Two-stage DC / DC converter

Abstract

The utility model discloses a two-stage DC / DC converter, which consists of a front-stage BUCK conversion unit, a rear-stage LLC resonant conversion unit, a secondary synchronous rectification circuit unit and a control unit; wherein the front-stage BUCK conversion unit is connected to the rear-stage LLC resonant conversion unit, and the rear-stage LLC resonant conversion unit is connected to the synchronous rectification circuit unit; the rear-stage LLC resonant conversion unit is an asymmetric half-bridge LLC resonant conversion unit; the control unit is connected to the front-stage BUCK conversion unit and the rear-stage LLC resonant conversion unit; the front-stage BUCK conversion unit stabilizes the output voltage at different input voltages and different output load currents, and the rear-stage LLC resonant conversion unit and the secondary synchronous rectification circuit unit isolate the input and output currents and rectify and filter the output currents. The two-stage DC / DC converter provided by the utility model has simple structure, simple control, high stability and high reliability.

Description

Technical Field

[0001] The present application belongs to the technical field of power conversion, and specifically refers to a two-stage DC / DC converter. Background Art

[0002] In the field of vehicles, new energy vehicles have developed rapidly in recent years, and pure electric vehicles have developed the fastest. In pure electric vehicles, the on-board DC / DC converter module that connects the power battery and the starting battery has very high requirements for system conversion efficiency, module volume, and reliability. At the same time, the on-board DC / DC converter is required to have a constant current function and an overcurrent output voltage retraction function. However, the harsh operating environment when the vehicle is driving requires the system to have very stringent requirements for functions such as short circuit, full load start, and constant current output. It is necessary to consider the requirements of the on-board DC / DC converter for overload power in actual use.

[0003] While the LLC converter has many excellent characteristics, the LLC converter's topological structure has a large starting current, and the system may damage the primary power MOSFET when it is fully loaded, and it is difficult for the system to achieve constant current and output voltage retraction functions. The phase-shifted full-bridge soft-switching converter can achieve zero voltage switching of the primary power MOSFET within a certain load range, but there are problems such as duty cycle loss and a small soft switching range. Therefore, it is difficult for the on-board DC / DC converter in the prior art to have both constant current function and overcurrent output voltage retraction function without damaging the primary power MOSFET. Utility Model Content

[0004] In order to overcome the shortcomings of the prior art, an embodiment of the present application provides a two-stage DC / DC converter, which combines a BUCK converter and a half-bridge LLC converter. It can combine the advantages of simple control and system stability of the BUCK converter and high efficiency of the LLC resonant converter to ensure that the entire system works stably, reliably and efficiently.

[0005] The utility model provides a two-stage DC / DC converter, which includes at least two-stage circuit topology structures, and is composed of a front-stage BUCK conversion unit, a rear-stage LLC resonant conversion unit, a secondary synchronous rectification circuit unit, and a control unit; wherein the front-stage BUCK conversion unit is connected to the rear-stage LLC resonant conversion unit, and the rear-stage LLC resonant conversion unit is connected to the synchronous rectification circuit unit; the rear-stage LLC resonant conversion unit is an asymmetric half-bridge LLC resonant conversion unit; the control unit is connected to the front-stage BUCK conversion unit and the rear-stage LLC resonant conversion unit;

[0006] The front-stage BUCK conversion unit stabilizes the output voltage at different input voltages and different output load currents, and the rear-stage LLC resonant conversion unit and the secondary synchronous rectification circuit unit isolate the input and output currents and rectify and filter the output currents.

[0007] According to the two-stage DC / DC converter provided by the utility model, the front-stage BUCK conversion unit includes a first switch tube Q1, a first diode D1, a first inductor L1 and a first capacitor C1;

[0008] The source of the first switch tube Q1 is connected to the cathode of the first diode D1 and one end of the first inductor L1, the drain of the first switch tube Q1 is connected to the positive electrode of the DC input voltage Vin, the anode of the first diode D1 is connected to the negative electrode of the DC input voltage Vin, the other end of the first inductor L1 is connected to one end of the first capacitor C1, and the other end of the first capacitor C1 is connected to the negative electrode of the DC input voltage Vin.

