Power conversion circuit

Abstract

The present disclosure provides a power conversion circuit, comprising an input positive electrode, an input negative electrode, an output positive electrode, an output negative electrode, an input inductor, a first bridge arm, a second bridge arm, a transformer, an output capacitor and an auxiliary capacitor. The input negative electrode is connected to the output negative electrode. The first end of the input inductor is coupled to the input positive electrode. Each bridge arm comprises three switches connected in series and two connection points located between the switches. The winding of the transformer is coupled in series between the connection points of the two bridge arms, and the two windings of the transformer are connected to form a connection point coupled to the output positive electrode. The two ends of the output capacitor are electrically connected to the output positive electrode and the output negative electrode, respectively. The two ends of the auxiliary capacitor are electrically connected to the second end of the input inductor and the output end, respectively.

Description

Technical Field

[0001] The present disclosure relates to a power conversion circuit, and in particular to a power conversion circuit with a variable input-output voltage gain ratio. Background Art

[0002] With the rapid development of fixed-line and mobile communications, the demand for high-power DC / DC power converters (especially proportional converters) is also increasing. Figure 1 , an existing step-down circuit with scalable duty cycle is shown, which has an input-output voltage gain ratio of 4:1 and has the advantage of high power density.

[0003] However, in this buck circuit, since the voltage at the connection point between the switches cannot be changed by adjusting the capacitance value, the input-output voltage gain ratio cannot be changed. This buck circuit can only work in applications with an input-output voltage gain ratio of 4:1, and has poor applicability.

[0004] Therefore, how to develop a power conversion circuit that can improve the above-mentioned prior art is an urgent need. Summary of the invention

[0005] The purpose of the present disclosure is to provide a power conversion circuit, which can change the input-output voltage gain ratio by adjusting the turns ratio of the transformer winding, so it can be applied to various applications with different input-output voltage gain ratio requirements and has good applicability.

[0006] To achieve the above-mentioned purpose, the present disclosure provides a power conversion circuit, comprising an input positive electrode, an input negative electrode, an output positive electrode, an output negative electrode, an input inductor, a first bridge arm, a second bridge arm, a transformer, an output capacitor and an auxiliary capacitor. The input negative electrode is connected to the output negative electrode. The first end of the input inductor is coupled to the input positive electrode. The first bridge arm comprises a first switch, a second switch and a third switch coupled in series. The first and third switches are respectively electrically connected to the second end of the input inductor and the input negative electrode, the first and second switches are connected to form a first connection point, and the second and third switches are connected to form a second connection point. The second bridge arm comprises a fourth switch, a fifth switch and a sixth switch coupled in series. The fourth and sixth switches are respectively electrically connected to the second end of the input inductor and the input negative electrode, the fourth and fifth switches are connected to form a third connection point, and the fifth and sixth switches are connected to form a fourth connection point. The transformer comprises a first winding, a second winding and a third winding. The first winding is coupled in series between the first and third connection points, the second and third windings are coupled in series between the second and fourth connection points, and the second and third windings are connected to form a fifth connection point, and the fifth connection point is connected to the output positive electrode. The output capacitor is connected between the output positive electrode and the output negative electrode. The first end of the auxiliary capacitor is electrically connected to the second end of the input inductor, and the second end of the auxiliary capacitor is electrically connected to the output positive electrode or the output negative electrode. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 The figure is a schematic diagram of the circuit structure of an existing step-down circuit with expandable duty cycle.

[0008] Figure 2A The figure is a schematic diagram of the circuit structure of the power conversion circuit of the preferred embodiment of the present disclosure.

[0009] Figure 2B exemplify Figure 2A The excitation inductance of the transformer in the power conversion circuit.

[0010] Figure 3 for Figure 2A Schematic diagram of the main waveforms of the power conversion circuit.

[0011] Figure 4A and Figure 4B for Figure 2A The power conversion circuit in Figure 3 Schematic diagram of working modes in different time periods.

