Cyclo-converter and methods of operation

1. A switched-mode cyclo-converter employing resonant switching comprising:

i. a first half bridge cyclo-converter including:

i. three phase voltage inputs;

ii. bidirectional switches between each input and an output line; and

iii. capacitors between each input and a common node;

ii. a second half bridge cyclo-converter including:

i. three phase voltage inputs;

ii. bidirectional switches between each input and an output line; and

iii. capacitors between each input and a common node;

iii. a series LLC resonant circuit connected across the output lines of the first and second half bridge cyclo-converters; and

iv. a controller that controls the switching periods of the bidirectional switches based on the three phase voltages;

wherein each of the bidirectional switches in the first half bridge cyclo-converter is configured to support four switching states including: full-on, full-off, forward-direction conducting diode and reverse-direction conducting diode.

2. A switched-mode cyclo-converter as claimed in claim 1 wherein the series LLC resonant circuit includes a primary coil of an output transformer as th center “L” in the series LLC resonant circuit.

3. A switched-mode cyclo-converter as claimed in claim 2 wherein the series LLC resonant circuit includes an additional inductive element as the first “L” in the series LLC resonant circuit.

4. A switched-mode cyclo-converter as claimed in claim 1 including a power supply transformer which supplies three phase input voltages to the three phase voltage inputs of the half bridge cyclo-converters.

5. A switched-mode cyclo-converter as claimed in claim 4 wherein the power supply transformer is a phase shifting transformer supplying first phase shifted three phase voltage inputs to one of the half bridge cyclo-converters and further supplying second phase shifted three phase voltage inputs to the other one of the half bridge cyclo-converters; and wherein each of the first phase shifted three phase voltage inputs is 60 degrees out-of-phase relative to a corresponding one of the second phase shifted three phase voltage inputs.

6. A switched-mode cyclo-converter as claimed in claim 1 including an output rectifier.

7. A switched-mode cyclo-converter as claimed in claim 6 wherein the output rectifier includes bidirectional switching elements enabling the converter to function as an inverter.

8. A switched-mode cyclo-converter as claimed in claim 1 wherein the controller controls the switching frequency of the bidirectional switches in dependence upon the output voltage of the cyclo-converter.

9. A switched-mode cyclo-converter as claimed in claim 1 wherein the switching frequency is controlled in dependence on a resonant transfer function for the resonant components.

10. A switched-mode cyclo-converter as claimed in claim 1 wherein elements of the bidirectional switches are pre-switched by the controller to ensure soft switching.

11. A switched-mode cyclo-converter, comprising:

a first half-bridge cyclo-converter containing first, second and third pairs of switches therein with each of the three pair of switches supporting four switching states including: full-on, full-off, forward-direction conducting diode and reverse-direction conducting diode;

a second half-bridge cyclo-converter containing first, second and third pairs of switches therein with each of the three pair of switches supporting four switching states including: full-on, full-off, forward-direction conducting diode and reverse-direction conducting diode;

a first plurality of capacitors having respective first terminals electrically connected to corresponding first, second and third input terminals of said first half-bridge cyclo-converter;

a second plurality of capacitors having respective first terminals electrically connected to corresponding first, second and third input terminals of said second half-bridge cyclo-converter; and

a resonant circuit having a first terminal electrically connected to an output terminal of said first half-bridge cyclo-converters and a second terminal electrically connected to an output terminal of said second half-bridge cyclo-converter, said resonant circuit comprising a series combination of at least a first inductor, a primary coil of a transformer and a first capacitor.

12. The switched-mode cyclo-converter of claim 11 , wherein said resonant circuit comprises a series combination of first and second inductors, the primary coil of the transformer and first and second capacitors.

13. The switched-mode cyclo-converter of claim 12 , wherein the first terminal of said resonant circuit is a current carrying terminal of the first inductor and the second terminal of said resonant circuit is a current carrying terminal of the second inductor.

14. The switched-mode cyclo-converter of claim 13 , wherein the primary coil of the transformer has a first terminal electrically connected to a terminal of the first capacitor and a second terminal electrically connected to a terminal of the second capacitor.

15. The switched-mode cyclo-converter of claim 12 , wherein said resonant circuit is a series LCLCL circuit with the primary coil of the transformer extending between the LC and CL portions of the series LCLCL circuit.

16. The switched-mode cyclo-converter of claim 11 , further comprising a phase shifting transformer responsive to three AC voltage signals that are 120 degrees out-of-phase relative to each other, said phase shifting transformer configured to supply the first, second and third input terminals of said first half-bridge cyclo-converter with three AC voltage signals that are 120 degrees out-of-phase relative to each other and supply the first, second and third input terminals of said second half-bridge cyclo-converter with three AC voltage signals that are 120 degrees out-of-phase relative to each other and 60 degrees out-of-phase relative to the three AC voltage signals supplied to the first, second and third input terminals of said first half-bridge cyclo-converter.