A multi-level isolated step-down DC / DC converter

Abstract

The present invention provides a multi-level isolated step-down DC / DC converter, including two transformers, the primary side of the transformer is a single winding, the secondary side is a double winding, the primary sides of the two transformers are connected in series and then connected to an LLC resonant circuit to complete isolation and voltage regulation, the other side of the LLC resonant circuit is connected to a flying capacitor clamping five-level half-bridge switch network to reduce voltage stress, the secondary sides of the two transformers are sequentially connected to a rectifier bridge and a BUCK circuit, the output ends of the two BUCK circuits are staggered in parallel, and the Buck circuit is responsible for closed-loop voltage regulation to achieve high-gain step-down of the converter. Compared with the traditional LLC converter, the voltage stress of the present invention is significantly reduced, a wider voltage range input and high-gain step-down can be achieved, switching loss is significantly reduced, and converter efficiency is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics, and in particular to a multi-level isolated step-down DC / DC converter. Background Art

[0002] For traditional DC / DC single-stage and two-stage buck converters, excessively high switching voltage stress will lead to increased converter losses and further reduced efficiency. At the same time, in order to improve efficiency, the input voltage range is narrowed, and it cannot be used in situations with a wide voltage input range. Due to the above-mentioned shortcomings of traditional DC / DC buck converters, it is necessary to improve the converter topology architecture and simultaneously achieve the converter's wide input voltage range, low voltage stress, high gain buck and high efficiency characteristics.

[0003] For wide input range high gain step-down DC / DC converter topologies, there are isolated topologies and non-isolated topologies. For high voltage step-down of traditional non-isolated DC / DC converters, a lower duty cycle is required to obtain a higher voltage step-down gain. However, too low a duty cycle will affect the response speed of the DC / DC converter and increase the output noise, which will make the system unstable and have a negative impact on the signal quality. More importantly, it will cause extremely high switching voltage stress, resulting in increased converter losses, thereby reducing the efficiency of the power supply. Compared with non-isolated converters, isolated DC / DC converters have high efficiency conversion. The use of isolation design can improve the safety and reliability of equipment. In addition, isolated converters have high integration, which can help customers efficiently convert and manage converters from power supply to load points, and achieve long product life, low cost and intelligence. However, isolated converters also face some challenges that need to be solved, such as high voltage stress, narrow input voltage range, and low efficiency.

[0004] In order to overcome the above-mentioned shortcomings of the converter, further reduce voltage stress and reduce losses, a variety of voltage stress reduction technologies such as flying capacitor clamping, diode clamping, and switched capacitor and diode mixed clamping have emerged to optimize the converter topology. Summary of the invention

[0005] The invention provides a multi-level isolated step-down DC / DC converter, which realizes high efficiency while realizing high step-down gain of the converter, and has small voltage stress and wide input voltage range.

[0006] The technical solution to achieve the purpose of the present invention is: a multi-level isolated step-down DC / DC converter, including two transformers T1 and T2, the primary sides of the transformers T1 and T2 are single windings, and the secondary sides are double windings, the primary sides of the two transformers are connected in series to an LLC resonant circuit, the other side of the LLC resonant circuit is connected to a flying capacitor clamped five-level half-bridge switch network, the secondary sides of the two transformers are connected in sequence to a rectifier bridge and a BUCK circuit, and the output ends of the two BUCK circuits are staggered in parallel;

[0007] The flying capacitor clamped five-level half-bridge switch network includes switch tubes S1, S2, S3, S1', S2', S3' with reverse parallel diodes, a first flying capacitor C1 and a second flying capacitor C2, the switch tubes S1, S2, S3, S3', S2', S1' are connected in series to form a bridge arm, and the drain of S1 is connected to the input power supply V in The positive terminal of S1' is connected to the input power supply V in The negative electrode of the first flying capacitor C1, one end of which is connected to the connection point of the switch tubes S2 and S3, and the other end of which is connected to the connection point of the switch tubes S2' and S3'; one end of the second flying capacitor C2 is connected to the connection point of the switch tubes S1 and S2, and the other end of which is connected to the connection point of the switch tubes S1' and S2';

