Switching parameter based discontinuous mode-critical conduction mode transition

1. An apparatus comprising:

a switching power converter;

a load, coupled to the switching power converter, wherein the load comprises one or more light emitting diodes;

a controller, coupled to the switching power converter, to provide at least dual-mode conduction control of the switching power converter, wherein the controller is configured to:

control transitions between discontinuous conduction mode and critical conduction mode of the switching power converter using a measured switching time parameter having a value corresponding with an approximately peak voltage of a time-varying supply voltage supplied to the switching power converter.

2. The apparatus of claim 1 wherein (i) the controller is further configured to control transfer of energy from a primary-side coil of the switching power converter to a secondary side coil, (ii) the measured switching time parameter value is a minimum flyback time of the secondary side coil corresponding to a fraction of a maximum flyback time for a particular cycle of the supply voltage, and (iii) the controller is further configured to:

transition the switching power converter from discontinuous conduction mode to critical conduction mode when the flyback time is greater than the minimum flyback time; and

transition the switching power converter from critical conduction mode to discontinuous conduction mode when the flyback time is less than the minimum flyback time.

3. The apparatus of claim 1 wherein the switching power converter includes a switch to control power conversion, and the switching parameter comprises a non-conductive time of the switch corresponding to a flyback time of a secondary coil of a transformer.

4. The apparatus of claim 1 wherein to control the transitions between discontinuous conduction mode and critical conduction mode of the switching power converter the controller is further configured to:

generate a switch control signal to control conductivity of the switch, wherein the switch control signal is periodic;

determine a flyback time of the secondary coil of the transformer;

detect a maximum flyback time of the secondary coil of the transformer;

determine a minimum non-conductive time of the switch for a period of the control signal, wherein the minimum non-conductive time of the switch is less than the maximum flyback time of the secondary coil of the transformer; and

control the switch to enforce the minimum non-conductive time of the switch per period of the switch control signal.

5. The apparatus of claim 4 wherein the controller is configured to control the switch of the switching power converter to:

transition from discontinuous conduction mode to critical conduction mode when power demand by the load coupled to the switching power converter results in the flyback time of the secondary coil exceeding the minimum flyback time.

6. The apparatus of claim 5 wherein the controller is configured to meet the power demand of the load by setting the peak current in the primary side coil of the transformer for each cycle of the switch control signal during a half cycle of the supply voltage so that the charge delivered to the load meets the power demand of the load.

7. The apparatus of claim 4 wherein the controller is configured to control the switch of the switching power converter to:

transition from critical conduction mode to discontinuous conduction mode when power demand by a load coupled to the switching power converter results in the flyback time of the secondary coil falling below the minimum flyback time.

8. The apparatus of claim 4 wherein to control a switch of the switching power converter to enforce the minimum non-conductive time of the switch per period of the switch control signal, the controller is configured to:

set a peak current in a primary side coil of the transformer for each cycle of the switch control signal.

9. The apparatus of claim 1 wherein the measured switch time parameter has a value occurring at approximately a peak voltage of a time-varying supply voltage supplied to the switching power converter.

10. The apparatus of claim 1 wherein the measured switch time parameter has a value occurring at a non-peak voltage of a time-varying supply voltage supplied to the switching power converter.

11. A method comprising:

providing at least dual-mode conduction control of a switching power converter, wherein providing at least dual-mode conduction control of a switching power converter comprises:

controlling transitions between discontinuous conduction mode and critical conduction mode of the switching power converter using a measured switching time parameter having a value corresponding with an approximately peak voltage of a time-varying supply voltage supplied to the switching power converter; and

providing power from the switching power converter to a load, wherein the load comprises one or more light emitting diodes.

12. The method of claim 11 further comprising:

controlling transfer of energy from a primary-side coil of the switching power converter to a secondary side coil, wherein the measured switching time parameter value is a minimum flyback time of the secondary side coil corresponding to a fraction of a maximum flyback time for a particular cycle of the supply voltage;

transitioning the switching power converter from discontinuous conduction mode to critical conduction mode when the flyback time is greater than the minimum flyback time; and

transitioning the switching power converter from critical conduction mode to discontinuous conduction mode when the flyback time is less than the minimum flyback time.

13. The method of claim 11 wherein controlling the transitions between discontinuous conduction mode and critical conduction mode of the switching power converter further comprises:

generating a switch control signal to control conductivity of the switch, wherein the switch control signal is periodic;

determining a flyback time of the secondary coil of the transformer;

detecting a maximum flyback time of the secondary coil of the transformer;

determining a minimum non-conductive time of the switch for a period of the control signal, wherein the minimum non-conductive time of the switch is less than the maximum flyback time of the secondary coil of the transformer; and

controlling the switch to enforce the minimum non-conductive time of the switch per period of the switch control signal.

14. The method of claim 13 further comprising:

controlling a switch of the switching power converter to transition from discontinuous conduction mode to critical conduction mode when power demand by a load coupled to the switching power converter results in the flyback time of the secondary coil exceeding the minimum flyback time.

15. The method of claim 13 further comprising:

controlling a switch of the switching power converter to transition from critical conduction mode to discontinuous conduction mode when power demand by a load coupled to the switching power converter results in the flyback time of the secondary coil falling below the minimum flyback time.

16. The method of claim 13 wherein controlling the switch to enforce the minimum non-conductive time of the switch per period of the switch control signal further comprises:

setting a peak current in a primary side coil of the transformer for each cycle of the switch control signal.

17. The method of claim 11 wherein the controller is configured of meeting the power demand of the load by setting the peak current in the primary side coil of the transformer for each cycle of the switch control signal during a half cycle of the supply voltage so that the charge delivered to the load meets the power demand of the load.

18. The method of claim 11 wherein the time-varying supply voltage is a rectified alternating current (AC) line voltage.

19. The method of claim 11 wherein the measured switch time parameter has a value occurring at approximately a peak voltage of a time-varying supply voltage supplied to the switching power converter.

20. The method of claim 11 wherein the measured switch time parameter has a value occurring at a non-peak voltage of a time-varying supply voltage supplied to the switching power converter.