[0012] Figure 5 The figure is a schematic diagram of the circuit structure of a power conversion circuit according to another preferred embodiment of the present invention.

[0013] Figure 6 for Figure 5 Schematic diagram of the main waveforms of the power conversion circuit.

[0014] Fig. 7A and Figure 7B for Figure 5 The power conversion circuit in Figure 6 Schematic diagram of working modes in different time periods.

[0015] Figure 8 for Figure 2A and Figure 5 Schematic diagram of the waveform of the oscillating current in the power conversion circuit.

[0016] Fig. 9 for Figure 2A A schematic diagram of the circuit structure of a variation of a power conversion circuit.

[0017] Fig.10 for Figure 5 A schematic diagram of the circuit structure of a variation of a power conversion circuit.

[0018] The following are the descriptions of the reference numerals:

[0019] 1: Power conversion circuit

[0020] 11: DC voltage source

[0021] Vin: input voltage

[0022] Vin+: input positive

[0023] Vin-: input negative pole

[0024] Lin: input inductance

[0025] 12: First bridge arm

[0026] Q1: First switch

[0027] Q2: Second switch

[0028] Q3: The third switch

[0029] P1: First connection point

[0030] P2: Second connection point

[0031] 13: Second bridge arm

[0032] Q4: The fourth switch

[0033] Q5: The fifth switch

[0034] Q6: The sixth switch

[0035] P3: Third connection point

[0036] P4: Fourth connection point

[0037] 14: Transformer

[0038] T1: First winding

[0039] T2: Second winding

[0040] T3: The third winding

[0041] N1, N2: number of turns

[0042] P5: Fifth connection point

[0043] Lk: Series inductance

[0044] Co: output capacitance

[0045] Vo+: output positive

[0046] Vo-: output negative pole

[0047] Vo: output voltage

[0048] Cr: auxiliary capacitor

[0049] VCr: terminal voltage

[0050] Lm: magnetizing inductance

[0051] Vgs1 / 3 / 5, Vgs2 / 4 / 6: Gate-source voltage

[0052] Vds1 / 5, Vds2 / 4: drain-source voltage

[0053] iLk: Oscillation current

[0054] iLm: Excitation current

[0055] iT2, iT3: current

[0056] t0, t1, t2, t3, t4: time

[0057] io: output current

[0058] 1a: Power conversion circuit

[0059] Lin1: First input inductance

[0060] Lin2: Second input inductance

[0061] Cr1: First auxiliary capacitor

[0062] VCr1: Voltage of the first auxiliary capacitor

[0063] Cr2: Second auxiliary capacitor

[0064] VCr2: Voltage of the second auxiliary capacitor

[0065] Cb: DC blocking capacitor

[0066] 1b, 1c: Power conversion circuit DETAILED DESCRIPTION

[0067] Some typical embodiments that embody the features and advantages of the present disclosure will be described in detail in the following description. It should be understood that the present disclosure can have various changes in different implementations without departing from the scope of the present disclosure, and the descriptions and illustrations therein are essentially for illustrative purposes rather than for limiting the present disclosure.

[0068] Figure 2A FIG. 1 is a schematic diagram of the circuit structure of a power conversion circuit according to a preferred embodiment of the present disclosure. Figure 2AAs shown, the power conversion circuit 1 is connected to a DC voltage source 11 and a load (not shown). The power conversion circuit 1 is used to convert the voltage of the DC voltage source 11 and then supply power to the load. The DC voltage source 11 provides an input voltage Vin, and is electrically connected to the input positive electrode Vin+ and the input negative electrode Vin- of the power conversion circuit 1. The load is electrically connected to the output positive electrode Vo+ and the output negative electrode Vo- of the power conversion circuit 1, and the input negative electrode Vin- is connected to the output negative electrode Vo-. The voltage between the output positive electrode Vo+ and the output negative electrode Vo- is the output voltage Vo. The power conversion circuit 1 includes an input inductor Lin, a first bridge arm 12, a second bridge arm 13, a transformer 14, a series inductor Lk, an output capacitor Co and an auxiliary capacitor Cr, and the first end of the input inductor Lin is coupled to the input positive electrode Vin+. In addition, the inductance of the input inductor Lin is much greater than the inductance of the series inductor Lk, so that the current flowing through the input inductor Lin is constant. Therefore, the DC voltage source 11 and the input inductor Lin can be equivalent to an input current source.

