JP2010252450A - Power conversion apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a power conversion apparatus capable of keeping an output voltage constant by gently varying a duty even in instantaneous voltage drop, and capable of reducing loss without need for increasing a breakdown voltage of a semiconductor device connected to the secondary side. <P>SOLUTION: The power conversion apparatus includes: a first series circuit in which first and second capacitors are connected in series at a first junction point; a second series circuit in which first and second switching elements are connected in series at a second junction point; a third series circuit in which third and fourth switching elements are connected in series at a third junction point; and a DC voltage source with which these series circuits are connected in parallel and which supplies direct-current input voltage to these circuits. The power conversion apparatus takes out an AC output voltage generated between the second junction point and the third junction point. The power conversion apparatus further includes a switch that is placed between the first junction point and the second junction point and connects these junction points together. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a power converter using a semiconductor switching element, and more particularly to a power converter suitable for reducing loss.

FIG. 9 shows a conceptual circuit diagram of a conventional inverter. In this figure, the first switching element [MOSFET (Q ₁ )] and the second switching element [MOSFET (Q ₂ )] are connected at a first connection point x to constitute a first series circuit. The third switching element [MOSFET (Q ₃ )] and the fourth switching element [MOSFET (Q ₄ )] are connected at the second connection point y to form a second series circuit. These first and second series circuits are connected in parallel, respectively, and a DC voltage is applied from the DC voltage source 1 to both ends of the circuits connected in parallel. The DC voltage source 1 is often obtained by rectifying commercial AC, for example.
The power converter configured as described above functions as an inverter that outputs an AC voltage between the connection points x and y by controlling the gates of the MOSFETs (Q _{1 to} Q ₄ ) by a gate driving unit (not shown). In detail, as shown in FIG. 10, the gate drive signals (V _{gs1 to} V _gs4 ) respectively applied to the MOSFETs (Q _{1 to} Q ₄ ) are in a period in which both the MOSFET (Q ₂ ) and the MOSFET (Q ₃ ) are off. The MOSFET (Q ₁ ) and the MOSFET (Q ₄ ) are turned on. Conversely, the MOSFET (Q ₂ ) and the MOSFET (Q ₃ ) are turned on while both the MOSFET (Q ₁ ) and the MOSFET (Q ₄ ) are off.

In this way, by driving and controlling the MOSFETs (Q _{1 to} Q ₄ ) by the gate drive unit, a desired AC voltage V _t can be obtained between the connection points x and y. Incidentally, the AC voltage V _t is controlled on-time and off-time period ratio of the gate drive signals (V _gs1 ~V _GS4) (duty) can be adjusted to a desired voltage value by a so-called PWM (pulse width control) .
The inverter described above may be configured as a DC-DC converter that obtains a DC voltage by providing a rectifier circuit that converts an AC voltage into a DC voltage at the connection points x and y, although not shown, or is transformed to the connection points x and y. In some cases, the input side (primary side) of the transformer is connected, and the output side (secondary side) of the transformer is further provided with a rectifier circuit.
When the DC voltage source 1 of the inverter or converter (hereinafter referred to as a power conversion device) having such a configuration is a rectified commercial AC, the commercial AC voltage is restored immediately after being reduced for a short time, so-called instantaneous. A voltage drop (instantaneous drop) may occur and the output voltage may fluctuate.

However, for example, when an electronic control device using a microcomputer or a semiconductor is connected as a load of the power conversion device, it is necessary to keep the output voltage constant even when an instantaneous drop occurs.
The power conversion device described above, sag order to maintain the output voltage when the generated, for example, 11 high duty ratio as shown in the gate drive signals _{(V gs1} ~V gs4) the MOSFET (Q ₁ to Q _By applying each to ₄ ), the amount of power transmitted to the output side is maintained, and the output voltage is maintained at a predetermined voltage. In other words, the circuit parameters are set in the above-described power conversion device so that the output voltage can be kept constant even when an instantaneous drop occurs. For this reason, during normal operation, the duty is reduced to reduce the output. Accordingly, in order to maintain the output voltage at a predetermined voltage value even when the duty is low, it is necessary to set the transformer ratio of the transformer small. In this case, a voltage generated in the MOSFET connected to the secondary side becomes high, which causes a new problem that an element having a high withstand voltage must be used.

