An EMI filter (Electromagnetic interference filter) is an electrical device or circuit used to suppress or filter out high-fr­equency noise current present on power and signal lines, i.e., it suppresses the conducted EMI on power and signal lines. By filtering out the noise current, the EMI filter protects sensitive electronic devices/systems connected on the line from harmful impacts of such noise current and ensures the reliable operation of the devices. This filter can be incorporated on a printed circuit board or utilized as a separate, standalone device. 

Figure 1: EMI filter conceptGenerally, the EMI filter is designed by using passive components such as inductors, capacitors, and resistors. But active filters are also available that use active components (e.g., transistor, Op-amp, etc.) with an external power supply. This filter works as a low-pass filter that filters out high-frequency currents present on power/signal lines and passes desirable currents to flow freely.    

The EMI filter is also called an EMC filter (Electromagnetic Compatibility filter) or EMI suppression filter. When it is used to suppress noise in a power line, it is known as an EMI power line filter that allows power frequency (DC or 50/60/400 Hertz) current while blocking high-frequency noise.

What is electromagnetic interference (EMI), conducted EMI, and radiated EMI?
Electromagnetic interference (EMI) refers to unwanted electromagnetic energy/signal emissions from manmade sources such as electrical and electronic devices or natural sources (e.g., lightning, solar flares) that can interfere with the operation of or sometimes destroy the nearby electrical and electronic devices (s). The Electromagnetic noise (EMI) is generated or emitted during the operation of devices such as motor drive inverters, DC-DC converters, SMPS, power supplies, electronic controls, inverters, microprocessors, clock circuits, etc., as well as emitted from natural sources. 

Conducted EMI and Radiated EMI are the two types of EMI. The conducted EMI is the high-frequency noise currents generated from an electrical and electronic device while operating that travels through the electrical conductors such as wires, PCB traces, or cables and can affect the other connected devices on the same line. Radiated EMI is the unwanted electromagnetic signal/wave emitted from a device structure and propagating via air and interfering with/affecting other nearby devices.

Figure 2: Understanding Conducted EMI (propagate via power line) and Radiated EMI (unwanted electromagnetic energy emissions, propagate via air)

Note that the EMI/EMC filters offer protection against conducted EMI only; hence, they are often used in conjunction with shields that block radiated EMI. An unshielded electromagnetic interference filter can still transmit noise via the air that may disturb or damage the device. Adding a shield at the attachment point of the EMI filter can effectively suppress all types of EMI. However, if there is a short length of conductor between the EMI source and the filter, using a filter alone is enough. Typically, this filter is used together with shields and other types of protection.

How does an EMI filter work? 
The EMI filter works as a low-pass filter that removes unwanted high-frequency noise current conducted through wires or cables (i.e., conducted EMI) while allowing desirable currents to flow freely. It effectively diverts high-frequency noise currents away from the device to be protected, either by diverting the currents to the ground/earth or, in some cases, absorbing them (see Figure 1). Some filters may route the noise currents back to the EMI noise source or cancel them. After the EMI filtration process, it provides a clean output signal at the output side.

Types of EMI filter: 
The EMI filters are broadly classified into two: Passive EMI filters and Active EMI filters.

Passive EMI filters: 
Generally, the EMI filters are designed as passive filters. The passive filter circuit uses passive components such as inductors, capacitors, and resistors to suppress the conducted EMI in an electronic circuit.

A simple passive EMI filter is designed with LC network topology. The inductor (L) allows DC and low-frequency signals to pass through while blocking high-frequency current (i.e., noise). The capacitor (C) provides a low impedance path for high-frequency current (i.e., noise) and directs the noise into the ground or earth. The multiple LC filters can be used in cascade form (i.e., multiple stages/higher order filter) to achieve a higher attenuation effect and a steeper rise in the attenuation curve.  EMI filters for power supply typically utilize passive components. 

Figure 3: The passive EMI filter installed at the AC side of the motor drive system 

There are various circuit configurations are used for passive filters, including L filter, C filter, LC filter, CL filter, T – filter, and Pi filter. Each filter configuration has its unique performance and characteristics that determine the level of noise suppression (attenuation) required at various frequencies. The following table provides various passive filter circuit configurations (C, L, Pi, T, 2Pi, and 2T filters) and their advantages and applications.

