Balancing multilevel DC-DC converter apparatus

1. An apparatus comprising:

a string of capacitors comprising at least two capacitors coupled in series;

a switching circuit comprising a first port having first and second terminals connected to a first node and a second end node, respectively, of the string of capacitors and a second port configured to be coupled to an energy storage device, the switching circuit configured to selectively connect first and second terminals of the second port to a first end node, a second end node, and at least one interconnection node of the string of capacitors;

at least one inductor configured to be coupled in series with the second port of the switching circuit and the energy storage device;

a charging switch configured to directly connect the first terminal of the second port to the second terminal of the second port; and

a control circuit configured to close the charging switch to charge the at least one inductor from the energy storage device in first intervals and to selectively close at least two switches of the switching circuit in second intervals to selectively charge the capacitors of the string of capacitors from the charged at least one inductor.

2. The apparatus of claim 1 :

wherein the switching circuit comprises at least one first switch configured to connect the first terminal of the first port to a first terminal of the second port, at least one second switch configured to connect the second terminal of the first port to a second terminal of the second port, and at least two third switches configured to connect the first and second terminals of the second port to the at least one interconnection node; and

wherein the control circuit configured to selectively close the at least two third switches in the second intervals to selectively charge the capacitors of the string of capacitors from the charged at least one inductor.

3. The apparatus of claim 2 , wherein at least two third switches comprise respective switches configured to connect respective ones of the first and second terminals of the second port to a midpoint interconnection node of the string of capacitors.

4. The apparatus of claim 2 , wherein the at least two third switches comprise:

a first plurality of switches configured to connect the first terminal of the second port to a first set of interconnection nodes of the string of capacitors; and

a second plurality of switches configured to connect the second terminal of the second port to a second set of interconnection nodes of the string of capacitors.

5. The apparatus of claim 4 , wherein the first and second sets of interconnection nodes each comprise a midpoint interconnection node.

6. The apparatus of claim 2 , wherein the control circuit is configured to sense at least one voltage across at least one of the capacitors of the string of capacitors and to selectively close the at least two third switches in the second intervals responsive to the sensed at least one voltage.

7. The apparatus of claim 6 , wherein the control circuit is configured to selectively close the at least two third switches in the second intervals responsive to the sensed at least one voltage to balance voltages across the capacitors of the string of capacitors.

8. The apparatus of claim 2 , wherein the at least two third switches comprise a first transistor having a first voltage rating and wherein the charging switch comprises a second transistor having a second voltage rating greater than the first voltage rating.

9. The apparatus of claim 1 , further comprising an inverter coupled to the first port of the switching circuit.

10. An apparatus comprising:

a string of capacitors comprising at least two capacitors coupled in series;

a switching circuit comprising:

a first port having first and second terminals connected to respective first and second end nodes of the string of capacitors;

a second port configured to be coupled to an energy storage device;

at least one first switch configured to connect the first terminal of the first port to a first terminal of the second port;

at least one second switch configured to connect the second terminal of the first port to a second terminal of the second port;

at least one third switch configured to connect the first terminal of the second port to a first interconnection node of the string of capacitors;

at least one fourth switch configured to connect the second terminal of the second port to a second interconnection node of the string of capacitors;

at least one fifth switch configured to connect the first terminal of the second port to a midpoint interconnection node of the string of capacitors; and

at least one sixth switch configured to connect the second terminal of the second port to the midpoint interconnection node;

at least one inductor configured to be coupled in series with the second port of the switching circuit and the energy storage device; and

a control circuit configured to cause the switching circuit to charge the at least one inductor in first intervals and to selectively close the at least one third switch, the at least one fourth switch, the at least one fifth switch, and the at least one sixth switch in second intervals to selectively charge the capacitors of the string of capacitors from the charged at least one inductor, wherein the control circuit is configured to sense at least one voltage across at least one of the capacitors of the string of capacitors and to selectively close the at least one third switch, the at least one fourth switch, the at least one fifth switch, and the at least one sixth switch in the second intervals responsive to the sensed at least one voltage.

11. The apparatus of claim 10 , wherein the control circuit is configured to close the at least one third switch and the at least one fourth switch to charge inner capacitors of the string of capacitors, to close the at least one fifth switch to charge a first outer capacitor of the string of capacitors, and to close the at least one sixth switch to charge a second outer capacitor of the string of capacitors.

12. The apparatus of claim 10 , wherein the control circuit is configured to close the at least one fifth switch and the at least one sixth switch in the first intervals to charge the at least one inductor.

13. The apparatus of claim 10 , wherein the switching circuit further comprises at least one seventh switch configured to directly connect the first terminal of the second port to the second terminal of the second port, and wherein the control circuit is configured to close the at least one seventh switch in the first intervals to charge the at least one inductor.

14. The apparatus of claim 10 , wherein the at least one third switch comprises two series-connected transistors and wherein the at least one fourth switch comprises two series-connected transistors.

15. An apparatus comprising:

a string of capacitors comprising at least two capacitors coupled in series;

a switching circuit comprising:

a first port having first and second terminals connected to respective first and second end nodes of the string of capacitors;

a second port configured to be coupled to an energy storage device;

at least one first switch configured to connect the first terminal of the first port to a first terminal of the second port;

at least one second switch configured to connect the second terminal of the first port to a second terminal of the second port;

at least one third switch configured to connect the first terminal of the second port to a first interconnection node of the string of capacitors;

at least one fourth switch configured to connect the second terminal of the second port to a second interconnection node of the string of capacitors;

at least one fifth switch configured to connect the at least one third switch to a midpoint interconnection node of the string of capacitors; and

at least one sixth switch configured to connect the at least one fourth switch to the midpoint interconnection node;

at least one inductor configured to be coupled in series with the second port of the switching circuit and the energy storage device; and

a control circuit configured to cause the switching circuit to charge the at least one inductor in first intervals and to selectively close the at least one third switch, the at least one fourth switch, the at least one fifth switch, and the at least one sixth switch in second intervals to selectively charge the capacitors of the string of capacitors from the charged at least one inductor.

16. The apparatus of claim 15 , wherein the at least one third switch is configured to connect the first terminal of the second port to the first interconnection node via a first diode and wherein the at least one fourth switch is configured to connect the second terminal of the second port to the second interconnection node via a second diode.

17. The apparatus of claim 15 , wherein the control circuit is configured to close the at least one third switch, the at least one fourth switch, the at least one fifth switch, and the at least one sixth switch in the first intervals to charge the at least one inductor.

18. The apparatus of claim 15 , wherein the switching circuit comprises at least one seventh switch configured to directly connect the first port of the second terminal to the second terminal of the second port, and wherein the control circuit is configured to close the at least one seventh switch in the first intervals to charge the at least one inductor.

19. The apparatus of claim 15 , wherein the control circuit is configured to sense at least one voltage across at least one of the capacitors of the string of capacitors and to selectively close the at least one third switch, the at least one fourth switch, the at least one fifth switch, and the at least one sixth switch in the second intervals responsive to the sensed at least one voltage.