TWI885401B - Bidirectional resonant conversion system - Google Patents

Abstract

The present invention discloses a bidirectional resonant conversion system comprising a first source, a second source, a first converter and a second converter. The first source provides a DC power. The second source provides an AC power. The first converter is electrically connected between the first source and the second converter. The second converter is electrically connected between the first converter and the second source. The bidirectional resonant conversion system transfers the DC power to the AC source or transfers the AC power to the DC source through the electric elements of the first converter and the second converter. Consequently, the bidirectional resonant conversion system transfers power bidirectionally so as to decrease the switch loss and increase the efficiency and the power factor. The current harmonics of the bidirectional resonant conversion system is reduced.

Description

Bidirectional resonant conversion system

This case relates to a resonant conversion system, particularly a two-way resonant conversion system.

The microinverter is used to convert DC power into AC power and includes two-stage converters, namely a DC/DC converter and a DC/AC converter, wherein the DC/DC converter boosts and regulates the DC input voltage, and the DC/AC converter converts the regulated DC input voltage into an AC output voltage.

Please refer to Figure 1, which is a circuit diagram of a conventional forward flyback converter. The conventional forward flyback converter includes two transformers T1 and T2 and an output resistor Ro. The two transformers T1 and T2 are respectively a forward transformer and a flyback transformer, and the two transformers T1 and T2 are connected in series for voltage conversion. The forward flyback converter uses the output resistor Ro to reduce the output ripple. However, the conventional forward flyback converter cannot achieve voltage resonance and has large switching losses. For example, when the input power is 100W and the power is fully loaded, the best efficiency of the conventional forward flyback converter is 84.9%. In addition, the conventional forward flyback converter can only provide a forward conversion function, that is, it can only provide a unidirectional conversion function, which makes its application range relatively small.

Therefore, how to develop a bidirectional resonant conversion system that overcomes the above-mentioned shortcomings is an urgent need at present.

The purpose of this case is to provide a bidirectional resonant conversion system, which provides the function of bidirectional power transmission, reduces switching loss, improves overall efficiency and power factor, and can also achieve the effect of reducing current harmonic components.

To achieve the above-mentioned purpose, one embodiment of the present case is a bidirectional resonant conversion system, comprising a first power source, a second power source, a first resonant converter and a second resonant converter. The first power source is used to provide direct current power and comprises a first input positive terminal and a first input negative terminal. The second power source is used to provide alternating current power and comprises a second input positive terminal and a second input negative terminal. The first resonant converter is electrically connected to the first power source and comprises a primary circuit, a transformer and a secondary circuit. The primary circuit comprises an input transistor and a first transistor, and the input transistor is connected to the first input negative terminal. The transformer includes a primary winding and a secondary winding. The primary winding and a first transistor are connected in series between a first input positive terminal and a first input negative terminal, and a connection point between the primary winding and the first transistor forms a first connection point. The secondary circuit includes a second capacitor, a third capacitor, a second transistor, a third transistor and a transition capacitor. The second capacitor and the third capacitor are connected in series to form a first bridge arm, and the connection between the second capacitor and the third capacitor forms a second connection point. The second connection point is electrically connected to the first end of the secondary winding. The second transistor and the third transistor are connected in series to form a second bridge arm, which is connected in parallel with the first bridge arm. The connection between the second transistor and the third transistor forms a third connection point. The third connection point is electrically connected to the second end of the secondary winding. The transition capacitor is connected in parallel to the first bridge arm and the second bridge arm. The second resonant converter is electrically connected between the first resonant converter and the second power source, and includes a fourth transistor, a fifth transistor, a sixth transistor, a seventh transistor and a high-frequency filter. The fourth transistor and the fifth transistor are connected in series to form a third bridge arm. The connection between the fourth transistor and the fifth transistor forms a fourth connection point. The sixth transistor and the seventh transistor are connected in series to form a fourth bridge arm. The fourth bridge arm is connected in parallel with the third bridge arm and the transition capacitor. The connection between the sixth transistor and the seventh transistor forms a fifth connection point. The high-frequency filter is connected to the fourth connection point, the fifth connection point and the second power source. The first resonant converter boosts the DC power provided by the first power source, and the second resonant converter converts the boosted DC power into output AC power, or the second resonant converter converts the AC power provided by the second resonant converter into transition DC power, and the first resonant converter reduces the transition DC power to output the output DC power.

