Insulation detecting device

1. An insulation detecting device comprising:

a flying capacitor configured to hold a charged voltage; and

a measurement and calculation unit configured to measure the charged voltage of the flying capacitor, and calculate a ground fault resistance formed between a direct-current power supply of a vehicle electrically insulated from a ground, and the ground, based on the measured voltage, wherein

the flying capacitor includes one or a plurality of first capacitors, one or a plurality of second capacitors connected with the first capacitor in parallel, and a parallel cancellation switch arranged between the first capacitor and the second capacitor, and which performs parallel connection, and cancellation of the parallel connection, between the first capacitor and the second capacitor, and

a capacitance of the flying capacitor is variably controlled by turning on or turning off of the parallel cancellation switch,

wherein the first capacitor has a first capacitance and the second capacitor has a second capacitance greater than the first capacitance,

wherein the insulation detecting device, in the case of a higher speed measurement mode including at the start of the vehicle when the parallel cancellation switch is turned off, monitors an output voltage of the direct current power supply and the ground fault resistance, and

wherein the capacitance of the flying capacitor at a time of the cancellation of the parallel connection between the first capacitor and the second capacitor is less than the capacitance of the second capacitor.

2. The insulation detecting device according to claim 1 , further comprising:

a first switch arranged between one end of the flying capacitor and a positive electrode of the direct-current power supply;

a second switch arranged between the other end of the flying capacitor and a negative electrode of the direct-current power supply;

a third switch arranged between the one end of the flying capacitor and the ground; and

a fourth switch arranged between the other end of the flying capacitor and the ground, wherein

the charged voltages of the flying capacitor charged with a first path, a second path, and a third path are measured by the measurement and calculation unit with a fourth path, the first path being formed by turning on of only the first and the second switches, the second path being formed by turning on of only the first and the fourth switches, the third path being formed by turning on of only the second and the third switches, and the forth path being formed by turning on of only the third and the fourth switches, and

the measurement and calculation unit calculates the ground fault resistance, based on the measured voltages of the charged voltages respectively charged in the flying capacitor in the first to the third paths.

3. The insulation detecting device according to claim 2 , wherein

the capacitance of the flying capacitor at a time of the cancellation of the parallel connection between the first capacitor and the second capacitor is less than half the capacitance of the flying capacitor at a time of the parallel connection between the first capacitor and the second capacitor.

4. The insulation detecting device according to claim 2 , further comprising:

a measurement mode configured to perform, in a state where the connection cancellation switch is kept off, full charge to the flying capacitor composed of only the first capacitor with the first path formed by turning on of the first and the second switches, measurement of the charged voltage of the flying capacitor fully charged with the fourth path formed by turning on of the third and the fourth switches, and calculation of an output voltage of the direct-current power supply, based on the measured voltage at the full charge measured by the measurement and calculation unit.

5. The insulation detecting device according to claim 3 , further comprising:

a measurement mode configured to perform, in a state where the connection cancellation switch is kept off, full charge to the flying capacitor composed of only the first capacitor with the first path formed by turning on of the first and the second switches, measurement of the charged voltage of the flying capacitor fully charged with the fourth path formed by turning on of the third and the fourth switches, and calculation of an output voltage of the direct-current power supply, based on the measured voltage at the full charge measured by the measurement and calculation unit.

6. The insulation detecting device according to claim 1 , wherein

the capacitance of the flying capacitor at a time of the cancellation of the parallel connection between the first capacitor and the second capacitor is less than half the capacitance of the flying capacitor at a time of the parallel connection between the first capacitor and the second capacitor.

7. The insulation detecting device according to claim 1 , wherein

the one or a plurality of first capacitors comprises a plurality of first capacitors, and

the plurality of first capacitors has a combined capacitance less than the second capacitance of the second capacitor.

8. The insulation detecting device according to claim 7 , wherein

the one or a plurality of second capacitors comprises a plurality of second capacitors, and

the plurality of second capacitors has a combined capacitance greater than the combined capacitance of the plurality of first capacitors.