CN102255537B - DC-AC conversion circuit - Google Patents

Abstract

The invention discloses a DC-AC conversion circuit, comprising: the switching circuit receives the DC power and converts the DC power to output an AC modulation voltage between the first and second output terminals, and includes: the first switch branch circuit is composed of a first switch element and a second switch element which are sequentially connected in series and electrically connected, and a first output end is electrically connected between the first switch element and the second switch element; the second switch branch circuit is composed of third to fifth switch elements which are sequentially connected in series and electrically connected, and a second output end is electrically connected between the fourth and fifth switch elements; a sixth switching element, one end of which is electrically connected between the third and fourth switching elements and the other end of which is electrically connected between the first and second switching elements; the first to sixth switching elements are turned on or off, so that the efficiency of the DC-AC conversion circuit is improved and the generation of leakage current is reduced.

Description

DC-AC conversion circuit

technical field

The present invention relates to a conversion circuit, and in particular to a DC-AC conversion circuit.

Background technique

At present, the main energy used by human beings is petroleum, which generates the required power or electricity by burning petroleum, such as automobiles or fuel-fired generators (factories). However, the high temperature and exhaust gas generated during the combustion of petroleum will not only cause air quality deterioration It will also worsen the global warming effect. In addition, according to the world's oil production statistics, oil production will reach its peak within ten years, and then production will decrease year by year. This not only means that oil prices (including electricity prices) will no longer be cheap, but may also lead to a real oil crisis, indirectly triggering global economic storm.

In view of this, the effective and economical conversion of renewable energy into general livelihood power supply or mechanical power has become an important industrial development policy for advanced technological countries taking into account both environmental protection and power generation. Among the renewable energy sources such as solar energy, wind energy, tidal energy, geothermal energy, and biological waste energy, the renewable energy power generation system using solar energy has become a popular technology due to its environmental protection, easy installation, mature commercial technology, and national planned auxiliary promotion. The main choice for advanced countries to develop distributed power systems.

Please refer to FIG. 1 , which is a schematic circuit diagram of a conventional DC-AC conversion circuit. As shown in Figure 1, the existing DC-AC conversion circuit 1 is applied to solar grid-connected systems, so it can also be called a photovoltaic inverter (Photovoltaic Inverter, PV inverter). The DC-AC conversion circuit 1 is non-isolated and fully The bridge circuit structure is mainly composed of an input filter circuit 10, a full bridge switching circuit 11 and an output filter circuit 12, wherein the input filter circuit 10 is composed of a first capacitor _C1 for receiving solar energy The DC input voltage V _DC generated by the board, and the DC input voltage V _DC is filtered. The full-bridge switching circuit 11 is electrically connected to the output filter circuit 12, and is composed of first to fourth switching elements S ₁ -S ₄ , the first switching element S ₁ and the second switching element S ₂ are electrically connected in series connected, the third switching element _S3 and the fourth switching element _S4 are also electrically connected in series, thereby forming a two-arm full-bridge structure, and the first to fourth switching elements _S1 - _S4 are passed through a control unit ( (not shown) is controlled to switch on or off, so that the full-bridge switching circuit 11 converts the filtered DC input voltage V _DC into an AC modulation voltage V _T . The output filter circuit 12 is electrically connected with the full-bridge switching circuit 11, and is composed of a first inductor L ₁ , a second inductor L ₂ and a second filter capacitor C ₂ , the output filter circuit ₁₂ is used for filtering The high frequency component of the AC modulated voltage V _T is removed, and an AC output voltage Vo is output to the power grid (Grid) G.

