CN111181407B - C-LLCT-LLT type resonance direct current converter - Google Patents

Abstract

The invention discloses a C-LLCT-LLT resonant DC converter, comprising a voltage-type half-bridge inverter circuit, a C-LLCT-LLT resonant cavity and a full-bridge rectifier circuit connected in sequence. The C-LLCT-LLT resonant cavity includes The first phase resonant circuit and the second phase resonant circuit, the first phase resonant circuit includes the first series branch inductance, the first parallel branch inductance, the first parallel branch capacitor and the first high frequency transformer, the second phase resonant circuit It includes a second series branch inductance, a second parallel branch inductance and a second high-frequency transformer. The input end of the first series branch inductance and the input end of the second series branch inductance are both connected to one end of the series branch capacitor. The other end of the branch capacitor is connected to the output port of the half-bridge inverter circuit, and the secondary windings of the first high-frequency transformer and the second high-frequency transformer are connected in series with the input port of the full-bridge rectifier circuit. The present invention has a wider voltage gain adjustment range and lower resonance capacitor voltage stress.

Description

A C-LLCT-LLT Resonant DC Converter

technical field

The invention belongs to the technical field of DC converters, in particular to a C-LLCT-LLT type resonant DC converter.

Background technique

In recent years, related technologies in the field of server data power systems have developed vigorously due to the continuous increase in demand. In these power supply systems, since the AC voltage generated by the AC grid is rectified and active power factor correction, the output DC voltage is often significantly higher than the voltage level at which the server data power supply works, so a high-voltage step-down power supply is often required. The DC-DC converter of the ratio converts the voltage efficiently. However, in order to prevent data loss caused by system power failure, it is necessary to ensure that the DC voltage finally provided to the data power supply of the server can be protected from the influence of the bus voltage drop to the maximum extent. For this reason, it is often necessary to connect a large-capacity stabilizing capacitor in parallel with the voltage input side of the server data power supply to suppress the voltage drop, so that the uninterruptible power supply (UPS) can provide backup protection for the system in time. Since the capacitance of the stabilizing capacitor is positively correlated with the volume, a substantial increase in the capacitance of the capacitor can suppress the possibility of server data power failure to a certain extent, but it will also bring about a substantial increase in the overall volume of the system. The cost also increases. For this reason, in recent years, most scholars have turned their attention from the stabilizing capacitor in the system to the DC-DC converter that provides voltage conversion, hoping to widen the voltage gain range of the DC-DC converter to maintain its output voltage from the bus voltage drop. Influence, and then improve the reliability of the server data power system.

Among the DC-DC converters used for this scenario, LLC-type resonant converters are widely used due to their excellent overall performance. The LLC converter can realize zero-voltage turn-on (ZVS) of the inverter switch tube and zero-current turn-off (ZCS) of the rectifier diode in a wide load range, thereby making the DC-DC voltage conversion process in the system obtain higher conversion efficiency. However, when the bus voltage drops, in order to ensure a higher voltage gain of the LLC converter, the inductance value of the parallel branch inductance (excitation inductance) needs to be as small as possible. Therefore, when the converter works in the normal state of the bus voltage and does not need high voltage gain, the reduction of the resonant cavity impedance caused by the low inductance value of the parallel branch inductance will lead to an increase in the current in the resonant cavity, which in turn makes the converter Efficiency is reduced. To this end, some scholars have proposed a LCLC resonant converter in recent years. Compared with LLC type converters, the only difference is that the parallel branch inductance (excitation inductance) is replaced by a pair of series connected inductors and capacitors (ie, parallel branch inductance and parallel branch capacitance). When the switching frequency is higher than the resonant frequency between the inductor and capacitor, the inductor and capacitor will form a "variable magnetizing inductance". Since the capacitor can be regarded as a negative inductor, the absolute value of its inductance value is opposite to the direction of change of the switching frequency, so when the switching frequency is high, the inductance value of the "variable excitation inductance" will naturally be larger, and when the switching frequency is low The inductance value of "variable magnetizing inductance" will naturally be smaller. Therefore, when the bus voltage is in a normal state, the switching frequency is high due to the low gain requirement. In order to obtain higher efficiency, the switching frequency at this time can be set equal to the resonant frequency of the converter. Therefore, the gain at this time is different from the excitation inductance The sense value is irrelevant. Due to the large inductance value of the "variable excitation inductance" at this time, the converter can further improve the conversion efficiency due to the large resonant cavity impedance and low resonant current; and when the bus voltage drops, the switching frequency can be reduced. A very high voltage gain is obtained due to the reduction of the inductance value of the "variable excitation inductance" (due to the action of the uninterruptible power supply (UPS), the high-gain state of the converter caused by the drop of the bus voltage is very short, so the bus voltage The efficiency of the converter in the drop state can be disregarded). To this end, it can be seen that the LCLC converter can obtain dual advantages in terms of efficiency and voltage gain.

