Field of the Invention
The present invention relates to a converter circuit
(DC/DC converter) for a hybrid power source including, e.g.,
a solar cell, a secondary battery as a backup power source
of the solar cell and the like.
Background of the Invention
In a photovoltaic system, an electric power generated
by a solar cell is influenced by weather conditions and the
like, and a voltage is changed by a temperature variation in
the solar cell itself and the like. Therefore, a secondary
battery is used as a backup power source, and when the
amount of the electric power generated by the solar cell is
small, the electric power is discharged from the secondary
battery to stabilize the electric power supplied to a load.
The voltage of the secondary battery is set to be slightly
lower than the voltage at which the electric power
generation of the solar cell is stable and is charged from
the solar cell when the electric power consumed by the load
is small.
In a hybrid power source including a solar cell, a
secondary battery for backup of the solar cell and the like,
since the voltage of the solar cell is different from the

rated voltage of the load and the secondary battery, the
voltage is stepped up/down by using a DC/DC converter to
supply an electric power to the load. In the general
circuit configuration, a DC/DC converter is provided for
each of the solar cell and the secondary battery.
Therefore, in designing the DC/DC converter, it is
important to achieve both the miniaturization and high
efficiency of the DC/DC converter, and it has been proposed
that a plurality of DC power sources share a transformer and
a rectifier circuit (see, e.g., Patent Document 1).
Patent Document 1 does not disclose a specific circuit
configuration of the DC/DC converter, but FIG. 12 shows a
circuit configuration of a DC/DC converter 50 using a
general MOSFET as a switch element. A first and a second
primary winding N51 and N52 corresponding to a first and a
second DC power source 51 and 52, respectively, are provided
on a primary side of a transformer 53, and one secondary
winding N53 corresponding to the load 57 is provided on the
secondary side of the transformer 53. A first oscillation
circuit 54 having a full-bridge structure, which is formed
of four switch elements Q51 to Q54, is connected to the
first primary winding N51. Further, a second oscillation
circuit 55 having a full-bridge structure, which is formed
of four switch elements Q55 to Q58, is connected to the
second primary winding N52. A rectifier circuit 56 is
connected to the secondary winding N53.

The first DC power source 51 is a solar cell and the
second DC power source 52 is a secondary battery. The
voltage of the solar cell is VDC1, and a reference voltage
of the solar cell is Vrefl. The voltage of the secondary
battery is VDC2, and a reference voltage of the secondary
battery is Vref2. The number of turns of the first primary
winding N51 and the number of turns of the second primary
winding N52 are n1 and n2, respectively.
In order that the output voltage of the load by the
discharge operation from the secondary battery and the power
generation of the solar cell is kept constant, it is
preferable to set a turns ratio n2/nl of the primary
windings N51 and N52 to satisfy Vrefl × n2/nl = Vref2.
However, considering that the secondary battery is
charged from the solar cell, it is preferable to set a turns
ratio n2/nl of the primary windings N51 and N52 to satisfy
Vrefl × n2/n1 > Vref2.
In practice, since the voltage applied to the load 57
is not constant and has a tolerance value, it is set to
satisfy Vrefl × n2/n1 > Vref2. However, in order to more
easily describe the nature of the problem in the present
invention, the problem will be described below on the
assumption that a turns ratio satisfies Vrefl × n2/nl =
Vref2.
FIG. 13 shows a state where in the case of VDC1 ×
n2/nl > VDC2, for example, under the condition that the

