ and ⑦, and C2 participates in the return path ⑧. The bus capacitor E1 supplies power to the load Rload through path ⑤. Figure 5 Analysis shows that within this half-cycle, the equivalent series connection of C1 and E1 eventually merges into parallel connection with C2. Since the total capacitance of C1 and E1 in series is smaller than C1 and close to its original value, the total resonant capacitance after merging with C2 in parallel is essentially consistent with the resonant parameters of existing mature circuits, and the resonance and control states are also essentially the same. Because the capacitance C2 > (C1 in series with E1), the resonant current flowing through C2 in this second half-cycle will be slightly greater than that of the C1 and E1 series circuit. At the end of the half-cycle, due to the resonance charging the bus capacitor, the voltage on capacitor C1 drops to a lower value (or close to 0V). After the second half-cycle ends, it re-enters the circuit. Figure 3 The first half of the cycle is shown, and this cycle repeats.

[0044] Combining the above and Figure 3 , 4 Analysis of points 5 and 6, and comparison of the circuit of this invention with existing circuits, shows that during DC-DC push-pull boost discharge, the circuit of this invention can introduce the DC high-voltage bus capacitor and the load into the resonant circuit. This allows the energy transferred from TX1 to charge the load and bus capacitor throughout the entire resonant cycle, without changing the operating state of the resonant circuit. It possesses the soft-switching effect of existing circuits and can effectively solve the problems of large ripple current, ripple voltage, and high temperature of the bus capacitor E1 in existing circuit schemes, as described above. Efficiency and reliability are further improved, and the design cost is more advantageous. Furthermore, by connecting resonant capacitors C2 and C1 of the same capacitance in series with the bus capacitor in the first and second half of a switching cycle, respectively, with a slightly larger current flowing through C1 in the first half and a slightly larger current flowing through C2 in the second half, the alternating handling of larger currents ensures balanced heating of C1 and C2. This effectively prevents the current and heating imbalance problems that occur when C1 and C2 are directly connected in parallel in existing circuits, where their capacitance values ​​differ. This results in higher reliability.

[0045] like Figure 7 and Figure 8 The diagram shows the current path of the DC-DC step-down circuit of this invention. A higher DC bus voltage (e.g., 380Vdc) is stepped down to 24Vdc through a half-bridge LLC resonant circuit to supply power to the battery (for charging). The push-pull voltage doubler resonant circuit and the half-bridge LLC resonant circuit are inverse circuits of each other; the analysis process is the reverse process of the push-pull boost circuit. Figure 7 As shown, in the first half of the cycle, Q2 and Q3 are turned on, which can be equivalent to C2 and E1 discharging, jointly charging capacitor C1 to store energy and transferring energy to the battery through transformer TX1. The return paths are ⑨ and ⑩ respectively. At the end of the first half of the cycle, the voltage of capacitor C1 reaches approximately 1 / VBUS = 190V (left positive, right negative); Figure 8 As shown, in the second half of the cycle, Q1 and Q4 are on, while Q2 and Q3 are off. Similarly, this can be considered equivalent to capacitor C1 discharging through L1 and Q4 and transferring energy to charge the battery through transformer TX1. The return current path is... Capacitor E1 passes through the path Charge C2. Similarly, it can be seen that in the equivalent circuit of the resonant cavity, the bus capacitor E1 is connected in series with capacitor C1 in the first half-cycle, and then in parallel with C2 at the end. In the second half-cycle, E1 is first connected in series with capacitor C2, and then in parallel with C1 at the end; the 380Vdc in the figure is supplied to other circuits at the back end (which can be regarded as a DC voltage source supply), combined with Figure 5 Figure 6It can be seen that replacing the Rload resistor with a voltage source results in the equivalent circuit of LLC resonant step-down. That is, the voltage source and the bus capacitor E1 participate in the resonant circuit throughout the entire switching cycle, and charge the battery through the isolation transformer TX1. During LLC resonant step-down, the ripple current and voltage of the bus capacitor E1 can also be effectively reduced. At the same time, the resonant capacitors C1 and C2 can play a natural current sharing effect.

[0046] like Figure 9 and Figure 10 The diagram shows the simulation waveforms of the existing circuit and the circuit of this invention during DC-DC boost: Under the same load, same input and output, and same component parameters, the circuit simulation results show that the original circuit scheme has a bus capacitor E1 ripple current of 10.36A and a bus ripple voltage of 0.08V (the bus capacitor used in the simulation has a larger capacitance and a smaller ripple voltage; this trend is not relevant here). However, using the connection of the circuit of this invention, the bus capacitor E1 ripple current is 4.706A and the bus ripple voltage is 0.018V. Therefore, it is evident that the circuit of this invention can significantly reduce the ripple current and heat generation of the E1 capacitor, and the bus ripple voltage can also be significantly reduced.

[0047] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0048] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A bidirectional DC-DC circuit, characterized by: Includes the circuit control section and the circuit power section; The power section of the circuit includes low-voltage MOSFETs Q1 and Q2, high-frequency isolation transformer TX1, resonant inductor L1, resonant capacitors C1 and C2, high-voltage MOSFETs Q3 and Q4, high-voltage DC bus capacitor E1, and output equivalent load Rload. The low-voltage MOSFETs Q1 and Q2 are located at the low-voltage end of the high-frequency isolation transformer TX1, forming a push-pull circuit with the low-voltage winding of the high-frequency isolation transformer TX1. The resonant inductor L1, high-voltage MOSFETs Q3 and Q4, high-voltage DC bus capacitor E1, output equivalent load Rload, resonant capacitors C1 and C2 are all located at the high-voltage end of the high-frequency isolation transformer TX1. The resonant capacitors C1 and C2 form independent current return paths, and together with the high-voltage DC bus capacitor E1 and the output equivalent load Rload, they are connected to the resonant circuit formed by the resonant inductor L1, so that the high-voltage DC bus capacitor E1 and the output equivalent load Rload are introduced into the resonant circuit.