[0069] The first bridge arm 12 includes a first switch Q1, a second switch Q2, and a third switch Q3 coupled in series. The first switch Q1 and the third switch Q3 are electrically connected to the second end of the input inductor Lin and the input negative electrode Vin-, respectively. The first switch Q1 and the second switch Q2 are connected in series to form a first connection point P1, and the second switch Q2 and the third switch Q3 are connected in series to form a second connection point P2. The second bridge arm 13 includes a fourth switch Q4, a fifth switch Q5, and a sixth switch Q6 coupled in series. The fourth switch Q4 and the sixth switch Q6 are electrically connected to the second end of the input inductor Lin and the input negative electrode Vin-, respectively. The fourth switch Q4 and the fifth switch Q5 are connected in series to form a third connection point P3, and the fifth switch Q5 and the sixth switch Q6 are connected in series to form a fourth connection point P4. The switches in the first bridge arm 12 and the second bridge arm 13 may be, for example, but not limited to, MOSFETs, SiC switches, or GaN switches.

[0070] The transformer 14 includes a first winding T1, a second winding T2, and a third winding T3 coupled to each other, wherein the number of turns of the first winding T1 is N1, and the number of turns of the second winding T2 and the third winding T3 are both N2. The first winding T1 and the series inductor Lk are coupled in series between the first connection point P1 and the third connection point P3. The second winding T2 and the third winding T3 are coupled in series between the second connection point P2 and the fourth connection point P4, and there is a fifth connection point P5 between the second winding T2 and the third winding T3, and the fifth connection point P5 is connected to the output positive electrode Vo+. In some embodiments, the series inductor Lk can be the leakage inductance of the transformer 14 or an external inductor, but it is not limited thereto. The series inductor Lk can also be the equivalent inductance of the sum of the leakage inductance of the transformer 14 and an external inductor.

[0071] The output capacitor Co is connected between the output positive electrode Vo+ and the output negative electrode Vo-. The two ends of the auxiliary capacitor Cr are electrically connected to the second end of the input inductor Lin and the output positive electrode Vo+, respectively. The terminal voltage VCr on the auxiliary capacitor Cr is the sum of the DC voltage component and the oscillating AC voltage component, wherein the DC voltage component is the difference between the input voltage Vin and the output voltage Vo. Preferably, the capacitance of the output capacitor Co is greater than the capacitance of the auxiliary capacitor Cr, but is not limited thereto.

[0072] Figure 2B exemplify Figure 2A The excitation inductance of the transformer in the power conversion circuit. Figure 2B As shown, the excitation inductance Lm of the transformer 14 can be equivalent to being connected in parallel with the first winding T1 . However, this is not limited to this. In other embodiments, the excitation inductance Lm can also be equivalent to being connected in parallel with the second winding T2 or the third winding T3 .

[0073] Figure 3 for Figure 2A The main waveform diagram of the power conversion circuit in the figure is as follows: Figure 4A and Figure 4B for Figure 2A The power conversion circuit in Figure 3 Schematic diagram of working modes in different time periods. Figure 3 , Vgs1 / 3 / 5 are the gate-source voltages of the first, third and fifth switches Q1, Q3 and Q5, Vgs2 / 4 / 6 are the gate-source voltages of the second, fourth and sixth switches Q2, Q4 and Q6, Vds1 / 5 are the drain-source voltages of the first and fifth switches Q1 and Q5, Vds2 / 4 are the drain-source voltages of the second and fourth switches Q2 and Q4, iLk is the current flowing through the series inductor Lk, iLm is the excitation current flowing through the excitation inductor Lm, iT2 and iT3 are the currents flowing through the second winding T2 and the third winding T3, respectively. Figure 4A and Figure 4B In the diagram, darker lines are used to show the paths through which current flows, and lighter lines are used to show the paths through which current does not flow.