voltage of the solar cell is varied to be larger than the
reference voltage Vrefl and the voltage of the secondary
battery is the reference voltage Vref2 (VDC1 > Vrefl, VDC2 =
Vref2), the switch elements Q55 to Q58 are turned off while
the switch elements Q51 and Q54 and the switch elements Q52
and Q53 are alternately turned on and off, so that the
electric power is supplied to the load 57 from the first DC
power source 51. In FIG. 13, the switch elements Q51 and
Q54 are being turned on. When supplying the electric power
to the load 57 from both the first and the second DC power
source 51 and 52, the switch elements Q51 and Q54, the
switch elements Q52 and Q53, the switch elements Q55 and
Q58, and the switch elements Q56 and Q57 may be sequentially
turned on by time division.
Under the conditions of VDC1 > Vrefl and VDC2 = Vref2,
the voltage VN52 of the primary winding N52 satisfies VN52 =
VDC1 × n2/nl = VDC1 × Vref2/Vrefl > VDC2 by the induced
electromotive force generated in the primary winding N52
from the primary winding N51. Thus, the voltage of the
primary winding N52 becomes larger than VDC2. Since MOSFET
has a body diode (parasitic diode), a reverse current flows
through the second DC power source 52 by the electromotive
force generated in the second primary winding N52 via the
body diodes of the switch elements Q55 and Q58. The same is
true when the switch elements Q52 and Q53 are turned on.
Since such a reverse current becomes a charging current to

the secondary battery, substantially, the first DC power
source 51 charges the secondary battery while supplying the
electric power to the load 57, thereby resulting in an
increase in the current flowing through the first
oscillation circuit 54 on the side of the first DC power
source 51.
Accordingly, the loss due to the switch elements Q51
to Q54 included in the first oscillation circuit 54 is
increased, and the power supply efficiency of the first DC
power source 51 is decreased. Further, the secondary
battery is charged through the body diodes of the switch
elements Q55 to Q58, and there occurs a problem such that it
cannot be charged at certain timings (even if charging is
not desired, it is charged arbitrarily).
Similarly, under the condition of VDC1 × n2/nl < VDC2,
when the electric power is outputted from the second DC
power source 52, a reverse current flows through the first
DC power source 51. That is, in the configuration of FIG.
13, the voltage of the first and the second DC power source
51 and 52 varies, and there occurs a problem such that the
efficiency is deteriorated in the case of VDC1 X n2/nl ≠
VDC2.
In another conventional example shown in FIG. 14, in
order to prevent the reverse current from flowing through
the first DC power source 51 or the second DC power source
52, backflow prevention diodes D51 to D58 are connected in

series to the switch elements Q51 to Q58, respectively, in
opposite directions to the body diodes (see Non-patent
Document 1).
However, when a current flows in the forward direction
through the backflow prevention diodes, the loss due to the
diodes is increased and the power supply efficiency from the
first DC power source 51 or the second DC power source 52 is
decreased. Further, it is necessary to add the backflow
prevention diodes D51 to D58 or choke coils C51 and C52 to
the oscillation circuits 54 and 55, which results in
reducing an advantage of the miniaturization of the DC/DC
converter 50 obtained by sharing the transformer 53 and the
rectifier circuit 56. In addition, since the reverse
current does not flow through the second DC power source 52
by the backflow prevention diodes, the secondary battery
cannot be charged by using this DC/DC converter.
Patent Document 1: Japanese Patent Laid-open
Publication No. 2005-229729 (FIG. 15)
Non-patent Document 1: Y. -M. Chen, Y. -C. Liu, F. -
Y. Wu, and T. -F. Wu "Multi-Input DC/DC Converter Based on
the Flux Additivity" IEEE IAS, Vol. 3, Oct. 2001, pp. 1866-
1873
Summary of the Invention
The present invention provides a converter circuit for

a hybrid power source in which a transformer and a rectifier
circuit are commonly used, capable of preventing a reverse
current from flowing through one DC power source when
supplying an electric power to a load from the other DC
power source and reducing a loss without reducing a power
supply efficiency. Further, if necessary, it is possible to
charge the secondary battery via a DC/DC converter.
In accordance with an embodiment of the present
invention, there is provided a converter circuit including:
a transformer having primary windings and at least one
secondary winding; a rectifier circuit connected to the
secondary winding; and oscillation circuits connected to the
primary windings. Each of the oscillation circuits includes
a switch element unit having no body diode.
In accordance with another embodiment of the present
invention, there is provided a converter circuit including:
a transformer having a first primary winding, a second
primary winding and a secondary winding; a rectifier circuit
connected between the secondary winding and a load; a first
oscillation circuit connected to a first DC power source and
the first primary winding; a second oscillation circuit
connected to a second DC power source and the second primary
winding; and a control circuit configured to control the
first and the second oscillation circuit. Each of the first
and the second oscillation circuit includes a switch element
unit having no body diode.