Generally speaking, the first to fourth switching elements S ₁ -S ₄ of the full bridge switching circuit 11 operate in a pulse width modulation manner, and the operating modes of the first to fourth switching elements S ₁ -S ₄ are different. It can also be divided into bipolar switching (Bipolar) or unipolar switching (Unipolar). Please refer to Figure 2 and Figure 3, where Figure 2 is the waveform diagram of the modulation voltage output by the full-bridge switching circuit shown in Figure 1 when it is operating in bipolar switching mode, and Figure 3 is the waveform diagram of the modulation voltage shown in Figure 1 The waveform diagram of the modulation voltage output by the full-bridge switching circuit when the modulation voltage output by the full-bridge switching circuit is in the unipolar switching working mode. As shown in Figures 1 to 3, when in the bipolar switching mode, the first to fourth switching elements S ₁ -S ₄ perform high-frequency switching operations, so that the AC output from the full-bridge switching circuit 11 When the modulation voltage V _T is in the positive half cycle or the negative half cycle, it changes between the positive DC input voltage V _DC and the negative DC input voltage V _DC , that is, as shown in Figure 2, when the unipolar In the switching mode, only a single bridge arm performs high-frequency switching when the switch is switched every half cycle, and the other bridge arm maintains a fixed switching state, that is, only the first switching element S constituting one of the bridge arms ₁ and the second switching element S ₂ or the third switching element S ₃ and the fourth switching element S ₄ that constitute another bridge arm operate in an alternate conducting manner, so that the AC modulation output by the full-bridge switching circuit 11 The variable voltage V _T varies between 0 and the positive DC input voltage VDC in the positive half cycle, and varies between 0 and the negative DC input voltage VDC in the negative half cycle, that is, as shown in Figure 3 Show.

When the full-bridge switching circuit 11 operates in a unipolar switching mode of operation, because only two switching elements included in a single bridge arm are performing high-frequency switching operations each time the switch is switched, instead of using bipolar When operating in the permanent switching mode, the first to fourth switching elements S ₁ -S ₄ switch at high frequency, so that the AC modulation voltage V _T is only between 0 and the positive DC input voltage V _DC or The DC input voltage V _DC varies between 0 and negative values, so the switching loss of the full-bridge switching circuit 11 in the unipolar switching mode is less than that in the bipolar switching mode, in other words, the efficiency higher. However, due to the parasitic capacitance C _P between the solar panels that generate the DC input voltage V _DC and the ground, as shown in Figure 1, when the full-bridge switching circuit 11 operates in a unipolar switching mode, the full-bridge The modulated voltage V _T output by the switching circuit 11 has high-frequency components, so the relative voltage of the first output terminal A' of the full-bridge switching circuit 11 to any point in the DC-AC converting circuit 1 is, for example, the same as The relative voltage of the common point N' electrically connected to the parasitic capacitance C _P , and the relative voltage of the second output terminal B' to the common point N', the summed average value of the two at any switching time point cannot maintain a constant value , resulting in a significant voltage change on the parasitic capacitance _CP , which in turn generates a leakage current that endangers the human body and equipment. On the other hand, the full-bridge switching circuit 11 operates in a bipolar switching mode, which has the effect of avoiding the generation of leakage current. .

Therefore, how to develop a DC-AC conversion circuit that can improve the above-mentioned deficiencies in the prior art and simultaneously improve efficiency and reduce leakage current generation is an urgent problem to be solved at present.

Contents of the invention

The main purpose of the present invention is to provide a DC-AC conversion circuit to solve the problem that the existing DC-AC conversion circuit cannot achieve high efficiency and reduce leakage current when it is applied to a solar grid-connected system.

To achieve the above purpose, a preferred embodiment of the present invention provides a DC-AC conversion circuit, including: a switching circuit, structured to receive DC power, convert it, and output it between the first output terminal and the second output terminal AC modulated voltage, and includes: a first switch branch circuit, including a first switch element and a second switch element electrically connected in series in sequence, the first switch element and the second switch element are electrically connected to the first output terminal ; The second switch branch is electrically connected in parallel with the first switch branch, and includes a third switch element, a fourth switch element, and a fifth switch element electrically connected in series in sequence, the fourth switch element and the fifth switch The elements are electrically connected to the second output end; and the sixth switch element, one end of which is electrically connected between the third switch element and the fourth switch element, and the other end is electrically connected to the first switch element and the second switch element and electrically connected to the first output terminal; wherein, during the positive half cycle, the first switching element and the fifth switching element are simultaneously and continuously switched on or off, and the sixth switching element is in a conducting state, In the negative half cycle, instead, the second switch element and the third switch element are switched on or off simultaneously and continuously, and the fourth switch element is in a conduction state.

