Variable on-Time Controlled Buck-Boost PFC Converter in Critical Conduction

For applications featuring a wide input voltage range (90–265 V), the buck-boost power factor correction (PFC) converter frequently adopts the constant on-time (COT) control strategy, which facilitates achieving low total harmonic distortion (THD) and high power factor (PF). However, due to phase differences and harmonic distortions between the average input current and voltage waveforms, PF theoretically cannot attain unity. To theoretically eliminate the impact of harmonics in buck-boost PFC converters, this paper introduces a variable on-time (VOT) control strategy. In this strategy, the on-time and off-time intervals within the current switching cycle are detected to correct the modulation wave of the traditional COT control strategy. During the subsequent switching cycle, the modified modulation wave and the peak inductor current are sent to the comparator to generate pulse-width modulation (PWM) pulses. Additionally, the switching frequency is calculated based on the on-time and off-time. When the switching frequency reaches a sufficiently high level, the converter automatically transitions to discontinuous conduction mode (DCM) to minimize switching losses. Experimental results, obtained using a 100 W buck-boost PFC converter platform, reveal that this strategy reduces THD by an average of 3.07% and enhances PF by 0.34% compared to the traditional COT control strategy.