Method and apparatus for determining zero-crossing of an AC input voltage to a power supply

A controller for a switched mode power supply, the controller comprising:

• a drive signal generator to be coupled to control switching of a switch coupled to an energy transfer element included in the power supply to regulate an output of the power supply, wherein energy is transferred across the energy transfer element to the output of the power supply in response to the switching of the switch;
• and
• a zero-crossing detector coupled to the drive signal generator and coupled to receive a current sense signal representative of a switch current flowing through the switch, wherein the zero-crossing detector generates a zero-crossing signal in response to comparing the current sense signal with a reference signal representative of a zero-crossing current threshold, wherein the zero-crossing signal indicates when a zero-crossing condition of an ac line input voltage of the power supply exists, and wherein the drive signal generator controls the switching of the switch in response to the zero-crossing signal.