DC-DC converter
The DC-DC converter has a configuration in which a first full-bridge circuit and a second full-bridge circuit are connected via a transformer. A control circuit controls soft switching of each switching element. An inductor current flowing through the transformer or an equivalent inductor equivalent to the transformer at a time of switching of turning on or off each switching element is greater than or equal to a threshold current. When the first full-bridge circuit and the second full-bridge circuit have different output voltages V 1 and V 2 , the control circuit causes the inductor current at start times t 4 and t 8 of a polarity inversion period to approach the inductor current at end times t 5 and t 9 , the polarity inversion period being a period in which V 1 and V 2 have reverse polarities. This suppresses an increase in loss resulting from a flow of large current and enables ZVS control.
1. A DC-DC converter comprising:
a first full-bridge circuit including four switching elements that include a capacitor serving as a parasitic capacitance or an external parallel-connected capacitor;
a second full-bridge circuit including four switching elements that include a capacitor serving as a parasitic capacitance or an external parallel-connected capacitor;
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 each of said first full-bridge circuit and said second full-bridge circuit,
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 that is not zero, the equivalent inductor being equivalent to said transformer and said inductance component, and
wherein when said first full-bridge circuit and said second full-bridge circuit have different output voltages, said control circuit controls a value of said inductor current at a start time of a polarity inversion period to approach a value of said inductor current at an end time of said polarity inversion period, 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.
2. The DC-DC converter according to claim 1 , wherein
said control circuit adjusts a voltage output period of said first full-bridge circuit in accordance with an input voltage of said first full-bridge circuit, and adjusts a voltage output period of said second full-bridge circuit in accordance with an input voltage of said second full-bridge circuit.
3. The DC-DC converter according to claim 2 , wherein said control circuit adjusts τ 1 and τ 2 to satisfy:
(τ 2 −τ c ) Vy =(τ 1 −τ c ) Vx
where τ c is the polarity inversion period in which the output of said first full-bridge circuit and the output of said second full-bridge circuit have reverse polarities, τ 1 is the voltage output period of said first full bridge circuit, τ 2 is the voltage output period of said second full bridge circuit, Vx is the input voltage of the first full-bridge circuit, and Vy is the input voltage of the second full-bridge circuit,
τ 1 , τ 2 , and τ c being expressed in radian notation.
4. The DC-DC converter according to claim 1 , wherein
said threshold current is set such that energy accumulated in said equivalent inductor becomes greater than or equal to energy accumulated in two of said capacitors.
5. The DC-DC converter according to claim 4 , wherein
the following expression is satisfied:
I ref =α·Vx √(2 C/L )
where I ref is said threshold current, Vx is the 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.