Taming EMI: How to Select EMC Filters for Power Electronics

Technical Overview: Electromagnetic interference (EMI) can disrupt power electronics, increase data errors, and prevent equipment from operating reliably. This article explains how radiated and conducted interference couples into circuits and distinguishes common-mode from differential-mode noise. It describes how electromagnetic compatibility (EMC) filters use common mode chokes, differential inductance, X capacitors, and Y capacitors to attenuate unwanted currents. Selection factors include insertion loss, source and load impedance, leakage current, direct current resistance, voltage and current ratings, phase configuration, mounting, and termination. TDK examples illustrate single-phase, three-phase, and direct current filters for demanding industrial, energy, information technology, and communications applications. (Overview courtesy of ChatGPT)

Designers of systems for demanding applications such as industrial, energy, and power electronics face growing pressure to maintain performance and reliability in harsh, electrically noisy environments. With higher-voltage electric vehicles (EVs), faster wide-bandgap (WBG) semiconductors, and higher serial data rates, electromagnetic interference (EMI) sources and severity are increasing. This raises the likelihood that EMI may degrade device performance, cause data errors or physical malfunctions, add cost, and, in the worst case, compromise safety. While global electromagnetic compatibility (EMC) regulations help minimize the effects of power-line noise and EMI, compliance can require redesigns and delay time-to-market.

Designers learn early to suppress EMI through filtering, shielding, and proper grounding. However, many EMI sources and filtering options require careful attention to techniques and device selection.

This article discusses the noise and interference challenges designers face in harsh environments. It then introduces and explains how to select and apply TDK's EMC filters to effectively address these challenges.

The sources and impact of EMI

EMI is an electrical disturbance, internal or from the environment, that affects an electrical circuit through magnetic, capacitive, or direct conduction coupling. EMI can degrade a circuit's performance or even prevent it from functioning. In serial data interfaces, EMI’s effects can range from an increased error rate to a total loss of data. Electromagnetic susceptibility (EMS) measures a device's vulnerability to EMI and determines an electronic device or system's ability to function correctly when exposed to external electromagnetic and electrostatic fields.

EMC is the ability of electrical and electronic devices and systems to operate correctly in the ambient electromagnetic environment without causing or experiencing unacceptable EMI. It can be thought of as the combination of EMI and EMS. EMC ensures that devices can function properly alongside one another without disrupting each other's operation.

EMI is generated by rapid changes in voltage or current within a circuit. Sources include the startup or shutdown of an electric motor, fast current switching in a switched-mode power supply (SMPS), high-speed logic switching in computers or serial data links, and radio frequency (RF) transmissions. These sources couple electromagnetic and electrostatic energy into a susceptible device through radiation or conductive paths (Figure 1).

Figure 1: EMI coupling paths include direct radiation, magnetic, and capacitive coupling. (Image source: TDK)

EMI propagates through radiation or conduction. Radiated emissions result from RF energy radiated through the air and picked up directly by a device's wiring. Radiated interference is most often observed at higher frequencies, typically 30 MHz to 1,000 MHz.

Conducted interference occurs through magnetic or capacitive coupling into a device's wires, boards, and housing. In practice, the transformer and capacitor shown are parasitic components, such as wires in a varying magnetic field or adjacent conductors on a circuit board. Conducted emissions have significant spectral content from 150 kHz to 30 MHz. This article focuses on reducing conducted noise.

Understanding EMI noise currents

EMI induces noise into a susceptible circuit's conductors depending on the conduction mode; there are two types of induced noise: common-mode and differential-mode (Figure 2).

Figure 2: The type of induced noise depends on the conduction mode: common-mode and differential-mode. (Image source: TDK)

Common-mode noise behaves as if the noise source drives both legs of the circuit in parallel. Induced current flows in the same direction on both legs of the affected circuit and returns to external ground through a leakage path. Differential noise behaves as if the noise source were in series with the signal source and flows in opposite directions on either side of the circuit. Suppressing these noise components requires two distinct strategies.

Reducing EMI

The two approaches to reducing EMI are controlling the physical layout and using appropriate filters. Magnetic and Faraday shields can attenuate radiated EMI, and careful grounding can provide return paths around sensitive circuits. However, filtering is the primary technique for controlling EMI. EMI filters use passive components such as capacitors, inductors, and resistors placed in the power line to reduce interfering currents (Figure 3).

Figure 3: Shown is a simplified schematic of an EMI filter designed to attenuate both common-mode and differential-mode noise. (Image source: TDK)

EMI filters block and shunt common-mode and differential-mode interfering currents. The common-mode choke (L_CM) impedes common-mode current on both the line and neutral conductors. In this example, the differential-mode choke (L_DM) is the common-mode choke's leakage inductance; it is in series with the differential-mode current and impedes its flow. The inductor's wire gauge is chosen to minimize its series DC resistance.

The C_X capacitors provide a shunt path for interfering differential-mode currents to return from line to neutral, preventing them from entering the protected circuit. The C_Y capacitors bypass each line to ground, providing a shunt path for those currents.

The resistor R_B is a discharge, or bleeder, resistor that discharges the capacitors during power-down to prevent electric shock. These resistors are high-value, usually in the hundreds of kilohms (kΩ) or higher.

Overall, the filter attenuates both common-mode and differential-mode EMI currents. These filters are typically symmetrical and attenuate noise on both the source and load sides. The filter's effectiveness is gauged by its insertion loss (Figure 4).

