WO2019163080A1

WO2019163080A1 - Voltage switching-type dc power supply

Info

Publication number: WO2019163080A1
Application number: PCT/JP2018/006653
Authority: WO
Inventors: 田中　正一
Original assignee: 田中　正一
Priority date: 2018-02-23
Filing date: 2018-02-23
Publication date: 2019-08-29

Abstract

Provided is a voltage switching-type DC power supply with which it is possible to reduce loss. The DC power supply has a connection switching circuit capable of selecting a series connection or a parallel connection of two or four batteries. The DC power supply applies a DC link voltage having two levels or three levels to an inverter for driving a variable speed motor. In the connection switching circuit, a series transistor is switched, whereby the connection switching circuit operates as a step-down chopper. If any of the batteries has failed, the series transistor is turned off.

Description

Voltage switching DC power supply

The present invention relates to a voltage switching DC power supply, and more particularly to a voltage switching DC power supply for a traction motor.

The DC link voltage applied to the inverter for the traction motor is preferably changed according to the motor rotation speed. FIG. 1 shows a known motor driving device in which a boost chopper 100 is disposed between a battery 101 and an inverter 102. The boost chopper 100 boosts the DC link voltage Vd applied to the smoothing capacitor 103 in the high speed region. However, the weight and loss of the step-up chopper 100 are disadvantages of this motor drive device.

A known voltage-switching DC power supply having a series switch that connects two batteries in series and two parallel switches that connect two batteries in parallel can avoid the switching loss of the boost chopper. One problem with this voltage-switching DC power supply is that the DC link voltage Vd changes abruptly. A sudden change in the DC link voltage Vd adversely affects the smoothing capacitor and the inverter. Another problem is that the short circuit current flowing between the two batteries increases the battery loss when the voltages of the two batteries connected in parallel are different.

FIG. 2 shows an example of a voltage-switching DC power source disclosed in Patent Document 1. Series relay 201 connects

batteries

202 and 203 in series. Two parallel relays 204 and 205 connect the

batteries

202 and 203 in parallel. The step-up chopper 208 shown in FIG. 2 gradually changes the DC link voltage Vd during a transition period for switching the battery connection. Thereby, it is possible to avoid an adverse effect on the smoothing capacitor 206 and the inverter 207 due to a sudden change in the DC link voltage at the time of voltage switching. However, the addition of this step-up chopper increases manufacturing cost and switching loss. Furthermore, it is not easy to control the cooperative operation of the three relays and the step-up chopper of the voltage-switching DC power supply.

JP 2012-060838 A

One object of the present invention is to provide a voltage-switching DC power source suitable for a traction motor.

According to the present invention, the connection switching circuit that can select either serial connection or parallel connection of two batteries changes the DC link voltage applied to the inverter that drives the traction motor. The connection switching circuit has one series transistor, one charging diode 4, and two parallel diodes, and an inductor. The series transistor applies a series voltage to the inverter, and the parallel diode applies a parallel voltage to the inverter. The charging diode charges the battery with the regenerative current of the inverter. This connection switching circuit can change the DC link voltage and reduce battery loss. In addition, when one of the batteries is defective, another normal battery can execute the parallel discharge mode. That is, this voltage switching type DC power supply has a DC link voltage changing function and a defective battery separating function.

Further, according to this voltage-switching DC power supply, the series transistor is PWM-switched during the switching period between the parallel connection and the series connection. This means that the series transistor, parallel diode, and inductor of the connection switching circuit operate as a step-down chopper. Thereby, the DC link voltage can be gradually changed during the switching period.

In a preferred embodiment, the inverter is operated as a boost chopper during the regenerative braking period of the traction motor. Thereby, the inverter can charge a plurality of batteries connected in series by the serial transistor.

In another preferred embodiment, a magnet contactor is connected in parallel with the parallel diode. The contact of the so-called relay called the magnetic contactor is consumed by the arc spark when it is off. In the worst case, a contact welding accident occurs. However, according to this aspect, since the parallel diode prevents arc spark, the failure rate of the magnetic contactor is reduced. Of course, the magnet contactor is turned off during the period when the series transistor is PWM-switched. It is also possible to individually connect two contacts of one magnetic contactor to two parallel diodes. Two magnetic contactors can be individually connected to two parallel diodes. According to this case, it is possible to avoid parallel charging and parallel discharging of a defective battery.

