Flux limited fast transient response in isolated DC-DC converters

1. A method of controlling an isolated DC-DC converter that includes primary side switching devices coupled to secondary side rectifying devices by a transformer having a core, the method comprising:

switching the primary side switching devices at a fixed first switching period and variable duty cycle during non-transient load conditions so as to transfer energy across the transformer during first energy transfer intervals separated by energy circulation intervals, such that the ratio of each first energy transfer interval to the first switching period is less than unity;

switching the primary side switching devices at a second switching period different than the first switching period during a transient load condition so as to transfer energy across the transformer during second energy transfer intervals of a duration determined so as to avoid saturation of the transformer core, and such that any energy circulation interval separating the second energy transfer intervals is shorter than the energy circulation intervals separating the first energy transfer intervals;

adjusting the duration of the second energy transfer intervals based on a new input voltage magnitude for the isolated DC-DC converter, so as to avoid saturation of the transformer core for the new input voltage magnitude during the transient load condition; and

adjusting the second switching period based on the adjusted duration of the second energy transfer intervals.

2. The method of claim 1 , wherein the second switching period is less than the first switching period.

3. The method of claim 1 , wherein the second switching period is set equal to twice the duration of the second energy transfer intervals.

4. The method of claim 1 , wherein the second switching period is set equal to twice the duration of the second energy transfer intervals plus dead time.

5. The method of claim 1 , further comprising:

truncating the duration of the second energy transfer interval being applied when a peak current value of the transformer is reached.

6. The method of claim 1 , further comprising:

increasing the duration of at least one of the second energy transfer intervals, so as to achieve zero or near-zero average magnetic flux density in the transformer core.

7. The method of claim 6 , wherein increasing the duration of at least one of the second energy transfer intervals comprises:

setting the duration of an initial one of the second energy transfer intervals at the beginning of the transient load condition, based on the last known duty cycle of the first energy transfer intervals just prior to the transient load condition; and

setting the duration of subsequent ones of the second energy transfer intervals to a pre-programmed value which is greater than the duration of the initial second energy transfer interval and which avoids saturation of the transformer core.

8. The method of claim 1 , further comprising:

preventing switching of the primary side switching devices at the second switching period after a peak current value of the transformer is reached, so that current in an output inductor of the isolated DC-DC converter begins to drop; and

resuming switching of the primary side switching devices at the first switching period after the current in the output inductor drops to a predetermined limit.

9. The method of claim 8 , further comprising:

applying a first recovery pulse to one of the primary side switching devices after the peak current value is reached and prior to resuming switching of the primary side switching devices at the first switching period.

10. The method of claim 9 , wherein the first recovery pulse has the same duration as the second energy transfer intervals.

11. The method of claim 9 , further comprising:

determining the duration of the first recovery pulse based on magnetic flux density measurements taken for the transformer core, so as to bring the magnetic flux in the transformer core to zero or near zero just prior to resuming switching of the primary side switching devices at the first switching period.

12. The method of claim 9 , further comprising:

truncating a final one of the second energy transfer intervals during the transient load event, so that the final second energy transfer interval during the transient load condition has a shorter duration than the other second energy transfer intervals; and

setting the duration of the first recovery pulse to the duration of the final second energy transfer interval plus the duration of the other second energy transfer intervals.

13. The method of claim 9 , further comprising:

applying a second recovery pulse to the opposite primary side switching device as the first recovery pulse, the second recovery pulse being narrower than the first recovery pulse.

14. The method of claim 1 , further comprising:

determining the duration of the second energy transfer intervals based on the last known duty cycle of the first energy transfer intervals just prior to the transient load condition.

15. The method of claim 1 , further comprising:

setting the duration of the second energy transfer intervals to a pre-programmed value that avoids saturation of the transformer core during the transient load condition.

16. The method of claim 1 , further comprising:

determining the duration of the second energy transfer intervals based on magnetic flux density measurements taken for the transformer core.

17. The method of claim 1 , further comprising:

truncating an initial one of the second energy transfer intervals at the beginning of the transient load condition, so as to avoid saturation of the transformer core when the isolated DC-DC converter begins responding to the transient load condition.

18. The method of claim 1 , wherein the duration of the second energy transfer intervals is based on a magnetic flux limit that is less stringent than a magnetic flux limit applied when the primary side switching devices are being switched at the first switching period.