In general, when the withstand voltage of the switching element is increased, the forward voltage drop during conduction is also increased, so that conduction loss is increased. In addition, since the current transformation ratio of the transformer is increased, when the same current is output on the secondary side of the transformer, the primary current of the transformer further increases. For this reason, the problem that conduction loss increases also arises.
As a method for solving such a problem, Patent Documents 1 and 2 propose a method of switching from a half-bridge operation to a full-bridge operation using a relay.

Japanese Patent Laid-Open No. 4-361872 JP 2002-96167 A

However, in the method described in the above-mentioned patent document, when switching from the half-bridge operation to the full-bridge operation, the voltage amplitude applied to the transformer is doubled. For this reason, the duty of the gate signal applied to the MOSFETs (Q _{1 to} Q ₄ ) must be changed instantaneously to maintain the output voltage at a predetermined voltage.
In general, the duty is often controlled by PI control (proportional / integral control). On the other hand, since the duty changes gradually, it is difficult to cope with a sudden change in voltage, which causes the output voltage to fluctuate. Of course, in the method using the relay described in the above-mentioned patent document, high-speed switching is difficult because of the delay time.
Furthermore, when the voltage amplitude is doubled, the resonance voltage due to the leakage inductance of the transformer generated on the secondary side of the transformer and the parasitic capacitance of the rectifier element is also doubled. For this reason, when there is a circuit having a semiconductor element on the secondary side, it is necessary to use a semiconductor element having a high withstand voltage so as to withstand this jumping voltage.

The present invention has been made to solve such a problem, and the object of the present invention is to maintain the output voltage constant by gradually changing the duty even at the time of a sag, and at the secondary side. An object of the present invention is to provide a power conversion device that does not need to increase the breakdown voltage of a semiconductor element even if there is a circuit having the semiconductor element.
Furthermore, this invention is providing the power converter device which can reduce the loss which generate | occur | produces at the time of switching of a semiconductor switching element.

In order to solve the above-described object, a power conversion device according to the present invention includes a first series circuit in which a first capacitor and a second capacitor are connected in series at a first connection point, and the first series circuit. And a second series circuit in which the first switching element and the second switching element are connected in series at the second connection point, and in parallel with the first and second series circuits. A third series circuit in which a third switching element and a fourth switching element are connected in series at a third connection point, and a DC input voltage is applied to these first to third series circuits. A power converter for taking out an AC output voltage generated between both the second connection point and the third connection point, further comprising: the first connection point; Connected to the second connection point to connect these two connection points. It is characterized by comprising a switch that.
In the power converter of the present invention, when the DC input voltage of the DC voltage source is equal to or higher than a predetermined voltage value, the switch is turned on, the first and second switching elements are turned off, and the first switching element is turned off. While the third and fourth switching elements are exclusively turned on or off at a predetermined cycle, when the DC input voltage falls below a predetermined voltage value, the switch is synchronized with the third and fourth switching cycles. The first and second switching elements are turned on at a predetermined cycle, and the switching pulse width is controlled according to the input DC voltage value.

In the power converter described above, the switch is opened at the time of steady state when the DC input voltage is equal to or higher than a predetermined voltage value, and each switching element is driven at a predetermined switching timing in the first series circuit and the second series circuit. When the DC input voltage falls below a predetermined voltage value, for example, when an instantaneous drop occurs, the switch is closed to maintain the output voltage at the predetermined voltage as a three-level operation.
The switch in the power converter of the present invention is configured as a fifth and sixth switching element connected in anti-series.
Preferably, the fifth and sixth switching elements are elements having a withstand voltage lower than those of the first to fourth switching elements.
The fifth and sixth switching elements are preferably elements having a switching speed slower than that of the first to fourth switching elements.