Description of drawings

FIG. 1 is a schematic circuit diagram of an existing DC-AC conversion circuit.

FIG. 2 is a waveform diagram of an AC modulation voltage output by the full-bridge switching circuit shown in FIG. 1 in a bipolar switching working mode.

FIG. 3 is a waveform diagram of an AC modulation voltage output by the full-bridge switching circuit shown in FIG. 1 in a unipolar switching working mode.

FIG. 4 is a schematic diagram of a circuit structure of a DC-AC conversion circuit according to a preferred embodiment of the present invention.

FIG. 5A is a schematic timing diagram of voltages and control signals in FIG. 4 .

FIG. 5B is a waveform diagram of the AC modulation voltage shown in FIG. 4 .

FIG. 6A is a schematic diagram of the circuit structure of the control unit shown in FIG. 4 .

FIG. 6B is a schematic timing diagram of voltages and control signals shown in FIG. 6A .

FIG. 7A is a schematic circuit structure diagram of another modification example of the control unit shown in FIG. 4 .

FIG. 7B is a timing diagram of the voltage and control signals shown in FIG. 7A .

Wherein, the reference signs are explained as follows:

1, 4: DC-AC conversion circuit; 10, 40: input filter circuit;

11: Full bridge switching circuit; 12, 42: Output filter circuit;

41: switch circuit; 411: first switch branch;

412: second switch branch; 43: control unit;

430～432: first to third comparators; 730～732: first to third comparators;

433, 733: NOT gate; 734-735: first-second AND gate;

736: Rectification device; 8: DC device;

9: AC load; C ₁ : first capacitor;

C ₂ : second capacitance; C _P : parasitic capacitance;

L ₁ : first inductance; L ₂ : second inductance;

Vo: AC output voltage; V _DC : DC input voltage;

VT: AC modulation voltage; V _C1 ~ V _C6 : first to sixth control signals;

V ₁ ～V ₂ : first to second sine wave signals; V _AN ～V _BN : first to second relative voltages;

V ₃ : sine wave signal; V ₄ : rectified sine wave signal;

V _TRI : triangle wave signal; Vr: specific voltage value;

G: power network; S ₁ ~ S ₆ : first to sixth switching elements;

A', A: the first output terminal; B', B: the second output terminal;

N', N: common contact; T ₁ ~ T ₂ : first to second time;

D ₁ -D ₆ : first to sixth body diodes.

Detailed ways

Some typical embodiments embodying the features and advantages of the present invention will be described in detail in the description in the following paragraphs. It should be understood that the present invention can have various changes in different ways without departing from the scope of the present invention, and the descriptions and drawings therein are used for illustration in nature rather than for limiting the present invention .

Please refer to FIG. 4 , which is a schematic diagram of a circuit structure of a DC-AC conversion circuit according to a preferred embodiment of the present invention. As shown in FIG. 4, the DC-AC conversion circuit 4 can be but not limited to be applied to solar grid-connected systems, and is a non-isolated circuit structure, which receives a DC input generated by a DC device 8, such as a solar panel. The voltage V _DC is converted into an AC output voltage Vo to provide an AC load 9 , such as an AC electrical device or a mains network system.

The DC-AC conversion circuit 4 mainly includes an input filter circuit 40 , a switching circuit 41 , an output filter circuit 42 and a control unit 43 . The input filter circuit 40 is electrically connected to the positive terminal and the negative terminal of the DC device 8 respectively to receive the DC input voltage V _DC , which is used to filter the DC input voltage V _DC . In this embodiment, the input filter circuit 40 can be It is but not limited to be constituted by a first capacitor _C1 .

