FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
[See section 10, Rule 13]
POWER CONVERTER AND POWER SUPPLY APPARATUS;
MITSUBISHI ELECTRIC CORPORATION, A CORPORATION
ORGANISED AND EXISTING UNDER THE LAWS OF JAPAN, WHOSE
ADDRESS IS 7-3, MARUNOUCHI 2-CHOME, CHIYODA-KU, TOKYO
1008310, JAPAN
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE
INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED

2
DESCRIPTION
TITLE OF THE INVENTION:
5 Field
[0001] The present disclosure relates to a power
converter for converting power supply voltage applied from
a DC power supply, into DC voltage to a DC load, and a
power supply apparatus.
10
Background
[0002] Patent Literature 1 below discloses a three-level
DC-DC converter employing a so-called zero-voltage and
zero-current switching scheme that switches a switching
15 element with zero voltage and current across the switching
element. Patent Literature 1 discloses a simply configured
three-level power converter including a main circuit having
a bidirectional switch and soft switching capacitors added
thereto without using an auxiliary circuit including a
20 high-withstand-voltage switching element, a saturation
reactor, etc. That converter controls switching elements
such that the switching elements are turned on/off, thereby
enabling zero-voltage and zero-current switching operation.
To expand a soft switching region, it is necessary to
25 increase the inductance value of an inductor or a reactor
involved in power transfer, the details of which will be
described later.
Citation List
30 Patent Literature
[0003] Patent Literature 1: Japanese Patent Application
Laid-open No. 2014-103725

3
Summary of Invention
Problem to be solved by the Invention
[0004] For the technique of Patent Literature 1, the
inductance value used at the time of power transfer is a
5 fixed value because the leakage inductance of an isolation
transformer is used. For this reason, an improvement in
power conversion efficiency is not necessarily sufficient.
[0005] The present disclosure has been made in view of
the above. It is an object of the present disclosure to
10 provide a power converter that further improves the power
conversion efficiency.
Means to Solve the Problem
[0006] To solve the above-described problem and achieve
15 the object, a power converter according to the present
disclosure is a power converter to convert a first DC
voltage applied from a DC power supply into a second DC
voltage for a load. The power converter comprises: an
inverter circuit; a variable reactor; a transformer; and a
20 converter circuit. The inverter circuit includes a
plurality of switching elements and snubber capacitors each
connected in parallel to a corresponding one of the
switching elements, and converts the first DC voltage into
a first AC voltage. The variable reactor is disposed on an
25 output side of the inverter circuit and configured to be
variable in inductance value. The transformer includes a
primary winding and a secondary winding that are
magnetically coupled to each other to insulate a primary
side and a secondary side from each other, and converts the
30 first AC voltage applied via the variable reactor, into a
second AC voltage. The converter circuit converts the
second AC voltage into the second DC voltage.

4
Effects of the Invention
[0007] The power converter according to the present
disclosure has the effect of further improving the power
conversion efficiency.
5
Brief Description of Drawings
[0008] FIG. 1 is a circuit diagram illustrating an
example configuration of a power converter according to an
embodiment.
10 FIG. 2 is a timing chart for explaining the basic
operation of an inverter circuit illustrated in FIG. 1.
FIG. 3 is a state transition diagram for explaining
the basic operation of the inverter circuit illustrated in
FIG. 1.
15 FIG. 4 is a diagram for explaining a condition for
achieving soft switching.
FIG. 5 is a diagram illustrating a detailed
configuration example of a variable reactor according to
the embodiment illustrated in FIG. 1.
20 FIG. 6 is a diagram illustrating an example
configuration of a control circuit for controlling the
variable reactor illustrated in FIG. 5, together with the
variable reactor.
FIG. 7 is a timing chart for explaining the operation
25 of the control circuit illustrated in FIG. 6.
FIG. 8 is a diagram for explaining the effect of the
power converter according to the embodiment.
Description of Embodiments
30 [0009] A power converter and a power supply apparatus
according to an embodiment of the present disclosure will
be hereinafter described in detail with reference to the
accompanying drawings. In the following description,

5
physical connection and electrical connection are simply
referred to as “connection” without being distinguished
from each other. That is, the term “connection” includes
both a case where components are directly connected to each
5 other, and a case where components are indirectly connected
to each other via another component.
[0010] Embodiment.
FIG. 1 is a circuit diagram illustrating an example
configuration of a power converter 50 according to an
10 embodiment. The power converter 50 according to the
embodiment is a DC-DC converter that converts a first DC
voltage into a second DC voltage for a load 19. The first
DC voltage is a power supply voltage applied from a DC
power supply 1. As illustrated in FIG. 1, the power
15 converter 50 includes an inverter circuit 60, a variable
reactor 20, a transformer 12, and a converter circuit 70.
[0011] The inverter circuit 60 includes an input
capacitor circuit 62 and an inverter main circuit 64. The
input capacitor circuit 62 holds the first DC voltage
20 applied from the DC power supply 1. The inverter circuit
60 converts the first DC voltage into a first AC voltage.
[0012] The input capacitor circuit 62 includes filter
capacitors 2A and 2B. The filter capacitor 2A and the
filter capacitor 2B are connected in series to each other.
25 One end of the filter capacitor 2A is connected to a highpotential line 3A, and the opposite end of the filter
capacitor 2A is connected to an intermediate-potential line
3B. One end of the filter capacitor 2B is connected to the
intermediate-potential line 3B, and the opposite end of the
30 filter capacitor 2B is connected to a low-potential line 3C.
A connection point at which the opposite end of the filter
capacitor 2A and the one end of the filter capacitor 2B are
connected to each other is referred to as a “midpoint” or a

