Novel three-level LLC direct-current converter

Abstract

The invention provides a novel three-level LLC direct-current converter. The novel three-level LLC direct-current converter comprises a converter primary side and a converter secondary side, wherein the primary side of the converter comprises a first switching tube, a second switching tube, a third switching tube and a fourth switching tube; a first capacitor, a second capacitor, a third capacitor, a fourth capacitor and a fifth capacitor; a first freewheel diode and a second freewheel diode; a resonant inductor; a primary winding of the first isolation transformer and a primary winding of the second isolation transformer; the secondary side of the converter comprises a first rectifier diode, a second rectifier diode, a third rectifier diode, a fourth rectifier diode, a fifth rectifier diode and a sixth rectifier diode; a secondary winding of the first isolation transformer and a secondary winding of the second isolation transformer; and a sixth capacitor. According to the invention, the soft switching range of the switching tube can be widened, the primary side circulation loss can be reduced, filter inductance does not need to be output, the size of the converter is reduced, and the conversion efficiency of the direct-current converter is improved.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and in particular to a novel three-level LLC DC converter. Background Art

[0002] The DC converter is an important component of modern power electronic devices. It is widely used in auxiliary power supply fields such as communications, aerospace, transportation, and new energy charging. It is mainly used to achieve conversion of different voltage levels. In the case of high voltage input and low voltage output, the current doubler rectifier three-level DC converter has been widely used. This converter can reduce the current stress of the filter inductor and the output current ripple. This rectification method is widely used in low voltage and high current output occasions. Similarly, the LLC converter is a widely used type of DC converter. The LLC resonant converter has the advantages of high efficiency, high power density, and a small number of switches.

[0003] With the industry's requirements for energy conservation and emission reduction, power electronic devices are also developing towards high efficiency, high power density, and high reliability. However, the traditional three-level DC converter has the following significant disadvantages in actual work: the soft switching range of the switch tube is narrow, and the light load efficiency is severely attenuated; there is circulating current loss on the primary side, which increases the filter requirements and conduction loss of the converter, which is not conducive to further improving the power density. The traditional LLC resonant converter usually has the following disadvantages: the switching frequency needs to be changed with the input voltage transformation, which is not conducive to the design of closed-loop control, and the converter may be out of control when lightly loaded, and is not suitable for wide-range output. Therefore, a new type of DC converter is needed to solve the deficiencies in the prior art. Summary of the invention

[0004] In response to the problems in the prior art, the present application provides a novel three-level LLC DC converter, which can broaden the soft switching range of the switch tube, thereby helping to reduce the primary side circulating current loss, eliminate the need for output filter inductance, reduce the size of the converter, and improve the conversion efficiency of the DC converter.

[0005] In order to solve the above technical problems, this application provides the following technical solutions:

[0006] The present application provides a novel three-level LLC DC converter, comprising: a converter primary side and a converter secondary side;

[0007] Wherein, the primary side of the converter includes a first switch tube, a second switch tube, a third switch tube and a fourth switch tube; a first capacitor, a second capacitor, a third capacitor, a fourth capacitor and a fifth capacitor; a first freewheeling diode and a second freewheeling diode; a resonant inductor; a primary winding of a first isolation transformer and a primary winding of a second isolation transformer; wherein the same-name end of the primary winding of the first isolation transformer is connected to one end of the third capacitor; the non-same-name end of the primary winding of the first isolation transformer is connected to the connection point of the first freewheeling diode and the second freewheeling diode; the same-name end of the primary winding of the second isolation transformer is connected to one end of the resonant inductor, and the non-same-name end of the primary winding of the second isolation transformer is connected to the connection point of the fourth capacitor and the fifth capacitor; the other end of the resonant inductor is connected to the connection point of the second switch tube and the third switch tube; the other end of the fourth capacitor is connected to the connection point of the second switch tube and the cathode of the first freewheeling diode; the other end of the fifth capacitor is connected to the connection point of the fourth switch tube and the anode of the second freewheeling diode;

