Field of the Invention
The present invention relates to a DC/DC converter
having DC input terminals.
Background of the Invention
Conventionally, a DC/DC converter having a DC input
terminal has been used to boost or drop a DC voltage and
output it (see, e.g., Japanese Patent Application
Publication No. 2010-022077). In such a type of the DC/DC
converter having the DC input terminal, when reverse
polarity is connected to the DC input terminal, an internal
circuit of the converter is broken. Various configurations
as shown in Figs. 28A to 28C have been known as
countermeasures against the reverse connection to the DC
input terminal.
Fig. 28A illustrates the configuration in which a
diode is connected in series to one input terminal, thereby
preventing a voltage from being applied to the internal
circuit of the DC/DC converter in reverse connection. In
this configuration, when polarity is normally connected to
the DC input terminal, voltage loss is always generated by
a voltage drop of the connected diode. Since the voltage
is not applied to the internal circuit of the DC/DC

converter in the reverse connection, the DC/DC converter
does not operate.
Fig. 28B illustrates the configuration in which a
diode is connected from one input terminal to the other
input terminal and the internal circuit is short-circuited
by the diode in reverse connection, thereby preventing a
voltage from being applied to the internal circuit of the
DC/DC converter in the reverse connection. In this
configuration, a protection circuit is additionally
required to protect a circuit connected to the DC input
terminal from the short-circuit current. Alternatively,
this configuration is limitedly applied to a power supply,
such as a solar photovoltaic power generating panel, to
which current limiting acts. Also, since the voltage is
not applied to the internal circuit of the DC/DC converter
in the reverse connection, the DC/DC converter does not
operate.
Fig. 28C illustrates the configuration in which a
bridge circuit is provided in an input unit, so that a
normal voltage is applied to the internal circuit of the
DC/DC converter even in any one of forward and reverse
connections, such as when an AC power supply is connected
to the DC/DC converter. In this configuration, the DC/DC
converter operates even in any one of the forward and
reverse connections, but voltage loss corresponding to the
voltage of two diodes included in the bridge circuit always
occurs.

Summary of the Invention
In view of the above, the present invention provides a
DC /DC converter which can normally operate without voltage
loss even in reverse connection.
In accordance with an embodiment of the present
invention, there is provided a DC/DC converter including:
DC input terminals to which a DC power is inputted; a
transformer; a bidirectional switching unit provided on a
primary side of the transformer.
The bidirectional switching unit may include a pair of
bidirectional switching devices forming a half-bridge
circuit.
The bidirectional switching unit may include two pairs
of bidirectional switching devices forming a full-bridge
circuit.
The bidirectional switching unit may include a pair of
bidirectional switching devices forming a push-pull
circuit.
The bidirectional switching unit may include a pair of
bidirectional switching devices forming a complex resonance
circuit.
One of the bidirectional switching devices may include
two switching elements, and one of the two switching
elements may be turned on at timing different from timing
at which the other of the two switching elements is turned
on.
One of the pair of bidirectional switching devices may

include two switching elements, and one of the two
switching elements may be turned on as soon as a switching
element included in the other of the pair of bidirectional
switching devices is turned off.
The DC/DC converter may further includes a pair of
switching elements on a secondary side of the transformer.
One switching element in the secondary side of the
transformer may be configured to be always on and the other
switching element in the secondary side of the transformer
may be configured to be always off to perform a forward
operation.
The bidirectional switching unit may be configured to
have on-time longer than off-time to perform a flyback
operation.
One of the bidirectional switching devices may include
two switching elements, and one of the two switching
elements may be always on.
The DC/DC converter may further include a polarity
determination circuit connected to the DC input terminals,
the DC/DC converter may be configured to control on-timing
of the two switching elements included in one of the
bidirectional switching devices in accordance with the
polarities of the DC input terminals determined by the
polarity determination circuit.
The DC/DC converter may further include a polarity
determination circuit connected to the DC input terminals.
The DC/DC converter may be configured to control on and off
operations of the switching element in accordance with the

polarities of the DC input terminals determined by the
polarity determination circuit.
The bidirectional switching device may include a
bidirectional switching device having a lateral transistor
structure using a GaN/AlGaN structure.
In accordance with a DC/DC converter of the present
invention, even in any one of forward and reverse
connections of a DC power supply, a proper current can be
allowed to flow into a primary side of a transformer and
the DC/DC converter can normally operate without voltage
loss.
Brief Description of the Drawings
Fig. 1 is a circuit diagram of a half-bridge type
DC/DC converter in accordance with an embodiment of the
present invention;
Figs. 2A and 2B are time charts illustrating an
operational example of the half-bridge type DC/DC
converter;
Figs. 3A and 3B are time charts illustrating another
operational example of the half-bridge type DC/DC
converter;
Figs. 4A and 4B are time charts illustrating still
another operational example of the half-bridge type DC/DC
converter;
Fig. 5 is a circuit diagram of a full-bridge type

DC/DC converter in accordance with a modification of the
embodiment;
Figs. 6A and 6B are time charts illustrating an
operational example of the full-bridge type DC/DC
converter;
Figs. 7A and 7B are time charts illustrating another
operational example of the full-bridge type DC/DC
converter;
Figs. 8A and 8B are time charts illustrating still
another operational example of the full-bridge type DC/DC
converter;
Fig. 9 is a circuit diagram of a push-pull type DC/DC
converter in accordance with another modification of the
embodiment;
Figs. 10A and 10B are time charts illustrating an
operational example of the push-pull type DC/DC converter;
Figs. 11A and 11B are time charts illustrating another
operational example of the push-pull type DC/DC converter;
Figs. 12A and 12B are time charts illustrating still
another operational example of the push-pull type DC/DC
converter;
Fig. 13 is a circuit diagram of a complex resonance
type DC/DC converter in accordance with still another
modification of the embodiment;
Figs. 14A and 14B are time charts illustrating an
operational example of the complex resonance type DC/DC
converter;
Figs. 15A and 15B are time charts illustrating another

operational example of the complex resonance type DC/DC
converter;
Fig. 16 is a circuit diagram of a single transistor
forward type DC/DC converter in accordance with still
another modification of the embodiment;
Figs. 17A and 17B are time charts illustrating an
operational example of the single transistor forward type
DC/DC converter;
Figs. 18A and 18B are time charts illustrating another
operational example of the single transistor forward type
DC/DC converter;
Fig. 19 is a circuit diagram of a single transistor
flyback type DC/DC converter in accordance with still
another modification of the embodiment;
Figs. 20A and 20B are time charts illustrating an
operational example of the single transistor flyback type
DC/DC converter;
Figs. 21A and 21B are time charts illustrating another
operational example of the single transistor flyback type
DC/DC converter;
Fig. 22 is a circuit diagram of a DC/DC converter
having a polarity determination circuit and the like in
accordance with still another modification of the
embodiment;
Fig. 23 is a plane view illustrating the configuration
of a bidirectional switching device (single gate);
Fig. 24 is an enlarged view of area A in Fig. 23;
Fig. 25 is a sectional view taken along line XXV-XXV

