Method and system for entering and exiting a frequency clamp mode for variable frequency, offline switch-mode power converters

1. A method of operating a switching power converter having an input operable to receive a periodic input signal and an output operable to provide a substantially constant output voltage, the method comprising:

measuring a switching time interval or a switching frequency of the switching power converter;

comparing the switching time interval or the switching frequency with a predetermined threshold value to obtain a comparison result;

recording an entry criteria in response to the comparison result, wherein the entry criteria indicates a transition of the switching power converter from a variable switching frequency state to a constant switching frequency state; and

defining an exit criteria based on the entry criteria and symmetric properties of the periodic input signal.

2. The method of claim 1 wherein the entry criteria is associated with a time value, a voltage level, or a phase angle of periodic input signal.

3. The method of claim 1 wherein defining the exit criteria comprises:

obtaining an amplitude level of the periodic input signal at which an entry transition occurs; and

determining the exit criteria based on the obtained amplitude level.

4. The method of claim 3 wherein the exit criteria comprises hysteresis to prevent the switching power converter from oscillating between the constant switching frequency state and the variable switching frequency state.

5. The method of claim 3 wherein obtaining the amplitude level of the periodic input signal comprises determining a change direction of the periodic input signal.

6. The method of claim 1 wherein the exit criteria is determined based on mirroring of the entry criteria with respect to a zero-crossing point of the periodic input signal or with respect to a peak voltage of the periodic input signal.

7. The method of claim 1 further comprising:

sampling an amplitude of the output voltage at a zero-crossing point of the periodic input signal; and

determining a load condition of the switching power converter; and

changing the constant switching frequency state to the variable switching frequency state in response to the determined load condition.

8. The method of claim 1 wherein the constant switching frequency state comprises a constant switching frequency that is lower than a variable switching frequency of the variable, switching frequency state.

9. The method of claim 1 wherein the constant switching frequency state is a discontinuous conduction mode and the variable switching frequency state is a critical conduction mode.

10. The method of claim 1 wherein the switching time interval or the switching frequency of the switching power converter is measured using a monitor device.

11. A controller for controlling a switching power converter having an input for receiving a periodic input signal and an output for providing a substantially constant output voltage to a load, the controller comprising:

a monitor device configured to compare a switching time interval or a switching frequency with a predetermined threshold value to obtain a comparison result; and

a parameter recorder configured to record an entry criteria in response to the comparison result, wherein the entry criteria indicates a transition of the switching power converter from a variable switching frequency state to a constant switching frequency state, and wherein the parameter recorder is further configured to define an exit criteria based on the entry criteria and symmetric properties of the periodic input signal.

12. The controller of claim 11 wherein the entry criteria is associated with a time value, a voltage level, or a phase angle of the periodic input signal.

13. The controller of claim 11 wherein the entry criteria comprises an amplitude level of the periodic input signal, and the exit criteria comprises the amplitude level of the periodic input signal.

14. The controller of claim 11 wherein the exit criteria comprises hysteresis to prevent the switching power converter from oscillating between the constant switching frequency state and the variable switching frequency state.

15. The controller of claim 11 wherein the exit criteria is determined based on time-value mirroring of the entry criteria with respect to a zero-crossing point of the periodic input signal or with respect to a peak voltage point of the periodic input signal.

16. The controller of claim 11 wherein the entry criteria is predetermined based on a voltage level of the periodic input signal.

17. The controller of claim 11 further comprising a zero-crossing detector configured to sample an amplitude of the output voltage of the switching power converter at a zero-crossing point of the periodic input signal to determine a load condition of the load, wherein the controller causes the switching power converter to transition from the constant switching frequency state to the variable switching frequency state in response to the load condition.