[0008] The secondary side of the converter includes a first rectifier diode, a second rectifier diode, a third rectifier diode, a fourth rectifier diode, a fifth rectifier diode and a sixth rectifier diode; a secondary winding of the first isolation transformer and a secondary winding of the second isolation transformer; a sixth capacitor; wherein the two ends of the secondary winding of the first isolation transformer are respectively connected to the connection point of the first rectifier diode and the fourth rectifier diode and the connection point of the second rectifier diode and the fifth rectifier diode; the two ends of the secondary winding of the second isolation transformer are respectively connected to the connection point of the second rectifier diode and the fifth rectifier diode and the connection point of the third rectifier diode and the sixth rectifier diode;

[0009] The other ends of the first rectifier diode, the second rectifier diode, and the third rectifier diode are all connected to one end of the sixth capacitor as the positive end of the output voltage; the other ends of the fourth rectifier diode, the fifth rectifier diode, and the sixth rectifier diode are all connected to one end of the sixth capacitor as the negative end of the output voltage.

[0010] Furthermore, the first capacitor, the second capacitor, the first switch tube, the second switch tube, the third switch tube and the fourth switch tube constitute a three-level integrated half-bridge inverter circuit; the three-level integrated half-bridge inverter circuit is connected in parallel with a DC power supply in a forward direction.

[0011] Further, one end of the first capacitor is connected to the positive output end of the DC power supply, the other end of the first capacitor is connected to one end of the second capacitor, and the other end of the second capacitor is connected to the negative output end of the DC power supply.

[0012] Furthermore, the primary side of the converter also includes a DC blocking capacitor; one end of the DC blocking capacitor is connected to the first isolation transformer, and the other end is connected to the resonant inductor.

[0013] Furthermore, the secondary side of the converter also includes an output capacitor; after the output capacitor is connected in parallel with the secondary load, one end of the output capacitor is connected to the negative end of the third rectifier diode, and the other end is connected to the positive end of the sixth rectifier diode.

[0014] Furthermore, the novel three-level LLC DC converter has a first switching mode; wherein the first switch tube, the fourth switch tube, the first rectifier diode and the sixth rectifier diode are turned on; after a first preset time, the three-level LLC DC converter enters an energy transmission stage, and the first isolation transformer and the first isolation transformer both transmit energy to the output end of the three-level LLC DC converter; the voltage and excitation current of the third capacitor increase linearly, and the current on the primary side of the converter and the current on the secondary side of the converter are both constant values.

[0015] Furthermore, the novel three-level LLC DC converter has a second switching mode; wherein, at a second preset moment, the first switch tube is turned off, and the junction capacitance of the first switch tube and the fourth switch tube is linearly charged and discharged; the primary voltage, primary current and secondary voltage and secondary current of the first isolation transformer all decrease.

[0016] Furthermore, the novel three-level LLC DC converter has a third switching mode; wherein, at a third preset moment, the first rectifier diode, the second rectifier diode and the sixth rectifier diode are turned on, and the junction capacitance of the first switch tube and the fourth switch tube completes charging and discharging.

[0017] Furthermore, the novel three-level LLC DC converter has a fourth switching mode; wherein, at a fourth preset moment, the fourth switch tube is turned on with zero voltage; the primary current of the first isolation transformer and the current of the third rectifier diode both decrease, and the current of the first rectifier diode increases; the first freewheeling diode starts to conduct; the voltage of the DC blocking capacitor acts on the first isolation transformer, causing the primary current of the first isolation transformer to decrease.

[0018] Furthermore, the novel three-level LLC DC converter has a fifth switching mode; wherein, at a fifth preset moment, the primary current of the first isolation transformer drops to zero, the first rectifier diode is turned off, and only the second isolation transformer transmits power to the secondary side.

[0019] Furthermore, the novel three-level LLC DC converter has a sixth switching mode; wherein, at a sixth preset moment, the current flowing through the resonant inductor drops to be equal to the transformer excitation current, entering a non-duty cycle period of the converter, the primary side of the second isolation transformer resonates, and the energy consumed by the secondary side load is provided by the output capacitor.

[0020] Furthermore, the novel three-level LLC DC converter has a seventh switching mode; wherein, at the seventh preset moment, the second switch tube is turned off, and the junction capacitance between the second switch tube and the third switch tube is linearly charged and discharged under the action of inductive reactance.

