WO2007129469A1

WO2007129469A1 - Power transducing device

Info

Publication number: WO2007129469A1
Application number: PCT/JP2007/000478
Authority: WO
Inventors: Yukimori Kishida; Akihiko Iwata; Shinichi Ogusa
Original assignee: Mitsubishi Electric Corporation
Priority date: 2006-05-08
Filing date: 2007-05-07
Publication date: 2007-11-15
Also published as: JP5049964B2; JPWO2007129469A1

Abstract

Provided is a power transducing device in a regeneratively controllable converter, which device can suppress higher harmonics and can reduce power losses and electromagnetic noises and which initially charges a DC power storage easily and reliably. A power transducer (7) is constituted by connecting the AC side of a single-phase sub-converter (3) having a DC voltage lower than the DC voltage of a three-phase main converter (2), with the AC input lines of the individual phases of the main converter (2), so that the phase voltage of the power transducer (7) is generated with the sum of the phase voltages of the individual converters (2), (3). Moreover, a charging resistor (23) is connected between a system power source (1) and the power transducer (7), so that filter condensers (4) and (5) of the main converter (2) and the sub-converter (3) are initially charged by a current passage through the charging resistor (23) from the system power source (1).

Description

Specification

Power converter

Technical field

TECHNICAL FIELD [0001] The present invention relates to a power conversion device, and more particularly to a converter that converts AC power into DC power. Background art

[0002] As a conventional power conversion device, a PWM converter composed of semiconductor elements with self-extinguishing capability, such as GTO, can be switched regardless of the polarity of the power supply voltage, the polarity of the current, or the size. It is possible to control the power factor to 1 for power and 1 to power for regeneration (for example, see Non-Patent Document 1).

[0003] Non-Patent Document 1: “Latest Electric Railway Engineering” (The Society of Electrical Engineers, Research Committee on Electric Railway 2000/9/1 1 issued by Corona) P 6 0 ~ 6 7

Disclosure of the invention

Problems to be solved by the invention

[0004] However, in the conventional PWM converter, a voltage is generated at the AC input terminal by PWM control of a pulse with a large voltage. If we try to suppress the harmonics by eliminating distortion for waveform formation, we will either increase the reactor of the system or increase the number of switching. However, increasing the reactor increases the size of the device, and increasing the number of switching increases the power loss and electromagnetic noise. In addition, if the number of times of switching is reduced, current distortion increases and there is a problem that harmonic current is generated in the system.

[0005] The present invention has been made to solve the above-mentioned problems, and is a converter that converts AC power into DC power and can control the power even during regeneration, and suppresses harmonics. In addition, power conversion devices with high conversion efficiency and reduced size are provided with reduced power loss and electromagnetic noise, and power storage devices that store the DC power of such power conversion devices can be easily trusted. The purpose is to perform initial charging with good quality. Means for solving the problem

[0006] The power conversion device according to the first aspect of the present invention has power storage units each having a different DC voltage on the DC side, converts AC power from an AC power source into DC power, and outputs the DC power to each power storage unit. A main converter and a sub-converter, wherein the sub-converter is arranged between the main converter and the AC power source and connected in series; and connected between the power converter and the AC power source. It has a charging resistor. At the time of initial charging of each power storage unit, at least the power storage unit of the sub-converter is charged in the current path from the AC power source through the charging resistor under the control of the main comparator and the sub-converter. Electricity.

[0007] The power conversion device according to the second invention has power storage units each having a different DC voltage on the DC side, converts AC power from an AC power source into DC power, and outputs the DC power to each power storage unit. A main converter and a sub-converter, wherein the sub-converter is arranged between the main converter and the AC power source and connected in series; and connected between the power converter and the AC power source. And a reactor. At the time of initial charging of each of the power storage units, at least the power storage unit of the sub-converter is controlled in the current path from the AC power source through the reactor by the control of the main comparator and the sub-computer. It is to be charged.

The invention's effect

[0008] In the power conversion devices according to the first and second inventions as described above, a voltage is generated on the AC input side as the sum of the voltages generated at the AC input terminals of the main converter and the sub-converter. Since the voltage can be shared between the main converter and the sub-converter in this way, it is not necessary to generate high voltage pulses by switching at a high frequency, reducing power loss and electromagnetic noise. For this reason, harmonics can be suppressed and power loss and electromagnetic noise can be reduced. In addition, since the power converter of the sub-converter is initially charged via a charging resistor or reactor, inrush current flowing to the power storage can be prevented, making it easy and reliable. The power storage can be initially charged.