6
“neutral point”. The midpoint between the filter
capacitors 2A and 2B has typically zero potential in the
inverter main circuit 64.
[0013] The inverter main circuit 64 includes switching
5 elements 7, 8, 9, and 10 (hereinafter, denoted as “7 to 10”
as appropriate) including anti-parallel-connected diodes.
An example of the switching elements 7 to 10 is metal-oxide
semiconductor field-effect transistors (MOSFETs) have the
anti-parallel-connected diodes incorporated therein, as
10 illustrated in the figure. Anti-parallel means that the
anodes of the diodes are connected to the sources of the
MOSFETs, and the cathodes of the diodes are connected to
the drains of the MOSFETs.
[0014] Another example of the switching elements 7 to 10
15 is insulated-gate bipolar transistors (IGBTs). Not only
silicon (Si) but also wide bandgap semiconductors such as
silicon carbide (SiC), gallium nitride (GaN), gallium oxide
(Ga2O3), and diamond can be used as material of the
switching elements. When the switching elements are formed
20 of a wide bandgap semiconductor material, lower losses and
higher-speed switching can be achieved.
[0015] The inverter main circuit 64 includes snubber
capacitors 7a, 8a, 9a, and 10a, clamp diodes 5A and 5B, and
a flying capacitor 6. Each of the snubber capacitors 7a,
25 8a, 9a, and 10a is connected in parallel to the
corresponding one of the switching elements 7 to 10. The
snubber capacitors 7a, 8a, 9a, and 10a (hereinafter,
denoted as “7a to 10a” as appropriate) are provided to
suppress surges when the switching elements 7 to 10 cut off
30 current. The flying capacitor 6 is provided to promote the
discharge of charges accumulated in the snubber capacitors
7a to 10a.
[0016] The switching elements 7 and 8, which are

7
connected in series to each other, define a positive arm.
The switching elements 9 and 10, which are connected in
series to each other in this order, define a negative arm.
The switching elements 7 to 10, which are connected in
5 series to each other in this order, define a half-bridge
circuit.
[0017] One end of the switching element 7 is connected
to the high-potential line 3A. The cathode of the clamp
diode 5A is connected to a connection point between the
10 opposite end of the switching element 7 and one end of the
switching element 8. The anode of the clamp diode 5A is
connected to the intermediate-potential line 3B. An AC
wire 4A is drawn from the opposite end of the switching
element 8. An AC wire 4B is drawn from the intermediate15 potential line 3B.
[0018] One end of the switching element 9 is connected
to the AC wire 4A. The anode of the clamp diode 5B is
connected to a connection point between the opposite end of
the switching element 9 and one end of the switching
20 element 10. The cathode of the clamp diode 5B is connected
to the intermediate-potential line 3B. The opposite end of
the switching element 10 is connected to the low-potential
line 3C.
[0019] The variable reactor 20 and the transformer 12
25 are disposed on the output side of the inverter circuit 60.
The variable reactor 20 is a variable inductance device
configured to be variable in inductance value. The
variable reactor 20 includes a first terminal 20A and a
second terminal 20B.
30 [0020] The transformer 12 includes a primary winding W1
and a secondary winding W2 that are magnetically coupled to
each other. When viewed from the transformer 12, the side
of the primary winding W1 is referred to as the “primary

8
side”, and the side of the secondary winding W2 is referred
to as the “secondary side”. The transformer 12 is provided
to electrically insulate the primary side and the secondary
side from each other.
5 [0021] The variable reactor 20 has the first terminal
20A connected to the AC wire 4A, and the second terminal
20B connected to one end of the primary winding W1 of the
transformer 12. The opposite end of the primary winding W1
is connected to the AC wire 4B. Both ends of the secondary
10 winding W2 are connected to the converter circuit 70. In
FIG. 1, the variable reactor 20 is disposed on the AC wire
4A, but is not limited to this configuration. The variable
reactor 20 can be disposed on the AC wire 4B.
[0022] On the primary side of the transformer 12, a
15 current sensor CT1 for detecting a transformer primary
current ip flowing through the primary winding W1 is
provided on the AC wire 4B. The current sensor CT1 can be
provided on the AC wire 4A. In this description, the
transformer primary current is sometimes simply referred to
20 as a “primary current”.
[0023] With this configuration, the first AC voltage is
applied to the primary winding W1 of the transformer 12 via
the variable reactor 20, and a second AC voltage is output
from the secondary winding W2 of the transformer 12. That
25 is, the transformer 12 converts, into the second AC voltage,
the first AC voltage applied via the variable reactor 20
and outputs the second AC voltage to the converter circuit
70.
[0024] The converter circuit 70 includes a rectifier
30 circuit 72 and an output filter circuit 74. The rectifier
circuit 72 includes four diodes 13, 14, 15, and 16
connected in a full bridge. The rectifier circuit 72
rectifies the second AC voltage applied from the converter