[0074] like Figure 3 As shown, time t0 to t4 is a switching cycle, the first, third and fifth switches Q1, Q3 and Q5 are turned on and off synchronously, and the second, fourth and sixth switches Q2, Q4 and Q6 are turned on and off synchronously. The control signals of the first, third and fifth switches Q1, Q3 and Q5 are 180 degrees out of phase with the control signals of the second, fourth and sixth switches Q2, Q4 and Q6, and the duty cycle of the multiple switches is about 50%.

[0075] See also Figure 3 and Figure 4A, during the period from time t0 to t1, the first, third and fifth switches Q1, Q3 and Q5 are in the on state, and the second, fourth and sixth switches Q2, Q4 and Q6 are in the off state. The corresponding working mode is as follows Figure 4A As shown. The excitation current iLm rises linearly, and the series inductor Lk and the auxiliary capacitor Cr oscillate. The generated oscillating current iLk and the voltage VCr of the auxiliary capacitor Cr are approximately sinusoidal waves. The current iT1 flowing through the first winding T1 is iLk-iLm. Through the magnetic coupling between the first winding T1, the second winding T2 and the third winding T3, a current iT2 is induced on the second winding T2 = iT1*(N2 / N1+1). Since the third winding T3 has the same number of turns as the second winding T2, the current iT3 induced on the third winding T3 is equal to the current iT2. The DC component of the sum of the currents iT2 and iT3 is the output current io, and the AC component of the sum of the currents iT2 and iT3 flows through the output capacitor Co and the auxiliary capacitor Cr.

[0076] In addition, the plurality of switches are turned on and off in a switching cycle, and an oscillation cycle is determined by the series inductor Lk and the auxiliary capacitor Cr. In this embodiment, the oscillation cycle is substantially equal to 1 / 2 of the switching cycle, wherein the oscillation cycle is equal to

[0077] At time t0, the initial value of the oscillating current iLk is equal to the excitation current iLm, and the initial value of the voltage VCr of the auxiliary capacitor Cr is equal to the input voltage Vin, wherein, due to the large inductance of the excitation inductance Lm, the excitation current iLm can be approximately zero. During the period from time t0 to t1, the oscillating current iLk completes an oscillation cycle, wherein when the oscillating current iLk is close to zero, the first, third and fifth switches Q1, Q3 and Q5 are turned off, thereby achieving zero current turn-off to reduce turn-off loss. Furthermore, during the period from time t0 to t1, the terminal voltage VCr on the auxiliary capacitor Cr completes an oscillation cycle, and the terminal voltage VCr returns to be equal to the input voltage Vin at time t1.

[0078] See also Figure 3 and Figure 4B , during the period from time t2 to t3, the second, fourth and sixth switches Q2, Q4 and Q6 are in the on state, and the first, third and fifth switches Q1, Q3 and Q5 are in the off state. The corresponding working mode is as follows Figure 4B The working principle of the power conversion circuit 1 during this period is similar to the working principle during the aforementioned period from time t0 to time t1, so it will not be described in detail here.

