Variable On-time Control of the Critical Conduction Mode Boost Power Factor Correction Converter to Improve Zero-crossing Distortion

The critical conduction mode boost power factor correction converter operating at the boundary of continuous conduction mode and discontinuous conduction mode has been widely used in low power application. But in many applications, it is hard to make the total harmonic distortion(THD) less than 10% when the input line voltage is very high. The increase of THD comes from the zero-crossing distortion near the ac line zero crossing point. In this paper, the reason of the zero-crossing distortion is investigated and a new variable on-time control method is proposed to improve THD. The proposed method uses the voltage mode PWM changing the ramp slope according to the ac line voltage to control the switch on-time. The performance was verified with a 100W boost PFC converter. Using the proposed method, THD improved greatly and was lower than 10% even the load was half of the full load.