9
circuit 70, generates a DC voltage including ripple, and
applies the thus generated DC voltage to the output filter
circuit 74.
[0025] The output filter circuit 74 includes a smoothing
5 reactor 17 and an output filter capacitor 18. The
smoothing reactor 17 and the output filter capacitor 18
define an LC filter circuit. The output filter circuit 74
smooths the DC voltage including the ripple and applies the
smoothed DC voltage to the load 19. Provided on the output
10 side of the output filter circuit 74 is a current sensor
CT2 for detecting a load current io flowing through the
load 19. The load 19 is a DC load that operates on
receiving DC power supply. The DC load referred to herein
also includes a load 19 incorporates therein both an
15 inverter for converting DC power into AC power, and a
device for operating on receiving supply of the AC power
from the inverter.
[0026] Next, the basic operation of the inverter circuit
60 illustrated in FIG. 1 will be described with reference
20 to FIGS. 2 and 3 in addition to FIG. 1. FIG. 2 is a timing
chart for explaining the basic operation of the inverter
circuit 60 illustrated in FIG. 1. FIG. 3 is a state
transition diagram for explaining the basic operation of
the inverter circuit 60 illustrated in FIG. 1. The
25 following description of the basic operation is based on
the assumption that the inductance Lk of the variable
reactor 20 is a fixed value.
[0027] The following operation of the inverter circuit
60 achieves zero-voltage soft switching (zero-voltage
30 switching (ZVS)) or zero-voltage zero-current soft
switching (zero-voltage zero-current switching (ZVZCS)).
ZVS and ZVZCS are collectively referred to as “soft
switching”.

10
[0028] FIG. 2 illustrates, from the top row, waveforms
of gate signals Q1, Q2, Q3, and Q4 (hereinafter, denoted as
“Q1 to Q4” as appropriate), a transformer primary voltage
vab, the transformer primary current ip, and operation modes
5 in this order. The gate signals Q1 to Q4 are drive signals
applied to the gates of the switching elements 7 to 10,
respectively. The phase difference between the gate signal
Q1 and the gate signal Q2 is a physical quantity called a
“phase shift angle”, and is represented by “α” in FIG. 2.
10 The phase shift angle α determines the phase at which the
transformer primary current ip rises, and the magnitude of
the transformer primary current ip. The phase shift angle
α also determines the phases of the gate signals Q2 to Q4
that are based on the gate signal Q1.
15 [0029] When the gate signals Q1 to Q4 are on, the
corresponding switching elements 7 to 10 are in on
operation. When the gate signals Q1 to Q4 are off, the
corresponding switching elements 7 to 10 are in off
operation.
20 [0030] The horizontal axis in FIG. 2 represents time.
In a period during which the transformer primary current ip
is positive, the inverter circuit 60 operates in mode 1 in
a period from time t0 to time t1, operates in mode 2 in a
period from time t1 to time t2, operates in mode 3 in a
25 period from time t2 to time t3, operates in mode 4 in a
period from time t3 to time t4, and operates in mode 5 in a
period from time t4 to time t5. In a period during which
the transformer primary current ip is negative, the
inverter circuit 60 operates in mode 6 in a period from
30 time t5 to time t6, operates in mode 7 in a period from
time t6 to time t7, operates in mode 8 in a period from
time t7 to time t8, operates in mode 9 in a period from
time t8 to time t9, and operates in mode 10 in a period

11
from time t9 to time t10. Thus, the inverter circuit 60
operates with modes 1 to 10 as one cycle period.
[0031] FIG. 3 illustrates the flow of current in modes 1
to 6. Portions shown in boldface are portions related to
5 operation in each mode. The directions of current flowing
during operation in each mode are indicated by arrows.
[0032] In mode 1, the switching elements 7 and 8 are
turned on. As a result, the transformer primary voltage
vab applied to the transformer 12 becomes “+Vd/2”. Vd
10 represents the voltage value of the power supply voltage
output from the DC power supply 1. The power supply
voltage Vd is divided into two by the filter capacitors 2A
and 2B, and thus vab=+Vd/2.
[0033] In mode 2, the switching element 7 is ZVS turned
15 off. As a result, the snubber capacitor 10a is discharged
through the switching element 8 and the flying capacitor 6,
and the anti-parallel-connected diode of the switching
element 10 comes into conduction.
[0034] In mode 3, after the anti-parallel-connected
20 diode of the switching element 10 comes into conduction,
the switching element 10 is ZVZCS turned on.
[0035] In mode 4, the switching element 8 is ZVS turned
off, and the snubber capacitor 9a is discharged.
[0036] In mode 5, the anti-parallel-connected diode of
25 the switching element 9 comes into conduction.
[0037] In mode 6, the switching element 9 is ZVZCS
turned on. Since the switching element 10 has been turned
on in mode 3, a voltage obtained by reversing the voltage
of the filter capacitor 2B is applied to the transformer 12.
30 As a result, vab=-Vd/2, and a transition to a “-Vd/2”
output is completed.
[0038] Operation in modes 7 to 10, which is the next
half cycle, is the above-described operation in which the

