JPH1066343A - Power converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make the modulation frequency of the PWM control higher, suppress high frequency components leaked into an AC system side and, further, reduce the power loss of switching devices and improve the operation efficiency when an NPC inverter is used as an NPC converter. SOLUTION: When a 1st and 3rd switching devices S1 and S3 are turned on and off alternately by PWM control while a 2nd switching device S2 is in an on-state and a 4th switching device S4 is in an on-state or, when the 2nd and 4th switching devices S2 and S4 are turned on and off alternately by PWM control while the 1st switching device S1 is in an off-state and the 3rd switching device S3 is in an on-state in order to convert an AC power into a DC power, a current is cut off by the switching devices S2 and S3 with the modulation frequency of the PWM control and the switching loss is proportional to the modulation frequency. Switching devices whose characteristics are such that switching lossess for one on-off operation are smaller than the loss of an ordinary devices are used as the switching devices S2 and S3 to improve the conversion efficiency.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power converter for converting AC power into DC power, and more particularly to reducing harmonic current flowing to an AC system, reducing power loss and improving operating efficiency. The present invention relates to a power conversion device as described above.

[0002]

2. Description of the Related Art An NPC inverter is used as a power converter for converting a DC voltage into an AC voltage having less harmonics by PWM control. As shown in FIG. 8, the NPC inverter converts a DC voltage supplied from the DC buses P and N of the DC voltage source 1 into a desired AC voltage by PWM control, and converts the converted DC voltage through AC terminals U, V and W. A conversion circuit 20 for supplying AC power to a load 30 is provided. A series circuit of two capacitors 2 and 3 for obtaining a neutral point potential C is connected between DC buses P and N, and the conversion circuit 20 is connected to AC terminals U and When an AC voltage is output to V and W, PWM control is performed at three levels of potentials so as to output one of the positive electrode P, the negative electrode N, and the neutral point C.

The conversion circuit 20 includes U-phase, V-phase, and W-phase bridge circuits 20u, 20v, and 20w. The U-phase bridge circuit 20u includes a series circuit of four switch elements Su1 to Su4. N and an intermediate point (switch element S
A connection point between u2 and Su3) is connected to the AC terminal U, and diodes Du1 to Du4 are connected in antiparallel to the respective switch elements Su1 to Su4. Also, a series circuit of neutral point clamp diodes Du5 and Du6 is connected in parallel with the series circuit of diodes Du2 and Du3, and neutral point clamp diodes Du5 and Du6.
Are connected to the neutral point C. The V-phase and W-phase bridge circuits 20v and 20w are similarly configured.

FIG. 9 shows an example of operation waveforms when AC power is supplied to an inductive load such as an induction motor by the above-mentioned NPC inverter, and shows the operation of the U-phase bridge circuit 20u as a representative. When the bridge circuit 20u outputs a positive voltage of the U-phase AC voltage, the switching element S
u2 is turned on, Su4 is turned off, and switch elements Su1 and Su3 are turned on.
Are alternately turned on and off by PWM control, and a positive voltage of a half sine wave is output. When outputting the negative voltage of the U-phase AC voltage, the switching element Su1 is turned off and the switching element Su3 is turned on, and the switching elements Su2 and Su4 are alternately turned on and off by the PWM control. Is output. FIG. 9 shows a state in which the U-phase load current Iu flows at a delayed power factor.

When operating as an inverter for converting a DC voltage to an AC voltage by performing PWM control in this manner, the switching elements Su1 and Su4 are connected to the U bus supplied from the DC buses P and N.
The load current Iu of the phase is repeatedly cut off and turned on at the modulation frequency of the PWM control, and the switch elements Su1 and Su4 have Iu1 of FIG.
And Iu4, a pulse-width modulated current flows.
Accordingly, the switching loss of the switching elements Su1 and Su4 is proportional to the modulation frequency of the PWM control.

On the other hand, the switching elements Su2 and Su3 are alternately turned on every half cycle of the AC voltage, so that the currents Iu1 and Iu4 flow and the neutral point clamp is performed when the switching elements Su1 and Su4 are turned off. Diodes Du5 and Du6
, The load current Iu flows back as Iu5d and Iu6d. After a lapse of a half cycle, the state is turned off, and on / off control is performed by PWM control in the next half cycle. Therefore, currents such as Iu2 and Iu3 in FIG. 9 flow through the switch elements Su2 and Su3, and when the fundamental wave component of the AC voltage becomes zero voltage, the load current remaining due to the delayed power factor is cut off. The current interruption by the PWM control is performed only for a short period of time until the current decreases in a sinusoidal manner and becomes zero current. Therefore, the switching loss of the switching elements Su2 and Su3 is almost proportional to the output frequency of the AC voltage.