In another preferred embodiment, two sub power supply sets are employed. Each of the two sub power supply sets includes two battery blocks and one connection switching circuit. Further, the two sub power supply sets are connected by a third connection switching circuit. As a result, the voltage-switching DC power supply can generate three levels of DC link voltage. Furthermore, this voltage-switching DC power supply can execute the 2-parallel discharge mode and the 4-parallel discharge mode when one battery block becomes defective. Therefore, the reliability of the battery is improved.

FIG. 1 is a wiring diagram showing a conventional step-up chopper type variable voltage DC power supply. FIG. 2 is a wiring diagram showing a conventional voltage-switching DC power supply. FIG. 3 is a wiring diagram showing the voltage-switching type DC power source of the first embodiment. FIG. 4 is a wiring diagram showing the step-down chopper operation of the connection switching circuit shown in FIG. FIG. 5 is a wiring diagram showing a voltage-switching DC power supply having a relay box for parallel charging. FIG. 6 is a schematic wiring diagram showing a boost chopper type regenerative braking operation. FIG. 7 is a schematic wiring diagram showing a boost chopper type regenerative braking operation. FIG. 8 is a wiring diagram showing the voltage switching type DC power source of the second embodiment. FIG. 9 is a diagram showing the relationship between the DC link voltage and the motor speed. FIG. 10 is a wiring diagram showing a voltage-switching DC power supply having a relay box for parallel charging. FIG. 11 is a wiring diagram showing a modification.

A preferred embodiment of the voltage-switching DC power source of the present invention will be described with reference to the drawings. This voltage-switching DC power supply is connected to an inverter that drives a traction motor of an electric vehicle. This voltage switching DC power supply can employ a capacitor instead of a battery. This voltage-switching DC power supply can be connected to an inverter that drives another variable speed motor.

First Embodiment A voltage-switching type DC power source according to a first embodiment will be described with reference to FIGS. This voltage-switching DC power source includes

batteries

1 and 2 and a connection switching circuit 10. The connection switching circuit 10 includes a series transistor 3, a charging diode 4,

parallel diodes

5 and 6, and an inductor 7. The voltage-switching DC power supply applies a DC link voltage Vd to the smoothing capacitor 20 and the inverter 30. Inverter 30 has three

legs

31, 32, and 33. Inverter 30 is connected to stator coil 40 of the three-phase motor. The stator coil 40 includes a U-phase coil 41, a V-phase coil 42, and a W-phase coil 43. Leg 31 is connected to phase coil 41, and leg 32 is connected to phase coil 42. The leg 33 is connected to the phase coil 43.

The negative electrode of the battery 1 is connected to the negative terminals of the smoothing capacitor 20 and the inverter 30. The positive electrode of the battery 2 is connected to the positive terminals of the smoothing capacitor 20 and the inverter 30 through the inductor 7. The series transistor 3 and the charging diode 4 connect the

batteries

1 and 2 in series. The series transistor 3 can turn off the discharge of the

batteries

1 and 2. A charging diode 4 connected in antiparallel to the series transistor 3 can charge the

batteries

1 and 2. The anode of the parallel diode 5 is connected to the negative electrode of the battery 1, and the cathode of the parallel diode 5 is connected to the negative electrode of the battery 2. The anode electrode of the parallel diode 6 is connected to the positive electrode of the battery 2, and the cathode electrode of the parallel diode 6 is connected to the positive electrode of the battery 1. For example,

batteries

1 and 2 each have a voltage of 320V. The connection switching circuit 10 and the inverter 30 are controlled by the controller 50. First, the precharge operation of the smoothing capacitor 20 when the key switch of the electric vehicle is turned on will be described. Since the series transistor 3 is off, the smoothing capacitor 20 is charged through the

parallel diodes

5 and 6. As a result, the inrush current flowing through the smoothing capacitor 20 is halved and the power loss is ¼.

The discharge operation of this voltage-switching DC power supply will be described. In the series discharge mode in which the series transistor 3 is turned on, the DC link voltage Vd becomes equal to the voltage sum of the batteries 1 and 2 (= 640 V). In the parallel discharge mode in which the series transistor 3 is turned off, the DC link voltage Vd becomes equal to the higher voltage (= about 320 V) of the

batteries

1 and 2. In other words, the voltage difference between the two

batteries

1 and 2 is automatically eliminated in the parallel discharge mode by the

parallel diodes

5 and 6.