12
switching elements 7 and 8 are replaced with the switching
elements 10 and 9, respectively, and the snubber capacitors
10a and 9a are replaced with the snubber capacitors 7a and
8a, respectively. Thus, in the operation, the transformer
5 primary voltage vab makes a transition from “-Vd/2” to
“+Vd/2”, and the operation in one cycle is completed.
[0039] Consider the above operation. First, to achieve
soft switching, it is necessary to satisfy a condition in
formula (1) below.
[0040] (1/2)Lk·ip1
2>Cs1·(Vd/2)2 10 ...(1)
[0041] In the above formula, Cs1 is the capacitances of
the snubber capacitors 7a to 10a. Assume that the
capacitances are equal among the snubber capacitors 7a to
10a.
15 [0042] By modifying formula (1) above, formula (2) below
is obtained.
[0043] Lk>{2·Cs1·(Vd/2)2}/ip1
2...(2)
[0044] In formula (2) above, Vd is regarded as almost
constant, and Cs1 can be considered constant. Then, since
20 the transformer primary current ip is proportional to the
load current io, the condition can be expressed as in
formula (3) below, using a proportionality constant K.
[0045] Lk>K/io
2...(3)
[0046] Formula (3) above is represented on a graph as
25 shown in FIG. 4. FIG. 4 is a diagram for explaining the
condition for achieving soft switching.
[0047] The illustrated curve is provided where the left
side and the right side in formula (3) above become equal
to each other. With this curve as a boundary line, the
30 lower side of the curve is a hard switching region, and the
upper side of the curve is a soft switching region. As is
well known, in the hard switching region, switching losses
occur when the switching elements 7 to 10 are turned on or

13
off. On the other hand, in the soft switching region, no
switching losses occur. Thus, in the curve of FIG. 4,
switching losses occur if the load current io is equal to
or lower than ioth, where ioth is the current value of the
5 load current io corresponding to the inductance Lk.
[0048] To expand the soft switching region to a region
where the load current io is as low as possible, it is
necessary to make the inductance Lk the highest possible
value. On the other hand, increasing the inductance Lk
10 increases the component of a voltage drop due to the
inductance Lk when the load current io is high. This
causes a problem of failure to obtain a sufficient inverter
output voltage. That means a trade-off relationship
between the prevention of switching losses and the inverter
15 output voltage. This trade-off relationship conventionally
prevents the inductance Lk from being increased so much,
which has been taken as a problem of failure to enlarge the
soft switching region.
[0049] Next, a configuration of a main part for solving
20 the above problem will be described. FIG. 5 is a diagram
illustrating a detailed configuration example of the
variable reactor 20 according to the embodiment illustrated
in FIG. 1. As illustrated in FIG. 5, the variable reactor
20 includes a reactor 105 and two bidirectional switches
25 SW1 and SW2. The variable reactor 20 includes gate
terminals 22A and 22B in addition to the first terminal 20A
and the second terminal 20B. The reactor 105 has its one
end side connected to the first terminal 20A, and the
opposite end side connected to the second terminal 20B.
30 The reactor 105 includes tap terminals 106 and 108.
[0050] The bidirectional switch SW1 includes switching
elements 101 and 102 including anti-parallel-connected
diodes. The bidirectional switch SW2 includes switching

14
elements 103 and 104 including anti-parallel-connected
diodes. The switching elements 101 and 102 are anti-series
connected so that the anodes of the anti-parallel-connected
diodes are in a face-to-face relation to each other. The
5 switching elements 103 and 104 are likewise configured.
Instead of this configuration, the switching elements can
be anti-series connected so that the cathodes of the antiparallel-connected diodes are in a face-to-face relation to
each other.
10 [0051] One end of the bidirectional switch SW1 is
connected to the tap terminal 106, and the opposite end of
the bidirectional switch SW1 is connected to the second
terminal 20B. The common gate of the bidirectional switch
SW1 is connected to the gate terminal 22A. When a gate
15 signal is input to the gate terminal 22A, the bidirectional
switch SW1 comes into conduction. When the bidirectional
switch SW1 comes into conduction, the inductance value
between the first terminal 20A and the second terminal 20B
is changed from Lk3 to Lk1.
20 [0052] One end of the bidirectional switch SW2 is
connected to the tap terminal 108, and the opposite end of
the bidirectional switch SW2 is connected to the second
terminal 20B. The common gate of the bidirectional switch
SW2 is connected to the gate terminal 22B. When a gate
25 signal is input to the gate terminal 22B, the bidirectional
switch SW2 comes into conduction. When only the
bidirectional switch SW2 comes into conduction without the
bidirectional switch SW1 not being in conduction, the
inductance value between the first terminal 20A and the
30 second terminal 20B is changed from Lk3 to Lk2. The
relationships among Lk1, Lk2, and Lk3 are Lk1io1 between the set value io1
and the set value io2. A detected value of the transformer
primary current ip is input from the current sensor CT1 to