Further, the switch element unit having no body diode
may have a lateral transistor structure using a GaN/AlGaN
structure.
Further, the switch element unit having no body diode
may be a bidirectional switch element.
Further, the switch element unit of each of the first
and the second oscillation circuit may include two pairs of
switch elements forming a full-bridge circuit.
Further, the switch element unit of each of the first
and the second oscillation circuit may include a pair of
switch elements forming a half-bridge circuit.
Further, the switch element unit of each of the first
and the second oscillation circuit may include one switch
element.
Further, the transformer may be a flyback transformer.
Further, the number of turns may be variable in at
least one of the first and the second primary winding.
In accordance with the present invention, since the
switch element unit having no body diode is used as a switch
element constituting the oscillation circuit, a reverse
current does not flow through the oscillation circuit when
the switch element is not turned on. Therefore, when
supplying the electric power to the load from one DC power
source, if the switch element of the oscillation circuit
connected to the other DC power source is turned off, a
reverse current does not flow through the other DC power

source, and the power supply efficiency is not reduced.
Further, there occurs no loss due to the body diode.
Further, if necessary, when one DC power source is a
secondary battery and the voltage of the other DC power
source is higher than the voltage of the secondary battery,
it is possible to charge the secondary battery through the
DC/DC converter.
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, in which:
FIG. 1 is a circuit diagram showing a circuit
configuration of a DC/DC converter in accordance with an
embodiment of the present invention;
FIG. 2 shows a modification example of the DC/DC
converter in accordance with the embodiment of the present
invention;
FIGS. 3A to 3C are time charts each showing waveforms
of the gate signals of the bidirectional switches elements
for driving the DC/DC converter;
FIG. 4 is a plan view showing a configuration of the
bidirectional switch element (single gate);
FIG. 5 is an enlarged view of area A shown in FIG. 4;

FIG. 6 is a cross-sectional view taken along line VI-
VI shown in FIG. 4;
FIG. 7 is a plan view showing a configuration of the
bidirectional switch element (dual gate);
FIG. 8 is a cross-sectional view taken along line
VIII-VIII shown in FIG. 7;
FIG. 9 shows another modification example of the DC/DC
converter in accordance with the embodiment of the present
invention;
FIG. 10 shows still another modification example of
the DC/DC converter in accordance with the embodiment of the
present invention;
FIG. 11 shows still another modification example of
the DC/DC converter in accordance with the embodiment of the
present invention;
FIG. 12 shows a circuit configuration of a
conventional DC/DC converter using a MOSFET as a switch
element;
FIG. 13 shows a state where the electric power is
supplied to the load from the first DC power source in the
conventional DC/DC converter; and
FIG. 14 shows a circuit configuration of the
conventional DC/DC converter in which a backflow prevention
diode is connected to the MOSFET.