Some typical embodiments that embody the features and advantages of the present invention will be described in detail in the following description. It should be understood that the present invention can have various variations in different aspects without departing from the scope of the present invention, and the descriptions and drawings therein are essentially for illustrative purposes rather than for limiting the present invention.

Please refer to FIG. 2, which is a circuit structure diagram of the bidirectional resonant conversion system of the present invention. As shown in the figure, the bidirectional resonant conversion system 1 can be applied to a solar power supply system, and includes a first power source 11, a second power source 12, a first resonant converter 2, and a second resonant converter 3. The first power source 11 is a DC power source for providing DC power, and includes a first input positive terminal Vin+ and a first input negative terminal Vin-, and the potential of the first input positive terminal Vin+ is higher than the potential of the first input negative terminal Vin-. The second power source 12 is an AC power source for providing AC power, and includes a second input positive terminal Vac+ and a second input negative terminal Vac-, and the potential of the second input positive terminal Vac+ is higher than the potential of the second input negative terminal Vac-.

The first resonant converter 2 is electrically connected between the first power source 11 and the second resonant converter 3, and includes a primary circuit 21, a transformer T, and a secondary circuit 22. The primary circuit 21 includes an input capacitor CB1, an input inductor Lin, an input transistor Qin, a first capacitor C1, a first resistor R1, a first diode D1, and a first transistor Q1. The input capacitor CB1 constitutes an energy storage element on the low voltage side, and the two ends of the input capacitor CB1 are electrically connected to the first input positive terminal Vin+ and the first input negative terminal Vin- of the first power source 11, respectively. The input inductor Lin and the input transistor Qin are connected in series between the first input positive terminal Vin+ and the first input negative terminal Vin- of the first power source 11 in sequence. The first capacitor C1, the first diode D1 and the first transistor Q1 are connected in series between the first positive input terminal Vin+ and the first negative input terminal Vin- of the first power source 11, wherein the connection between the first diode D1 and the first transistor Q1 forms a first connection point A1. The first resistor R1 is connected in parallel with the first capacitor C1.

The transformer T is an isolation transformer and includes a primary winding 23 and a secondary winding 24. A first end of the primary winding 23 is electrically connected to the first positive input terminal Vin+ of the first power source 11, and a second end of the primary winding 23 is electrically connected to a first connection point A1, that is, the primary winding 23 and the first transistor Q1 are connected in series between the first positive input terminal Vin+ and the first negative input terminal Vin-.

The secondary circuit 22 includes a second capacitor C2, a third capacitor C3, a second transistor Q2, a third transistor Q3 and a transition capacitor CB2. The second capacitor C2 and the third capacitor C3 are resonant capacitors, respectively, and are connected in series to form a first bridge arm, wherein the connection between the second capacitor C2 and the third capacitor C3 forms a second connection point A2, and the second connection point A2 is electrically connected to the first end of the secondary winding 24 of the transformer T. The second transistor Q2 and the third transistor Q3 are connected in series to form a second bridge arm, and the second bridge arm is connected in parallel with the first bridge arm, wherein the connection between the second transistor Q2 and the third transistor Q3 forms a third connection point A3, and the third connection point A3 is electrically connected to the second end of the secondary winding 24 of the transformer T. The transition capacitor CB2 constitutes an energy storage element on the high voltage side and is connected in parallel to the first bridge arm and the second bridge arm.

The second resonant converter 3 is electrically connected between the first resonant converter 2 and the second power source 12, and includes a fourth transistor Q4, a fifth transistor Q5, a sixth transistor Q6, a seventh transistor Q7, a first inductor L1, a second inductor L2, and an output capacitor Co. The fourth transistor Q4 and the fifth transistor Q5 are connected in series to form a third bridge arm, and the third bridge arm is connected in parallel with the first bridge arm, the second bridge arm, and the transition capacitor CB2, wherein the connection between the fourth transistor Q4 and the fifth transistor Q5 forms a fourth connection point A4. The sixth transistor Q6 and the seventh transistor Q7 are connected in series to form a fourth bridge arm, and the fourth bridge arm is connected in parallel with the first bridge arm, the second bridge arm, the third bridge arm and the transition capacitor CB2, wherein the connection between the sixth transistor Q6 and the seventh transistor Q7 forms a fifth connection point A5. The first inductor L1 is electrically connected between the fourth connection point A4 and the second input positive terminal Vac+ of the second power source 12. The second inductor L2 is electrically connected between the fifth connection point A5 and the second input negative terminal Vac- of the second power source 12. The output capacitor Co is electrically connected between the second input positive terminal Vac+ and the second input negative terminal Vac- of the second power source 12.

