Insulation resistance detecting apparatus

1. An insulation resistance detecting apparatus which detects insulation resistance between a DC power supply and a ground of an external unit including said DC power supply, comprising:

a pulse generator that generates a rectangular pulse that has a constant period and a constant duty ratio;

a comparator to which a reference voltage is supplied as one input and an output from said pulse generator is supplied as the other input, said comparator outputting a signal of a first level when a level of said other input exceeds said reference voltage, and outputting a signal of a second level different from said first level when a level of said other input is lower than said reference voltage;

a first resistance provided in series on an output line of said pulse generator;

a coupling capacitor one end of which is connected to a line by which said first resistance is connected to said other input of said comparator and the other end of which is connected to an output line of said DC power supply;

a capacitor one end of which is connected to a portion on said line, said portion being located between said first resistance and said coupling capacitor, said line being connected to said other input of said comparator, and the other end of which is connected to said ground; and

a pulse width measuring device that measures a pulse width of an output waveform of said comparator to calculate a duty ratio of said output waveform and that calculates a value of said insulation resistance on the basis of said calculated duty ratio.

2. The insulation resistance detecting apparatus according to claim 1 , wherein a circuit in which a diode and a second resistance are connected in series is provided on an output line of said pulse generator in parallel with said first resistance.

3. The insulation resistance detecting apparatus according to claim 2 , wherein the value of said second resistance is smaller than the value of said first resistance.

4. The insulation resistance detecting apparatus according to claim 1 , wherein said comparator is a hysteresis circuit in which a part of an output from said comparator is positively fed back to said another input of said comparator.

5. The insulation resistance detecting apparatus according to claim 1 , wherein said pulse width measuring apparatus comprises:

a counter;

an edge detector that detects a rising edge and a falling edge of an output waveform of said comparator;

a first memory in which data indicating a correspondence relationship between a duty ratio of an output waveform of said comparator and a value of said insulation resistance is stored beforehand; and

a processor that controls said counter at a timing when said edge detector performs edge detection to obtain a pulse width of an output waveform of said comparator on the basis of the count value of said counter and that refers to data indicating a correspondence relationship stored in said first memory to obtain a value of said insulation resistance based on a duty ratio calculated from said obtained pulse width.

6. The insulation resistance detecting apparatus according to claim 5 , comprising a second memory in which insulation resistance values calculated by said processor are stored in time series;

wherein said processor determines that an insulation failure occurs when a change in the insulation resistance values in time-series insulation resistance value data stored in said second memory exceeds a predetermined value.

7. The insulation resistance detecting apparatus according to claim 5 , wherein the percentage of change in a duty ratio in a predetermined range of insulation resistance values in the correspondence data stored in said first memory is greater than the percentage of change in a duty ratio outside said predetermined range of insulation resistance values.