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 converter circuit (DC/DC converter) in accordance
with an embodiment of the present invention will be
described. FIG. 1 is a circuit diagram showing a circuit
configuration of a DC/DC converter 1. In order to simplify
the explanation, a case with two power sources of a first DC
power source 11 and a second DC power source 12 will be
described, but the number of power sources is not limited to
two.
The DC/DC converter 1 is a DC/DC converter for a
hybrid power source for supplying an electric power supplied
from a plurality of DC power sources including the first and
the second DC power source 11 and 12 to a load 57. The
DC/DC converter 1 functions as a DC/DC converter for
supplying an electric power to the load 57 from the first DC
power source 11, and also functions as a DC/DC converter for
supplying an electric power to the load 57 from the second
DC power source 12. The DC/DC converter 1 may be regarded

as two DC/DC converters which share a transformer 13, a
rectifier circuit 16 and a control circuit 17.
A first primary winding N1 and a second primary
winding N2 are provided at the primary side of the
transformer 13, and one secondary winding N3 is provided at
the secondary side of the transformer 13. A first
oscillation circuit 14 is connected to the first primary
winding Nl, and the first DC power source 11 is connected to
the first oscillation circuit 14. Similarly, a second
oscillation circuit 15 is connected to the second primary
winding N2, and the second DC power source 12 is connected
to the second oscillation circuit 15. A rectifier circuit
16 is connected to the secondary winding N3, and the load 57
is connected to the rectifier circuit 16. The first
oscillation circuit 14 and the second oscillation circuit 15
have basically the same configuration. The first
oscillation circuit 14 includes a full bridge circuit formed
of four bidirectional switch elements Q1 to Q4. Similarly,
the second oscillation circuit 15 includes a full bridge
circuit formed of four bidirectional switch elements Q5 to
Q8.
In FIG. 1, a wiring connecting the gate of each of the
bidirectional switch elements Ql to Q8 to the control
circuit 17 is omitted. Further, in this embodiment, one
secondary winding N3 is merely provided at the secondary
side of the transformer 13, but two or more secondary

windings may be provided without being limited thereto.
For example, it is assumed that the first DC power
source 11 is a solar cell and the second DC power source 12
is a secondary battery. When supplying an electric power to
the load 57 from the first DC power source 11 alone, as
shown in FIG. 3A, the control circuit 17 alternately turns
on and off a pair of the bidirectional switch elements Ql
and Q4 and a pair of the bidirectional switch elements Q2
and Q3 of the first oscillation circuit 14. In the
meantime, the control circuit 17 turns off all of the
bidirectional switch elements Q5 to Q8 of the second
oscillation circuit 15.
On the other hand, when supplying an electric power to
the load 57 from the second DC power source 12 alone, as
shown in FIG. 3B, the control circuit 17 alternately turns
on and off a pair of the bidirectional switch elements Q5
and Q8 and a pair of the bidirectional switch elements Q6
and Q7 of the second oscillation circuit 15. In the
meantime, the control circuit 17 turns off all of the
bidirectional switch elements Ql to Q4 of the first
oscillation circuit 14.
Further, when an electric power is alternately
supplied from the first and the second DC power source 11
and 12 to the load 57, as shown in FIG. 3C, a pair of the
bidirectional switch elements Ql and Q4, a pair of the
bidirectional switch elements Q2 and Q3, a pair of the

bidirectional switch elements Q5 and Q8, and a pair of the
bidirectional switch elements Q6 and Q7 are sequentially
turned on and off.
In this case, assuming VDC1 x n2 /n1 > VDC2, where a
voltage of the solar cell is VDC1, a voltage of the
secondary battery is VDC2, and the number of turns of the
first primary winding Nl and the number of turns of the
second primary winding N2 are nl and n2, respectively.
In a conventional example shown in FIGS. 12 and 13,
even if the bidirectional switch elements Q55 to Q58 of a
second oscillation circuit 55 are turned off, an
electromotive force is generated in the second primary
winding N52, and a current flows in a body diode (parasitic
diode) of each of the switch elements. Accordingly, a
reverse current flows in the second DC power source 52 via
the switch elements Q55 to Q58.
In contrast, as will be described below, since each of
the bidirectional switch elements Q1 to Q8 has no body
diode, a reverse current does not flow therethrough. That
is, in accordance with the configuration of the DC/DC
converter 1 of this embodiment shown in FIG. 1, even in the
case of VDC1 x n2/nl > VDC2, a reverse current does not flow
in the second DC power source 12. The first DC power source
11 does not charge the secondary battery while supplying an
electric power to the load 57, and the current, which flows
through the first oscillation circuit 14 on the side of the

