Input AC line control for AC-DC converters

1. An AC-DC converter comprising:

a first rectifier circuit that is configured to rectify an input AC line voltage to generate a rectified input AC line voltage;

a DC-DC converter that is configured to receive the rectified input AC line voltage and to control a switching operation of a main switch to convert the rectified input AC line voltage to an output DC voltage; and

an input AC line control circuit that is configured to:

detect, over a half cycle of the input AC line voltage, a voltage indicative of the rectified input AC line voltage;

prohibit switching of the main switch within the half cycle when the voltage indicative of the rectified input AC line voltage is below a low threshold voltage;

enable switching of the main switch within the half cycle when the voltage indicative of the rectified input AC line voltage is between the low threshold voltage and a high threshold voltage; and

prohibit switching of the main switch within the half cycle when the voltage indicative of the rectified input AC line voltage is above the high threshold voltage;

wherein the low threshold voltage is positive and has a first magnitude, the high threshold voltage is positive and has a second magnitude, and the second magnitude is higher than the first magnitude.

2. The AC-DC converter of claim 1 , wherein the input AC line control circuit comprises:

an input voltage sensor that is configured to detect the voltage indicative of the rectified input AC line voltage; and

a comparator circuit that compares the voltage indicative of the rectified input AC line voltage to the low threshold voltage and the high threshold voltage.

3. The AC-DC converter of claim 2 , wherein the input voltage sensor comprises a second rectifier circuit.

4. The AC-DC converter of claim 1 , wherein the input AC line control circuit comprises:

an input voltage sensor that rectifies the input AC line voltage to generate the voltage indicative of the rectified input AC line voltage;

a band generator that generates the low threshold voltage and the high threshold voltage; and

an operating phase generator that is configured to compare the voltage indicative of the rectified input AC line voltage to the low threshold voltage and the high threshold voltage to determine whether or not to prohibit switching of the main switch.

5. The AC-DC converter of claim 4 , wherein the band generator is configured to adjust at least the high threshold voltage depending on load conditions.

6. The AC-DC converter of claim 4 , wherein the band generator is configured to adjust at least the high threshold voltage depending on a level of the input AC line voltage.

7. The AC-DC converter of claim 1 , further comprising a soft ramp up/down circuit for soft-starting a current through the main switch.

8. A method of operating an AC-DC converter, the method comprising:

rectifying an input AC line voltage to generate a rectified AC line voltage; and

controlling a switching operation of a main switch to generate an output DC voltage;

during a half cycle of the input AC line voltage, the controlling comprises:

preventing the main switch from switching when a level of the rectified AC line voltage is below a regulation band;

switching the main switch when the level of the rectified AC line voltage is within the regulation band;

preventing the main switch from switching when the level of the rectified AC line voltage is above the regulation band; and

wherein the regulation band has a low threshold voltage that is positive and has a first magnitude, a high threshold voltage is positive and has a second magnitude, and the second magnitude is higher than the first magnitude.

9. The method of claim 8 , further comprising:

varying the regulation band according to load conditions.

10. The method of claim 8 , wherein the switching operation of the main switch is controlled by PWM when the level of the rectified AC line voltage is within the regulation band.

11. The method of claim 10 , wherein switching the main switch further comprises switching of the main switch when the level of the rectified AC line voltage is higher than a low band threshold of the regulation band and lower than a high band threshold of the regulation band.

12. The method of claim 8 , further comprising:

adjusting the regulation band to control a peak current or turn-on time of the main switch.

13. The method of claim 8 , further comprising:

soft-starting a current through the main switch so that the current through the main switch ramps up when the main switch is turned ON.

14. The method of claim 8 , further comprising:

ramping down a current through the main switch before the main switch is prevented from switching.

15. An AC-DC converter comprising:

an input AC line voltage sensor circuit that detects an input AC line voltage;

a DC-DC converter comprising a main switch and a pulse width modulation (PWM) controller that controls a switching operation of the main switch to generate a regulated DC output voltage; and

a control circuit that, within a single half cycle of the input AC line voltage, prevents switching of the main switch when the input AC line voltage is not within a PWM zone defined by a range of input AC line voltage levels that are positive, non-zero, and less than a peak positive voltage of the input AC line voltage, and the control circuit enables switching of the main switch when the input AC input voltage is within the PWM zone.

16. The AC-DC converter of claim 15 , wherein the control circuit comprises:

a comparator circuit that compares a level of the input AC line voltage to a low threshold and a high threshold; and

a disable circuit that prevents the PWM controller from turning ON the main switch when the level of the input AC line voltage is not between the low threshold and the high threshold.

17. The AC-DC converter of claim 16 , wherein a rectifier rectifies the input AC line voltage to generate a first rectified input AC line voltage, the comparator circuit compares the first rectified input AC line voltage to the low threshold and the high threshold, and the disable circuit prevents the PWM controller from turning ON when the first rectified input AC line voltage is not between the low threshold and the high threshold.

18. The AC-DC converter of claim 15 , wherein the DC-DC converter is a buck converter.

19. The AC-DC converter of claim 15 , wherein the DC-DC converter is a boost converter.

20. The AC-DC converter of claim 15 , further comprising a bridge rectifier that rectifies the input AC line voltage to generate a second rectified input AC line voltage and the DC-DC converter converts the second rectified input AC line voltage to the DC output voltage by the switching operation of the main switch.