In this embodiment, the bidirectional resonant conversion system 1 may further include a control unit (not shown) to control the operation of the first power source 11, the second power source 12 and internal electronic components, thereby converting the DC power provided by the first power source 11 into output AC power, or converting the AC power provided by the second power source 12 into output DC power. When the bidirectional resonant conversion system 1 is to convert the DC power provided by the first power source 11 into output AC power (hereinafter referred to as forward operation), the control unit controls the first power source 11 to operate and the second power source 12 to be turned off, and the first resonant converter 2 constitutes a forward-flyback resonant converter to boost and stabilize the DC power provided by the first power source 11 and simultaneously achieve the effect of Maximum Power Point Tracking (MPPT), and the second resonant converter 3 constitutes a full-bridge resonant converter to convert the boosted DC power into output AC power. And during forward operation, the first capacitor C1, the first resistor R1 and the first diode D1 of the primary side circuit 21 of the first resonant converter 2 constitute a buffer circuit, the first transistor Q1 constitutes a primary side switching element, the second capacitor C2 and the third capacitor C3 of the secondary side circuit 22 of the first resonant converter 2 constitute a resonant capacitor, the fourth transistor Q4, the fifth transistor Q5, the sixth transistor Q6 and the seventh transistor Q7 of the second resonant converter 3 constitute a switching element, the first inductor L1, the second inductor L2 and the output capacitor Co constitute a high-frequency filter, and the operation of the above components will be explained later. When the bidirectional resonant conversion system 1 wants to convert the AC power provided by the second power source 12 into DC power (hereinafter referred to as reverse operation), the control unit controls the first power source 11 to be turned off and the second power source 12 to operate, the second resonant converter 3 constitutes a full-bridge resonant converter to convert the AC power provided by the second power source 12 into DC power, and the first resonant converter 2 constitutes a half-bridge resonant converter to step down the DC power. And during reverse operation, the fourth transistor Q4, the fifth transistor Q5, the sixth transistor Q6 and the seventh transistor Q7 of the second resonant converter 3 constitute a switching element, the first inductor L1 and the second inductor L2 constitute a boost inductor, the second transistor Q2 and the third transistor Q3 of the secondary side circuit 22 of the first resonant converter 2 constitute a switching element, the second capacitor C2 and the third capacitor C3 constitute a resonant capacitor, and the first transistor Q1 constitutes a rectifying element. The operation of the above elements will be explained later.

The following will further describe the conversion operation of the bidirectional resonant conversion system 1 between DC power and AC power. Please refer to Figures 3A, 3B, 3C and 3D in conjunction with Figure 2, wherein Figures 3A and 3B are circuit operation diagrams of the first resonant converter of the bidirectional resonant conversion system shown in Figure 2 in forward operation, and Figures 3C and 3D are circuit operation diagrams of the second resonant converter of the bidirectional resonant conversion system shown in Figure 2 in forward operation. When the bidirectional resonant conversion system 1 is to convert the DC power provided by the first power source 11 into AC power and perform forward operation, the bidirectional resonant conversion system 1 uses the first resonant converter 2 to boost and stabilize the DC power. As shown in FIG. 3A , the control unit controls the input transistor Qin of the first resonant converter 2 to be open, the first transistor Q1 and the third transistor Q3 to be turned on, and the second transistor Q2 to be turned off. In this state, the internal components of the first resonant converter 2 of the bidirectional resonant conversion system 1 meet the following equations:

in, The DC power provided by the first power source 11, is the voltage on the primary winding 23 of transformer T, is the voltage on the secondary winding 24 of transformer T, n is the winding turns ratio of transformer T, is the voltage on the third capacitor C3.

