Controlling power factor in a switching power converter operating in discontinuous conduction mode

1. An apparatus comprising:

a controller to control a switching power converter, wherein the controller is configured to automatically transition operation of the switching power converter, during each cycle of an input voltage to the switching power converter, between operating in discontinuous conduction mode and critical conduction mode, wherein during each cycle of a control signal to a switch that controls power delivered to a load coupled to the switching power converter, to automatically transition operation of the switching power converter between operating in discontinuous conduction mode and critical conduction mode, the controller is configured to:

determine a first period of a control signal to a switch of the switching power converter in accordance with a discontinuous conduction mode control rule, wherein the switch controls power delivered to an output of the switching power converter;

determine a second period of the control signal in accordance with a critical conduction mode control rule;

compare the first period and the second period;

set an actual period of the control signal equal to the first period if the first period is less than the second period and set an actual period of the control signal equal to the second period if the first period is greater than the second period; and

calculate an ON time of the switch in accordance with one of: (i) the discontinuous conduction mode control rule if the actual period is set to the first period and (ii) the critical conduction mode control rule if the actual period is set to the second period.

2. The apparatus of claim 1 wherein the controller is further configured to operate the switching power converter in critical conduction mode when a determined switching period of the switch in the switching power converter that controls power delivered to the load of the switching power converter is more efficient than a determined switching period of the switch for operating the switching power converter in discontinuous conduction mode.

3. The apparatus of claim 1 further comprising:

the switching power converter coupled to the controller, and

the load coupled to the switching power converter.

4. The apparatus of claim 3 wherein the load comprises a light emitting diode.

5. A method comprising:

controlling a switching power converter to automatically transition operation of the switching power converter, during each cycle of an input voltage to the switching power converter, between operating in discontinuous conduction mode and critical conduction mode, wherein to automatically transition operation of the switching power converter, during each cycle of an input voltage to the switching power converter, between operating in discontinuous conduction mode and critical conduction mode comprises:

determining a first period of a control signal to a switch of the switching power converter in accordance with a discontinuous conduction mode control rule, wherein the switch controls power delivered to an output of the switching power converter;

determining a second period of the control signal in accordance with a critical conduction mode control rule;

comparing the first period and the second period;

setting an actual period of the control signal equal to the first period if the first period is less than the second period and set an actual period of the control signal equal to the second period if the first period is greater than the second period; and

calculating an ON time of the switch in accordance with one of: (i) the discontinuous conduction mode control rule if the actual period is set to the first period and (ii) the critical conduction mode control rule if the actual period is set to the second period.

6. The method of claim 5 wherein the method further comprises:

operating the switching power converter in critical conduction mode when a determined switching period of a switch in the switching power converter that controls power delivered to a load of the switching power converter is more efficient than a determined switching period of the switch for operating the switching power converter in discontinuous conduction mode.

7. The method of claim 5 further comprising:

providing power from the switching power converter to a load.

8. The method of claim 7 wherein the load comprises a light emitting diode.

9. A lighting system comprising:

a switching power converter, wherein the switching power converter include an inductor, a link capacitor coupled to the inductor, and a switch having a first terminal coupled between the inductor and the link capacitor;

a load coupled to the link capacitor, and

a controller coupled to a conduction control terminal of the switch, wherein the controller is configured to:

(a) determine a first period of a control signal for the switch in accordance with a discontinuous conduction mode control rule;

(b) determine a second period of the control signal for the switch in accordance with a critical conduction mode control rule;

(c) compare the first period and the second period;

(d) set an actual period of the control signal equal to the first period if the first period is less than the second period and set an actual period of the control signal equal to the second period if the first period is greater than the second period;

(e) calculate an ON time of the switch in accordance with one of: (i) the discontinuous conduction mode control rule if the actual period is set to the first period and (ii) the critical conduction mode control rule if the actual period is set to the second period; and

(f) provide a control signal to the switch, wherein the control signal comprises a period equal to the actual period and a pulse width equal to the ON time calculated.

10. The lighting system of claim 9 wherein the load comprises one or more light emitting diodes.

11. The lighting system of claim 9 wherein the controller is configured to perform (a)-(f) for each cycle of the switch.

12. A method to control a switching power converter and provide power to a load, the method comprising:

(a) determining a first period of a control signal to a switch of the switching power converter in accordance with a discontinuous conduction mode control rule, wherein the switch controls power delivered to an output of the switching power converter;

(b) determining a second period of the control signal in accordance with a critical conduction mode control rule;

(c) comparing the first period and the second period;

(d) setting an actual period of the control signal equal to the first period if the first period is less than the second period and set an actual period of the control signal equal to the second period if the first period is greater than the second period;

(e) calculating an ON time of the switch in accordance with one of: (i) the discontinuous conduction mode control rule if the actual period is set to the first period and (ii) the critical conduction mode control rule if the actual period is set to the second period; and

(f) providing a control signal to the switch, wherein the control signal comprises a period equal to the actual period and a pulse width equal to the ON time calculated.

13. The method of claim 12 wherein the load comprises one or more light emitting diodes.

14. The method of claim 12 further comprising:

performing (a)-(f) for each cycle of the switch.