in Fig. 23;
Fig. 26 is a plane view illustrating the configuration
of a bidirectional switching device {dual gate);
Fig. 27 is a sectional view taken along line XXVII-
XXVII in Fig. 26; and
Figs. 28A to 28C show circuit diagrams illustrating
countermeasures against reverse connection of a DC power
supply in a conventional DC/DC converter.
Detailed Description of the Embodiments
Hereinafter, embodiments of the present invention will
be described in detail with reference to the accompanying
drawings which form a part hereof. Throughout the
drawings, the same reference numerals are used to designate
the same or similar elements and a redundant description
thereof will be omitted.
(First Embodiment)
A DC/DC converter in accordance with an embodiment of
the present invention will be described with reference to
the accompanying drawings. For example, the DC/DC
converter is built in a notebook PC. The DC/DC converter
converts one DC voltage into another DC voltage and then
supplies the converted DC voltage to each unit of the PC.
Fig. 1 illustrates the circuit configuration of a half-
bridge type DC/DC converter. The DC/DC converter 1 has a
pair of DC input terminals 2a and 2b to which the voltage

of a DC power supply 11 is inputted, a transformer 3 having
a primary side connected to the DC input terminals 2a and
2b, a pair of bidirectional switching devices 4 and 5 on
the primary side of the transformer 3, and a pair of
capacitors C1 and C2. A half-bridge circuit is formed on
the primary side of the transformer 3 with the transformer
3 and the bidirectional switching devices 4 and 5. The
bidirectional element 4 has switching elements Q1 and Q2.
The bidirectional element 5 has switching elements Q3 and
Q4. A load 12 is connected to a secondary side of the
transformer 3, and the DC voltage, which is converted by
the transformer 3 and then smoothed, is applied to the load
12.
The DC power supply 11 is connected to the DC input
terminals 2a and 2b- Hereinafter, a case where the DC
power supply 11 is connected to the DC input terminals 2a
and 2b with the polarity shown in A of Fig. 1 refers to
forward connection, and a case where the DC power supply 11
is connected to the DC input terminals 2a and 2b with the
polarity shown in B of Fig. 1 refers to reverse connection.
In the DC/DC converter 1, a proper current can be allowed
to flow into the primary side of the transformer 3 through
the bidirectional switching devices 4 and 5 even when any
one of the forward and reverse connections of the DC power
supply 11 is made, as will be described below.
Fig. 2 illustrates examples of the opening-closing
timing (gate voltage of each switching element) of the
switching elements Q1, Q2, Q3 and Q4 included in the

bidirectional switching devices 4 and 5. As a driving
signal outputted from a driving circuit provided in each
element is inputted to the gate, the switching elements Q1,
Q2, Q3 and Q4 are turned on/off. The driving circuit
outputs the driving signal based on a control signal
inputted from a control circuit.
Fig. 2A illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 when the forward
connection of the DC power supply 11 is made as shown in A
of Fig. 1. The switching elements Q1 and Q2 included in
the bidirectional switching device 4 are switched on/off
repeatedly at the same timing. Further, the switching
elements Q3 and Q4 included in the bidirectional switching
device 5 are switched on/off repeatedly at the same timing.
The bidirectional switching device 4 and the bidirectional
switching devices 5 are alternately switched on/off. That
is, the switching elements Q3 and Q4 are on during the
period in which the switching elements Q1 and Q2 are off,
and the switching elements Q1 and Q2 are on during the
period in which the switching elements Q3 and Q4 are off.
When the switching elements Q1 and Q2 are on and the
switching elements Q3 and Q4 are off, electric charges
charged in a capacitor CI flow through the bidirectional
switching device 4 and the primary side of the transformer
3 sequentially as a current. When the switching elements
Q1 and Q2 are off and the switching elements Q3 and Q4 are
on, electric charges charged in a capacitor C2 flow through
the primary side of the transformer 3 and the bidirectional

switching device 5 sequentially as a current. By repeating
these operations, the current alternately flows through the
primary side of the transformer 3 in different directions.
Fig. 2B illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 when the reverse
connection of the DC power supply 11 is made as shown in B
of Fig. 1. Even in this case, the switching elements Q1,
Q2, Q3 and Q4 included in the bidirectional switching
devices 4 and 5 are also switched on/off repeatedly. That
is, when the switching elements Q3 and Q4 are on and the
switching elements Q1 and Q2 are off, the electric charges
charged in the capacitor C2 flow through the bidirectional
switching device 5 and the primary side of the transformer
3 sequentially as a current. When the switching elements
Q3 and Q4 are off and the switching elements Q1 and Q2 are
on, the electric charges charged in the capacitor CI flow
through the primary side of the transformer 3 and the
bidirectional switching device 4 sequentially as a current.
By repeating these operations, the current alternately
flows through the primary side of the transformer 3 in
different directions.
In accordance with the DC/DC converter 1, the
bidirectional switching devices 4 and 5 are driven as shown
in Fig. 2A and 2B, so that a proper current can be allowed
to flow into the primary side of the transformer 3 even
when any one of the forward and reverse connections of the
DC power supply 11 is made. Accordingly, the diodes of the
input unit shown in Figs. 28A to 28C can be removed.

When the switching elements Q1 and Q2 included in the
bidirectional switching device 4 are turned on, a current
flows into FETs constituting the switching elements Q1 and
Q2. Similarly, when the switching elements Q3 and Q4
included in the bidirectional switching device 5 are turned
on, a current flows into FETs constituting the switching
elements Q3 and Q4. However, since the FETs have an on-
resistance smaller than the resistance of the diodes
forming the bridge circuit shown in Fig. 28C, it is
. possible to reduce voltage loss.
Figs. 3A and 3B illustrate another example of the
opening-closing timing of the switching elements Q1, Q2, Q3
and Q4 included in the bidirectional switching devices 4
and 5. In the operational examples, since the switching
elements Q1, Q2, Q3 and Q4 have different opening-closing
timings depending on whether the forward or reverse
connection of the DC power supply 11 is made, a polarity
determination circuit (see Fig. 22) is additionally
provided.
Fig. 3A illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 when the forward
connection of the DC power supply 11 is made. In this
operational example, a body diode of the switching element
Q2 connected to the reverse direction side of a coil is
turned on in order to absorb a surge voltage when the
switching element Q4 is turned off. For this reason, the
on-timing of the switching element Q1 is shifted to be
earlier than the on-timing of the switching element Q2 so

that the on-timing of the switching element Q1 is
synchronized with the off-timing of the switching element
Q4. Similarly, the on-timing of the switching element Q3
is shifted to be earlier than the on-timing of the
switching element Q4 so that the on-timing of the switching
element Q3 is synchronized with the off-timing of the
switching element Q2.
Fig. 3B illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 when the reverse
connection of the DC power supply 11 is made as shown in B
of Fig. 1. In this operational example, the on-timing of
the switching element Q2 is shifted to be earlier than the
on-timing of the switching element Q1 so that the on-timing
of the switching element Q2 is synchronized with the off-
timing of the switching element Q3. Similarly, the on-
timing of the switching element Q4 is shifted to be earlier
than the on-timing of the switching element Q3 so that the
on-timing of the switching element Q4 is synchronized with
the off-timing of the switching element Q1.
In accordance with these operational examples, the
switching element can be protected from the surge voltage
when being turned off, even if any one of the forward and
reverse connections of the DC power supply 11 is made.
Figs. 4A and 4B illustrate still another example of
the opening-closing timing of the switching elements Q1, Q2
and Q3 and Q4 included in the bidirectional switching
devices 4 and 5. In the operational examples, since the
switching elements Q1, Q2, Q3 and Q4 have different

