What is EMI Filter : Working & Its Applications

Electronic devices in different fields mainly work through electricity but sometimes, the performance of these devices may interrupt or they will break down or crash when an Electromagnetic interference (EMI) occurs. So, based on the severe interruption, Electromagnetic interference may impact the signal quality of the device received, so the component or device may get damaged permanently. So to protect these devices from damage, an EMI filter is used.

There are two types of electromagnetic interference conducted & radiated. The conducted type EMI travels through conductors like power lines or wires whereas radiated types EMI goes through the air & does not need a conductor. These two EMIs can affect the performance of the electronic device through degradation, impairment, system failure, or malfunctioning. So this article discusses an overview of the EMI filter and it’s working.

What is EMI Filter/Define EMI Filter?

EMI Filters (electromagnetic interference filters) are electrical devices or circuits, used to diminish the high-frequency Electromagnetic noise which is generated from different electronic & electrical devices like electric motors, power supplies, electronic controls, inverters, microprocessors, clock circuits, etc. The frequency range of this electromagnetic noise ranges from 9KHz – 10GHz & it can avoid the signal transmissions & also electrical or electronic devices performance.

EMI Filter
EMI Filter

Electromagnetic compatibility is an essential indicator to calculate the quality of the electronic product & it has become the key within the design of the electronic product. In the power system design process, the beginning of EMC design can enhance the overall anti-interference capacity of the power system and also expand the power system service life. So, the electromagnetic interference filter (EMC filter) is a device used to provide good electromagnetic compatibility.

How does an EMI Filter Work?

Most electronic devices include an EMI filter or fixed within circuit boards. The main function of an EMI filter is to decrease high-frequency electronic noise that may occur due to interference with other devices. Electro-Magnetic Interference is mainly generated from the electrical current switching and also from electronic power supplies.

Generally, an EMI filter for a power supply includes passive components like capacitors & inductors which are connected to make LC circuits. The inductors allow low-frequency currents or DC to supply through while blocking the unnecessary high-frequency currents whereas capacitors provide a low-impedance lane to redirect the noise away from the i/p of the EMI filter either into the power supply otherwise into the GND connection.


EMI filter has to meet protection standards to meet EMI regulations. The increase in inductor temperature can be measured & for mains operation, the least electrical spacing among line, ground & neutral are controlled. So, this decreases the risk of electrical shock or fire.

Types of EMI Filter

There are two types of EMI filters active EMI filter and passive EMI filter.

Active EMI Filters

Active EMI filters are mainly designed with active components like op-amps in conjunction with passive components to develop low-frequency attenuation and to achieve the desired filtering. The Active EMI filter is used to detect the noise current from the current transformer and amplifies then injects it back like the compensation current throughout an RC branch coupled to an op-amp circuit.

There are different types of EMI filters like VSVC or voltage sensing voltage cancellation, CSVC or current sensing voltage cancellation, VSCC or voltage sensing current cancellation & CSCC or current sensing current cancellation. These filters play a key role in low-level circuits as compared to high voltage or high input current AC power circuits. The bandwidth effectivity of these filters can be restricted relative to passive filters.

Passive EMI Filters

Passive EMI filters include capacitors and inductors which are connected in particular circuits to get the desired filtering. This filter performance mainly depends on the source of the host circuit & load impedances.

These filters are mainly used to enhance EMI noise attenuation because of less cost & no need for control circuit otherwise components. These filters are available in bulk size & need multi-stage LC circuits to attain significant high-frequency noise drop. So this results from higher cost & larger size of the power converter.

Electronic Ballast using EMI Filter Circuit Diagram

Electronic ballasts were manufactured by different companies where each company maintains the same block diagram but the circuit will be different. The circuit diagram of electronic ballast using EMI filter can be divided into five blocks EMI filter, rectifier, dc filter, inverter, and control circuit.

Ballast Circuit Diagram using EMI Filter
Ballast Circuit Diagram using EMI Filter
  • EMI filter in the first block includes a capacitor and inductor which blocks or decreases the EMI or electromagnetic interference.
  • The rectifier circuit in the second block is used to change AC to DC.
  • The DC filter circuit in the third block includes a capacitor that filters the impure DC which is coming from the rectifier circuit.
  • In the inverter circuit in the fourth block, the DC can be changed into high-frequency AC & there is an arrangement of a step-up transformer. This transformer increases the level of voltage.
  • The final block in the above circuit is the control circuit. This circuit takes feedback of an o/p to control the rectifier, filter & inverter circuits.

Circuit Working

In the above circuit, a current limit resistor or series resistor is connected to restrict the short circuit and overload current. In some types of electronic ballasts, a fuse can be used in place of this resistor. The resistance value of this current limit resistor is very less up to 22 Ohm. After that, an EMI filter is connected which includes one inductor within series & one capacitor within parallel.

A bridge rectifier circuit includes 4 PN junction diodes, used to convert AC to DC. A single capacitor can be connected in parallel for filtering of impure DC which is coming from the rectifier circuit.

An inverter circuit includes two transistors which create high-frequency AC & give to the step-up transformer. The frequency which is created in electronic ballast mainly ranges from 20 KHz – 80 KHz. The step-up transformer in the circuit will increase the level of voltage up to 1000V. So at the beginning time, the voltage across the light is 1000V. When the tube light glows then the voltage will be decreased to 230V. So, the electronic ballast allows a low current flow toward the tube light.

Advantages and Disadvantages

The advantages of the EMI filter include the following.

  • It avoids failure and malfunction of equipment
  • It is a special type filter, used to filter EMI on different cables.
  • The working frequency of this filter is wide which ranges from 10 kHz to above 1GHZ.
  • The filter impedance which is connected to the electronic circuit is fixed, however, the circuit impedance which is connected to the EMI filter is not fixed, although changes through the frequency.
  • It obeys the European EMC regulation 2014/30/EU
  • Comply through standard EN 61000
  • Comply through EMC plans arranged by engineers or consultants or installers.
  • Conquer EMC disturbances from outside
  • Resolves EMC disturbances caused by harmonics

The disadvantages of the EMI filter include the following.

  • Some EMI filters may also route unwanted currents back to the noise source or absorb them. Because these filters defend against conducted EMI only, they frequently couple with shields that chunk radiated EMI.
  • An unshielded EMI filter transmits noise throughout the air to harm the device.


The applications of the EMI filter include the following.

  • Military or Aerospace systems and subsystems
  • Energy Management Systems
  • Test Chambers or Shielded Rooms
  • Appliances
  • MRI Rooms
  • Computers
  • Automotive Battery Charger
  • Factory Automation Equipment
  • Exercise Equipment
  • Industrial Equipment
  • Medical Imaging or Diagnostic/Patient Devices

Thus, this is all about an overview of an EMI filter or electromagnetic interference filter. These filters can be used in different ways to protect from electromagnetic noise. These filters simply block noise with different frequencies to meet unstable regulations within different industries. Here is a question for you, what are the different types of filters available?