[0008] The LLC resonant circuit includes a first resonant inductor L r , the first resonant capacitor C r , the first excitation inductor L m1 , the second excitation inductor L m2 , the first resonant inductor L r One end is connected to the connection point of the switch tubes S3 and S3', and the first resonant inductor L r The other end is connected to the first resonant capacitor C r The positive electrode of the first resonant capacitor C r The negative electrode is connected to the primary end of the first transformer T1, and the first excitation inductor L m1 The first transformer T1 is connected in parallel with the primary side, one end of the primary side of the second transformer T2 is connected to the other end of the primary side of the first transformer T1, and the second excitation inductor L m2 Connected in parallel with the primary side of the second transformer T2, the other end of the primary side of the second transformer T2 is connected to the source of the switch tube S1';

[0009] The rectifier bridge connected to the secondary side of the transformer T1 includes a first diode D1, a second diode D2 and a first output capacitor C3. The secondary side of the first transformer T1 is two windings connected in series. The negative electrode of the first output capacitor C3 is connected to the common end of the two windings of the secondary side of the first transformer T1. The positive electrodes of the first diode D1 and the second diode D2 are respectively connected to the other ends of the two windings of the secondary side of the first transformer T1. The negative electrode of the first diode D1, the negative electrode of the second diode D2, and the positive electrode of the first output capacitor C3 are connected;

[0010] The Buck circuit connected to the secondary side of the transformer T1 includes switches S4 and S6 with reverse parallel diodes, a first output filter inductor L1, and an output filter capacitor Co. The drain of the switch S4 is connected to the cathode of the first diode D1, the cathode of the second diode D2, and the anode of the first output capacitor C3. The source of the switch S4 is connected to the drain of the switch S6 and one end of the first output filter inductor L1. The other end of the first output filter inductor L1 is connected to the anode of the output filter capacitor Co and the anode of the output power supply Vo. The cathode of the first output capacitor C3, the source of the switch S6, and the output filter capacitor C o The negative pole of the output power supply V o The common end of the negative electrodes of the two transformers is connected to the common end of the two secondary windings of the first transformer T1;

[0011] The rectifier bridge connected to the secondary side of the transformer T2 includes a third diode D3, a fourth diode D4 and a second output capacitor C4. The secondary side of the second transformer T2 is two windings connected in series. The negative electrode of the second output capacitor C4 is connected to the common end of the two windings of the secondary side of the second transformer T2. The positive electrodes of the third diode D3 and the fourth diode D4 are respectively connected to the other ends of the two windings of the secondary side of the second transformer T2. The negative electrode of the third diode D3, the negative electrode of the fourth diode D4, and the positive electrode of the second output capacitor C4 are connected;

[0012] The Buck circuit connected to the secondary side of the transformer T2 includes switches S5 and S7 with reverse parallel diodes, and a second output filter inductor L2. The drain of the switch S5 is connected to the cathode of the third diode D3, the cathode of the fourth diode D4, and the anode of the second output capacitor C4. The source of the switch S5 is connected to the drain of the switch S7 and one end of the second output filter inductor L2. The other end of the second output filter inductor L2 is connected to the other end of the first output filter inductor L1. The cathode of the second output capacitor C4, the source of the switch S7, and the output filter capacitor C o The negative pole of the output power supply V o The common end of the connected negative electrodes is connected to the common end of the two secondary windings of the second transformer T2.

