Bus leakage resistance estimation for electrical isolation testing and diagnostics

1. An electrified vehicle comprising:

a positive bus connectable to a positive output of a DC power source;

a negative bus connectable to a negative output of the DC power source;

a chassis ground distributed within the vehicle;

a first detector circuit comprising a first limiting resistance and a first sense resistance selectably connected between the positive bus and the chassis ground in order to sense a resulting first current flowing through a negative bus leakage resistance and a balanced leakage resistance;

a second detector circuit comprising a second limiting resistance and a second sense resistance selectably connected between the negative bus and the chassis ground in order to sense a resulting second current flowing through a positive bus leakage resistance and the balanced leakage resistance; and

a control circuit identifying the positive and negative bus leakage resistances in response to respective ratios of the first and second currents.

2. The vehicle of claim 1 wherein the detector circuits include respective sampling switches closed by the control circuit.

3. The vehicle of claim 1 wherein the control circuit is configured to disconnect the positive and negative buses from the DC power source when an isolation value is less than a threshold, the isolation value being determined using a smaller one of the positive and negative bus leakage resistances.

4. The vehicle of claim 1 wherein the control circuit is configured to detect a fault condition when an isolation value is less than a threshold, the isolation value being determined using a smaller one of the positive and negative bus leakage resistances.

5. The vehicle of claim 1 wherein the positive bus leakage resistance is identified according a formula:

R

lp

=

V

b

I

RSP

-

R

C

⁡

(

1

+

I

RSM

I

RSP

)

where R lp is the positive bus leakage resistance, V b is a voltage from the DC power source, I RSP is the second current, I RSM is the first current, and R C is the combined sense circuit resistance.

6. The vehicle of claim 1 wherein the negative bus leakage resistance is identified according a formula:

R

lm

=

V

b

I

RSM

-

R

C

⁡

(

1

+

I

RSP

I

RSM

)

where R lm is the negative bus leakage resistance, V b is a voltage from the DC power source, I RSM is the first current, I RSP is the second current, and R C is the combined sense circuit resistance.

7. The vehicle of claim 1 wherein the control circuit identifies a balanced leakage resistance as a higher one of the positive and negative bus leakage resistances, and wherein the control circuit monitors changes over time of the balanced leakage resistance to identify a potential malfunction of the vehicle.

8. The vehicle of claim 1 wherein the control circuit identifies an unbalanced resistance according to a formula:

R

unbal

=



R

lp

·

R

lm

R

lp

-

R

lm



where R lp and R lm are the positive and negative bus leakage resistances, respectively, and wherein the control circuit monitors changes over time of the unbalanced resistance to identify a potential malfunction of the vehicle.

9. A method of detecting bus isolation for an electrified vehicle, the vehicle having a DC power source connectable to a positive bus and a negative bus and a chassis ground distributed within the vehicle, the method comprising the steps of:

connecting a first fixed resistance between the positive bus and chassis ground;

sensing a resulting first current flowing through a negative bus leakage resistance and a balanced leakage resistance;

connecting a second fixed resistance between the negative bus and chassis ground;

sensing a resulting second current flowing through a positive bus leakage resistance and a balanced leakage resistance;

estimating the positive and negative bus leakage resistances in response to respective ratios of the first and second currents;

comparing an isolation value to a threshold, wherein the isolation value is responsive to a voltage of the DC power source and a smaller one of the positive and negative bus leakage resistances; and

signaling an atypical isolation when the isolation value is less than the threshold.

10. The method of claim 9 wherein the signaling of an atypical isolation includes disconnecting the DC power source from the positive and negative buses.

11. The method of claim 9 further comprising the step of measuring the voltage of the DC power source, wherein the measured voltage is used in determining the isolation value.

12. The method of claim 9 wherein the voltage used in determining the isolation value is comprised of a predetermined constant.

13. The method of claim 9 wherein the positive bus leakage resistance is estimated according a formula:

R

lp

=

V

b

I

RSP

-

R

C

⁡

(

1

+

I

RSM

I

RSP

)

where R lp is the positive bus leakage resistance, V b is a measured voltage from the DC power source, I RSP is the second current, I RS is the first current, and R C is the combined sense circuit resistance.

14. The method of claim 9 wherein the negative bus leakage resistance is estimated according a formula:

R

lm

=

V

b

I

RSM

-

R

C

⁡

(

1

+

I

RSP

I

RSM

)

where R lm is the negative bus leakage resistance, V b is a measured voltage from the DC power source, I RSM is the first current, I RSP is the second current, and R C is the combined sense circuit resistance.

15. An electrified vehicle comprising:

a positive bus connectable to a positive output of a DC power source;

a negative bus connectable to a negative output of the DC power source;

a chassis ground distributed within the vehicle;

a first detector circuit comprising a first limiting resistance and a first sense resistance selectably connected between the positive bus and the chassis ground in order to sense a resulting first current flowing through a negative bus leakage resistance and a balanced leakage resistance;

a second detector circuit comprising a second limiting resistance and a second sense resistance selectably connected between the negative bus and the chassis ground in order to sense a resulting second current flowing through a positive bus leakage resistance and the balanced leakage resistance; and

a control circuit identifying the positive and negative bus leakage resistances in response to respective ratios of the first and second currents, wherein the control circuit identifies a balanced resistance as a higher one of the positive and negative bus leakage resistances, and wherein the control circuit monitors changes over time of the balanced resistance to identify a potential malfunction of the vehicle;

wherein the DC power source is a fuel cell with a deionizer, and wherein the monitoring of changes in the balanced resistance is comprised of comparing the balanced resistance with a threshold to detect a malfunction of the deionizer.