Since the modulation frequency of the PWM control is much higher than the fundamental frequency of the AC voltage, the switching loss of the switching elements Su1 and Su4 is much larger than the switching loss of the switching elements Su2 and Su3. However, since the time during which the U-phase load current Iu is supplied during one cycle period is longer for the switch elements Su2 and Su3 than for the switch elements Su1 and Su4, the conduction loss is lower for the switch elements Su2 and Su3 than for the switch element Su1. And larger than Su4. The V-phase and W-phase bridge circuits 20v and 20w also output AC voltages having a phase difference of 120 ° and operate in the same manner. The U-phase and V-phase line voltages Vuv are PWM at three levels of potentials as shown in FIG.
Controlled AC voltage with less harmonics can be obtained. In FIG. 9, Iu1d to Iu4d are currents flowing through the diodes Du1 to Du4, and Iu5d and Iu6d are currents flowing through the neutral point clamp diodes Du5 and Du6.

Utilizing such operational characteristics of the NPC inverter, the present applicant uses high-speed switching elements with small switching loss for the switching elements Su1 and Su4, and turns on the on-state voltage for the switching elements Su2 and Su3. Japanese Patent Application Laid-Open No. Hei 7-245960 proposes an NPC inverter using a switch element having a small conduction loss and reducing the power loss of the switch element to improve the operation efficiency.

[0009]

In recent years, GTO (Ga
Large-capacity switch elements such as te Turn off Thyristor) have been developed, and NPC inverters have been increasing in capacity, and high-voltage, large-capacity devices with a DC voltage of several kV have been put to practical use. The NPC inverter is capable of reversible power conversion and can be used as an NPC converter (rectifier) for converting an AC voltage to a DC voltage. In this case, harmonic current flowing into the AC system is reduced, and the NPC inverter has a high power factor. It is noted that control becomes possible.

However, the NPC inverter shown in FIG.
When used as a converter, the NPC inverter is operated in a regenerative operation mode, and the switching elements Su2 and Su3 operate to cut off the load current at a PWM control modulation frequency as described later. It is larger than Su1 and Su4, and the operating efficiency is reduced, and there is a problem that the capacity of the converter and the modulation frequency are limited.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and when an NPC inverter is used as an NPC converter, the modulation frequency of PWM control is increased to reduce harmonic components flowing out to the AC system side. Another object of the present invention is to provide a power conversion device in which power loss of a switch element is reduced and operation efficiency is improved.

[0012]

In order to achieve the above object, a power converter according to the present invention is characterized in that the first, second, third and fourth switch elements having diodes connected in anti-parallel are connected to a positive terminal of a DC bus. And a negative electrode are sequentially connected in series, a connection point of the second and third switch elements is connected to an AC power supply via a reactor, and a DC voltage between the positive electrode and the negative electrode is divided to obtain a neutral point. Two connected capacitors, and a neutral point connected between the connection point of the first and second switch elements and the connection point of the third and fourth switch elements and the neutral point, respectively. When a clamp diode is provided for each phase of the AC power supply, and the second and third switching elements have switching characteristics per switching loss smaller than usual, and convert AC power to DC power. The conversion efficiency of (Claim 1) Further, the number of the second and third switch elements arranged in parallel is made larger than the number of the first and fourth switch elements arranged in parallel. (Claim 2) The rated currents of the second and third switch elements are selected to be larger than the rated currents of the first and fourth switch elements. (Claim 3) Further, current reference generating means for outputting a sinusoidal current reference synchronized with the voltage of the AC power supply based on a current command;
A current control unit that outputs a current control signal based on a deviation between the current reference and a current detection value supplied from the AC power supply, based on a synchronization signal synchronized with the AC voltage of the AC power supply and the current control signal. The first, second, third, and fourth switch elements are turned on and off so that the deviation is reduced, and the potential at the connection point of the second and third switch elements is set to the positive and negative DC bus and PWM control means for controlling the potential to any one of the neutral points, and converts the AC power to the DC power while controlling the current supplied from the AC power supply in a sine wave shape. (Claim 4) Further, the second and third switch elements are switch elements having a characteristic in which a tail current at the time of turning off is smaller than usual or a characteristic in which the tail current flows for a shorter time than usual, and convert AC power into DC power. The switching loss at the time of the conversion is reduced and the conversion efficiency is improved. (Claim 5) Further, as the diode connected in anti-parallel to the first and fourth switch elements, a diode having a characteristic having less accumulated carriers than the diode connected in anti-parallel to the second and third switch elements is used. Then, the switching loss when converting the AC power to the DC power is reduced, and the conversion efficiency is improved.
(Claim 6)

