In today’s rapidly advancing electronic world, selecting the right EMI/EMC filter is no longer a luxury—it’s a necessity. Whether you’re designing a medical device, developing a power supply for industrial automation, or simply trying to meet international compliance standards, the success of your system often hinges on the filter you choose.

But with so many variables—voltage, current, leakage, insertion loss, industry certifications—how do you know what filter you need?

This guide breaks down the process step-by-step so you can make an informed, confident decision.

Understanding EMI and Why It Matters

Electromagnetic Interference (EMI), also called electromagnetic noise, is unwanted high-frequency energy that disrupts the normal operation of electrical or electronic systems. EMI can be radiated through the air or conducted through cables and power lines. Sources of EMI include:

Switched-mode power supplies (SMPS)
Motors and relays
Variable frequency drives (VFDs)
Mobile phones, radios, and Wi-Fi
Natural phenomena like lightning and static electricity

Left unfiltered, EMI can cause malfunctions, reduce equipment lifespan, or even lead to dangerous failures in critical systems.

What Does an EMI Filter Do?

EMI filters, also known as EMI suppression filters, are designed to attenuate high-frequency noise. They:

Prevent EMI from entering the equipment
Stop EMI generated by the equipment from reaching the power grid
Help electronic systems meet EMI compliance standards (FCC, EN, MIL-STD, etc.)

Most EMI filters are low-pass filters composed of inductors and capacitors that block high-frequency noise while allowing low-frequency power to pass.

Step 1: Define Electrical and Operational Requirements

✔ Rated Voltage

Choose a filter with a voltage rating equal to or greater than your system’s input voltage.

Single-phase filters are typically rated at 250VAC.
Three-phase filters are available up to 600VAC.
For DC systems, filters up to 1200VDC exist.
Do not operate filters beyond their rated voltage—this can damage internal capacitors.

✔ Rated Current

The filter must handle the maximum steady-state current of your device.

Always derate by 20–25% to ensure long-term performance.
Inrush currents can be tolerated briefly, but sustained overcurrent can lead to filter failure.

✔ Operating & Ambient Temperature

Most filters operate from -25°C to +85°C, but some support military-grade ranges from -40°C to +100°C.

Always check the derating curve if your system runs hot.
Mount the filter in a well-ventilated location inside the enclosure.

✔ Leakage Current

Regulatory bodies place strict limits on leakage current:

<0.5mA for general electronics
<0.1mA (100µA) for medical devices (IEC 60601)
Filters with “Y” capacitors contribute to leakage current—select low-leakage variants if necessary.

Step 2: Match the EMI Filter to Your Power System

Different countries and industries use different power system configurations:

Single-phase (2-wire)
Three-phase Delta (3-wire)
Three-phase Wye (4-wire with neutral)
Split phase / corner-grounded delta / hi-leg delta (special cases)

You must select a filter compatible with your system layout—or consult with the manufacturer for a custom configuration.

Step 3: Consider the Number of Stages

Filters come in:

Single-stage: Basic attenuation
Two-stage / Multi-stage: Enhanced performance

More stages mean better noise suppression but also increased size and cost. Consider the level of EMI you’re dealing with.

Step 4: Identify System and Application Constraints

✔ Equipment Type

Your application determines your filter:

AC/DC converters → Moderate EMI
VFDs, motors → High EMI
RF modules → Wide frequency spread
Medical devices → Ultra-low leakage
Military gear → Extreme temperature and ruggedness

✔ Industry Regulations

Your filter must support compliance with:

FCC Part 15 (USA)
EN 55011 / 55032 / 61000 (Europe)
MIL-STD-461 (Defense)
IEC 60601 (Medical)
UL, CSA, CE, UKCA, etc.

✔ Mounting Style

Choose from:

PCB mount
Chassis mount
Bulkhead / flange
IEC inlet filters (for small form factor equipment)

✔ Size Constraints

Tight enclosures or portable equipment may limit filter options. If space is an issue, ask about compact or custom-built filters.

✔ Grounding

Many EMI filters require chassis ground to function properly. If grounding is not available, a special floating ground design may be needed.

Step 5: Understand EMI Filter Performance—Insertion Loss

Insertion loss describes how well a filter attenuates noise, measured in decibels (dB).

10 dB = 90% attenuation
20 dB = 99%
40 dB = 99.99%

✔ Common Mode (CM) vs Differential Mode (DM)

CM noise: Flows identically on all lines to ground.
DM noise: Flows between line and neutral, 180° out of phase.
You may need a filter that suppresses both modes, or a filter optimized for one depending on your emissions profile.

Step 6: Use Insertion Loss Graphs to Match to Your Device

✔ Run Conducted Emissions Testing

Perform a scan (150kHz–30MHz range) to determine noise amplitude and frequency.
Identify which frequencies fail to meet standards.
Note the margin of failure in dB.

✔ Compare to Filter Datasheets

Use the filter’s insertion loss graphs (for both CM and DM) to match attenuation levels at failure points.
Example:

If your device fails by 25 dB at 500kHz, choose a filter with >30 dB insertion loss at 500kHz.

Step 7: Evaluate Real-World Performance

Manufacturers test filters under ideal lab conditions:

50Ω source/load impedance
Minimal crosstalk and noise

In your system, impedance mismatches, wiring layout, or long leads can reduce effectiveness. Always validate performance in your actual setup.

Step 8: Watch for Special Requirements

✔ Medical Devices

Total system leakage current must be under 100µA
Filters should support double fusing and DPST switches to break both AC lines during faults.

✔ Immunity Protection

For systems exposed to power surges, relay spikes, or lightning, choose filters with amorphous core chokes instead of MOVs, which degrade over time.

✔ Safety Certifications

Ensure filters comply with standards like:

UL 1283 / UL 60939
IEC 60939
CSA 22.2 Look for agency markings (UL, CSA, VDE, TUV, etc.) to simplify system certification.

Step 9: Consider EMI Testing Options

If you don’t have access to a full EMI test lab:

Use a LISN and spectrum analyzer
Work with an EMI filter manufacturer who can scan your system and recommend the right product
Rent time in a certified chamber before final testing

Step 10: Consult a Local EMI Expert

Even with all the right specs, filter selection can be tricky. A local EMI filter specialist can help with:

Performance evaluation
Custom filter design
On-site testing
Mounting and termination solutions

Look for manufacturers with in-house RFI chambers and full EMI compliance labs.

Quick Checklist Before You Buy

Ask yourself:

What are my device’s voltage, current, and temperature requirements?
What power system is used (AC, DC, single/three-phase)?
What are the EMI problem frequencies?
Do I need an IEC inlet, chassis mount, or PCB filter?
Is this for a low-leakage or medical application?
How much space is available for the filter?
What are the required agency certifications?
Do I have test scan data to guide filter selection?

Looking for the right EMI filter for your application?

Visit our Product Page to explore a wide range of standard EMI/EMC filters designed for industrial, medical, and commercial environments. Whether you’re dealing with common mode noise, differential mode interference, or strict leakage current requirements, we offer high-performance filters in various form factors and ratings to meet your needs.

Still need something more specific?

Contact us today for expert guidance or request a custom-designed EMI filter solution tailored to your system’s unique electrical, mechanical, and regulatory requirements.