Field of the Invention
The present invention relates to a DC power supply
system which includes a secondary battery as a backup power
source.
Background of the Invention
Recently, there has been supplied a DC power supply
system including a photovoltaic (solar) power generator or a
household fuel cell.
However, an output power generated by solar cells is
not stable because the amount of power generation changes
due to weather conditions and temperature. Thus, it is
generally provided with a secondary battery as a backup
power source (see, e.g., Patent Document 1).
In the DC power supply system, a DC power supply such
as a solar cell is connected in parallel with a circuit in
which the secondary battery is connected in series with a
parallel circuit of a DC/DC converter for charging the
secondary battery and a DC/DC converter for discharging the
secondary battery. Typically, the secondary battery has a
voltage lower than a constant voltage of the DC power
supply. When the voltage of the DC power supply is higher
than the voltage of the secondary battery, a charging

current flows in the secondary battery through the charging
DC/DC converter to charge the secondary battery. In
contrast, when the voltage of the DC power supply is lower
than the voltage of the secondary battery, a discharging
current flows from the secondary battery through the
discharging DC/DC converter to supply an electric power from
the secondary battery.
In the case of the photovoltaic power generator, the
solar cell generates an electric power even when the
electric power is not consumed at the load, and the electric
power generated during this period of time is charged in the
secondary battery. However, when the temperature of the
solar cell itself becomes excessively high due to, e.g., an
excessively sunny condition, the output voltage of the solar
cell is rather reduced.
Patent Document 1 does not directly describe that the
secondary battery is charged when the voltage of the DC
power supply is lower than the voltage of the secondary
battery, and only describes that the secondary battery can
be charged by using, e.g., a DC/DC converter capable of
stepping up a voltage. However, in the case of charging the
secondary battery by stepping up the output voltage of the
solar cell to be equal to or higher than the voltage of the
secondary battery, the current flowing into the secondary
side of the DC/DC converter becomes a value obtained by
multiplying the current flowing into the primary side of the

DC/DC converter by the conversion efficiency and the
reciprocal of the boosting ratio, and the current value
decreases considerably. In other words, the charging
efficiency is reduced while the energy loss is increased.
Patent Document 1: Japanese Patent Application
Publication No. 2008-48544
Summary of the Invention
In view of the above, the present invention provides a
DC power supply system capable of realizing a high charging
efficiency and charging a secondary battery even when a
voltage of a DC power supply is lower than a voltage of the
secondary battery.
In accordance with an embodiment of the present
invention, there is provided a DC power supply system
including: a DC power supply; and a secondary battery for
backing up the DC power supply. Further, when a voltage of
the DC power supply is lower than a voltage of the secondary
battery, the DC power supply system adds an extra voltage to
the voltage of the DC power supply to provide an input
voltage to the secondary battery higher than the voltage of
the secondary battery by using an electric power of the
secondary battery and charges the secondary battery.
In accordance with another embodiment of the present
invention, there is provided a DC power supply system

including: a DC power supply; a secondary battery for
backing up the DC power supply; and a backup power supply
circuit connected in parallel to the DC power supply, the
backup power supply circuit including the secondary battery.
The backup power supply circuit includes: a first charging
DC/DC converter configured to charge the secondary battery
when a voltage of the DC power supply is higher than a
voltage of the secondary battery; a first discharging DC/DC
converter configured to discharge an electric power charged
in the secondary battery when a voltage of the DC power
supply is higher than a voltage of the secondary battery; a
second charging DC/DC converter configured to add an extra
voltage to a voltage of the DC power supply by using an
electric power of the secondary battery to charge the
secondary battery when the voltage of the DC power supply is
lower than a voltage of the secondary battery; a second
discharging DC/DC converter configured to discharge an
electric power charged in the secondary battery when a
voltage of the DC power supply is lower than a voltage of
the secondary battery; and a control circuit configured to
control the first charging DC/DC converter, the first
discharging DC/DC converter, the second charging DC/DC
converter and the second discharging DC/DC converter.
Further, a first DC/DC converter may serve as the
first charging DC/DC converter and the second discharging
DC/DC converter, and a second DC/DC converter different from

