Cuk and SEPIC converters operating in discontinuous current mode (DCM), used as power factor preregulators (PFP), present some advantages over other topologies: sinusoidal current shaping using constant frequency and duty-cycle, low input current harmonic content, low input filter requirements, isolation between line and load, etc. The main drawbacks are the voltage and current stresses that devices are submitted to. In this paper, an auxiliary circuit is proposed which limits the overvoltage on the main switch and allows its zero-voltage turn off. Zero-current turn on is provided by the DCM operation. In this way switch commutations are soft, thus improving efficiency and reducing RF noise. Design criteria and experimental results are reported.
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