[0009] According to the two-stage DC / DC converter provided by the utility model, the first switch tube Q1 further includes a parasitic diode.

[0010] According to the two-stage DC / DC converter provided by the utility model, the control unit adjusts the duty cycle of the first switch tube Q1 and stabilizes the output voltage at different input voltages and different output load currents.

[0011] According to the two-stage DC / DC converter provided by the utility model, the post-stage LLC resonant conversion unit includes a second switch tube Q2, a third switch tube Q3, a second capacitor Cr, a second inductor Lr and a primary side structure of a transformer T1;

[0012] The source of the second switch tube Q2 is connected to one end of the second capacitor Cr, the drain of the second switch tube Q2 is connected to one end of the first inductor L1, the source of the third switch tube Q3 is connected to the negative electrode of the DC input voltage Vin, the drain of the third switch tube Q3 is connected to one end of the second capacitor Cr, the other end of the second capacitor Cr is connected to one end of the second inductor Lr, the other end of the second inductor Lr is connected to the primary side structure of the transformer T1, and the other end of the primary side structure of the transformer T1 is connected to the negative electrode of the DC input voltage Vin.

[0013] According to the two-stage DC / DC converter provided by the utility model, the second switch tube Q2 and the third switch tube Q3 both include parasitic diodes, and the second capacitor Cr is a resonant capacitor.

[0014] According to the two-stage DC / DC converter provided by the utility model, the second inductor Lr is a resonant inductor.

[0015] According to the two-stage DC / DC converter provided by the utility model, the secondary synchronous rectification circuit unit includes a fourth switch tube Qr1, a fifth switch tube Qr2, a third capacitor Co, a first resistor Ro and a secondary side structure of a transformer T1, and the secondary side structure of the transformer T1 includes a first secondary winding and a second secondary winding;

[0016] The source of the fourth switch tube Qr1 is connected to the source of the fifth switch tube Qr2, the drain of the fourth switch tube Qr1 is connected to the first secondary winding, the drain of the fifth switch tube Qr2 is connected to the second secondary winding, the first secondary winding and the second secondary winding are connected to the first resistor Ro, and the third capacitor Co is connected in parallel with the first resistor Ro.

[0017] According to the two-stage DC / DC converter provided by the present invention, the fourth switch tube Qr1 and the fifth switch tube Qr2 both include parasitic diodes.

[0018] According to the two-stage DC / DC converter provided by the utility model, the second switch tube Q2 and the third switch tube Q3 are open-loop controlled with fixed frequency and duty cycle.

[0019] The beneficial effects of the utility model are as follows: the two-stage DC / DC converter provided by the utility model combines the BUCK conversion unit and the LLC resonant conversion unit to form a two-stage circuit topology structure with simple control and high conversion efficiency. The front-stage BUCK conversion unit can stabilize the output voltage at different input voltages and different output load currents, while the rear-stage LLC resonant conversion unit and the secondary synchronous rectification circuit unit isolate the input and output currents and rectify and filter the output. Moreover, the rear-stage LLC resonant conversion unit and the secondary synchronous rectification circuit unit can be equivalent to a DC transformer circuit, so the two-stage DC / DC converter provided by the utility model can be equivalent to a BUCK converter and a DC transformer circuit connected in series, and can be further equivalent to a basic BUCK converter. The equivalent basic BUCK converter has simple control, high system stability and high reliability, and can achieve the characteristics of simple control process, simple circuit structure and high system conversion efficiency. It well combines the advantages of simple structure, simple control, high stability and high reliability of the BUCK converter and high conversion efficiency of the asymmetric half-bridge LLC converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The technical solution and other beneficial effects of the present application will be made apparent by describing in detail the specific implementation methods of the present application in conjunction with the accompanying drawings.

[0021] Figure 1 A schematic diagram of the circuit topology of a two-stage DC / DC converter provided in an embodiment of the utility model.

