Advice on designing EMI filters for power supplies

This article is part of the TechXchange: Dive into EMI, EMC and Noise

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What you will learn:

  • Types of EMI and filters used to address them.
  • The steps for creating an EMI filter for your application.

Most environments are full of some type of electromagnetic interference (EMI). EMI can wreak havoc or, at a minimum, cause a circuit design to perform less than optimally. Designers may be able to create a very simple EMI filter to prevent EMI from affecting the performance level of their circuit.

However, in many cases, a more extensive EMI filter design may be required to suppress, or at least attenuate, EMI so as not to impede achievement of desired performance goals.

EMI comes in two varieties:

  • Realized : This can penetrate wires and cables that carry power and/or signals.
  • Radiated: This type of interference spreads through the air from radiators (eg electric motors) and RF transmissions such as cell phones and other radio products.

EMI will most likely disturb other electrical/electronic devices such as speakers, motor controls, communication systems, etc., especially in industrial environments.

When deciding on a suitable EMI filter design, designers should determine the regional EMI regulations that must be met to pass the performance levels needed for qualification. For example, you may need to follow the International Special Committee on Radio Interference (CISPR) (or Federal Communications Commission, FCC) EMI specifications and other similar guides in various countries.

Design of an EMI filter for power supplies

Filter circuit architectures, which are typically used in power supplies, can fall into passive and/or active filter categories. To get started, identify the main offenders of EMI and find the coupling paths.

Designers may need to block common or differential mode noise, depending on the device and its operation. Common mode current (noise or signals flowing in the same relative direction in a pair of lines or in all lines) and differential mode current (noise or signals present in one of the lines or flowing in opposite directions in a pair lines) could both cause interference that may require filtering.

In this case, time will need to be spent engineering the circuit layout to mitigate the interference, and then adding filters and snubbers as needed. Trial and error will probably be the best way to zero in on the best filter design.

The main forms of passive filtering are capacitor filters, inductor filters, and complex filters (including inverted L type, LC filter, LCπ type filter, and RCπ type filter, etc.).

The usual form of active filters is the active RC filter, also called an electronic filter. The larger the value of S – the pulsation coefficient – the worse the filtering effect. The pulsating component is caused by a unbalanced load which will generate a current flowing in the capacitor of the intermediate circuit, thus constituting the reactive power. If the capacitance value in the DC link is large enough, the DC voltage distortion (or ripple) will not be present.

The amplitude of the pulsating component in direct current is represented by the pulsation coefficient S:

Pulse coefficient (S) = fundamental maximum of AC component of output voltage / DC component of output voltage

Many online filter design tools are currently available that can create an unobtrusive and flexible design fairly quickly. Designers can also decide to purchase a standard or custom modular filter, or design a discrete one that can be integrated into a PCB.

Design steps

We then begin to select or design a filter design by understanding the type of filter architecture needed: band-pass, band-stop, high-pass, low-pass, or all-pass.

Subsequently, we determine where the filter will be located in the overall design. The filter may be integrated into a larger printed circuit board or it may be a separate module that must be properly located in or near the circuit most affected by EMI.

The height, length and width of the overall filter must be determined so that the filter can be integrated into, on or near a printed circuit board area. Capacitors and magnetic inductors/transformers are likely to be the main components that will take up the most space.

Now the designers have to decide how many phases are needed. Temperature, humidity, and possible airflow and heat dissipation will likely also be part of the design. The probability of a global shield may also be needed.

Active EMI filters in automotive applications

EMI represents a formidable challenge for designers of electrical systems, particularly in the automotive industry with the arrival of autonomous and semi-autonomous vehicles. Enter the built-in active EMI filter (AEF).1 This unique filter design has active circuitry to detect noise and then inject a noise cancellation signal which will reduce EMI (see picture).

The AEF will detect the noise voltage (VL), then amplify the noise voltage and inject a cancellation current (Ito cancel) in the system.

As a bonus, when comparing an AEF to a crossover, the AEF will be about 50% smaller in size and 75% less in volume, for the same attenuation.

Finally, AEF is a less expensive solution than a passive filter design because AEF does not use large inductors and capacitors in the design architecture.

Adding the EMI Filter to Your Design

We now need to properly integrate the EMI filter into the overall design architecture. Important points to keep in mind:

  • Take care when bundling cables. Do not combine the charging end and the power cable end together.
  • Keep power lines as short as possible.


In summary, designers will need to choose their filter type and design architecture based on local EMI regulations, electrical specifications that the designer must meet to achieve the best performance in the particular system. Many other system design requirements may depend on the system architecture, such as filter placement, cooling, physical size, heatsink, shielding, etc.

In most cases, a standard standard filter can meet the requirements of an application. However, in some designs, a custom EMI filter solution becomes necessary to meet application specific parameters.

Read more articles in the TechXchange: Diving into EMI, EMC and Noise


1. Active EMI filters to reduce the size and cost of EMI filters in automotive systems, 2021

2. PCB Design Guidelines to Reduce EMI

3. The Importance of EMI Shielding in Medical Devices – SAT Plating

4. FDA/CDRH Recommendations for EMC/EMI in Healthcare Facilities | FDA

5. EMI Filter Components and Their Non-Idealities for Automotive DC/DC Regulators

Rosemary C. Kearney