The fifth and sixth switching elements in the above-described power conversion device use elements whose breakdown voltage is lower than that of the first to fourth switching elements and whose switching speed is slow, so that loss can be reduced. It is.
Each of the fifth and sixth switching elements has a parasitic diode, and is configured to turn on the switching element having the parasitic diode when a current flows through the parasitic diode.
In the power conversion device described above, since the current flowing through the parasitic diode flows through the switching element, the loss can be reduced.
Or the said 5th and 6th switching element is comprised with the bidirectional | two-way switch which connected two reverse element type switching elements in antiparallel.

In the power conversion device described above, the switch is configured by applying a switching element such as an IGBT having no parasitic diode such as a MOSFET.
Moreover, the power converter device of this invention is equipped with the transformer which transforms and outputs the alternating voltage which arises between both the connection points of the said 2nd connection point and the said 3rd connection point.
The power converter described above can obtain a desired AC voltage obtained by transforming an AC voltage obtained by switching.
Alternatively, the power conversion device of the present invention includes a DC-DC including a rectifier circuit that rectifies an AC output voltage generated between both the second connection point and the third connection point to output a DC voltage. Provided as a converter.
The power converter described above can obtain a desired DC voltage output from the input DC voltage.

Moreover, the power converter device of this invention comprises the insulation type DC-DC converter provided with the rectifier circuit which rectifies | straightens the alternating voltage output from the said transformer, and outputs a direct voltage.
In the power converter described above, the input side and the output side are insulated by a transformer, and a desired DC voltage output can be obtained from the input DC voltage.

Since the power conversion device of the present invention can be operated at the maximum duty in a steady state, the withstand voltage of the switching element can be kept low by optimizing the transformer ratio so as to meet this condition. Therefore, the power conversion device of the present invention can suppress an increase in conduction loss accompanying an increase in input side current. In addition, since the power converter according to the present invention gradually changes the duty as a three-level operation during a sag, the DC output voltage can be kept constant.
In addition, an element having a lower withstand voltage than the other switching elements (first to fourth switching elements) can be selected as the fifth and sixth switching elements constituting the switch, and the conduction loss can be further reduced. Is possible. Furthermore, since the power converter of this invention has always turned on the 5th and 6th switching element, an element with a slow switching speed can be applied. For this reason, since the element with the lowest forward voltage drop can be selected, the power conversion device of the present invention can further reduce the conduction loss.

In the power converter of the present invention, only half of the DC input voltage is applied to the third and fourth switching elements in the normal state, so that the switching loss can be halved. . Furthermore, since there are only two elements that switch, switching loss can be further reduced.
Moreover, since the control signals are input to the fifth and sixth switching elements so that the period during which the current flows in the parasitic diode is shortened at the time of the instantaneous drop, the loss can be reduced. Of course, by combining a reverse blocking semiconductor element with the switch, the number of semiconductor elements through which current passes can be reduced, and loss can be reduced.
Furthermore, since the power conversion apparatus according to the present invention operates in three levels, the voltage change on the secondary side of the transformer, that is, the jumping voltage generated on the secondary side of the transformer changes in two steps, so that the voltage fluctuation range. Can be reduced to half of the conventional level.

  Note that the power conversion device described in the above-mentioned prior art document uses a relay, whereas the power conversion device of the present invention uses a semiconductor switching element, so that the operation mode in a steady state and the time of a sag are reduced. The operation mode can be switched instantaneously, and the practically excellent effect that the stability of the output voltage can be improved can be obtained.