Next, the switching operation from the parallel discharge mode to the series discharge mode will be described. In this switching operation, the series transistor 3 is switched at a predetermined PWM carrier frequency. Its PWM duty ratio, which is equal to the ratio of on period / (on period + off period), is gradually increased from 0 to 1. When the series transistor 3 is turned on, a voltage sum (640V) is applied to the smoothing capacitor 20 through the inductor 7, and the inductor 7 stores magnetic energy. When the series transistor 3 is turned off, the inductor 7 suppresses a decrease in current flowing through the inductor 7. As a result, the DC link voltage Vd gradually increases from 320V to 640V.

Next, the switching operation from the series discharge mode to the parallel discharge mode will be described. In this switching operation, the series transistor 3 is switched at a predetermined PWM carrier frequency. Its PWM duty ratio, which is equal to the ratio of on period / (on period + off period), is gradually reduced from 1 to 0. As a result, the DC link voltage Vd gradually decreases from 640V to 320V. Eventually, the

parallel diodes

5 and 6, the series transistor 3, and the inductor 7 serve as a known step-down chopper.

FIG. 4 is a schematic diagram showing the operation of the step-down chopper. The circuit 300 on the left shows a state where the series transistor 3 is turned on, and the circuit 400 on the right shows a state where the series transistor 3 is turned off. The inductor 7 can have a lower inductance value than the inductor of the boost chopper shown in FIG. The switching frequency of the series transistor 3 can have a higher switching frequency value than that of the boost chopper shown in FIG. As a result, the switching loss of the step-down chopper increases. However, since the high-frequency switching of the series transistor 3 is only in the connection switching period, this increase in switching loss can be ignored.

Next, charging of the

batteries

1 and 2 by the external DC power supply will be described with reference to FIG. In order to charge the

batteries

1 and 2 in parallel, the external DC power supply applies a voltage of about 320V to the

batteries

1 and 2. For this parallel charging, the connection switching circuit 10 has a relay box 8 and a connector 9. The relay box 8 accommodates two

magnet contactors

81 and 82. The positive electrode of the battery 2 is connected to the positive terminal 91 of the connector 9, and the negative electrode of the battery 1 is connected to the negative terminal 92 of the connector 9.

Terminals

91 and 92 are connected to an external DC power supply (not shown).

Contactor 81 connects negative electrode terminal 92 to the negative electrode of battery 2, and contactor 82 connects positive electrode terminal 91 to the positive electrode of battery 1. When the

contactors

81 and 82 are turned on, the contactor 81 is connected in parallel with the parallel diode 5, and the contactor 82 is connected in parallel with the parallel diode 6. Thereby, the external DC power supply charges the

batteries

1 and 2 in parallel. In the parallel discharge mode in which the

contactors

81 and 82 are turned on, the

batteries

1 and 2 apply a DC link voltage Vd of about 320V to the inverter 30. Thereby, the loss of the

parallel diodes

5 and 6 is reduced.

According to this embodiment, the contactor 81 is connected in parallel with the parallel diode 5, and the contactor 82 is connected in parallel with the parallel diode 6. Accordingly, in the parallel discharge mode, the

batteries

1 and 2 are already in the parallel discharge mode by the

diodes

5 and 6 before the

contactors

81 and 82 are turned on. Therefore, when the

contactors

81 and 82 are turned on, the voltage difference between the

batteries

1 and 2 can be sufficiently reduced. This means that when the

contactors

81 and 82 are turned on, the short-circuit current flowing between the

batteries

1 and 2 becomes substantially zero. Furthermore, when the contactor 81 and / or the contactor 82 is turned off, arcing of the

contactor

81 and 82 is prevented by the

diodes

5 and 6. Thereby, the lifetime and reliability of the

ts contactors

81 and 82 are improved.

In the parallel charging period of the

batteries

1 and 2, when only the voltage of the battery 1 exceeds a predetermined value, only the contactor 81 is turned off. Similarly, when only the voltage of the battery 2 exceeds a predetermined value, only the contactor 82 is turned off. In the parallel discharge period of the

batteries

1 and 2, when only the voltage of the battery 1 becomes less than a predetermined value, only the contactor 81 is turned off. Similarly, when only the voltage of the battery 2 exceeds a predetermined value, only the contactor 82 is turned off. Eventually, the bad battery is separated by two

independent contactors

81 and 82.

Next, the charging operation in the regenerative braking period of this voltage-switching DC power supply will be described with reference to FIGS.