Brief Description of Drawings

FIG. 1 is a configuration diagram of a power conversion device according to Embodiment 1 of the present invention.

FIG. 2 is a circuit diagram of a sub-converter according to Embodiment 1 of the present invention.

FIG. 3 is a flow chart showing initial charging of a filter capacitor according to Embodiment 1 of the present invention.

FIG. 4 is a circuit diagram illustrating charging of a filter condenser of a sub-converter according to Embodiment 1 of the present invention.

FIG. 5 is a circuit diagram illustrating charging of a filter capacitor of a main converter according to Embodiment 1 of the present invention.

FIG. 6 is a flowchart showing initial charging of a filter condenser according to Embodiment 2 of the present invention.

FIG. 7 is a flowchart showing initial charging of a filter condenser according to another example of the second embodiment of the present invention.

FIG. 8 is a circuit diagram illustrating charging of the filter capacitor shown in FIG.

FIG. 9 is a configuration diagram of a power conversion device according to another example of the second embodiment of the present invention.

FIG. 10 is a configuration diagram of a power conversion device according to Embodiment 3 of the present invention.

FIG. 11 shows the output logic and output gradation (voltage level) of each converter of the power conversion device according to embodiment 3 of the present invention.

FIG. 12 is a voltage waveform of each converter in the power conversion device according to embodiment 3 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0010] Embodiment 1.

Hereinafter, a power converter according to Embodiment 1 of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing a configuration of a power conversion device according to Embodiment 1 of the present invention. This is applied to the formation of an intermediate DC voltage of a power converter that supplies AC power to a load such as a motor or a light or an AC system. This power converter is medium In order to maintain the DC voltage of the filter capacitor as the DC voltage between the AC systems, power conversion is performed between the AC system and the DC voltage link, so switching is performed so that the same voltage as the system voltage is output to the AC system side.

[001 1] As shown in Fig. 1, the power conversion device connects the AC side of the single-phase sub-converter 3 in series to each phase AC input line 9 of the main converter 2 consisting of a three-phase two-level converter. The power converter 7 is configured as described above. The main converter 2 and each sub-converter 3 are provided with filter capacitors 4 and 5 as power storages on the DC output side. The DC voltage of filter capacitor 4 of main converter 2 is assumed to be greater than the DC voltage of filter capacitor 5 of sub-converter 3. The power converter 7 converts the AC power supplied from the three-phase AC system power source 1 as the AC power source through the system reactor 6 into DC power, and supplies this DC power to the filter capacitor 4 of the main converter 2. 8 is, for example, a motor load and an inverter for driving the motor load or a DC load.

Each phase AC input line 9 has a main high-order switch 21 as a switching switch between the sub-converter 3 and the system power supply 1, and a sub-high-level switch 2 2 as a second switch and a charging resistor. A series circuit in which 2 and 3 are connected in series is connected in parallel to the main upper switch 21.

The main converter 2 is a three-phase two-level converter composed of a plurality of self-extinguishing semiconductor switching elements such as I G B T and the like that have diodes connected in antiparallel and a filter capacitor 4. Further, as shown in FIG. 2, the sub-converter 3 is composed of a plurality of self-extinguishing semiconductor switching elements 11 such as IGBTs each having a diode 12 connected in antiparallel and a filter capacitor 5. . The self-extinguishing type semiconductor switching element is not limited to I G B T but may be a forced commutation operation even with G C, G T O, transistor, M O S F E T, or a thyristor without a self-extinguishing function.

The ratio of the voltage of filter capacitor 4 to the voltage of filter capacitor 5 is, for example, 2: 1, 3: 1, 4: 1, 5: 1, 6: 1. Used.

Instead of the filter capacitors 4 and 5, a battery that stores power may be used for the power storage, and the same effect can be obtained.

[0013] In the power conversion device configured as described above, the main converter 2 and the sub-compactor 3 are connected in series to form the power converter 7, and power conversion is performed by the sum of the phase voltages of the converters 2 and 3. Because the phase voltage of generator 7 is generated, it is not necessary to generate large voltage pulses at a high switching frequency, harmonics can be suppressed without increasing the reactor of the system, and power loss and electromagnetic noise can also be reduced. As a result, a power conversion device with high conversion efficiency and reduced size can be obtained.

