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In a power converter apparatus, such as an uninterruptible power supply, a phase reference signal is generated responsive to a DC voltage on a DC bus. A magnitude reference signal may be generated responsive to a phase current of an AC bus and/or the DC voltage on the DC bus. Power is transferred be

In a power converter apparatus, such as an uninterruptible power supply, a phase reference signal is generated responsive to a DC voltage on a DC bus. A magnitude reference signal may be generated responsive to a phase current of an AC bus and/or the DC voltage on the DC bus. Power is transferred between the AC and DC busses responsive to the phase reference signal and the magnitude reference signal. Generation of the phase reference signal may include generating a DC voltage error signal responsive to the DC voltage on the DC bus, generating a phase offset signal responsive to the DC voltage error signal, and generating the phase reference signal responsive to the phase offset signal. Generation of the magnitude reference signal may include generating a volt-amperes reactive (VAR) error signal responsive to the phase current and generating the magnitude reference signal responsive to the VAR error signal.

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That which is claimed: 1. A method of transferring power between a DC bus and an AC bus, the method comprising: generating a DC voltage error signal responsive to a DC voltage on the DC bus; differentiating the DC voltage error signal to generate a phase offset signal; generating a phase reference

That which is claimed: 1. A method of transferring power between a DC bus and an AC bus, the method comprising: generating a DC voltage error signal responsive to a DC voltage on the DC bus; differentiating the DC voltage error signal to generate a phase offset signal; generating a phase reference signal responsive to the phase offset signal; generating a magnitude reference signal; and transferring power between the AC and DC busses responsive to the phase offset signal and the magnitude reference signal. 2. A method according to claim 1: wherein generating a DC voltage error signal comprises: generating a series of samples of the DC voltage on the DC bus; and generating respective DC voltage error signal values for respective ones of the samples of the DC voltage; wherein differentiating the DC voltage error signal comprises determining weighted differences between successive ones of the DC voltage error signal values to generate a series of phase offset signal values; and wherein generating a phase reference signal responsive to the phase offset signal comprises generating a series of phase reference signal values from the series of phase offset signal values. 3. A method according to claim 2, wherein generating a series of phase reference signal values from the series of phase offset signal values comprises generating a phase reference signal value from a previously computed phase reference signal value, a phase offset signal value, and a phase angle-per-step value. 4. A method according to claim 3, further comprising generating the phase-angle-per step value responsive to an AC voltage on the AC bus. 5. A method according to claim 1, wherein generating a phase reference signal responsive to the phase offset signal comprises generating the phase reference signal responsive to the phase offset signal and a frequency of an AC voltage on the AC bus. 6. A method according to claim 1, wherein generating a phase reference signal comprises updating the phase reference signal at a rate that is independent of a frequency of an AC voltage on the AC bus. 7. A method according to claim 1, wherein generating a phase reference signal comprises updating the phase reference signal at a frequency greater than twice a number of phases of the AC bus times a fundamental frequency of an AC voltage on the AC bus. 8. A method according to claim 1, wherein transferring power between the AC and DC busses comprises operating a power converter responsive to the phase offset signal to provide power transfer between the AC and DC busses, and wherein the phase offset signal represents an amount of phase shift between an input of the power converter and an AC source driving the AC bus needed to maintain a desired voltage on the DC bus. 9. A method according to claim 1, wherein generating a magnitude reference signal comprises generating the magnitude reference signal responsive to a phase current of the AC bus and/or the DC voltage on the DC bus. 10. A method according to claim 9, wherein generating a magnitude reference signal comprises: generating a volt-amperes reactive (VAR) error signal responsive to the phase current; and generating the magnitude reference signal responsive to the VAR error signal. 11. A method according to claim 10, wherein generating a VAR error signal comprises: generating a VAR estimate signal responsive to the phase current, the magnitude reference signal and the phase reference signal; and generating the VAR error signal responsive to the VAR estimate signal and a VAR reference signal. 12. A method according to claim 9, wherein generating a magnitude reference signal comprises generating the magnitude reference signal responsive to the DC voltage error signal. 13. A method according to claim 1, wherein transferring power between the AC and DC busses responsive to the phase reference signal and the magnitude reference signal comprises: generating a sinusoidal reference signal responsive to the phase reference signal; scaling the sinusoidal reference signal according to the magnitude reference signal; and transferring power between the AC and DC busses responsive to the scaled sinusoidal reference signal. 14. A method according to claim 13: wherein generating a sinusoidal reference signal comprises generating a plurality of sinusoidal reference signals responsive to the phase reference signal, the sinusoidal reference signals being phase offset from one another by predetermined phase angles corresponding to phase offsets among phases of the AC bus; wherein scaling the sinusoidal reference signal comprise scaling each of the plurality of sinusoidal reference signals according to the magnitude reference signal; and wherein transferring power between the AC and DC busses responsive to the scaled sinusoidal reference signal comprises selectively coupling respective ones of the phases of the AC bus to the DC bus responsive to respective ones of the scaled sinusoidal reference signals. 