Power conversion device

1. A power conversion device comprising:

a rectification circuit having a plurality of diodes with a full-bridge configuration and rectifying input from an AC power supply;

a reactor connected to a positive DC terminal of the rectification circuit;

an inverter circuit in which a first leg having a first switching element and a second switching element connected in series to each other with a first connection point therebetween as a first AC end, a second leg having a third switching element and a fourth switching element connected in series to each other with a second connection point therebetween as a second AC end, and a DC capacitor are connected in parallel between DC buses, only the first AC end of the first and second AC ends being connected to the positive DC terminal of the rectification circuit, a negative one of the DC buses being connected to a negative DC terminal of the rectification circuit;

an isolation transformer having a primary winding and a secondary winding, the first AC end and the second AC end of the inverter circuit being connected to respective ends of the primary winding; and

a control circuit configured to perform output control of the inverter circuit, wherein the control circuit performs power factor control of circuit current flowing from the AC power supply via the rectification circuit, by PWM control for only the first leg of the first and second legs, and controls voltage of the DC capacitor by PWM control for the second leg using duty cycles of the third and fourth switching elements equal to or smaller than both duty cycles of the first and second switching elements in the first leg, thereby controlling power outputted to a secondary side of the isolation transformer, and

the control circuit limits the duty cycles of the third switching element and the fourth switching element using, as an upper limit, a smaller one of the duty cycles of the first and second switching elements.

2. The power conversion device according to claim 1 , further comprising:

a second rectification circuit connected to the secondary winding of the isolation transformer; and

a second reactor connected to a DC terminal of the second rectification circuit, wherein

the control circuit controls output power to a DC circuit connected to the second rectification circuit, by performing PWM control for the second leg.

3. The power conversion device according to claim 2 , wherein the control circuit

performs PWM control for the first leg and the second leg with equal drive cycles synchronized with each other,

controls the first leg so that the circuit current becomes target sinewave current, and

controls the second leg so that the voltage of the DC capacitor becomes target voltage higher than peak voltage of the AC power supply.

4. The power conversion device according to claim 1 , wherein the control circuit

performs PWM control for the first leg and the second leg with equal drive cycles synchronized with each other,

controls the first leg so that the circuit current becomes target sinewave current, and

controls the second leg so that the voltage of the DC capacitor becomes target voltage higher than peak voltage of the AC power supply.

5. The power conversion device according to claim 4 , wherein the control circuit

controls the duty cycles of the third switching element and the fourth switching element to be equal to each other.

6. The power conversion device according to claim 5 , wherein the control circuit

controls the second switching element so that ON and OFF thereof are inverted from those of the first switching element,

controls the fourth switching element which is diagonal with respect to the first switching element, so as to have an ON period only within an ON period of the first switching element, and

controls the third switching element which is diagonal with respect to the second switching element, so as to have an ON period only within an ON period of the second switching element.

7. The power conversion device according to claim 4 , wherein the control circuit

controls the second switching element so that ON and OFF thereof are inverted from those of the first switching element,

controls the fourth switching element which is diagonal with respect to the first switching element, so as to have an ON period only within an ON period of the first switching element, and

controls the third switching element which is diagonal with respect to the second switching element, so as to have an ON period only within an ON period of the second switching element.

8. The power conversion device according to claim 7 , wherein the control circuit

uses a saw-tooth wave as a carrier wave of PWM control for the first leg and the second leg,

synchronizes rising of a gate signal pulse for the first switching element and rising of a gate signal pulse for the fourth switching element, and

synchronizes rising of a gate signal pulse for the second switching element and rising of a gate signal pulse for the third switching element.

9. The power conversion device according to claim 7 , wherein the control circuit

uses a triangular wave as a carrier wave of PWM control for the first leg and the second leg,

synchronizes a center of a gate signal pulse for the first switching element and a center of a gate signal pulse for the fourth switching element, and

synchronizes a center of a gate signal pulse for the second switching element and a center of a gate signal pulse for the third switching element.

10. The power conversion device according to claim 9 , wherein the control circuit

generates a command value of a duty cycle for one element of the third and fourth switching elements,

generates the gate signal pulse for the one element by comparison between the command value of the duty cycle, and the triangular wave, and

generates the gate signal pulse for another element of the third and fourth switching elements by comparison between a value obtained by subtracting the command value of the duty cycle from 1, and the triangular wave.