first DC power source 11, is not increased. In addition,
the bidirectional switch elements Q1 to Q8 have a much lower
loss compared with the MOSFET since they have no body diode.
Accordingly, there is less loss due to the bidirectional
switch elements Ql to Q4, and the power supply efficiency of
the first DC power source 11 is improved as compared with
the conventional example described above.
When charging the secondary battery, the control
circuit 17 turns on/off the bidirectional switch elements Q5
to Q8 of the second oscillation circuit 15 in
synchronization with the on/off of the bidirectional switch
elements Q1 to Q4 of the first oscillation circuit 14.
Since the loss due to the bidirectional switch elements Q5
to Q8 is small, it is possible to flow a larger current as a
charging current. Thus, the charging efficiency becomes
higher than that in the conventional example.
Further, when an electric power is outputted from the
second DC power source 12, the reverse current does not flow
in the first DC power source 11 in the similar manner
described above. Therefore, the power supply efficiency
when supplying an electric power from the second DC power
source 12 is also improved as compared with the conventional
example described above.
FIG. 2 shows a modification example of the DC/DC
converter 1. In this modification example, the number of
turns of the second primary winding N2 of the transformer 13

is varied. In FIG. 2, although a changeover switch for
changing the number of turns is not illustrated in detail,
it is possible to configure a non-contact switch with low
loss by using the bidirectional switch element as described
above.
As shown in FIG. 3C, if the electric power is
outputted alternately to the load 57 from the first and the
second DC power source 11 and 12, it is preferable to output
the voltage of the second DC power source 12 after the
voltage of the second DC power source 12 is stepped up to
the same level as that of the first DC power source 11.
On the other hand, when charging the second DC power
source 12 serving as the secondary battery, the voltage of
the electromotive force generated in the second primary
winding N2 needs to be higher than the voltage of the second
DC power source 12. Therefore, the second primary winding
N2 has at least two types of the number of turns
corresponding to a turns ratio n2/nl satisfying Vref1 ×
n2/nl = Vref2 and a turns ratio n2/nl satisfying Vref1 ×
n2/nl > Vref2 (n1 is constant). Further, the second primary
winding N2 may have another type of the number of turns
depending on the voltage variation of the first DC power
source 11. In this case, in order to satisfy the setting
condition of the turns ratio, n2 may be constant while nl
may be varied. Alternatively, both of nl and n2 may be
varied.

As a specific example of the bidirectional switch
elements Q1 to Q8, a bidirectional switch element 100 having
a lateral transistor structure using a GaN/AlGaN structure
will be described in detail. FIG. 4 is a plan view showing
a configuration of the bidirectional switch element 100.
FIG. 5 is an enlarged view of area A shown in FIG. 4, and
FIG. 6 is a cross-sectional view taken along line VI-VI
shown in FIG. 4. Further, the bidirectional switch element
100 in which only one gate G is provided between two
electrodes D1 and D2 is referred to as a single gate type.
As shown in FIG. 6, 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. 4, a first electrode Dl and a
second electrode D2 respectively connected in series with
respect to the DC power source 11 or 12 and the winding Nl
or N2, and an intermediate potential portion S having an
intermediate potential relative to the potential of the
first electrode Dl and the potential of the second electrode
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 D1 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 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. 5, the electrode portion 111 of the first
electrode Dl and the electrode portion 121 of the second
electrode D2 are arranged such that 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 Dl 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 Dl and the second electrode
D2.
Therefore, 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 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).
On the other hand, 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 Dl (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. 7 and 8 show a configuration of another
bidirectional switch element 300 having a lateral transistor
structure using a GaN/AlGaN structure. FIG. 7 is a plan
view showing the configuration of the bidirectional switch
element 300. FIG. 8 is a cross-sectional view taken along
line VIII-VIII shown in FIG. 7. Further, the bidirectional
switch element 300 is referred to as a dual gate type
because two gates Gl and G2 are provided between two
electrodes Dl and D2.
As shown in FIGS. 7 and 8, the bidirectional switch
element 300 of the lateral dual 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 D1 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. On the
other hand, when a voltage is applied to. the gate electrodes
Gl and G2, a current flows in the AlGaN/GaN heterogeneous
interface from the drain electrode D1 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
first electrode Dl 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 Dl 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).