opening-closing timings depending on whether the forward or
reverse connection of the DC power supply 11 is made, the
polarity determination circuit (see Fig. 22) is
additionally provided.
Fig. 4A illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 when the forward
connection of the DC power supply 11 is made as shown in A
of Fig. 1. In this operational example, the switching
elements Q1 and Q3 are always in an on-state. As shown in
Fig. 2A, when the switching element Q2 is on and the
switching element Q4 is off, the electric charges charged
in the capacitor C1 flow through the bidirectional
switching device 4 and the primary side of the transformer
3 sequentially as a current. When the switching element Q2
is off and the switching element Q4 is on, the electric
charges charged in the capacitor C2 flow through the
primary side of the transformer 3 and the bidirectional
switching device 5 sequentially as a current. By repeating
these operations, the current flows through the primary
side of the transformer 3 in different directions.
Fig. 4B illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 when the reverse
connection of the DC power supply 11 is made as shown in B
of Fig. 1. In this operational example, the switching
elements Q2 and Q4 are always in an on-state. As shown in
Fig. 2B, when the switching element Q3 is on and the
switching element Q1 is off, the electric charges charged
in the capacitor C2 flow through the bidirectional

switching device 5 and the primary side of the transformer
3 sequentially as a current. When the switching element Q3
is off and the switching element Q1 is on, the electric
charges charged in the capacitor C1 flow through the
primary side of the transformer 3 and the bidirectional
switching device 4 sequentially as a current.
In accordance with these operational examples, any one
of the switching elements Q1 and Q2 included in the
bidirectional switching device 4 and any one of the
switching elements Q3 and Q4 constituting the bidirectional
switching device 5 are always in an on-state. Thus,
although the bidirectional switching devices 4 and 5
increase their losses when being controlled, the control
thereof can be simplified to reduce the cost of the control
circuit.
(Modification)
Fig. 5 illustrates a full-bridge type DC/DC converter
as a modification of the circuit configuration of the DC/DC
converter. The full-bridge type DC/DC converter 1 has a
pair of DC input terminals 2a and 2b, a transformer 3, two
pairs of bidirectional switching devices 4, 5, 6 and 7 on a
primary side of the transformer 3, and a capacitor C3. A
full-bridge circuit is formed on the primary side of the
transformer 3 by the transformer 3 and the bidirectional
switching devices 4, 5, 6 and 7. The bidirectional
switching device 4 has switching elements Q1 and Q2. The
bidirectional switching device 5 has switching elements Q3
and Q4. The bidirectional switching device 6 has switching

elements Q5 and Q6. The bidirectional switching device 7
has switching elements Q1 and Q8. The load 12 is connected
to a secondary side of the transformer 3, and the DC
voltage, which is converted by the transformer 3 and then
smoothed, is applied to the load 12.
Figs. 6A and 6B illustrate an example of the opening-
closing timing of the switching elements Q1 to Q8 included
in the bidirectional switching devices 4, 5, 6 and 7.
Fig. 6A illustrates the opening-closing timing of the
switching elements Q1 to Q8 in the DC/DC converter 1 of
Fig. 5 when the forward connection of the DC power supply
11 is made as shown in A of Fig. 1. The switching elements
Q1, Q2, Q7 and Q8 included in the bidirectional switching
devices 4 and 7 are switched on/off repeatedly at the same
timing. In the meantime, the switching elements Q3, Q4, Q5
and Q6 included in the bidirectional switching devices 5
and 6 are switched on/off repeatedly at the same timing.
That is, the bidirectional switching devices 4 and 7 are
switched on/off repeatedly at the same timing as each
other. Similarly, the bidirectional switching devices 5
and 6 are switched on/off repeatedly at the same timing as
each other. Also, the bidirectional switching devices 4
and 5 are switched on/off alternately and repeatedly.
Similarly, the bidirectional switching devices 6 and 7 are
switched on/off alternately and repeatedly.
That is, the switching elements Q3, Q4, Q5 and Q6 are
on during the period in which the switching elements Q1,
Q2, Q7 and Q8 are off, and the switching elements Q1, Q2,

Q7 and Q8 are on during the period in which the switching
elements Q3, Q4, Q5 and Q6 are off. When the switching
elements Q1, Q2, Q7 and Q8 are on and the switching
elements Q3, Q4, Q5 and Q6 are off, a current flows from
the DC power supply 11 through the bidirectional switching
device 4, the primary side of the transformer 3, and the
bidirectional switching device 7 sequentially, and returns
back to the DC power supply 11. When the switching
elements Q1, Q2, Q7 and Q8 are off and the switching
elements Q3, Q4, Q5 and Q6 are on, a current flows from the
DC power supply 11 through the bidirectional switching
device 6, the primary side of the transformer 3, and the
bidirectional switching device 5 sequentially, and returns
back to the DC power supply 11. By repeating these
operations, the current alternately flows through the
primary side of the transformer 3 in different directions.
Fig. 6B illustrates the opening-closing timing of the
switching elements Q1 to Q8 in the DC/DC converter 1 of
Fig. 5 when the reverse connection of the DC power supply
11 is made as shown in B of Fig. 1. Even in this case, the
switching elements Q1 to Q8 included in the bidirectional
switching devices 4, 5, 6 and 7 are also switched on/off
repeatedly. That is, when the switching elements Q3, Q4,
Q5 and Q6 are on and the switching elements Q1, Q2, Q7 and
Q8 are off, a current flows from the DC power supply 11
through the bidirectional switching device 5, the primary
side of the transformer 3, and the bidirectional switching
device 6 sequentially, and returns back to the DC power

supply 11. When the switching elements Q3, Q4, Q5 and Q6
are off and the switching elements Q1, Q2, Q1 and Q8 are
on, a current flows from the DC power supply 11 through the
bidirectional switching device 7, the primary side of the
transformer 3, and the bidirectional switching device 4
sequentially, and returns back to the DC power supply 11.
By repeating these operations, the current alternately
flows into the primary side of the transformer 3 in
different directions.
In accordance with the DC/DC converter 1 of this
modification, the bidirectional switching devices 4, 5, 6
and 7 are driven as shown in Figs. 6A and 6B, so that an
appropriate current can be allowed to flow into the primary
side of the transformer 3 even when any one of the forward
and reverse connections of the DC power supply 11 is made.
Accordingly, the diodes of the input unit shown in Figs.
28A to 28C can be removed. Like the DC/DC converter 1
shown in Fig. 1, since the FETs have an on-resistance
smaller than the resistance of the diodes forming the
bridge circuit shown in Fig. 28C, it is possible to reduce
voltage loss. Further, the full-bridge circuit is
configured to apply a substantially intact voltage of the
DC power supply 11 to the primary side of the transformer
3, so that the efficiency of the transformer 3 can be
improved.
Figs. 7A and 7B illustrate another example of the
opening-closing timing of the switching elements Q1 to Q8
included in the bidirectional switching devices 4, 5, 6 and

