Improvement of CM EMI Attenuation Ability of Transformer with Negative Capacitor

The capacitance balance technique to reduce CM EMI noise for isolated converters would be less effective at high frequency. A third-order CM model of the transformer is derived and verified from the two coupled windings and the six-capacitor model, which has the advantage of a wider effective frequency range compared to the two-capacitor model. The failure of the capacitance balance technique can be explained by the proposed model. With the understanding of the failure mechanism, a negative capacitance is applied to extend the effective frequency range of the balancing technique, which is also verified by experiments.