Next, as shown in FIG. 3B , the control unit controls the input transistor Qin of the first resonant converter 2 to be open, the first transistor Q1 and the third transistor Q3 to be closed, and the second transistor Q2 to be turned on. In this state, the internal components of the first resonant converter 2 of the bidirectional resonant conversion system 1 meet the following equations:

in, is the voltage on the primary winding 23 of transformer T, is the voltage on the secondary winding 24 of transformer T, n is the winding turns ratio of transformer T, is the voltage on the transition capacitor CB2. And the following equation can be obtained from the volt-second balance,

Where D is the duty cycle of the transistor. Substituting equations (3) and (7) into equation (5) yields the following equation:

The bidirectional resonant conversion system 1 uses the second resonant converter 3 to convert the boosted DC power into AC power. As shown in FIG. 3C , the control unit controls the fourth transistor Q4 and the seventh transistor Q7 of the second resonant converter 3 to be turned on, and the fifth transistor Q5 and the sixth transistor Q6 to be turned off. In this state, the internal components of the second resonant converter 3 of the bidirectional resonant conversion system 1 meet the following equations:

in, is the voltage on the fourth connection point A4, is the voltage on the fifth connection point A5, is the voltage on the transition capacitor CB2, is the voltage on the output capacitor Co.

Next, as shown in FIG. 3D , the control unit controls the fifth transistor Q5 and the seventh transistor Q7 of the second resonant converter 3 to be turned on, and the fourth transistor Q4 and the sixth transistor Q6 to be turned off. The equations satisfied by the internal components in this state are similar to the aforementioned equations, which will not be described in detail here.

Please refer to Figures 4A, 4B, 4C and 4D in conjunction with Figure 2, wherein Figures 4A and 4B are circuit operation diagrams of the second resonant converter of the bidirectional resonant conversion system shown in Figure 2 when operating in reverse and the input voltage is a positive half cycle, and Figures 4C and 4D are circuit operation diagrams of the second resonant converter of the bidirectional resonant conversion system shown in Figure 2 when operating in reverse and the input voltage is a negative half cycle. When the bidirectional resonant conversion system 1 is to convert the AC power provided by the second power source 12 into DC power and perform reverse operation, the bidirectional resonant conversion system 1 uses the second resonant converter 3 to convert the AC power into DC power. As shown in FIG. 4A , the AC power provided by the second power source 12 is a positive half cycle, and when it is used to charge the transition capacitor CB2, the control unit controls the fifth transistor Q5 and the seventh transistor Q7 to be turned on, and the fourth transistor Q4 and the sixth transistor Q6 to be turned off, so that the AC power provided by the second power source 12 flows through the first inductor L1, the fifth transistor Q5, the seventh transistor Q7 and the second inductor L2 in sequence. In this state, the internal components of the first resonant converter 2 of the bidirectional resonant conversion system 1 meet the following equations,

in, is the inductance of the first inductor, is the inductance of the second inductor, is the voltage on the first inductor L1, is the voltage on the second inductor L2.

Next, as shown in FIG. 4B , the AC power provided by the second power source 12 is in the positive half cycle, and when the transition capacitor CB2 is discharged, the control unit controls the fourth transistor Q4 and the seventh transistor Q7 to be turned on, and the fifth transistor Q5 and the sixth transistor Q6 to be turned off, so that the AC power provided by the second power source 12 flows through the first inductor L1, the fourth transistor Q4, the transition capacitor CB2, the seventh transistor Q7 and the second inductor L2 in sequence. In this state, the internal components of the first resonant converter 2 of the bidirectional resonant conversion system 1 meet the following equations,

in, The AC power provided by the second power source 12, is the voltage on the transition capacitor CB2. According to the volt-second balance, the above equation (16) is equal to equation (19), and the following equation can be obtained:

As shown in FIG. 4C , when the AC power provided by the second power source 12 is in the negative half cycle and is used to charge the transition capacitor CB2, the control unit controls the fifth transistor Q5 and the seventh transistor Q7 to be turned on, and the fourth transistor Q4 and the sixth transistor Q6 to be turned off, so that the AC power provided by the second power source 12 flows through the second inductor L2, the seventh transistor Q7, the fifth transistor Q5 and the first inductor L1 in sequence. In this state, the internal components of the first resonant converter 2 of the bidirectional resonant conversion system 1 meet the following equations,

in, is the inductance of the first inductor, is the inductance of the second inductor, is the voltage on the first inductor L1, is the voltage on the second inductor L2.