It is a circuit diagram which shows the fundamental structure of the inverter in the power converter device which concerns on one Embodiment of this invention. FIG. 2 is a diagram showing a schematic waveform of a gate pulse signal and an output AC voltage when a DC voltage value input in the inverter shown in FIG. 1 is equal to or higher than a predetermined voltage value (in a steady state). FIG. 2 is a diagram schematically showing waveforms of a gate pulse signal and an output AC voltage when a DC voltage value input in the inverter shown in FIG. FIG. 2 is a waveform schematic diagram showing a relationship between a DC voltage value input to the inverter shown in FIG. 1, a gate pulse signal, and an output voltage. It is a figure which shows another waveform outline of the gate pulse signal and output AC voltage when the direct-current voltage value input in the inverter shown in FIG. 1 falls below a predetermined voltage value (at the time of instantaneous drop). It is a circuit diagram which shows a principle structure when a DC-DC converter is comprised as a power converter device which concerns on another embodiment of this invention. It is a circuit diagram which shows another embodiment of the inverter shown in FIG. FIG. 8 is a circuit diagram showing another configuration example of the bidirectional switch applied to FIG. 7. It is a circuit diagram which shows the fundamental structure of the conventional inverter. FIG. 10 is a diagram schematically illustrating waveforms of a gate pulse signal and an output AC voltage when a DC voltage value input in the inverter illustrated in FIG. 9 is equal to or higher than a predetermined voltage value (in a steady state). FIG. 10 is a diagram schematically illustrating waveforms of a gate pulse signal and an output AC voltage when a DC voltage value input in the inverter illustrated in FIG. 9 falls below a predetermined voltage value (at the time of instantaneous drop).

  Hereinafter, a power supply device according to an embodiment of the present invention will be described with reference to the accompanying drawings. 1-7 shows an example of embodiment in this invention, Comprising: These inventions are not limited by these drawings. Also, in the figure, the parts denoted by the same reference numerals as those in FIG.

FIG. 1 shows an inverter as a first embodiment of the power conversion apparatus of the present invention. This inverter differs from the conventional inverter shown in FIG. 9 in that capacitors C ₁ and C ₂ connected in series at a connection point a are connected in parallel to the DC voltage source 1 and the first switching element [MOSFET ( Q ₁ )] and the second switching element [MOSFET (Q ₂ )] are connected at the connection point b, and the fifth and sixth switchings connected in anti-series connecting these two connection points a and b. The element [MOSFET (Q ₅ , Q ₆ )] is provided.
The operation of the inverter according to the power conversion device of the present invention having such a characteristic configuration will be described with reference to FIGS.
When the capacitances of the two capacitors C ₁ and C ₂ connected in series are equal, the voltages of the respective capacitors are equal to each other and become half the voltage of the DC voltage source 1, that is, [V _in / 2].

Now, when the DC voltage V _in of the DC voltage source 1 has a predetermined voltage value, a gate drive unit (not shown) turns on the MOSFETs (Q ₅ , Q ₆ ). Next, the gate driver turns on the MOSFET (Q ₄ ). Then, capacitor C ₂ (connection point a) → MOSFET (Q ₅ ) → MOSFET (Q ₆ ) → connection point b → load (not shown) → connection point c → MOSFET (Q ₄ ) → current in the path of capacitor C ₂ Flows. At this time, the voltage V _t applied to the load (not shown) is equal to the voltage across the capacitor C ₂ , that is, [V _in / 2] (period t ₁ ).
Next, the gate driver turns off the MOSFET (Q ₄ ) and turns on the MOSFET (Q ₃ ). Then, capacitor C ₁ → MOSFET (Q ₃ ) → connection point c → load (not shown) → connection point b → MOSFET (Q ₆ ) → MOSFET (Q ₅ ) → connection point a → current flows in the path of capacitor C _1. Flowing (period t ₂ ). At this time, the voltage V _t applied to the load is equal in magnitude to the voltage of the capacitor C ₁ and has a reverse polarity. That is, [V _t = −V _in / 2].

