Digital Implementation of a Current Observer with On-Line Current Sample Correction for PFC Rectifiers

A novel method to ascertain the switching inductor current in a half-bridge-based power electronic circuits is presented in this paper. It is composed of one current transformer (CT) as a physical current sensor, auxiliary windings on the inductor, and a digital current observer based on a low-cost digital signal processor (DSP). The observer estimates the DC content and the inductor current ripple by computing the voltage-time area from the output of the auxiliary windings by utilizing the analog inputs and the time capture modules of the DSP. The mean estimated current from the observer is used for the inner current control and for sample correction to improve power factor correction (PFC) during discontinuous conduction mode (DCM). For a PFC with bipolar inductor current, such as the Totem-Pole half-bridge boost topology, the sample correction in DCM is not accurate with only one CT sensor because either the regular sample is zero or if the sampling is taken close to the switching instant it contains noise. But with the presented concept an accurate estimation of the current is possible for such cases. The proposed method is also promising as one CT in the commutation path introduces less parasitic inductance and thus the PFC-converter with modern SiC- or GaN-based devices can be operated at higher frequencies, leading to an increase in power density and cost efficiency.