[0021] In response to the problems in the prior art, the novel three-level LLC DC converter provided in the present application forms a novel LLC structure by sharing two switching tubes, a freewheeling capacitor, an introduced auxiliary transformer and a resonant inductor of the three-level converter; the integrated LLC structure solves the circulating current problem, and can effectively reduce the output ripple and improve the power density of the converter while also improving the efficiency when only a filter capacitor structure is used, and can achieve zero voltage switching within a wide load range and operate within a wide input voltage variation range. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0023] Figure 1 A circuit connection diagram of a novel three-level LLC DC converter according to an embodiment of the present application;

[0024] Figure 2 A simplified equivalent circuit diagram of an embodiment of the present application;

[0025] Figure 3 The present invention provides Figure 2 Schematic diagram of the main working waveforms;

[0026] Figures 4 to 10 The present invention provides Figure 2 Equivalent circuit diagrams in different modes. DETAILED DESCRIPTION

[0027] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0028] In one embodiment, see Figure 1 , in order to widen the soft switching range of the switch tube, thereby helping to reduce the primary side circulation loss, eliminating the need for output filter inductance, reducing the converter volume, and improving the conversion efficiency of the DC converter, the present application provides a novel three-level LLC DC converter, including: a converter primary side and a converter secondary side;

[0029] Among them, the primary side of the converter includes a first switch tube, a second switch tube, a third switch tube and a fourth switch tube; a first capacitor, a second capacitor, a third capacitor, a fourth capacitor and a fifth capacitor; a first freewheeling diode and a second freewheeling diode; a resonant inductor; a primary winding of a first isolation transformer and a primary winding of a second isolation transformer; wherein the same-name end of the primary winding of the first isolation transformer is connected to one end of the third capacitor; the non-same-name end of the primary winding of the first isolation transformer is connected to the connection point of the first freewheeling diode and the second freewheeling diode; the same-name end of the primary winding of the second isolation transformer is connected to one end of the resonant inductor, and the non-same-name end of the primary winding of the second isolation transformer is connected to the connection point of the fourth capacitor and the fifth capacitor; the other end of the resonant inductor is connected to the connection point of the second switch tube and the third switch tube; the other end of the fourth capacitor is connected to the connection point of the second switch tube and the cathode of the first freewheeling diode; the other end of the fifth capacitor is connected to the connection point of the fourth switch tube and the anode of the second freewheeling diode;

[0030] The secondary side of the converter includes a first rectifier diode, a second rectifier diode, a third rectifier diode, a fourth rectifier diode, a fifth rectifier diode and a sixth rectifier diode; a secondary winding of a first isolation transformer and a secondary winding of a second isolation transformer; a sixth capacitor; wherein the two ends of the secondary winding of the first isolation transformer are respectively connected to the connection point of the first rectifier diode and the fourth rectifier diode and the connection point of the second rectifier diode and the fifth rectifier diode; the two ends of the secondary winding of the second isolation transformer are respectively connected to the connection point of the second rectifier diode and the fifth rectifier diode and the connection point of the third rectifier diode and the sixth rectifier diode;

[0031] The other ends of the first rectifier diode, the second rectifier diode, and the third rectifier diode are all connected to one end of the sixth capacitor as the positive end of the output voltage; the other ends of the fourth rectifier diode, the fifth rectifier diode, and the sixth rectifier diode are all connected to one end of the sixth capacitor as the negative end of the output voltage.

[0032] In one embodiment, the first capacitor, the second capacitor, the first switch tube, the second switch tube, the third switch tube and the fourth switch tube constitute a three-level integrated half-bridge inverter circuit; the three-level integrated half-bridge inverter circuit is connected in parallel with the DC power supply in the forward direction.

[0033] In one embodiment, one end of the first capacitor is connected to the positive output end of the DC power supply, the other end of the first capacitor is connected to one end of the second capacitor, and the other end of the second capacitor is connected to the negative output end of the DC power supply.

[0034] In one embodiment, the primary side of the converter further includes a DC blocking capacitor; one end of the DC blocking capacitor is connected to the first isolation transformer, and the other end is connected to the resonant inductor.