When the bus voltage drops and the switching frequency decreases, so that the inductance value of the "variable excitation inductance" decreases, for the series branch capacitor in the LCLC converter, even if the input impedance of the resonant cavity decreases, due to the input The voltage drops at the same time, so that the current flowing through the series branch capacitor does not increase significantly, so when the bus voltage drops, the voltage stress of the series branch capacitor does not increase significantly. However, in order to maintain the same output voltage, the reduction of the inductance value of the "variable magnetizing inductance" will directly lead to a significant increase in the current flowing through the parallel branch capacitor. In addition, in order to ensure that the inductance value of the "variable excitation inductance" changes significantly in the process of reducing the switching frequency, so that the gain can be improved significantly, the capacitance value of the parallel branch capacitor will be as small as possible. Therefore, both factors will lead to a sharp increase in the voltage stress of the parallel branch capacitor. Since too high capacitor voltage stress will reduce the reliability of the converter, the voltage stress of the parallel branch capacitors has become a major factor restricting the performance of traditional LCLC converters.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a C-LLCT-LLT resonant DC converter in view of the deficiencies of the prior art, which can obtain wide voltage gain characteristics consistent with the LCLC converter, and can also avoid parallel connection under the condition of bus voltage drop. At the same time, the winding loss of the magnetic element when the converter works in the normal state of the bus voltage is further reduced by means of parallel shunt, and the power conversion efficiency is improved.

In order to achieve the above object, the present invention adopts the following technical solutions:

A C-LLCT-LLT resonant DC converter, comprising a voltage-type half-bridge inverter circuit, a C-LLCT-LLT resonant cavity and a full-bridge rectifier circuit connected in sequence, the C-LLCT-LLT resonant cavity comprising a first A phase resonant tank and a second phase resonant tank, the first phase resonant tank includes a first series branch inductance L _r1 , a first parallel branch inductance L _p1 , a first parallel branch capacitor C _p1 and a first high frequency transformer T ₁ , the second phase resonant circuit includes a second series branch inductance L _r2 , a second parallel branch inductance L _m2 and a second high frequency transformer T ₂ , the input end of the first series branch inductance L _r1 The input end of the second series branch inductor L _r2 is both connected to one end of the series branch capacitor _Cr , and the other end of the series branch capacitor _Cr is connected to the output port of the half-bridge inverter circuit, so The secondary windings of the _first high-frequency transformer T1 and the _second high-frequency transformer T2 are connected in series with the input port of the full-bridge rectifier circuit.

As an improvement of the C-LLCT-LLT resonant DC converter described in the present invention, the half-bridge inverter circuit includes an input voltage stabilization capacitor C _i , a first high-frequency power switching device S ₁ and a second The high frequency power switching device S ₂ , the input voltage stabilization capacitor C _i , the first high frequency power switching device S ₁ and the second high frequency power switching device S ₂ form a loop.

As an improvement of the C-LLCT-LLT resonant DC converter described in the present invention, the output port of the half-bridge inverter circuit is set at the first high-frequency power switching device S1 and the _first high-frequency power switching device S1 and the second between _two high-frequency power switching devices S2.