[0079] In addition, if Figure 3As shown in , the period from time t1 to t2 and the period from time t3 to t4 are dead time. During the dead time, the excitation current iLm charges and discharges the parasitic capacitance of the corresponding switch, and causes the drain-source voltage of the switch to be turned on to drop to zero at the end of the dead time. Specifically, in the dead time of the first half cycle, during the period from real time t1 to t2, the excitation current iLm charges the parasitic capacitance of the first, third and fifth switches Q1, Q3 and Q5, and discharges the parasitic capacitance of the second, fourth and sixth switches Q2, Q4 and Q6, so that the drain-source voltage Vds2 / 4 of the second, fourth and sixth switches Q2, Q4 and Q6 to be turned on drops to zero at time t2. In the dead time of the second half cycle, during the period from real time t3 to t4, the excitation current iLm charges the parasitic capacitance of the second, fourth and sixth switches Q2, Q4 and Q6, and discharges the parasitic capacitance of the first, third and fifth switches Q1, Q3 and Q5, so that the drain-source voltage Vds1 / 5 of the first, third and fifth switches Q1, Q3 and Q5 to be turned on drops to zero at time t4. Accordingly, at the end of each dead time, the drain-source voltage of the switch to be turned on drops to zero, thereby achieving zero voltage turn-on of the switch and reducing the turn-on loss of the switch. At the same time, since the dead time is shorter than the turn-on time of the switch (approximately less than one tenth of the turn-on time of the switch), the changes of other variables during the dead time can be ignored. In another embodiment, at the end of each dead time, the drain-source voltage of the switch to be turned on is reduced to less than half of the drain-source voltage at the beginning of the dead time. Although zero voltage turn-on of the switch cannot be achieved, the turn-on loss of the switch can be reduced.

[0080] in accordance with Figure 2A The circuit structure of the power conversion circuit shown in FIG. Figure 3 The control strategy shown can be used to obtain that the input voltage Vin and the output voltage Vo of the power conversion circuit 1 satisfy the relationship: Vin = Vo*(2 + N1 / N2). That is, the input-output voltage gain ratio of the power conversion circuit 1 is (2 + N1 / N2): 1. For example, when the turns ratio of the first, second and third windings T1, T2 and T3 is 2:1:1 (i.e., N1 / N2 = 2), the input-output voltage gain ratio of the power conversion circuit 1 is 4:1; and when the turns ratio of the first, second and third windings T1, T2 and T3 is 3:1:1 (i.e., N1 / N2 = 3), the input-output voltage gain ratio of the power conversion circuit 1 is 5:1. It can be seen that the power conversion circuit 1 of the present disclosure can change the input-output voltage gain ratio by adjusting the turns ratio of the windings of the transformer 14. Therefore, the input-output voltage gain ratio can be flexibly adjusted according to actual needs, so that the power conversion circuit 1 of the present disclosure has excellent applicability.

[0081] In some embodiments, such as Figure 5As shown, the input inductor of the power conversion circuit 1a includes a first input inductor Lin1 and a second input inductor Lin2, and the auxiliary capacitor of the power conversion circuit 1a includes a first auxiliary capacitor Cr1 and a second auxiliary capacitor Cr2, wherein the inductance of the first and second input inductors Lin1 and Lin2 are much larger than the series inductor Lk. The first ends of the first input inductor Lin1 and the second input inductor Lin2 are both electrically connected to the input positive electrode Vin+. The second end of the first input inductor Lin1 is electrically connected to the first switch Q1 of the first bridge arm 12 and the first end of the first auxiliary capacitor Cr1. The second end of the second input inductor Lin2 is electrically connected to the fourth switch Q4 of the second bridge arm 13 and the first end of the second auxiliary capacitor Cr2. The second ends of the first auxiliary capacitor Cr1 and the second auxiliary capacitor Cr2 are both electrically connected to the output positive electrode Vo+. In addition, as shown Figure 5 As shown, the power conversion circuit 1a also includes a DC blocking capacitor Cb, which is coupled in series with the first winding T1 and the series inductor Lk between the first and third connection points P1 and P3 to prevent the transformer 14 from being magnetically saturated when the capacitances of the first auxiliary capacitor Cr1 and the second auxiliary capacitor Cr2 are not equal.

[0082] At Figure 5 In, with Figure 2A and Figure 2B Similar elements are denoted by the same reference numerals and are not described again herein. Figure 6 for Figure 5 The main waveform diagram of the power conversion circuit in the figure is as follows: Fig. 7A and Figure 7B for Figure 5 The power conversion circuit in Figure 6 Schematic diagram of working modes in different time periods. Figure 6 , Fig. 7A and Figure 7B As shown, since the operation mode of the switch is similar to that described above, the relationship between the input-output voltage gain ratio of the power conversion circuit 1a and the winding turns ratio in the transformer 14 is also the same as that described above, so they are not repeated here.