Figure 4: The attenuation curves show an EMI filter’s frequency-dependent response to common-mode and differential-mode noise. (Image source: TDK)

Attenuation in decibels (dB) for both modes is measured and displayed separately. Greater attenuation indicates better filter performance. Insertion loss is frequency-dependent and varies across the noise-related frequency range. The vertical line at 300 kHz intersects the differential-mode curve at -40 dB and the common-mode curve at -30 dB, which are the insertion-loss values at that frequency. These curves are based on standard 50 Ω source and load impedances. In practice, the source and load impedances are generally lower. When comparing filters, it is important to analyze the graphs for relative attenuation only.

Filters come in a variety of sizes and shapes, depending on the rated voltage and current they support and the number of phases. They are also available with various connection options, including a terminal strip (Figure 5).

Figure 5: Shown is the physical layout of a single-phase EMI filter with terminal strip connections. (Image source: TDK)

The common-mode choke, bifilar-wound on a ferrite toroidal core, is the largest component. TDK, with a long tradition of ferrite design, typically uses higher-value inductors and lower-value capacitors to achieve the desired response. Larger inductors limit inrush current, reduce earth leakage currents, and flatten the filter's frequency response. TDK filter connection options include printed circuit board (pc board) pins for through-hole mounting, quick-connect tabs, wire leads, threaded posts, or an IEC 320-C14 appliance inlet for direct line-cord connection.

A three-phase EMI filter essentially uses a filter circuit for each phase (Figure 6).

Figure 6: Shown is an example of a three-phase, three-wire (delta-configured) EMI filter. (Image source: TDK)

The circuit includes a single three-winding common-mode choke. The X capacitors connect between phases to attenuate differential-mode noise, while the Y capacitors connect the phases to a common point referenced to ground to attenuate common-mode noise. TDK also offers three-phase, four-wire filters for wye systems that include a neutral conductor.

EMI filter examples

TDK EMI filters are available in a broad range of current and voltage ratings, including single-phase, three-phase three-wire, and three-phase four-wire configurations. They also come in a wide selection of physical packages and connection types (Figure 7).

Figure 7: TDK EMI filters offer a wide range of power ratings, case sizes, and connection types. (Image source: TDK)

A widely used EMI filter is the power entry type. These filters mount on an external panel and integrate with an IEC 320-C14 power input connector. The TDK RPA-2003RL (Figure 7, top left) is a single-phase line filter rated at 3 A and 250 V_AC. It has a maximum DC resistance of 150 milliohms (mΩ). This filter integrates an IEC 320-C14 appliance inlet on the input and quick-connect terminals on the output. It offers leakage currents below 0.01 milliamperes (mA). This model provides at least 10 dB of attenuation for common-mode noise from 0.4 MHz to 10 MHz and 15 dB of differential-mode attenuation from 1 MHz to 30 MHz.

The TDK RSEN-2020D (Figure 7, top right) is a single-phase line filter rated for 20 A at 250 V_AC with a maximum DC resistance of 10 mΩ. It includes a DIN rail mounting bracket for simplified installation. Connections are made via a terminal block. The connection terminals are integrated into the packaging, and both the screws and terminal covers are captive, preventing dropped or lost hardware. This filter provides at least 25 dB of attenuation for common-mode noise from 0.4 MHz to 20 MHz and 25 dB of differential-mode attenuation from 0.3 MHz to 30 MHz.

The RTEN-5006 filter (Figure 7, bottom left) is a three-wire (delta-connected) three-phase filter rated for 6 A at 500 VAC. It has a maximum DC resistance of 145 mΩ. Connections are made through a terminal block with captive screws and covers. This filter provides at least 25 dB of attenuation for common-mode noise from 0.2 to 10 MHz and 25 dB of differential-mode attenuation from 0.2 to 30 MHz.

For DC lines, designers can use the RDEN-048050 (Figure 7, bottom right), an EMI filter rated for 50 A at 48 V_DC with a maximum DC resistance of 3 mΩ. It is intended for information technology and communications applications. The RDEN-048050 provides at least 30 dB of common-mode attenuation from 0.2 to 10 MHz and 25 dB of differential-mode attenuation from 0.7 to 30 MHz. Threaded studs or posts are used for interconnects.

Selecting TDK EMI Filters

TDK makes it easy to select from their broad range of EMI filter products via their DigiKey Advanced Power Line Noise Suppression Solutions webpage (Figure 8). The top row allows selection of the power source type: single-phase, three-phase (three-wire), three-phase (four-wire), or DC. Clicking any of these selections shifts the view to the corresponding product group.

Figure 8: Shown is TDK’s Advanced Power Line Noise Suppression Solutions EMI filter selection guide. (Image source: DigiKey)

Within each model group, the left-hand column lists the product shape and performance. The top row identifies the special characteristics of the products listed below the label, such as multi-purpose, pulse attenuation, or wide-range high-attenuation characteristics. The graphic on the right side displays the current range for each filter model.

Individual products are placed within each model group aligned with the appropriate characteristic label. Clicking any product outline box opens the product's detailed specifications for all its variants. Selecting a specific variant opens the product details with a link to the datasheet.

Conclusion

TDK offers a robust portfolio of filters to mitigate EMI across a wide range of electronic applications, including single-phase, three-phase, and DC power systems. They provide designers in the industrial, energy, and power electronics sectors with a broad range of noise-suppression solutions to support EMC compliance.