the first DC/DC converter may serve as the second charging
DC/DC converter and the first discharging DC/DC converter.
Further, the second DC/DC converter may include input
terminals connected in parallel to both terminals of the
secondary battery; and output terminals connected in series
with between a high voltage side terminal of the DC power
supply and a high voltage side terminal of the secondary
battery.
Further, the first DC/DC converter may include input
terminals connected to the output terminals of the second
DC/DC converter; and output terminals connected to the
terminals of the secondary battery.
Further, an input side circuit of the first DC/DC
converter and an output side circuit of the second DC/DC
converter may include a bidirectional switch element having
a lateral transistor structure using a GaN/AlGaN structure.
In accordance with the present invention, even when
the voltage of the DC power supply is lower than the voltage
of the secondary battery, for example, when an electric
power consumed at the load is small, the extra voltage can
be added to the voltage of the DC power supply by using an
electric power of the secondary battery to charge the
secondary battery. Accordingly, the electric power
generated by a DC power supply such as photovoltaic power
generator can be effectively stored and used. Further,
since the voltage of the DC power supply is not directly

stepped up by using a step-up DC/DC converter, the current
supplied from the DC power supply may be directly used as a
charging current, thereby obtaining high charging
efficiency.
Brief Description of the Drawings
The objects and features of the present invention will
become apparent from the following description of
embodiments, given in conjunction with the accompanying
drawings.
FIG. 1 is a circuit diagram showing a basic
configuration of a DC power supply system in accordance with
an embodiment of the present invention.
FIG. 2 is a circuit diagram showing an operation of
the DC power supply system when a voltage of the DC power
supply is higher than a voltage of the secondary battery,
and the secondary battery is being charged.
FIG. 3 is a circuit diagram showing an operation of
the DC power supply system when a voltage of the DC power
supply is higher than a voltage of the secondary battery,
and the secondary battery is being discharged.
FIG. 4 is a circuit diagram showing an operation of
the DC power supply system when a voltage of the DC power
supply is lower than a voltage of the secondary battery, and
the secondary battery is being charged.

FIG. 5 is a circuit diagram showing an operation of
the DC power supply system when a voltage of the DC power
supply is lower than a voltage of the secondary battery, and
the secondary battery is being discharged.
FIG. 6 shows a specific circuit configuration of the
first DC/DC converter and the second DC/DC converter in the
DC power supply system.
FIG. 7 is a plan view showing a configuration of a
bidirectional switch element (single gate type).
FIG. 8 is an enlarged view of area A shown in FIG. 7.
FIG. 9 is a cross-sectional view taken along line IX-
IX shown in FIG. 7.
FIG. 10 is a plan view showing a configuration of a
bidirectional switch element (dual gate type).
FIG. 11 is a cross-sectional view taken along line XI-
XI shown in FIG. 10.
Detailed Description of the Embodiments
Hereinafter, an embodiment of the present invention
will be described in detail with reference to the
accompanying drawings which form a part hereof. Throughout
the specification and drawings, like reference numerals will
be given to like parts having substantially the same
function and configuration, and a redundant description
thereof will be omitted.

A DC power supply system in accordance with an
embodiment of the present invention will be described. A DC
power supply system 1 includes: a DC power supply 2, such as
a solar cell, a household fuel cell, or the like; and a
backup power supply circuit 4 connected in parallel with the
DC power supply 2 and including a secondary battery 3 for
backing up the DC power supply.
The backup power supply circuit 4 includes a first
DC/DC converter 5 and a second DC/DC converter 6, which are
connected in parallel to each other; the secondary battery 3
connected in series with the above parallel circuit; and a
control circuit 7 for controlling the first DC/DC converter
5 and the second DC/DC converter 6. Further, the backup
power supply circuit 4 includes: a first voltage detector 8
for detecting a voltage V1 of the DC power supply 2; and a
second voltage detector 10 for detecting a voltage V2 of the
secondary battery 3. In addition, if necessary, the backup
power supply circuit 4 may include: a first current detector
9 for detecting a current I1 flowing through a load 20; and
a second current detector 11 for detecting a charging
current or a discharging current 12 flowing through the
backup power supply circuit 4.
Input terminals of the second DC/DC converter 6 are
connected in parallel to both terminals of the secondary
battery 3. Output terminals of the second DC/DC converter 6
are connected in series with between a high voltage side

