REDEXPERT EMV Filter Designer

EMC filter design at the push of a button

EMC compliance of a product is annoying but unavoidable. At the same time, EMC filter design is far from trivial. The new RedExpert filter designer tool simplifies filter design, helping to save time and money.

EMC (electromagnetic compatibility) compliance is often the last hurdle an electronic device has to overcome in the development process. It is not rare at all for circuit- and layout-specific changes to be required in the process. These then extend the product development cycle and increase costs. For this reason, Würth Elektronik now offers the new “EMI Filter Designer” as part of its “RedExpert” tool family, which can be used to design EMC filters for conducted differential mode interference signals. A typical application is an input filter for DC/DC converters, especially switching regulators which can generate a lot of interference. In the same way, filters for attenuating broadband interference in other applications can be calculated. The goal of a filter circuit is to realize a defined insertion loss in the selected frequency range. This is achieved with a high impedance over the desired frequency range by the largest possible mismatch between load and source. Here, the insertion loss (a; equation 1) is defined as the ratio of the voltage without filter compared to the same circuit with a filter (Fig. 1). For a 2nd order LC filter, equation 2 applies to the cutoff frequency fc.

Figure 1a                                          Figure 1b

Figure 1. Circuit with interference-prone voltage source (left) and load (right) without (a) and with EMI filter (b).


However, equation 2 assumes ideal components. The inductance does not take into account the winding resistance (Rdc) or the capacitance between the windings, and for the capacitor the equivalent series resistance (ESR) and the equivalent series inductance (ESL), etc.


RedExpert Filter Designer

RedExpert EMI Filter Designer takes these parasitic elements into account in the frequency range up to 30 MHz, so that the simulation reflects much more accurately the real electrical behavior of the components and the filter characteristics up to 4th order.

The EMI Filter Designer determines the most suitable topology as a suggestion from the input variables operating voltage, current, load/LISN (Line Impedance Stabilization Network) and interference source impedance, cutoff frequency and attenuation at a defined frequency.

A total of six topologies are available for filters from 2nd to 4th order: LC, CL, Pi (CLC), T (LCL), LC-LC and CL-CL.

After selecting the topology, the software calculates the discrete component values and simulates the frequency responses of gain, input and output impedance of the filter. A summary shows again the input values, the circuit, a BOM with ordering function and the simulated frequency responses. The calculation and the following automatic selection of the component values is based on a Butterworth characteristic with corresponding position of the poles.


Practical example with RedExpert EMI Filter Designer

RedExpert EMI Filter Designer is an online tool at that is suitable for designing an input or output filter for AC/DC or DC/DC converters, etc. The main goal of the filter tool is to calculate a clean signal attenuation for defined input parameters by achieving maximum impedance mismatch.


Asymmetrical input filter for DC/DC converters

A good filter design requires the most accurate input specifications possible. In reality, DC/DC converters have a bulk capacitor in parallel with the input impedance on the input side, so it is common to take the equivalent series resistance (ESR) as the input impedance. The ESR value is usually in the range of 0.1 to 1 Ω.

For example, if we assume the WCAP-AS5H 865230557007 from Würth Elektronik as the bulk capacitor, then the ESR value is 100 mΩ. If one now wants to achieve a filter characteristic with an attenuation of 35 dB at 350 kHz, the tool recommends a CL circuit (Fig. 2) and determines the values of the components to be C1 = 47.0 pF and L1 = 240 nH. Figure 3 also shows the frequency response of attenuation, input impedance and output impedance.

Figure 2. Start page of the RedExpert EMI filter designer for entering filter parameters followed by a suggestion for a suitable filter topology.


Figure 3. Selection of filter components and frequency response of attenuation, input and output impedance.


Output filter for DC/DC converters

Because of the high switching frequencies commonly used in DC/DC converters today, an output filter is required to reduce high-frequency noise and ripple. For example, the output voltage might have a ripple component of 50 mV at 1 MHz, which needs to be reduced to 15 mV. The required attenuation (a) in dB is obtained according to equation 3.

With the input parameters “Attenuation 11 dB” and “at Frequency 1 MHz” the tool now recommends an LC topology with L1 = 470 nH and C1 = 220 nF.



RedExpert EMI Filter Designer:


Gerhard Stelzer studied electrical engineering and information technology at the Technical University of Munich, where he graduated with a degree in engineering. Afterwards he worked in the development of high-rate optical communication technology at Siemens AG. In 1995 he changed to technical journalism at the magazine “Elektronik”. Since 2021, he serves as Senior Technical Editor at Würth Elektronik eiSos.