Study on Subharmonics in a Flyback Converter Considering Transformer Inductance and Parasite Capacitance of Switching Device

This paper presents the subharmonics phenomenon of switching frequency in a flyback converter (FBC). Discontinuous conduction mode (DCM) and continuous conduction mode (CCM) are interleaving, which generates 1/2, 3/2, 5/2... switching frequency subharmonics. The subharmonics are related to the oscillation between the transformer inductance and the parasite capacitance of the switching device. Numerical analysis validates the subharmonic phenomenon in a 132 kHz 15 W FBC, the analysis results suggest that the subharmonics are minimum when the primary and equivalent secondary side leakage impedances are equal in the transformer.