Method and apparatus for sensing the input voltage of a power converter

1. A method of detecting zero crossings in an AC input voltage at an input of a power converter, the method comprising:

measuring, at a first time, a first voltage at the input of the power converter;

if the first voltage is positive:

determining that the first time corresponds to a positive half-cycle of the AC input voltage; and

if the first voltage is equal to approximately zero:

a) determining that the first time corresponds to a negative half-cycle of the AC input voltage;

b) turning on a high-side switch of the power converter for a first time period; and

c) measuring, during the first time period, a second voltage at the input of the power converter;

if the second voltage is less than a first threshold, turning off the high-side switch and repeating b) and c) after a time delay; and

if the second voltage is less than a second threshold, maintaining the high-side switch in an ON state for a second time period.

2. The method of claim 1 wherein the first threshold corresponds to a forward voltage drop of a diode corresponding to a low-side switch.

3. The method of claim 1 wherein the first threshold is on the order of 3 V.

4. The method of claim 1 wherein the second threshold is a negative value.

5. The method of claim 1 wherein the second time period is longer than the first time period.

6. The method of claim 1 wherein the first time period is chosen so that the AC input voltage at the input of the power converter is able to be sampled.

7. The method of claim 1 further comprising turning on a low-side switch during the positive half-cycle.

8. The method of claim 7 wherein the low-side switch and the high-side switch are operated as mutually exclusive switches.

9. The method of claim 1 further comprising, prior to turning on the high-side switch, measuring a DC bus voltage.

10. The method of claim 9 wherein, measuring the second voltage comprises computing a third voltage equal to the second voltage minus the DC bus voltage.

11. A method of forming guard bands at zero crossings of an AC input signal, the method comprising:

turning on a low-side switch;

measuring a first voltage corresponding to the AC input signal;

determining that the first voltage is less than or equal to a first threshold;

turning off the low-side switch;

waiting a predetermined time;

turning on a high-side switch for a first time period;

measuring a second voltage corresponding to the AC input signal;

computing a third voltage using the second voltage;

determining that the third voltage is less than a second threshold;

turning on the high-side switch for a second time period;

measuring a fourth voltage corresponding to the AC input signal;

computing a fifth voltage using the fourth voltage;

determining that the fifth voltage is greater than a third threshold;

turning off the high-side switch; and

turning on the low-side switch.

12. The method of claim 11 wherein the first threshold corresponds to a forward voltage drop of a diode corresponding to the low-side switch.

13. The method of claim 11 further comprising measuring a DC bus voltage, wherein computing the third voltage comprises subtracting the DC bus voltage from the second voltage.

14. The method of claim 11 wherein the second threshold is a negative value.

15. The method of claim 11 wherein the third threshold is a negative value.

16. The method of claim 11 further comprising waiting a second predetermined time before turning on the low-side switch.

17. The method of claim 11 further comprising:

measuring a sixth voltage corresponding to the AC input signal; and

determining that the sixth voltage is greater than zero before turning on the low-side switch.

18. The method of claim 17 wherein the sixth voltage is greater than a forward voltage drop of a diode corresponding to the low-side switch.

19. A method of detecting a positive to negative crossing in an AC input signal, the method comprising:

measuring a first voltage corresponding to the AC input signal;

determining that the first voltage is less than or equal to a first threshold;

turning off a low-side switch;

measuring a second voltage corresponding to the AC input signal, wherein the second voltage is measured through a diode of the low-side switch;

determining that the second voltage is less than or equal to a second threshold;

waiting a first time period;

turning on a high-side switch for a second time period;

measuring, during the second time period, a third voltage corresponding to the AC input signal;

if the third voltage is less than a third threshold, turning off the high-side switch and repeating the steps of turning on the high-side switch and measuring the third voltage after a third time period; and

if the third voltage is less than a fourth threshold, maintaining the high-side switch in an ON state for a remainder of the AC half-cycle.

20. The method of claim 19 , wherein

the first threshold is approximately 3 V;

the second threshold is approximately 0 V; and

the fourth threshold is a negative value.