Industry White Paper

EMC Basics: Using EMI Filters

October 20, 2022 by Würth Elektronik
Topics Covered
  • Why EMC Filters?
  • Challenges of Designing EMI Filters
  • REDEXPERT EMI Filter Designer


When creating an electronic-based product, engineers must ensure that their designs are EMC compliant before they can be released. This can potentially mean lots of rework if it isn’t addressed until later in the design process, especially if there is a failure to meet the standards. EMI filters can be a great solution for managing unwanted noise if they are integrated early in the design.

In this article, we’ll explain the importance of EMI filters and how to design input filters for DC-DC power supplies. If you want to brush up on some other EMC topics, check out our other articles here.

Why EMI Filters?

Electronic circuits may face interference from both outside sources or within the design. EMI filters work to attenuate this noise so that it does not interfere with the system or any outside devices. The goal of a filter is to provide the desired insertion loss or attenuation at a desired frequency, which is done by providing high impedance over a wide range of frequencies and maximum mismatch between load and source. Figure 1 shows an example of a typical input filter. 

Figure 1. Example of a typical input filter for a DC/DC converter. 


Switch mode power supplies are one type of electronic system that always requires EMI filters. This is because they create broadband interference emissions in the form of interfering voltage and interference fields which can make other devices malfunction. The main cause of the interfering voltage is the discontinuous current that results from switching. These currents generate voltage ripple as they flow through the ESR of the input capacitor. Additionally, the parasitic capacitance and inductance of the components (input capacitor, switch, inductor) and layout cause high frequency ringing at the switching transitions that needs to be attenuated.

This can be corrected by using an input filter to reduce the amplitude of the interfering voltage and suppress the harmonics. 

Challenges of Designing EMI Filters

One of the challenges of designing your filter is the need to know the results across a range of frequencies. The impedance of reactive components like capacitors and inductors changes with frequency. Many formulas used for selecting components do so at a specific frequency. For example, the cut off frequency is for a specific attenuation at a specific frequency.

Cut-off frequency: f=12πLC

It’s assumed at lower frequencies the attenuation (or insertion loss) is less and at higher frequencies it is more. With ideal components this would be the case, but all components have parasitic elements such as winding resistance (both AC and DC), inductance, and capacitance, which primarily limit their high frequency response. Thus, filter response always needs curves that show impedance vs. frequency over the range of interest. 

Additionally, the Q of the filter may cause unwanted gain that will need to be damped. Trying to include all these non-ideal properties in your calculations can quickly become tedious and complex. 

REDEXPERT EMI Filter Designer

Because manual calculations using equations have many shortcomings, more advanced tools can help find the proper components for your EMI filter. That is why Würth Elektronik provides our REDEXPERT EMI Filter Designer for engineers looking to create their own filter. 

Filter Designer allows you to enter your input parameters and then see the recommendations for your design. You will receive component recommendations based on the noise source and load impedance, cut-off frequency, and other input parameters. It also shows you the insertion loss and the input/output impedance. If you’re not sure which topology you should use for your filter, you can also use the recommended topology based on the impedance, insertion loss, and cutoff frequency. 

Another benefit of REDEXPERT is that it will show you simulation results with the real parasitics of the selected components. Typically, the ideal is assumed and you are not able to take into account the non-ideal components. 

Input Filter for DC-DC Converter Example

Let’s walk through an example of how to design an input filter for a DC-DC converter. Note that Filter Designer can be used for other filter types as well, but for now we will focus on an input filter. When designing a filter, the power supply generates the noise and is the source. For input filters, the noise travels back to the input supply making it the load, which is usually a LISN and for output filters the noise travels to the output load, hence the term ‘Load/LISN’. Since most power supplies have a bulk or DC link capacitor on the input, the ESR of that capacitor is usually dominant over the calculated input impedance.

We will use the following parameters for the design:

Operating Voltage: 12 V

Operating Current: 3 A

Load/LISN impedance: 50 Ω (LISN impedance)

Noise source impedance: 0.1 Ω (input capacitor ESR)

The reason for providing the operating voltage and current is so that you can receive the proper selection of capacitors based on the voltage rating and inductors based on the saturation current. You can select what frequency you would like to attenuate noise at and the amount of attenuation you need. For this example, we want 30 dB of attenuation at 500 kHz. It is recommended to enter the cut-off frequency for a more precise insertion loss and topology recommendation. 

Then, you can select your desired filter topology. Generally, the inductor is placed on the low impedance side and the capacitor is placed on the higher impedance side. Thus the Pi filter works best when both source and load are high impedance and the T-filter works well when they are both low. If you’re not sure which topology to choose, don’t worry! REDEXPERT will provide an automatic recommendation. 

Figure 2 shows the page where you will input the parameters on REDEXPERT. 


Figure 2. Parameters for Input Filter example. 


Now, you can select “Next” to see the component selection and simulation page. This page will show the recommended components; for our example, it is recommended that we use a 27.0 µF capacitor and a 160 nH inductor as shown in Figure 3.  You will also see the C1 capacitance, L1 inductance, and insertion loss values; in this example, they are 25.3 µF, 127 nH, and -31.5 dB at 500 kHz, respectively. The charts to the right showing the Insertion Loss, Input Impedance, and Output Impedance are based on the real parasitics of the components. The buttons with the double arrows opens a window where you can select alternates parts or values and even add specific parts that are not listed.

Figure 3: Selection and Simulation page of REDEXPERT Filter Designer.


Hitting “Get Summary” will take you to the summary page where you can see the exact components in the Bill of Materials for your filter as well as an overview of the specifications and graphs. 

For our design, we were recommended this WCAP-PSHP Aluminum Polymer Capacitor and this WE-LQS SMT Power Inductor.  

Figure 4: REDEXPERT Filter Designer Summary Page. 


EMI filters are a great way to ensure designs and products are EMC compliant. Tools like REDEXPERT EMI Filter Designer make the process of filter design less complex and take out the trial-and-error of determining the proper components. A filter with suitable components for your design can help you achieve the attenuation and EMC compliance you need.