Power supply circuit and power conversion device

1. A power conversion device comprising:

an inverter circuit which converts DC power into AC power and which includes a plurality of switching devices constituting an upper arm and a lower arm;

a control circuit which controls the plurality of switching devices;

a drive circuit which drives the plurality of switching devices based on a signal from the control circuit; and

a power supply circuit which supplies power to the drive circuit; wherein:

the control circuit controls a power supply voltage to be outputted from the power supply circuit to the drive circuit;

the drive circuit drives the plurality of switching devices based on a carrier frequency and the power supply voltage;

the power supply circuit includes a transformer and a feedback output circuit, the transformer including a primary coil to which a voltage is supplied from a battery, and a plurality of secondary coils to which voltages are supplied through the primary coil respectively;

among the secondary coils, a first secondary coil outputs a voltage to the drive circuit;

among the secondary coils, a second secondary coil outputs a voltage to the feedback output circuit; and

the feedback output circuit includes a dummy load circuit which controls the voltage to be outputted to the primary coil based on a change of the carrier frequency.

2. A power conversion device according to claim 1 , wherein:

the inverter circuit includes U-phase, V-phase and W-phase circuits being series circuits in each of which the upper arm and the lower arm are connected in series;

the drive circuit has a plurality of gate drive circuits corresponding to the upper arm and the lower arm forming the U-phase, V-phase and W-phase circuits, respectively;

the dummy load circuit includes a first switch unit, a second switch unit, a capacitor and a resistor;

the first switch unit and the second switch unit are connected in series to form a series circuit, the first switch unit being disposed on a higher potential side than the second switch unit;

the capacitor is connected in parallel with the second switch unit; and

the resistor is connected in parallel with the series circuit.

3. A power conversion device according to claim 2 , wherein:

capacitance of the capacitor is substantially equal to capacitance of a capacitor connected in parallel between a gate and an emitter of one of the switching devices forming the inverter circuit.

4. A power conversion device according to claim 2 , wherein:

a resistance value of the resistor is equal to a resistance value between a positive electrode connection point of each gate drive IC belonging to the drive circuit and a negative electrode connection point of the gate drive IC.

5. A power conversion device according to claim 3 , wherein:

a resistance value of the resistor is equal to a resistance value between a positive electrode connection point of each gate drive IC belonging to the drive circuit and a negative electrode connection point of the gate drive IC.

6. A power conversion device according to claim 1 , wherein:

the second secondary coil has a smaller number of turns than the primary coil.

7. A power conversion device according to claim 2 , wherein:

the second secondary coil has a smaller number of turns than the primary coil.

8. A power conversion device according to claim 1 , wherein:

a ratio between the number of turns of the primary coil and the number of turns of the second secondary coil is equal to a ratio between the voltage of the battery and the voltage for driving the switching devices.

9. A power conversion device according to claim 2 , wherein:

a ratio between the number of turns of the primary coil and the number of turns of the second secondary coil is equal to a ratio between the voltage of the battery and the voltage for driving the switching devices.

10. A power conversion device according to claim 1 , wherein:

the dummy load circuit includes a plurality of switching circuits and a first resistor, each of the switching circuits including a resistor and a switching device connected in series;

the first resistor is connected in parallel with the switching circuits; and

the control circuit changes the number of switching circuits in which electric conduction should be secured, in accordance with a change of the carrier frequency.

11. A power conversion device according to claim 10 , wherein:

the plurality of switching circuits include a first switching circuit, a second switching circuit and a third switching circuit;

the control circuit secures electric conduction in the first switching circuit when the carrier frequency is not lower than a first predetermined value, secures electric conduction in the first switching circuit and the second switching circuit when the carrier frequency is not lower than a second predetermined value, and secures electric conduction in the first switching circuit, the second switching circuit and the third switching circuit when the carrier frequency is not lower than a third predetermined value; and

the first predetermined value is smaller than the second predetermined value, and the second predetermined value is smaller than the third predetermined value.

12. A power conversion device according to claim 2 , wherein:

the feedback output circuit includes a plurality of dummy load circuits corresponding to the U-phase circuit, the V-phase circuit and the W-phase circuit, respectively.

13. A power conversion device according to claim 12 , wherein:

information of the carrier frequency is outputted from the control circuit.