19. An isolated DC-DC converter, comprising:

primary side switching devices;

secondary side rectifying devices;

a transformer having a core, the transformer coupling the primary side switching devices to the secondary side rectifying devices; and

a controller operable to:

switch the primary side switching devices at a fixed first switching period and variable duty cycle during non-transient load conditions so as to transfer energy across the transformer during first energy transfer intervals separated by energy circulation intervals, such that the ratio of each first energy transfer interval to the first switching period is less than unity;

switch the primary side switching devices at a second switching period different than the first switching period during a transient load condition so as to transfer energy across the transformer during second energy transfer intervals of a duration determined so as to avoid saturation of the transformer core, and such that any energy circulation interval separating the second energy transfer intervals is shorter than the energy circulation intervals separating the first energy transfer intervals;

adjust the duration of the second energy transfer intervals based on a new input voltage magnitude for the isolated DC-DC converter, so as to avoid saturation of the transformer core for the new input voltage magnitude during the transient load condition; and

adjust the second switching period based on the adjusted duration of the second energy transfer intervals.

20. The isolated DC-DC converter of claim 19 , wherein the controller is operable to truncate the duration of the second energy transfer interval being applied when a peak current value of the transformer is reached.

21. The isolated DC-DC converter of claim 19 , wherein the controller is operable to increase the duration of at least one of the second energy transfer intervals, so as to achieve zero or near-zero average magnetic flux density in the transformer core.

22. The isolated DC-DC converter of claim 21 , wherein the controller is operable to:

set the duration of an initial one of the second energy transfer intervals at the beginning of the transient load condition, based on the last known duty cycle of the first energy transfer intervals just prior to the transient load condition; and

set the duration of subsequent ones of the second energy transfer intervals to a pre-programmed value which is greater than the duration of the initial second energy transfer interval and which avoids saturation of the transformer core.

23. The isolated DC-DC converter of claim 19 , wherein the controller is operable to:

prevent switching of the primary side switching devices at the second switching period after a peak current value of the transformer is reached, so that current in an output inductor of the isolated DC-DC converter begins to drop; and

resume switching of the primary side switching devices at the first switching period after the current in the output inductor drops to a predetermined limit.

24. The isolated DC-DC converter of claim 23 , wherein the controller is operable to apply a first recovery pulse to one of the primary side switching devices after the peak current value is reached and prior to resuming switching of the primary side switching devices at the first switching period.

25. The isolated DC-DC converter of claim 24 , wherein the controller is operable to determine the duration of the first recovery pulse based on magnetic flux density measurements taken for the transformer core, so as to bring the magnetic flux in the transformer core to zero or near zero just prior to resuming switching of the primary side switching devices at the first switching period.

26. The isolated DC-DC converter of claim 24 , wherein the controller is operable to:

truncate a final one of the second energy transfer intervals during the transient load event, so that the final second energy transfer interval during the transient load condition has a shorter duration than the other second energy transfer intervals; and

set the duration of the first recovery pulse to the duration of the final second energy transfer interval plus the duration of the other second energy transfer intervals.

27. The isolated DC-DC converter of claim 24 , wherein the controller is operable to apply a second recovery pulse to the opposite primary side switching device as the first recovery pulse, the second recovery pulse being narrower than the first recovery pulse.

28. The isolated DC-DC converter of claim 19 , wherein the controller is operable to determine the duration of the second energy transfer intervals based on the last known duty cycle of the first energy transfer intervals just prior to the transient load condition.

29. The isolated DC-DC converter of claim 19 , wherein the controller is operable to set the duration of the second energy transfer intervals to a pre-programmed value that avoids saturation of the transformer core during the transient load condition.

30. The isolated DC-DC converter of claim 19 , wherein the controller is operable to determine the duration of the second energy transfer intervals based on magnetic flux density measurements taken for the transformer core.

31. The isolated DC-DC converter of claim 19 , wherein the controller is operable to truncate an initial one of the second energy transfer intervals at the beginning of the transient load condition, so as to avoid saturation of the transformer core when the isolated DC-DC converter begins responding to the transient load condition.

32. The isolated DC-DC converter of claim 19 , wherein the controller is operable to determine the duration of the second energy transfer intervals based on a magnetic flux limit that is less stringent than a magnetic flux limit applied when the primary side switching devices are being switched at the first switching period.

33. A method of controlling an isolated DC-DC converter that includes primary side switching devices coupled to secondary side rectifying devices by a transformer having a core, the method comprising:

switching the primary side switching devices at a fixed first switching period and variable duty cycle during non-transient load conditions so as to transfer energy across the transformer during first energy transfer intervals separated by energy circulation intervals, such that the ratio of each first energy transfer interval to the first switching period is less than unity; and

switching the primary side switching devices at a second switching period different than the first switching period during a transient load condition so as to transfer energy across the transformer during second energy transfer intervals of a duration determined so as to avoid saturation of the transformer core, and such that any energy circulation interval separating the second energy transfer intervals is shorter than the energy circulation intervals separating the first energy transfer intervals,

wherein the method further comprises:

truncating the duration of the second energy transfer interval being applied when a peak current value of the transformer is reached; and/or

increasing the duration of at least one of the second energy transfer intervals, so as to achieve zero or near-zero average magnetic flux density in the transformer core.