[0035] In one embodiment, the secondary side of the converter further includes an output capacitor; after the output capacitor is connected in parallel with the secondary load, one end of the output capacitor is connected to the negative end of the third rectifier diode, and the other end is connected to the positive end of the sixth rectifier diode.

[0036] It should be noted that Figure 1 The circuit connection diagram of a novel three-level LLC DC converter provided by the present invention. Wherein, Vin is a DC power supply, Q1, Q2, Q3, Q4 are the first to fourth switch tubes, and their junction capacitances are C1, C2, C3, C4, respectively. d1 , C d2 , C b , C ss1 , C ss2 , C o are the first to sixth capacitors, T r1 、T r2 are the first and second transformers, n p1 、n p2 They are the primary windings of the first and second transformers, n s1 、n s2 is the secondary winding of the first transformer, D1 and D2 are the first and second freewheeling diodes respectively, D1, D2, D3, D4, D5 and D6 are the first to sixth rectifier diodes respectively, L r is the first resonant inductor, and R is the load resistance.

[0037] It can be understood that the first and second capacitors and the first to fourth switching tubes constitute a three-level integrated half-bridge inverter circuit, which is connected in parallel in the positive and negative output ends of the DC power supply; the first transformer includes a primary winding and a secondary winding, the same-name end of the primary winding of the first transformer is connected to one end of the third capacitor, and the non-same-name end is connected to the connection point of the first freewheeling diode and the second freewheeling diode, forming the primary structure of the three-level converter; the two ends of the secondary winding of the first transformer are respectively connected to the connection point of the first rectifier diode and the fourth rectifier diode and the connection point of the second rectifier diode and the fifth rectifier diode, forming a full-bridge rectifier structure; the second transformer includes a primary winding and a secondary winding, the same-name end of the primary winding of the second transformer is connected to one end of the first resonant inductor, and the non-same-name end is connected to the connection point of the fourth and fifth capacitors. The first resonant inductor and the second resonant inductor are connected to form the primary part of the LLC resonant circuit, the other end of the first resonant inductor is connected to the connection point of the second and third switch tubes, and the two ends of the secondary winding of the second transformer are respectively connected to the connection point of the second rectifier diode and the fifth rectifier diode and the connection point of the third rectifier diode and the sixth rectifier diode to form a full-bridge rectifier structure; the other end of the fourth capacitor is connected to the connection point of the cathode of the second switch tube and the first freewheeling diode, and the other end of the fifth capacitor is connected to the connection point of the anode of the fourth switch tube and the second freewheeling diode. The fourth and fifth capacitors integrate a half-bridge structure in the three-level converter; the other ends of the first, second and third rectifier diodes are connected to one end of the sixth capacitor as the positive end of the output voltage, and the other ends of the fourth, fifth and sixth rectifier diodes are connected to one end of the sixth capacitor as the negative end of the output voltage.

[0038] Specifically, C. d1 , C d2 , Q1~Q4 switch tube, T r1 , C b and D R1 , D R2 It forms the primary side structure of the traditional three-level converter, by sharing Q2, Q3 and T r2 , C ss1 , C ss2 , forming an LLC structure. On the secondary side, the rectifier circuit consists of T r1 、T r2 It consists of the secondary winding, D1, D2, D3, D4, D5, and D6.

[0039] Below Figure 2 The simplified equivalent circuit, combined with the attached Figures 3 to 10 The specific working principle of the novel three-level LLC DC converter provided by the present application is described. Figure 3It can be seen that the entire converter has 14 switching modes in one switching cycle, namely [t0~t1], [t1~t2], [t2~t3], [t3~t4], [t4~t5], [t5~t6], [t6~t7], [t7~t8], [t8~t9], [t9~t 10 ]、[t 10 ~t 11 ]、[t 11 ~t 12 ]、[t 12 ~t 13 ]、[t 13 ~t 14 ]. Among them, [t0~t7] is the first half cycle, [t8~t 14 ] is the second half cycle, and the working conditions of each switching mode are analyzed in detail below.

[0040] To simplify the analysis, the following assumptions are made: 1) All devices are ideal devices; 2) The parasitic devices of the switch tube only consider the body diode and junction capacitance; 3) T r1 、T r2 The transformer's magnetizing inductance and leakage inductance; 4) Output capacitor C o It is equivalent to a constant pressure source.