2. A bidirectional DC-DC circuit according to claim 1, characterized in that: The circuit control section includes a DSP control chip, drive circuit 2 and drive circuit 1, isolation drive circuit 3 and isolation drive circuit 4. Drive circuit 2 and drive circuit 1 amplify the complementary PWM control signals emitted by DSP control chip OUTA and DSP control chip OUTB to drive low-voltage MOSFETs Q1 and Q2. Isolation drive circuit 3 and isolation drive circuit 4 isolate and amplify the complementary PWM control signals emitted by DSP control chip OUTC and DSP control chip OUTD to drive high-voltage MOSFETs Q3 and Q4.

3. A bidirectional DC-DC circuit according to claim 1, characterized in that: The circuit control section and the circuit power section form a push-pull voltage doubler resonant circuit. The push-pull voltage doubler resonant circuit boosts the 24Vdc battery voltage to a DC bus voltage of 380Vdc. The push-pull voltage doubler resonant circuit reverses to form a half-bridge LLC resonant circuit. The half-bridge LLC resonant circuit reduces the DC bus voltage to 24Vdc to supply battery charging. The push-pull voltage doubler resonant circuit and the half-bridge LLC resonant circuit are inverse circuits of each other.

4. A bidirectional DC-DC circuit according to claim 3, characterized in that: The main current path of the push-pull voltage doubler resonant circuit in the first half cycle includes paths ①, ②, ③, and ④, and their combination relationships. When the low-voltage MOSFET Q1 and the high-voltage MOSFET Q4 are turned on, and the low-voltage MOSFET Q2 and the high-voltage MOSFET Q3 are turned off, the winding N4 of the high-frequency isolation transformer TX1 is connected in series with the winding N1. It charges the resonant capacitor C1 through current path ①, charges C2 through current path ④ while simultaneously supplying power to the output equivalent load Rload, and charges the high-voltage DC bus capacitor E1 through current path ③. Finally, the current sums up as ① + ③ + ④ and return to the transformer Pin4.

5. A bidirectional DC-DC circuit according to claim 4, characterized in that: In the main current path of the first half cycle of the push-pull voltage doubler resonant circuit, the high-voltage DC bus capacitor E1 simultaneously supplies power to the output equivalent load Rload through path ②.

6. A bidirectional DC-DC circuit according to claim 3, characterized in that: The main current path in the second half-cycle of the push-pull voltage doubler resonant circuit includes paths ⑤, ⑥, ⑦, and ⑧, and their combinations. When the low-voltage MOSFET Q2 and the high-voltage MOSFET Q3 are turned on, and the low-voltage MOSFET Q1 and the high-voltage MOSFET Q4 are turned off, the polarity of the series windings N4 and N1 of the high-frequency isolation transformer TX1 is reversed. The voltage between Pin4 and Pin6 of the high-frequency isolation transformer TX1 is superimposed on the charging voltage of the resonant capacitor C1 in the first half-cycle of the push-pull voltage doubler resonant circuit, and charges the high-voltage DC bus capacitor E1 and supplies power to the load through the resonant inductor L1. The resonant return path in which the resonant capacitor C1 participates is ⑥ and ⑦, and the return path in which the resonant capacitor C2 participates is ⑧.

7. A bidirectional DC-DC circuit according to claim 6, characterized in that: In the main current path of the second half cycle of the push-pull voltage doubler resonant circuit, the high-voltage DC bus capacitor E1 supplies power to the output equivalent load Rload through path ⑤.

8. A bidirectional DC-DC circuit according to claim 3, characterized in that: During the first half of the push-pull voltage doubler resonant circuit, the resonant capacitor C1 and the high-voltage DC bus capacitor E1 are connected in series, and then in parallel with the resonant capacitor C2. During the first half of the push-pull voltage doubler resonant circuit, the high-voltage DC bus capacitor E1 is first connected in series with the resonant capacitor C2, and then in parallel with the resonant capacitor C1.

9. A bidirectional DC-DC circuit according to claim 3, characterized in that: When the low-voltage MOSFET Q2 and the high-voltage MOSFET Q3 are turned on, and the low-voltage MOSFET Q1 and the high-voltage MOSFET Q4 are disconnected, the main return paths of the half-bridge LLC resonant circuit in the first half cycle are ⑨ and ⑩. The resonant capacitor C2 and the high-voltage DC bus capacitor E1 discharge together to charge and store energy for the resonant capacitor C1, and the energy is transferred to charge the battery through the high-frequency isolation transformer TX1.

10. A bidirectional DC-DC circuit according to claim 3, characterized in that: When the low-voltage MOSFET Q1 and the high-voltage MOSFET Q4 are turned on, and the low-voltage MOSFET Q2 and the high-voltage MOSFET Q3 are turned off, the main return paths of the half-bridge LLC resonant circuit in the second half cycle are ⑪ and ⑫. The resonant capacitor C1 discharges through path ⑫ via the resonant inductor L1 and Q4, and transfers energy to charge the battery through the high-frequency isolation transformer TX1. The high-voltage DC bus capacitor E1 charges the resonant capacitor C2 through path ⑪.

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