This application note explains how to design a transition-mode PFC pre-regulator using the L6563S and L6563H. The L6563S and L6563H are the latest devices from STMicroelectronics for low and middle power applications such as lamp ballasts, high-end adapters, flatscreen TVs and monitors, PC power supplies, and all SMPS requiring harmonics reduction. The L6563S is a current-mode PFC controller operating in transition mode (TM) and is packaged in the same SO14 pinout as its predecessor L6563, offering improved performance and additional functions. The L6563H is the SO16 pinout version, embedding the same features as the L6563S with the addition of a high-voltage startup power source. These functions make the L6563H especially suitable for applications that need to be compliant with energy-saving regulations and where the PFC pre-regulator works as the master stage without an auxiliary SMPS.