Power supply with active power buffer

1. A power converter comprising:

a DC-DC converter stage having an input coupled to an input of the power converter and an output coupled to an output of the power converter; and

an active power buffer coupled to the output of the power converter, the active power buffer further comprising:

an energy storage capacitor; and

one or more switching devices selectively coupling the energy storage capacitor to the output of the power converter so as to alternately store energy in and discharge energy from the energy storage capacitor; and

wherein control circuitry of the power converter comprises a first control loop that operates the DC-DC converter stage to regulate an average voltage across the energy storage capacitor of the active power buffer and a second control loop that operates the one or more switching devices of the active power buffer to regulate an output voltage of the power converter.

2. The power converter of claim 1 wherein the power converter is an AC-DC converter, and the DC-DC converter stage is coupled to the input of the power converter by a rectifier.

3. The power converter of claim 2 wherein the first control loop has a bandwidth less than twice the line frequency of an AC input to the power converter.

4. The power converter of claim 3 wherein the second control loop has a bandwidth higher than the first control loop.

5. The power converter of claim 4 wherein the second control loop employs duty cycle control.

6. The power converter of claim 2 wherein the first control loop employs constant Ton control.

7. The power converter of claim 1 wherein the DC-DC converter stage is an isolated converter.

8. The power converter of claim 7 wherein the DC-DC converter stage is a flyback converter.

9. The power converter of claim 8 wherein the flyback converter employs a split magnetics arrangement having two or more flyback transformers with their secondary windings connected in parallel.

10. The power converter of claim 9 wherein the two or more flyback transformers have their primary windings connected in series and driven by a single switching device.

11. The power converter of claim 9 wherein the two or more flyback transformers have their primary windings connected in parallel and driven by respective switching devices for each primary winding.

12. An AC-DC converter comprising:

a rectifier coupled to an AC input of the AC-DC converter;

a flyback converter having an input coupled to the rectifier and an output coupled to an output of the AC-DC converter;

an active power buffer coupled to the output of the AC-DC converter, the active power buffer further comprising:

an energy storage capacitor; and

one or more switching devices selectively coupling the energy storage capacitor to the output of the AC-DC converter so as to alternately store energy in and discharge energy from the energy storage capacitor; and

control circuitry including:

a first control loop, having a bandwidth less than twice the line frequency of the AC input, that operates the flyback converter to regulate an average voltage across the energy storage capacitor of the active power buffer while providing unity power factor at the AC input of the AC-DC converter; and

a second faster control loop that operates the one or more switching devices of the active power buffer to regulate an output voltage of the AC-DC converter.

13. The AC-DC converter of claim 12 wherein the flyback converter employs a split magnetics arrangement having two or more flyback transformers with their secondary windings connected in parallel.

14. The AC-DC converter of claim 13 wherein the two or more flyback transformers have their primary windings connected in series and driven by a single switching device.

15. The AC-DC converter of claim 13 wherein the two or more flyback transformers have their primary windings connected in parallel and driven by respective switching devices for each primary winding.

16. The AC-DC converter of claim 12 wherein the active power buffer further comprises an inductor coupling the one or more switching devices to output of the AC-DC converter, wherein the inductor and one or more switching devices are operated as a bi-directional buck-boost converter.

17. A method of operating an AC-DC converter having a rectifier, a DC-DC converter, and an active power buffer coupled to an output of the DC-DC converter, the method comprising:

operating the DC-DC converter to maintain an average voltage across an energy storage capacitor of the active power buffer; and

operating one or more switching devices of the active power buffer to regulate an output voltage of the AC-DC converter.

18. The method of claim 17 wherein the average voltage is selected to be approximately half-way between the output voltage of the AC-DC converter and a voltage rating of the energy storage capacitor.

19. The method of claim 17 wherein operating the DC-DC converter to maintain an average voltage across an energy storage capacitor of the active power buffer further comprises operating the DC-DC converter to present unity power factor at an input of the AC-DC converter.

20. The method of claim 17 wherein:

operating the DC-DC converter to maintain an average voltage across an energy storage capacitor of the active power buffer comprises generating PWM signals for one or more switching devices of the DC-DC converter using a constant on time control loop; and

operating one or more switching devices of the active power buffer to regulate an output voltage of the AC-DC converter comprises generating PWM signals for the one or more switching devices of the active power buffer using a duty cycle control loop.