The switching circuit 41 is electrically connected to the input filter circuit 40, and includes first to sixth switching elements S ₁ -S ₆ , and the switching circuit 41 operates by switching on or off the first to sixth switching elements S ₁ -S ₆ The filtered DC input voltage V _DC is converted to output an AC modulated voltage V _T between a first output terminal A and a second output terminal B.

In this embodiment, the first switching element _S1 and the second switching element _S2 are electrically connected in series in order to form a first switching branch 411, and one end of the first switching element _S1 is connected to the positive end of the DC device 8 is electrically connected to the positive end of the input filter circuit 40 , and one end of the second switch element S ₂ is electrically connected to the negative end of the DC device 8 and the negative end of the input filter circuit 40 . The third switch element S ₃ , the fourth switch element S ₄ , and the fifth switch element S ₅ are electrically connected in series in sequence to form a second switch branch 412 electrically connected in parallel with the first switch branch 411 , and the third switch branch 412 is electrically connected in parallel. One end of the switch element _S3 is electrically connected to the positive end of the DC device 8 and the positive end of the input filter circuit 40, and one end of the fifth switch element _S5 is connected to the negative end of the DC device 8 and the negative end of the input filter circuit 40. electrical connection. One end of the sixth switching element _S6 is electrically connected between the third switching element _S3 and the fourth switching element _S4 of the second switching branch 412, and the other end is electrically connected to the first switching element S4 of the first switching branch 411. The switch element _S1 and the second switch element _S2 are electrically connected to the first output terminal A.

In some embodiments, the first to sixth switching elements S ₁ -S ₆ may be but not limited to be made of Metal-Oxide-SemiConduCtor Field-EffeCt Transistor (MOSFET), and the first to The sixth switching elements S ₁ -S ₆ each have a corresponding body diode (body diode), such as the first to sixth body diodes D ₁ -D ₆ shown in FIG. 4 , wherein the first body diode D ₁ and the second body diode The conduction direction of the diode _D2 is the direction from the second switch element _S2 to the first switch element _S1 , and the conduction direction of the third body diode _D3 , the fourth body diode _D4 and the fifth body diode _D5 is From the fifth switching element _S5 to the third switching element _S3 , the conduction direction of the sixth body diode _D6 is from the first switching branch 411 to the second switching branch 412.

The control unit 43 is electrically connected to the control terminals of the first to sixth switching elements S ₁ -S ₆ , and generates first to sixth control signals V _C1 to V _C6 in the form of pulse width modulation to control the first to sixth switching elements respectively. The six switching elements S ₁ -S ₆ are turned on or off.

The output filter circuit 42 is electrically connected to the first output terminal A and the second output terminal B of the switching circuit 41, and is electrically connected to the AC load 9 for receiving the AC modulation voltage V _T and filtering the AC modulation voltage The high-frequency component of V _T is used to output the AC output voltage Vo to the AC load 9 . In this embodiment, the output filter circuit 4 is composed of a first inductor L ₁ , a second inductor L ₂ and a second capacitor C ₂ , wherein one end of the first inductor L ₁ is electrically connected to the first output end A One end of the second inductor L ₂ is electrically connected to the second output terminal B, and the second capacitor C ₂ is electrically connected to the first inductor L ₁ , the second inductor L ₂ and the AC load 9 .

