What is the difference between Single Stage and Two Stage Power Supply?

There are 2 types of Power Factor Correction circuits; one is Single Stage; the other is Two Stage.

Single Stage power supply combines functions of power factor correction and converter in one circuit but Two Stage use two separate circuits. Compared to Single Stage, Two Stage design is more complex and costly, but the immunity performance of Two Stage power supply against AC mains is much better than that of Single Stage power supply. In addition, Two Stage power supplies manifest better ripple noises performance on output. Owing to that Single Stage is only suitable for fields with quality AC mains but Two Stage can be used in serious circumstances for LED drivers or as industrial switching power supplies.