Design criteria for power factor preregulators based on Sepic and Cuk converters in continuous conduction mode

Sepic and Cuk converters used as power factor preregulators in continuous conduction mode are analyzed. The application of the average current mode control technique to obtain sinusoidal input current is investigated. It is shown that, unlike the usual boost topology, the gain of the internal current loop depends strongly on the instantaneous input voltage. Moreover, the presence of undamped complex poles and zeroes makes difficult the design of a stable control. The paper shows that stability can be achieved by using a suitable damping network, which properly shapes the current loop transfer function without introducing significant power losses. With this provision, a standard PI controller can be used to ensure the desired phase margin in any operating condition. Detailed design criteria of both power and control stages are given in the paper. Results of a 300 W prototype based on the Sepic topology are also reported