As an improvement of the C-LLCT-LLT resonant DC converter described in the present invention, the full-bridge rectifier circuit includes a first diode D ₁ , a second diode D ₂ , a third diode D 1 , and a third diode D 2 . tube D ₃ , the fourth diode D ₄ and the output voltage regulator filter capacitor C _o , the first diode D ₁ , the second diode D ₂ , the third diode D ₃ and The fourth diode D ₄ forms a loop, and the output voltage stabilization filter capacitor C _o is connected in parallel with the third diode D ₃ and the fourth diode D ₄ .

As an improvement of the C-LLCT-LLT resonant DC converter of the present invention, the full bridge is provided between the first diode D ₁ and the second diode D ₂ The first input port of the rectifier circuit, the first input port is connected to the secondary winding of the _first high-frequency transformer T1.

As an improvement of the C-LLCT-LLT resonant DC converter of the present invention, the anode of the _first diode D1 is connected to the cathode of the _second diode D2.

As an improvement of the C-LLCT-LLT resonant DC converter according to the present invention, the full bridge is provided between the third diode D ₃ and the fourth diode D ₄ The second input port of the rectifier circuit, the second input port is connected to the secondary winding of the _second high frequency transformer T2.

As an improvement of the C-LLCT-LLT resonant DC converter of the present invention, the anode of the third diode _D3 is connected to the cathode of the _fourth diode D4.

As an improvement of the C-LLCT-LLT resonant DC converter according to the present invention, the first diode D ₁ , the second diode D ₂ , and the third diode Both _D3 and the _fourth diode D4 are unidirectional diodes.

As an improvement of the C-LLCT-LLT resonant DC converter described in the present invention, one end of the first parallel branch capacitor C _p1 is connected to one end of the second parallel branch inductance L _m2 .

The beneficial effects of the present invention are:

(1) When the switching frequency of the converter is higher than the resonant frequency of the inductor L _p1 and the capacitor C _p1 , the inductor L _p1 and the capacitor C _p1 can be equivalent to a "variable excitation" whose inductance value decreases with the decrease of the switching frequency inductance". Therefore, when the bus voltage drops, the inductance value of the "variable excitation inductance" can be reduced only by reducing the switching frequency, thereby greatly increasing the voltage gain of the converter and finally stabilizing the output voltage, so that the output voltage is free from the busbar. When the bus voltage is in a normal state, in order to reduce the voltage gain, the switching frequency needs to increase. If the switching frequency at this time is set equal to the resonant frequency of the converter, then the gain at this time has nothing to do with the inductance value of the excitation inductance. Therefore, the increase of the resonant cavity impedance caused by the "variable excitation inductance" with a higher inductance value can reduce the size of the resonant current, thereby obtaining a higher conversion efficiency.

(2) The C-LLCT-LLT resonant DC converter can rely on its special resonant cavity structure. When the inductance value of the "variable excitation inductance" decreases, the voltage across the "variable excitation inductance" will also decrease at the same time, and then It can prevent the current flowing through the "variable excitation inductance" from excessively increasing in the state of bus voltage drop. Therefore, compared with the traditional LCLC resonant DC converter, the capacitance in the "variable excitation inductance" (ie, the parallel branch capacitor) can be Avoid excessive voltage stress, thereby reducing the fabrication cost of capacitors and improving system reliability.

(3) Compared with the single-phase resonant circuit composed of 5 elements in the traditional LCLC type converter, the C-LLCT-LLT type converter can expand the resonant circuit into two phases under the condition that only 2 elements are added. When the number of components is increased in a small proportion, the function of two-phase parallel shunt can be used to further reduce the winding loss of the magnetic components when the converter works in the normal state of the bus voltage, and improve the power conversion efficiency.

Description of drawings

FIG. 1 is a schematic circuit diagram of the present invention.

FIG. 2 is a fundamental wave equivalent circuit diagram of the C-LLT-LLT resonant structure in the present invention.