[0041] In one embodiment, a novel three-level LLC DC converter has a first switching mode; wherein the first switch tube, the fourth switch tube, the first rectifier diode and the sixth rectifier diode are turned on; after a first preset time, the three-level LLC DC converter enters an energy transmission stage, and the first isolation transformer and the first isolation transformer both transmit energy to the output end of the three-level LLC DC converter; the voltage and excitation current of the third capacitor increase linearly, and the current on the primary side of the converter and the current on the secondary side of the converter are both constant values.

[0042] Understandably, see Figure 4 , switching mode 1 [t0~t1]: Q1, Q4, D1, D6 are turned on. After t0, it enters the energy transfer stage. The three-level transformer T r1 and LLC transformer T r2 Both transfer energy to the output side. The primary capacitor resonates with the resonant inductor, and the DC blocking capacitor C b The voltage and excitation current increase linearly, and the primary and secondary currents are approximately constant.

[0043] In one embodiment, a novel three-level LLC DC converter has a second switching mode; wherein, at a second preset moment, the first switch tube is turned off, and the junction capacitance of the first switch tube and the fourth switch tube is linearly charged and discharged; the primary voltage, primary current, secondary voltage, and secondary current of the first isolation transformer all decrease.

[0044] Understandably, see Figure 5 , Switching mode 2 [t1~t2]: At t1, Q1 is turned off. Since the filter inductor is mapped to the primary side, the junction capacitance of Q1 and Q4 is linearly charged and discharged. r1 The transformer primary and secondary voltages begin to drop, and the primary and secondary currents begin to drop.

[0045] In one embodiment, a novel three-level LLC DC converter has a third switching mode; wherein, at a third preset moment, the first rectifier diode, the second rectifier diode and the sixth rectifier diode are turned on, and the junction capacitance of the first switch tube and the fourth switch tube completes charging and discharging.

[0046] Understandably, see Figure 6 , Switching mode 3 [t2~t3]: At t2, D2 is turned on, and D1 and D6 are turned on at the same time, T r1 The voltage across the secondary winding is clamped and short-circuited by D1 and D2, the current of D1 decreases, and the current of D2 increases. The junction capacitance of Q1 and Q4 is charged and discharged, creating conditions for soft switching.

[0047] In one embodiment, a novel three-level LLC DC converter has a fourth switching mode; wherein, at a fourth preset moment, the fourth switch tube is turned on with zero voltage; the primary current of the first isolation transformer and the current of the third rectifier diode both decrease, and the current of the first rectifier diode increases; the first freewheeling diode starts to conduct; the voltage of the DC blocking capacitor acts on the first isolation transformer, causing the primary current of the first isolation transformer to decrease.

[0048] Understandably, see Figure 7 , Switching mode 4 [t3~t4]: At t3, Q4 is turned on with zero voltage. r1 The primary current and the current of D3 continue to decrease, and the current of D1 continues to increase. R1 Starts to conduct. DC blocking capacitor C b The voltage applied to T r1 On, make T r1 The primary current drops rapidly and the circulating loss is greatly reduced.

[0049] In one embodiment, a novel three-level LLC DC converter has a fifth switching mode; wherein, at a fifth preset moment, the primary current of the first isolation transformer drops to zero, the first rectifier diode is turned off, and only the second isolation transformer transmits power to the secondary side.

[0050] Understandably, see Figure 8 , Switching mode 5 [t4~t5]: At time t4, T r1 The primary current drops to 0, and the rectifier diode D1 is turned off, leaving only T r2 Transfer power to the secondary side.

[0051] In one embodiment, a novel three-level LLC DC converter has a sixth switching mode; wherein, at a sixth preset moment, the current flowing through the resonant inductor drops to be equal to the transformer excitation current, entering a non-duty cycle period of the converter, the primary side of the second isolation transformer resonates, and the energy consumed by the secondary side load is provided by the output capacitor.

[0052] Understandably, see Fig. 9 , switching mode 6 [t5~t6]: At t5, the current i p2 The current drops to the same level as the transformer excitation current. At this time, the converter enters the non-duty cycle period. LLC resonance is generated on the primary side of Tr2, and the energy consumed by the secondary side load R is supplied by the output capacitor C o supply.

