Non-isolated charger with bi-polar inputs

1. An uninterruptable power supply, comprising:

a positive DC bus, a neutral DC bus, and a negative DC bus;

a battery charger circuit having an inductor, a first charger output and a second charger output;

a first switch connected to a first end of the inductor and configured to selectively couple the positive DC bus with the first charger output; and

a second switch connected to a second end of the inductor and configured to selectively couple the negative DC bus with the inductor;

wherein the neutral DC bus is directly coupled to the second charger output; and

wherein the battery charger circuit is configured to draw power from at least one of the positive DC bus and the negative DC bus to charge a battery directly coupled to the first charger output and the second charger output.

2. The uninterruptible power supply of claim 1 , comprising:

a control module configured to switch the first switch and the second switch in unison.

3. The uninterruptable power supply of claim 1 , comprising:

a control module configured to direct the first switch to repeatedly couple and decouple the positive DC bus with the first charger output during a first time period.

4. The uninterruptable power supply of claim 3 , wherein the control module is configured to direct the second switch to repeatedly couple and decouple the negative DC bus with the inductor during a second time period.

5. The uninterruptable power supply of claim 4 , wherein the control module is configured to direct the second switch to repeatedly couple and decouple the negative DC bus with the inductor during a time period that is subsequent to the first time period.

6. The uninterruptable power supply of claim 4 , wherein the first time period and the second time period are each between 8 and 12 milliseconds.

7. The uninterruptable power supply of claim 1 , wherein:

the first switch is configured to pass current intermittently from the positive DC bus during a first continuous time period; and

the second switch is configured to pass current intermittently from the neutral DC bus during a second continuous time period.

8. The uninterruptable power supply of claim 7 , wherein the first continuous time period and the second continuous time period at least partially overlap.

9. The uninterruptable power supply of claim 1 , wherein the battery charger circuit is configured to concurrently charge the battery with current from the positive DC bus and from the negative DC bus.

10. The uninterruptable power supply of claim 1 , further comprising:

a DC power source coupled to at least one of the positive DC bus, the neutral DC bus, and the negative DC bus.

11. The uninterruptable power supply of claim 1 , further comprising:

a control module configured to drive an inductor current of the inductor toward a value between an upper current threshold and a lower current threshold.

12. The uninterruptable power supply of claim 11 , wherein the lower current threshold is 0 amperes.

13. The uninterruptable power supply of claim 11 , wherein:

the battery charger circuit includes a current transformer and a resistor; and

the control module is configured to sample at least one of a transformer voltage and a resistor voltage to determine a value of the inductor current.

14. The uninterruptable power supply of claim 1 , wherein the battery charger circuit is configured such that the battery draws a substantially equal amount of power from each of the positive DC bus and the negative DC bus.

15. A method for charging a battery of an uninterruptable power supply having a positive DC bus, a neutral DC bus, and a negative DC bus, comprising:

coupling:

a first charger output of a battery charger circuit with the positive DC bus; and

an inductor of the battery charger circuit with the negative DC bus;

coupling a second charger output of the battery charger circuit directly with the neutral DC bus; and

applying current from at least one of the positive DC bus and the neutral DC bus through the inductor to the battery, wherein the battery is directly coupled to the first charger output and the second charger output.

16. The method of claim 15 , comprising:

performing a first switching operation at a first end of the inductor to couple the positive DC bus with the first battery output; and

performing a second switching operation at a second end of the inductor to couple the inductor with the negative DC bus.

17. The method of claim 16 , comprising:

performing the first switching operation and the second switching operation in unison.

18. The method of claim 16 , comprising:

concurrently coupling the positive DC bus with the first battery output, and the negative DC bus with the inductor.

19. The method of claim 16 , comprising:

performing the first switching operation to repeatedly couple and decouple the positive DC bus with the first charger output during a first time period; and

performing the second switching operation to repeatedly couple and decouple the negative DC bus with the inductor during a second time period.

20. The method of claim 16 , comprising:

generating an upper inductor current threshold and a lower inductor current threshold;

controlling an inductor current of the inductor to a value between the upper current threshold and the lower current threshold.

21. The method of claim 19 , comprising:

sampling at least one of a battery charger circuit transformer voltage and a battery charger circuit resistor voltage to determine a value of the inductor current.

22. The method of claim 19 , comprising:

adjusting at least one of the upper current threshold and the lower current threshold based at least in part on a voltage of the battery.

23. The method of claim 15 , comprising:

drawing a substantially equal amount of power from each of the positive DC bus and the negative DC bus through the battery charger circuit to charge the battery.

24. An uninterruptable power supply, comprising:

a positive DC bus, a neutral DC bus, and a negative DC bus;

a battery charger circuit having an inductor, a first charger output, and a second charger output;

means for selectively coupling:

the first charger output with the positive DC bus; and

the inductor with the negative DC bus;

the second charger output directly coupled to the neutral DC bus; and

the battery charger circuit configured to pass current from at least one of the positive DC bus and the neutral DC bus through the inductor to charge a battery directly coupled to the first charger output and the second charger output.