FIG. 3 is a change trend diagram of the voltage gain of the C-LLT-LLT resonant structure in the present invention with the change of the inductance value of L _m1 .

FIG. 4 is a change curve of the voltage gain with the switching frequency of the present invention.

FIG. 5 is a change trend diagram of the output voltage E _o1 of the first-phase resonant circuit and the output voltage E _o2 of the second-phase resonant circuit of the C-LLT-LLT resonant structure in the present invention with the change of the inductance value of L _m1 .

FIG. 6 is a waveform comparison diagram of the voltages at both ends of L _p1 and C _p1 and the voltages at both ends of L _m2 in the present invention.

Detailed ways

As used in the specification and claims, certain terms are used to refer to particular components. It should be understood by those skilled in the art that hardware manufacturers may refer to the same component by different nouns. The description and claims do not use the difference in name as a way to distinguish components, but use the difference in function of the components as a criterion for distinguishing. As mentioned in the entire specification and claims, "comprising" is an open-ended term, so it should be interpreted as "including but not limited to". "Approximately" means that within an acceptable error range, those skilled in the art can solve technical problems within a certain error range, and basically achieve technical effects.

In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "upper", "lower", "front", "rear", "left", "right", horizontal" etc. is based on the accompanying drawings The orientation or positional relationship shown is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the indicated device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a reference to the present invention. limits.

In the invention, unless otherwise expressly specified and limited, the terms "installation", "connection", "connection", "fixation" and other terms should be understood in a broad sense, for example, it may be a fixed connection or a detachable connection, Or integrally connected; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, and it can be the internal communication between the two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

The present invention will be described in further detail below with reference to the accompanying drawings 1 to 6, but it is not intended to limit the present invention.

Example 1

A C-LLCT-LLT type resonant DC converter, the schematic diagram of which is shown in Figure 1, includes a voltage-type half-bridge inverter circuit, a C-LLCT-LLT resonant cavity and a full-bridge rectifier circuit connected in sequence, C-LLCT The LLT resonant cavity includes a first phase resonant tank and a second phase resonant tank, the first phase resonant tank includes a first series branch inductance L _r1 , a first parallel branch inductance L _p1 , a first parallel branch capacitance C _p1 and The first high frequency transformer T ₁ , the second phase resonant circuit includes the second series branch inductance L _r2 , the second parallel branch inductance L _m2 and the second high frequency transformer T ₂ , the input of the first series branch inductance L _r1 The terminal and the input terminal of the second series branch inductor L _r2 are both connected to one end of the series branch capacitor _Cr , and the other end of the series branch capacitor _Cr is connected to the output port of the half-bridge inverter circuit. The first high-frequency transformer T ₁ and the secondary winding of the second high frequency transformer T ₂ are connected in series with the input port of the full bridge rectifier circuit.

Preferably, the half-bridge inverter circuit includes an input voltage stabilization capacitor C _i , a first high frequency power switching device S ₁ and a second high frequency power switching device S ₂ , an input voltage stabilization capacitor C _i , and a first high frequency power switching device S ₁ and the second high frequency power switching device S ₂ form a loop.

Preferably, the output port of the half-bridge inverter circuit is arranged between the _first high-frequency power switching device S1 and the _second high-frequency power switching device S2.

Preferably, the full-bridge rectifier circuit includes a first diode D ₁ , a second diode D ₂ , a third diode D ₃ , a fourth diode D ₄ and an output voltage stabilization filter capacitor C _o . The diode D ₁ , the second diode D ₂ , the third diode D ₃ and the fourth diode D ₄ form a loop, and the output voltage stabilization filter capacitor C _o and the third diode D ₃ and the fourth diode D 3 Diode _D4 is connected in parallel.

Preferably, a first input port of the full-bridge rectifier circuit is provided between the _first diode D1 and the _second diode D2, and the first input port is connected to the secondary winding of the _first high-frequency transformer T1.

Preferably, the anode of the _first diode D1 is connected to the cathode of the _second diode D2.