[0053] In one embodiment, the novel three-level LLC DC converter has a seventh switching mode; wherein, at the seventh preset moment, the second switch tube is turned off, and the junction capacitance between the second switch tube and the third switch tube is linearly charged and discharged under the action of inductive reactance.

[0054] Understandably, see Fig.10 , switching mode 7 [t6~t7]: At t6, Q2 is turned off, and the junction capacitance of Q2 and Q3 is linearly charged and discharged under the action of inductive reactance, providing the zero voltage turn-on condition for Q3 at the next moment.

[0055] The second half cycle [t7~t 14 The working principle of ] is basically the same as that of the first half cycle [t0~t7], except that the current and voltage are in opposite directions, so it will not be repeated.

[0056] From the above description, it can be seen that the three-level DC converter based on the integrated half-bridge current doubler rectifier proposed by the present invention has the following advantages: the converter can work in a wide input voltage range, and all switch tubes can realize soft switching in a wide load range; all switch tubes can realize zero voltage switching; the three-level and LLC hybrid structure greatly reduces the loss in circuit conversion and eliminates the primary side circulation loss phenomenon; at the same time, due to the characteristics of LLC, no output filter inductor is required; it is conducive to the design of magnetic components of the circuit. The transformer excitation current in the integrated half-bridge helps to broaden the soft switching range of the switch tube.

[0057] The various embodiments in this specification are described in a progressive manner, and the same or similar parts between the various embodiments can be referenced to each other, and each embodiment focuses on the differences from other embodiments.

[0058] The above is a description of a specific embodiment of the specification. Other embodiments are within the scope of the appended claims. In some cases, the actions or steps recorded in the claims can be performed in an order different from that in the embodiments and still achieve the desired results. In addition, the processes depicted in the drawings do not necessarily require the specific order or continuous order shown to achieve the desired results. In some embodiments, multitasking and parallel processing are also possible or may be advantageous.

[0059] Each embodiment in this specification is described in a progressive manner, and the same and similar parts between the embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments. In the description of this specification, the description of reference terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the embodiment of this specification. In this specification, the schematic representation of the above terms does not necessarily target the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples without contradiction.

[0060] The above is only an example of the embodiment of the present specification and is not intended to limit the embodiment of the present specification. For those skilled in the art, the embodiment of the present specification may have various changes and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the embodiment of the present specification shall be included in the scope of the claims of the embodiment of the present specification.

Claims

1. A novel three-level LLC DC converter, characterized in that: include: A converter primary side and a converter secondary side; Wherein, the primary side of the converter includes a first switch tube, a second switch tube, a third switch tube and a fourth switch tube; a first capacitor, a second capacitor, a third capacitor, a fourth capacitor and a fifth capacitor; a first freewheeling diode and a second freewheeling diode; a resonant inductor; a primary winding of a first isolation transformer and a primary winding of a second isolation transformer; wherein the same-name end of the primary winding of the first isolation transformer is connected to one end of the third capacitor; the non-same-name end of the primary winding of the first isolation transformer is connected to the connection point of the first freewheeling diode and the second freewheeling diode; the same-name end of the primary winding of the second isolation transformer is connected to one end of the resonant inductor, and the non-same-name end of the primary winding of the second isolation transformer is connected to the connection point of the fourth capacitor and the fifth capacitor; the other end of the resonant inductor is connected to the connection point of the second switch tube and the third switch tube; the other end of the fourth capacitor is connected to the connection point of the second switch tube and the cathode of the first freewheeling diode; the other end of the fifth capacitor is connected to the connection point of the fourth switch tube and the anode of the second freewheeling diode; The secondary side of the converter includes a first rectifier diode, a second rectifier diode, a third rectifier diode, a fourth rectifier diode, a fifth rectifier diode and a sixth rectifier diode; a secondary winding of the first isolation transformer and a secondary winding of the second isolation transformer; a sixth capacitor; wherein the two ends of the secondary winding of the first isolation transformer are respectively connected to the connection point of the first rectifier diode and the fourth rectifier diode and the connection point of the second rectifier diode and the fifth rectifier diode; the two ends of the secondary winding of the second isolation transformer are respectively connected to the connection point of the second rectifier diode and the fifth rectifier diode and the connection point of the third rectifier diode and the sixth rectifier diode; The other ends of the first rectifier diode, the second rectifier diode, and the third rectifier diode are all connected to one end of the sixth capacitor as the positive end of the output voltage; the other ends of the fourth rectifier diode, the fifth rectifier diode, and the sixth rectifier diode are all connected to one end of the sixth capacitor as the negative end of the output voltage.

