Total Harmonic Distortion and Power Factor Improvement Technique for CRM Flyback PFC Converters

In many of today's solid-state lighting applications, Critical Conduction Mode (CCM) Flyback PFC converter is a popular solution due to its limited component count and simple structure with respect to other SMPS topologies. As described in international standards documents (IEC/ EN 61000-3-2), drivers are required to demonstrate a power factor results greater than 0.9 and lower THD for power levels above 25W. Due to non-sinusoidal input current waveform of the traditional constant-ontime control method, achieving improved PF and THD is quite difficult. In this paper, a new control method is introduced in an effort to improve power factor and total harmonic distortion of Voltage Mode CRM Flyback PFC Converter. The work proposes injection of a compensating sin(omega t) signal component in the feedback path utilizing an additional primary transformer winding to linearize the current from the source. The detailed theoretical analysis is presented along with measurement results of a 30W high-power system prototype.