[0083] And, in Figure 5 In the embodiment shown, the inductances of the two input inductors Lin1 and Lin2 are designed to be equal, and the capacitances of the two auxiliary capacitors Cr1 and Cr2 are designed to be equal, so that the switching period of the multiple switches is equal to the oscillation period. However, in actual production, the two inductors and the two capacitors have parameter distribution (for example, the design value ±15%), so that the switching period of the switches is approximately equal to the oscillation period. Figure 6As shown, at time t0, the initial value of the oscillating current iLk is equal to the excitation current iLm, the initial value of the voltage VCr1 of the first auxiliary capacitor Cr1 is the maximum value in a switching cycle, and the initial value of the voltage VCr2 of the second auxiliary capacitor Cr2 is the minimum value in a switching cycle. Since the inductance of the excitation inductance Lm is relatively large, the excitation current iLm can be approximately zero. During the period from time t0 to t1 or from time t2 to t3, the oscillating current iLk completes the oscillation of half an oscillation cycle, wherein at time t1 or t3, the oscillating current iLk is close to zero, and the corresponding switch is turned off at this time, thereby achieving zero current turn-off to reduce turn-off loss. Furthermore, during the period from time t0 to t1, the terminal voltage VCr1 on the first auxiliary capacitor Cr1 completes the oscillation of half an oscillation cycle and oscillates to the minimum value in a switching cycle, and the voltage VCr2 of the second auxiliary capacitor Cr2 is charged by the second input inductor Lin2 to the maximum value in a switching cycle. During the period from t0 to t1, the voltage VCr1 of the first auxiliary capacitor Cr1 is charged by the first input inductor Lin1 to the maximum value in one switching cycle, and the voltage VCr2 of the second auxiliary capacitor Cr2 completes the oscillation of half an oscillation cycle and oscillates to the minimum value in one switching cycle.

[0084] In the above-mentioned embodiment, the power conversion circuits 1 and 1a both work in the preferred working state, that is, the switching period is at an ideal value. Specifically, in the power conversion circuit 1, the switching period is equal to twice the oscillation period, and in the power conversion circuit 1a, the switching period is equal to the oscillation period. However, even if the power conversion circuits 1 and 1a work in a non-preferred working state, that is, when the switching period in the power conversion circuit 1 is not equal to twice the oscillation period (for example, the switching period is greater than 3 / 2 of the oscillation period and less than 5 / 2 of the oscillation period), or when the switching period in the power conversion circuit 1a is not equal to the oscillation period (for example, the switching period is greater than 1 / 2 of the oscillation period and less than 3 / 2 of the oscillation period), the switch action and working principle in the power conversion circuits 1 and 1a are the same as those described above, and have the same technical effects. However, since the switching period is not at an ideal value, the switch will be turned off in advance or later, resulting in the inability to achieve zero current turn-off and increase the turn-off loss, thereby reducing the efficiency of the power conversion circuits 1 and 1a.

[0085] When the power conversion circuits 1 and 1a are both operating in the preferred working state, assuming that the input voltage Vin, the output voltage Vo, the output current io and other parameters are the same, and the excitation current iLm is ignored, the oscillation current in the power conversion circuits 1 and 1a is as follows: Figure 8 As shown. Figure 8 In FIG. 1 , the solid line represents the oscillating current in the power conversion circuit 1, and the dotted line represents the oscillating current in the power conversion circuit 1a. Figure 8As shown, both the power conversion circuits 1 and 1a can realize zero current shutdown of the switch, wherein the peak value of the oscillating current in the power conversion circuit 1 is larger, and the corresponding current effective value is also larger. Therefore, the parasitic resistance loss on the current path in the power conversion circuit 1 is greater than the parasitic resistance loss on the circuit path in the power conversion circuit 1a, wherein the parasitic resistance may include the turn-on impedance of the switch, the parasitic resistance on the line, and the parasitic resistance of the capacitor or inductor, etc.

