In adapters with input power over 75 W, power factor correction is used to be in compliance with European Union standards. For power level under 200 W, a CrCM or DCM controller would be used and around 200 W the use of a CCM controller is preferred. The PWM may be a Flyback converter or a resonant switching half bridge, a flyback is shown here. Standby mode is detected by an opto-coupler monitoring the output voltage and shuts down the PFC to minimize standby power requirements. The VCC power for the PFC and PWM IC is derived from the aux supply winding for the PWM supply.