In this paper, a primary-side control integrated circuits for single stage AC-DC flyback converter is realized. With the primary-side control, the system area and cost can be reduced. The proposed controller samples the feedback voltage on the auxiliary winding, which is proportional to the output voltage plus the turn-on voltage of the output diode. A simple knee point detector is adopted to ensure the sampling is near the instant when the inductor current is zero. Thus, the feedback voltage is more accurate under various load conditions because the diode voltage is not affected by the diode current. Since the auxiliary winding voltage can not accurately reflect the output voltage under low input voltage condition, the duty ratio is kept as constant under this condition. Thus, good load regulation can be achieved. Finally, the chip is integrated with TSMC 0.25 urn CMOS high voltage mixed signal general purpose process.