16
the positive terminal of the comparator 210. A zero value,
that is, “0” is input to the negative terminal of the
comparator 210.
[0057] The output of the comparator 208 is input to the
5 data input terminal D of the latch circuit 204. The output
of the comparator 209 is input to the data input terminal D
of the latch circuit 205. The output of the comparator 210
is input to the clock terminal 206 of the latch circuit 204
and the clock terminal 207 of the latch circuit 205.
10 [0058] Next, the operation of the control circuit 30
will be described with reference to FIGS. 7 and 8 in
addition to FIG. 6. FIG. 7 is a timing chart for
explaining the operation of the control circuit 30
illustrated in FIG. 6. FIG. 8 is a diagram for explaining
15 the effect of the power converter 50 according to the
embodiment.
[0059] The comparator 208 compares the load current io
with the set value io2. When the load current io is higher
than the set value io2, a signal indicating logic “H” is
20 output from the comparator 208 and held in the latch
circuit 204. When the load current io is equal to or lower
than the set value io2, a signal indicating logic “L” is
output from the comparator 208 and held in the latch
circuit 204. Likewise, the comparator 209 compares the
25 load current io with the set value io1. When the load
current io is higher than the set value io1, a signal
indicating logic “H” is output from the comparator 209 and
held in the latch circuit 205. When the load current io is
equal to or lower than the set value io1, a signal
30 indicating logic “L” is output from the comparator 209 and
held in the latch circuit 205.
[0060] The comparator 210 compares the transformer
primary current ip with the zero value. When the

17
transformer primary current ip is higher than the zero
value, a signal indicating logic “H” is output from the
comparator 210 and input to each of the clock terminals 206
and 207 of the latch circuits 204 and 205. When the
5 transformer primary current ip is equal to or lower than
the zero value, a signal indicating logic “L” is output
from the comparator 210 and input to each of the clock
terminals 206 and 207 of the latch circuits 204 and 205.
[0061] The latch circuits 204 and 205 hold comparison
10 signals output from the comparators 208 and 209,
respectively. Then, the latch circuits 204 and 205 output
the held signals from their corresponding data output
terminals Q at the timing when the output of the comparator
210 changes from logic “L” to logic “H”.
15 [0062] In this description, the outputs of the
comparators 208 and 209 are sometimes referred to as “first
comparison results”, and the output of the comparator 210
as a “second comparison result”.
[0063] In FIG. 7, waveforms of the gate signals Q1 to Q4,
20 the transformer primary voltage vab, and the transformer
primary current ip are the same as those illustrated in FIG.
2. FIG. 7 illustrates an output waveform of the comparator
210 and how the value of the inductance Lk of the reactor
105 is changed. Specifically, FIG. 7 illustrates how the
25 inductance Lk is changed from Lk2 to Lk1. As illustrated in
FIG. 7, the value of the inductance Lk switches from Lk2 to
Lk1 at the timing when the output of the comparator 210
changes from logic “L” to logic “H”, that is, at a zerocrossing point where the transformer primary current ip
30 switches from negative to positive. Note that as
illustrated in the middle of the timing chart, the value of
the inductance Lk does not switch at the timing when the
output of the comparator 210 changes from logic “H” to

18
logic “L”.
[0064] As described with reference to FIG. 4, the soft
switching region depends on the load current io. When the
load current io is higher than the set value io2, both the
5 outputs of the comparators 208 and 209 are logic “H”, so
that the bidirectional switches SW1 and SW2 come into
conduction, and the value of the inductance Lk becomes Lk1.
This results in an operation in a region where the load
current io is higher than the set value io2, as illustrated
10 in FIG. 8, although the value of Lk1 is relatively low.
Accordingly, switching drive in the soft switching region
is possible.
[0065] When the load current io is equal to or lower
than the set value io2, and the load current io is higher
15 than the set value io1, only the output of the comparator
209 becomes logic “H”, so that only the bidirectional
switch SW2 comes into conduction, and the value of the
inductance Lk becomes Lk2. This operation can enlarge the
soft switching region as also illustrated in FIG. 8.
20 [0066] FIG. 8 illustrates ioth. ioth is the value of the
load current io corresponding to Lk3. Lk3 is the inductance
value of the reactor 105, that is, the value of the
inductance Lk when both the bidirectional switches SW1 and
SW2 are not in conduction. Thus, when the load current io
25 is equal to or higher than ioth, switching drive in the soft
switching region is possible. When the load current io is
less than ioth, drive is performed in the hard switching
region. As the value of the load current io less than ioth
is low, the transformer primary current ip becomes low, and
30 the current flowing through the switching elements 7 to 10
becomes low as well. Consequently, the conduction losses
of the switching elements 7 to 10 are small, and thus
increases in the switching losses of the switching elements