[0086] In addition, the location of the auxiliary capacitor is not limited to the implementation shown in the above embodiments. Figure 2A In the embodiment shown, the first end of the auxiliary capacitor Cr is electrically connected to the second end of the input inductor Lin, and the second end of the auxiliary capacitor Cr is electrically connected to the output positive electrode Vo+. However, this is not limited to this. In other embodiments, such as Fig. 9 As shown in FIG. 1 , the second end of the auxiliary capacitor Cr can be electrically connected to the output negative electrode Vo- instead. Figure 5 In the embodiment shown, the first ends of the first and second auxiliary capacitors Cr1 and Cr2 are electrically connected to the second ends of the first and second input inductors Lin1 and Lin2, respectively, and the second ends of the first and second auxiliary capacitors Cr1 and Cr2 are electrically connected to the output positive electrode Vo+. However, this is not limited to this. In other embodiments, such as Fig.10 As shown, the second ends of the first and second auxiliary capacitors Cr1 and Cr2 can be electrically connected to the output negative electrode Vo- instead.

[0087] In summary, the present disclosure provides a power conversion circuit, which can change the input-output voltage gain ratio by adjusting the turns ratio of the transformer winding, and thus can be applied to various applications with different input-output voltage gain ratio requirements, and has good applicability.

[0088] It should be noted that the above are only preferred embodiments for illustrating the present disclosure, and the present disclosure is not limited to the embodiments described, and the scope of the present disclosure is determined by the claims. Those skilled in the art may make various modifications and alterations to the present disclosure, but they do not depart from the scope of the appended claims.

Claims

1. A power conversion circuit, comprising: An input positive electrode, an input negative electrode, an output positive electrode and an output negative electrode, wherein the input negative electrode and the output negative electrode are connected; an input inductor, wherein a first end of the input inductor is coupled to the input positive electrode; a first bridge arm, comprising a first switch, a second switch and a third switch coupled in series, wherein the first and third switches are electrically connected to the second end of the input inductor and the input negative electrode respectively, the first and second switches are connected to form a first connection point, and the second and third switches are connected to form a second connection point; a second bridge arm, comprising a fourth switch, a fifth switch and a sixth switch coupled in series, wherein the fourth and sixth switches are electrically connected to the second end of the input inductor and the input negative electrode respectively, the fourth and fifth switches are connected to form a third connection point, and the fifth and sixth switches are connected to form a fourth connection point; A transformer, comprising a first winding, a second winding and a third winding, wherein the first winding is coupled in series between the first and third connection points, the second and third windings are coupled in series between the second and fourth connection points, and the second and third windings are connected to form a fifth connection point, and the fifth connection point is connected to the output positive electrode; an output capacitor connected between the output positive electrode and the output negative electrode; and An auxiliary capacitor, wherein a first end of the auxiliary capacitor is electrically connected to the second end of the input inductor, and a second end of the auxiliary capacitor is electrically connected to the output positive electrode or the output negative electrode.

2. The power conversion circuit as described in claim 1, wherein the first, third and fifth switches are turned on and off synchronously, the second, fourth and sixth switches are turned on and off synchronously, and the control signals of the first, third and fifth switches are 180 degrees out of phase with the control signals of the second, fourth and sixth switches. 3 . The power conversion circuit as claimed in claim 2 , wherein a duty cycle of the plurality of switches is substantially 50%.

4. The power conversion circuit as claimed in claim 1, wherein the voltage between the input positive electrode and the input negative electrode is an input voltage, the voltage between the output positive electrode and the output negative electrode is an output voltage, and the relationship between the input voltage and the output voltage conforms to the following equation: Vo=Vin*[N2 / (N1+N2*2)] Wherein Vo is the output voltage, Vin is the input voltage, N1 is the number of turns of the first winding, N2 is the number of turns of the second winding, and the number of turns of the second and third windings are the same.

