Balancing and/or discharge resistor arrangements

1. A sub-module for use in a multilevel modular converter, the sub-module comprising:

an energy storage device;

a resistor; and

a switch arranged in series with the resistor, the switch comprising a series of semiconductor switching elements and a potential divider, the series of semiconductor switching elements comprising a first switching element and a second switching element;

the first switching element comprising a first gate terminal that is configured to control a state between a first source terminal and a first drain terminal;

the second switching element comprising a second gate terminal that is configured to control a state between a second source terminal and a second drain terminal;

the potential divider connecting an output of the first switching element to the second gate terminal of the second switching element such that a proportion of a drain-source voltage of the first switching element is configured to be provided as a gate voltage to the second gate terminal; and

wherein the energy storage device is arranged in parallel with the switch and the resistor.

2. The sub-module of claim 1 , wherein the sub-module is configured to be controlled by a controller, and the resistor draws a current which exceeds that drawn by the controller.

3. The sub-module of claim 1 , wherein a change in a switching state of the first switching element provides a switching signal for the second switching element.

4. The sub-module of claim 3 , wherein the potential divider is arranged to divide the drain-source voltage of the first switching element.

5. The sub-module of claim 4 , wherein the potential divider comprises a first resistance and a second resistance.

6. The sub-module of claim 5 , wherein the potential divider comprises a first capacitance and a second capacitance in association with the first and second resistances, respectively.

7. The sub-module of claim 6 , wherein a ratio of the first and second capacitances is approximately equal to the reciprocal of the ratio of the first and second resistances.

8. The sub-module of claim 4 , wherein the potential divider comprises a gate resistor and a source resistor.

9. The sub-module of claim 8 , wherein the gate resistor and the source resistor are configured to supply a proportion of the drain-source voltage of the first switching element to the second gate terminal of the second switching element.

10. The sub-module of claim 8 , wherein the potential divider further comprises a gate capacitor and a source capacitor.

11. The sub-module of claim 8 , wherein the gate resistor and the source resistor are configured to balance the voltage sharing of the first switching element and the second switching element.

12. The sub-module of claim 8 , wherein the gate resistor and the source resistor are configured to provide static sharing.

13. The sub-module of claim 1 , wherein the potential divider is configured such that when the drain-source voltage of the first switching element reaches a pre-determined value, the gate voltage of the second switching element reaches a turn-off threshold.

14. The sub-module of claim 1 , wherein the first gate terminal is controlled by a bias voltage from a controller.

15. The sub-module of claim 14 , wherein the switch is on when the bias voltage is zero.

16. The sub-module of claim 1 , wherein the switch is a normally on switch.

17. The sub-module of claim 1 , wherein the potential divider couples the first switching element to the second switching element.

18. The sub-module of claim 1 , wherein at least one of the series of semiconductor switching elements comprises a JFET or a MOSFET.

19. The sub-module of claim 1 ,

wherein the potential divider is a first potential divider and the switch further comprises a second potential divider;

the series of semiconductor switching elements further comprising a third switching element, the third switching element comprising a third gate terminal that is configured to control a state between a third source terminal and a third drain terminal; and

the second potential divider connecting an output of the second switching element to the third gate terminal of the third switching element such that a proportion of a drain-source voltage of the second switching element is configured to be provided as a gate voltage to the third gate terminal.

20. The sub-module of claim 19 ,

the first potential divider comprising a first gate resistor, a first source resistor, a first gate capacitor, and a first source capacitor;

the second potential divider comprising a second gate resistor, a second source resistor, a second gate capacitor, and a second source capacitor; and

wherein the first source capacitor has a greater capacitance than the second gate capacitor.