Optimizing EMI Filtering with Active EMI Filter Circuits

EMI is a pest. It’s a persistent challenge in high-density power applications. Traditional passive EMI filters rely on large, bulky common-mode (CM) chokes and capacitors, leading to increased board size, weight, and cost. However, Active EMI Filter (AEF) circuits present an innovative approach to mitigating EMI while reducing filter size and improving performance.

Despite these challenges, EMI filters remain necessary to meet electromagnetic compatibility (EMC) standards, such as CISPR and IEC 61000-3-2/-12, which define emission limits for industrial and automotive applications.

The Challenge of Passive EMI Filters

CM EMI filters are constrained by safety regulations that limit the total Y-capacitance, requiring high-inductance CM chokes to meet EMI suppression requirements. The drawbacks include:

• Bulky and heavy components.
• Increased thermal management requirements.
• Electromagnetic coupling issues in compact designs.
• High costs due to large magnetic components.

Despite these challenges, EMI filters remain necessary to meet electromagnetic compatibility (EMC) standards, such as CISPR and IEC 61000-3-2/-12, which define emission limits for industrial and automotive applications.

Active EMI Filtering: A Game-Changer

AEF circuits offer an alternative by using active components to sense and inject compensatory signals, effectively canceling EMI disturbances. Unlike passive filters, which rely on impedance mismatching to block noise, AEFs leverage signal processing techniques to counteract noise directly, enabling:

• Significant size reduction of CM chokes.
• Lower power losses and improved thermal performance.
• Enhanced reliability and easier mechanical integration.

AEF Circuit Configurations

AEFs are designed based on different noise sensing and cancellation techniques. The primary approaches include:

1. Feedback (FB) Designs – These sense EMI disturbances at the victim end and inject an opposing signal to cancel the noise.
2. Feedforward (FF) Designs – These detect noise at the source and inject an anti-noise signal at the victim end.

While there are many AEF Circuit topologies, feedback control with voltage sensing and current injection, or FB-VSCI designs are particularly effective, as they enable high-attenuation filtering without the need for additional magnetic components.

Practical Implementation of AEF in Power Systems

The Texas Instruments TPSF12C1-Q1 and TPSF12C3-Q1 stand-alone AEF ICs illustrate the benefits of AEFs in single-phase and three-phase power systems. These ICs interface with power lines using Y-rated sense capacitors and provide high-frequency noise attenuation. Key benefits include:

• 30 dB CM noise attenuation from 100 kHz to 3 MHz.
• 52% reduction in CM choke volume.
• 60% reduction in copper losses.
• Improved high-frequency performance due to reduced parasitic capacitances.

Capacitive Amplification and EMI Mitigation

AEFs amplify the apparent capacitance of injection capacitors, reducing the required CM choke inductance. For instance, a 4.7-nF injection capacitor in an FB-VSCI AEF setup can achieve high-frequency EMI suppression while maintaining safety compliance. This results in:

• Higher self-resonant frequencies.
• Improved high-frequency attenuation.
• Lower component operating temperatures and increased lifespan.

Performance Results and Industry Applications

Measured results from a 3.3-kW power factor correction (PFC) AC/DC regulator demonstrate the effectiveness of AEFs. High-voltage testing with a TPSF12C1-Q1-based filter shows:

• Equivalent EMI suppression as a passive filter using 12-mH CM chokes but with only 2-mH chokes.
• Reduced box volume and improved thermal efficiency.
• Compliance with CISPR 11 and CISPR 25 standards.

Conclusion

AEFs revolutionize EMI filtering by providing superior attenuation with smaller and more efficient components. By replacing bulky passive filters with active solutions, engineers can achieve:

• Space and weight savings.
• Lower power losses.
• Improved design flexibility.

For next-generation power applications, adopting AEFs ensures compliance with stringent EMC regulations while optimizing system performance and cost.

For further technical insights on EMI suppression and AEF circuit design, explore the Texas Instruments and Würth Elektronik technical white paper on active EMI filtering solutions.