Soft-transition controller of a synchronous converter

1. A soft-transition controller for use with a synchronous converter having primary and secondary rectifiers, an output inductor and an output capacitor, comprising:

a primary driver configured to provide a primary drive signal to said primary rectifier operating in a synchronous mode, while said secondary rectifier is operating in a diode mode, to provide an output voltage of said synchronous converter; and

a complementary driver configured to provide a soft-transition drive signal during a transition period to transfer said secondary rectifier from said diode mode to a synchronous mode while maintaining said output voltage within a predetermined voltage range based on a voltage measured at a node between said output inductor and said output capacitor.

2. The controller as recited in claim 1 wherein said soft-transition drive signal provides a monotonically changing pulse amplitude during said transition period.

3. The controller as recited in claim 1 wherein said soft-transition drive signal provides a monotonically increasing pulse width during said transition period.

4. The controller as recited in claim 1 wherein said soft-transition drive signal is provided when said output voltage has reached a predetermined level.

5. The controller as recited in claim 1 wherein said soft-transition drive signal is provided for all output current conditions of said synchronous converter up to a rated load current.

6. The controller as recited in claim 1 wherein said soft-transition drive signal is employed to transition between a start-up mode and a normal mode of operation of said synchronous converter.

7. The controller as recited in claim 1 wherein said predetermined voltage range provides an output voltage deviation from a continued diode mode operation of said secondary rectifier during said transition period that is defined by an allowable change per transition step.

8. The controller as recited in claim 1 wherein said secondary rectifier includes a diode parallel coupled to a controllable switch.

9. A method of controlling a soft transition for use with a synchronous converter having primary and secondary rectifier, an output inductor and an output capacitor, comprising:

providing a primary drive signal to said primary rectifier operating in a synchronous mode, while said secondary rectifier is operating in a diode mode, to provide an output voltage of said synchronous converter; and

further providing a soft-transition drive signal during a transition period thereby transferring said secondary rectifier from said diode mode to a synchronous mode while maintaining said output voltage within a predetermined voltage range based on a voltage measured at a node between said output inductor and said output capacitor.

10. The method as recited in claim 9 wherein said soft-transition drive signal provides a monotonically changing pulse amplitude during said transition period.

11. The method as recited in claim 9 wherein said soft-transition drive signal provides a monotonically increasing pulse width during said transition period.

12. The method as recited in claim 9 wherein said soft-transition drive signal is provided when said output voltage has reached a predetermined level.

13. The method as recited in claim 9 wherein said soft-transition drive signal is provided for all output current conditions of said synchronous converter up to a rated load current.

14. The method as recited in claim 9 wherein said soft-transition drive signal is employed to transition between a start-up mode and a normal mode of operation of said synchronous converter.

15. The method as recited in claim 9 wherein said predetermined voltage range provides an output voltage deviation from a continued diode mode operation of said secondary rectifier during said transition period that is defined by an allowable change per transition step.

16. The method as recited in claim 9 wherein said secondary rectifier includes a diode parallel coupled to a controllable switch.

17. A synchronous converter, comprising:

primary and secondary rectifiers coupled to an output inductor and an output capacitor, and

a soft-transition controller coupled to said primary and secondary rectifiers, including:

a primary driver that provides a primary drive signal to said primary rectifier operating in a synchronous mode, while said secondary rectifier is operating in a diode mode, to provide an output voltage of said synchronous converter, and

a complementary driver that provides a soft-transition drive signal during a transition period to transfer said secondary rectifier from said diode mode to a synchronous mode while maintaining said output voltage within a predetermined voltage range based on a voltage measured between said output inductor and said output capacitor.

18. The converter as recited in claim 17 wherein said soft-transition drive signal provides a monotonically changing pulse amplitude during said transition period.

19. The converter as recited in claim 17 wherein said soft-transition drive signal provides a monotonically increasing pulse width during said transition period.

20. The converter as recited in claim 17 wherein said soft-transition drive signal is provided when said output voltage has reached a predetermined level.

21. The converter as recited in claim 17 wherein said soft-transition drive signal is provided for all output current conditions of said synchronous converter up to a rated load current.

22. The converter as recited in claim 17 wherein said soft-transition drive signal is employed to transition between a start-up mode and a normal mode of operation of said synchronous converter.

23. The converter as recited in claim 17 wherein said predetermined voltage range provides an output voltage deviation from a continued diode mode operation of said secondary rectifier during said transition period that is defined by an allowable change per transition step.

24. The converter as recited in claim 17 wherein said secondary rectifier includes a diode parallel coupled to a controllable switch.