Electromagnetic interference (EMI) filtering is critical in modern power electronic systems to meet regulatory requirements while maintaining electromagnetic compatibility (EMC) with other equipment. EPCOS components provide excellent performance in EMI filter applications, with specialized capacitors, inductors, and resistors designed specifically for EMI suppression. This technical article covers advanced techniques for implementing effective EMI filters using EPCOS components.

Understanding EMI Mechanisms

EMI in power electronic systems manifests as both conducted and radiated emissions. Conducted emissions travel along power and signal lines, while radiated emissions propagate through space. Both types must be controlled to meet regulatory requirements such as those defined in CISPR 22 (information technology equipment), CISPR 11 (industrial equipment), and FCC Part 15.

Conducted emissions are classified as either differential mode (DM) or common mode (CM). Differential mode noise appears between the line and neutral conductors, while common mode noise appears between each conductor and the equipment ground. The noise sources differ fundamentally:

• Differential Mode Noise: Generated by the switching action of power semiconductors creating current ripple that modulates the DC input
• Common Mode Noise: Generated by displacement currents through parasitic capacitances to ground, typically at high frequencies

Effective EMI filters must address both types of noise appropriately, often requiring different component technologies and topologies for each mode.

Basic Filter Topologies

The most common EMI filter topology is the LCπ configuration, which provides both differential and common mode suppression. A typical single-stage filter consists of:

• One or two series inductors for differential mode suppression
• One or more X capacitors across the line for differential mode filtering
• Two Y capacitors from line to ground for common mode filtering

The series inductors present high impedance to high-frequency noise, while the capacitors provide low-impedance paths for noise to bypass back to the source. For effective filtering, the impedance of the inductors must be significantly higher than the circuit impedances at the frequencies of concern, while the capacitors' impedance must be significantly lower than the load impedances.

At higher frequencies, stray capacitance and inductance in the inductors and capacitors become significant, limiting filter effectiveness. This is where EPCOS component technologies provide advantages through optimized designs for high-frequency performance.

EPCOS Component Solutions for EMI Filtering

EPCOS provides specialized components designed specifically for EMI filtering applications:

Common Mode Chokes (B78105 Series)

EPCOS common mode chokes use bifilar winding techniques on ferrite cores optimized for common mode impedance while maintaining low differential mode inductance. B78105 series components feature:

• High common mode impedance (50Ω to 2000Ω) across wide frequency ranges
• Low differential mode inductance to avoid DC voltage drop
• Optimized for high current applications (1-40A ratings)
• Robust construction for harsh industrial environments

The key design parameter is the common mode impedance as a function of frequency. For most applications, 100Ω at 100MHz is considered the minimum effective value, though higher values (500Ω+) provide better attenuation.

Safety-Approved X and Y Capacitors (B32669 Series)

EPCOS safety capacitors are specifically designed for EMI filtering with appropriate safety certifications. X capacitors are connected across the line (between line and neutral) and are designed to fail open to prevent fire hazards. Y capacitors are connected line-to-ground and must fail in a way that doesn't create shock hazards.

B32669 series X capacitors feature X1 and X2 safety classifications with self-healing metallized film construction. Y capacitors are available in Y1 and Y2 classifications with enhanced safety features including flame-resistant housings and internal safety mechanisms.

Advanced Filter Design Techniques

For applications with more challenging EMI requirements, advanced filter topologies provide enhanced performance:

Multi-Stage Filters

Two-stage or three-stage filters provide additional attenuation by cascading filter sections. Each stage adds approximately 40dB of theoretical attenuation (20dB per LC section) at frequencies well above the cutoff frequency.

However, stage interaction must be prevented through proper impedance matching. The output impedance of the first stage should be significantly lower than the input impedance of the second stage to prevent resonance effects that could reduce overall filter performance.

Typical multi-stage approaches include:

• First stage: High-current components for gross filtering
• Second stage: Lower-current, higher-frequency components for fine filtering
• Isolation: Resistive or inductive elements to prevent stage interaction

Virtual Ground Filters

In applications where Y capacitors are undesirable due to leakage current concerns, virtual ground filters eliminate line-to-ground connections while maintaining common mode filtering. This topology uses a floating reference point created by equal capacitors to each line conductor.

While more complex than conventional filters, virtual ground filters are valuable in medical equipment, battery chargers, and other applications where leakage current must be minimized.

LCL Filters

For applications with particularly stringent EMI requirements, LCL filters provide superior attenuation compared to simple LC filters. The additional inductor provides steeper roll-off characteristics.

LCL filters can achieve 60dB or more attenuation above the resonant frequency while maintaining good DC characteristics. However, they require more components and can be prone to resonance issues if not properly designed.