19
7 to 10 do not become a problem.
[0067] The control circuit 30 in FIG. 6 is configured
such that the inductance value is changed at a zerocrossing point where the transformer primary current ip
5 switches from negative to positive, but the configuration
of the control circuit is not limited to this example. The
inductance value can be changed at a zero-crossing point
where the transformer primary current ip switches from
positive to negative.
10 [0068] Operation for starting the inverter circuit 60 or
operation for driving the inverter circuit 60 from no load
is opposite to that in the above description. The then
operation will be described with reference to FIG. 8.
[0069] When the load current io increases from zero, the
15 operation starts from hard switching. At the point in time
when the load current io exceeds ioth, the operation
switches to soft switching. When the load current io
exceeds io1, the value of the inductance Lk switches to Lk2.
In a region where the load current io is from ioth to io1,
20 soft switching drive is possible. Further, when the load
current io exceeds io2, the value of the inductance Lk
switches to Lk1. Even in a region where the load current io
is from io1 to io2, soft switching drive is possible. This
achieves the effect of obtaining a required inverter output
25 voltage, maintaining soft switching drive.
[0070] When the control circuit 30 includes a
microcomputer, the value of the proportionality constant K
in formula (3) above is recalculated to thereby change the
values of the set values io1 and io2. Even when the
30 characteristics of the load 19 vary, thus, the control
circuit 30 is applicable to the power converter 50 without
being changed in design.
[0071] Although FIG. 6 illustrates the configuration

20
example in which the value of the inductance Lk is changed
in three stages, Lk1, Lk2, and Lk3, the present invention is
not limited to this example. The value of the inductance
Lk may be changed in two stages, or the value of the
5 inductance Lk may be changed in four or more stages. This
can be done by changing, in accordance with the number of
change stages, the number of the tap terminals 106 and 108
provided to the reactor 105, the numbers of the latch
circuits 204 and 205 and the comparators 208 and 209 in the
10 control circuit 30, and the corresponding number of the GD
circuits 110 and 112.
[0072] It is conceivable that a saturable reactor is
used as another method for making the value of the
inductance Lk variable. However, it is difficult to
15 maintain proper characteristics by using a saturable
reactor in view of the use environment of the power
converter 50, aging, etc. In addition, a problem with a
saturable reactor is that an expandable soft switching
region is narrow. Furthermore, a saturable reactor suffers
20 from a problem of leading to increases in dimensions and
weight, etc., and hence it is difficult to make the value
of the inductance Lk variable using the saturable reactor.
By contrast, the method of the embodiment, which can
enlarge the soft switching region by the electrical method,
25 can thus prevent increases in dimensions and weight. In
addition, since the values of the set values io1 and io2 can
be changed by a processor such as a microcomputer, the
problems of the use environment, aging, etc. can be solved.
[0073] When the value of the inductance Lk is changed,
30 the characteristics on the load side as viewed from the
inverter circuit 60 change. In this regard, the method of
the embodiment, in which the value of the inductance Lk is
changed at the timing when the transformer primary current

21
ip becomes zero, thus can reduce the effect of change of
the load-side characteristics on the inverter circuit 60.
Thus, even the configuration that discretely changes the
inductance Lk can obtain an effect equivalent to that of a
5 configuration that continuously changes the inductance Lk.
[0074] The power converter 50 and its technology
according to the embodiment can be applied to various power
supply apparatuses, and are particularly suitable for use
in a power supply apparatus that supplies power to an
10 auxiliary load installed in a railway vehicle. The
auxiliary load is a name referring to a load other than a
main motor among loads installed in the railway vehicle.
Examples of the auxiliary load include an air conditioner,
a vehicle interior lighting device, a door opening and
15 closing device, a safety device, a compressor, a battery,
and a control power supply. The compressor is a device
that generates an air source for a vehicle brake.
[0075] For a power supply apparatus used in a railway
vehicle, the load current io increases because the load of
20 an air conditioner increases when train operation starts or
when a passenger load factor is high in the hot summer
season. On the other hand, as cooling in the vehicle
proceeds, the load current io decreases. The time during
which the load current io is high is short, and the time
25 during which the load current io is low is long. When the
power supply apparatus is applied to, for example, a
railway vehicle, it is required to improve efficiency at
low load at which operating time is long as well as
achieving high power supply capacity. Thus, the power
30 converter according to the present embodiment that can
reduce losses at low load is suitable as a power supply
apparatus for a railway vehicle.
[0076] A train made up of a plurality of railway

