Introduction
This application note describes step-by-step design considerations for a power supply using the FAN480X controller. The FAN480X combines a PFC controller and a PWM controller. The PFC controller employs average current mode control for Continuous Conduction Mode (CCM) boost converter in the front end. The PWM controller can be used in either current mode or voltage mode for the downstream converter. In voltage mode, feed-forward from the PFC output bus can be used to improve the line transient response of PWM stage. In either mode, the PWM stage uses conventional trailingedge duty cycle modulation, while the PFC uses leadingedge modulation. This proprietary leading/trailing-edge modulation technique can significantly reduce the ripple current of the PFC output capacitor.

The synchronization of the PWM with the PFC simplifies the PWM compensation due to the controlled ripple on the PFC output capacitor (the PWM input capacitor). In addition to power factor correction, a number of protection features have been built in to the FAN480X. These include programmable soft-start, PFC over-voltage protection, pulse-by-pulse current limiting, brownout protection, and under-voltage lockout.