Analysis of Line Current Harmonics for Single-Phase PFC Converters

Line current distortion for active PFC converters is usually governed by the IEC61000-3-2 harmonic standard. Feedback and feed-forward loop design for these circuits generally involves the inherent tradeoff between input current distortion and dynamic response. Typically, simulations are performed to help determine the optimal compromise between these two parameters. However creating a valid simulation model can be quite complicated and time consuming. This is generally the case with PFC converters due to the presence of multiple, low and high frequency feedback loops. Complicating matters even further, the model must properly account for crossover-distortion, which at the higher line frequencies can easily dominate the harmonic spectrum. Proposed here is an alternative approach in which we write all of the governing equations and allow the powerful symbolic processor contained within a MathCAD routine to carry out the actual computation. This method is not only faster but also provides a means for error checking not possible with a simulation. This paper outlines an elegant method using the classical Fourier series and integrals for the analysis of these harmonics. Here we predict the input line current distortion with excellent convergence between theoretical and actual results. This paper is based on a conventional boost topology utilizing the L4981 PFC chip. The equations and techniques laid out in this paper also provide a convenient basis for the evaluation of more sophisticated loops.