Next, as shown in FIG. 4D , when the AC power provided by the second power source 12 is in the negative half cycle and the transition capacitor CB2 is discharged, the control unit controls the fifth transistor Q5 and the sixth transistor Q6 to be turned on, and the fourth transistor Q4 and the seventh transistor Q7 to be turned off, so that the AC power provided by the second power source 12 flows through the second inductor L2, the sixth transistor Q6, the transition capacitor CB2, the fifth transistor Q5 and the first inductor L1 in sequence. In this state, the internal components of the first resonant converter 2 of the bidirectional resonant conversion system 1 meet the following equations,

in, The AC power provided by the second power source 12, is the voltage on the transition capacitor CB2. According to the volt-second balance, the above equation (25) is equal to equation (28), and the following equation can be obtained:

Please refer to FIG. 5 in conjunction with FIG. 2, where FIG. 5 is an equivalent circuit diagram of the first resonant converter of the bidirectional resonant conversion system shown in FIG. 2 in reverse operation. First, the winding turns ratio of the secondary winding 24, the primary winding 23 and the input inductor Lin of the transformer T is defined as n:1:1, and the transformer T has a resonant inductor Lr and a resonant capacitor Cr, where the resonant capacitor Cr is equivalent to the second capacitor C2 plus the third capacitor C3 in FIG. 2. When the bidirectional resonant conversion system 1 is in reverse operation, the first resonant converter 2 is used to step down the DC power.

First, the control unit controls the second transistor Q2 to be on, the third transistor Q3 to be off, the input transistor Qin to be on, and the first transistor Q1 to be off. The current on the transition capacitor CB2 flows through the second transistor Q2, the resonant inductor Lr and the resonant capacitor Cr in sequence, and generates resonance, so that the parasitic capacitor Coss3 of the third transistor Q3 is charged. Due to the polarity relationship of the transformer T, the input transistor Qin is forward biased and the first transistor Q1 is reverse biased, so that the energy of the primary circuit 21 is transmitted to the secondary circuit 22, and the current relationship of the equivalent circuit is as follows:

in, is the current on the resonant inductor Lr, is the voltage on the transition capacitor CB2, is the voltage on the resonant capacitor Cr, is the voltage on the input capacitor CB1. The above current relationship can be obtained by inverse Laplace transformation as follows:

Next, the control unit controls the second transistor Q2 to be turned off, the third transistor Q3 to be turned off, the input transistor Qin to be turned on, and the first transistor Q1 to be turned off. The current on the transition capacitor CB2 flows through the parasitic capacitor Coss2, the resonant inductance Lr and the resonant capacitor Cr of the second transistor Q2 in sequence, and generates resonance. When the capacitance of the parasitic capacitor Coss2 of the second transistor Q2 is close to 0, the voltage on the parasitic capacitor Coss2 of the second transistor Q2 rises, and the voltage on the parasitic capacitor Coss3 of the third transistor Q3 drops to 0. If the current of the secondary circuit 22 is greater than the load current at this time, the input transistor Qin remains turned on. At this time, the current relationship of the equivalent circuit is as follows:

Wherein, C is the capacitance of the parasitic capacitor Coss2 of the second transistor Q2 and is the capacitance of the parasitic capacitor Coss3 of the third transistor Q3. Then, the control unit controls the second transistor Q2 to be turned off, the third transistor Q3 to be turned on, the input transistor Qin to be turned off, and the first transistor Q1 to be turned on. Since the voltage on the parasitic capacitor Coss3 of the third transistor Q3 is 0 at this time, zero voltage switching is achieved. At this time, the current relationship of the equivalent circuit is as follows,

Next, the control unit controls the second transistor Q2 to be turned off, the third transistor Q3 to be turned off, the input transistor Qin to be turned off, and the first transistor Q1 to be turned on. At this time, the current on the inductor causes the voltage on the parasitic capacitor Coss2 of the second transistor Q2 to drop to 0, and causes the voltage on the parasitic capacitor Coss3 of the third transistor Q3 to rise, thereby making the cross-voltage of the second transistor Q2 0 when it is turned on next time, achieving zero voltage switching. At this time, the current relationship of the equivalent circuit is as follows,