[0013] Furthermore, the gates of the switch tubes S1, S2, S3, S1', S2', and S3' receive external control drive signals to control the converter to work in a frequency modulation mode. Half a switching cycle includes five working states, which are as follows:

[0014] Mode 1: Switches S1, S2, and S3 are turned on, i Lr (-)>i Lm1 / Lm2 (-), creating ZVS turn-on conditions for the converter;

[0015] Mode 2: Switches S1, S2, and S3 are turned on, i Lr (+)>i Lm1 / Lm2 (-), converter ZVS is turned on;

[0016] Mode 3: Switches S1, S2, and S3 are turned on, i Lr (+)>i Lm1 / Lm2 (+), creating ZCS shutdown conditions for the converter;

[0017] Mode 4: Switches S1, S2, and S3 are turned on, i Lr (+)=i Lm1 / Lm2 (+), the converter secondary side ZCS is turned off;

[0018] Mode 5: Switches S1, S2, and S3 are ready to turn off, and S1', S2', and S3' are ready to turn on. Lr (+)=i Lm1 / Lm2 (+), creating the ZVS start-up conditions for the converter.

[0019] Furthermore, the gates of the switch tubes S1, S2, S3, S1', S2', and S3' receive external control drive signals to control the converter to work in a phase-shift mode. Half a switching cycle includes four working states, which are as follows:

[0020] Mode 1: switch tube S1 is ready to turn on, switch tube S1' is ready to turn off, switch tubes S2' and S3 are turned on, i Lr (+)>i Lm1 / Lm2 (-), creating ZVS turn-on conditions for the converter;

[0021] Mode 2: Switches S1, S2', and S3 are turned on, i Lr (+)>i Lm1 / Lm2 (-), converter ZVS is turned on;

[0022] Mode 3: Switches S1, S2', and S3 are turned on, i Lr (+)=i Lm1 / Lm2 (-), the converter secondary side ZCS is turned off;

[0023] Mode 4: Switch S3' is ready to turn on, switch S3 is ready to turn off, switches S1 and S2' are turned on, i Lr (-) Lm1 / Lm2 (+), creating the ZVS start-up conditions for the converter.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] Compared with the traditional LLC+Buck cascade buck converter, the voltage stress is significantly reduced by replacing the half-bridge switch network of the front-stage LLC resonant converter with a five-level half-bridge switch network based on flying capacitor clamping. Compared with the traditional LLC converter, the voltage stress is reduced by 0.5 to 0.75 times, which can achieve a wider voltage range input and high gain buck, significantly reduce switching losses, and improve converter efficiency; at the same time, it has the advantages of high efficiency and high power density of the traditional LLC resonant converter and the advantages of good dynamic performance, precise voltage regulation, and small ripple of the Buck circuit. Based on the above advantages, the converter can be used in places that require a wide input range, low voltage stress, and high gain buck, and can achieve a higher buck gain at an appropriate duty cycle, and the converter loss is significantly reduced, with a higher conversion efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0027] Figure 1 A schematic diagram of a topological structure of a multi-level isolated step-down DC / DC converter provided in an embodiment of the present disclosure;

[0028] 2(a) to (e) are working mode diagrams of a multi-level isolated buck DC / DC converter in a frequency modulation mode provided by an embodiment of the present disclosure;

[0029] FIG3 (a) to (d) are working mode diagrams of a multi-level isolated buck DC / DC converter in a phase-shift mode provided by an embodiment of the present disclosure;

[0030] Figure 4 The topological structure diagram of the converter in which the previous five-level switch network is expanded to a 2n-order converter;

[0031] Figure 5 The topological structure diagram of the converter with the rear stage extended to m-phase interleaved parallel connection;

[0032] Figure 6 ​The schematic diagram of the converter topology structure is that the front-stage five-level switch network is expanded to 2n-level and the rear-stage is expanded to m-phase interleaved parallel. DETAILED DESCRIPTION

[0033] Embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although certain embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be construed as being limited to the embodiments described herein, which are instead provided for a more thorough and complete understanding of the present invention. It should be understood that the drawings and embodiments of the present invention are only for exemplary purposes and are not intended to limit the scope of protection of the present invention.

[0034] The technical solution of the present invention is described in detail with specific embodiments below. The following specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described in detail in some embodiments.