[0022] Figure 2 GS voltage waveform of the first switch tube Q1, DS waveform of the fourth switch tube Qr1 and current waveform of the second inductor Lr.

[0023] Figure 3 GS voltage waveform of the first switch tube Q1, voltage waveform of the first capacitor C1 and current waveform of the second inductor Lr.

[0024] Figure 4 GS voltage waveform of the first switch tube Q1, DS voltage waveform of the third switch tube Q3 and current waveform of the second inductor Lr.

[0025] Figure 5 The conversion efficiency curve of a two-stage DC / DC converter at an input voltage of 310VDC, an output voltage of 14VDC, and different output currents. DETAILED DESCRIPTION

[0026] The technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present application.

[0027] In the description of the present application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first" and "second" are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present application, the meaning of "multiple" is two or more, unless otherwise clearly and specifically defined.

[0028] The disclosure below provides many different embodiments or examples to realize the different structures of the present application. In order to simplify the disclosure of the present application, the parts and settings of specific examples are described below. Of course, they are only examples, and the purpose is not to limit the present application. In addition, the present application can repeat reference numbers and / or reference letters in different examples, and this repetition is for the purpose of simplification and clarity, which itself does not indicate the relationship between the various embodiments and / or settings discussed. In addition, the various specific processes and material examples provided by the present application, but those of ordinary skill in the art can appreciate the application of other processes and / or the use of other materials.

[0029] Figure 1 A schematic diagram of the circuit topology of a two-stage DC / DC converter provided in an embodiment of the utility model.

[0030] like Figure 1 As shown, the two-stage DC / DC converter provided by the embodiment of the utility model includes at least two-stage circuit topology, which is composed of a front-stage BUCK conversion unit, a rear-stage LLC resonant conversion unit, a secondary synchronous rectification circuit unit and a control unit; wherein the front-stage BUCK conversion unit is connected to the rear-stage LLC resonant conversion unit, and the rear-stage LLC resonant conversion unit is connected to the synchronous rectification circuit unit; the rear-stage LLC resonant conversion unit is an asymmetric half-bridge LLC resonant conversion unit; the control unit is connected to the front-stage BUCK conversion unit and the rear-stage LLC resonant conversion unit; the front-stage BUCK conversion unit stabilizes the output voltage at different input voltages and different output load currents, and the rear-stage LLC resonant conversion unit and the secondary synchronous rectification circuit unit isolate the input and output currents and rectify and filter the output currents.

[0031] Among them, the front-stage BUCK conversion unit includes a first switch tube Q1, a first diode D1, a first inductor L1 and a first capacitor C1; the source of the first switch tube Q1 is connected to the cathode of the first diode D1 and one end of the first inductor L1, the drain of the first switch tube Q1 is connected to the positive electrode of the DC input voltage Vin, the anode of the first diode D1 is connected to the negative electrode of the DC input voltage Vin, the other end of the first inductor L1 is connected to one end of the first capacitor C1, and the other end of the first capacitor C1 is connected to the negative electrode of the DC input voltage Vin.

[0032] The post-stage LLC resonant conversion unit includes a second switch tube Q2, a third switch tube Q3, a second capacitor Cr, a second inductor Lr and a primary side structure of a transformer T1; the source of the second switch tube Q2 is connected to one end of the second capacitor Cr, the drain of the second switch tube Q2 is connected to one end of the first inductor L1, the source of the third switch tube Q3 is connected to the negative electrode of the DC input voltage Vin, the drain of the third switch tube Q3 is connected to one end of the second capacitor Cr, the other end of the second capacitor Cr is connected to one end of the second inductor Lr, the other end of the second inductor Lr is connected to the primary side structure of the transformer T1, and the other end of the primary side structure of the transformer T1 is connected to the negative electrode of the DC input voltage Vin.

