Advanced EMI filters keep brush DC motors “low cost”: Page 5 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.

Despite the popularity of common mode chokes, a better alternative may be monolithic EMI filters.

Compared to common mode chokes, monolithic EMI filters provide significantly more RFI suppression in a substantially smaller package. A monolithic EMI filter also rejects a much wider frequency band and is not affected by the amount of DC current required because it is mounted in shunt (between lines and ‘ground’).

EMI filters combine two balanced shunt capacitors in a single package, with mutual inductance cancellation and shielding effect. These filters from Johanson Dielectrics utilize two separate electrical pathways within a single device attached to four external connections.

Like other EMI filters, monolithic EMI filters attenuate all energy above a specified cut-off frequency, only selecting to pass required signal energy while diverting unwanted noise to ‘ground’.

The key, however, is the very low inductance and matched impedance. With monolithic EMI filters, the terminations connect internally to a common reference (shield) electrode within the device, and the plates are separated by the reference electrode.   

Monolithic EMI filters can be effective from 50 KHz to 6 GHz and it filters both common-mode and differential mode noise. The filter also has virtually no limit to the amount of DC current, because it is designed to work in parallel to motor and no DC current flows through it.

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