Advanced EMI filters keep brush DC motors “low cost”: Page 4 of 6

April 27, 2020 //By Christophe Cambrelin, Johanson Dielectrics
Advanced EMI filters keep brush DC motors “low cost”
Brush DC motors need to meet higher EMC requirements as electronic environments get more crowded and “noisy”. Further, EMI issues are going to become more of a problem with the higher frequencies with Bluetooth, Wi-Fi and now 5G devices. EMI filters will have to handle wider frequency ranges while allowing the appropriate signals to pass through.

To meet increasing EMC requirements, however, low cost solutions such as 2-capacitor differential filters are insufficient because unmatched capacitors generate a different filtering of each line, and therefore mode conversion (i.e. part of common-mode noise is transformed into differential-mode noise, and vice-versa). Traditional 3-capacitor filters are adequate, provided the EMC requirements are only at relatively low frequencies (i.e., < 150 MHz, such as AM/FM radios in automotive).  

Although they provide good filtering performance, 3-capacitor filters are generally ineffective when filtering noise in telecom frequency bands. Other solutions like feed-thru filters offer good rejection over a wide frequency band, but become expensive when the power line must carry a current of several amperes.  

Additionally, feed-through filters are single-ended devices, and therefore may introduce mode conversions (like 2-cap filters).

Regardless of the noise generated, if a high DC current is required you will need a very large, expensive feed-through filter, which eliminates the brush DC motor as a low-cost solution.

For brush DC motors, a possible alternative to low-pass filter is a common mode choke.

When a common-mode signal (same AC current) is going through each winding of the common-mode choke, the magnetic field coming from each winding adds up, and therefore the impedance increases significantly. On the other hand, when a differential signal (opposite AC current) is going through each winding, the magnetic field coming from each winding will subtract to each other and therefore the impedance decreases significantly.

That is why common mode chokes block common mode noise, but let a differential signal go through. Similar to feed-thru filters, a bigger and more expensive common choke is required to be able to carry a significant current (i.e., more than 1-A rms).

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