terminal of the DC power supply 2 and a high voltage side
terminal of the secondary battery 3.
Further, input terminals of the first DC/DC converter
5 are connected to the output terminals of the second DC/DC
converter 6. Output terminals of the first DC/DC converter
5 are connected to both terminals of the secondary battery
3.
Each of the first DC/DC converter 5 and the second
DC/DC converter 6 may function as a discharging DC/DC
converter, or may function as a charging DC/DC converter
depending on whether or not the voltage V1 of the DC power
supply 2 is higher than the voltage V2 of the secondary
battery 3.
Specifically, when the voltage V1 of the DC power
supply 2 is higher than the voltage V2 of the secondary
battery 3 (V1 > V2), the first DC/DC converter 5 functions
as a first charging DC/DC converter for charging the
secondary battery 3, and the second DC/DC converter 6
functions as a first discharging DC/DC converter for
discharging the electric power charged in the secondary
battery 3.
In contrast, when the voltage V1 of the DC power
supply 2 is lower than the voltage V2 of the secondary
battery 3 (V2 > V1), the second DC/DC converter 6 functions
as a second charging DC/DC converter for charging the
secondary battery 3, and the first DC/DC converter 5

functions as a second discharging DC/DC converter for
discharging the electric power charged in the secondary
battery 3.
The first DC/DC converter 5 includes a transformer 51,
an input side (primary side) circuit 52, and an output side
(secondary side) circuit 53. Further, the second DC/DC
converter 6 includes a transformer 61, an input side
(primary side) circuit 62, and an output side (secondary
side) circuit 63. FIG. 6 shows a specific circuit
configuration of the first DC/DC converter 5 and the second
DC/DC converter 6. The circuit configuration will be later
described.
The control circuit 7 compares the voltage V1 of the
DC power supply 2 that has been detected by the first
voltage detector 8 with the voltage V2 of the secondary
battery 3 that has been detected by the second voltage
detector 10. The control circuit 7, based on the comparison
result, controls the input side circuits 52 and 62 of the
first DC/DC converter 5 and the second DC/DC converter 6.
Hereinafter, a specific operation will be described for the
case when the voltage V1 of the DC power supply 2 is higher
than the voltage V2 of the secondary battery 3 (V1 > V2) and
the case when the voltage V1 of the DC power supply 2 is
lower than the voltage V2 of the secondary battery 3 (V2 >
V1) . Further, for the sake of convenience, the voltage of
the secondary battery 3 is fixed to 380 V, but a current

value may be determined optionally.
(Specific Operation Example 1)
FIG. 2 shows the case when the voltage V1 of the DC
power supply 2 is higher than the voltage V2 of the
secondary battery 3 (V1 > V2) and the secondary battery 3 is
being charged. This corresponds to, e.g., a case when the
electric power consumption at the load 2 0 is small, and the
electric power generated in the DC power supply 2 is large
enough. The first DC/DC converter 5 functions as a first
charging DC/DC converter.
As shown in FIG. 2, it is assumed that the voltage V1
of the DC power supply 2 is 400 V, and the voltage V2 of the
secondary battery 3 is 380 V. The control circuit 7 drives
the input side (primary side) circuit 52 of the first DC/DC
converter 5 to make a current flow from the DC power supply
2 into the backup power supply circuit 4 based on the result
of comparing the voltage V1 of the DC power supply 2 with
the voltage V2 of the secondary battery 3.
It will be assumed that a current of 2 A flows into
the backup power supply circuit 4 from the DC power supply
2. Since the input side of the first DC/DC converter 5 has
a potential difference of 20 V and the current of 2 A flows
into the input side (primary side) circuit 52, the input
power becomes 40 W (20 V x 2 A) . Assuming that the first
DC/DC converter 5 has an efficiency of 90%, the output side
(the output side circuit 53) of the first DC/DC converter 5