[0033] The secondary synchronous rectification circuit unit includes a fourth switch tube Qr1, a fifth switch tube Qr2, a third capacitor Co, a first resistor Ro and a secondary structure of a transformer T1, wherein the secondary structure of the transformer T1 includes a first secondary winding and a second secondary winding; the source of the fourth switch tube Qr1 is connected to the source of the fifth switch tube Qr2, the drain of the fourth switch tube Qr1 is connected to the first secondary winding, the drain of the fifth switch tube Qr2 is connected to the second secondary winding, the first secondary winding and the second secondary winding are connected to the first resistor Ro, and the third capacitor Co is connected in parallel with the first resistor Ro.

[0034] In this embodiment, the first switch tube Q1, the second switch tube Q2, the third switch tube Q3, the fourth switch tube Qr1 and the fifth switch tube Qr2 all include parasitic diodes, and the conduction directions of the parasitic diodes and the sources and drains of the switch tubes are opposite.

[0035] In this embodiment, the control unit adjusts the duty cycle of the first switch tube Q1 to achieve the operation of stabilizing the output voltage at different input voltages and different output load currents. The post-stage LLC resonant conversion unit is an asymmetric half-bridge LLC resonant conversion unit, and the secondary output is a synchronous rectification circuit to achieve input-output isolation and output side rectification and filtering functions.

[0036] The control unit includes at least a DSP chip and an intelligent synchronous rectification chip. The DSP chip is used to control the switching of the first switch tube Q1, the second switch tube Q2 and the third switch tube Q3, while the intelligent synchronous rectification chip is used to control the switching of the fourth switch tube Qr1 and the fifth switch tube Qr2.

[0037] In the software setting, the control unit uses the DSP chip as the main control chip to directly use the DC input voltage, the output current and output voltage of the front-stage BUCK conversion unit, and the primary current of the rear-stage LLC resonant conversion unit, and performs system control through a double PI loop, using the current signal output by the system as the current outer loop, and the output current signal of the front-stage BUCK conversion unit as the current inner loop to work. Among them, the switching frequency of the front-stage BUCK conversion unit is set to 70KHz and controlled by PWM1A, and the loop calculation interrupt is generated by the underflow interrupt of PWM1A. At the same time, in order to sample the midpoint of the switch tube current of the front-stage BUCK conversion unit, the ADC sampling start time is set at the midpoint of the duty cycle, that is, CMPB is used as the AD conversion start trigger, and the current sampling is placed in the first channel.

[0038] In this embodiment, the front-stage BUCK conversion unit is set to a discontinuous working mode, which can reduce the problem of reverse recovery of the first diode D1, so that a relatively high conversion efficiency can be obtained even when the current is small. However, the output voltage of the front-stage BUCK conversion unit does not participate in the closed-loop control of the DC / DC converter as a key parameter, so the output voltage of the front-stage BUCK conversion unit has a large voltage ripple. Therefore, in this embodiment, a thin film capacitor is used as the output voltage filter capacitor of the front-stage BUCK conversion unit, that is, the first capacitor C1 is a thin film capacitor.

[0039] In this embodiment, since the input voltage range is relatively wide, the operating range of the post-stage LLC resonant conversion unit is set at a fixed frequency and fixed duty cycle. Therefore, the post-stage LLC resonant conversion unit and the secondary synchronous rectification circuit unit operate at the optimal operating point, that is, the resonant frequency point of the second inductor Lr and the second capacitor Cr, so that the two-stage DC / DC converter provided by this embodiment can achieve the highest conversion efficiency. In order to reduce the parasitic parameters of the transformer and the losses caused by the parasitic diode of the switch tube. At the same time, the excitation inductance of the transformer is determined according to the switching frequency point of the switch tube, the relevant parameters of the switch tube, and the load level.

[0040] In this embodiment, the operating frequency of the post-stage LLC resonant conversion unit is set near the resonant frequency point of the second inductor Lr and the second capacitor Cr of the primary structure of the transformer, which can ensure that the switch tube of the primary structure of the transformer operates in a zero-voltage soft switching state, and can also ensure that the synchronous rectification switch tube of the secondary structure of the transformer operates in an optimal state. At the same time, the sufficiently large inductance value of the excitation inductance of the transformer ensures that the switch tube of the primary structure of the transformer has a small turn-off current and operates in an approximately zero-current turn-off state. This ensures that the entire system operates at the minimum loss state point and reaches the optimal working efficiency point.

