Product # MCOTS-F-270-P-QT

Phone 1-888-567-9596

Doc.# 005-0005521

Rev. H

10/20/16

Page 4

MCOTS-F-270-P-QT

-300

-250

-200

-150

-100

-50

0

50

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

Frequency (Hz)

Common Mode Attenuation

Differential Mode Attenuation

LISN

LISN

Source

50Ω

50Ω

Vin(+)

Vin(-)

Vout(-)

Inoise

Imeas

Vout(+)

EMI Filter

Com In

Com Out

LISN

LISN

Source

50Ω

50Ω

EMI Filter

Vin(+)

Com In

Vin(-)

Vout(-)

Com Out

Inoise

Imeas

Vout(+)

Basic Operation and Features

This module is a multi-stage differential-mode and common-mode passive

EMI filter designed to interface a power source with one or more Mil-COTS

dc-dc converters (or other loads that create EMI). Each stage of this filter

is well damped to avoid resonances and oscillations, and only X7R multi-

layer ceramic capacitors are used. This Mil-COTS EMI filter includes a large

bulk capacitor with a series damping resistor to correct for the unstabilizing

effect of a converter’s negative input resistance. A white paper discussing

this negative input resistance and the need for corrective damping can

be found on the SynQor website (see Input System Instability application

note).

A typical application would place the Mil-COTS filter close to the input of

the dc-dc converter. The input-side common-mode pin would be connected

to the chassis ground that is common with the system input line filter or

other earthed point used for EMI measurement. The output-side common-

mode pin would be connected to the output ground or plane of the

power converters with as low inductance a path as possible. There are no

connections to the metal baseplate, which may also be connected to the

chassis ground if desired.

Do not connect the outputs of multiple Mil-COTS filters in parallel.

Connecting filters in this manner may result in slightly unequal currents to

flow in the positive and return paths of each filter. These unequal currents

may cause the internal common-mode chokes to saturate and thus cause

degraded common-mode rejection performance.

Figure A: Simulation of calculated common mode and differential mode

attenuation. The curves plot the ratio of noise current in a 50Ω LISN sensing

port connected to the noise output side of the filter (the power input side, pins 1

and 3) to the noise current on the input side (the power output side, pins 4 and

6). Refer to Figures B and C.

Figure B: Differential-Mode Current Attenuation, Imeas / Inoise

Figure C: Common-Mode Current Attenuation, Imeas / Inoise