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of a power converter according to claims 1 to 4 of the present invention. In FIG. 1, 2
Reference numeral 0 denotes a conversion circuit that operates as a converter that converts AC power into DC power, and its circuit configuration is the same as that of the related art (FIG. 8). However, in this case, the AC buses U, V, W of the conversion circuit 20 are connected to the AC power supply 5 via the AC reactor 4, and the load 6 is connected to the DC buses P, N. Also, the switching elements Su2, Su3, Sv2, S
v3, Sw2, and Sw3 include switch elements Su1, Su4, Sv1,
A high-speed switching element having a characteristic of a smaller turn-off loss than Sv4, Sw1, and Sw4 is used.

The control unit includes a voltage detector 7 for detecting a DC voltage, a voltage detector 8 for detecting an AC voltage, a current detector 9 for detecting an AC current, a synchronization circuit 10 for outputting a synchronization signal,
It comprises a voltage control circuit 11 for controlling a DC voltage, a current reference circuit 12 for generating a sine-wave current reference, a current control circuit 13 for controlling an AC current, and a PWM control circuit 14 for performing switching control of the conversion circuit 20.

In the above configuration, the AC voltage supplied from the AC power supply 5 is converted into the DC voltage Vd by the conversion circuit 20 and supplied to the load 6. This DC voltage Vd is applied to the voltage detector 7
, And compared with the voltage reference V ^* by the voltage control circuit 11 to output the current amplitude reference Im such that the voltage deviation decreases. On the other hand, AC voltage V supplied from AC power supply 5
a is detected by the voltage detector 8, a synchronization signal S synchronized with the phase of the AC voltage Va is output from the synchronization circuit 10, input to the current reference circuit 12, and input to the PWM control circuit 14. The current reference circuit 12 generates a unit sine wave based on the synchronization signal S and multiplies the unit sine wave by a current amplitude reference Im to obtain an AC power supply 5.
And outputs a current reference I ^* having the same phase as the AC voltage. The current reference I ^* is compared with the AC current Ia detected by the current detector 9 in the current control circuit 13, and a control signal C is output and input to the PWM control circuit 14 so that the current deviation is reduced. The PWM control circuit 14 determines the polarity of the line voltage of each phase of the AC power supply 5 based on the synchronization signal S, determines the pulse width of the PWM control based on the control signal C, and controls the conversion circuit 20 to perform PWM control. A gate signal G for controlling is output. The conversion circuit 20 uses the gate signal G to generate an AC bus U,
PWM control of the potentials of V and W to any one of P, N, and C to control the load currents Iu and Iv supplied from the AC power supply 5.
, Iw are controlled in a sinusoidal manner. (Claim 4) For example, during the positive half cycle of the U-phase and V-phase line voltage Vuv of the AC power supply 5, the U-phase switch element Su2 is turned on, the Su4 is turned off, and the switch elements Su1 and Su3 are PWM-switched.
When it is turned on and off alternately by control, Su1 is off and S
While u3 is on, U-phase AC reactor 4-U-phase switch element Su3-U-phase neutral point clamp diode Du6
-Capacitor 3-current flows from AC power supply 5 via V-phase diodes Dv4, Dv3-V-phase AC reactor 4,
Energy is stored in the U-phase and V-phase AC reactors 4. Iu3 and Iu6d in FIG. 2 indicate currents flowing through the switch element Su3 and the neutral point clamp diode Du6. Also, during the period when Su1 is on and Su3 is off, the U-phase AC reactor 4-U-phase diode Du2, Du1-capacitor 2, 3-V-phase diode Dv3, Dv4-V-phase AC reactor 4 The discharge current flows, and the energy stored in the U-phase and V-phase AC reactors 4 is transferred to the capacitors 2 and 3.
Move to Iu1d and Iu2d in FIG. 2 indicate currents flowing through the diodes Du1 and Du2. Only the regenerative current due to the delayed power factor flows through the switch element Su1 slightly as Iu1. Although the current waveform in FIG. 2 is shown as a sine wave, the actual waveform is PWM-controlled so that the average value becomes a sine wave.

