DC-DC converter including first and second full-bridge circuits for performing soft switching and reducing switching losses

1. A DC-DC converter for performing soft switching and reducing switching losses, the DC-DC converter comprising:

a first full-bridge circuit including four switching elements;

a second full-bridge circuit including four switching elements;

a transformer including a first winding and a second winding, the first winding being connected to said first full-bridge circuit, and the second winding being connected to said second full-bridge circuit and magnetically coupled to said first winding;

an inductance component connected in series with said first winding or said second winding; and

a control circuit that controls soft switching of each switching element in said first full-bridge circuit and said second full-bridge circuit,

wherein each switching element of said first and second full-bridge circuits includes a capacitor,

wherein said capacitor is a parasitic capacitance or a combination of a parasitic capacitance and an actual element,

wherein an inductor current flowing through an equivalent inductor at a time of switching of turning on or off each switching element is greater than or equal to a threshold current, the equivalent inductor being equivalent to said transformer and said inductance component,

wherein said control circuit changes a voltage output ON time of said first full-bridge circuit and a voltage output ON time of said second full-bridge circuit by controlling a phase of each switching element of said first and second full-bridge circuits to output power from said DC-DC converter, while fixing a switching frequency of said first and second full-bridge circuits and keeping a polarity inversion period at a value greater than or equal to a fixed value, the polarity inversion period being a period in which an output of said first full-bridge circuit and an output of said second full-bridge circuit have reverse polarities,

wherein said control circuit changes the voltage output ON time of said first full-bridge circuit and the voltage output ON time of said second full-bridge circuit while making them equal,

wherein said control circuit performs control to satisfy the following expression:

P

=

V

x

·

V

y

2

⁢

π

⁢

⁢

L

⁢

⁢

ω

⁡

[

-

3

⁢

⁢

τ

2

+

4

⁢

(

τ

C

+

π

)

-

2

⁢

τ

C

2

-

2

⁢

πτ

C

-

π

2

]

where P is output power, τ is the voltage output ON time of said first full-bridge circuit and the voltage output ON time of said second full-bridge circuit, τ c is said polarity inversion period, V x is an input voltage of said first full-bridge circuit, V y is an input voltage of said second full-bridge circuit, and L is an inductance of said equivalent inductor, and

wherein in the above expression, τ and τ c are parameters indicating times and expressed in radian measure.

2. The DC-DC converter according to claim 1 , wherein

said control circuit changes the voltage output ON time of said first full-bridge circuit and the voltage output ON time of said second full-bridge circuit while making them different from each other.

3. The DC-DC converter according to claim 1 , wherein

said threshold current is set to make energy accumulated in said equivalent inductor greater than or equal to energy accumulated in two of said capacitors.

4. A DC-DC converter for performing soft switching and reducing switching losses, the DC-DC converter comprising:

a first full-bridge circuit including four switching elements;

a second full-bridge circuit including four switching elements;

a transformer including a first winding and a second winding, the first winding being connected to said first full-bridge circuit, and the second winding being connected to said second full-bridge circuit and magnetically coupled to said first winding;

an inductance component connected in series with said first winding or said second winding; and

a control circuit that controls soft switching of each switching element in said first full-bridge circuit and said second full-bridge circuit,

wherein each switching element of said first and second full-bridge circuits includes a capacitor,

wherein said capacitor is a parasitic capacitance or a combination of a parasitic capacitance and an actual element,

wherein an inductor current flowing through an equivalent inductor at a time of switching of turning on or off each switching element is greater than or equal to a threshold current, the equivalent inductor being equivalent to said transformer and said inductance component,

wherein said control circuit changes a voltage output ON time of said first full-bridge circuit and a voltage output ON time of said second full-bridge circuit by controlling a phase of each switching element of said first and second full-bridge circuits to output power from said DC-DC converter, while fixing a switching frequency of said first and second full-bridge circuits and keeping a polarity inversion period at a value greater than or equal to a fixed value, the polarity inversion period being a period in which an output of said first full-bridge circuit and an output of said second full-bridge circuit have reverse polarities,

wherein said control circuit performs control to satisfy the following expression:

P

=

V

X

·

V

y

2

⁢

π

⁢

⁢

L

⁢

⁢

ω

⁢

[

2

⁢

τ

C

2

-

2

⁢

τ

2

⁢

τ

C

-

2

⁢

τ

1

⁢

τ

C

+

2

⁢

πτ

C

+

τ

2

2

+

τ

1

⁢

τ

2

-

2

⁢

πτ

2

+

τ

1

2

-

2

⁢

πτ

1

+

π

2

]

where P is output power, τ 1 is the voltage output ON time of said first full-bridge circuit, τ 2 is the voltage output ON time of said second full-bridge circuit, τ c is said polarity inversion period, V x is an input voltage of said first full-bridge circuit, V y is an input voltage of said second full-bridge circuit, and L is an inductance of said equivalent inductor, and

wherein in the above expression, τ 1 , τ 2 , and τ c are parameters indicating times and expressed in radian measure.

5. A DC-DC converter for performing soft switching and reducing switching losses, the DC-DC converter comprising:

a first full-bridge circuit including four switching elements;

a second full-bridge circuit including four switching elements;

a transformer including a first winding and a second winding, the first winding being connected to said first full-bridge circuit, and the second winding being connected to said second full-bridge circuit and magnetically coupled to said first winding;

an inductance component connected in series with said first winding or said second winding; and

a control circuit that controls soft switching of each switching element in said first full-bridge circuit and said second full-bridge circuit,

wherein each switching element of said first and second full-bridge circuits includes a capacitor,

wherein said capacitor is a parasitic capacitance or a combination of a parasitic capacitance and an actual element,

wherein an inductor current flowing through an equivalent inductor at a time of switching of turning on or off each switching element is greater than or equal to a threshold current, the equivalent inductor being equivalent to said transformer and said inductance component,

wherein said control circuit changes a voltage output ON time of said first full-bridge circuit and a voltage output ON time of said second full-bridge circuit by controlling a phase of each switching element of said first and second full-bridge circuits to output power from said DC-DC converter, while fixing a switching frequency of said first and second full-bridge circuits and keeping a polarity inversion period at a value greater than or equal to a fixed value, the polarity inversion period being a period in which an output of said first full-bridge circuit and an output of said second full-bridge circuit have reverse polarities,

wherein said control circuit performs control to satisfy the following expression:

I ref =α·V x √(2 C/L )

where L ref is said threshold current, V x is an input voltage of said first full-bridge circuit, C is a capacitance of said capacitors, L is an inductance of said equivalent inductor, and α is a correction factor.