has an output power of 36 W (40 W x 0.9) . In the first
DC/DC converter 5, the boosting ratio is determined to
generate a voltage of 380 V on the secondary side thereof in
order to charge the secondary battery 3. The current of
0.095 A (36 W ÷ 380 V) flows into the output side circuit 53
of the first DC/DC converter 5.
Since the current of 2 A, which flows through the
input side circuit 52 of the first DC/DC converter 5,
directly flows to the ground through the secondary battery
3, the current of 2 A on the input side and the current of
0.095 A on the output side are combined to flow a current of
2.095 A through the secondary battery 3. The DC power
supply 2 outputs an electric power of 800 W (400 V × 2 A)
therefrom, and the secondary battery 3 charges an electric
power of 796 W (380 V × 2.095 A) therein. In other words,
the DC power supply system has a total charging efficiency
of 0.995 (796 W ÷ 800 W) , so that the charging circuit
having a very high efficiency can be obtained. During this
period of time, the second DC/DC converter 6 is not
operated. Further, the current flowing through the output
side circuit 53 of the first DC/DC converter 5 is controlled
by detecting the charging current 12 through the second
current detector 11, thereby controlling the charging
current to be constant.
(Specific Operation Example 2)
FIG. 3 shows the case when the voltage V1 of the DC

power supply 2 is higher than the voltage V2 of the
secondary battery 3 (V1 > V2) and the secondary battery 3 is
being discharged. This corresponds to, e.g., a case when
the output power of the DC power supply 2 such as a solar
cell decreases, or the electric power generated in the DC
power supply 2 is insufficient to cover an electric power
demanded at the load 20 because the electric power is large.
The second DC/DC converter 6 functions as a first
discharging DC/DC converter.
The control circuit 7 drives the input side (primary
side) circuit 62 of the second DC/DC converter 6 based on
the comparative result of the voltage V1 of the DC power
supply 2 with the voltage V2 of the secondary battery 3, and
flows the current of, e.g., 0.1 A into the input side
circuit 62 of the second DC/DC converter 6. The second
DC/DC converter 6 generates an extra voltage to be added to
the voltage V2 (380 V) of the secondary battery 3 to provide
a voltage equal to or higher than the voltage V1 (400 V) of
the DC power supply 2.
Since the current of 0.1 A flows into the input side
circuit of the second DC/DC converter 6, the second DC/DC
converter 6 will obtain an input power of 38 W (380 V × 0.1
A) . Assuming that the second DC/DC converter 6 has an
efficiency of 90%, the second DC/DC converter 6 will obtain
an output power of 34.2 W (38 W × 0.9} in the output side.
A step-down ratio of the second DC/DC converter 6 is

designed such that the second DC/DC converter 6 obtains the
extra voltage of 20 V on the secondary side thereof in order
to supplement the voltage difference between the DC power
supply 2 and the secondary battery 3. The current of 1.71 A
(34.2 W ÷ 20 V) flows into the output side circuit 63 of
the second DC/DC converter 6.
Since the current flows into the secondary battery 3
as well as flows through the output side circuit 63 of the
second DC/DC converter 6, the current flowing through the
input side circuit 62 and the current flowing through the
output side circuit 63 are combined to flow a current of
1.81 A (0.1 A + 1.71 A) through the secondary battery 3.
The electric power of 687.8 W (380 V × 1.81 A) is
discharged from the secondary battery 3. In contrast, the
electric power supplied to the load 20 from the secondary
battery 3 is 684 W (400 V × 1.71 A). Accordingly, the DC
power supply system has a total discharging efficiency of
0.994 (684 W ÷ 687.8 W), thereby obtaining very high
efficiency in the discharging circuit. During this period
of time, the first DC/DC converter 5 is not operated.
(Specific Operation Example 3)
FIG. 4 shows the case when the voltage V1 of the DC
power supply 2 is lower than the voltage V2 of the secondary
battery 3 (V2 > V1) and the secondary battery 3 is being
charged. This corresponds to, e.g., a case when the
electric power consumption at the load 20 is small, and the