In summary, the bidirectional resonant conversion system of the present invention includes a first power source, a second power source, a first resonant converter and a second resonant converter. The first power source provides direct current power, the second power source provides alternating current power, the first resonant converter is electrically connected between the first power source and the second resonant converter, and the second resonant converter is electrically connected between the first resonant converter and the second power source. The bidirectional resonant conversion system utilizes the arrangement structure of the electronic components in the first resonant converter and the second resonant converter to enable it to convert direct current power into alternating current power and also to convert alternating current power into direct current power. Therefore, compared with the conventional forward-flyback converter which only has a unidirectional conversion function, the bidirectional resonant conversion system of the present invention can provide bidirectional power transmission. In addition, the bidirectional resonant conversion system of this case can further utilize its circuit architecture to reduce switching losses, improve overall efficiency and power factor, and achieve the effect of reducing current harmonic components.

T1, T2: Transformer Ro: Output resistor 1: Bidirectional resonant converter system 11: First power supply Vin+: First input positive terminal Vin-: First input negative terminal 12: Second power supply Vac+: Second input positive terminal Vac-: Second input negative terminal 2: First resonant converter 21: Primary circuit CB1: Input capacitor Lin: Input inductor Qin: Input transistor C1: First capacitor R1: First resistor D1: First diode Q1: First transistor A1: First connection point T: Transformer 23: Primary winding 24: Secondary winding 22: Secondary circuit C2: Second capacitor C3: Third capacitor Q2: Second transistor Q3: Third transistor CB2: Transition capacitor A2: Second connection point A3: Third connection point 3: Second resonant converter Q4: Fourth transistor Q5: Fifth transistor Q6: Sixth transistor Q7: Seventh transistor L1: First inductor L2: Second inductor Co: Output capacitor A4: Fourth connection point A5: Fifth connection point Lr: Resonant inductor Cr: Resonant capacitor Coss2, Coss3: Parasitic capacitor

Figure 1 is a circuit diagram of a conventional forward-flyback converter; Figure 2 is a circuit diagram of a bidirectional resonant conversion system of the present invention; Figures 3A and 3B are circuit diagrams of the first resonant converter of the bidirectional resonant conversion system shown in Figure 2 in forward operation; Figures 3C and 3D are circuit diagrams of the second resonant converter of the bidirectional resonant conversion system shown in Figure 2 in forward operation; Figures 4A and 4B are circuit diagrams of the second resonant converter of the bidirectional resonant conversion system shown in Figure 2 in reverse operation and when the input voltage is a positive half cycle; Figures 4C and 4D are circuit operation diagrams of the second resonant converter of the bidirectional resonant conversion system shown in Figure 2 when operating in reverse and the input voltage is a negative half cycle; and Figure 5 is an equivalent circuit diagram of the first resonant converter of the bidirectional resonant conversion system shown in Figure 2 when operating in reverse.

1: Bidirectional resonant conversion system

11: First Power Source

Vin+: first input positive terminal

Vin-: first input negative end

12: Second power source

Vac+: Second input positive terminal

Vac-: Second input negative terminal

2: First resonant converter

21: Primary circuit

CB1: Input capacitor

Lin: Input inductor

Qin: Input transistor

C1: first capacitor

R1: first resistor

D1: First diode

Q1: First transistor

A1: First connection point

T: Transformer

23: Primary side winding group

24: Secondary side winding group

22: Secondary circuit

C2: Second capacitor

C3: The third capacitor

Q2: Second transistor

Q3: The third transistor

CB2: Transition capacitor

A2: Second connection point

A3: The third connection point

3: Second resonant converter

Q4: The fourth transistor

Q5: The fifth transistor

Q6: The sixth transistor

Q7: The seventh transistor

L1: First inductor

L2: Second inductor

Co: output capacitance

A4: The fourth connection point

A5: The fifth connection point

Claims (9)