2. The novel three-level LLC DC converter according to claim 1 is characterized in that: The first capacitor, the second capacitor, the first switch tube, the second switch tube, the third switch tube and the fourth switch tube constitute a three-level integrated half-bridge inverter circuit; the three-level integrated half-bridge inverter circuit is connected in parallel with a DC power supply in the forward direction.

3. The novel three-level LLC DC converter according to claim 2 is characterized in that: One end of the first capacitor is connected to the positive output end of the DC power supply, the other end of the first capacitor is connected to one end of the second capacitor, and the other end of the second capacitor is connected to the negative output end of the DC power supply.

4. The novel three-level LLC DC converter according to claim 1 is characterized in that: The primary side of the converter also includes a DC blocking capacitor; one end of the DC blocking capacitor is connected to the first isolation transformer, and the other end is connected to the resonant inductor.

5. The novel three-level LLC DC converter according to claim 1 is characterized in that: The secondary side of the converter also includes an output capacitor; after the output capacitor is connected in parallel with the secondary load, one end of the output capacitor is connected to the negative end of the third rectifier diode, and the other end is connected to the positive end of the sixth rectifier diode.

6. The novel three-level LLC DC converter according to claim 1 is characterized in that: It has a first switching mode; wherein the first switch tube, the fourth switch tube, the first rectifier diode and the sixth rectifier diode are turned on; after a first preset time, the three-level LLC DC converter enters the energy transmission stage, and the first isolation transformer and the first isolation transformer both transmit energy to the output end of the three-level LLC DC converter; the voltage and excitation current of the third capacitor increase linearly, and the current on the primary side of the converter and the current on the secondary side of the converter are both constant values.

7. The novel three-level LLC DC converter according to claim 1 is characterized in that: It has a second switching mode; wherein, at a second preset moment, the first switch tube is turned off, and the junction capacitance of the first switch tube and the fourth switch tube is linearly charged and discharged; the primary voltage, primary current and secondary voltage and secondary current of the first isolation transformer all decrease.

8. The novel three-level LLC DC converter according to claim 1 is characterized in that: It has a third switching mode; wherein, at a third preset moment, the first rectifier diode, the second rectifier diode and the sixth rectifier diode are turned on, and the junction capacitance of the first switch tube and the fourth switch tube completes charging and discharging.

9. The novel three-level LLC DC converter according to claim 4 is characterized in that: It has a fourth switching mode; wherein, at a fourth preset moment, the fourth switch tube is turned on with zero voltage; the primary current of the first isolation transformer and the current of the third rectifier diode both decrease, and the current of the first rectifier diode increases; the first freewheeling diode starts to conduct; the voltage of the DC blocking capacitor acts on the first isolation transformer, causing the primary current of the first isolation transformer to decrease.

10. The novel three-level LLC DC converter according to claim 1, characterized in that: It has a fifth switching mode; wherein, at a fifth preset moment, the primary current of the first isolation transformer drops to zero, the first rectifier diode is turned off, and only the second isolation transformer transmits power to the secondary side.

11. The novel three-level LLC DC converter according to claim 5, characterized in that: It has a sixth switching mode; wherein, at a sixth preset moment, the current flowing through the resonant inductor drops to be equal to the transformer excitation current, entering a non-duty cycle period of the converter, the primary side of the second isolation transformer resonates, and the energy consumed by the secondary side load is provided by the output capacitor.

12. The novel three-level LLC DC converter according to claim 1, characterized in that: It has a seventh switching mode; wherein, at a seventh preset moment, the second switch tube is turned off, and the junction capacitance between the second switch tube and the third switch tube is linearly charged and discharged under the action of inductive reactance.

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