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

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.

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.