Thereafter, the gate drive unit alternately turns on and off the MOSFETs (Q ₃ and Q ₄ ) alternately to apply an AC voltage to the load.
By the way, when the gate drive unit detects that the voltage of the DC voltage source 1 has dropped due to an instantaneous drop or the like, the gate drive unit turns on the MOSFETs (Q ₄ and Q ₅ ). Then, a current flows through the path of the capacitor C ₂ → the connection point a → the MOSFET (Q ₅ ) → the MOSFET (Q ₆ ) → the connection point b → the load → the connection point c → the MOSFET (Q ₄ ) → the capacitor C ₂ , and the load A voltage of [V _t = V _in / 2] is applied (period t ′ ₁ ). During this period t ′ _1, the gate drive unit further turns on the MOSFET (Q ₁ ) and performs pulse width control to perform the capacitor C ₁ → MOSFET (Q ₁ ) → connection point b → load → connection point c → MOSFET. (Q ₄ ) → A current is passed through the capacitor C ₂ . Then, a voltage of [V _t = V _in ] is applied to the load.

Next, the gate drive unit turns off the MOSFETs (Q ₁ , Q ₄ , Q ₅ ), turns on the MOSFETs (Q ₃ , Q ₆ ), and sets the capacitor C ₁ → MOSFET (Q ₃ ) → connection point → load → connection. A current is passed through the path of point b → MOSFET (Q ₆ ) → MOSFET (Q ₅ ) → capacitor C ₁ (period t ′ ₂ ). The voltage of the load during this period t ′ ₂ is [V _t = −V _in / 2]. In this period t ′ ₂ , the gate driver turns on the MOSFET (Q ₂ ) and performs pulse width control so that the capacitor C ₁ → MOSFET (Q ₃ ) → connection point c → load → connection point b → MOSFET (Q ₆ ) → MOSFET (Q ₅ ) → A current is passed through the capacitor C ₁ . At this time, the voltage of [V _t = −V _in ] is applied to the load. When the voltage of the DC voltage source 1 drops instantaneously (instantaneously decreases) as described above, the gate drive unit is in the period t ′ ₁ . The MOSFETs (Q ₄ , Q ₅ ) are turned on, and the MOSFETs (Q ₃ , Q ₆ ) are turned on during the period t ′ ₂ , so-called full bridge operation is performed, and the MOSFETs (Q ₁ , Q ₂ ) are turned on. adjusting the voltage time product of the load voltage V _t by performing PWM control for controlling the width of the time.

Thus a power conversion apparatus according to a first embodiment of the present invention, when the DC voltage V _in of the DC voltage source 1 is lowered to a range of up to half of the voltage in a steady state, the gate control unit MOSFET (Q _1, Q ₂ ) Since the time for turning on is controlled, the same [V _in / 2] as _{in the} steady state can be applied to the load. Also when the DC voltage V _in of the DC voltage source 1 has dropped to half of the steady state, the power conversion device of the present invention, the load since the gate controller is to maximize the MOSFET (Q _1, Q ₂₎ pulse width Can apply the same voltage as in the steady state. In other words, the power conversion device of the present invention can maintain the AC output voltage at a predetermined voltage value as long as the voltage drop due to the instantaneous drop is within [1/2] of the steady state.
Further, the conventional power conversion device requires a steep duty change at the time of instantaneous determination. However, the power conversion device of the present invention has three levels during the period t ′ ₁ and the period t ′ ₂ shown in FIG. By performing the operation, the duty can be changed gently, and a stable output voltage can be obtained.

A method for generating a gate signal generated by the gate controller will be described with reference to FIG. 4 in more detail. The gate control unit, a steady state based on the value of the DC voltage V _in of the DC voltage source 1 detected by the voltage detection circuit (not shown) is operated at a point which is the conversion ratio 0.5.
Sag gate driver when the input voltage drops occurring, to turn on the MOSFET (Q _1, Q ₂₎ when the voltage of comparing the reference wave A and the DC voltage V _in the reference wave A is high. This increases the conversion ratio, also the DC voltage V _in is lowered can maintain the output voltage V _t to be constant.
However, in a steady state, the MOSFETs (Q ₁ , Q ₂ , Q ₅ , Q ₆ ) are not switched, so that no loss due to switching occurs. However, the MOSFETs (Q ₅ , Q ₆ ) are always turned on, causing conduction loss. . When a voltage sag occurs, the gate controller switches the MOSFETs (Q ₁ , Q ₂ , Q ₅ , Q ₆ ), but the voltage sag is short and has little effect.