[0041] The working range of the post-stage LLC resonant conversion unit is set at a fixed frequency and a fixed duty cycle. The second inductor Lr connected to the primary side structure of the transformer is a resonant inductor, which can be an independent inductor or a parasitic leakage inductor of the transformer. Since the inductance value of the parasitic leakage inductor is generally small, the capacitance value of the second capacitor Cr of the resonant capacitor can be relatively large.

[0042] In a specific embodiment, the input voltage of the DC input voltage Vin is 230~370VDC, the output voltage is 14VDC, and the output current is 43A. The parameters of the two-stage DC / DC converter are the filter inductance of the front-stage BUCK conversion unit, that is, the first inductor L1=50uH, the output capacitor, that is, the first capacitor C1=12uF, the switching frequency of the first switch tube Q1 is 53kHz, and the switching frequency of the second switch tube Q2 and the third switch tube Q3 in the rear-stage LLC resonant conversion unit is 120kHz, the transformer turns ratio n=7, the resonant inductor, that is, the second inductor Lr=8uH, the resonant inductor Lm=100uH, and the resonant capacitor, that is, the second capacitor Cr=220nF.

[0043] like Figure 2 As shown in FIG. 1 , the GS voltage waveform of the first switch tube Q1 , the DS waveform of the fourth switch tube Qr1 and the current waveform of the second inductor Lr are shown. Figure 2 It can be seen that the voltage stress across the fourth switch tube Qr1 in the secondary synchronous rectification circuit unit is twice the DC output voltage, and there is no voltage oscillation. A switch tube with low voltage stress can be selected as the output synchronous rectification device, which can reduce the rectification conduction loss and help improve the conversion efficiency of the rectification circuit. This shows that the asymmetric half-bridge LLC provided in this embodiment works at the resonance point, ensuring that the secondary synchronous rectification circuit unit works at the optimal working point state, eliminating the reverse recovery problem of the parasitic diode of the switch tube in the secondary synchronous rectification circuit unit.

[0044] like Figure 3 As shown in FIG. 1 , the GS voltage waveform of the first switch tube Q1, the voltage waveform of the first capacitor C1, and the current waveform of the second inductor Lr are shown in FIG. Figure 3 It can be seen that the primary current of the asymmetric half-bridge LLC resonant conversion unit is a standard sinusoidal waveform, and the bus voltage of the front-stage BUCK conversion unit is stable, indicating that the asymmetric half-bridge LLC resonant conversion unit operates at the resonant frequency point.

[0045] like Figure 4 As shown in FIG. 1 , the GS voltage waveform of the first switch tube Q1, the DS voltage waveform of the third switch tube Q3, and the current waveform of the second inductor Lr are shown. Figure 4 It can be seen that the voltage stress across the primary power switch tube of the asymmetric half-bridge LLC resonant conversion unit is the input voltage, and there is no burr caused by voltage oscillation, indicating that the primary switch tube of the asymmetric half-bridge LLC resonant conversion unit achieves zero voltage turn-on.

