Earth fault detection circuit and power source device

1. An earth fault detection circuit for detecting an earth fault between a high-voltage battery and a motor, said earth fault detection circuit comprising:

an A.C. signal generation portion for generating an A.C. signal;

a first capacitive element provided between said A.C. signal generation portion and one end side of said high-voltage battery;

a voltage dividing circuit for dividing a voltage on said one end side of said high-voltage battery;

an earth fault detection portion for detecting the earth fault between said high-voltage battery and said motor based on an earth fault detection signal inputted thereto; and

a second capacitive element provided between said voltage dividing circuit and said earth fault detection portion, the second capacitive element being used for inputting to said earth fault detection portion as the earth fault detection signal an A.C. component of the voltage, on the one end side of said high-voltage battery, which is obtained by the voltage division in said voltage dividing circuit,

wherein said earth fault detection portion determines a level of a noise between said high-voltage battery and said motor based on a result of measurement of the earth fault detection signal made when the A.C. signal is not generated by said A.C. signal generation portion, and a result of measurement of the earth fault detection signal made when the A.C. signal is generated by said A.C. signal generation portion, thereby detecting the earth fault, and

wherein said earth fault detection portion compares a result of measurement of the earth fault detection signal made when no A.C. signal is generated by said A.C. signal generation portion, with a result of measurement of the earth fault detection signal made when the A.C. signal is generated by said A.C. signal generation portion, and determines the level of a noise between said high-voltage battery and said motor based on the comparison result.

2. The earth fault detection circuit according to claim 1 , wherein when said earth fault detection portion judges that the noise level is low when a predetermined comparison result is within a predetermined range, said earth fault detection portion judges presence or absence of the earth fault based on the result of the measurement of the earth fault detection signal made when the A.C. signal is generated by said A.C. signal generation portion; and

when said earth fault detection portion judges that the noise level is high when a predetermined comparison result is beyond the predetermined range, said earth fault detection portion postpones or stops detecting of the earth fault.

3. The earth fault detection circuit according to claim 1 , further comprising: an inverter for driving said motor which is connected between said high-voltage battery and said motor, and

wherein resistance values of resistors composing said voltage dividing circuit are set in such a way that the voltage of the earth fault detection signal provided when a noise from said inverter between said high-voltage battery and said motor is superimposed on the earth fault detection signal falls within a predetermined voltage range.

4. The earth fault detection circuit according to claim 1 , further comprising: a total voltage detection circuit for detecting voltage of said high-voltage battery by using said voltage dividing circuit.

5. A power source device, comprising:

the earth fault detection circuit according to claim 1 ; and

a battery controller for managing said high-voltage battery as an assembled battery composed of plural battery cells.

6. The power source device according to claim 5 , further comprising: a relay for switching a state of connection between said high-voltage battery, and a load driven by said high-voltage battery,

wherein said battery controller carries out interruption between said high-voltage battery and said load by controlling said relay in response to the earth fault being detected by said earth fault detection portion.