15. An apparatus for transferring power between an AC bus and a DC bus, the apparatus comprising: a power converter circuit operative to transfer power between the AC and DC busses responsive to a phase reference signal and a magnitude reference signal; a phase reference signal generator circuit operative to generate the a DC voltage error signal responsive to the DC voltage on the DC bus, to differentiate the DC voltage error signal to generate a phase offset signal, and to generate the phase reference signal responsive to the phase offset signal; and a magnitude reference signal generator circuit operative to generate the magnitude reference signal. 16. An apparatus according to claim 5, wherein the phase reference signal generator circuit is operative to generate a series of samples of the DC voltage on the DC bus, to generate respective DC voltage error signal values for respective ones of the samples of the DC voltage, to determine weighted differences between successive ones of the DC voltage error signal values to generate a series of phase offset signal values and to generate a series of phase reference signal values from the series of phase offset signal values, and wherein the power converter circuit is operative to transfer power between the AC and DC busses responsive to the phase reference signal values. 17. An apparatus according to claim 16, wherein the phase reference signal generator circuit is operative to generate a phase reference signal value from a previously computed phase reference signal value, a phase offset signal value, and a phase angle-per-step value. 18. An apparatus according to claim 17, wherein the phase reference signal generator circuit is further operative to generate the phase-angle-per step value responsive to an AC voltage on the AC bus. 19. An apparatus according to claim 16, wherein the phase reference signal generator circuit is operative to generate the phase reference signal responsive to the phase offset signal and a frequency of an AC voltage on the AC bus. 20. An apparatus according to claim 15, wherein the magnitude reference signal generator circuit is operative to generate the magnitude reference signal responsive to a phase current of the AC bus and/or the DC voltage on the DC bus. 21. An apparatus according to claim 20, wherein the magnitude reference signal generator circuit is operative to generate a volt-amperes reactive (VAR) error signal responsive to the phase current and to generate the magnitude reference signal responsive to the VAR error signal. 22. An apparatus according to claim 21, wherein the magnitude reference signal generator circuit is operative to generate a VAR estimate signal responsive to the phase current, the magnitude reference signal and the phase reference signal and to generate the VAR error signal responsive to the VAR estimate signal and a VAR reference signal. 23. An apparatus according to claim 20, wherein the magnitude reference signal generator circuit is operative to generate the magnitude reference signal responsive to the DC voltage error signal. 24. An apparatus according to claim 15, wherein the power converter circuit comprises an active bridge circuit. 25. An apparatus according to claim 15, wherein the power converter circuit is operative to generate a sinusoidal reference signal responsive to the phase reference signal, to scale the sinusoidal reference signal according to the magnitude reference signal, and to transfer power between the AC and DC busses responsive to the scaled sinusoidal reference signal. 26. An uninterruptible power supply (UPS), comprising: an AC bus; a DC bus; a power converter circuit operative to transfer power between the AC and DC busses responsive to a phase reference signal and a magnitude reference signal; a phase reference signal generator circuit operative to generate a DC voltage error signal responsive to the DC voltage on the DC bus, to differentiate the DC voltage error signal to generate a phase offset signal, and to generate the phase reference signal responsive to the phase offset signal; and a magnitude reference signal generator circuit operative to generate the magnitude reference signal. 27. A UPS according to claim 26, wherein the magnitude reference signal generator circuit is operative to generate the magnitude reference signal responsive to a phase current of the AC bus and/or the DC voltage on the DC bus. 28. A UPS according to claim 27, wherein the magnitude reference signal generator circuit is operative to generate a volt-amperes reactive (VAR) error signal responsive to the phase current and to generate the magnitude reference signal responsive to the VAR error signal. 29. A UPS according to claim 28, wherein the magnitude reference signal generator circuit is operative to generate a VAR estimate signal responsive to the phase current, the magnitude reference signal and the phase reference signal and to generate the VAR error signal responsive to the VAR estimate signal and a VAR reference signal. 30. A UPS according to claim 27, wherein the phase reference signal generator circuit is operative to generate a DC voltage error signal responsive to the DC voltage on the DC bus and to generate the phase reference signal responsive to the DC voltage error signal, and wherein the magnitude reference signal generator circuit is operative to generate the magnitude reference signal responsive to the VAR error signal and the DC voltage error signal. 31. A UPS according to claim 26, wherein the power converter circuit is operative to generate a sinusoidal reference signal responsive to the phase reference signal, to scale the sinusoidal reference signal according to the magnitude reference signal, and to transfer power between the AC and DC busses responsive to the scaled sinusoidal reference signal.