output voltage of the DC power supply 2 such as a solar cell
decreases. The second DC/DC converter 6 functions as a
second charging DC/DC converter.
The control circuit 7 drives the input side (primary
side) circuit 62 of the second DC/DC converter 6 based on
the comparative result of the voltage V1 of the DC power
supply 2 with the voltage V2 of the secondary battery 3, and
flows the current of, e.g., 0.1 A into the input side
circuit 62 of the second DC/DC converter 6. The second
DC/DC converter 6 generates an extra voltage to be added to
the voltage V1 (360 V) of the DC power supply 2 to provide a
voltage equal to or higher than the voltage V2 (380 V) of
the secondary battery 3. The extra voltage generated in the
output side (secondary side) circuit 63 of the second DC/DC
converter 6 has an opposite polarity as compared with the
case shown in FIG. 3.
As in the above case, when the current of 0.1 A flows
into the input side circuit 62 of the second DC/DC converter
6, the second DC/DC converter 6 will obtain an input power
of 38 W (380 V × 0.1 A). Assuming that the second DC/DC
converter 6 has an efficiency of 90%, the second DC/DC
converter 6 will obtain an output power of 34.2 W (38 W ×
0.9) in the output side. A boosting ratio of the second
DC/DC converter 6 is designed such that the second DC/DC
converter 6 obtains the extra voltage on the secondary side
thereof in order to supplement the voltage difference of 20

V between the DC power supply 2 and the secondary battery 3.
The current of 1.71 A (34.2 W 4 ÷ 20 V) flows into the output
side circuit 63 of the second DC/DC converter 6. However,
the direction of the current flow is opposite.
Flowing the current through the output side circuit 63
of the second DC/DC converter 6 causes the DC power supply 2
to supply an electric power to the backup power supply
circuit 4 for charging the secondary battery 3. The current
of 1.61 A (1.71 A - 0.1 A), which is obtained by subtracting
the current flowing through the input side circuit 62 from
the current flowing through the output side circuit 63 of
the second DC/DC converter 6, actually flows into the
secondary battery 3 as a charging current.
The electric power of 616 W (360 V X 1.71 A) is
outputted from the DC power supply 2, and the electric power
of 612 W (380 V × 1.61 A) is charged in the secondary
battery 3. In other words, the DC power supply system has
an total charging efficiency of 0.994 (612 W ÷ 616 W) ,
thereby obtaining very high efficiency in the charging
circuit. During this period of time, the first DC/DC
converter 5 is not operated. Further, the current flowing
through the input side circuit 62 of the second DC/DC
converter 6 is controlled by detecting the charging current
12 through the second current detector 11, thereby
controlling the charging current to be constant.
(Specific Operation Example 4)

FIG. 5 shows the case when the voltage V1 of the DC
power supply is lower than the voltage V2 of the secondary
battery 3 (V2 > V1) , and the secondary battery 3 is being
discharged. For example, in this case, although the voltage
of the DC power supply 2 is low, the load 20 may continue to
be driven. In this example, the secondary battery 3 merely
functions to assist the DC power supply 2, and the backup
power supply circuit 4 drops the voltage of the secondary
battery 3 to the voltage of the DC power supply 2 and then
outputs it. The first DC/DC converter 5 functions as a
second charging DC/DC converter.
The control circuit 7 drives the input side (primary
side) circuit 52 of the first DC/DC converter 5 based on the
value of the load current I1 detected by the first current
detector 9, and flows the current of, e.g., 2 A into the
input side circuit 52 of the first DC/DC converter 5. The
input side circuit 52 of the first DC/DC converter 5 steps
down the voltage V2 (380 V) of the secondary battery 3 to
the voltage V1 (360 V) of the DC power supply 2.
Simultaneously, at the output side of the first DC/DC
converter 5, a voltage of 380 V is generated to charge the
secondary battery 3.
When the current of 2 A flows into the input side
circuit 52 of the first DC/DC converter 5, the first DC/DC
converter 5 will obtain an input power of 40 W ((380 V - 360
V) × 2 A) . Assuming that the first DC/DC converter 5 has

an efficiency of 90%, the first DC/DC converter 5 will
obtain an output power of 36 W (40 W × 0.9) in the output
side. The charging current of 0.095 A (36 W ÷ 380 V) flows
into the output side circuit 53 of the first DC/DC converter
5.
Since this charging current is returned to the
secondary battery 3, the current of 1.905 A, which is
obtained by subtracting the charging current of 0.095 A from
the current of 2 A, flows out of the secondary battery 3,
whereby the electric power of 724 W (380 V × 1.905 A) is
discharged from the secondary battery 3. In contrast, the
electric power supplied to the load from the secondary
battery 3 is 720 W (360 V × 2 A) . Accordingly, the DC
power supply system has a total discharging efficiency of
0.994 (720 W ÷ 724 W) , thereby obtaining very high
efficiency in the discharging circuit. During this period
of time, the second DC/DC converter 6 is not operated.
Further, the current (charging current) flowing through the
output side circuit 53 of the first DC/DC converter 5 is
controlled, thereby controlling the discharging current
flowing to the load 2 0 from the secondary battery 3 to be
constant.
As described above, each of the first DC/DC converter
5 and the second DC/DC converter 6 functions as a
discharging DC/DC converter or a charging DC/DC converter
depending on whether or not the voltage of the DC power

