Basic Guidelines for EMI Filtering

October 20, 2006

All switching power supplies generate signals that can potentially interfere with other electronic circuits as a result of the switching action. Vicor’s quasi-resonant, zero-current switching topology generates far less conducted and radiated noise than other topologies, both in magnitude and frequency spectrum. EMI filtering can reduce the noise by an additional 40-60 dB.The most effective means to reduce common mode noise current is to bypass both input leads as well as both output leads with “Y” capacitors, keeping the leads short to reduce parasitic inductance. Additionally, a common mode choke is usually required to meet common EMI standards.

Figure 1 – shows a typical filter circuitry for a Vicor converter with its corresponding noise spectra.Figures 2a, 2b, 2c – Conducted input noise, with common-mode choke

Some basic guidelines to follow for successful EMI filtering are:

  1. Keep current loops small. The ability of a conductor to couple energy by induction and radiation is proportional to the loop area.
  2. For conductor pairs, use wide (low Z) copper traces aligned above and below each other.
  3. Locate filters close to the source of interference, i.e. the power converter.
  4. Filter component values should be chosen with consideration to the desired frequency range of attenuation. For example, capacitors are self-resonant at some frequencies, beyond which they look inductive. Keep bypass capacitor leads as short as possible.
  5. Bear in mind the proximity of noise sources to potentially susceptible circuits when locating components on the board.

Further information on EMI Filtering may be found in the EMI Considerations section of the VI-200/VI-J00 Design Guide

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