The operation of the DC-AC conversion circuit 4 of the present invention will be described exemplarily below. Please refer to FIG. 5A and FIG. 5B , together with FIG. 4 , wherein FIG. 5A and FIG. 5B are respectively a timing diagram of the voltage and control signals and a waveform diagram of the AC modulation voltage in FIG. 4 . As shown in Fig. 4, Fig. 5A and Fig. 5B, when in the positive half cycle, for example, between 0 and the first time _T1 , the first control signal V _C1 and the fifth control signal V _C5 are pulse-width modulated change, that is, the interactive change of the disabled level (disabled) and the enabled level (eanbled), so the first switching element _S1 and the fifth switching element _S5 are simultaneously and continuously switched on or off. In addition, the second The second control signal V _C2 , the third control signal V _C3 and the fourth control signal V _C4 are continuously maintained at the disabled level, so the second switch element S ₂ , the third switch element S ₃ and the fourth switch element S ₄ are Furthermore, the sixth control signal V _C6 is continuously maintained at the enable level, so the sixth switch element S ₆ is in the on state.

Therefore, when the first switching element S ₁ and the fifth switching element S ₅ are in the conduction state in the positive half cycle, the current output by the DC device 8 flows through the first switching element S ₁ , the first inductor L ₁ , the The second capacitor C ₂ , the second inductance L ₂ and the fifth switching element S ₅ , so that the DC power output by the DC device 8 can be converted and filtered by the DC-AC conversion circuit 4 and transmitted to the AC in the form of AC. Load 9, while the first inductance L ₁ and the second inductance L ₂ store energy, when the first switching element S ₁ and the fifth switching element S ₅ are switched to the cut-off state in the positive half cycle, due to the current continuity characteristic of the inductance, Therefore, the energy stored in the first inductor _L1 and the second inductor _L2 will sequentially flow through the fourth body diode D4 of the fourth switching element _S4 in the off state and the fourth body diode _D4 in the on state in the form of current. Six switching elements S ₆ , so the AC load 9 can also continuously receive the electric energy output by the DC device 8 .

When in the negative half cycle, for example, between the first time T ₁ and the second time T ₂ , the second control signal V _C2 and the third control signal V _C3 change in a pulse width modulation manner, that is, the disable level and The enable level changes alternately, so the second switch element S ₂ and the third switch element S ₃ are simultaneously and continuously switched on or off. In addition, the first control signal V _C1 , the fifth control signal V _C5 , and the The sixth control signal V _C6 is changed to continuously maintain at the disabled level, so the first switch element S ₁ , the fifth switch element S ₅ and the sixth switch element S ₆ are in the cut-off state. Furthermore, the fourth control signal V _C4 Instead, the fourth switching element _S4 is turned on by continuously maintaining the enabling level.

Therefore, when the second switching element S ₂ and the third switching element S ₃ are in the conduction state in the negative half cycle, the current output by the DC device 8 flows through the third switching element S ₃ and the fourth switching element S _{4 in} sequence. , the second inductance L ₂ , the second capacitor C ₂ , the first inductance L ₁ and the second switching element S ₂ , so the electric energy in the form of DC output by the DC device 8 can be converted and filtered by the DC-AC conversion circuit 4 And it is transmitted to the AC load 9 in the form of AC, while the first inductance _L1 and the second inductance _L2 store energy. When the second switching element _S2 and the third switching element _S3 are switched to the cut-off state in the negative half cycle, Due to the continuous current characteristic of the inductor, the energy stored in the first inductor L ₁ and the second inductor L ₂ will sequentially flow through the sixth body diode D ₆ of the sixth switching element S ₆ in the off state in the form of current and the fourth switching element S ₄ in the conduction state, so the AC load 9 can also continuously receive the electric energy output by the DC device 8 .

Please refer to FIG. 5B again. Through the setting of the fourth switching element _S4 and the sixth switching element _S6 , the AC modulation voltage V _T output by the switching circuit 41 is a specific voltage from 0 to a positive value in the positive half cycle. Vr varies, and in the negative half cycle, it varies between 0 and a negative specific voltage value Vr, so the actual action mode of the switching circuit 41 is the same as that of the existing DC-AC conversion circuit shown in FIG. 1 The full-bridge switching circuit 11 of 1 is similar to the unipolar switching mode of operation, so the DC-AC conversion circuit 4 of the present invention can reduce the switching loss of these switching elements inside the switching circuit 41, thereby improving efficiency. In addition, It can be seen from the figure that the first output terminal A and the second output terminal B of the switching circuit 41 are respectively relative to a specific point of the internal circuit of the DC-AC conversion circuit 4, for example, for the parasitic capacitance C _P generated by the DC device 8 (As shown in FIG. 4 ) The summed average value of the first relative voltage V _AN and the second relative voltage V _BN of the electrically connected common contact point N at any switching time point maintains a fixed value, so the parasitic capacitance There is no obvious voltage change on _CP , so that the generation of leakage current can be reduced, thereby reducing the risk of harming human body and equipment.