When the line element Vuv is in the negative half cycle, the U-phase switch element Su1 is turned off and Su3 is turned on, and the switch elements Su2 and Su4 are turned on and off alternately by PWM control. In the period when Su4 is off, a V-phase AC reactor 4-V-phase diode Dv2,
Dv1-capacitor 2-U-phase neutral point clamp diode Du5-U-phase switch element Su2-Current flows from the AC power supply 5 via the U-phase AC reactor 4, and energy is supplied to the U-phase and V-phase AC reactors 4. During the period when Su2 is off and Su4 is on, the V-phase AC reactor 4-V-phase diodes Dv2, Dv1-condenser 2, 3-U-phase diodes Du4, Dv3-U-phase AC reactor 4 A discharge current flows through the capacitor, and energy stored in the U-phase and V-phase AC reactors 4 is transferred to the capacitors 2 and 3.

Similar PWM control is performed for the other phases, so that AC power can be converted to DC power at a high power factor with less harmonics, and a DC voltage corresponding to the voltage reference V ^* can be obtained. When the PWM control is performed in this manner, the switching elements Su2, Su3, Sv2, Sv3, Sw2, and Sw3 turn on and off the current in the modulation cycle of the PWM control.
u2, Su3, Sv2, Sv3, Sw2, Sw3 are switch elements S
U1, Su4, Sv1, Sv4, Sw1, and Sw4 use a high-speed switch element having characteristics of smaller turn-off loss, so that the switching frequency is increased to reduce the harmonic components of the current flowing to the AC power supply side and reduce the switching loss. Thus, the power conversion efficiency as a converter can be improved and the size can be reduced.

FIG. 3 is a characteristic diagram showing the relationship between the ON voltage VTM of the GTO and the turn-off loss Eoff per switching operation. There is a trade-off relationship that the loss Eoff increases. As shown in FIG. 3, when the ON voltage VTM1 of GTO1 is lower than the ON voltage VTM2 of GTO2, the conduction loss of GTO1 is smaller than the conduction loss of GTO2, but the turn-off loss Eoff1 of GTO1 is larger than the turn-off loss Eoff2 of GTO2. .

For example, if the ON voltage VTM1 of GTO1 is 3
V, turn-off loss Eoff1 is 10 joules, GT0
2 has an on-voltage VTM2 of 4 V and a turn-off loss Eoff2 of 5
When operating at 2000 A and 500 Hz, the power loss P1 of GTO1 and the power loss P2 of GTO2 are as follows:

[0020]

## EQU1 ## P1 = 3.times.2000 / 2 + 10.times.500 = 80
00 (W) P2 = 4 × 2000/2 + 5 × 500 = 5500
(W), and the power loss P1 of GTO1 becomes larger than the power loss P2 of GT02. Also, the power loss P1 of the GTO 1 when operated at 2000 A and 50 Hz (commercial frequency)
And the power loss P2 of GTO2 is

[0021]

P1 = 3 × 2000/2 + 10 × 50 = 3500 (W) P2 = 4 × 2000/2 + 5 × 50 = 4250 (W), and the power loss P2 of the GTO2 is larger than the power loss P1 of the GT01.