7 in the DC/DC converter 1 of Fig. 5. In the operational
examples, since the switching elements Q1 to Q8 have
different opening-closing timings depending on whether the
forward or reverse connection of the DC power supply 11 is
made, the polarity determination circuit (see Fig. 22) is
additionally provided.
Fig. 7A illustrates the opening-closing timing of the
switching elements Q1 to Q8 in the DC/DC converter 1 of
Fig. 5 when the forward connection of the DC power supply
11 is made as shown in A of Fig. 1. In this operational
example, like the case of Fig. 3A, the on-timing of the
switching elements Q1 and Q7 is shifted to be earlier than
the on-timing of the switching elements Q2 and Q8 so that
the on-timing of the switching elements Q1 and Q7 is
synchronized with the off-timing of the switching elements
Q4 and Q6. Also, the on-timing of the switching elements
Q3 and Q5 is shifted to be earlier than the on-timing of
the switching elements Q4 and Q6 so that the on-timing of
the switching elements Q3 and Q5 is synchronized with the
off-timing of the switching elements Q2 and Q8.
Fig. 7B illustrates the opening-closing timing of the
switching elements Q1 to Q8 in the DC/DC converter 1 of
Fig. 5 when the reverse connection of the DC power supply
11 is made as shown in B of Fig. 1. In this operational
example, the on-timing of the switching elements Q2 and Q8
is shifted to be earlier than the on-timing of the
switching elements Q1 and Q7 so that the on-timing of the
switching elements Q2 and Q8 is synchronized with the off-

timing of the switching elements Q3 and Q5. Similarly, the
on-timing of the switching elements Q4 and Q6 is shifted to
be earlier than the on-timing of the switching elements Q3
and Q5 so that the on-timing of the switching elements Q4
and Q6 is synchronized with the off-timing of the switching
elements Q1 and Q1.
In accordance with these operational examples, the
switching element can be protected from the surge voltage
when being turned off even if any one of the forward and
reverse connections of the DC power supply 11 is made.
Figs. 8A and 8B illustrate still another example of
the opening-closing timing of the switching elements Q1 to
Q8 included in the bidirectional switching devices 4, 5, 6
and 7 in the DC/DC converter 1 of Fig. 5. In the
operational examples, since the switching elements Q1 to Q8
have different opening-closing timings depending on whether
the forward or reverse connection of the DC power supply 11
is made, the polarity determination circuit (see Fig. 22)
is additionally provided.
Fig. 8A illustrates the opening-closing timing of the
switching elements Q1 to Q8 in the DC/DC converter 1 of
Fig. 5 when the forward connection of the DC power supply
11 is made as shown in A of Fig. 1. In this operational
example, as shown in Fig. 6A, the switching elements Q1,
Q3, Q5 and Q1 are always in an on-state. When the
switching elements Q2 and Q8 are on and the switching
elements Q4 and Q6 are off, a current flows from the DC
power source 11 through the bidirectional switching device

4, the primary side of the transformer 3, and the
bidirectional switching device 7 sequentially, and returns
back to the DC power supply 11. When the switching
elements Q2 and Q8 are off and the switching elements Q4
and Q6 are on, a current flows from the DC power source 11
through the bidirectional switching device 6, the primary
side of the transformer 3, and the bidirectional switching
device 5 sequentially, and returns back to the DC power
supply 11. By repeating these operations, the current
alternately flows through the primary side of the
transformer 3 in different directions.
Fig. 8B illustrates the opening-closing timing of the
switching elements Q1 to Q8 in the DC/DC converter 1 of
Fig. 5 when the reverse connection of the DC power supply
11 is made as shown in B of Fig. 1. In this operational
example, the switching elements Q2, Q4, Q6 and Q8 are
always in an on-state. Like the case of Fig. 6B, when the
switching elements Q3 and Q5 are on and the switching
elements Q1 and Q7 are off, a current flows from the DC
power source 11 through the bidirectional switching device
5, the primary side of the transformer 3, and the
bidirectional switching device 6 sequentially, and returns
back to the DC power supply 11. When the switching
elements Q3 and Q5 are off and the switching elements Q1
and Q7 are on, a current flows from the DC power source 11
through the bidirectional switching device 7, the primary
side of the transformer 3, and the bidirectional switching
device 4 sequentially, and returns back to the DC power

supply 11. By repeating these operations, the current
alternately flows through the primary side of the
transformer 3 in different directions.
In accordance with these operational examples, any one
of the switching elements Q1 and Q2 included in the
bidirectional switching device 4 and any one of the
switching elements Q3 and Q4 included in the bidirectional
switching device 5 are always in an on-state. In addition,
any one of the switching elements Q5 and Q6 included in the
bidirectional switching device 6 and any one of the
switching elements Q1 and Q8 included in the bidirectional
switching device 7 are always in an on-state. Thus,
although the bidirectional switching devices 4, 5, 6 and 7
increase their losses when being controlled, the control
thereof can be simplified to reduce the cost of the control
circuit.
(Modification)
Fig. 9 illustrates a push-pull type DC/DC converter as
a modification of the circuit configuration of the DC/DC
converter. The push-pull type DC/DC converter 1 has a pair
of DC input terminals 2a and 2b, a transformer 3, a pair of
bidirectional switching devices 4 and 5 on a primary side
of the transformer 3, and a capacitor C4. A push-pull
circuit is formed on the primary side of the transformer 3
by the transformer 3 and the bidirectional switching
devices 4 and 5. The bidirectional switching device 4 has
switching elements Q1 and Q2. The bidirectional switching
device 5 has switching elements Q3 and Q4. A load 12 and.

if necessary, a smoothing coil are connected to a secondary
side of the transformer 3, and the DC voltage, which is
converted by the transformer 3 and smoothed, is applied to
the load 12.
Figs. 10A and 10B illustrate an example of the
opening-closing timing of the switching elements Q1, Q2, Q3
and Q4 included in the bidirectional switching devices 4
and 5 in the DC/DC converter 1 of Fig. 9.
Fig. 10A illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 in the DC/DC converter
1 of Fig. 9 when the forward connection of the DC power
supply 11 is made as shown in A of Fig. 1. The switching
elements Q1 and Q2 included in the bidirectional switching
device 4 are switched on/off repeatedly at the same timing.
In the meantime, the switching elements Q3 and Q4 included
in the bidirectional switching device 5 are switched on/off
repeatedly at the same timing. The bidirectional switching
devices 4 and 5 are switched on/off alternately and
repeatedly. That is, the switching elements Q3 and Q4 are
on during the period in which the switching elements Q1 and
Q2 are off, and the switching elements Q1 and Q2 are on
during the period in which the switching elements Q3 and Q4
are off.
When the switching elements Q1 and Q2 are on and the
switching elements Q3 and Q4 are off, a current flows from
the DC power supply 11 through a coil N11 on the primary
side of the transformer 3 and the bidirectional switching
device 4 sequentially, and returns back to the DC power

supply 11. When the switching elements Q1 and Q2 are off
and the switching elements Q3 and Q4 are on, a current
flows from the DC power supply 11 through a coil N12 on the
- primary side of the transformer 3 and the bidirectional
switching device 5 sequentially, and returns back to the DC
power supply 11. By repeating these operations, the
current flows through the primary side of the transformer 3
in different directions.
Fig. 10B illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 in the DC/DC converter
1 of Fig. 9 when the reverse connection of the DC power
supply 11 is made as shown in B of Fig. 1. Even in this
case, the switching elements Q1, Q2, Q3 and Q4 included in
the bidirectional switching devices 4 and 5 also are
switched on/off repeatedly. That is, when the switching
elements Q3 and Q4 are on and the switching elements Q1 and
Q2 are off, a current flows from the DC power supply 11
through the bidirectional switching device 5 and the coil
N12 on the primary side of the transformer 3 sequentially,
and returns back to the DC power supply 11. When the
switching elements Q3 and Q4 are off and the switching
elements Q1 and Q2 are on, a current flows from the DC
power supply 11 through the bidirectional switching device
4 and the coil N11 on the primary side of the transformer 3
sequentially, and returns back to the DC power supply 11.
By repeating these operations, the current flows through
the primary side of the transformer 3 in different
directions.

