Important aspects on effective common mode noise filtering by means of passive cancellation

The basic idea of passive cancellation of common mode (cm) noise has already been described in literature. It is a very promising alternative to conservative cm filtering. A thorough analysis of parasitic effects as well as the effects of a finite differential mode (dm) filter shows that a significant reduction of cm noise is achievable. However, in a lot of applications the electromagnetic compatibility (EMC) standard has not yet been fulfilled by using only the cancellation method and an additional compact conservative cm filter is necessary. Combining the passive cancellation filter with a compact conservative cm filter yields excellent results. However, two important aspects have not been addressed so far in literature. First, the asymmetries in the boost converter and the parasitic equivalent series inductance (ESL) in the dm filter generate a noteworthy dm noise in the frequency range above 10MHz. Hence, a detailed investigation of the resulting circuit is essential. Second, the component tolerances have to be taken into account. Especially the mismatch between the parasitic capacitor and the compensation capacitor limits the achievable attenuation. An analysis based on the frequency-response allows quantifying the influences of different design parameters and thus the circuit can be optimized. The theoretical results are verified in a test set-up.