Preferably, a second input port of the full-bridge rectifier circuit is provided between the third diode _D3 and the _fourth diode D4, and the second input port is connected to the secondary winding of the _second high-frequency transformer T2.

Preferably, the anode of the third diode _D3 is connected to the cathode of the _fourth diode D4.

Preferably, the first diode D ₁ , the second diode D ₂ , the third diode D ₃ and the fourth diode D ₄ are all unidirectional diodes.

Preferably, one end of the first parallel branch capacitor C _p1 is connected to one end of the second parallel branch inductor L _m2 .

The working principle of the present invention is:

Similar to the LCLC converter, if the switching frequency is higher than the resonant frequency of L _p1 and C _p1 , then L _p1 and C _p1 can form a "variable magnetizing inductance" L _m1 . When L _p1 and C _p1 can be regarded as "variable excitation inductance" L _m1 , the C-LLT-LLT resonant cavity can be regarded as a kind of capacitor-inductor-inductor-transformer-inductor-inductor-transformer (C-LLT-LLT ) resonant structure, and its fundamental wave equivalent circuit can be shown in Figure 2. The two branches in the resonator can be seen as two LLT units (LLT-1 and LLT-2). E _o1 and E _o2 represent the output voltages of LLT-1 and LLT-2, respectively. E _i , E _o and Re _eq are the input voltage, output voltage and equivalent load resistance of the entire C-LLT-LLT resonant structure, respectively. I ₁ , I ₂ and I _o are the currents flowing through L _r1 , L _r2 and Re _eq , respectively. s is the Laplace operator. From Figure 2 the following system of equations can be derived:

According to formula (1), it can be deduced that the voltage gain M of the converter is:

in:

According to formula (2), let the imaginary part in the expression of M be zero, the resonant frequency f _r of the converter can be obtained as

Combining formula (2) and formula (3), when L _m1 takes different values, the voltage gain curve of the C-LLT-LLT type resonant structure can be drawn, as shown in Figure 3. It can be seen that in the C-LLT-LLT structure, the reduction of the inductance value of L _m1 can also make the converter obtain a higher voltage gain in the process of reducing the switching frequency, which means that the C-LLT-LLT converter It is also possible to obtain the same excellent characteristics as the LCLC converter - high gain characteristics when the bus voltage drops and high efficiency characteristics when the bus voltage is normal.

For the "variable magnetizing inductance" L _m1 , its relationship with L _p1 and C _p1 can be expressed as:

Combining Equation (2), Equation (3) and Equation (5), the complete voltage gain curve of the C-LLCT-LLT resonant DC converter can be drawn, as shown in Figure 4. It can be seen that due to the existence of L _p1 and C _p1 , the converter also has a resonance zero. The frequency f ₀ of the resonance zero point can be obtained by making M=0, and f ₀ is shown in formula (6).

Parallel branch capacitor voltage stress:

Define the inductance ratio:

Combining Equation (1) and Equation (7), the expression of the output voltage E _o1 of the first phase LLT (LLT-1) can be obtained as:

It can be seen from formula (8) that if L _r2 n ₁ -L _r1 n ₂ =0, then E _o1 has nothing to do with s and load Re _eq . To this end, considering the current sharing between the two branches, L _r1 =L _r2 , n ₁ =n ₂ can be set. Therefore, the expressions for E _o1 and E _o2 can be simplified to:

Since L _m1 in Fig. 2 is "variable excitation inductance", its inductance value will decrease with the decrease of switching frequency. Therefore, _k1 will increase as the switching frequency decreases. For this reason, according to formula (9), when k ₂ =0.1 is constant, the trend curves of E _o1 and E _o2 with k ₁ can be drawn, as shown in FIG. 5 .