supply 2 is higher than the voltage of the secondary battery
3.
When the first DC/DC converter 5 functions as the
first charging DC/DC converter, both the direction of the
current flowing to the input side and the polarity of the
voltage of the input terminals are reversed as compared with
the case where the first DC/DC converter 5 functions as the
second discharging DC/DC converter. Similarly, when the
second DC/DC converter 6 functions as the second charging
DC/DC converter, both the direction of the current flowing
to the output side and the polarity of the voltage of the
output terminals are reversed as compared with the case
where the second DC/DC converter 6 functions as the first
discharging DC/DC converter.
Therefore, as shown in FIG. 6, bidirectional switch
elements are used as switch elements included in the input
side circuit 52 of the first DC/DC converter 5 and the
output side circuit 63 of the second DC/DC converter 6.
As shown in FIG. 6, the input side circuit 52 of the
first DC/DC converter 5 has two bidirectional switches Q1
and Q2 which are connected in series. A series circuit of a
capacitor, an inductor and a primary winding of the
transformer 51 is connected in parallel to the bidirectional
switch Q2. By turning on and off the bidirectional switches
Q1 and Q2 at a predetermined frequency, a pulse current
flows through the primary winding of the transformer 51,

thereby generating an electromotive force in a secondary
winding. Since the current bi-directionally flows through
the input side circuit 52, the polarity of the voltage
inputted to the output side circuit 53 is alternately
changed according to the direction of the current.
Therefore, the output side circuit 53 includes diodes such
that the current produced by the electromotive force
generated in the secondary winding of the transformer 51
always flows in the same direction in order to charge the
secondary battery 3.
The input side circuit 62 of the second DC/DC
converter 6 has two switches Q3 and Q4 which are connected
in series. A series circuit of a capacitor and a primary
winding of the transformer 61 is connected in parallel to
the switch Q4. By turning on and off the switches Q3 and Q4
at a predetermined frequency, a pulse current flows through
the primary winding of the transformer 61, thereby
generating an electromotive force in a secondary winding.
In addition, since the current always flows in the same
direction in the input side circuit 62, the polarity of the
voltage inputted to the output side circuit 63 is constant.
However, as described above, when the second DC/DC
converter 6 functions as a discharging DC/DC converter, the
direction of the current is reversed as compared with the
case where the second DC/DC converter 6 functions as a
charging DC/DC converter. Therefore, the output side

circuit 63 includes two bidirectional switches Q5 and Q6
connected to the secondary winding of the transformer 61.
Then, by turning off the bidirectional switch Q6 when the
bidirectional switch Q5 is turned on and turning off the
bidirectional switch Q5 when the bidirectional switch Q6 is
turned on, the direction of the current flowing into the
output side circuit 63 is reversed. Further, since the
current always flows in the same direction in the input side
circuit 62, the switches Q3 and Q4 are not necessary to be
bidirectional switches, and may be, e.g., MOSFETs.
As a specific example of the bidirectional switch, a
bidirectional switch element 100 having a lateral transistor
structure using a GaN/AlGaN structure will be described in
detail. FIG. 7 is a plan view showing a configuration of
the bidirectional switch element 100. FIG. 8 is an enlarged
view of area A shown in FIG. 7, and FIG 9 is a cross-
sectional view taken along line IX-IX shown in FIG. 7.
Further, the bidirectional switch element 100 is referred to
as a single gate type because only one gate G is provided
between two electrodes D1 and D2.
As shown in FIG. 9, a substrate 101 of the
bidirectional switch element 100 includes a conductive layer
101a, and a GaN layer 101b and an AlGaN layer 101c which are
formed on the conductive layer 101a. In this embodiment, a
two-dimensional electron gas layer, which is generated at a
hetero-interface between AlGaN and GaN, is used as a channel

