Ac signal detector and the method thereof

An AC/DC converter, comprising:

• a X-cap having a first terminal and a second terminal, wherein the X-cap is coupled in parallel with an AC signal;
• a rectify circuit having a first input terminal and a second input terminal configured to receive the AC signal, and an output terminal configured to provide a rectified signal based on the AC signal;
• a detecting circuit having an input terminal coupled to the output terminal of the rectify circuit to receive the rectified signal, and an output terminal configured to provide a square signal based on the rectified signal;
• an unplug indicate circuit having an input terminal coupled to the detecting circuit to receive the square signal, and an output terminal configured to provide an unplug indicate signal based on the square signal;
• and
• a discharging circuit having a first terminal coupled to the first terminal of the X cap, a second terminal connected to the ground and a control terminal configured to receive the unplug indicate signal;
• wherein the discharging circuit discharges the X-cap when the unplug indicate signal is valid.

A method for detecting an AC signal, comprising:

• rectifying an AC signal to a rectified signal by a rectifier;
• generating a delayed rectified signal by delaying the rectified signal;
• generating a square signal based on the comparison of the rectified signal and the delayed rectified signal;
• detecting the square signal to generate a pulse at every switching cycle of the square signal;
• and
• generating an unplug indicate signal based on comparing the interval time between the adjacent pulses with a preset value;
• wherein the unplug indicate signal is valid to indicate that the AC signal is unplugged when the interval time between the adjacent pulses is larger than the preset value, and the unplug indicate signal is invalid to indicate that the AC signal is plugged properly when the interval time between the adjacent pulses is smaller than the preset value.