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A primary resonant flyback converter may include a primary winding, a resonant capacitor in series with the primary winding, a secondary winding magnetically coupled to the primary winding, and an output electrically coupled to the secondary winding. A main switch may be operated to energize the pri

A primary resonant flyback converter may include a primary winding, a resonant capacitor in series with the primary winding, a secondary winding magnetically coupled to the primary winding, and an output electrically coupled to the secondary winding. A main switch may be operated to energize the primary winding when closed and transfer energy stored in the primary winding to the secondary winding when open. An auxiliary switch may be configured to switch complimentarily to the main switch, thereby allowing a resonant current to circulate through the primary winding and capacitor. Switch timing may be controlled to produce a desired output voltage. The switching frequency may be varied as a function of output load, input voltage and/or voltage ripple on a DC bus of the converter. Switching may also be temporarily disabled responsive to a decrease in output load and re-enabled responsive to an increase in output load.

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1. A primary resonant flyback converter comprising: a primary winding;a secondary winding magnetically coupled to the primary winding and electrically coupled to an output rectifier, and an output terminal;a resonant capacitor coupled in series with the primary winding;a main switch configured to sw

1. A primary resonant flyback converter comprising: a primary winding;a secondary winding magnetically coupled to the primary winding and electrically coupled to an output rectifier, and an output terminal;a resonant capacitor coupled in series with the primary winding;a main switch configured to switch on to energize the primary winding and resonant capacitor from a DC voltage bus and to switch off to transfer energy stored in the primary winding to the secondary winding;an auxiliary switch configured to switch on during an off time of the main switch thereby allowing a resonant current to circulate through the primary winding and the resonant capacitor; anda control circuit configured to: vary a duty cycle of the main switch and the auxiliary switch to produce a desired voltage at the output terminal; andvary a switching frequency of the main switch and the auxiliary switch responsive to a change in output load on the primary resonant flyback converter in combination with at least one of an input voltage into the primary resonant flyback converter and a DC bus voltage of the primary resonant flyback converter. 2. The primary resonant flyback converter of claim 1 wherein the control circuit is configured to vary the switching frequency by increasing the switching frequency responsive to a decrease in output load or by decreasing the switching frequency responsive to an increase in output load. 3. The primary resonant flyback converter of claim 1 wherein the control circuit detects a change in output load with reference to a feedback signal from an output voltage control loop. 4. The primary resonant flyback converter of claim 1 wherein the control circuit is configured to vary the switching frequency of the main switch and the auxiliary switch responsive to both the change in output load and the input voltage. 5. The primary resonant flyback converter of claim 4 wherein the control circuit is further configured to maintain a constant ratio between the DC voltage bus and the switching frequency of the main switch and the auxiliary switch. 6. The primary resonant flyback converter of claim 1 wherein the control circuit configured to vary the switching frequency of the main switch and the auxiliary switch responsive to the DC voltage bus of the primary resonant flyback converter is further configured to vary the switching frequency of the main switch and the auxiliary switch responsive to a voltage ripple on the DC voltage bus. 7. The primary resonant flyback converter of claim 6 wherein the control circuit is configured to vary the switching frequency by increasing the switching frequency in response to a decreased ripple voltage or by decreasing the switching frequency in response to an increased ripple voltage. 8. A method of operating a power converter, the method comprising: sensing an output voltage of the power converter;sensing at least one of a load on the power converter and an input voltage into the power converter;varying a duty cycle of a main switch of the power converter to produce a desired output voltage; andvarying a switching frequency of the main switch responsive to the load on the power converter in combination with at least one of the input voltage into the power converter and a DC voltage bus of the power converter. 9. The method of claim 8 wherein varying the switching frequency of the main switch responsive to at least one of the load and the input voltage comprises at least one of: increasing the switching frequency responsive to a decrease in output load; anddecreasing the switching frequency responsive to an increase in output load. 10. The method of claim 8 wherein sensing a load on the power converter comprises sensing a feedback signal from an output voltage control loop. 11. The method of claim 8 wherein varying the switching frequency of the main switch responsive to at least one of the load on the power converter and the input voltage into the power converter comprises varying the switching frequency responsive to both the load and the input voltage. 12. The method of claim 11 wherein varying the switching frequency responsive to both the load and the input voltage comprises maintaining a constant ratio between a DC voltage bus of the power converter and the switching frequency. 13. The method of claim 8 wherein varying the switching frequency of the main switch responsive to the DC voltage bus of the power converter further comprises varying a switching frequency of the main switch responsive to a ripple voltage on a DC bus of the power converter. 14. The method of claim 13 wherein varying the switching frequency of the main switch responsive to a ripple voltage on a DC bus of the power converter further comprises at least one of: increasing the switching frequency in response to a decreased ripple voltage; anddecreasing the switching frequency in response to an increased ripple voltage. 15. The method of claim 8 wherein varying a switching frequency of the main switch responsive to the load on the power converter comprises temporarily disabling operation of the main switch responsive to a decrease in output load on the power converter. 16. The method of claim 15 further comprising re-enabling operation of the main switch responsive to an increase in output load on the power converter. 17. A primary resonant flyback converter comprising: a primary winding;a secondary winding magnetically coupled to the primary winding and electrically coupled to an output rectifier, and an output terminal;a resonant capacitor coupled in series with the primary winding;a main switch configured to switch on to energize the primary winding and resonant capacitor from a DC voltage bus and to switch off to transfer energy stored in the primary winding to the secondary winding;an auxiliary switch configured to switch on during an off time of the main switch thereby allowing a resonant current to circulate through the primary winding and the resonant capacitor; anda control circuit configured to: implement a hysteretic control algorithm to temporarily disable and re-enable operation of the main switch and the auxiliary switch responsive to a decrease in output load on the primary resonant flyback converter. 18. The primary resonant flyback converter of claim 17 wherein the control circuit is configured to re-enable operation of the main switch and the auxiliary switch responsive to an increase in output load. 19. The primary resonant flyback converter of claim 18 wherein the hysteretic control algorithm references a high threshold corresponding to a load condition at which switching should be enabled and a low threshold corresponding to a load condition at which switching should be disabled. 20. The primary resonant flyback converter of claim 19 wherein the high and low thresholds are compared to a feedback signal from the output voltage control loop.