FIG. 9 shows another modification example of the DC/DC
converter 1. In this modification example, each of the.
first and the second oscillation circuit 14 and 15 has a
half-bridge circuit. The first oscillation circuit 14
includes a pair of bidirectional switch elements Q11 and Q12
connected in series, and a series circuit of capacitors C11
and C12 connected in parallel to the series circuit of the
bidirectional switch elements Q11 and Q12. The first
primary winding N1 is connected between a midpoint of the
series circuit of the bidirectional switch elements Q11 and
Q12 and a midpoint of the series circuit of the capacitors
C11 and C12.
Similarly, the second oscillation circuit 15 includes
a pair of bidirectional switch elements Q13 and Q14
connected in series, and a series circuit of capacitors C13
and C14 connected in parallel to the series circuit of the
bidirectional switch elements Q13 and Q14. The second
primary winding N2 is connected between a midpoint of the
series circuit of the bidirectional switch elements Q13 and
Q14 and a midpoint of the series circuit of the capacitors
C13 and C14.
When the bidirectional switch element Q11 is turned on
and the bidirectional switch element Q12 is turned off, the
current flows in the bidirectional switch element Q11 and
the first primary winding Nl of the transformer 13 by the
electric charges charged in the capacitor C11. Further,

when the bidirectional switch element Q12 is turned on and
the bidirectional switch element Q11 is turned off, the
current flows in the opposite direction in the bidirectional
switch element Q12 and the first primary winding N1 of the
transformer 13 by the electric charges charged in the
capacitor C12. This operation is repeated, and the current
flows alternately in different directions in the first
primary winding Nl of the transformer 13. The same applies
to the second oscillation circuit 15.
In the case of this modification example, as compared
with the case where each of the first and the second
oscillation circuit 14 and 15 shown in FIG. 1 has a full-
bridge circuit, the voltage applied to the first and the
second primary winding Nl and N2 of the transformer 13 is
1/2 of the voltage of the first and the second DC power
source 11 and 12. However, it has an advantage of
simplifying the structure and facilitating the control of
the first and the second oscillation circuit 14 and 15
FIG. 10 shows still another modification example of
the DC/DC converter 1. In this modification example, each
of the first and the second oscillation circuit 14 and 15 is
configured to include a forward converter having only one of
bidirectional switch elements Q21 and Q22. Further, in the
bidirectional switch elements Q21 and Q22, there are
provided reset circuits R21 and R22 for preventing the
magnetization of the core. The control circuit 17 controls

the bidirectional switch element Q21 or Q22 so as to
repeatedly turn on and off at a predetermined frequency.
When the bidirectional switch element Q21 or Q22 is turned
on, a current flows in the first primary winding Nl or the
second primary winding N2 of the transformer 13, and an
electromotive force is generated and a current flows in the
secondary winding N3 of the transformer 13. Thus, by
configuring the forward converter, it becomes possible to
simplify the configuration of the first oscillation circuit
14, the second oscillation circuit 15 and the rectifier
circuit 16.
FIG. 11 shows still another modification example of
the DC/DC converter 1. In this modification example, a
flyback converter is configured by using a flyback
transformer serving as the transformer 13.
Each of the first and the second oscillation circuit
14 and 15 uses only one of bidirectional switch elements Q31
and Q32. The control circuit 17 controls the bidirectional
switch element Q31 or Q32 so as to repeatedly turn on and
off at a predetermined frequency.
When the bidirectional switch element Q31 or Q32 is
turned on, a current flows in the first primary winding Nl
or the second primary winding N2 of the transformer 13 to
magnetize the core of the transformer 13. Further, when the
bidirectional switch element Q31 or Q32 is turned on, no
current flows in the secondary winding N3 of the transformer

