The R2A20112A controls a boost converter to provide an active power factor correction. The R2A20112A adopts critical conduction mode for power factor correction and realizes high efficiency and a low switching noise by zero current switching. The interleaving function improves ripple current on the input or output capacitor by a 180-degree phase shift. Soft-star, the feedback loop short detection, two-mode overvoltage protection, overcurrent protection, overcurrent ON/OFF timer protection for boost diode short and slave ZCD open detection are built in the R2A20112A, and can constitute a power supply system of high reliability with few external parts.