22
vehicles includes a train information management system
that manages train information. The train information
management system can determine the operating condition of
an auxiliary load on the basis of the train information,
5 and can control the operation of the auxiliary load. Thus,
the train information management system can determine
whether the operating condition of the auxiliary load is
low load or high load. Using this function, the value of
the inductance Lk in the variable reactor 20 can be changed
10 on the basis of load information that is information about
the operating condition of the auxiliary load. This can
simplify the configuration of the control circuit 30. To
change the value of the inductance Lk with high precision,
attention should be paid to outputting a signal to change
15 the value of the inductance Lk at a transmission timing
synchronized with, for example, the timing of transmitting
a control signal to the air conditioner, etc.
[0077] When the operating conditions of the auxiliary
load are classified into two categories, low load and high
20 load, the variable reactor 20 is only required to include
any one of the bidirectional switches SW1 and SW2.
[0078] When the operating conditions of the auxiliary
load are classified into three categories, low load, medium
load, and high load, and the value of the inductance Lk is
25 changed according to the three categories, the variable
reactor 20 having the configuration illustrated in FIG. 6
can be used. The control circuit 30 illustrated in FIG. 6
can use the functions of the comparator 210 and the latch
circuits 204 and 205.
30 [0079] For example, the control circuit 30 can be
configured such that the latch circuit 204 outputs a signal
to bring the bidirectional switch SW1 into conduction when
the operating condition of the auxiliary load is low load.

23
The control circuit 30 can be configured such that the
latch circuit 205 outputs a signal to bring only the
bidirectional switch SW2 into conduction when the operating
condition of the auxiliary load is medium load. The
5 control circuit 30 can be configured such that neither of
the bidirectional switches SW1 and SW2 is in conduction
when the operating condition of the auxiliary load is high
load.
[0080] As described above, the power converter according
10 to the embodiment includes the variable reactor disposed on
the output side of the inverter circuit and configured to
be variable in inductance value. By using the variable
reactor, the inductance value between the inverter circuit
and the primary winding of the transformer can be changed,
15 on the basis of the load current. Consequently, the power
conversion efficiency can be further improved.
[0081] The power converter according to the embodiment
includes the control circuit that changes the inductance
value of the variable reactor in two or more stages. On
20 the basis of the load current flowing through the load, the
control circuit performs the control for changing the
inductance value. The variable reactor according to the
embodiment includes the reactor having the first terminal
connected to the inverter circuit and the second terminal
25 connected to the primary winding of the transformer, and
includes the one or more bidirectional switches including
the two anti-series-connected switching elements including
the anti-parallel-connected diodes. The one or more
bidirectional switches each have its one end connected to
30 the first terminal or the second terminal, and the opposite
end connected to the corresponding one of the different tap
terminals of the variable reactor. By using the control
circuit and the variable reactor configured as described

24
above, the inductance value between the inverter circuit
and the primary winding of the transformer can be changed
in two or more stages. This enables the provision of a
required inverter output voltage, maintaining soft
5 switching drive.
[0082] It is preferable to change the inductance value
of the variable reactor according to the embodiment at a
zero-crossing point where the value of the transformer
primary current switches from negative to positive or from
10 positive to negative. This can reduce the effect of change
of the load-side characteristics on the inverter circuit.
Thus, even the configuration that discretely changes the
inductance value can obtain an effect equivalent to that of
a configuration that continuously changes the inductance
15 value.
[0083] The control circuit according to the embodiment
can include the same number of the first comparators as the
number of the bidirectional switches, the same number of
the latch circuits as the number of the first comparators,
20 and the second comparator that outputs the second
comparison result obtained by comparing the primary current
with the zero value. Each of the first comparators outputs
the first comparison result obtained by comparing the load
current with the predetermined set value. Each of the
25 latch circuits receives input of the first comparison
result from the corresponding first comparator, and the
second comparison result from the second comparator. Each
of the latch circuits holds the first comparison result.
Each of the latch circuits outputs a signal based on the
30 first comparison result to the corresponding bidirectional
switch in accordance with the timing at which the second
comparison result is input. Using the control circuit thus
configured enables proper change of the inductance value in