13.
On the other hand, when the bidirectional switch
element Q31 or Q32 is turned off, a current flows in the
secondary winding N3 of the transformer 13 so as to release
the magnetic energy from the core. Thus, by configuring a
flyback converter using a flyback transformer, it is
possible to simplify the configuration of the first
oscillation circuit 14, the second oscillation circuit 15
and the rectifier circuit 16. In the case of the flyback
converter, as compared with the forward converter, the
direction of the current flowing through the secondary
winding N3 of the transformer 13 is reversed.
As described above, in accordance with the embodiments
of the present invention, for a hybrid power source
including a plurality of DC power sources such as a solar
cell, a secondary battery as a backup power source of the
solar cell and the like, it is possible to provide a DC/DC
converter which shares a transformer and a rectifier circuit
and includes a plurality of oscillation circuits with low
loss.
Further, since bidirectional switch elements, each
having no body diode, are used as switch elements included
in the oscillation circuit, when supplying an electric power
to the load from one DC power source while the bidirectional
switches elements of the oscillation circuit connected to
the other DC power source are turned off, no reverse current

flows through the other DC power source. Therefore, the
power supply efficiency of the DC power source (the one that
supplies the electric power) is not reduced, and there
occurs no loss due to the body diode.
Further, if necessary, when one DC power source is
used as a secondary battery and the voltage of the other DC
power source is set to be higher than the voltage of the
secondary battery, it is possible to charge the secondary
battery through the DC/DC converter.
Further, in addition to the solar cell and the
secondary battery, other DC power sources such as a fuel
cell and the like may be used as a DC power source.
Further, in the description of the above embodiments, a
bidirectional switch element having no body diode has been
illustrated as an example of a switch element included in
the DC/DC converter, but any element can be used as long as
it has no body diode. For example, two unidirectional
switches may be used in combination.
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 converter circuit comprising:
a transformer having primary windings and at least one
secondary winding;
a rectifier circuit connected to the secondary
winding; and
oscillation circuits connected to the primary
windings,
wherein each of the oscillation circuits includes a
switch element unit having no body diode.
2. A converter circuit comprising:
a transformer having a first primary winding, a second
primary winding and a secondary winding;
a rectifier circuit connected between the secondary
winding and a load;
a first oscillation circuit connected to a first DC
power source and the first primary winding;
a second oscillation circuit connected to a second DC
power source and the second primary winding; and
a control circuit configured to control the first and
the second oscillation circuit,
wherein each of the first and the second oscillation
circuit includes a switch element unit having no body diode.

3. The converter circuit of claim 1 or 2, wherein the
switch element unit having no body diode has a lateral
transistor structure using a GaN/AlGaN structure.
4. The converter circuit of any one of claims 1 to 3,
wherein the switch element unit having no body diode is a
bidirectional switch element.
5. The converter circuit of any one of claims 2 to 4,
wherein the switch element unit of each of the first and the
second oscillation circuit comprises two pairs of switch
elements forming a full-bridge circuit.
6. The converter circuit of any one of claims 2 to 4,
wherein the switch element unit of each of the first and the
second oscillation circuit comprises a pair of switch
elements forming a half-bridge circuit.
7. The converter circuit of any one of claims 2 to 4,
wherein the switch element unit of each of the first and the
second oscillation circuit comprises one switch element.
8. The converter circuit of any one of claims 1 to 4,
wherein the transformer is a flyback transformer.
9. The converter circuit of any one of claims 1 to 8,

wherein the number of turns is variable in at least one of
the first and the second primary winding.