[0035] Figure 1 A multi-level isolated step-down DC / DC converter is provided in an embodiment of the present disclosure, such as Figure 1 As shown, it includes two transformers T1 and T2, the primary sides of the transformers T1 and T2 are single windings, and the secondary sides are double windings. The primary sides of the two transformers are connected in series to an LLC resonant circuit, and the other side of the LLC resonant circuit is connected to a flying capacitor clamped five-level half-bridge switch network. The secondary sides of the two transformers are connected to a rectifier bridge and a BUCK circuit in sequence, and the output ends of the two BUCK circuits are staggered in parallel;

[0036] The flying capacitor clamped five-level half-bridge switch network includes switch tubes S1, S2, S3, S1', S2', S3' with reverse parallel diodes, a first flying capacitor C1 and a second flying capacitor C2. The switch tubes S1, S2, S3, S3', S2', and S1' are connected in series in sequence as bridge arms, and the drain of S1 is connected to the input power supply V in The positive terminal of S1' is connected to the input power supply V in The negative electrode of the first flying capacitor C1, one end of which is connected to the connection point of the switch tubes S2 and S3, and the other end of which is connected to the connection point of the switch tubes S2' and S3'; one end of the second flying capacitor C2 is connected to the connection point of the switch tubes S1 and S2, and the other end of which is connected to the connection point of the switch tubes S1' and S2';

[0037] The LLC resonant circuit includes a first resonant inductor L r , the first resonant capacitor C r , the first excitation inductor L m1 , the second excitation inductor L m2 , the first resonant inductor L r One end is connected to the connection point of the switch tubes S3 and S3', and the first resonant inductor L rThe other end is connected to the first resonant capacitor C r The positive electrode of the first resonant capacitor C r The negative electrode is connected to the primary end of the first transformer T1, and the first excitation inductor L m1 The first transformer T1 is connected in parallel with the primary side, one end of the primary side of the second transformer T2 is connected to the other end of the primary side of the first transformer T1, and the second excitation inductor L m2 Connected in parallel with the primary side of the second transformer T2, the other end of the primary side of the second transformer T2 is connected to the source of the switch tube S1';

[0038] The rectifier bridge connected to the secondary side of the transformer T1 includes a first diode D1, a second diode D2 and a first output capacitor C3. The secondary side of the first transformer T1 is two windings connected in series. The cathode of the first output capacitor C3 is connected to the common end of the two windings of the secondary side of the first transformer T1. The anodes of the first diode D1 and the second diode D2 are respectively connected to the other ends of the two windings of the secondary side of the first transformer T1. The cathode of the first diode D1, the cathode of the second diode D2 and the anode of the first output capacitor C3 are connected.

[0039] The Buck circuit connected to the secondary side of the transformer T1 includes switches S4 and S6 with reverse parallel diodes, a first output filter inductor L1, and an output filter capacitor Co. The drain of the switch S4 is connected to the cathode of the first diode D1, the cathode of the second diode D2, and the anode of the first output capacitor C3. The source of the switch S4 is connected to the drain of the switch S6 and one end of the first output filter inductor L1. The other end of the first output filter inductor L1 is connected to the anode of the output filter capacitor Co and the anode of the output power supply Vo. The cathode of the first output capacitor C3, the source of the switch S6, and the output filter capacitor C o The negative pole of the output power supply V o The common end of the negative electrodes of the two transformers is connected to the common end of the two secondary windings of the first transformer T1;

[0040] The rectifier bridge connected to the secondary side of the transformer T2 includes a third diode D3, a fourth diode D4 and a second output capacitor C4. The secondary side of the second transformer T2 is two windings connected in series. The cathode of the second output capacitor C4 is connected to the common end of the two windings of the secondary side of the second transformer T2. The anodes of the third diode D3 and the fourth diode D4 are respectively connected to the other ends of the two windings of the secondary side of the second transformer T2. The cathode of the third diode D3, the cathode of the fourth diode D4 and the anode of the second output capacitor C4 are connected.