Therefore, the conversion circuit 20 shown in FIG.
When the frequency of the AC power supply 5 is operated at 50 Hz and the modulation frequency of the PWM control is operated at 500 Hz, the switch elements Su2,
By using Su3 as a characteristic of GTO2, when using as a converter for converting AC power to DC power by increasing the modulation frequency, it is possible to perform operation with reduced power loss and improved conversion efficiency. (Claim 1) FIG. 4 is a waveform diagram showing a current i, a voltage v, and a power loss p when a switch element such as a GTO or an electrostatic induction thyristor is turned off. When the switching element is turned off, when the current i decreases rapidly and the voltage v rises rapidly and the switching element recovers the insulation, a tail current i1 due to the discharge of accumulated carriers flows. The power loss p is determined by the product of the voltage v and the current i, and the power loss p due to the tail current i1
The addition of 1 has a large effect on turn-off loss. The tail current is determined by the characteristics of the switch element. The tail current peak value Itail is small and the tail time ttail is short i in FIG.
If the switching element has the characteristic shown in FIG. 2, the power loss due to the tail current i2 is reduced as shown by p2, and the turn-off loss is reduced. Therefore, by using an element having a small tail current peak value and a short tail time for the switching elements Su2 and Su3 of the conversion circuit 20 of FIG. 1, the switching frequency can be increased to increase the modulation frequency and be used as a converter for converting AC power to DC power. In this case, it is possible to perform operation with reduced power loss and improved conversion efficiency. (Claim 5) In the case of a large-capacity power converter, a method of increasing the current capacity by connecting a plurality of switch elements in parallel is used. FIG. 5 shows a U-phase bridge circuit 20u of the conversion circuit 20, in which switch elements Su21 and Su31 are respectively connected in parallel to switch elements Su2 and Su3 to form two parallel circuits, and the other phase circuits are similarly configured. It is an example.

According to this embodiment, when operating the conversion circuit 20 as a three-level converter for converting AC power to DC power, the switching elements Su2, Su3, Su21, S
The current flowing through u31 is halved, the conduction loss is halved, and PWM
Even if the current is interrupted by the number of times of the control modulation frequency, the current can be kept within the allowable power loss of the switching elements Su2, Su3, Su21, and Su31, and the capacity of the conversion circuit 20 can be increased.

In this embodiment, the switching elements Su2,
Although the example in which Su3 has two parameters has been described, the switch elements Su1,
The switch elements Su2 and Su3 may be composed of three parameters with Su4 being 2 paras. The same effect can be obtained by making the parallel number of the switch elements Su2 and Su3 larger than the parallel number of the switch elements Su1 and Su4. . (Claim 2) When each switch element is composed of one parameter, the rated currents of the switch elements Su1 and Su4 are changed to the switch elements Su2 and Su3.
Select a switch element of a type smaller than the rated current of
The size can be reduced, and the load factor of each switch element can be balanced. (Claim 3) FIG. 6 shows an operation state when the switch elements Su1 and Su3 are alternately turned on and off by PWM control while the switch element Su2 is on and the switch element Su4 is off.
FIG. 6A shows that the switching element Su1 is turned on and the switching element Su3 is turned on.
Is turned off, the energy stored in the AC reactor 4 flows as the discharge current I1 through the diodes Du1 and Du2 as described above, and the capacitors 2 and 3 are charged. FIG. 6B shows a state in which the switching element Su1 is turned off and the switching element Su3 is turned on, the charging current I0 flows through the neutral point clamp diode Du6 as described above, and energy is accumulated in the AC reactor 4. When the switching element Su1 is turned off and the switching element Su3 is turned on in the state shown in FIG. 6A to shift to the state shown in FIG.
, The charging voltage Ed1 of the capacitor 2 is applied as a reverse voltage, the diode Du1 reversely recovers, and the current I1 flowing through the diodes Du1 and Du2 becomes zero. When the diode recovers the reverse voltage immediately after the forward current flows, the accumulated charge Qrr in the semiconductor flows out as shown in FIG.
rr flows. At this time, a power loss occurs due to the product of the reverse recovery voltage Ed1 and the reverse current Irr. Diodes Du1, Du4, Dv
By selecting 1, Dv4, Dw1, and Dw4 as those having a small accumulated charge amount Qrr and a short time for the reverse current Irr to flow, the switching loss is reduced, the power loss is reduced, and the conversion efficiency can be improved. (Claim 6)

[0025]

According to the power converter of the present invention, the NPC
When converting AC power to DC power with a converter, P
The modulation frequency of the WM control is increased to reduce the harmonic component current flowing out to the AC system side, and the switching loss is suppressed, the power loss is reduced, the power conversion efficiency is improved, and the size can be reduced.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an embodiment of a power converter according to claims 1 to 4 of the present invention.

FIG. 2 is a current waveform diagram for explaining the operation of the embodiment.

FIG. 3 is a characteristic diagram showing a relationship between an ON voltage VTM of a GTO and a turn-off loss Eoff per one switching operation.

FIG. 4 is a waveform diagram of current and voltage when a switch element is turned off.

