12/13/2015© x2y attenuators, llc1. common mode filters test comparisons, x2y ® versus cm chokes...

04/21/23 © X2Y Attenuators, LLC 1

Common Mode Filters

Test comparisons, X2Y® versus CM Chokes and PI

Filters


Common Mode and EMI


Most EMI compliance problems are common mode emissions.

Only 10’s of uAs in external cables are enough to violate EMC standards.

Figure 1, FCC Emissons Limits

Common Mode Noise Model


The E field developed between any lead exiting a shielded enclosure and the enclosure outer skin radiates

Ability to radiate depends on:– Power in the noise source– Coupling efficiency between the effective antenna structure and the

surrounding space The power leads and the case form the antenna

Figure 2, Common Mode Noise Model

Common Mode Noise Model


To Reduce Radiation:– Reduce the noise potential between the case and

leads, AND/OR– Reduce the coupling efficiency to surrounding space

Figure 2, Common Mode Noise Model

Reduce Noise Potential

Reduce HF current in product– Rarely an option

Decrease shunt impedance to case– Optionally insert series

impedance between the noise source and the EMI shunt


Figure 3, Reduce Noise Potential

Reduce Coupling

Reduce antenna efficiency

– Change the cable length Keep cables short Avoid odd multiples of quarter-wavelengths to

dominant noise sources

– Cable routing / shielding Route cables to minimize radiated noise pick-up

Mismatch antenna impedance

– Increase driving impedance to much higher than the antenna characteristic impedance*

– Decrease driving impedance to much lower than the antenna characteristic impedance

*Antenna impedance may be anywhere from 10’s to

100’s of Ohms

04/21/23© X2Y Attenuators, LLC

Confidential, Internal X2Y® use ONLY7

Figure 4, Reduce Antenna Coupling Efficiency

Differential Noise


Voltage(s) between multiple leads that form an antenna in the area between.

Figure 5, Differential Noise Radiation

Mode Conversion

Occurs when individual filters are not matched

Differential signal energy converts into common-mode energy

Common-mode energy converts into differential energy

Avoid by matching filters throughout stop-band


CM Chokes as EMI Filters

Ideally, CM chokes work by increasing the noise source impedance, mismatching it to the antenna.

A CM choke is a 1:1 transformer where the primary and secondary are both driven.– Both windings act as both

primary and secondary.– Current through one winding

induces an opposing image current in the other winding

– For K close to 1.0, effective impedance is:

Z ≈ 2πF*LMAG


Figure 6, Common Mode Choke Operation


In an ideal 1:1 transformer, primary and secondary currents are identically equal and opposite

– Real transformers, currents match to between +/-1% and +/-5% over the optimum design frequency range.

– Real CM chokes work over a limited frequency range due to parasitic capacitance and stray leakage inductance.

For a given core material;– the higher the inductance used to

obtain lower frequency filtering, the greater the number of turns required and consequent parasitic capacitance that defeats high frequency filtering, and leakage inductance.


Figure 7, CM Choke Parasitic Capacitance


Mismatch between windings from mechanical manufacturing tolerance causes mode conversion.– A percentage of signal

energy converts to common mode, and vice-versa.

– This gives rise to EMC issues as well as immunity issues.


Figure 8, CM Choke Mode Conversion


CM chokes have one really good application:– Signals must be passed

that operate in the same frequency range as CM noise that must be suppressed.

Otherwise, CM chokes are: – Large– Heavy – Expensive– Subject to vibration

induced failure.


Figure 9, CM Choke Mode

X2Y® Capacitors

Nearly Ideal Shunts:– When properly applied,

X2Y® capacitors filter CM noise by both attenuating source energy, and mismatching antenna impedance.

– The key is very low, and matched inductance.

– Proper application must mind inductance in the common path: G1/G2 terminals


Figure 10, X2Y Capacitor as EMI Filter

X2Y® Capacitors

X2Y® capacitor shunts between A, B, and G1/G2 attachments.– Component inductance is

very low: ≈110pH from each A or B to G1/G2.

Low impedance shunt serves two purposes:– Divides noise voltage

– Mismatches external antenna impedance

Reflects inside noise back inside Reflects external noise: EFT/ESD

back towards outside.



X2Y® Capacitors

Performance is typically limited by external capacitor wiring inductance:– L3A/L3B, L4A, L4B

Maximize performance by minimizing L3x, and L4x inductances.– Follow X2Y® mounting

guidelines.

L1x, and L2x inductance is OK and even beneficial when balanced.– Limitation on L2 is to keep

connection close to egress.




Confidential, Internal X2Y® use ONLY 17

X2Y® Capacitors

Example, Good Circuit 1 Mounting Practice– Minimize, L3A, L3B

Connect internal A, B pad connections near base of pads

Connect external A, B pad connections near base of pads

– Minimize L4A, L4B: Connect through minimum

length, maximum width connections to chassis edge.

