Modelling & a New Control Strategy for CCM Buck-Boost PFC

In PFC applications, ripple-based control offers lower total harmonic distortion (THD) than average current mode control (ACM), at a cost of potential sub-harmonic oscillation caused by its sample-hold behavior [1]. Although introducing ramp compensation to the charge control eliminates the harmful sub-harmonic oscillation [2], it will compromise the input THD. In contrast, ACM doesn't suffer from sub-harmonic oscillation [3]. However, to date, direct input current feedback of an ACM Buck-Boost PFC hasn't been proper [2]ly modelled and the results are unsatisfactory. This work proves that a previously unnoticed moving zero exists on the small signal transfer function, and it leads to a severe oscillation near zero crossing. Researchers have found that in ACM converters, a feedforward path exists, and it causes unwanted phase shift in the input current. This work adopts the leading phase admitance concept in a Buck-Boost PFC with comprehensive analytical solution, the proposed compensation method reduces THD and eliminates the phase shift, allowing the Buck-Boost PFC to operate at a much higher grid frequency (400HZ to 1000HZ) for the use of powergrid in MEA. In addition, the compensation alleviates the zero-crossing oscillation.