Contactors

81 and 82 may or may not be turned on. In this regenerative braking, when the generated voltage of the inverter 30 is lower than the DC link voltage, the controller 50 instructs the inverter 30 to perform a boost chopper operation. In this step-up chopper operation, the three legs 31-33 of the inverter 30 are switched synchronously at a predetermined PWM carrier frequency. Each PWM cycle period is composed of a sum of a clamp period and an output period. In the clamp period shown in FIG. 6, the lower arm transistors of the legs 31-33 are simultaneously turned on. In the output period shown in FIG. 7, the lower arm transistors of the legs 31-33 are simultaneously turned off.

When the PWM duty ratio equal to the ratio of output period / (clamp period + output period) is low, inverter 30 generates a high output voltage. Thereby, the inverter 30 which functions as a step-up chopper can generate a high charging voltage. The PWM duty ratio is controlled based on the battery charging current. Since the inverter 30 is operated as a step-up chopper, the ripple rate of the charging current supplied from the inverter 30 to the

batteries

1 and 2 is increased. However, this ripple rate is reduced by the inductor 7 and the smoothing capacitor 20. The loss of the

batteries

1 and 2 is reduced by this ripple rate reduction. Similarly, the losses of the

batteries

1 and 2 are reduced in the parallel discharge mode compared to the series discharge mode under low load conditions of the traction motor. This suppresses an increase in battery temperature. In particular, this effect is superior in older batteries with increased internal resistance.

Second Embodiment A voltage-switching DC power source according to a second embodiment will be described with reference to FIG. According to this embodiment, a second connection switching circuit 10A and a third connection switching circuit 10B are added to the voltage-switching DC power supply of the first embodiment. The battery 1 of the first embodiment is divided into two battery blocks 1A and 1B each having a rated voltage of 160V. Similarly, the battery 2 of the first embodiment is divided into two

battery blocks

2A and 2B each having a rated voltage of 160V. The inductor 7 of the first embodiment is divided into two

inductors

71 and 72.

Inductors

71 and 72 can have a common core. Each of the added connection switching circuits 10A and 10B has substantially the same circuit configuration as that of the connection switching circuit 10.

The connection switching circuit 10A includes a series transistor 3A, a charging diode 4A, parallel diodes 5A and 6A, and an inductor 7. The series transistor 3A connects the block 2A and the block 2B. The charging diode 4A is connected in antiparallel with the series transistor 3A. The parallel diode 5A connects the negative electrode of the block 2A and the negative electrode of the block 2B. The parallel diode 6A connects the positive electrode of the block 2A and the positive electrode of the block 2B.

The connection switching circuit 10B includes a series transistor 3B, a charging diode 4B,

parallel diodes

5B and 6B, and an inductor 7. The serial transistor 3B connects the block 1A and the block 1B. The charging diode 4B is connected in antiparallel with the series transistor 3B. The parallel diode 5B connects the negative electrode of the block 1A and the negative electrode of the block 1B. The parallel diode 6B connects the positive electrode of the block 1A and the positive electrode of the block 1B.

The discharge operation of this voltage-switching DC power supply will be described. The controller 50 has a series discharge mode, a 2-parallel discharge mode, and a 4-parallel discharge mode. The 2-parallel discharge mode is essentially the same as the parallel discharge mode of the first embodiment. In the series discharge mode in which the three

series transistors

3, 3A, and 3B are turned on, the DC link voltage Vd is equal to the voltage sum (= 640V) of the four

blocks

2A, 2B, 1A, and 1B. In the 2-parallel discharge mode, the series transistor 3 is turned off and the

series transistors

3A and 3B are turned on. The DC link voltage Vd is approximately 320V. In the 4-parallel discharge mode, the

series transistors

3, 3A and 3B are turned off. The DC link voltage Vd is approximately 160V.

Next, the switching operation from the 2-parallel discharge mode to the series discharge mode will be described. In this switching operation, the

series transistors

3A and 3B are turned off. The serial transistor 3 is switched at a predetermined PWM carrier frequency, and its PWM duty ratio is gradually increased from 0 to 1. As a result, the DC link voltage Vd gradually increases from 320V to 640V. Next, the switching operation from the series discharge mode to the 2-parallel discharge mode will be described. In this switching operation, the serial transistor 3 is switched at a predetermined PWM carrier frequency, and its PWM duty ratio is gradually reduced from 1 to 0. As a result, the DC link voltage Vd gradually decreases from 640V to 320V. Eventually, the

parallel diodes

Next, the switching operation from the 4-parallel discharge mode to the 2-parallel discharge mode will be described. In this switching operation, the series transistor 3 is turned off. The