In accordance with the DC/DC converter 1 of this
modification, the bidirectional switching devices 4 and 5
are driven as shown in Fig. 10, so that a proper current
can be allowed to flow into the primary side of the
transformer 3 even when any one of the forward and reverse
connections of the DC power supply 11 is made.
Accordingly, the diodes of the input unit shown in Figs.
28A to 28C can be removed. Like the DC/DC converter 1
shown in Fig. 1, since the FETs have an on-resistance
smaller than the resistance of the diodes forming the
bridge circuit shown in Fig. 28C, it is possible to reduce
voltage loss.
Figs. 11A and 11B illustrate another example of the
opening-closing timing of the switching elements Q1, Q2, Q3
and Q4 included in the bidirectional switching devices 4
and 5 in the DC/DC converter 1 of Fig. 9. In the
operational examples, since the switching elements Q1, Q2,
. Q3 and Q4 have different opening-closing timings depending
on whether the forward or reverse connection of the DC
power supply 11 is made, the polarity determination circuit
{see Fig. 22) is additionally provided.
Fig. 11A illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 in the DC/DC converter
1 of Fig. 9 when the forward connection of the DC power
supply 11 is made as shown in A of Fig. 1. In this
operational example, the body diode of the switching
element Q2 connected to the reverse direction side of the
coil is conducted in order to absorb the surge voltage when

the switching element Q4 is turned off. For this reason,
the on-timing of the switching element Q1 is shifted to be
earlier than the on-timing of the switching element Q2 so
that the on-timing of the switching element Q1 is
synchronized with the off-timing of the switching element
Q4. Similarly, the on-timing of the switching element Q3
is shifted to be earlier than the on-timing of the
switching element Q4 so that the on-timing of the switching
element Q3 is synchronized with the off-timing of the
switching element Q2.
Fig. 11B illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 in the DC/DC converter
1 of Fig. 9 when the reverse connection of the DC power
supply 11 is made as shown in B of Fig. 1. In this
operational example, the on-timing of the switching element
Q2 is shifted to be earlier than the on-timing of the
switching element Q1 so that the on-timing of the switching
element Q2 is synchronized with the off-timing of the
switching element Q3. Similarly, the on-timing of the
switching element Q4 is shifted to be earlier than the on-
timing of the switching element Q3 so that the on-timing of
the switching element Q4 is synchronized with the off-
timing of the switching element Q1.
In accordance with these operational examples, the
switching element can be protected from the surge voltage
when being turned off even if any one of the forward and
reverse connections of the DC power supply 11 is made.
Figs. 12A and 12B illustrate still another example of

the opening-closing timing of the switching elements Q1,
Q2, Q3 and Q4 included in the bidirectional switching
devices 4 and 5 in the DC/DC converter 1 of Fig. 9. In the
operational examples, since the switching elements Q1, Q2,
Q3 and Q4 have different opening-closing timings depending
on whether the forward or reverse connection of the DC
power supply 11 is made, the polarity determination circuit
(see Fig. 22) is additionally provided.
Fig. 12A illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 in the DC/DC converter
1 of Fig. 9 when the forward connection of the DC power
supply 11 is made as shown in A of Fig. 1. In this
operational example, the switching elements Q1 and Q3 are
always in an on-state. As shown in Fig. 10A, when the
switching element Q2 is on and the switching element Q4 is
off, a current flows from the DC power supply 11 through
the coil N11 on the primary side of the transformer 3 and
the bidirectional switching device 4 sequentially, and
returns back to the DC power supply 11. When the switching
element Q2 is off and the switching element Q4 is on, a
current flows from the DC power supply 11 through the coil
N12 on the primary side of the transformer 3 and the
bidirectional switching device 5 sequentially, and returns
back to the DC power supply 11. By repeating these
operations, the current alternately flows through the
primary side of the transformer 3 in different directions.
Fig. 12B illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 in the DC/DC converter

1 of Fig. 9 when the reverse connection of the DC power
supply 11 is made as shown in B of Fig. 1. In this
operational example, the switching elements Q2 and Q4 are
always in an on-state. As shown in Fig. 10B, when the
switching element Q3 is on and the switching element Q1 is
off, a current flows from the DC power supply 11 through
the bidirectional switching device 5 and the coil N12 on
the primary side of the transformer 3 sequentially, and
returns back to the DC power supply 11. When the switching
element Q3 is off and the switching element Q1 is on, a
current flows from the DC power supply 11 through the
bidirectional switching device 4 and the coil N11 on the
primary side of the transformer 3 sequentially, and returns
back to the DC power supply 11. By repeating these
operations, the current alternately flows through the
primary side of the transformer 3 in different directions.
In accordance with these operational examples, any one
of the switching elements Q1 and Q2 included in the
bidirectional switching device 4 and any one of the
switching elements Q3 and Q4 included in the bidirectional
switching device 5 are always in an on-state. Thus,
although the bidirectional switching devices 4 and 5
increase their losses when being controlled, the control
thereof can be simplified to reduce the cost of the control
circuit.
(Modification)
Fig. 13 illustrates a complex resonance type DC/DC
converter as a modification of the circuit configuration of

the DC/DC converter. The complex resonance type DC/DC
converter 1 has a pair of input terminals 2a and 2b, a
transformer 3, a pair of bidirectional switching devices 4
and 5 on a primary side of the transformer 3, capacitors C5
and C6, and a coil L. A complex resonance circuit is
formed on the primary side of the transformer 3 by the
transformer 3, the bidirectional switching devices 4 and 5,
the capacitor C6 and the coil L. The bidirectional
switching device 4 has switching elements Q1 and Q2. The
bidirectional switching device 5 has switching elements Q3
and Q4. A load 12 is connected to a secondary side of the
transformer 3 and the DC voltage, which is converted by the
transformer 3 and then smoothed, is applied to the load 12.
Figs. 14A and 14B illustrate an example of the
opening-closing timing of the switching elements Q1, Q2, Q3
and Q4 included in the bidirectional switching devices 4
and 5 in the DC/DC converter 1 of Fig. 13. Since the
switching elements Q1, Q2, Q3 and Q4 have different
opening-closing timings depending on whether the forward or
reverse connection of the DC power supply 11 is made, the
polarity determination circuit (see Fig. 22) is
additionally provided.
Fig. 14A illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 in the DC/DC converter
1 of Fig. 13 when the forward connection of the DC power
supply 11 is made as shown in A of Fig. 1. When the
switching elements Q1 and Q2 are on and the switching
elements Q3 and Q4 are off, a current flows from the DC

power supply 11 through the bidirectional switching device
4 and the primary side of the transformer 3 sequentially,
and electric charges are charged in the capacitor C6. When
the switching elements Q1 and Q2 are off and the switching
elements Q3 and Q4 are on, a current flows from the
capacitor C6 through the primary side of the transformer 3
and the bidirectional switching device 5 sequentially. By
repeating these operations, the current alternately flows
through the primary side of the transformer 3 in different
directions. In this modification, in order to flow a
recovery current, like the operation shown in Fig. 3A, the
on-timing of the switching element Q1 is shifted to be
earlier than the on-timing of the switching element Q2 so
that the on-timing of the switching element Q1 is
synchronized with the off timing of the switching element
Q4. Similarly, the on-timing of the switching element Q3
is shifted to be earlier than the on-timing of the
switching element Q4 so that the on-timing of the switching
element Q3 is synchronized with the off timing of the
switching element Q2.
Fig. 14B illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 in the DC/DC converter
1 of Fig. 13 when the reverse connection of the DC power
supply 11 is made as shown in B of Fig. 1. When the
switching elements Q3 and Q4 are off and the switching
elements Q1 and Q2 are on, a current sequentially flows
through the primary side of the transformer 3 and the
bidirectional switching device 4, and electric charges are