The voltage applied to the MOSFETs (Q ₅ , Q ₆ ) is [V _in / 2], which is the voltage across the capacitors C ₁ and C ₂ . Therefore, these MOSFETs (Q ₅ and Q ₆ ) may have a lower breakdown voltage than the other MOSFETs (Q _{1 to} Q ₄ ).
Generally, the lower the breakdown voltage of the switching element, the lower the forward voltage drop during conduction and the smaller the loss. Accordingly, current flows through the three MOSFETs in the steady state, but the loss can be reduced by applying a MOSFET having a low element breakdown voltage.
Further, when the MOSFET (Q ₁ ) is off in the period t ′ ₁ , the MOSFET (Q ₆ ) passing through the same current path as that in the period t ₁ shown in FIG. 2 may be on or off. There is no effect on its operation. Therefore, the gate controller turns on the MOSFET (Q ₆ ) except during the period when the MOSFET (Q ₁ ) is on as shown in FIG. Then, current flows between the drain and source of the MOSFET (Q ₆ ) without passing through the parasitic diode, and the loss can be reduced.

Similarly period t 'MOSFET as shown in FIG. 5 in ₂ (Q _2), except periods in an on MOSFET (Q ₅₎ loss because current does not flow to the parasitic diode can reduced by turning on It is.
As a modification of the first embodiment, as shown in FIG. 6, a transformer T is connected to the output side of the inverter, and a rectifier circuit that converts alternating current appearing on the secondary side of the transformer T into direct current is provided. An insulated DC-DC converter may be configured.
In FIG. 6, the cathodes of rectifying diodes D ₁ and D ₂ as switching elements are connected to the secondary winding of the transformer T, and the anodes of these rectifying diodes D ₁ and D ₂ are connected. The transformer T is provided with a center tap on the secondary side, one end of the smoothing reactor L and a smoothing capacitor C ₃ is connected in series to the midpoint tap. The other end of the smoothing capacitor C ₃ is connected to the anodes of the rectifying diodes D ₁ and D ₂ to constitute a center tap rectifier circuit that obtains a DC output voltage V _out from both ends of the smoothing capacitor C ₃ .

In the isolated DC-DC converter configured as described above, when the voltage drop occurs in the DC voltage source 1 as described above, the inverter performs a three-level operation, so that it is applied to the primary side of the transformer T. that the voltage V _t changes stepwise (two steps). For this reason, the voltage fluctuation width becomes 1/2. Therefore, the jumping voltage generated on the secondary side of the transformer can be kept low, and the withstand voltage of the rectifying diode (switching element) can be made lower than that of the conventional DC-DC converter. Of course, in place of the center tap rectification, for example, a rectifier circuit such as full-bridge rectification or double current rectification, or a switching element other than the rectifying diode can be used to obtain the same effect.

Next, a second embodiment according to the power converter of the present invention will be described with reference to FIG. The second embodiment differs from the first embodiment in that a reverse blocking semiconductor element (for example, IGBT) connected in antiparallel instead of the MOSFETs (Q ₅ , Q ₆ ) connecting the connection points a and b is used. : Q ₇ , Q ₈ ) are used to form a bidirectional switch.
By using IGBTs (Q ₇ , Q ₈ ) as switching elements that connect the connection points a and b in this way, the number of switching elements through which current passes can be reduced, and the conduction loss can be further reduced. Can do.
As the bidirectional switch, for example, a switch as shown in FIG. FIG. 8A shows two IGBTs (Q ₃₂ , Q ₃₃ ) connected in series so that their conduction directions are opposite to each other, and diodes (D ₄₂ , D ₄₃ ) are connected in antiparallel to each IGBT. Connected to form a bidirectional switch.