In the above embodiment, the first control signal V _C1 , the second control signal V _C2 , the third control signal V _C3 and the fifth control signal V _C5 are high-frequency pulse width modulation signals, and the fourth control signal V _C4 and The sixth control signal V _C6 is a low-frequency PWM signal.

The circuit configuration of the control unit 43 shown in FIG. 4 will be briefly described below. Please refer to FIG. 6A and FIG. 6B, wherein FIG. 6A is a schematic diagram of the circuit structure of the control unit shown in FIG. 4, and FIG. 6B is a schematic diagram of the timing sequence of the voltage and control signals shown in FIG. , a second comparator 431, a third comparator 432 and a NOT gate 433, wherein the positive input terminal of the first comparator 430 receives a first sine wave signal V ₁ , and the negative input terminal of the first comparator 430 is grounded, the output terminal of the first comparator 430 is electrically connected to the control terminal of the sixth switch element _S6 and outputs the sixth control signal V _C6 , the positive input terminal of the second comparator 431 receives the first sinusoidal signal V 1 , and the second comparator 431 receives the first sinusoidal signal V ₁ . The negative input terminal of the second comparator 431 receives a triangular wave signal V _TRI , the output terminal of the second comparator 431 is electrically connected to the control terminal of the first switching element _S1 and the control terminal of the fifth switching element _S5 and outputs the first For the control signal V _C1 and the fifth control signal V _C5 , the positive input terminal of the third comparator 432 receives a second sinusoidal signal V ₂ , and the phase difference between the first sinusoidal signal V ₁ and the second sinusoidal signal V ₂ is 180 degrees, the negative input terminal of the third comparator 432 receives the triangular wave signal V _TRI , and the output terminal of the third comparator 432 is electrically connected to the control terminal of the second switching element _S2 and the control terminal of the third switching element _S3 And output the second control signal V _C2 and the third control signal V _C3 , the input terminal of the NOT gate 433 is electrically connected to the output terminal of the first comparator 430, and the output terminal of the NOT gate 433 is electrically connected to the fourth switching element S ₄ , the inverter 433 inverts the sixth control signal V _C6 to output the fourth control signal V _C4 .

Certainly, the control unit 43 is not limited to the above-mentioned circuit structure. In some embodiments, as shown in FIGS. 7A and 7B , the control unit 43 may also include a first comparator 730 and a second comparator 731. , a third comparator 732, a NOT gate 733, a first AND gate 734, a second AND gate 735 and a rectifying device 736, wherein the rectifying device 736 receives a sine wave signal V ₃ and rectifies it into a rectified sine wave Signal V ₄ .

The positive input terminal of the first comparator 730 is electrically connected to the rectification device 736 to receive the rectified sine wave signal V ₄ , the negative input terminal of the first comparator 730 receives the triangular wave signal V _TRI , and the output terminal of the first comparator 730 is electrically connected. Connected to the first input terminal of the first AND gate 734. The positive input terminal of the second comparator 731 receives the sine wave signal V ₃ , the negative input terminal of the second comparator 731 is grounded, the output terminal of the second comparator 731 is electrically connected to the control terminal of the sixth switching element S ₆ and outputs Sixth control signal V _C6 . The positive input terminal of the third comparator 732 is electrically connected to the rectification device 736 to receive the rectified sine wave signal V ₄ , the negative input terminal of the third comparator 732 receives the triangular wave signal V _TRI , and the output terminal of the third comparator 732 is electrically connected Connected to a first input end of the second AND gate 735 .