Note that the main converter 2 and the sub-converter 3 may perform a pulse width modulation control output, that is, a so-called PMW output, on the voltage to be output. By placing a small filter at the output, the accuracy of the voltage output can be improved and harmonics of voltage and current can be suppressed.

In particular, if the main converter with a high voltage reduces the number of output pulses and the sub-converter with a low voltage outputs PWM, it is possible to improve both conversion efficiency and suppress harmonics, which is effective.

If the power converter 7 does not perform regenerative operation, the power converter may be configured using a diode full bridge main converter instead of the main comparator 2 shown in FIG. . In this case as well, harmonics can be suppressed by PWM control of the sub-compartment 3.

[0014] Next, initial charging of the filter capacitors 4 and 5 at the time of starting up the power conversion device will be described below.

The main upper switch 2 1, sub upper switch 2 2, and charging resistor 2 3 provided between the sub-converter 3 and the system power supply 1 in each phase AC input line 9 are connected to the main converter 2 and the sub-converter 3. Configure a resistor circuit for initial charging of filter capacitors 4 and 5. The charging resistor 23 is a number, and the sub upper switch 22 is a small-capacity contactor or relay.

FIG. 3 is a flowchart showing the initial charging of the filter capacitors 4 and 5. The

First, when the power converter is started, the main upper switch 2 1 and the sub upper switch 2 2 are cut off, and the filter capacitors 4 and 5 are not charged (step s 1), and the sub upper switch 2 2 is turned on. The filter capacitor 5 of the sub-converter 3 is charged while suppressing the inrush current. At this time, only all the upper arms or only all the lower arms of the main converter 2 are turned on, and as shown in FIG. 4 (a), the sub-converter is connected by the current path from the system power supply 1 through the charging resistor 23. Charge filter capacitor 5 of 3. For convenience, FIG. 4 shows the case where the filter capacitor 5 of the R-phase sub-converter 3 is charged by the current flowing from the R-phase to the S-phase, but the same applies to the other phases. When the charging voltage of the filter capacitor 5 of the sub-converter 3 exceeds the target value, the switch state of the sub-converter 3 is changed, and the filter capacitor 5 of the sub-converter 3 is discharged by the current path shown in Fig. 4 (b). The charge voltage is made close to the target value by repeating the charge mode and the discharge mode shown in the figure (step s 2).

Next, it is determined whether charging of the filter capacitor 5 of the sub-converter 3 is completed (step s 3). When charging is completed, the filter capacitor 4 of the main converter 2 is charged. At this time, the sub-converter 3 is switched to a state where the current does not flow to the filter capacitor 5 (hereinafter referred to as “through mode”).

As shown in (a), the switch state of the main converter 2 is fully turned off, and the filter capacitor 4 of the main converter 2 is charged from the system power supply 1 through the charging resistor 23. For convenience, FIG. 5 shows the case where the filter capacitor 4 of the main converter 2 is charged by the current flowing from the R phase to the S phase, but the same applies to the other phases. In this case, by changing the switch state of the main converter 2 and using the discharge mode in which the filter capacitor 4 of the main converter 2 is discharged by the current path shown in Fig. 5 (b), the charge mode and the discharge mode shown in the figure are repeated. Bring the charging voltage close to the target value (Step s 4) Next, it is determined whether or not the charging of the filter capacitor 4 of the main converter 2 is completed (step s 5). When charging is completed, the sub upper switch 2 2 is shut off and the main upper switch 2 1 is turned on. The operation of the power converter is started (step s 6).

[001 7] In this way, the initial charging of the filter capacitors 4 and 5 of the main converter 2 and the subconverter 3 through the current path from the system power supply 1 through the charging resistor 2 3 by the control of the main converter 2 and the subconverter 3 As a result, the inrush current flowing through the filter capacitors 4 and 5 can be suppressed, and initial charging can be performed with high reliability.