[0041] The Buck circuit connected to the secondary side of the transformer T2 includes switches S5 and S7 with reverse parallel diodes, and a second output filter inductor L2. The drain of the switch S5 is connected to the cathode of the third diode D3, the cathode of the fourth diode D4, and the anode of the second output capacitor C4. The source of the switch S5 is connected to the drain of the switch S7 and one end of the second output filter inductor L2. The other end of the second output filter inductor L2 is connected to the other end of the first output filter inductor L1. The cathode of the second output capacitor C4, the source of the switch S7, and the output filter capacitor C o The negative pole of the output power supply V o The common end of the connected negative electrodes is connected to the common end of the two secondary windings of the second transformer T2.

[0042] The multi-level isolated step-down DC / DC converter of this embodiment realizes the on and off of the switches S1, S2, S3, S1', S2', and S3' by receiving an external control drive signal, thereby controlling the output voltage of the converter. The converter can operate in a frequency modulation mode and a phase shift mode, as follows:

[0043] When the converter works in frequency modulation mode, half a switching cycle includes 5 working states, as follows:

[0044] Mode 1: As shown in Figure 2(a), switches S1, S2, and S3 are turned on, i Lr (-)>i Lm1 / Lm2 (-), creating ZVS turn-on conditions for the converter;

[0045] Mode 2: As shown in Figure 2(b), switches S1, S2, and S3 are turned on, i Lr (+)>i Lm1 / Lm2 (-), converter ZVS is turned on;

[0046] Mode 3: As shown in Figure 2(c), switches S1, S2, and S3 are turned on, i Lr (+)>i Lm1 / Lm2 (+), creating ZCS shutdown conditions for the converter;

[0047] Mode 4: As shown in Figure 2(d), switches S1, S2, and S3 are turned on, i Lr (+)=i Lm1 / Lm2 (+), the converter secondary side ZCS is turned off;

[0048] Mode 5: As shown in Figure 2(e), the switches S1, S2, and S3 are ready to turn off, and S1', S2', and S3' are ready to turn on. Lr (+)=i Lm1 / Lm2 (+), creating ZVS turn-on conditions for the converter;

[0049] When the converter works in phase-shift mode, half a switching cycle includes four working states, as follows:

[0050] Mode 1: As shown in Figure 3(a), switch S1 is ready to turn on, switch S1' is ready to turn off, switch S2' and S3 are turned on, i Lr (+)>i Lm1 / Lm2 (-), creating ZVS turn-on conditions for the converter;

[0051] Mode 2: As shown in Figure 3(b), switches S1, S2', and S3 are turned on, i Lr (+)>i Lm1 / Lm2 (-), converter ZVS is turned on;

[0052] Mode 3: As shown in Figure 3(c), switches S1, S2', and S3 are turned on, i Lr (+)=i Lm1 / Lm2 (-), the converter secondary side ZCS is turned off;

[0053] Mode 4: As shown in Figure 3(d), the switch tube S3' is ready to turn on, the switch tube S3 is ready to turn off, the switch tubes S1 and S2' are turned on, i Lr (-) Lm1 / Lm2 (+), creating the ZVS start-up conditions for the converter.

[0054] Through analysis and derivation, the voltage gain M of the converter is obtained as:

[0055]

[0056] Where D is the duty cycle of the Buck circuit, K is the inductance ratio, and f n is the normalized frequency, Q is the quality factor;

[0057] The converter voltage stress is:

[0058]

[0059]

[0060] ​Compared with the traditional LLC+Buck cascade buck converter, the voltage stress is significantly reduced by replacing the half-bridge switch network of the front-stage LLC resonant converter with a five-level half-bridge switch network based on flying capacitor clamping. Compared with the traditional LLC converter, the voltage stress is reduced by 0.5 to 0.75 times, which can achieve a wider voltage range input and high gain buck, significantly reduce switching losses, and improve converter efficiency; at the same time, it has the advantages of high efficiency and high power density of the traditional LLC resonant converter and the advantages of good dynamic performance, precise voltage regulation, and small ripple of the Buck circuit. Based on the above advantages, the converter can be used in places that require a wide input range, low voltage stress, and high gain buck, and can achieve a higher buck gain at an appropriate duty cycle, and the converter loss is significantly reduced, with a higher conversion efficiency.