FIG. 5 is a configuration diagram of an embodiment of a U-phase conversion circuit 20u of FIG. 1;

FIG. 6 is a diagram for explaining the commutation operation of the U-phase conversion circuit 20u of FIG. 1;

FIG. 7 is a waveform diagram of a current voltage at the time of reverse voltage recovery of a diode.

FIG. 8 is a configuration diagram of a conventional NPC inverter.

FIG. 9 is a time chart for explaining the operation of the NPC inverter.

[Explanation of symbols]

2, 3, a capacitor 4, an AC reactor 5, an AC power supply 6, a load 7, a voltage detector (DC voltage) 8, a voltage detector (AC voltage) 9, a current detector 10, a synchronous circuit 11, a voltage control circuit 12, and the like. Current reference circuit 13 ... Current control circuit 14 ... PWM
Control circuit 20 ... Conversion circuit Su1, Sv1, Sw1 ... First switch element Su2, Sv2, Sw2 ... Second switch element Su3, Sv3, Sw3 ... Third switch element Su4, Sv4, Sw4 ... Fourth switch element Du1 to Du4, Dv1 to Dv4, Dw1 to Dw4 ... diodes Du5, Du6, Dv5, Dv6, Dw5, Dw6 ... neutral point clamp diodes

Claims (6)

[Claims] A first, second, third, and fourth switch element having a diode connected in anti-parallel is connected in series between a positive electrode and a negative electrode of a DC bus, and the second and third switch elements are connected in series. Is connected to the AC power supply via the reactor,
Two capacitors connected in series to divide a DC voltage between the positive electrode and the negative electrode to obtain a neutral point, a connection point of the first and second switch elements, and a connection of the third and fourth switch elements A neutral point clamp diode connected between each point and the neutral point for each phase of the AC power supply, and the switching loss per operation of the second and third switch elements is normal. Switch element with smaller characteristics
A power conversion device for improving conversion efficiency when converting AC power into DC power. 2. The first, second, third, and fourth switch elements having diodes connected in anti-parallel are sequentially connected in series between a positive electrode and a negative electrode of a DC bus, and the second and third switch elements are connected in series. Is connected to the AC power supply via the reactor,
Two capacitors connected in series to divide a DC voltage between the positive electrode and the negative electrode to obtain a neutral point, a connection point of the first and second switch elements, and a connection of the third and fourth switch elements A neutral point clamp diode connected between each point of the AC power supply and a neutral point connected between the neutral point and the neutral point.
A power conversion device, wherein the number of switch elements is larger than the number of switch elements in parallel. 3. A first, second, third, and fourth switch element having a diode connected in anti-parallel is connected in series between a positive electrode and a negative electrode of a DC bus, and the second and third switch elements are connected in series. Is connected to the AC power supply via the reactor,
Two capacitors connected in series to divide a DC voltage between the positive electrode and the negative electrode to obtain a neutral point, a connection point of the first and second switch elements, and a connection of the third and fourth switch elements A neutral point clamp diode connected between a point and the neutral point for each phase of the AC power supply, and the rated current of the second and third switch elements is set to the first and fourth switch elements. A power converter, wherein the power converter is selected to be larger than the rated current of the switch element. 4. A power converter according to claim 1, wherein a current reference generating means for outputting a sine-wave current reference synchronized with a voltage of the AC power supply based on a current command; A current control unit that outputs a current control signal based on a deviation between the current reference and a current detection value supplied from the AC power supply, based on a synchronization signal synchronized with the AC voltage of the AC power supply and the current control signal. The first, second, third, and fourth switch elements are turned on and off so that the deviation is reduced, and the potential at the connection point of the second and third switch elements is set to the positive and negative DC bus and A power converter, comprising: PWM control means for controlling a potential at any one of neutral points, and converting AC power to DC power while controlling a current supplied from an AC power supply in a sine wave shape. 5. The power converter according to claim 1,
The second and third switch elements are switch elements having a characteristic in which the tail current at the time of turn-off is smaller than usual or a characteristic in which the tail current flows for a shorter time than usual, thereby reducing switching loss when converting AC power to DC power. A power converter characterized by improved conversion efficiency. 6. The power converter according to claim 1, wherein a diode connected in antiparallel to said first and fourth switch elements is connected to said second and third switch elements. A power conversion device using a diode having less characteristics of accumulated carriers than a diode connected in anti-parallel, reducing switching loss when converting AC power to DC power, and improving conversion efficiency.