Figure 4: Filter configurations

Another commonly used passive EMI filter is the ferrite bead or ferrite choke/ferrite clamp. The ferrite bead or ferrite clamp is a type of choke commonly used on a cable and wire to suppress high-frequency radiated noise. It absorbs the conducted EMI on power lines/cables and dissipates the noise in the form of heat. The laptop’s power cord uses a ferrite bead/ferrite clamp to remove high-frequency noise on the input power supply line.

Figure 4: Ferrite bead

Active EMI filters: 

Figure 5: Active EMI filter circuit example

An active EMI filter uses active electronic components such as transistors, operational amplifiers, voltage converters, etc., in conjunction with passive components to suppress conducted EMI in the electronic circuit. This filter requires an external power supply for its operation. The active filter first senses any high-frequency noise on the supply lines and then generates noise-canceling currents. These noise-canceling currents are injected back into the supply lines to cancel out EMI. This EMI filter typically consists of a noise sensing circuit, resonant controller, noise injection circuit, and other supporting components/circuits to suppress noise and ensure stability. The active EMI filter ICs typically come with inherent protection features such as UVLO protection and thermal protection.    

Types of active EMI filters include:

• Current-Sensing Voltage-Cancellation (CSVC)
• Voltage-Sensing Voltage-Cancellation (VSVC)
• Current- Sensing Current Cancellation (CSCC)
• Voltage-Sensing Current Cancellation (VSCC).

Active vs Passive filter:

• Active EMI filtering (AEF) technology needs an external power supply for its operation.
• As compared to passive filters, the active EMI filters have a simple structure with improved performance and reduced CM choke size, lower volume, weight, and cost.  
• Active filter takes up less space than the Passive EMC filter. 
• Active filter has a complex design and has to deal with the stability of electronic components.
• Active filters indeed have a limited bandwidth of operation compared to passive filters because active electronic components have a finite bandwidth.

Note: Based on the type of power line network or application, the EMI/EMC filters can also be classified as 2-line filters (used for DC or single-phase applications), 3-line filters (no neutral conductor) or 4-line filters (with neutral conductor) for three-phase applications. The signal/data line filters are also available.

How do EMI filters help product manufacturers to comply with the requirements of EMC standards?
EMC (electromagnetic compatibility) means the ability of an electrical and electronic device/system to operate satisfactorily/properly in its intended electromagnetic environment without introducing intolerable electromagnetic disturbances (EMI) to anything in that environment.

Government organizations have developed EMC standards and regulations. The objective of the EMC standards/regulations is to limit EMI emission from electrical and electronic products and also to ensure that the product has sufficient immunity to withstand EMI from external devices while operating in its intended electromagnetic environment. 

An electrical and electronic product can get into to market of a country when it meets the requirements of EMC emission and immunity standards/regulations relevant to the product. When a product meets the requirements of the relevant EMC standards, it is said to be an EMC-compatible or EMC-certified product. EMC-compatible product means that the product does not emit intolerable EMI emission, can withstand external EMI from nearby devices (i.e., it has immunity against EMI from external devices), and operates satisfactorily in its intended electromagnetic environment. EMC testing is a process performed to verify that a product complies with its relevant EMC emission and immunity standards for EMC certification purposes.

• The EMI/EMC filter prevents conducted EMI on the power line/signal line from entering into the equipment, thereby enhancing the equipment's immunity to external noise currents or disturbances (i.e., protecting the equipment from external noise currents or disturbances). 
• Additionally, the filter limits the emission of conducted EMI generated within the device (i.e., stops internally generated EMI from leaking out), thereby safeguarding other connected devices on the same line from the conducted EMI interference.

Figure 6: EMI/EMC filter prevents EMI from entering into equipment and also prevents EMI emission leak out from the equipment

In this way, EMI filters help product manufacturers meet the emission and immunity requirements of EMC standards and regulations, enhancing the overall performance and reliability of electrical and electronic devices/products. Hence, the EMI/EMC filter has become a critical component of electrical and electronic equipment design. 

Note: These filters must be designed to filter out both common-mode and differential-mode noises.  

Where to place an EMI/EMC filter?
For EMI compliance, the EMI filter is placed at the power entry point (POE) of the device.  The filter is mounted such that the filter's metal case makes direct contact with the device enclosure.