[0018] It should be noted that when charging the filter capacitor 4 of the main converter 2 in steps s4 and s5 in the initial charging flow, the filter capacitor 4 is passed at a predetermined time or at a predetermined rate in step s5. As shown in step s6, when the sub upper switch 2 2 is turned off and the main upper switch 2 1 is turned on to operate the power converter, the remaining filter capacitor 4 remains. The voltage may be charged. In this case, the filter capacitor 4 is charged to the target voltage by controlling the current so as to suppress the current by the control of the sub-computer 2. In this way, the main high-order switch 21 is turned on and charging is performed without going through the charging resistor 23, so that high-speed charging is possible.

[0019] When the circuit disconnection is performed by a higher-order switch, the sub-higher switch 22 may not be provided.

[0020] Embodiment 2.

In the first embodiment, it has been described that the filter capacitor 4 of the main converter 2 is charged after the filter capacitor 5 of the sub-converter 3 is charged. In the second embodiment, as shown in the flowchart of FIG. After charging the filter capacitor 4 of the main converter 2, the filter capacitor 5 of the sub-converter 3 is charged.

First, when main upper switch 21 and sub upper switch 22 are shut off and filter capacitors 4 and 5 are not charged (step ss 1), Turn on the sub-higher switch 2 2 to charge the filter capacitor 4 of the main converter 2 while suppressing the inrush current (see step ss 1, Fig. 5).

Next, it is determined whether or not the charging of the filter capacitor 4 of the main converter 2 is completed (step s s 3). When the charging is completed, the filter capacitor 5 of the sub-converter 3 is charged (see step s s 4 and FIG. 4).

Next, it is determined whether or not the charging of the filter capacitor 5 of the sub-converter 3 is completed (step ss 5). When the charging is completed, the sub-higher switch 2 2 is shut off and the main higher-level switch 2 1 is turned on. Start the converter operation (step ss 6).

Furthermore, the filter capacitors 4 and 5 of the main converter 2 and the sub-converter 3 may be charged at the same time as shown in the flowchart of FIG. 7, for example. In this case, the filter capacitors 4 and 5 of the main converter 2 and the sub-converter 3 are simultaneously charged while switching between the charge mode and the discharge mode. First, with the main upper switch 2 1 and sub upper switch 2 2 shut off and the filter capacitors 4 and 5 are not charged (step t 1), the sub upper switch 2 2 is turned on while suppressing the inrush current. The filter capacitor 4 of the main converter 2 and the filter capacitor 5 of the sub-converter 3 are charged simultaneously. At this time, as shown in FIG. 8 (a), the switch state of the main converter 2 is completely turned off, and the filter capacitor 5 and the main converter 3 of the sub-converter 3 are connected by the current path from the system power supply 1 to the charging resistor 23. Charge filter capacitor 4 of converter 2. For convenience, Fig. 8 shows the case where the filter capacitor 5 of the R-phase sub-converter 3 and the filter capacitor 4 of the main converter 2 are charged by the current flowing from the R-phase to the S-phase. Is the same (step t 2).

[0022] Next, it is determined whether the charging of the filter capacitor 5 of the sub-converter 3 is completed (step t3). If the charging is not completed, the charging of the filtering capacitor 5 of the main converter 2 is subsequently completed. If the charging is incomplete, the process returns to step t2, and the filter capacitor of the main converter 2 Simultaneous charging of the capacitor 4 and the filter capacitor 5 of the sub-converter 3 is continued. When charging of the filter capacitor 5 of the sub-converter 3 is completed at step t3, the switch state of the sub-converter 3 is set to the through mode, and the filter capacitor 5 of the sub-converter 3 is charged by the current path shown in Fig. 8 (b). Instead, only the filter capacitor 4 of the main converter 2 is charged. Next, it is determined whether the charging of the filter capacitor 5 of the main converter 2 is completed (step t6). When charging is completed, the sub upper switch 2 2 is shut off and the main upper switch 2 1 is turned on to convert the power. Start equipment operation (step t7).

When charging of the filter capacitor 4 of the main converter 2 is completed at step t4, the filter capacitor 4 of the main converter 2 is charged and discharged, and the filter capacitor 5 of the sub-converter 3 is charged. At this time, the discharge mode in which the filter capacitor 4 of the main converter 2 is discharged by the current path shown in FIG. 8 (c) is used, and the charge mode shown in FIG. Move closer to the value (step t8). Then, it is determined whether or not the charging of the filter capacitor 5 of the sub-converter 3 is completed (step t 9). When the charging is completed, the process proceeds to step t 7 where the sub upper switch 2 2 is shut off and the main upper switch 2 1 is turned on. To start operation of the power converter.