charged in the capacitor C6. When the switching elements
Q3 and Q4 are on and the switching elements Q1 and Q2 are
off, a current flows from the capacitor C6 through the
bidirectional switching device 5 and the primary side of
the transformer 3 sequentially- By repeating these
operations, the current alternately flows through the
primary side of the transformer 3 in different directions.
In this modification, in order to flow a recovery current,
like the operation shown in Fig. 3B, the on-timing of the
switching element Q2 is shifted to be earlier than the on-
timing of the switching element Q1 so that the on-timing of
the switching element Q2 is synchronized with the off
timing of the switching element Q3. Similarly, the on-
timing of the switching element Q4 is shifted to be earlier
than the on-timing of the switching element Q3 so that the
on-timing of the switching element Q4 is synchronized with
the off timing of the switching element Q1.
In accordance with the DC/DC converter 1 of this
modification, the bidirectional switching devices 4 and 5
are driven as shown in Fig. 14A and 14B, so that an
appropriate current can be allowed to flow into the primary
side of the transformer 3 even when any one of the forward
and reverse connections of the DC power supply 11 is made.
Accordingly, the diodes of the input unit shown in Figs.
28A to 28C can be removed. Like the DC/DC converter 1
shown in Fig. 1, since the FETs have an on-resi stance
smaller than the resistance of the diodes forming the
bridge circuit shown in Fig. 28C, it is possible to reduce

voltage loss. Further, the switching element can be
operated under soft-switching by using a resonance
phenomenon of the complex resonance circuit. Furthermore,
it is possible to prevent noise and to reduce switching
loss.
Figs. 15A and 15B illustrate another example of the
opening-closing timing of the switching elements Q1, Q2, Q3
and Q4 included in the bidirectional switching devices 4
and 5 in the DC/DC converter 1 of Fig. 13. In these
operational examples, since the switching elements Q1, Q2,
Q3 and Q4 have different opening-closing timings depending
on whether the forward or reverse connection of the DC
power supply 11 is made, the polarity determination circuit
(see Fig. 22) is additionally provided.
Fig. 15A illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 in the DC/DC converter
1 of Fig. 13 when the forward connection of the DC power
supply 11 is made as shown in A of Fig. 1. In this
operational example, the switching elements Q1 and Q3 are
always in an on-state. Fig. 15B illustrates the opening-
closing timing of the switching elements Q1, Q2, Q3 and Q4
in the DC/DC converter 1 of Fig. 13 when the reverse
connection of the DC power supply 11 is made as shown in B
of Fig. 1. In this operational example, the switching
elements Q2 and Q4 are always in an on-state. Either
operation of the DC/DC converter 1 is similar to that of
Fig. 14, and its description will be omitted.
In accordance with these operational examples, any one

of the switching elements Q1 and Q2 included in the
bidirectional switching device 4 and any one of the
switching elements Q3 and Q4 included in the bidirectional
switching device 5 are always in an on-state. Thus,
although the bidirectional switching devices 4 and 5
increase their losses when being controlled, the control
thereof can be simplified to reduce the cost of the control
circuit.
(Modification)
Fig. 16 illustrates a single transistor forward type
DC/DC converter as a modification of the circuit
configuration of the DC/DC converter. The single
transistor forward type DC/DC converter 1 has a pair of DC
input terminals 2a and 2b, a transformer 3, a bidirectional
switching device 4 on a primary side of the transformer 3,
a capacitor C7, and switching elements Q3 and Q4 on a
secondary side of the transformer 3.
Figs. 17A and 17B illustrate an example of the
opening-closing timing of switching elements Q1 and Q2
included in the bidirectional switching device 4 and the
switching elements Q3 and Q4 in the DC/DC converter 1 of
Fig. 16. Since the switching elements Q1, Q2, Q3 and Q4
have different opening-closing timings depending on whether
the forward or reverse connection of the DC power supply 11
is made, the polarity determination circuit (see Fig. 22)
is additionally provided in the DC/DC converter 1.
Fig. 17A illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 in the DC/DC converter

1 of Fig. 16 when the forward connection of the DC power
supply 11 is made as shown in A of Fig. 1. In this
operational example, the switching elements Q1 and Q2 on
the primary side of the transformer 3 are simultaneously
switched on/off. Further, the switching element Q3 on the
secondary side of the transformer 3 is always on and the
switching element Q4 on the secondary side of the
transformer 3 is always off, so that a secondary side coil
N22 on the flyback side is in an open-state, thereby
performing a forward operation.
Fig. 17B illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 in the DC/DC converter
1 of Fig. 16 when the reverse connection of the DC power
supply 11 is made as shown in B of Fig. 1. In this
operational example, the switching elements Q1 and Q2 on
the primary side of the transformer 3 are simultaneously
switched on/off. Further, the switching element Q4 on the
secondary side of the transformer 3 is always on and the
switching element Q3 on the secondary side of the
transformer 3 is always off, so that a secondary side coil
N21 on the flyback side is in an open-state, thereby
performing the forward operation.
In accordance with the DC/DC converter 1 of this
modification, the bidirectional switching device 4 is
driven as shown in Figs. 17A and 17B, so that a proper
current can be allowed to flow into the primary side of the
transformer 3 even when any one of the forward and reverse
connections of the DC power supply 11 is made.

Accordingly, the diodes of the input unit shown in Figs.
28A to 28C can be removed. Like the DC/DC converter 1
shown in Fig. 1, since the FETs have an on-resistance
smaller than the resistance of the diodes forming the
bridge circuit shown in Fig. 28C, it is possible to reduce
voltage loss.
Figs. 18A and 18B illustrate another example of the
opening-closing timing of the switching elements Q1 and Q2
included in the bidirectional switching device 4 and the
switching elements Q3 and Q4 in the DC/DC converter 1 of
Fig. 16. In the operational examples, since the switching
elements Q1, Q2, Q3 and Q4 have different opening-closing
timings depending on whether the forward or reverse
connection of the DC power supply 11 is made, the polarity
determination circuit (see Fig. 22) is additionally
provided.
Fig. 18A illustrates the opening-closing timing of the
switching elements Q1, Q2, Q3 and Q4 in the DC/DC converter
1 of Fig. 16 when the forward connection of the DC power
supply 11 is made as shown in A of Fig. 1. In this
operational example, the switching element Q1 on the
primary side of the transformer 3 is always in an on-state.
The switching element Q3 on the secondary side of the
transformer 3 is always on and the switching elements Q4 on
the secondary side of the transformer 3 is always off, so
that the secondary side coil N22 on the flyback side is in
an open-state, thereby performing the forward operation.
Fig. 18B illustrates the opening-closing timing of the

switching elements Q1, Q2, Q3 and Q4 in the DC/DC converter
1 of Fig. 16 when the reverse connection of the DC power
supply 11 is made as shown in B of Fig. 1. In this
operational example, the switching element Q2 on the
primary side of the transformer 3 is always in an on-state.
The switching element Q3 on the secondary side of the
transformer 3 is always off and the switching elements Q4
on the secondary side of the transformer 3 is always on, so
that the secondary side coil N21 on the flyback side is in
an open-state, thereby performing the forward operation.
In accordance with these operational examples, any one
of the switching elements Q1 and Q2 included in the
bidirectional switching device 4 is always in an on-state.
Thus, although the bidirectional switching device 4
increases its loss when being controlled, the control
thereof can be simplified to reduce the cost of the control
circuit.
(Modification)
Fig. 19 illustrates a single transistor flyback type
DC/DC converter as a modification of the circuit
configuration of the DC/DC converter. The single
transistor flyback type DC/DC converter 1 has a pair of DC
input terminals 2a and 2b, a transformer 3, a bidirectional
switching device 4 on a primary side of the transformer 3,
and a capacitor 7. In the DC/DC converter 1 configured as
shown in Fig. 19, the output voltage Vfw at the forward
side and the output voltage Vfb at the flyback side are
represented by the following equations.

