NEW - X2Y® EMI Filter Evaluation & PCB Design Guide
By Steve Cole - X2Y Product Mgr. Johanson Dielectrics, Inc Mar. 18, 2009	Click here to download pdf File Size: 383kB

X2Y® Capacitors are capable of extraordinary EMI Filtering performance when effectively applied to the PCB. Understanding X2Y® primary advantages, different connection options, and following a few basic guidelines will help achieve successful lab evaluation and optimal production designs. This guide is focused on PCB EMI and RFI filtering however many of the same principals apply to DC Motor Filtering and IC Power Bypassing. For additional information on those topics please refer to “X2Y_DC_Motor_Filtering_Basics” and “X2Y_Power_Bypass_Mounting”.

X2Y IN A NUTSHELL (Basic Theory) X2Y is an ultra-low inductance 3-node capacitor circuit consisting of two balanced Y capacitors of equal value. This configuration enables filtering of two signal lines (or positive AND negative power lines) simultaneously. A special electrode design provides broadband filtering characteristics for both differential and common-mode noise signals into the GHz band. EMI Immunity and Radiated Emissions performance far exceeds that of standard ceramic MLCs and multi-component filters. X2Y’s low inductance maintains insertion loss over a wide frequency band, not just a narrow bandwidth around the capacitor SRF. This means that for a given capacitance value and corresponding pass band Fc, X2Y’s deliver far better high frequency filtering then standard MLCs.

IDENTITY CRISIS (X2Y vs. Feed-Thru Filter) Externally an X2Y® closely resembles a chip feed-thru EMI filter but has major internal, schematic, and performance differences. A feed-thru capacitor drives one filtered signal through the capacitor along the long component axis and must handle both DC current and AC noise currents. X2Y® capacitors filter two signal lines simultaneously shunting only noise. Since no DC currents flow through the X2Y®, there is no DC current limitation. (In EMI applications AC noise currents are orders of magnitude smaller than DC currents.)
HIT THE GROUND RUNNING (X2Y® Connection Options)
X2Y® capacitors require a low inductance, dual ground connection as seen in “Circuit 1.” Chassis or system ground is preferred, but if unavailable, an isolated or “floating” ground may be used very effectively. Because the filtered noise currents are small, adding a copper flood layer in the PCB and connecting it with vias to G1 & G2 create an image plane. Circuit 3 may be used in applications where a signal line is common with system ground. Comparative radiated emissions testing shows nearly the same level of effectiveness.
NO-FLY ZONE (Minimize Connection Inductance) Fly-wire connections introduce large blocking inductances that do not work in EMC applications. For best results use an X2Y® specific layout for evaluations per the PCB Layout Guide. If this isn’t possible make every attempt to make a well-grounded, low inductance placement. Occasionally X2Y® can be kludged to an existing PCB for testing by careful cutting or exposing traces and/or addition of copper foil or braid. Such kludges should follow basic EMC design principals and have minimal inductance for effective EMI filtering.

WHERE’S THE BEEF? (PCB Layout Guidelines) Component specific terminal pad dimensions are listed in the JDI X2Y® datasheet. PCB via layout dimensions depend on PCB fabrication and assembly capabilities. Use of solder mask beneath component is NOT recommended

LOCATION, LOCATION, LOCATION (Preventing EMI Coupling)

AVOID CRACK-UPS (Lab Soldering Precautions)

1. 4 terminals to solder can increase hand-soldering time
2. Pb-free solders have higher reflow temperatures
3. Low inductance connections to ground are inherently good heat-sinks

SUMMARY OF CRITICAL GUIDELINES

A HELPING HAND (X2Y® Design Assistance Available)

Notice: Specifications are subject to change without notice. Contact your nearest Johanson Dielectrics, Inc. Sales Office for the latest specifications. All statements, information and data given herein are believed to be accurate and reliable, but are presented without guarantee, warranty, or responsibility of any kind, expressed or implied. Statements or suggestions concerning possible use of our products are made without representation or warranty that any such use is free of patent infringement and are not recommendations to infringe any patents. The user should not assume that all safety measures are indicated or that other measures may not be required. Specifications are typical and may not apply to all applications.