The input terminal of the NOT gate 733 is electrically connected to the output terminal of the second comparator 731 to receive the sixth control signal V _C6 , the output terminal of the NOT gate 733 is electrically connected to the fourth switching element S ₄ , and the NOT gate 733 controls the sixth control signal V C6 . The signal V _C6 is reversed so as to output the fourth control signal V _C4 from the output end of the NOT gate 733 . The second input end of the first AND gate 734 is electrically connected to the output end of the second comparator 731 to receive the sixth control signal V _C6 , and the output end of the first AND gate 734 is electrically connected to the control of the first switching element S ₁ terminal and the control terminal of the fifth switching element _S5 and outputs the first control signal V _C1 and the fifth control signal V _C5 , the second input terminal of the second AND gate 735 is electrically connected with the output terminal of the NOT gate 733 to receive the first Four control signals V _C4 , the output end of the second AND gate 735 is electrically connected to the control end of the second switch element S ₂ and the control end of the third switch element S ₃ and outputs the second control signal V _C2 and the third control signal V _C3 .

In summary, the DC-AC conversion circuit of the present invention reduces the switching loss of these switching elements inside the switching circuit by setting the fourth switching element and the sixth switching element, thereby improving the efficiency. The parasitic capacitance formed by the ground will not produce significant voltage changes, so the generation of leakage current can be reduced, thereby reducing the risk of harming the human body and equipment.

The present invention can be modified in various ways by those who are familiar with this technology, but all of them do not break away from the intended protection of the scope of the appended patent application.

Claims (15)

1. a dc-ac conversion circuit is characterized in that, comprising: one switches circuit, and framework is in receiving direct current energy, and changes, and output one exchanges demodulating voltage between one first output and one second output, and comprises:

One first switching branches comprises one first switch element and a second switch element that sequentially series connection is electrically connected, is electrically connected this first output between this first switch element and this second switch element; And

One second switch branch road, with the electric connection in parallel of this first switching branches, and comprise one the 3rd switch element, one the 4th switch element and one the 5th switch element that sequentially series connection is electrically connected, be electrically connected this second output between the 4th switch element and the 5th switch element; And

One the 6th switch element, one end are electrically connected between the 3rd switch element and the 4th switch element, and the other end is electrically connected between this first switch element and this second switch element and with this first output and is electrically connected;

Wherein, when positive half cycle, this first switch element and the 5th switch element while and constantly conducting or cut-off switching, the 6th switch element is conducting state, when negative half period, change by this second switch element and the 3rd switch element while and constantly conducting or cut-off switching, the 4th switch element is conducting state.

2. dc-ac conversion circuit according to claim 1, it is characterized in that, when positive half cycle, this second switch element, the 3rd switch element and the 4th switch element are cut-off state, when negative half period, this first switch element, the 5th switch element and the 6th switch element are cut-off state.

3. dc-ac conversion circuit according to claim 2, it is characterized in that, also have a control unit, be electrically connected to the 6th switch element with this first switch element, framework is in the action of controlling respectively this first switch element and carry out to the 6th switch element conducting or cut-off.

4. dc-ac conversion circuit according to claim 3 is characterized in that, this control unit comprises:

One first comparator, the positive input terminal of this first comparator receive a first string ripple signal, the negative input end ground connection of this first comparator, and the output of this first comparator is electrically connected the control end of the 6th switch element;

One second comparator, the positive input terminal of this second comparator receives this first string ripple signal, the negative input end of this second comparator receives a triangular signal, and the output of this second comparator is electrically connected the control end of this first switch element and the control end of the 5th switch element;

One the 3rd comparator, the positive input terminal of the 3rd comparator receives one second string ripple signal, the negative input end of the 3rd comparator receives this triangular signal, and the output of the 3rd comparator is electrically connected the control end of this second switch element and the control end of the 3rd switch element; And

One not gate, the input of this not gate is electrically connected at the output of this first comparator, and the output of this not gate is electrically connected at the control end of the 4th switch element.