[0023] Also in the second embodiment, the main converter 2 and the sub-converter

Under the control of 3, the filter capacitors 4 and 5 of the main converter 2 and sub-converter 3 are initially charged through the current path from the system power supply 1 through the charging resistor 2 3, so that inrush flows into the filter capacitors 4 and 5. Current can be suppressed and initial charging can be performed reliably.

[0024] Next, the stop operation of the power converter in a normal operation stop or accident (2) overload operation will be described. First, the main upper switch 21 opens the contact, and an arc voltage is generated. The current is limited and cut off at the natural current zero point. After that, the current is commutated to the parallel circuit of the sub upper switch 2 2 and the charging resistor 2 3. The circuit recovery voltage between the contacts of the main upper switch 21 is low. The charging resistor 23 suppresses the current flowing through the sub upper switch 22 2, and the oscillation of the circuit recovery voltage at the time of disconnection can be reduced, so that the sub upper switch 22 can easily cut off the current.

In this way, the circuit breaking performance is greatly improved, and the disconnecting surge at the time of breaking can be suppressed by the charging resistor 23.

In the first and second embodiments, the charging resistor 23 is provided, but as shown in FIG. 9, a reactor 23 a may be used instead of the charging resistor 23. In this case, the filter capacitors 4 and 5 of the main converter 2 and the sub-converter 3 are initially charged by the same control as in the first and second embodiments through the current path from the system power supply 1 through the reactor 23 3a. As a result, the inrush current flowing through the filter capacitors 4 and 5 can be suppressed, and initial charging can be performed with high reliability. Also in this case, after charging the filter capacitor 5 of the sub-converter 3 through the reactor 2 3 a, the sub-higher switch 2 2 is shut off and the main higher-level switch 2 1 is turned on so that the filter capacitor 4 of the main converter 2 is turned on. You may charge to a target voltage. At this time, since the current is controlled so as to suppress the current by controlling the sub-converter 2 without passing through the reactor 2 3 a and the filter capacitor 4 of the main converter 2 is charged to the target voltage, high-speed charging becomes possible.

Embodiment 3

In the first and second embodiments, the number of sub-converters 3 is one for each phase. However, in this third embodiment, a plurality of single-phase sub-converters 3 are connected to each phase AC input line 9 of the main converter 2. Connect the AC side in series. FIG. 10 is a diagram showing a configuration of a power conversion device according to Embodiment 3 of the present invention. In this case, the main converter 2 is a three-phase three-level converter in which each phase 2 a is configured as shown in the figure, and the filter capacitor 4 uses two filter capacitors 4 a connected in series.

The filter converter of the main converter 2 of the power converter configured as described above. The DC voltage VG of Densa 4 and the DC voltages Vb1 and Vb2 of the filter capacitors 5 of the two sub-converters 3 are different values (Vc>Vb1> Vb2), respectively, 4: 2: 1, 4: 3: 1, 5 : 3: 1, 6: 3: 1, 7: 3: 1, etc. It should be noted that other values may be used in accordance with the product specifications, and the types of components may be reduced by making the DC voltages Vb1 and Vb2 of the filter capacitors 5 of the two sub-converters 3 equal. In each case, the relationship between the output logic of each converter and the output gradation (voltage level) of the power converter connected in series is shown in the logic table of A to E in Fig. 11.

[0027] Here, the case of Table A will be described below.

Vc, Vb1, and Vb2 have a 4: 2: 1 relationship. By combining the three converters 2 and 3, the sum of these generated voltages is used to generate an 8-level phase voltage (absolute value) of 0 to 7 that is an AC input. Occurs at the terminal. Figure 12 shows the voltage waveform of each comparator 2 and 3 to obtain a gradation voltage that is almost sinusoidal. Fig. 1 2 (a) is the voltage waveform of the entire power converter, Fig. 1 2 (b) is the voltage waveform of sub-converter 3 having DC voltage Vb2, and Fig. 1 2 (c) is the voltage waveform of sub-converter 3 having DC voltage Vb1. Voltage waveform, Fig. 1 2 (d) shows the voltage waveform of main converter 2 with DC voltage VG. It can be seen that a smooth voltage gradation waveform is obtained by combining the voltages generated by converters 2 and 3.