25
the variable reactor.
[0084] The power converter according to the embodiment
can be configured as a power supply apparatus installed in
a railway vehicle or a train. The power converter
5 according to the embodiment can reduce losses at low load.
For this reason, the power converter can be suitably used
in a railway vehicle or a train that requires improved
efficiency at low load at which operating time is long as
well as achieving high power supply capacity.
10 [0085] The power converter according to the embodiment
can be configured as a power supply apparatus installed in
a train including a train information management system
that manages train information. The power converter
includes the control circuit that changes the inductance
15 value of the variable reactor in two or more stages. The
control circuit performs the control for changing the
inductance value, on the basis of load information output
from the train information management system. The power
supply apparatus thus configured provides the effect of
20 simplifying the configuration of the control circuit.
[0086] The configuration described in the above
embodiment illustrates an example, and can be combined with
another known art, and can be partly omitted or changed
without departing from the gist.
25 [0087] For example, the inverter circuit 60 illustrated
in FIG. 1 has the three-level circuit configuration, but is
not limited to this. The inverter circuit 60 can have a
two-level circuit configuration. The inverter circuit 60
illustrated in FIG. 1 has the half-bridge circuit
30 configuration, but is not limited to this. The inverter
circuit 60 can have a single-phase bridge circuit
configuration. The converter circuit 70 illustrated in FIG.
1 uses the rectifier circuit 72 having the four diodes

26
connected in the full bridge, but is not limited to this.
Instead of the rectifier circuit 72, an AC-DC conversion
circuit including at least one switching element can be
used.
5
Reference Signs List
[0088] 1 DC power supply; 2A, 2B filter capacitor; 3A
high-potential line; 3B intermediate-potential line; 3C
low-potential line; 4A, 4B AC wire; 5A, 5B clamp diode; 6
10 flying capacitor; 7, 8, 9, 10, 101, 102, 103, 104
switching element; 7a, 8a, 9a, 10a snubber capacitor; 12
transformer; 13, 14, 15, 16 diode; 17 smoothing reactor;
18 output filter capacitor; 19 load; 20 variable
reactor; 20A first terminal; 20B second terminal; 22A,
15 22B gate terminal; 30 control circuit; 50 power
converter; 60 inverter circuit; 62 input capacitor
circuit; 64 inverter main circuit; 70 converter circuit;
72 rectifier circuit; 74 output filter circuit; 105
reactor; 106, 108 tap terminal; 110, 112 GD circuit; 204,
20 205 latch circuit; 206, 207 clock terminal; 208, 209, 210
comparator; CT1, CT2 current sensor; SW1, SW2 bidirectional
switch; W1 primary winding; W2 secondary winding.

We Claim:
[Claim 1] A power converter to convert a first DC voltage
5 applied from a DC power supply into a second DC voltage for
a load, the power converter comprising:
an inverter circuit including a plurality of switching
elements and snubber capacitors each connected in parallel
to a corresponding one of the switching elements, to
10 convert the first DC voltage into a first AC voltage;
a variable reactor disposed on an output side of the
inverter circuit and configured to be variable in
inductance value;
a transformer including a primary winding and a
15 secondary winding that are magnetically coupled to each
other to insulate a primary side and a secondary side from
each other, to convert the first AC voltage applied via the
variable reactor, into a second AC voltage; and
a converter circuit to convert the second AC voltage
20 into the second DC voltage.
[Claim 2] The power converter according to claim 1,
comprising
a control circuit to change the inductance value of
25 the variable reactor in two or more stages, wherein
the control circuit changes the inductance value, on a
basis of a load current flowing through the load.
[Claim 3] The power converter according to claim 2, wherein
30 the inductance value is changed at a zero-crossing
point where a primary current flowing through the primary
winding switches from negative to positive or from positive
to negative.

28
[Claim 4] The power converter according to claim 3, wherein
the variable reactor includes
a reactor including first and second terminals and one
5 or more tap terminals, the first terminal being connected
to the inverter circuit, the second terminal being
connected to the primary winding of the transformer, and
one or more bidirectional switches each including two
switching elements including anti-parallel-connected diodes,
10 the two switching elements being anti-series connected to
each other, and
the one or more bidirectional switches each have one
end connected to the first terminal or the second terminal,
and an opposite end connected to a corresponding one of the
15 different tap terminals.
[Claim 5] The power converter according to claim 4, wherein
the control circuit includes
the same number of first comparators as the number of
20 the bidirectional switches, to output first comparison
results obtained by comparing the load current with
predetermined set values,
a second comparator to output a second comparison
result obtained by comparing the primary current with a
25 zero value, and
the same number of latch circuits as the number of the
first comparators, to receive input of the first comparison
results from the corresponding first comparators, and the
second comparison result from the second comparator, and
30 the latch circuits each hold a corresponding one of
the first comparison results and output a signal based on
the held first comparison result to a corresponding one of
the bidirectional switches in accordance with a timing at

29
which the second comparison result is input.
[Claim 6] A power supply apparatus to be installed in a
railway vehicle, the power supply apparatus comprising the
5 power converter according to any one of claims 1 to 5,
wherein
the power supply apparatus supplies power to a load
installed in the railway vehicle, using the power converter.
10 [Claim 7] A power supply apparatus to be installed in a
train including a train information management system to
manage train information, the power supply apparatus
comprising
the power converter according to claim 1, wherein the
15 power supply apparatus supplies power to a load installed
in the train, using the power converter,
the power converter includes a control circuit to
change the inductance value of the variable reactor in two
or more stages, and
20 the control circuit changes the inductance value on a
basis of load information output from the train information
management system.