A bidirectional resonant conversion system comprises: A first power source for providing DC power and comprising a first input positive terminal and a first input negative terminal; A second power source for providing AC power and comprising a second input positive terminal and a second input negative terminal; A first resonant converter electrically connected to the first power source and comprising: A primary circuit comprising an input transistor and a first transistor, the input transistor being connected to the first input negative terminal; A transformer, comprising a primary winding and a secondary winding, the primary winding and the first transistor are connected in series between the first input positive terminal and the first input negative terminal, and the connection point between the primary winding and the first transistor forms a first connection point; and A secondary circuit, comprising a second capacitor, a third capacitor, a second transistor, a third transistor and a transition capacitor, the second capacitor and the third capacitor are connected in series to form a first bridge arm, and the connection between the second capacitor and the third capacitor forms a second connection point, the second connection point is electrically connected to a first end of the secondary winding, the second transistor and the third transistor are connected in series to form a second bridge arm, connected in parallel with the first bridge arm, the connection between the second transistor and the third transistor forms a third connection point, the third connection point is electrically connected to a second end of the secondary winding, and the transition capacitor is connected in parallel to the first bridge arm and the second bridge arm; and A second resonant converter, electrically connected between the first resonant converter and the second power source, and comprising a fourth transistor, a fifth transistor, a sixth transistor, a seventh transistor and a high-frequency filter, the fourth transistor and the fifth transistor are connected in series to form a third bridge arm, the connection between the fourth transistor and the fifth transistor forms a fourth connection point, the sixth transistor and the seventh transistor are connected in series to form a fourth bridge arm, the fourth bridge arm is connected in parallel with the third bridge arm and the transition capacitor, the connection between the sixth transistor and the seventh transistor forms a fifth connection point, and the high-frequency filter is connected to the fourth connection point, the fifth connection point and the second power source; The first resonant converter boosts the DC power provided by the first power source, and the second resonant converter converts the boosted DC power into an output AC power, or the second resonant converter converts the AC power provided by the second resonant converter into a transition DC power, and the first resonant converter reduces the voltage of the transition DC power to output an output DC power. A bidirectional resonant conversion system as described in claim 1, wherein when the first resonant converter boosts the DC power provided by the first power source, the first power source operates and the second power source is turned off, the first transistor of the primary circuit of the first resonant converter constitutes a primary switching element, and the second capacitor and the third capacitor of the secondary circuit of the first resonant converter constitute a resonant capacitor. As described in claim 2, the bidirectional resonant conversion system, wherein when the second resonant converter converts the boosted DC power into the output AC power, the fourth transistor, the fifth transistor, the sixth transistor and the seventh transistor of the second resonant converter constitute a switching element. A bidirectional resonant conversion system as described in claim 1, wherein when the second resonant converter converts the AC power provided by the second resonant converter into the transition DC power, the second power supply operates and the first power supply is turned off, and the fourth transistor, the fifth transistor, the sixth transistor and the seventh transistor of the second resonant converter constitute a switching element. A bidirectional resonant conversion system as described in claim 4, wherein when the first resonant converter steps down the transition DC power to output the output DC power, the second transistor and the third transistor of the secondary circuit of the first resonant converter constitute a switching element, the second capacitor and the third capacitor constitute a resonant capacitor, and the first transistor of the primary circuit constitutes a rectifying element. In the bidirectional resonant conversion system as described in claim 1, the primary-side circuit of the first resonant converter includes an input capacitor, and two ends of the input capacitor are electrically connected to the first input positive terminal and the first input negative terminal of the first power source. In the bidirectional resonant conversion system as described in claim 1, the primary circuit of the first resonant converter includes an input inductor, and the input inductor and the input transistor are connected in series between the first input positive terminal and the first input negative terminal. A bidirectional resonant conversion system as described in claim 1, wherein the primary circuit of the first resonant converter includes a first capacitor, a first resistor, and a first diode, the first capacitor and the first diode are connected in series between the first input positive terminal and the first connection point, the first resistor is connected in parallel with the first capacitor, and when the first resonant converter boosts the DC power provided by the first power source, the first capacitor, the first resistor, and the first diode form a buffer circuit. A bidirectional resonant conversion system as described in claim 1, wherein the high-frequency filter of the second resonant converter comprises a first inductor, a second inductor and an output capacitor, the first inductor is electrically connected between the fourth connection point and the second input positive terminal, the second inductor is electrically connected between the fifth connection point and the second input negative terminal, the output capacitor is electrically connected between the second input positive terminal and the second input negative terminal, wherein when the second resonant converter converts the AC power provided by the second resonant converter into the transition DC power, the first inductor and the second inductor constitute a boost inductor.

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