G1 immediate connection to Chassis metal

G2 via to wide polygon on PCB layer 2


Figure 12, Good X2Y CM Mounting Practice


Confidential, Internal X2Y® use ONLY 18

X2Y® Capacitors,

Circuit 1 Mount Practices to Avoid– Avoid introducing

unnecessary Common inductance

between I/O traces and the A or B pads of the X2Y capacitor

Common inductance between the G1 / G2 pads of the X2Y capacitor and chassis

– Any of the above practices impairs performance at high frequency


Figure 13, Poor X2Y CM Mounting Practice

Example, Single Board Computer Power Feed

Single board computer– 68HC11 processor

5uH CM choke tested PI filter w/ 5uH CM choke tested Seven values of X2Y® capacitors tested

– 47pF, 100pF, 220pF, 330pF, 470pF 560pF, 1000pF


Comparative Performance 1MHz – 500MHz

CM chokes and PI Filters– Both exhibit similar

performance Filter cut-off ≈ 32MHz

Attenuation effective to about

450MHz

Parasitic capacitance

completely defeats CM

choke and PI filter above

450MHz– No attenuation compared to no

filter case


Comparative Performance 1MHz – 500MHz


CM chokes and PI Filters

Comparative Performance 50MHz –1GHz

X2Y® capacitors effective to 1GHz and beyond Capacitance value determines low frequency

rejection Very small X2Y® caps (47pF) superior solution

vs. CM chokes or PI filters down to 300MHz 470pF and larger X2Y® caps superior to choke

based filters over all frequencies X2Y® 1000pF vastly better radiated emissions

than 5uH CM choke or PI filter


Comparative Performance 50MHz –1GHz, 47pF


47pF Superior to CM chokeAbove 300MHz

GSM ambient



100pF Superior to CM chokeAbove 150MHz



220pF Comparable/Superior to CM chokeAbove 50MHz

Comparative Performance 50MHz –1GHz 47pF, 1000pF




X2Y® 1000pF vastly better radiated emissions than 5uH CM choke or PI filter

X2Y® Capacitor Selection

X2Y® capacitors operate as shunts.– X2Y’s attenuate all energy above cut-off

frequency– Select value large enough to block offensive

HF noise– Select value small enough to pass required

signal energy


X2Y® Capacitor Selection Methods

METHOD 1 - Pass Source Signals w/ Known TRISE / TFALL

– Establish TRISE / TFALL

C <= TRISE_10%_90% _MIN/(2.2*ZSOURCE)

– Example: CAN BUS 1Mbps TRISE_10%_90% <= 50ns

ZSOURCE = 120 Ohms / 2 = 60 Ohms

CMAX <= 50ns/(2.2*60 Ohms)

CMAX <= 380pF Recommended value = 330pF TRISE_10%_90% <= 44ns

– TRISE_10%_90% / TFALL_90%_10% < 5-10% of bit period is usually OK 5%

– C <= 1/(44*Bit_Frequency*ZSOURCE)

– CAN BUS» C <= 1/(44*1MHz*60Ohms) <= 380pF

10%– C <= 1/(22*Freq*ZSOURCE)


X2Y® Capacitor Selection

Method 2 – Known Cut-off Frequency– Noise cut-off frequency FCO is known, source

impedance ZSOURCE

C <= 1/(2π*FCO*ZSOURCE)

– Example: Switching power supply harmonic suppression

FCO = 2MHz

ZSOURCE = transmission line impedance 1 Ohm

CMIN >= 1/(2π*2MHz*1 Ohm) = 1/1.26E7 = 80nF

Recommended minimum value = 100nF


Summary

Most EMI problems are Common Mode Reduce common mode by attenuating driving

voltage and/or mismatching antenna impedance– Properly mounted X2Y® caps do both

Select X2Y® capacitor values based on known source impedance and either required signal pass-band ( sets max value ), or required noise stop-band ( sets min value )


X2Y vs. CMC


Application X2Y Part Used Comments

CanBus, EMI filtering 0603 330pF NPO 100V X2Y less overshoot on signal than 5uH CMC, lowered radiated emissions

5uH CMC, power filter X2Y 0805 1nF 100V X2Y much lower radiated emissions

10/100 BaseT, EMI suppression 0603 5.6pF NPO 100V Pass 100BaseT requirements, BER testing

Telecom, Power Filter 0603 47nF X7R 16V Steve checking w/Siemens for this info

Data / Signal lines 1206 0.1uF X7R 50V X2Y on PCB at I/O , replaced 51uH CMC

Input filter, Instrumentation amplifier

X2Y 0603 10nF 50V See MT-070 TUTORIAL

The following X2Y® components are in production replacing common mode chokes in various application uses.


Johanson X2Y® Products

Scott MullerJohanson DielectricsDirect: 480 753-3644Cell: 480 220-2510E-Fax: 602 532-7898Email: [email protected]

Steve ColeX2Y Product ManagerJohanson Dielectrics, Inc.TEL 603.433.6328Email: [email protected]

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