5. dc-ac conversion circuit according to claim 4 is characterized in that, the phase difference of this first string ripple signal and this second string ripple signal is 180 degree.

6. dc-ac conversion circuit according to claim 3 is characterized in that, this control unit comprises:

One rectifying device receives a string ripple signal, and is rectified into a rectification string ripple signal;

One first comparator, the positive input terminal of this first comparator is electrically connected this rectifying device, and the negative input end of this first comparator receives a triangular signal;

One second comparator, the positive input terminal of this second comparator receive this string ripple signal, the negative input end ground connection of this second comparator, and the output of this second comparator is electrically connected the control end of the 6th switch element;

One the 3rd comparator, the positive input terminal of the 3rd comparator is electrically connected this rectifying device, and the negative input end of the 3rd comparator receives this triangular signal;

One not gate, the input of this not gate is electrically connected the output of this second comparator, and the output of this not gate is electrically connected the control end of the 4th switch element;

One first with the door, this first is electrically connected the output of this first comparator with the first input end of door, this first is electrically connected the output of this second comparator with the second input of door, and this first is electrically connected the control end of this first switch element and the control end of the 5th switch element with the output of door; And

One second with the door, this second is electrically connected the output of this not gate with the first input end of door, this second is electrically connected the output of the 3rd comparator with the second input of door, and this second is electrically connected the control end of this second switch element and the control end of the 3rd switch element with the output of door.

7. dc-ac conversion circuit according to claim 1, it is characterized in that this first switch element, this second switch element, the 3rd switch element, the 4th switch element, the 5th switch element and the 6th switch element operate in the mode of pulse width modulation.

8. dc-ac conversion circuit according to claim 1, it is characterized in that, this first switch element, this second switch element, the 3rd switch element and the 5th switch element carry out conducting in the mode of high frequency or cut-off is switched, and the 4th switch element and the 6th switch element carry out conducting in the mode of low frequency or cut-off is switched.

9. dc-ac conversion circuit according to claim 1 is characterized in that, this dc-ac conversion circuit is non-isolation type.

10. dc-ac conversion circuit according to claim 1 is characterized in that, this dc-ac conversion circuit is applied to the sunlight grid-connected system.

11. dc-ac conversion circuit according to claim 1 is characterized in that, this first to the 6th switch element is made of metal-oxide half field effect transistor.

12. dc-ac conversion circuit according to claim 11, it is characterized in that, this first to the 6th switch element has a body diode separately, and the conducting direction of those body diodes that this second switch element and this first switch element the have direction of this second switch element to this first switch element of serving as reasons, the 3rd switch element, the conducting direction of those body diodes that the 4th switch element and the 5th switch element have is served as reasons the 5th switch element to the direction of the 3rd switch element, the conducting direction of this body diode that the 6th switch element the has direction of this first switching branches to this second switch branch road of serving as reasons.

13. dc-ac conversion circuit according to claim 1, it is characterized in that this of this commutation circuit the first output maintains a fixed value to this second output of one first relative voltage of the altogether contact in this dc-ac conversion circuit and this commutation circuit to the addition mean value of one second relative voltage of this common contact.

14. dc-ac conversion circuit according to claim 1, it is characterized in that, this dc-ac conversion circuit also has an input filter circuit, be electrically connected with this commutation circuit, framework is in receiving a direct current input voltage, and this DC input voitage carried out filtering, change with this DC input voitage behind the output filtering to this commutation circuit.

15. dc-ac conversion circuit according to claim 1, it is characterized in that this dc-ac conversion circuit also has an output filter circuit, be electrically connected with this commutation circuit, framework is somebody's turn to do the high frequency composition that exchanges demodulating voltage in filtering, to export an ac output voltage.

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