[0061] In practical applications, when a higher voltage drop gain is required, the converter can be expanded according to the site requirements. There are three converter expansion methods:

[0062] 1. The five-level switch network in the front stage of the proposed converter topology is expanded to 2n levels, increasing to 2n flying capacitors, and the number of switches is increased to 2n, which becomes a flying capacitor clamped 2n+1 level switch network, such as Figure 4 shown.

[0063] At this time, the voltage stress of the converter is:

[0064]

[0065] 2. Expand the proposed converter topology to an m-phase interleaved parallel connection, such as Figure 5 shown.

[0066] 3. The five-level switch network in the front stage of the proposed converter topology is expanded to 2n-level, and the back stage is expanded to m-phase interleaved parallel, such as Figure 6 shown.

[0067] At this time, the voltage stress of the converter is:

[0068]

Claims

1. A multi-level isolated step-down DC / DC converter, characterized in that: It comprises two transformers T1 and T2, wherein the primary sides of the transformers T1 and T2 are single windings and the secondary sides are double windings, the primary sides of the two transformers are connected in series to an LLC resonant circuit, the other side of the LLC resonant circuit is connected to a flying capacitor clamped five-level half-bridge switch network, the secondary sides of the two transformers are connected in sequence to a rectifier bridge and a BUCK circuit, and the output ends of the two BUCK circuits are staggered in parallel; The flying capacitor clamped five-level half-bridge switch network includes switch tubes S1, S2, S3, S1', S2', S3' with reverse parallel diodes, a first flying capacitor C1 and a second flying capacitor C2, the switch tubes S1, S2, S3, S3', S2', S1' are connected in series to form a bridge arm, and the drain of S1 is connected to the input power supply V in The positive terminal of S1' is connected to the input power supply V in The negative electrode of the first flying capacitor C1, one end of which is connected to the connection point of the switch tubes S2 and S3, and the other end of which is connected to the connection point of the switch tubes S2' and S3'; one end of the second flying capacitor C2 is connected to the connection point of the switch tubes S1 and S2, and the other end of which is connected to the connection point of the switch tubes S1' and S2'; The LLC resonant circuit includes a first resonant inductor L r , the first resonant capacitor C r , the first excitation inductor L m1 , the second excitation inductor L m2 , the first resonant inductor L r One end is connected to the connection point of the switch tubes S3 and S3', and the first resonant inductor L r The other end is connected to the first resonant capacitor C r The positive electrode of the first resonant capacitor C r The negative electrode is connected to the primary end of the first transformer T1, and the first excitation inductor L m1 The first transformer T1 is connected in parallel with the primary side, one end of the primary side of the second transformer T2 is connected to the other end of the primary side of the first transformer T1, and the second excitation inductor L m2 Connected in parallel with the primary side of the second transformer T2, the other end of the primary side of the second transformer T2 is connected to the source of the switch tube S1'; The rectifier bridge connected to the secondary side of the transformer T1 includes a first diode D1, a second diode D2 and a first output capacitor C3. The secondary side of the first transformer T1 is two windings connected in series. The negative electrode of the first output capacitor C3 is connected to the common end of the two windings of the secondary side of the first transformer T1. The positive electrodes of the first diode D1 and the second diode D2 are respectively connected to the other ends of the two windings of the secondary side of the first transformer T1. The negative electrode of the first diode D1, the negative electrode of the second diode D2, and the positive electrode of the first output capacitor C3 are connected; The Buck circuit connected to the secondary side of the transformer T1 includes switches S4 and S6 with reverse parallel diodes, a first output filter inductor L1, and an output filter capacitor Co. The drain of the switch S4 is connected to the cathode of the first diode D1, the cathode of the second diode D2, and the anode of the first output capacitor C3. The source of the switch S4 is connected to the drain of the switch S6 and one end of the first output filter inductor L1. The other end of the first output filter inductor L1 is connected to the anode of the output filter capacitor Co and the anode of the output power supply Vo. The cathode of the first output capacitor C3, the source of the switch S6, and the output filter capacitor C o The negative pole of the output power supply V o The common end of the negative electrodes of the two transformers is connected to the common end of the two secondary windings of the first transformer T1; The rectifier bridge connected to the secondary side of the transformer T2 includes a third diode D3, a fourth diode D4 and a second output capacitor C4. The secondary side of the second transformer T2 is two windings connected in series. The negative electrode of the second output capacitor C4 is connected to the common end of the two windings of the secondary side of the second transformer T2. The positive electrodes of the third diode D3 and the fourth diode D4 are respectively connected to the other ends of the two windings of the secondary side of the second transformer T2. The negative electrode of the third diode D3, the negative electrode of the fourth diode D4, and the positive electrode of the second output capacitor C4 are connected; The Buck circuit connected to the secondary side of the transformer T2 includes switches S5 and S7 with reverse parallel diodes, and a second output filter inductor L2. The drain of the switch S5 is connected to the cathode of the third diode D3, the cathode of the fourth diode D4, and the anode of the second output capacitor C4. The source of the switch S5 is connected to the drain of the switch S7 and one end of the second output filter inductor L2. The other end of the second output filter inductor L2 is connected to the other end of the first output filter inductor L1. The cathode of the second output capacitor C4, the source of the switch S7, and the output filter capacitor C o The negative pole of the output power supply V o The common end of the connected negative electrodes is connected to the common end of the two secondary windings of the second transformer T2.

