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Page 40


www.us-tech.com


Advanced EMI Filters Keep Brushed DC Motor Costs Low


By Jeff Elliott


price in industries that include auto- motive, aerospace, medical, industri- al, appliance, consumer, and home automation. As a result, billions are manufactured annually worldwide, a figure that is expected to increase over the next 10 years. However, increasing electro-


I


magnetic compatibility (EMC) re- quirements along with more crowded and “noisy” electronic environments are threatening to upset the balance, by driving the cost of these low-end solutions to a level on par with more expensive brushless alternatives. The issue is the electromagnetic


interference (EMI) generated by the brushes as they rub the commutator — an inherent drawback of the de- sign. To counteract the noise gener- ated, a combination of shielding and filtering components is required. This not only drives up the cost,


but many EMI/RF filtering solutions for brushed DC motors on the market are not satisfactory to meet today’s higher EMC requirements. “Many EMI filtering solutions


do not filter out all forms of noise that are generated and many cannot handle higher DC currents without a corresponding escalation of the cost,” explains Christophe Cambrelin of Jo- hanson Dielectrics, a company that


Brushed DC motor with monolithic EMI filter.


of brush DC motors slightly, while meeting the evolving EMC require- ments.


Noisy Electronics When electronic devices receive


strong electromagnetic waves, un- wanted electric currents can be in- duced in the circuit and interfere


expand the affected frequency range and the miniaturization of electronic devices that shrinks the distance be- tween source and victim. Many electronic devices are also


more easily affected by noise, even with less energy, due to circuits today that operate at very low voltages. As a result, industries such as the automo-


nexpensive and easy to operate, brushed DC motors provide a bal- ance of performance at the right


manufactures a variety of multilayer ceramic capacitors and EMI filters. To address these concerns, com-


panies, such as Johanson Dielectrics, are now offering more advanced EMI filtering solutions that increase costs


with intended operations. EMI can even cause physical damage in oper- ational equipment. Exacerbating the issue are in-


creases in operating circuit frequen- cy, noises of higher frequencies that


tive sector are increasingly turning to brushless DC motors. With brushless DC motors, the commutation is done electronically. Therefore there is sig- nificantly less noise generation, but the complexity and cost of implemen- tation are increased.


EMI Filtering Solutions Electromagnetic and RF inter-


ference is either radiate or conducted in a wide frequency range, from sever- al hundred hertz to several gigahertz. Radiated noise occurs when voltage is applied at varying levels to the wiring. To keep the radiation confined to the motor housing, several precau- tions should be taken by the manufac- turers of brushed DC motors. The most important is the mate-


rial used for the motor housing, which should be metal, as well as a metal cap (not plastic) on top. When the cap is made of plastic, the used must cover it with a metal shield, such as a metallized PCB. When EMI/RFI is conducted,


the noise generated travels along the electrical power leads and is then ra- diated. Shielding is ineffectual against conducted noise, so filtering is required with a separate device. Traditional common-mode fil-


tering approaches include low-pass filters made up of capacitors that pass signals with a frequency lower


Continued on next page


March, 2020


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