series transistors

3A and 3B are switched at a predetermined PWM carrier frequency, and the PWM duty ratio is gradually increased from 0 to 1. As a result, the DC link voltage Vd gradually increases from 160V to 320V. Next, the switching operation from the 2-parallel discharge mode to the 4-parallel discharge mode will be described. In this switching operation, the

series transistors

3A and 3B are switched at a predetermined PWM carrier frequency, and the PWM duty ratio is gradually reduced from 1 to 0. As a result, the DC link voltage Vd gradually decreases from 320V to 160V. Eventually, the parallel diodes 5A and 6A, the series transistor 3A, and the inductor 7 function as a known step-down chopper. Similarly,

parallel diodes

5B and 6B, series transistor 3B, and inductor 7 also function as a known step-down chopper.

FIG. 9 is a diagram showing the relationship between the motor rotation speed and the DC link voltage Vd when the motor torque is the maximum value. The DC link voltage Vd is, for example, 160 V in a low speed region less than 40 km / h, for example, 320 V in a medium speed region in the range of 40-80 km / h, and 640 V in a high speed region exceeding 80 km / h.

Next, charging of the four

blocks

1A, 1B, 2A, and 2B by the external DC power supply will be described with reference to FIG.

Series transistors

3A and 3B are turned on and series transistor 3 is turned off. Thereby, the blocks 1A and 1B connected in series substantially become the battery 1. Similarly, the

blocks

2A and 2B connected in series become the battery 2 substantially. In order to charge the

batteries

1 and 2. For this parallel charging, the connection switching circuit 10 has a relay box 8 and a connector 9 as in the first embodiment.

The

contactors

81 and 82 can be turned on in the 2-parallel discharge mode and the regenerative braking mode. Thereby, the contactor 81 is connected in parallel with the parallel diode 5, and the contactor 82 is connected in parallel with the diode 6. The DC link voltage Vd is 320 V in the 2-parallel discharge mode and the regenerative braking mode. Since the regenerative braking in this embodiment is essentially the same as in the first embodiment, description thereof is omitted.

According to this embodiment, the contactor 81 is connected in parallel with the parallel diode 5, and the contactor 82 is connected in parallel with the parallel diode 6. Therefore, in the 2-parallel discharge mode, the

batteries

1 and 2 are already in the 2-parallel discharge mode by the

diodes

5 and 6 before the

contactors

81 and 82 are turned on. Therefore, when the

contactors

81 and 82 are turned on, the voltage difference between the

batteries

1 and 2 can be sufficiently reduced. This means that when the

contactors

81 and 82 are turned on, the short-circuit current flowing between the

batteries

1 and 2 becomes substantially zero. Furthermore, when the contactor 81 and the contactor 82 are turned off, the arc discharge of the

contactors

81 and 82 is prevented by the

diodes

5 and 6. Thereby, the lifetime and reliability of the

contactors

81 and 82 are improved.

Next, the discharge mode in the block failure state which means the case where the voltage of one of the

blocks

1A, 1B, 2A, and 2B is out of the allowable range will be described. 4-When the parallel discharge mode is executed, three blocks except the defective block supply the discharge current. In other words, 75% of the battery cells can continue to discharge. In the 2-parallel discharge mode, one of the

batteries

1 and 2 that does not include a defective block supplies a discharge current. In other words, 50% of the battery cells can continue to discharge.

Next, the charging mode in the block failure state will be described. The serial transistor 3 is turned off and the 2-parallel charging mode is employed. Further, the

serial transistor

3A, 3B connected to the defective block is turned off. Thereby, 50% of battery cells can continue charging. Eventually, according to the second embodiment using three series transistors, it is possible to select a three-stage DC link voltage Vd and to disconnect a defective battery block.

The advantages of this embodiment are described. According to this embodiment, the 4-parallel discharge mode employed in the low speed region significantly reduces battery loss. For this reason, the rise in battery temperature can be suppressed, and the battery life can be extended particularly in a high temperature environment. This effect is particularly noticeable in old batteries with high internal resistance.

Variations of the first and second embodiments will be described. According to this modification, the charging diode 4 shown in FIG. 5 or FIG. 8 is omitted. The

contactors

81 and 82 are turned on during the regenerative braking period, and the inverter 30 charges the

batteries

1 and 2 in parallel through the

contactors

81 and 82. A power transistor can be employed in place of the

contactors

81 and 82. FIG. 11 shows another variation. In this variation, the

series transistors

3, 3A, and 3B shown in FIG. 8 each comprise a bidirectional insulated gate transistor. The insulated gate transistor can have a body diode.