[0028] In such a serial connection configuration of the main converter 2 having different DC voltages and the plurality of sub-converters 3, the voltage generated at the AC input terminal is multi-leveled and becomes a smooth voltage gradation waveform. Harmonics can be suppressed. In addition, by suppressing the number of switching times of the main converter 2 having a high voltage, switching loss can be suppressed, the efficiency of the power converter can be improved, and electromagnetic noise can be reduced.

In addition, as shown in FIG. 10, an initial charging resistor comprising a main upper switch 21, a sub upper switch 22, and a charging resistor 23 is provided between the sub-converter 3 and the system power source 1 in each phase AC input line 9. Provide a circuit. As a result, the main converter 2 and the current path from the grid power supply 1 through the charging resistor 23 The initial charging of the filter capacitors 4 and 5 of the sub-converter 3 can be performed. Also in this case, the initial charging of the filter capacitors 4 and 5 can be performed as in the first and second embodiments, and the same effect can be obtained.

In this case as well, the reactor 2 3 a may be used instead of the charging resistor 2 3.

Industrial applicability

It can be widely applied to power converters that are equipped with a power storage that requires initial charging on the DC side and that convert power from AC to DC and can control power even during regeneration.

Claims

The scope of the claims

[1] A main converter and a sub-converter, each having a power storage unit having a different DC voltage on the DC side, converting the AC power from an AC power source into DC power and outputting the DC power to the power storage unit. A power converter arranged in series between the main converter and the AC power supply, and a charging resistor connected between the power converter and the AC power supply,

At the time of initial charging of each power storage unit, at least the power storage unit of the sub-converter is charged from the AC power source through the charging resistor under the control of the main converter and the sub-converter. Power converter.

[2] The power conversion device according to [1], wherein a switching switch for bypassing the charging resistor is arranged in parallel with the charging resistor.

3. The power conversion device according to claim 2, wherein a second circuit is connected in series to the charging resistor to form a series circuit, and the series circuit and the switching switch are connected in parallel. .

[4] At the time of initial charging of each of the power storage units, each power storage unit of the main converter and the sub-converter in a current path from the AC power source through the charging resistor under the control of the main converter and the sub-converter The power conversion device according to any one of claims 1 to 3, wherein the power conversion device is charged.

[5] At the time of initial charging of each power storage unit, the power storage unit of the sub-converter is charged from the AC power source through the charging resistor by the control of the main converter and the sub-converter, The power converter according to claim 2 or 3, wherein the power storage of the main converter is charged from the AC power source through a current path bypassing the charging resistor with the sub-converter.

[6] The sub-converter is characterized in that a plurality of stages are connected in series.

The power conversion device according to any one of 1 to 3.

[7] A main converter and a sub-converter, each having a power storage unit having a different DC voltage on the DC side, converting the AC power from the AC power source to DC power and outputting the DC power to the power storage unit. A power converter arranged in series between the main converter and the AC power supply, and a reactor connected between the power converter and the AC power supply,

At the time of initial charging of each power storage unit, at least the power storage unit of the sub-converter is charged in a current path from the AC power source through the reactor under the control of the main converter and the sub-converter. Conversion device.

8. The power conversion device according to claim 7, wherein a switching switch for bypassing the reactor is arranged in parallel with the reactor.

9. The power converter according to claim 8, wherein a series circuit is configured by connecting a second switch in series with the reactor, and the series circuit and the switching switch are connected in parallel.

[10] At the time of initial charging of each of the power storage units, the main power storage unit and the sub-converter are charged in a current path from the AC power source through the reactor under the control of the main converter and the sub-converter. The power conversion device according to any one of claims 7 to 9, wherein the power conversion device is charged.

[1 1] During initial charging of each power storage unit, the power storage unit of the sub-converter is charged from the AC power source through a current path through the reactor under the control of the main converter and the sub-converter. 10. The power conversion device according to claim 8, wherein the power storage device of the main converter is charged while current is controlled by the sub-converter through a current path that bypasses the reactor from an AC power source.

[12] The power converter according to any one of claims 7 to 9, wherein the sub-converter has a plurality of stages connected in series.