It can be seen from Fig. 5 that only when k ₁ =k ₂ , E _o1 and E _o2 are both equal to E _o /2. Once k ₁ becomes higher and higher than k ₂ (L _m1 becomes lower and lower than L _m2 ), E _o1 will become lower and lower than E _o /2, and E _o2 will become lower and lower than E _o /2 is getting higher and higher. Since the first phase LLT (LLT-1) and the second phase LLT (LLT-2) are connected in series at the output, it can be concluded that whenever L _m1 becomes lower than L _m2 , the obtained due to L _m1 A higher voltage gain is increasingly reflected on the total output voltage by the second phase LLT (LLT-2). Therefore, through this "functional exchange", C-LLT-LLT can not only obtain high voltage gain by virtue of the existence of "variable excitation inductance", but also make the voltage across the "variable excitation inductance" increase with the gain and continued to decrease. Therefore, when L _m1 is replaced by L _p1 and C _p1 , compared with the situation where the voltage across the "variable excitation inductance" in the LCLC converter remains unchanged, the C-LLCT-LLT converter can While the inductance value of the "variable excitation inductance" decreases, the current flowing through the "variable excitation inductance" can be prevented from rising sharply due to the drop of the terminal voltage. Therefore, the voltage stress of the capacitor (parallel branch capacitor) in the "variable magnetizing inductance" can be alleviated to a certain extent. Therefore, the biggest defect of LCLC converter can be solved to a certain extent.

In order to confirm that the terminal voltage of the "variable excitation inductance" of the C-LLCT-LLT converter will decrease with the decrease of the inductance value of the "variable excitation inductance", Figure 6 shows that the C-LLCT-LLT converter works when the bus voltage is normal. (Fig. 6(a), converter input voltage 400V, low gain state, high switching frequency: 150kHz) and the bus voltage drops (Fig. 6(b), converter input voltage 170V, high gain state, low switching frequency: 90kHz) ) simulated waveforms in both cases. When the bus voltage is normal, according to the secondary side currents of the transformers T ₁ and T ₂ , it can be seen that the converter is working at the resonant frequency. Since the inductance value of the "variable excitation inductance" is the same as that of L _m2 , the "variable excitation inductance" and The voltage at both ends of the inductor L _m2 is the same, the converter can obtain a small resonant current due to the large resonant cavity impedance, and it works in a resonant state, so that a high power conversion efficiency can finally be obtained; when the bus voltage drops, according to the From the secondary side currents of transformers T ₁ and T ₂ , it can be seen that the switching frequency of the converter is lower than the resonant frequency, so the inductance value of the "variable excitation inductance" will decrease, and the increase in gain will keep the output voltage unchanged (50V). , the voltage at both ends of the "variable excitation inductance" is also lower than when the bus voltage is normal, and more output voltage is added to the two ends of the inductance L _m2 . With this special "function exchange" effect, C- Parallel branch capacitors in LLCT-LLT converters are protected from excessive voltage stress.

Combined advantages in size and efficiency:

As we all know, in order to reduce the winding loss of the magnetic element in the resonant cavity of the converter, the current can be shunted by multiple phases in parallel. The more phases shunt in parallel, the smaller the total winding loss of the converter. However, as the number of parallel phases increases, the volume of the converter also increases due to the increase in the number of components. Therefore, volume and efficiency often become two factors that restrict each other. For the LCLC converter, due to the existence of the parallel branch capacitance, the parallel branch inductance cannot be realized by the excitation inductance of the transformer. In the same way, for the C-LLCT-LLT converter in Fig. 1, due to the existence of the capacitor C _p1 , the inductance L _p1 cannot be realized by the excitation inductance of the transformer T ₁ , however, the inductance in the C-LLCT-LLT converter However, L _m2 can be realized by the magnetizing inductance of the transformer T ₂ . Therefore, there are 5 elements in the cavity of the LCLC converter and 7 elements in the cavity of the C-LLCT-LLT converter. Therefore, it can be seen that the C-LLCT-LLT converter can increase the number of parallel phases in the resonant cavity by 100% (1 phase to 2 phases) with only a 40% increase in the number of resonant elements (5 to 7). That is, the C-LLCT-LLT converter can further reduce the winding loss of the magnetic components when the converter works in the normal state of the bus voltage through the effect of two-phase parallel shunt when the number of components increases in a small proportion, and improves the power conversion efficiency. .

