EMI Filter Query

[Techman_7]

The EMI filter can be a pi filter or with common mode choke.
When to use and why to use a pi filter configuration OR common mode choke filter configuration for -

1. Buck converter power supply and
2. BLDC motor inverter.

 

[scopeprobe]

For ANY converter a pi filter can be usually to attenuate differential mode emissions and can be used to filter both the input, output or both to meet conducted emissions targets. A common mode filter can be used as well as a pi filter for common mode currents and sometimes even to provide both common mode and differential mode (this usually uses the leakage inductance as a differential element).

A CM filter is usually necessary in any design where the parasitic elements in the design can cause circulating currents to form in the same direction on all conductors which can then radiate (usually on wires but can be anything). A CM Choke impedes (provides a high resisance) against these common currents and is usually used with capacitors to provide a alternative low impedance path as part of a CM filter.

 

[Techman_7]

Thanks scopeprobe for reply.
Generally it is seen that for
a SMPS (AC-DC) converter, CM filter is used.
And pi filter is used for a buck power supply (DC-DC)

So, any particular reason for this?
As you said about parasitics, so CM filter should be used for buck power supply instead of pi filter.

 

[scopeprobe]

Thanks scopeprobe for reply.
Generally it is seen that for
a SMPS (AC-DC) converter, CM filter is used.
And pi filter is used for a buck power supply (DC-DC)

So, any particular reason for this?
As you said about parasitics, so CM filter should be used for buck power supply instead of pi filter.

AC converters or DC converters are the same, they both have the ability to create both CM and DM emissions, the choice of filtering usually comes down to the EMC requirements and the topology used for the supply being filtered. AC Supplies often have bespoke magnetics where its easier to deal with the differential emissions at source (by generating as little as possible through a well design magnetic) this usually leaves the CM Mode emissions as the bigger issue to resolve through filtering but AC supplies like DC ones can require both CM and DM mode filters, some CM chokes are also high leakage so does differential and common mode. Often AC supplies as complete products so have to meet regulations so are designed with these in mind.

DC to DC converters such as the Buck are usually available as development boards and arn't part of a final product so usually doesn't have to meet any regulations. They sometime use filtering using a PI network to control the DM mode emissions.