How To Choose an EMI Filter for Your Electronics Product

Electromagnetic interference (EMI) is unwanted noise or interference in an electrical circuit caused by an electromagnetic field (EMF). EMI can be radiated from sources such as radio transmitters, or conducted through wires or cables due to the flow of electrical current. It can cause noise or operational errors, affecting the performance of electronic devices and potentially leading to operational issues. Understanding and addressing EMI is crucial for ensuring the safety and functionality of electronic devices.

Where Does EMI Originate?

EMI can come from natural sources such as lightning and solar storms, as well as from human-made sources such as power lines, radio transmitters, and household appliances. EMI can travel through wires, disrupt sensitive electronic components, and spread through the air as electromagnetic waves. Internally, EMI can be generated by switching circuits, motors, transformers, and high-speed data lines. These sources create high-frequency noise that can interfere with other circuits, leading to data distortion, malfunctions, and even damage to components.

EMI can be either conducted or radiated. Radiated EMI occurs when the electrical paths on a circuit board cover large areas, creating loop antennas or dipoles that propagate the signal. This noise, which ranges from 30 MHz to 1 GHz, can come from both inside and outside sources. Conducted EMI, on the other hand, comes from imbalances in circuits or unwanted elements in the circuit. This type can magnetically or electrically connect to other parts of the circuit.

Unwanted currents or voltages can travel through wires or cables because of common-mode or differential-mode noise. Both types of EMI can impact signal quality, leading to distorted data, malfunctions, and even damage to components. Inside the equipment, switching circuits create high-frequency transients that can radiate or couple conductively, while motors and transformers can generate both conducted and radiated EMI due to the interaction of magnetic fields within windings. High-speed clocks and data lines can also generate high-frequency energy, potentially interfering with other circuits.

Why are EMI Filters Needed?

EMI filters are crucial for reducing electromagnetic noise in electronic devices. These filters help ensure devices meet electromagnetic compatibility (EMC) standards and operate smoothly by blocking unwanted frequencies while allowing desired signals to pass through. Here are the benefits of EMI filters:

• Reduces EMI: Enhances device performance and reliability by minimizing noise.
• Ensures EMC compliance: Helps devices meet regulatory requirements.
• Protects sensitive equipment: Safeguards critical applications from electromagnetic disturbances.
• Extends device longevity: Reduces stress and wear on components.
• Improves communication quality: Enhances data transmission by reducing interference.
• Promotes safety: Contributes to a safer environment for electronic devices.

A typical setup of a DC input filter for a DC/DC switching controller is shown in Figure 1. An LC filter is used to minimize differential mode interference to prevent noise from entering the device from the power supply and vice versa. With a well-designed input filter, and carefully chosen components, you can achieve the highest possible insertion loss while maintaining the stability of the switching controller.

Figure 2: DC input filter. Image Source: Wurth Elektronik

EMI filters provide the desired insertion loss or attenuation at a desired frequency by providing high impedance over a wide range of frequencies. Selecting the right EMI filters early in the design process can effectively manage this noise.

What are the Types of EMI Filters?

EMI filters come in various types and configurations. They can be passive or active:

• Active Filters: Generate electricity to counteract EMI by detecting voltage input and creating an opposing current.
• Passive Filters: Absorb unwanted energy using components such as capacitors, resistors, transformers, and inductors.

EMI filters come in different types: low pass, high pass, bandpass, band reject, etc. Each type is designed to block specific ranges of frequencies to reduce unwanted noise. They can be set up in various ways to achieve different levels of noise suppression. Common designs include LC, CL, Pi, and T-Filter forms, as well as more complex architectures, as shown in Figure 4. These filters are carefully adjusted to resonate at a particular frequency or range of frequencies to block unwanted noise.

Figure 4: Topologies of implementing EMI filters. Image Source: Wurth Elektronik

A ferrite choke is a passive EMI filter, a capacitance-based inductor that suppresses high frequency signals around a power line, providing low pass filtration for common mode or differential mode conducted EMI.

Other common types of EMI filters include:

• Power Line Filters (PLFs): Target conducted EMI on power lines.
• Signal Line Filters: Address conducted and radiated EMI on signal lines.
• High-Frequency (HF) Filters: Attenuate EMI at high frequencies.
• Feedthrough Capacitors: Filter low-frequency conducted EMI.
• Common Mode Filters: Target common-mode noise on differential signal pairs.

What Factors Should You Consider When Choosing an EMI Filter?

When choosing an EMI filter, consider these factors:

• Frequency range and interference levels: Identify the specific frequencies and levels of interference to address.
• Filter type: Select a filter that effectively suppresses the identified EMI.
• Environmental conditions: Ensure the filter performs well under expected conditions such as temperature and humidity.
• Mounting options: Choose a mounting method compatible with your PCB layout and production process.
• System compatibility: Verify that the filter works well with your system's connections, cables, and power supply.

What Is the REDEXPERT Filter Designer?

The REDEXPERT Filter Designer is a tool for designing input filters in DC-DC converters, AC-DC converters, and output filters. It simplifies the process of designing differential mode input filters or noise filters by calculating components based on input parameters. The tool uses non-ideal component models, including parasitics, to determine filter performance. By using the correct input parameters, the tool reduces the time and cost of EMC compliance.

This design kit offers a quick and cost-effective way to build filter designs using various components. Initially, we recommend using the REDEXPERT Filter Designer for calculations. This tool simplifies the process of designing differential mode input filters or noise filters by calculating components based on input parameters. By utilizing non-ideal component models, including parasitics, it accurately determines filter performance and reduces the time and cost of EMC compliance.

After completing the initial calculations with REDEXPERT, you can directly transition to creating prototypes using the physical design kit. This approach enables the verification of the filter circuit's behavior regarding the desired filtering effect and the integration of the selected filter circuit into your application. Consequently, this method helps you to minimize development times and associated costs.

Conclusion

Electromagnetic interference (EMI) is unwanted noise or interference in an electrical circuit caused by an electromagnetic field (EMF). EMI can come from natural sources like such as lightning and solar storms, as well as from human-made sources such as power lines, radio transmitters, and household appliances. EMI filters are crucial for reducing electromagnetic noise in electronic devices. These filters help ensure devices meet electromagnetic compatibility (EMC) standards and operate smoothly by blocking unwanted frequencies while allowing desired signals to pass through. When choosing an EMI filter, frequency range and interference levels, filter type, environmental conditions, mounting options, and system compatibility should be considered.

About the Sponsor

Würth Elektronik eiSos Group is a manufacturer of electronic and electromechanical components for the electronics industry and a technology company that spearheads pioneering electronic solutions. Würth Elektronik eiSos is one of the largest European manufacturers of passive components and is active in 50 countries. Production sites in Europe, Asia and North America supply a growing number of customers worldwide.

The product range includes EMC components, inductors, transformers, RF components, varistors, capacitors, resistors, quartz crystals, oscillators, power modules, Wireless Power Transfer, LEDs, sensors, radio modules, connectors, power supply elements, switches, push-buttons, connection technology, fuse holders and solutions for wireless data transmission. The portfolio is complemented by customized solutions.

The unrivaled service orientation of the company is characterized by the availability of all catalog components from stock without minimum order quantity, free samples and extensive support through technical sales staff and selection tools. For more information go here