Vfw=(N21/N11)*Vin
Vfb=(N21/N11)*(Ton/Toff)*Vin
N11: the number of turns at a primary side of
transformer
N21=N22: the number of turns at a secondary side
of transformer
Ton: on-time of switching elements Q1 and Q2
Toff: off-time of switching elements Q1 and Q2
Vin: input voltage
From the two equations, if the operation is performed
under Ton>Toff, Vfb>Vfw and the flyback operation can be
performed.
Figs. 20A and 20B illustrate an example of the
opening-closing timing of the switching elements Q1 and Q2
included in the bidirectional switching device 4 in this
modification. In this modification, the switching elements
Q1 and Q2 on the primary side of the transformer 3 are
simultaneously on/off. Also, the on-time Ton of the
switching elements Q1 and Q2 is set to be longer than the
off-time Toff thereof. In the operational examples shown
in Figs. 20A and 20B, a normal operation can be performed
even when any one of the forward and reverse connections of
the DC power supply 11 is made, so that the polarity
determination circuit is unnecessary.
In accordance with the DC/DC converter 1 of this
modification, the bidirectional switching device 4 is
driven as shown in Figs- 20A and 20B, whereby the normal
operation can be performed even when any one of the forward

and reverse connections of the DC power supply 11 is made.
Accordingly, the diodes of the input unit shown in Figs.
28A to 28C can be removed. Like the DC/DC converter 1
shown in Fig. 1, since the FETs have an on-resistance
smaller than the resistance of the diodes forming the
bridge circuit shown in Fig. 28C, it is possible to reduce
voltage loss.
Figs. 21A and 21B illustrate another example of the
opening-closing timing of the switching elements Q1 and Q2
included in the bidirectional switching device 4 in the
DC/DC converter 1 of Fig. 19. In these operational
examples, since the switching elements Q1 and Q2 have
different opening-closing timings depending on whether the
forward or reverse connection of the DC power supply 11 is
made, the polarity determination circuit (see Fig. 22) is
additionally provided.
Fig. 21A illustrates the opening-closing timing of the
switching elements Q1 and Q2 in the DC/DC converter 1 of
Fig. 19 when the forward connection of the DC power supply
11 is made as shown in A of Fig. 1. In this operational
example, the switching element Q1 on the primary side of
the transformer 3 is always in an on-state. Fig. 21B
illustrates the opening-closing timing of the switching
elements Q1 and Q2 in the DC/DC converter 1 of Fig. 19 when
the reverse connection of the DC power supply 11 is made as
shown in B of Fig. 1. In this operational example, the
switching element Q2 on the primary side of the transformer
3 is always in an on-state.

According to the operational example, any one of the
switching elements Q1 and Q2 included in the bidirectional
switching device 4 is always in an on-state. Thus, the
bidirectional switching device 4 increases its loss when
being controlled, but the control thereof can be simplified
to reduce the cost of the control circuit.
(Modification)
Fig. 22 illustrates a modification of the half-bridge
type DC/DC converter shown in Fig. 1. The modified DC/DC
converter 1 further has an input polarity determination
circuit 8 and the like, in addition to the constitutions of
the DC/DC converter of Fig. 1. The input polarity
determination circuit 8 determines the polarity of the DC
input terminals. A control circuit 9 outputs a control
signal to the driving circuit 10 according to the polarity
of the DC input terminals determined by the polarity
determination circuit, and controls a driving circuit 10.
The driving circuit 10 is provided with each of switching
elements Q1, Q2, Q3 and Q4 included in bidirectional
switching devices 4 and 5. The driving circuit 10 drives
each gate of the switching elements Q1, Q2, Q3 and Q4,
based on the control signal outputted from the control
circuit 9.
In the operational examples shown in Figs. 3A to 4B,
the switching elements Q1, Q2, Q3 and Q4 have different
opening-closing timings depending on whether the forward or
reverse connection of the DC power supply 11 is made.
Thus, the DC/DC converter 1 having the polarity

determination circuit 8, the control circuit 9, and the
driving circuit 10 is suitable. Also, the polarity
determination circuit 8, the control circuit 9 and the
driving circuit 10 may be applied to the DC/DC converters 1
shown in Figs. 5, 9, 13, 16 and 19.
Figs. 23 to 27 illustrate a bidirectional switching
device 100 having a lateral transistor structure, which is
applicable to the bidirectional switching devices 4, 5, 6
and 7 included in the DC/DC converter 1 in accordance with
the embodiments of the present invention. The
bidirectional switching device having the lateral
transistor structure using a GaN/AlGaN structure has no
loss due to a diode structure, and the loss thereof is low
as compared with an FET. Further, it is possible to
implement the integration of the control circuit.
Hereinafter, the bidirectional switching device 100 having
the lateral transistor structure using a GaN/AlGaN
structure will be described in detail.
Fig. 23 is a plane view illustrating the configuration
of the bidirectional switching device 100. Fig. 24 is an
enlarged view of an area A in Fig. 23. Fig. 25 is a
sectional view taken along a line XXV-XXV in Fig. 23.
Further, in the bidirectional switching device 100, only
one gate G is provided between two electrodes D1 and D2.
Therefore, the bidirectional switching device 100 is
referred to as a single gate type.
As shown in Fig. 25, a substrate 101 of the
bidirectional switching device 100 includes a conductive

layer 101a, and a GaN layer 101b and an AlGaN layer 101c
laminated on the conductive layer 101a. In the embodiment,
a two-dimensional electron gas layer generated on an
AlGaN/GaN heterogeneous interface is used as a channel
layer. As shown in Fig. 23, on a surface 101d of the
substrate 101, there are formed the first electrode D1 and
the second electrodes D2 respectively connected in series
to a DC power supply 2 and a load 3; and an intermediate
potential portion S that has an intermediate potential
between the potentials of the first electrode Dl and the
second electrode D2. Further, the control electrode (gate)
G is formed on the intermediate potential portion S. For
example, a Schottky electrode is used as the control
electrode G. The first electrode Dl and the second
electrode D2 are respectively formed in comb shapes having
electrode portions 111, 112, 113 ... and 121, 122 and 123 ...
arranged in parallel with each other, and the comb-shaped
electrode portions are arranged opposite to each other.
The intermediate potential portion S and the control
electrode G are respectively disposed between the comb-
shaped electrode portions 111, 112, 113 ... and 121, 122 and
123 ..., and have a shape (approximately backbone of fish)
similar to the plan shape of the space defined between the
electrode portions.
Next, the lateral transistor structure included in the
bidirectional switching device 100 will be described. As
shown in Fig. 24, the electrode portion 111 of the first
electrode Dl and the electrode portion 121 of the second