Based on the disclosure and teaching of the above specification, those skilled in the art to which the present invention pertains can also make changes and modifications to the above-described embodiments. Therefore, the present invention is not limited to the above-mentioned specific embodiments, and any obvious improvement, replacement or modification made by those skilled in the art on the basis of the present invention falls within the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims (9)

1. a C-LLCT-LLT type resonant DC converter is characterized in that: comprise the voltage type half-bridge inverter circuit, C-LLCT-LLT resonant cavity and full bridge rectifier circuit connected in turn, the C-LLCT-LLT- The LLT resonant cavity includes a first-phase resonant tank and a second-phase resonant tank. The first-phase resonant tank includes a first series branch inductance L _r1 , a first parallel branch inductance L _p1 , and a first parallel branch capacitance C _p1 and the first high frequency transformer T ₁ , the second phase resonant circuit includes a second series branch inductance L _r2 , a second parallel branch inductance L _m2 and a second high frequency transformer T ₂ , the first series branch The input end of the inductor L _r1 and the input end of the second series branch inductor L _r2 are both connected to one end of the series branch capacitor _Cr , and the other end of the series branch capacitor _Cr is connected to the half-bridge inverter circuit. The output port of the _first high frequency transformer T1 and the _second high frequency transformer T2 are connected in series with the input port of the full bridge rectifier circuit, and one end of the first parallel branch capacitor C _p1 is connected with one end of the second parallel branch inductance L _m2 , when the switching frequency of the converter is higher than the resonant frequency of L _p1 and C _p1 , then L _p1 and C _p1 can form a “variable excitation inductance” L _m1 ,

where f _s is the switching frequency. 2. A C-LLCT-LLT resonant DC converter as claimed in claim 1, characterized in that: the half-bridge inverter circuit comprises an input voltage-stabilizing capacitor C _i , a first high-frequency power switching device S ₁ and a second high frequency power switch device S ₂ , the input voltage stabilization capacitor C _i , the first high frequency power switch device S ₁ and the second high frequency power switch device S ₂ form a loop. 3. A C-LLCT-LLT resonant DC converter as claimed in claim 2, characterized in that: the output port of the half-bridge inverter circuit is arranged on the first high frequency power switching device S ₁ and between the _second high frequency power switching device S2. 4 . The C-LLCT-LLT resonant DC converter according to claim 1 , wherein the full-bridge rectifier circuit comprises a first diode D ₁ , a second diode D ₂ , a first diode D 2 , a Three diodes D ₃ , a fourth diode D ₄ and an output voltage regulator filter capacitor C _o , the first diode D ₁ , the second diode D ₂ , and the third diode D ₃ and the fourth diode D ₄ form a loop, and the output voltage stabilization filter capacitor C _o is connected in parallel with the third diode D ₃ and the fourth diode D ₄ . 5. A C-LLCT-LLT type resonant DC converter as claimed in claim 4, characterized in that: a space between the _first diode D1 and the _second diode D2 is provided The first input port of the full-bridge rectifier circuit is connected to the secondary winding of the _first high-frequency transformer T1. 6. A C-LLCT-LLT resonant DC converter according to claim 5, wherein the anode of the _first diode D1 is connected to the cathode of the _second diode D2 . 7. A C-LLCT-LLT type resonant DC converter as claimed in claim 4, characterized in that: a space is provided between the third diode D ₃ and the fourth diode D ₄ The second input port of the full-bridge rectifier circuit is connected to the secondary winding of the _second high-frequency transformer T2. 8. A C-LLCT-LLT resonant DC converter as claimed in claim 7, wherein the anode of the third diode _D3 is connected to the cathode of the _fourth diode D4 . 9 . The C-LLCT-LLT resonant DC converter according to claim 4 , wherein the first diode D ₁ , the second diode D ₂ , the third diode D 2 , and the third diode D 1 . Both the diode _D3 and the _fourth diode D4 are unidirectional diodes.

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