The R2A20113A controls a boost converter to provide an active power factor correction. The R2A20113A adopts critical conduction mode for power factor correction and realizes high efficiency and a low switching noise by zero current switching. Because the zero current is detected by using the GND current, the ZCD Auxiliary winding is unnecessary. The feedback loop open detection, two mode overvoltage protection, overcurrent protection are built in the R2A20113A, and can constitute a power supply system of high reliability with few external parts.