electrode D2 are arranged so that their center lines in the
width direction are aligned. Further, each of the
intermediate potential portion S and the control electrode
G is provided in parallel with the electrode portion 111 of
the first electrode D1 and the electrode portion 121 of the
second electrode D2. The distances from the electrode
portion 111 of the first electrode Dl and the electrode
portion 121 of the second electrode D2 to the intermediate
potential portion S and the control electrode G in the
width direction are set so that a predetermined withstand
voltage can be maintained. The distances in the
longitudinal direction of the electrode portion 111 of the
first electrode Dl and the electrode portion 121 of the
second electrode D2, i.e., perpendicular to the width
direction are also set in the same manner. Such a
relationship is also applied to another electrode portions
112 and 122, 113 and 123, .... That is, the intermediate
potential portion S and the control electrode G are
disposed at positions where the predetermined withstand
voltage can be maintained with respect to the first
electrode Dl and the second electrode D2.
For this reason, assuming that the first electrode Dl
is in a high potential side and the second electrode D2 is
in a low potential side, when the bidirectional switching
device 100 is turned off, a current is completely
interrupted between at least the first electrode D1 and the
control electrode G and intermediate potential portion S
(the current is blocked immediately under the control

electrode G).
Meanwhile, when the bidirectional switching device 100
is turned on, i.e., when a signal having a voltage of a
predetermined threshold value or more is applied to the
control electrode G, a current flows along the path of the
first electrode D1 (the electrode portions 111 ...) , the
intermediate potential portion S and the second electrode
D2 {the electrode portions 121 ...) as shown by arrows in
Fig. 24, and vice versa. As a result, even if the
threshold voltage of the signal applied to the control
electrode G is lowered to a necessary minimum level, the
bidirectional switching device 100 can be surely switched
on/off, thereby enabling a low on-resistance. Further, the
electrode portions 111, 112, 113 ... of the first electrode
Dl and the electrode portions 121, 122, 123 ... of the second
electrode D2 may be arranged in a comb shape, and thus, a
high current can be obtained without increasing the chip
size of the bidirectional switching device 100.
Figs. 26 and 27 illustrate the configuration of
another bidirectional switching device 300 having the
lateral transistor structure using a GaN/AlGaN structure.
Fig. 26 is a plane view illustrating the configuration of
the bidirectional switching device 300. Fig. 27 is a
sectional view taken along the line XXVII-XXVII in Fig. 26.
Also, two gates G1 and G2 are provided between two
electrodes D1 and D2, so that the bidirectional switching
device 300 is referred to as a dual gate type.
As shown in Figs. 26 and 27, the main switching device

300 of the lateral dual-gate transistor structure is
configured to have a single portion for maintaining a
withstand voltage, so that it is possible to implement a
bidirectional switching device with a small loss. That is,
the drain electrodes D1 and D2 are formed to reach the GaN
layer, and the gate electrodes G1 and G2 are formed on the
AlGaN layer. In a state where no voltage is applied to the
gate electrodes G1 and G2, an electron depletion region
occurs in the two-dimensional electron gas layer generated
on the AlGaN/GaN heterogeneous interface immediately under
the gate electrodes G1 and G2, and no current flows.
Meanwhile, when a voltage is applied to the gate
electrodes G1 and G2, a current flows in the AlGaN/GaN
heterogeneous interface toward the drain electrode D2 from
the drain electrode Dl (or reversely). To obtain a
withstand voltage, a predetermined distance is required
between the gate electrodes G1 and G2. However, no
withstand voltage is required between the drain electrode
D1 and the gate electrode G1, and between the drain
electrode D2 and the gate electrode G2. For this reason,
the drain electrode Dl and the gate electrode G1 or the
drain electrode D2 and the gate electrode G2 may be
overlapped with each other through an insulation layer In
interposed therebetween. Also, the element with such a
configuration is required to be controlled based on the
voltages of the drain electrodes D1 and D2, and therefore
driving signals are necessarily inputted to the respective
gate electrodes G1 and G2 (hence, referred to as the dual

gate transistor structure).
The present invention is not limited to the
configurations of the aforementioned embodiments but may be
configured to have at least a DC input terminal to which a
voltage of a DC power supply is inputted, a transformer,
and a bidirectional switching device provided on a primary
side of the transformer. The present invention may also be
variously modified. For example, the present invention is
limitedly applied to the power circuit of the notebook PC,
but may be applied to a large-scale power circuit in the
range where the withstand voltage of the element is
allowable.
A variety of aforementioned embodiments may be
appropriately combined with each other. For example, the
bidirectional switching device shown in Fig. 26 may be
applied to the DC/DC converter 1 shown in Fig. 13.
While the invention has been shown and described with
respect to the embodiments, it will be understood by those
skilled in the art that various changes and modification
may be made without departing from the scope of the
invention as defined in the following claims.

WE CLAIM:
1. A DC/DC converter, comprising:
DC input terminals to which a DC power is inputted;
a transformer; and
a bidirectional switching unit provided on a primary
side of the transformer.
2. The DC/DC converter of claim 1, wherein the bidirectional
switching unit comprises a pair of bidirectional switching
devices forming a half-bridge circuit.
3. The DC/DC converter of claim 1, wherein the bidirectional
switching unit comprises two pairs of bidirectional switching
devices forming a full-bridge circuit.
4. The DC/DC converter of claim 1, wherein the bidirectional
switching unit comprises a pair of bidirectional switching
devices forming a push-pull circuit.
5. The DC/DC converter of claim 1, wherein the bidirectional
switching unit comprises a pair of bidirectional switching
devices forming a complex resonance circuit.
6. The DC/DC converter of any one of claims 2 to 5, wherein
one of the bidirectional switching devices comprises two
switching elements, and
wherein one of the two switching elements is turned on

at timing different from timing at which the other of the two
switching elements is turned on.
7. The DC/DC converter of any one of claims 2, 4 and 5,
wherein one of the pair of the bidirectional switching
devices comprises two switching elements, and
wherein one of the two switching elements is turned on
as soon as a switching element included in the other of the
pair of bidirectional switching device is turned off.
8. The DC/DC converter of any one of claims 1 to 7, further
comprising a pair of switching elements on a secondary side
of the transformer, wherein one switching element in the
secondary side of the transformer is configured to be always
on and the other switching element in the secondary side of
the transformer is configured to be always off to perform a
forward operation.
9. The DC/DC converter of claim 1, wherein the bidirectional
switching unit is configured to have on-time longer than off-
time to perform a flyback operation.
10. The DC/DC converter of any one of claims 2 to 5, wherein
one of the bidirectional switching devices comprises two
switching elements, and
wherein one of the two switching elements is always on.
11. The DC/DC converter of claim 6 or 7, further comprising a

polarity determination circuit connected to the DC input
terminals, wherein the DC/DC converter is configured to
control on-timing of the two switching elements included in
one of the bidirectional switching devices in accordance with
the polarities of the DC input terminals determined by the
polarity determination circuit.
12. The DC/DC converter of claim 8, further comprising a
polarity determination circuit connected to the DC input
terminals, wherein the DC/DC converter is configured to
control on and off operations of the switching element in
accordance with the polarities of the DC input terminals
determined by the polarity determination circuit.
13. The DC/DC converter of any one of claims [[1 to 11]] 1 to
12 and 14, wherein the bidirectional switching device
comprises a bidirectional switching device having a lateral
transistor structure using a GaN/AlGaN structure.
14. The DC/DC converter of claims 2, 4 and 5, wherein one of
the pair of bidirectional switching devices comprises two
switching elements,
wherein one of the two switching elements is turned on
at timing different from timing at which the other of the two
switching elements is turned on, and
wherein one of the two switching elements is turned on
as soon as a switching element included in the other of the
pair of bidirectional switching devices is turned off.