[0046] Figure 5 The conversion efficiency curve of the two-stage DC / DC converter at the input voltage of 310VDC, the output voltage of 14VDC, and different output currents. Figure 5 It can be seen from the test efficiency curve that the two-stage DC / DC converter provided in this embodiment can achieve a conversion efficiency of 94% when the output current is 10A-43A. Since the two-stage DC / DC converter system ensures the full resonance of the second inductor Lr and the second capacitor Cr of the primary structure of the asymmetric half-bridge LLC resonant conversion unit, it ensures that the second switch tube Q2 and the third switch tube Q3 are turned on when the voltage stress at both ends is zero, and zero voltage is turned on; at the same time, the inductance of the magnetizing inductance of the primary structure of the transformer T1 is optimized, and the turn-off current of the second switch tube Q2 and the third switch tube Q3 is reduced to an acceptable range, and an approximately zero current turn-off state is achieved. Since the switch tube of the primary structure of the asymmetric half-bridge LLC resonant conversion unit achieves zero voltage turn-on and approximately zero current turn-off, the two-stage DC / DC converter system has the smallest conversion loss and reaches the highest efficiency conversion working point. The fourth switch tube Qr1 and the fifth switch tube Qr2 of the secondary synchronous rectification circuit unit are controlled by the intelligent synchronous rectification chip, and the control frequency is consistent with the second switch tube Q2 and the third switch tube Q3 of the primary side result of the transformer T1. The duty cycle of the output switch tube is intelligently adjusted by detecting the output current, eliminating the diode loss and electromagnetic interference caused by the reverse recovery of the parasitic diode of the switch tube. At the same time, the control process is optimized by using the DSP chip as the main control unit, and the system output constant current function and overcurrent output voltage retraction function are well realized. When the output voltage retracts, by adjusting the duty cycle of the first switch tube Q1, as the duty cycle of the first switch tube Q1 decreases, the input voltage of the asymmetric half-bridge LLC resonant conversion unit decreases. Since the rear-stage asymmetric half-bridge LLC resonant conversion unit is equivalent to a DC transformer circuit, the output voltage decreases in the same proportion when the input voltage decreases, realizing the function of output voltage retraction. When the constant current is output, the duty cycle of the first switch tube Q1 is gradually reduced, and the input voltage of the asymmetric half-bridge LLC resonant conversion unit is reduced. Since the rear-stage asymmetric half-bridge LLC resonant conversion unit is equivalent to a DC transformer circuit, the output voltage is reduced in the same proportion when the input voltage is reduced. At this time, the output power is reduced, the output voltage is reduced, and the output current is constant. Since the system controls the duty cycle of the first switch tube Q1Q1, the control is simple and the loop is stable and reliable.

[0047] The two-stage DC / DC converter provided by the utility model combines the BUCK conversion unit and the LLC resonant conversion unit to form a two-stage circuit topology structure with simple control and high conversion efficiency. The front-stage BUCK conversion unit can stabilize the output voltage at different input voltages and different output load currents, while the rear-stage LLC resonant conversion unit and the secondary synchronous rectification circuit unit isolate the input and output currents and rectify and filter the output. Moreover, the rear-stage LLC resonant conversion unit and the secondary synchronous rectification circuit unit can be equivalent to a DC transformer circuit, so the two-stage DC / DC converter provided by the utility model can be equivalent to a BUCK converter and a DC transformer circuit connected in series, and can be further equivalent to a basic BUCK converter. The equivalent basic BUCK converter has simple control, high system stability and high reliability, and can achieve the characteristics of simple control process, simple circuit structure and high system conversion efficiency. It well combines the advantages of simple structure, simple control, high stability and high reliability of the BUCK converter and high conversion efficiency of the asymmetric half-bridge LLC converter.

[0048] Although the preferred embodiments of the utility model embodiments have been described, those skilled in the art may make additional changes and modifications to these embodiments once they are aware of the basic creative concepts. Therefore, the attached claims are intended to be interpreted as including the preferred embodiments and all changes and modifications falling within the scope of the utility model embodiments. Finally, 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 that there is any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variant thereof are intended to cover non-exclusive inclusion, so that the process, method, article or terminal device including a series of elements includes not only those elements, but also other elements that are not explicitly listed, or also includes elements inherent to such process, method, article or terminal device. In the absence of more restrictions, the elements defined by the sentence "including one..." do not exclude the existence of other identical elements in the process, method, article or terminal device including the elements.

[0049] The two-stage DC / DC converter provided in the embodiment of the present application is introduced in detail above. Specific examples are used in this article to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the technical solution and its core idea of ​​the present application. Ordinary technicians in this field should understand that they can still modify the technical solutions recorded in the aforementioned embodiments, or replace some of the technical features therein with equivalents; and these modifications or replacements do not make the essence of the corresponding technical solution deviate from the scope of the technical solution of the embodiments of the present application.

