Method for controlling a converter

1. A method of controlling a converter, including a resonant circuit having an inductor in which a direction of a current through the inductor is dependent on a state of at least a first control switch and a second control switch, the method comprising:

a. switching the first control switch into a first state upon occurrence of a first zero crossing event, wherein the first zero crossing event occurs when a voltage across the first control switch reaches zero;

b. switching the first control switch into a second state in response to expiration of a first predetermined time interval, wherein the first predetermined time interval is triggered to begin upon the second control switch switching to the second state;

c. switching the second control switch into the first state upon occurrence of a second zero crossing event, wherein the second zero crossing event occurs when a voltage across the second control switch reaches zero; and

d. switching the second control switch into the second state in response to expiration of a second predetermined time interval, wherein the second predetermined time interval is triggered to begin upon the first control switch switching to the second state.

2. The method as claimed in claim 1 , wherein the first state is an on state and the second state is an off state.

3. The method as claimed in claim 1 , wherein the first state is an off state and the second state is an on state.

4. The method as claimed in claim 1 , wherein the predetermined time interval is the same as half the desired period of operation of the converter.

5. The method as claimed in claim 1 , wherein the predetermined time interval is the same as one cycle of operation of the converter.

6. The method as claimed in claim 1 , wherein the desired period of operation of the converter is chosen to be similar to the unloaded natural resonant period of the converter.

7. A converter including a resonant circuit having an inductor in which a direction of a current through the inductor is dependent on a state of at least a first control switch and a second control switch, wherein the converter includes control circuitry adapted to control the control switches so that:

a. the first control switch is switched into a first state upon occurrence of a first zero crossing event, wherein the first zero crossing event occurs when a voltage across the first control switch reaches zero;

b. the first control switch is switched into a second state in response to expiration of a first predetermined time interval, wherein the first predetermined time interval is triggered to begin upon the second control switch switching to the second state;

c. the second control switch is switched into the first state upon occurrence of a second zero crossing event, wherein the second zero crossing event occurs when a voltage across the second switch reaches zero; and

d. the second control switch is switched into the second state in response to expiration of a second predetermined time interval, wherein the second predetermined time interval is triggered to begin upon one of the first control switch and the second control switch switching to the second state.

8. The converter of claim 7 , wherein the first state is an on state and the second state is an off state.

9. The converter of claim 7 , wherein the first state is an off state and the second state is an on state.

10. The converter of claim 7 , wherein the control circuitry is adapted to detect the first zero crossing event and the second zero crossing event.

11. The converter of claim 8 , wherein the second switching event is the end of a predetermined time interval after a control switch being switched off.

12. An inductive power transfer transmitter including a converter as claimed claim 7 .

13. The converter of claim 1 , wherein the occurrence of the second switching event is related to a substantially constant converter frequency.

14. The converter of claim 7 , wherein the occurrence of the second switching event is related to a substantially constant converter frequency.

15. The method of claim 1 , wherein the second switch is switched to the first state while the first switch is in the first state.

16. The method of claim 1 , wherein the second switch is switched to the first state while the first switch is in the second state.

17. The method of claim 1 , wherein the first predetermined time interval and the second predetermined time interval are of equal duration.

18. A method of controlling a converter, including a resonant circuit having an inductor in which a direction of a current through the inductor is dependent on a state of at least a first control switch and a second control switch, the method comprising:

a. switching the first control switch into a first state upon occurrence of a first zero crossing event, wherein the first zero crossing event occurs when a voltage across the first control switch reaches zero;

b. switching the first control switch into a second state in response to expiration of a first predetermined time interval, wherein the first predetermined time interval is triggered to begin upon the second control switch switching to the second state;

c. switching the second control switch into the first state upon occurrence of a second zero crossing event, wherein the second zero crossing event occurs when a voltage across the second control switch reaches zero; and

d. switching the second control switch into the second state in response to expiration of a second predetermined time interval, wherein the second predetermined time interval is triggered to begin upon the first control switch switching to the second state.

19. The method of claim 12 , wherein the first predetermined time interval and the second predetermined time interval are of equal duration.