2. The multi-level isolated step-down DC / DC converter according to claim 1, characterized in that: The gates of the switch tubes S1, S2, S3, S1', S2', and S3' receive external control drive signals to control the converter to work in a frequency modulation mode. Half a switching cycle includes five working states, which are as follows: Mode 1: Switches S1, S2, and S3 are turned on, i Lr (-)>i Lm1 / Lm2 (-), creating ZVS turn-on conditions for the converter; Mode 2: Switches S1, S2, and S3 are turned on, i Lr (+)>i Lm1 / Lm2 (-), converter ZVS is turned on; Mode 3: Switches S1, S2, and S3 are turned on, i Lr (+)>i Lm1 / Lm2 (+), creating ZCS shutdown conditions for the converter; Mode 4: Switches S1, S2, and S3 are turned on, i Lr (+)=i Lm1 / Lm2 (+), the converter secondary side ZCS is turned off; Mode 5: Switches S1, S2, and S3 are ready to turn off, and S1', S2', and S3' are ready to turn on. Lr (+)=i Lm1 / Lm2 (+), creating the ZVS start-up conditions for the converter.

3. The multi-level isolated step-down DC / DC converter according to claim 1, characterized in that: The gates of the switch tubes S1, S2, S3, S1', S2', and S3' receive external control drive signals to control the converter to work in a phase shift mode. Half a switching cycle includes four working states, which are as follows: Mode 1: switch tube S1 is ready to turn on, switch tube S1' is ready to turn off, switch tubes S2' and S3 are turned on, i Lr (+)>i Lm1 / Lm2 (-), creating ZVS turn-on conditions for the converter; Mode 2: Switches S1, S2', and S3 are turned on, i Lr (+)>i Lm1 / Lm2 (-), converter ZVS is turned on; Mode 3: Switches S1, S2', and S3 are turned on, i Lr (+)=i Lm1 / Lm2 (-), the converter secondary side ZCS is turned off; Mode 4: Switch S3' is ready to turn on, switch S3 is ready to turn off, switches S1 and S2' are turned on, i Lr (-) Lm1 / Lm2 (+), creating the ZVS start-up conditions for the converter.​

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