In some converter applications, this filter is used on both the input and output side.  

Safety standards for EMI/EMC filters:
The EMC/EMI filters must be designed to meet the requirements of safety standards like UL 1283, IEC 60939, CSA 22.2, etc. Compliance with these standards is important to ensure the reliability and safety of EMI filters and the equipment they are installed in.

For use in an application, it is good to select the EMI/EMC filter that is already tested for safety and has safety approval marks such as UL recognition, CSA/cUL approval, ENEC approval, etc. This will simplify the safety approval process of the equipment.  

 Benefits of EMI Filters: 

• Protects electrical and electronic devices/equipment from conducted EMI on power as well as signal lines, ensuring the smooth and uninterrupted operation of the equipment. 
• It prevents conducted EMI on the power/signal line from entering into the equipment while also stopping internally generated EMI from leaking out. Reduce the risk of electrical noise affecting nearby electronic equipment.
• It helps the product manufacturers to ensure compliance with electromagnetic compatibility (EMC) standards. 
• The performance and longevity of sensitive electronic components are enhanced. 
• Improve signal quality and minimize signal distortion in electronic circuits. 
• Enhance the overall efficiency and functionality of electrical systems.

Drawbacks of EMI filters:

• It only suppresses conducted EMI noises. To block both conducted and radiated EMI, it is necessary to use the filter together with a shield that blocks radiated EMI. 
• Some EMI filters reflect the noise current back to the noise source.
• An unshielded filter may transmit noise via air that may disturb other nearby equipment.

EMI filter applications:
Electromagnetic interference filters are used in various applications to protect electrical and electronic devices from conducted disturbance on power and signal lines to guarantee reliable operation.  They are used in application including power supplies, wind turbines, motor drives, energy management systems, Telecommunications Equipment, Medical Devices, Automotive Electronics, industrial electronics, consumer electronics, battery chargers, etc.

Key parameters needed to consider while selecting the filter:
The EMI/EMC filters come in various types, designs, shapes, and configurations. Here are the key specification parameters to consider while selecting an EMI/EMC filter. 

Type: It represents the type of EMI/EMC filter. The filters are available in various types, including the DC Filter, Single Phase EMI/EMC Filter, Three Phase EMI/EMC filter, Dual phase EMI/EMC filter, signal/data line filters, and more. 

Rated voltage (V): It represents the maximum line voltage level that can be handled by the filter. Select the filter that has the rated voltage more than or equal to the maximum input line voltage applied to the load. Note that EMI filters for AC power supply lines can also be utilized for DC power supply lines. 

Rated current (A): It is the maximum steady-state current load current (nominal value) that can be handled by the filter without exceeding the safe temperature range.  The rated current of the filter must be greater than or equal to the maximum load current that the device will take when powered. Filters can handle in-rush current, but the filter will fail when handling current higher than its rated current for a longer time.

Leakage current: It refers to the current that flows from the 'line' and 'neutral' to the 'ground' connection in the AC power line connected EMI filter.  This current is caused by ‘line’ to ‘ground’ (Y) capacitors in the filter and expressed in the mA range.  In so many applications, standards limit the total leakage current of the device. Hence, it is necessary to consider leakage current to avoid compliance issues with safety standards, such as IEC60601 for medical equipment, EN 60950-1 for information technology equipment, and EN 55014 for appliances.

Resistance: It is the resistance value (DC resistance) between the input and output of the filter in both directions. This value includes the resistances of the filter's coils as well as any resistance introduced by connections between the coils and terminals. It is measured in ohms. The voltage drop due to the EMI filter is represented by the following expression.

Voltage drop = DC resistance x Load current

Insulation resistance: It is the resistance value that indicates the degree of insulation between the conductive components and the ground or other conductive elements in the filter. It is usually in the megohms range.  A high insulation resistance indicates good insulation that prevents leakage current between the conductive components and the ground or other conductive elements in the filter. 

Dielectric Strength: It is the high DC voltage (VDC) applied between the lines and the mounting plate (ground) of the filter to check the insulation strength. 

Package type: The EMI filters are available in various package types like Screw Mount, Chassis Mount, Panel Mount, and Flange Mount. 

Operating temperature: It is the safe operating temperature range of the filter, measured in ºC.

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