The R2A20112 controls a boost converter to provide a active power factor correction. The R2A20112 adopts critical conduction mode for power factor correction and realizes high efficiency and a low switching noise by zero current switching. Interleaving function improve ripple current on input or output capacitor by 180 degrees phase shift. The feedback loop open detection, two mode overvoltage protection, overcurrent protection are built in the R2A20112, and can constitute a power supply system of high reliability with few external parts.