Further, as shown in FIG. 8B, a bridge circuit is constituted by four rectifying diodes (D _{50 to} D ₅₃ ), and the (cathode) connection point of the rectifying diodes D ₅₀ and D ₅₂ and the IGBT (Q ₄₀ ). The collector, the (anode) connection point of the rectifying diodes D ₅₁ and D ₅₃ and the emitter of the IGBT (Q ₄₀ ) are respectively connected to form a bidirectional switch to control the on / off of the IGBT (Q ₄₀ ). May be.
That is, in this bidirectional switch, when the potential of the terminal a is higher than the potential of the terminal b, when the IGBT (Q ₄₀ ) is turned on, the current is rectified diode D ₅₀ → IGBT (Q ₄₀ ) → rectifier diode D ₅₃ → terminal It flows with b. Conversely, when the potential of the terminal b is higher than the potential of the terminal a, when the IGBT (Q ₄₀ ) is turned on, the current is changed from the terminal b → rectifier diode D ₅₂ → IGBT (Q ₄₀ ) → rectifier diode D ₅₁ → terminal a And flow.

  Even the power supply device according to the second embodiment of the present invention configured as described above can obtain the same effects as those of the first embodiment described above. In other words, the output voltage can be kept constant by changing the duty gradually even during a sag, and the breakdown voltage of the switching element used in the DC-DC converter having the switching element connected to the secondary side is increased. There is an excellent effect that it is not necessary.

1 DC voltage source C ₁ and C ₂ capacitors Q _{1 to} Q ₆ MOSFET

Claims (10)

A first series circuit in which a first capacitor and a second capacitor are connected in series at a first connection point;
A second series circuit connected in parallel to the first series circuit, wherein the first switching element and the second switching element are connected in series at a second connection point;
A third series circuit connected in parallel with the first and second series circuits, wherein a third switching element and a fourth switching element are connected in series at a third connection point;
A DC voltage source for applying a DC input voltage to the first to third series circuits connected in parallel;
A power conversion device for extracting an AC output voltage generated between both connection points of the second connection point and the third connection point,
The power conversion device further comprises a switch interposed between the first connection point and the second connection point to connect the two connection points. The power conversion device according to claim 1,
When the DC input voltage of the DC voltage source is greater than or equal to a predetermined voltage value, the switch is turned on, the first and second switching elements are turned off, and the third and fourth switching elements are While exclusively on or off with a period of
When the DC input voltage falls below a predetermined voltage value, the switch is turned on or off in synchronization with the third and fourth switching periods, and the first and second switching elements are A power converter that is turned on in a cycle and controls a switching pulse width according to an input DC voltage value.   The power converter according to claim 1 or 2, wherein the switch is obtained by connecting fifth and sixth switching elements in anti-series.   The power converter according to claim 3, wherein the fifth and sixth switching elements are elements having a withstand voltage lower than those of the first to fourth switching elements.   5. The power converter according to claim 3, wherein the fifth and sixth switching elements are elements having a switching speed slower than that of the first to fourth switching elements. The fifth and sixth switching elements each have a parasitic diode;
The power converter according to any one of claims 3 to 5, wherein when a current flows through the parasitic diode, the switching element having the parasitic diode is turned on.   The power converter according to claim 1 or 2, wherein the fifth and sixth switching elements are bidirectional switches in which two reverse element type switching elements are connected in antiparallel. The power conversion device according to any one of claims 1 to 7,
Furthermore, the power converter device provided with the transformer which transforms and outputs the alternating voltage which arises between both the connection points of the said 2nd connection point and the said 3rd connection point. The power conversion device according to any one of claims 1 to 7,
And a rectifier circuit that rectifies an AC output voltage generated between the second connection point and the third connection point to output a DC voltage. The power conversion device according to claim 8, wherein
And a rectifier circuit that rectifies the AC voltage output from the transformer and outputs a DC voltage.

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