Modulation of switching signals in power converters

1. A method for reducing noise in a power converter by controlling a switching device in the power converter according to a modulation scheme, the switching device for coupling a direct current (DC) source to provide an alternating current (AC) output at a particular switching frequency, the method comprising the step of:

in each switching period, switching the switching device between an active configurations providing a finite voltage at the AC output and an inactive configurations providing a zero voltage at the AC output;

wherein a ratio between a total period of time in which the switching device is in the active configuration and the total period of time in which the switching device is in the inactive configuration is the same for each switching period and is determined according to a desired voltage at the AC output; and

wherein, in each switching period, there are at least two time periods in which the switching device is in the inactive configuration, and a ratio between those at least two time periods is changed every switching period.

2. The method of claim 1 , wherein the ratio between the at least two time periods in which the switching device is in the inactive configuration is changed every switching period in dependence upon a random sequence generated for each switching period.

3. The method of claim 2 , wherein the random sequence comprises 256 elements.

4. The method of claim 1 , wherein the power converter comprises a three phase power inverter.

5. The method of claim 4 , wherein at least one of the switches in the three phase inverter is an insulated gate bipolar transistor (IGBT).

6. The method of claim 4 , wherein the ratio between the at least two time periods in which the switching device is in the inactive configuration is weighted towards configurations in which a selected switch in each phase of the three phase inverter is on.

7. A control system for reducing noise in a power converter switching device, the switching device for coupling a direct current (DC) source to provide an alternating current (AC) output at a particular switching frequency, the control system comprising:

a controller for switching the switching device in the power converter according to a modulation scheme, the controller being arranged to switch the switching device, in each switching period, between an active configurations providing a finite voltage at the AC output and an inactive configurations providing a zero voltage at the AC output;

wherein a ratio between a total period of time in which the switching device is in the active configuration and a total period of time in which the switching device is in the inactive configuration is the same for each switching period and is determined according to a desired voltage at the AC output; and

wherein, in each switching period, there are at least two time periods in which the switching device is in the inactive configuration, and a ratio between those at least two time periods is changed every switching period.

8. The control system of claim 7 , wherein the ratio between the at least two time periods in which the switching device is in the inactive configuration is changed every switching period in dependence upon a random sequence generated for each switching period.

9. The control system of claim 8 , wherein the random sequence comprises 256 elements.

10. The control system of claim 7 , wherein the power converter comprises a three phase power inverter.

11. The control system of claim 10 , wherein at least one of the switches in the three phase inverter is an insulated gate bipolar transistor (IGBT).

12. The control system of claim 10 , wherein the ratio between the at least two time periods in which the switching device is in the inactive configuration is weighted towards configurations in which a selected switch in each phase of the three phase inverter is on.

13. A switching power converter comprising:

a switching device for coupling a direct current (DC) source to provide an alternating current (AC) output at a particular switching frequency; and

a controller for reducing noise in the power converter by switching the switching device according to a modulation scheme, the controller being arranged to switch the switching device, in each switching period, between active configurations providing a finite voltage at the AC output and inactive configurations providing a zero voltage at the AC output;

wherein a ratio between a total period of time in which the switching device is in the active configuration and a total period of time in which the switching device is in the inactive configuration is the same for each switching period and is determined according to a desired voltage at the AC output; and

wherein, in each switching period, there are at least two time periods in which the switching device is in the inactive configuration, and a ratio between those at least two time periods is changed every switching period.

14. The switching power converter of claim 13 , wherein the ratio between the at least two time periods in which the switching device is in the inactive configuration is changed every switching period in dependence upon a random sequence generated for each switching period.

15. The switching power converter of claim 14 , wherein the random sequence comprises 256 elements.

16. The switching power converter of claim 13 , wherein the power converter comprises a three phase power inverter.

17. The switching power converter of claim 16 , wherein at least one of the switches in the three phase inverter is an insulated gate bipolar transistor (IGBT).

18. The switching power converter of claim 16 , wherein the ratio between the at least two time periods in which the switching device is in the inactive configuration is weighted towards configurations in which a selected switch in each phase of the three phase inverter is on.