layer. As shown in FIG. 7, a first electrode D1 and a
second electrode D2, and an intermediate potential portion S
having an intermediate potential between the potentials of
the first and the second electrode Dl and D2 are formed on a
surface 101d of the substrate 101. Further, a 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 has a comb shape having
electrode portions 111, 112, 113 ••• arranged in parallel to
one another, and the second electrode D2 has a comb shape
having electrode portions 121, 122, 123••• arranged in
parallel to one another. The comb-shaped electrode portions
of the first electrode Dl and the comb-shaped electrode
portions of the second electrode D2 are arranged opposite to
each other. Since 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, 123 •••, they have a shape (substantially fish spine
shape) similar to the planar shape of the space defined
between the electrode portions.
Next, a lateral transistor structure of the
bidirectional switch element 100 will be described. As
shown in FIG. 8, the electrode portion 111 of the first
electrode D1 and the electrode portion 121 of the second
electrode D2 are arranged such that the center lines in the

width direction thereof are aligned. In addition, the
intermediate potential portion S and the control electrode G
are positioned in parallel to the electrode portion 111 of
the first electrode D1 and the electrode portion 121 of the
second electrode D2. Distances in the width direction 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
are set such that a predetermined withstand voltage can be
maintained. Distances in the longitudinal direction of the
electrode portion 111 of the first electrode D1 and the
electrode portion 121 of the second electrode D2, i.e.,
perpendicular to the width direction are also set in the
same manner.
In addition, such relationships are the same as those
of the other electrode portions 112 and 122, and 113 and
123. That is, the intermediate potential portion S and the
control electrode G are disposed at positions at which a
predetermined withstand voltage can be maintained with
respect to the first electrode D1 and the second electrode
D2.
Therefore, assuming that the first electrode D1 is in
a high potential side and the second electrode D2 is in a
low potential side, when the bidirectional switch element
100 is turned off, the current is completely interrupted
between at least the first electrode Dl, and the control

electrode G and the intermediate potential portion S (the
current is blocked directly under the control electrode
(gate) G).
In contrast, when the bidirectional switch element 100
is turned on, i.e., when a signal having a voltage equal to
or higher than a predetermined threshold is applied to the
control electrode G, a current flows through a path of the
first electrode D1 (electrode portion 111 •••), the
intermediate potential portion S, and the second electrode
D2 (electrode portion 121 •••) as indicated by the arrow in
the figure, and vice versa.
As a result, even though a threshold voltage of the
signal applied to the control electrode G is lowered to the
required minimum level, it is possible to securely turn
on/off the bidirectional switch element 100, thereby
enabling a low on-resistance. Further, since the electrode
portions 111 112, 113 ••• of the first electrode Dl and the
electrode portions 121, 122, 123 ••• of the second electrode
D2 can be arranged in a comb shape, a high current can be
obtained without increasing a chip size of the bidirectional
switch element 100.
FIGS. 10 and 11 show a configuration of another
bidirectional switch element 300 having a lateral transistor
structure using a GaN/AlGaN structure. FIG. 10 is a plan
view showing the configuration of the bidirectional switch
element 300. FIG. 11 is a cross-sectional view taken along

line XI-XI shown in FIG. 10. Further, the bidirectional
switch element 300 is referred to as a dual gate type
because two gates G1 and G2 are provided between two
electrodes D1 and D2.
As shown in FIGS. 10 and 11, the bidirectional switch
element 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 switch element with a small loss. In other
words, the drain electrodes Dl and D2 are formed on 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
at the AlGaN/GaN heterogeneous interface directly below the
gate electrodes G1 and G2, and no current flows. In
contrast, when a voltage is applied to the gate electrodes
G1 and G2, a current flows in the AlGaN/GaN heterogeneous
interface from the drain electrode Dl toward the drain
electrode D2 (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. Therefore,
the drain electrode D1 and the gate electrode G1, or the