Claims

In a voltage-switching DC power supply having a connection switching circuit (10) capable of selecting one of a series connection and a parallel connection of a plurality of batteries (1, 2) and connected to an inverter (30) for driving a variable speed motor ,
The connection switching circuit (10)
A series transistor (3) connecting the battery (1) and the battery (2) in series;
A parallel diode (5) connecting the negative electrodes of the two batteries (1, 2);
A parallel diode (6) connecting the positive electrodes of the two batteries (1, 2);
An inductor (7) connecting the voltage-switching DC power source and the inverter (30);
A controller (50) for controlling the series transistor (3),
The controller (50) includes a series discharge mode for discharging the two batteries (1, 2) in series, a parallel discharge mode for discharging the two batteries (1, 2) in parallel, and the connection switching circuit ( A voltage-switching type DC power supply characterized by having a chopper mode in which 10) is operated as a step-down chopper.
The controller (50) turns on the series transistor (3) in the series discharge mode, turns off the series transistor (3) in the parallel discharge mode, and turns on the series transistor (3) in the chopper mode. 2. The voltage switching type DC power source according to claim 1, wherein switching is performed at a PWM carrier frequency.
The voltage-switching DC power supply according to claim 1, wherein the connection switching circuit (10) has a charging diode (4) connected in antiparallel with the series transistor (3).
The voltage-switching DC power supply according to claim 3, wherein the controller (50) operates the inverter (30) as a step-up chopper during a regenerative braking period of the motor.
The voltage-switching DC power supply according to claim 1, wherein the controller (50) adopts the chopper mode in a predetermined switching period between the series discharge mode and the parallel discharge mode.
The voltage-switching DC power supply according to claim 1, wherein the controller (50) adopts the parallel discharge mode under a condition that one voltage of the two batteries (1, 2) is less than a predetermined value.
The connection switching circuit (10) has two contactors (81, 82) individually connected in parallel with the two parallel diodes (5, 6),
The voltage-switching DC power supply according to claim 1, wherein the controller (50) turns on at least one of the two contactors (81, 82) when the batteries (1, 2) are charged by an external DC power supply. .
The voltage-switching DC power supply according to claim 7, wherein the controller (50) turns on at least one of the two contactors (81, 82) in the parallel discharge mode.
The battery (1) is composed of two blocks (1A, 1B) connected through a second connection switching circuit (10B).
The battery (2) is connected through a third connection switching circuit (10A) and consists of two blocks (2A, 2B).
The voltage-switching DC power supply according to claim 1, wherein the second and third connection switching circuits (10A, 10B) have essentially the same circuit configuration as the connection switching circuit (10).
The controller (50) turns on all the series transistors (3, 3A, 3B) of the three connection switching circuits (10, 10A, 10B) in the series discharge mode, and the series transistors (3, 3A, 3B) in the parallel discharge mode. The voltage-switching type DC power supply according to claim 9, wherein only 3) is turned off.
The voltage switching DC power supply according to claim 10, wherein the controller (50) operates only the connection switching circuit (10) as a step-down chopper in the chopper mode for switching between the series discharge mode and the parallel discharge mode.
The voltage-switching DC power supply according to claim 10, wherein the controller (50) has a 4-parallel discharge mode in which all of the three series transistors (3, 3A, 3B) are turned off.
The controller (50) operates the second and third connection switching circuits (10A, 10B) as a step-down chopper in a second chopper mode for switching between the 4-parallel discharge mode and the parallel discharge mode,
The controller (50) switches the series transistors (3A, 3B) of the second and third connection switching circuits (10A, 10B) at a predetermined PWM carrier frequency in the second chopper mode. 12. The voltage switching type DC power source according to 12.
The voltage-switching DC according to claim 10, wherein each of the connection switching circuits (10, 10A, 10B) has a charging diode (4, 4A, 4B) connected in antiparallel with the series transistor (3, 3A, 3B). Power supply.
The voltage-switching DC power supply according to claim 10, wherein the controller (50) operates the inverter (30) as a step-up chopper during a regenerative braking period of the motor.
The voltage-switching DC power supply according to claim 10, wherein the controller (50) prohibits the series discharge mode when one voltage of the four blocks (1A, 1B, 2A, 2B) becomes less than a predetermined value.