5. The power conversion circuit as described in claim 1 further includes a series inductor, wherein the series inductor includes the leakage inductance of the transformer and / or an external inductor, and the first winding and the series inductor are coupled in series between the first and third connection points. 6 . The power conversion circuit as claimed in claim 5 , further comprising a DC blocking capacitor, wherein the DC blocking capacitor is coupled in series with the first winding and the series inductor between the first and third connection points.

7. The power conversion circuit as claimed in claim 5, wherein the plurality of switches are turned on and off in a switching cycle, an oscillation cycle is determined by the series inductor and the auxiliary capacitor, and the switching cycle is substantially equal to twice the oscillation cycle.

8. The power conversion circuit as described in claim 5, wherein the plurality of switches are turned on and off in a switching cycle, an oscillation cycle is determined by the series inductor and the auxiliary capacitor, wherein the switching cycle is greater than 3 / 2 of the oscillation cycle and less than 5 / 2 of the oscillation cycle.

9. The power conversion circuit as claimed in claim 7 or 8, wherein there is a dead time in the switching cycle, and within the dead time, the excitation current charges and discharges the corresponding parasitic capacitance of the switch to achieve zero voltage switching.

10. A power conversion circuit as described in claim 7 or 8, wherein there is a dead time in the switching cycle, and at the end of the dead time, the drain-source voltage of the switch to be turned on is reduced to less than half of the drain-source voltage at the beginning of the dead time.

11. The power conversion circuit as described in claim 5, wherein the input inductor includes a first input inductor and a second input inductor, the auxiliary capacitor includes a first auxiliary capacitor and a second auxiliary capacitor, the first ends of the first and second input inductors are both electrically connected to the input positive electrode, the second end of the first input inductor is electrically connected to the first switch and the first end of the first auxiliary capacitor, the second end of the second input inductor is electrically connected to the fourth switch and the first end of the second auxiliary capacitor, and the second ends of the first and second auxiliary capacitors are both electrically connected to the output positive electrode or the output negative electrode. 12 . The power conversion circuit as claimed in claim 11 , wherein the second ends of the first and second auxiliary capacitors are both electrically connected to the output positive electrode or the output negative electrode.

13. The power conversion circuit according to claim 11, in, The inductance of the first input inductor is the same as that of the second input inductor, the capacitance of the first auxiliary capacitor is equal to that of the second auxiliary capacitor, the multiple switches are turned on and off in a switching cycle, the series inductor and the auxiliary capacitor oscillate in an oscillation cycle, wherein the switching cycle is substantially equal to the oscillation cycle.

14. The power conversion circuit according to claim 11, in, The inductance of the first input inductor is the same as that of the second input inductor, the capacitance of the first auxiliary capacitor is equal to that of the second auxiliary capacitor, the multiple switches are turned on and off in a switching cycle, the series inductor and the auxiliary capacitor oscillate in an oscillation cycle, wherein the switching cycle is greater than 1 / 2 of the oscillation cycle and less than 3 / 2 of the oscillation cycle. 15 . The power conversion circuit as claimed in claim 5 , wherein when any of the switches is turned off, an oscillating current flowing through the series inductor oscillates to substantially zero. 16 . The power conversion circuit as claimed in claim 5 , wherein the inductance of the input inductor is greater than the inductance of the series inductor.

17. A power conversion circuit as described in claim 1, wherein the voltage between the input positive electrode and the input negative electrode is an input voltage, the voltage between the output positive electrode and the output negative electrode is an output voltage, and the voltage of the auxiliary capacitor is the sum of a DC voltage component and an AC voltage component, wherein the DC voltage component is the difference between the input voltage and the output voltage.

18. The power conversion circuit as claimed in claim 1, wherein the capacitance of the output capacitor is greater than the capacitance of the auxiliary capacitor.

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