Power factor correction circuit and method therefor

1. A power factor correction circuit comprising:

an ac input configured to receive a first ac signal;

a zero crossing detector configured to receive the first ac signal and generate a detect signal representing a zero crossing of the first ac signal;

a switching output configured to control a power switch to regulate an output voltage to a target value within a range of values that includes a first value that is greater than the target value and a second value that is less than the target value;

a sample and hold circuit configured to receive a feedback signal that is representative of the output voltage, to store a value of the feedback signal on a storage element, to couple a stored value of the feedback signal to an output of the sample and hold circuit responsively to the detect signal, and to couple the feedback signal to the output of the sample and hold circuit responsively to the feedback signal increasing to a third value that is greater than the first value or decreasing to a fourth value than is less than the second value; and

an error amplifier configured to receive a first signal that includes a second signal from the output of the sample and hold circuit and responsively provide an error signal.

2. The power factor correction circuit of claim 1 further including a multiplier circuit coupled to multiply the output of the sample and hold circuit with the first ac signal wherein the first signal received by the error amplifier includes an output signal of the multiplier circuit.

3. The power factor correction circuit of claim 2 further including a squaring circuit coupled to receive a dc signal and mathematically square the second ac signal to form a squared signal.

4. The power factor correction circuit of claim 3 further including a divider circuit coupled to receive the squared signal and divide the output signal of the multiplier circuit by the squared signal wherein the first signal received by the error amplifier includes an output signal of the divider circuit.

5. The power factor correction circuit of claim 1 wherein the sample and hold circuit includes a first comparator configured to detect the feedback signal increasing to the third value.

6. The power factor correction circuit of claim 5 wherein the sample and hold circuit includes a second comparator configured to detect the feedback signal decreasing to the fourth value.

7. The power factor correction circuit of claim 6 wherein the sample and hold circuit couples the feedback signal to the output of the sample and hold circuit responsively to an output of either the first or second comparators.

8. The power factor correction circuit of claim 6 wherein the sample and hold circuit includes an OR gate coupled to receive an output of the first comparator and an output of the second comparator and also includes an output coupled to control the sample and hold circuit to couple the feedback signal to the output of the sample and hold circuit.

9. The power factor correction circuit of claim 1 wherein the sample and hold circuit includes a first switch having a first terminal coupled to receive the feedback signal and a second terminal coupled to the output of the sample and hold circuit, and a second switch having a first terminal coupled to receive a stored value of the feedback signal from the storage element and a second terminal coupled to the output of the sample and hold circuit.

10. The power factor correction circuit of claim 1 wherein the sample and hold circuit includes an amplifier having an input coupled to a storage element to receive the stored value of the feedback signal.

11. A method of forming a power factor circuit comprising:

configuring a switching portion of the power factor circuit to form a control signal to control a value of an output voltage within a range having an upper value and a lower value;

configuring a sample and hold circuit of the power factor circuit to sample a value of a feedback signal wherein the feedback signal is representative of a value of the output voltage; and

configuring the power factor circuit to by-pass the sample and hold circuit and couple the feedback signal to an output of the sample and hold circuit responsively to the output voltage being no less than the upper value or no greater than the lower value.

12. The method of claim 11 wherein configuring the sample and hold circuit of the power factor circuit to sample the value of the feedback signal includes configuring the sample and hold circuit to sample the value of the feedback signal responsively to a zero crossing of an ac input signal received by the power factor circuit.

13. The method of claim 12 wherein configuring the sample and hold circuit to sample the value of the feedback signal responsively to the zero crossing includes configuring the sample and hold circuit to store a value of the feedback signal and to sample a stored value of the feedback signal responsively to the zero crossing of the ac input signal.

14. The method of claim 11 further including configuring an error amplifier coupled to receive a first signal that includes a second signal on an output of the sample and hold circuit.

15. The method of claim 11 further including coupling a multiplier to receive a signal on the output of the sample and hold circuit and multiply the signal by a first ac input signal received by the power factor circuit.

16. The method of claim 15 further including coupling a squaring circuit to square a dc input signal.

17. The method of claim 16 further including coupling a divide circuit to divide an output of the multiplier by an output of the squaring circuit.

18. The method of claim 17 further including coupling an error amplifier to receive an output of the divide circuit and responsively form an error signal.

19. The method of claim 11 further including coupling a first comparator to compare the feedback signal to a first reference signal and form a first control signal to cause the power factor circuit to couple the feedback signal to the output of the sample and hold circuit.

20. A method of forming a power factor circuit comprising:

configuring a switching portion of the power factor circuit to form a control signal to control a value of an output voltage within a range having an upper value and a lower value;

configuring a sample and hold circuit of the power factor circuit to sample a value of a feedback signal wherein the feedback signal is representative of a value of the output voltage;

configuring the power factor circuit to by-pass the sample and hold circuit and couple the feedback signal to an output of the sample and hold circuit responsively to the output voltage being no less than the upper value or no greater than the lower value;

coupling a first comparator to compare the feedback signal to a first reference signal and form a first control signal to cause the power factor circuit to couple the feedback signal to the output of the sample and hold circuit; and

coupling a second comparator to compare the feedback signal to a second reference signal and form a second control signal to cause the power factor circuit to couple the feedback signal to the output of the sample and hold circuit.