34. The method of claim 33 , wherein increasing the duration of at least one of the second energy transfer intervals comprises:

setting the duration of an initial one of the second energy transfer intervals at the beginning of the transient load condition, based on the last known duty cycle of the first energy transfer intervals just prior to the transient load condition; and

setting the duration of subsequent ones of the second energy transfer intervals to a pre-programmed value which is greater than the duration of the initial second energy transfer interval and which avoids saturation of the transformer core.

35. A method of controlling an isolated DC-DC converter that includes primary side switching devices coupled to secondary side rectifying devices by a transformer having a core, the method comprising:

switching the primary side switching devices at a fixed first switching period and variable duty cycle during non-transient load conditions so as to transfer energy across the transformer during first energy transfer intervals separated by energy circulation intervals, such that the ratio of each first energy transfer interval to the first switching period is less than unity;

switching the primary side switching devices at a second switching period different than the first switching period during a transient load condition so as to transfer energy across the transformer during second energy transfer intervals of a duration determined so as to avoid saturation of the transformer core, and such that any energy circulation interval separating the second energy transfer intervals is shorter than the energy circulation intervals separating the first energy transfer intervals;

preventing switching of the primary side switching devices at the second switching period after a peak current value of the transformer is reached, so that current in an output inductor of the isolated DC-DC converter begins to drop; and

resuming switching of the primary side switching devices at the first switching period after the current in the output inductor drops to a predetermined limit.

36. The method of claim 35 , further comprising:

applying a first recovery pulse to one of the primary side switching devices after the peak current value is reached and prior to resuming switching of the primary side switching devices at the first switching period.

37. The method of claim 36 , wherein the first recovery pulse has the same duration as the second energy transfer intervals.

38. An isolated DC-DC converter, comprising:

primary side switching devices;

secondary side rectifying devices;

a transformer having a core, the transformer coupling the primary side switching devices to the secondary side rectifying devices; and

a controller operable to:

switch the primary side switching devices at a fixed first switching period and variable duty cycle during non-transient load conditions so as to transfer energy across the transformer during first energy transfer intervals separated by energy circulation intervals, such that the ratio of each first energy transfer interval to the first switching period is less than unity; and

switch the primary side switching devices at a second switching period different than the first switching period during a transient load condition so as to transfer energy across the transformer during second energy transfer intervals of a duration determined so as to avoid saturation of the transformer core, and such that any energy circulation interval separating the second energy transfer intervals is shorter than the energy circulation intervals separating the first energy transfer intervals,

wherein the controller is further operable to:

truncate the duration of the second energy transfer interval being applied when a peak current value of the transformer is reached; and/or

increase the duration of at least one of the second energy transfer intervals, so as to achieve zero or near-zero average magnetic flux density in the transformer core.

39. The isolated DC-DC converter of claim 38 , wherein the controller is operable to:

set the duration of an initial one of the second energy transfer intervals at the beginning of the transient load condition, based on the last known duty cycle of the first energy transfer intervals just prior to the transient load condition; and

set the duration of subsequent ones of the second energy transfer intervals to a pre-programmed value which is greater than the duration of the initial second energy transfer interval and which avoids saturation of the transformer core.

40. An isolated DC-DC converter, comprising:

primary side switching devices;

secondary side rectifying devices;

a transformer having a core, the transformer coupling the primary side switching devices to the secondary side rectifying devices; and

a controller operable to:

switch the primary side switching devices at a fixed first switching period and variable duty cycle during non-transient load conditions so as to transfer energy across the transformer during first energy transfer intervals separated by energy circulation intervals, such that the ratio of each first energy transfer interval to the first switching period is less than unity;

switch the primary side switching devices at a second switching period different than the first switching period during a transient load condition so as to transfer energy across the transformer during second energy transfer intervals of a duration determined so as to avoid saturation of the transformer core, and such that any energy circulation interval separating the second energy transfer intervals is shorter than the energy circulation intervals separating the first energy transfer intervals;

prevent switching of the primary side switching devices at the second switching period after a peak current value of the transformer is reached, so that current in an output inductor of the isolated DC-DC converter begins to drop; and

resume switching of the primary side switching devices at the first switching period after the current in the output inductor drops to a predetermined limit.

41. The isolated DC-DC converter of claim 40 , wherein the controller is operable to apply a first recovery pulse to one of the primary side switching devices after the peak current value is reached and prior to resuming switching of the primary side switching devices at the first switching period.