drain electrode D2 and the gate electrode G2 may be
overlapped with each other via an insulating layer In
interposed therebetween. Further, the element with such a
configuration needs to be controlled based on the voltages
of the drain electrodes D1 and D2, and therefore it is
necessary to input a drive signal to the respective gate
electrodes G1 and G2 (thus, referred to as a dual gate
transistor structure).
As described above, in accordance with the present
invention, even when the voltage V1 of the DC power supply 2
is lower than the voltage V2 of the secondary battery 3, the
current from the secondary battery 3 is made to flow into
the input side circuit 62 of the second DC/DC converter 6 to
generate a voltage in the secondary winding of the
transformer 61. Then, the extra voltage outputted to the
output side circuit 63 is added to provide a voltage higher
than the voltage of the secondary battery 3, and the
secondary battery 3 is charged. Accordingly, the electric
power generated by a DC power supply such as photovoltaic
power generator can be effectively stored and used.
Furthermore, by using the bidirectional switches in
the input side circuit 52 of the first DC/DC converter 5 and
the output side circuit 63 of the second DC/DC converter 6,
the current can flow through the identical DC/DC converter
in the opposite directions depending on the charging and
discharging operation. That is, since a single DC/DC

converter can be used as a charging DC/DC converter and a
discharging DC/DC converter, it is possible to achieve the
downsizing and cost reduction of the backup power supply
circuit 4. Further, by using a bidirectional switch, element
having a lateral transistor structure using a GaN/AlGaN
structure, which does not include a parasitic diode, as a
bidirectional switch, it is possible to realize the backup
power supply circuit 4 with a low loss.
Further, the present invention is not limited to the
configuration of the above embodiment, but various
modifications may be made. For example, as a bidirectional
switch, there may be used two MOSFETs whose parasitic diodes
are connected in series opposed to each other, or other
bidirectional switches such as a triac may be used. In
addition, the numerical values described in the above
embodiment are used for making the description of the
present invention understandable, so it is needless to say
that they are not actual values.
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 power supply system comprising:
a DC power supply; and
a secondary battery for backing up the DC power
supply,
wherein, when a voltage of the DC power supply is
lower than a voltage of the secondary battery, the DC power
supply system adds an extra voltage to the voltage of the DC
power supply to provide an input voltage to the secondary
battery higher than the voltage of the secondary battery by
using an electric power of the secondary battery and charges
the secondary battery.
2. A DC power supply system comprising:
a DC power supply;
a secondary battery for backing up the DC power
supply; and
a backup power supply circuit connected in parallel to
the DC power supply, the backup power supply circuit
including the secondary battery,
wherein the backup power supply circuit includes:
a first charging DC/DC converter configured to charge
the secondary battery when a voltage of the DC power supply
is higher than a voltage of the secondary battery;
a first discharging DC/DC converter configured to

discharge an electric power charged in the secondary battery
when a voltage of the DC power supply is higher than a
voltage of the secondary battery;
a second charging DC/DC converter configured to add an
extra voltage to a voltage of the DC power supply by using
an electric power of the secondary battery to charge the
secondary battery when the voltage of the DC power supply is
lower than a voltage of the secondary battery;
a second discharging DC/DC converter configured to
discharge an electric power charged in the secondary battery
when a voltage of the DC power supply is lower than a
voltage of the secondary battery; and
a control circuit configured to control the first
charging DC/DC converter, the first discharging DC/DC
converter, the second charging DC/DC converter and the
second discharging DC/DC converter.
3. The DC power supply system of claim 2, wherein a first
DC/DC converter serves as the first charging DC/DC converter
and the second discharging DC/DC converter, and a second
DC/DC converter different from the first DC/DC converter
serves as the second charging DC/DC converter and the first
discharging DC/DC converter.
4. The DC power supply system of claim 3, wherein the
second DC/DC converter comprises: input terminals connected

in parallel to both terminals of the secondary battery; and
output terminals connected in series with between a high
voltage side terminal of the DC power supply and a high
voltage side terminal of the secondary battery.
5. The DC power supply system of claim 4, wherein the
first DC/DC converter comprises: input terminals connected
to the output terminals of the second DC/DC converter; and
output terminals connected to the terminals of the secondary
battery.
6. The DC power supply system of any one of claims 3 to 5,
wherein an input side circuit of the first DC/DC converter
and an output side circuit of the second DC/DC converter
comprise a bidirectional switch element having a lateral
transistor structure using a GaN/AlGaN structure.