Component Selection Guidelines

Proper component selection is critical for achieving desired EMI performance:

Common Mode Choke Selection

Key parameters for common mode choke selection include:

• Current Rating: Select with 20-50% derating from maximum operating current to prevent saturation and ensure long-term reliability
• Impedance: Target 100-500Ω at the frequencies of greatest concern
• DC Resistance: Minimize to reduce losses and voltage drop
• Core Material: Material optimized for the frequency range of interest

For EPCOS B78105 series, select the smallest package that meets current requirements to minimize parasitics while ensuring adequate current handling capability.

Capacitor Selection

For X capacitors, select values based on required differential mode attenuation and leakage current limits. Typical values range from 10nF to 2.2µF depending on the application.

For Y capacitors, leakage current regulations limit values at power line frequencies. For 230Vac systems, typical limits are:

• Class I: 3.5mA maximum
• Class II: 0.5mA maximum

Calculate Y capacitance limits as: CYmax ≤ Ileak_max / (2πfVAC) where f is the line frequency.

Layout and Implementation Considerations

Even the best filter design performs poorly with poor implementation. Key layout considerations include:

Minimize Loop Areas

EMI filters are inductive elements, and their effectiveness depends on minimizing parasitic inductance. Keep input and output traces physically separated to prevent coupling that bypasses the filter. The input power and return path should form a tight loop, as should the output power and return path.

Grounding Considerations

Y capacitors must connect to a clean earth ground for effective common mode filtering. The ground connection impedance at the frequencies of interest directly affects filter performance. Bonding Y capacitor ground connections directly to the equipment ground point minimizes ground impedance.

For systems without safety grounds, common mode noise may not be adequately filtered, potentially limiting the effectiveness of Y-capacitor based filtering. In such cases, enhanced common mode chokes may be necessary.

Component Placement

Place filter components at the point where input power enters the equipment. This prevents noise from coupling to other circuits within the equipment. Maintain isolation between input and output connections to prevent coupling around the filter.

For high-power filters, consider thermal management as components may experience significant heating from high-frequency currents and losses.

Verification and Testing

EMI filter performance must be verified through proper testing:

• Conducted emissions measurements using LISN (Line Impedance Stabilization Network)
• Impedance analysis of individual components across frequency range
• Thermal analysis under maximum operating conditions
• Vibration and environmental testing for critical applications

Pre-compliance testing helps identify potential issues before formal testing, saving time and cost. EPCOS components are well-documented with impedance and performance data to aid simulation and design verification.

Case Study: SMPS EMI Filter Implementation

Consider a 2kW switching power supply requiring compliance with EN 55032 Class A emissions limits:

Design Requirements:

• Input: 85-265 Vac, 50-60Hz
• Output: 48V, 42A
• Switching frequency: 100kHz
• Required attenuation: 60dB at 150kHz

Solution:

• Common Mode Choke: B78105S1012M (10mH, 10A)
• X Capacitor: B32669 C1032K (10nF, 275VAC, X2)
• Y Capacitors: 2× B32669 K1022K (1000pF, 250VAC, Y2)
• Additional high-frequency filtering: B32673 for IGBT snubber applications

This implementation achieved 75dB attenuation at 150kHz with 45dB margin to the Class A limit, demonstrating the effectiveness of properly designed EPCOS-based EMI filters.

Specialized Applications

Different applications have unique EMI filtering requirements:

Automotive Applications

Automotive EMI requirements are particularly stringent, with requirements across 150kHz to 2GHz for radiated emissions. Automotive-grade components must withstand temperature extremes and vibration while meeting AEC-Q200 qualification standards.

Medical Equipment

Medical devices have strict leakage current requirements to protect patients from electric shock. Virtual ground topologies are often required, and components must meet IEC 60601-1 safety requirements.

Telecommunications

Telecom equipment must maintain signal integrity while meeting EMI requirements. Low-capacitance Y capacitors and special core materials may be required to avoid interference with high-speed data signals.

Future Trends in EMI Filtering

As switching frequencies increase with new semiconductor technologies (GaN, SiC), EMI filter design becomes more challenging. Higher frequencies require components with better high-frequency performance and potentially new topologies to meet emissions requirements.

Integration of filtering components is becoming more common, with ICs that include filtering functions and modules that combine multiple components. EPCOS continues to develop new products to meet these evolving requirements.

Emerging standards for systems with digital control require attention to new frequency ranges and measurement techniques, pushing filter design toward even broader frequency coverage and more sophisticated approaches.

Conclusion

Effective EMI filter design is both an art and a science, requiring understanding of noise sources, component limitations, and implementation challenges. EPCOS components provide the high-performance, reliable solutions needed for modern EMI filtering applications across all market segments.

LiTong Electronics provides comprehensive support for EMI filter design, from component selection through implementation and testing. Our application engineering team has extensive experience with EPCOS components in EMI filtering applications and can provide design assistance for your specific requirements.