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March, 2020


www.us-tech.com


Advanced EMI Filters Keep Brushed DC Motor Costs Low


Continued from previous page


than a selected cutoff frequency and attenuate signals with frequencies higher than the cutoff frequency. Among the options for OEMs


are two-capacitor differential, three- capacitor (one x-cap and two y-caps), feed-through filters, common-mode chokes, LC filters, or combinations of these.


To meet increasing EMC re-


quirements, however, low-cost solu- tions, such as two-capacitor differen- tial filters are insufficient, because unmatched capacitors generate a dif- ferent filtering of each line, and therefore allow mode conversion (part of common-mode noise is trans- formed into differential-mode noise


Increasing EMC


requirements are driving the cost of brushed DC motors to a level on par with more expensive brushless alternatives.


and vice versa). Traditional three-ca- pacitor filters are adequate, provided the EMC requirements are only at relatively low frequencies (i.e., less than 150 MHz, such as AM/FM ra- dios in cars). While they provide decent filter-


ing performance, three-capacitor fil- ters are generally ineffective when fil- tering noise in telecom frequency bands. Other solutions, like feed- through filters, offer good rejection over a wide frequency band, but be- come expensive when the power line must carry a current of several Amps. In addition, feed-through filters


are single-ended devices and may in- troduce mode conversions like two- cap filters. “Regardless of the noise generated, if a high-DC current is re- quired, you will need a very large, ex- pensive feed-through filter, which eliminates the brushed DC motor as a low-cost solution,” says Cambrelin.


Monolithic EMI Filters For brushed DC motors, a possi-


ble alternative to a low-pass filter is a common-mode choke, while a bet- ter alternative may be monolithic EMI filters. When a common-mode signal (same AC current) is going through each winding of the com- mon-mode choke, the magnetic field coming from each winding adds up, increasing impedance significantly. On the other hand, when a differen- tial signal (opposite AC current) is going through each winding, the magnetic field coming from each winding will subtract to each other, reducing impedance. This is why common-mode


chokes block common-mode noise, but let a differential signal go through. Similar to feed-through filters, a big- ger and more expensive common choke is required to carry a significant current (i.e., more than 1A rms). Compared with common-mode


chokes, monolithic EMI filters pro- vide significantly more RFI suppres- sion in a substantially smaller pack- age. A monolithic EMI filter also re- jects 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 bal-


anced shunt capacitors in a single package, with mutual inductance can- cellation and shielding effect. These filters from Johanson Dielectrics uti- lize two separate electrical pathways within a single device attached to four external connections. Like other EMI filters, mono-


lithic EMI filters attenuate all ener- gy above a specified cut-off frequen- cy, only selecting to pass required signal energy while diverting un-


wanted noise to ground. The key, however, is the very


low inductance and matched imped- ance. With monolithic EMI filters, the terminations connect internally to a common reference (shield) elec- trode within the device, and the plates are separated by the reference electrode.


Monolithic EMI filters can be ef-


fective from 50 kHz to 6 GHz and fil- ter both common-mode and differen- tial-mode noise. The filter also has virtually no limit to the amount of DC current, because it is designed to


work in parallel to the motor and no DC current flows through it.


Pulse Width Modulation Regardless of the type of filter,


an often-overlooked factor is the fact that many brushed DC motors are controlled by pulse width modulated signals. With PWM signals, the volt- age is turned on and off at a very fast rate between a few and tens of kHz. The total power supplied is based on the time the switch is on compared to its off periods.


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