Claims

1. A two-stage DC / DC converter, characterized in that: The two-stage DC / DC converter includes at least two-stage circuit topology structures, which are composed of a front-stage BUCK conversion unit, a rear-stage LLC resonant conversion unit, a secondary synchronous rectification circuit unit and a control unit; wherein the front-stage BUCK conversion unit is connected to the rear-stage LLC resonant conversion unit, and the rear-stage LLC resonant conversion unit is connected to the synchronous rectification circuit unit; the rear-stage LLC resonant conversion unit is an asymmetric half-bridge LLC resonant conversion unit; the control unit is connected to the front-stage BUCK conversion unit and the rear-stage LLC resonant conversion unit; The front-stage BUCK conversion unit stabilizes the output voltage at different input voltages and different output load currents, and the rear-stage LLC resonant conversion unit and the secondary synchronous rectification circuit unit isolate the input and output currents and rectify and filter the output currents.

2. The two-stage DC / DC converter according to claim 1, characterized in that: The front-stage BUCK conversion unit includes a first switch tube Q1, a first diode D1, a first inductor L1 and a first capacitor C1; The source of the first switch tube Q1 is connected to the cathode of the first diode D1 and one end of the first inductor L1, the drain of the first switch tube Q1 is connected to the positive electrode of the DC input voltage Vin, the anode of the first diode D1 is connected to the negative electrode of the DC input voltage Vin, the other end of the first inductor L1 is connected to one end of the first capacitor C1, and the other end of the first capacitor C1 is connected to the negative electrode of the DC input voltage Vin.

3. The two-stage DC / DC converter according to claim 2, characterized in that: The first switch tube Q1 also includes a parasitic diode.

4. The two-stage DC / DC converter according to claim 3, characterized in that: The control unit adjusts the duty cycle of the first switch tube Q1 and stabilizes the output voltage at different input voltages and different output load currents.

5. The two-stage DC / DC converter according to claim 2, characterized in that: The post-stage LLC resonant conversion unit includes a second switch tube Q2, a third switch tube Q3, a second capacitor Cr, a second inductor Lr and a primary side structure of a transformer T1; The source of the second switch tube Q2 is connected to one end of the second capacitor Cr, the drain of the second switch tube Q2 is connected to one end of the first inductor L1, the source of the third switch tube Q3 is connected to the negative electrode of the DC input voltage Vin, the drain of the third switch tube Q3 is connected to one end of the second capacitor Cr, the other end of the second capacitor Cr is connected to one end of the second inductor Lr, the other end of the second inductor Lr is connected to the primary side structure of the transformer T1, and the other end of the primary side structure of the transformer T1 is connected to the negative electrode of the DC input voltage Vin.

6. The two-stage DC / DC converter according to claim 5, characterized in that: The second switch tube Q2 and the third switch tube Q3 both include parasitic diodes, and the second capacitor Cr is a resonant capacitor.

7. The two-stage DC / DC converter according to claim 5, characterized in that: The second inductor Lr is a resonant inductor.

8. The two-stage DC / DC converter according to claim 5, characterized in that: The secondary synchronous rectification circuit unit includes a fourth switch tube Qr1, a fifth switch tube Qr2, a third capacitor Co, a first resistor Ro and a secondary side structure of a transformer T1, wherein the secondary side structure of the transformer T1 includes a first secondary side winding and a second secondary side winding; The source of the fourth switch tube Qr1 is connected to the source of the fifth switch tube Qr2, the drain of the fourth switch tube Qr1 is connected to the first secondary winding, the drain of the fifth switch tube Qr2 is connected to the second secondary winding, the first secondary winding and the second secondary winding are connected to the first resistor Ro, and the third capacitor Co is connected in parallel with the first resistor Ro.

9. The two-stage DC / DC converter according to claim 8, characterized in that: The fourth switch tube Qr1 and the fifth switch tube Qr2 both include parasitic diodes.

10. The two-stage DC / DC converter according to claim 8, characterized in that: The second switch tube Q2 and the third switch tube Q3 are open-loop controlled with fixed frequency and duty cycle.

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