New Fixed-Off-Time PWM modulator provides constant frequency operation in boost PFC pre-regulators

The objective of this paper is to describe a new fixed-off-time (FOT) PWM modulator and its application in boost PFC pre-regulators. The uniqueness of this new modulator, suitable for integration in a control IC, is that it provides the OFF-time modulated with the instantaneous line voltage in such a way that the resulting switching frequency is constant and not dependent on the rms input voltage or on the output load as long as the boost stage operates in continuous conduction mode (CCM).