Cyclo-converter and methods of operation
A three phase full resonant cyclo-converter suitable for converting a three phase AC supply to a DC output. In one embodiment the cycloconverter controls switching frequency to control converter output and adjusts phase on times for power factor correction. A switching sequence is employed which provides resonant switching to reduce losses and component ratings. The converter provides high conversion efficiency with a simple power component design.
1. A converter comprising:
three phase inputs configured to be coupled to respective phases of an AC source;
a cyclo-converter coupled to the three phase inputs and having an output port comprising first and second output power lines, the cyclo-converter comprising respective controllable bidirectional switches between respective ones of the phase inputs and the first output power line and respective power capacitors between respective ones of the phase inputs and the second output power line;
a resonant circuit connected across the output port of the cyclo-converter;
a rectifier coupled to the resonant circuit and configured to produce a DC output responsive to a voltage across an inductive element of the resonant circuit; and
a controller configured to control the bidirectional switches.
2. A converter as claimed in claim 1 wherein the resonant circuit includes a primary coil of a transformer and a capacitor, wherein the inductive element includes the primary coil and wherein the rectifier is coupled to a secondary coil of the transformer.
3. A converter as claimed in claim 2 wherein the resonant circuit includes an additional inductive element.
4. A converter as claimed in claim 3 wherein the additional inductive element is provided by gapping the transformer.
5. A converter as claimed in claim 1 wherein the bidirectional switches can switch on or off or switch in a diode of either polarity.
6. A converter as claimed in claim 5 wherein each bidirectional switch consists of back to back MOSFET transistors, each having a body diode in parallel.
7. A converter as claimed in claim 5 wherein each bidirectional switch consists of back to back IGBT devices with anti-parallel diodes in parallel.
8. A converter as claimed in claim 5 wherein each bidirectional switch consists of MOSFET transistors in parallel with diodes in parallel with each transistor and in series with each transistor.
9. A three converter as claimed in claim 5 wherein each bidirectional switch consists of a back to back configuration of a MOSFET and IB GT transistor in parallel.
10. A converter as claimed in claim 1 wherein the controller controls the switching frequency of the bidirectional switches responsive to an output of the rectifier.
11. A converter as claimed in claim 10 wherein the controller controls the switching frequency of the bidirectional switches responsive to an output voltage of the rectifier.
12. A converter as claimed in claim 10 wherein the switching frequency is controlled in dependence on a resonant transfer function of the resonant circuit.
13. A converter as claimed in claim 1 wherein the controller reduces an output voltage of the converter responsive to an output current of the rectifier exceeding a threshold.
14. A converter as claimed in claim 1 wherein the controller achieves power factor correction by adjusting the periods of time for the bidirectional switches couple the phase inputs to the first output power line.
15. A converter as claimed in claim 1 wherein the controller achieves power factor correction by adjusting a ratio of time periods the bidirectional switches couple two of the phase inputs to the first output power line.
16. A converter as claimed in claim 15 wherein the controller is configured to couple the phase input with the largest absolute voltage to the first output power line for a constant time.
17. A converter as claimed in claim 15 wherein the controller is configured to couple the phase input with the largest absolute voltage to the first output power line for about half the time.
18. A converter as claimed in claim 15 wherein the controller is configured to determine the ratio using a look up table.
19. A converter as claimed in claim 1 wherein the controller controls switching of the bidirectional switches to predominantly switch in an order of a one of the phase inputs having a greatest absolute voltage, followed by a one of the phase inputs having a middle absolute voltage followed by a one of the phase inputs having a lowest absolute voltage.
20. A converter as claimed in claim 19 wherein, near a zero crossing of a given one of the phase inputs, only the other two phase inputs are switched.
21. A converter as claimed in claim 19 wherein, near magnitude crossings of a given two of the phase inputs, the other phase input is switched alternately with the given two phases.
22. A converter as claimed in claim 19 wherein elements of the bidirectional switches are preswitched by the controller to ensure soft switching.
23. A converter as claimed in claim 1 wherein the rectifier includes bidirectional switching elements enabling the converter to function as an inverter.
24. A converter as claimed in claim 1 wherein the controller adjusts time periods the phase inputs are coupled to the first power output line to control the DC output.
25. A converter as claimed in claim 1 , wherein the controller is configured to control the bidirectional switches responsive to an output voltage of the rectifier.
26. A converter as claimed in claim 1 , wherein the controller is configured to control the bidirectional switches responsive to an output current of the rectifier.