“EMC (Electromagnetic Compatibility) certification is often the last hurdle that must be overcome in the development of an Electronic product. During the certification process, changes to specific circuits or wiring are often required, which can extend the product development cycle and increase costs. To this end, Würth Electronics’ “REDEXPERT” simulation platform has added a new “EMI filter design tool”, which can be used to design filters that suppress conducted differential mode noise.

“

Author: Gerhard Stelzer

**REDEXPERT EMI Filter Design ToolOne-click EMC filter design**

EMC certification of products is challenging, but inevitable, and EMC filter design is no easy task. The new REDEXPERT filter design tool simplifies the filter design process, saving you time and money.

EMC (Electromagnetic Compatibility) certification is often the last hurdle that must be overcome in the development of an electronic product. During the certification process, changes to specific circuits or wiring are often required, which can extend the product development cycle and increase costs. To this end, Würth Electronics’ “REDEXPERT” simulation platform has added a new “EMI filter design tool”, which can be used to design filters that suppress conducted differential mode noise.

Typical applications are input filters for DC/DC conversion circuits, especially switching power supplies that generate a lot of interference. Filters used to filter out broadband noise in circuits can do the same. The goal of filter design is to achieve the desired insertion loss over the required frequency range. By mismatching the load and source impedances as much as possible, the target can be achieved over the desired frequency range. Insertion loss (“a” in Equation 1) is defined here as the ratio of the voltage without the filter to the voltage with the filter in the same circuit (Figure 1). For a second-order LC filter, the cutoff frequency fC is usually calculated using Equation 2.

(1)

(2)

The calculation of Equation 2 uses ideal components and does not consider the winding resistance (Rdc) and winding capacitance of the Inductor, as well as the equivalent series resistance (ESR) and equivalent series inductance (ESL) of the capacitor.

Figure 1: Power (left) and load (right) circuits containing noise, (a) without EMI filter, (b) with EMI filter.

**REDEXPERT EMI Filter Design Tool**

The REDEXPERT EMI filter design tool can account for these parasitics in the frequency range of 30MHz, enabling simulations to more accurately reflect the true electrical performance of components, as well as up to fourth-order filter characteristics.

The EMI filter design tool determines the most suitable topology based on parameters such as input operating voltage, current, load/LISN (Line Impedance Stabilization Network), interferer impedance, cutoff frequency, and attenuation in the desired frequency band.

There are 6 topologies to choose from, from 2nd to 4th order: LC, CL, Pi (CLC), T (LCL), LC-LC and CL-CL.

After the topology is determined, the software calculates the values of the discrete components and simulates the filter’s gain frequency response, input and output impedances. Finally, the tool provides a summary page with input information, recommended circuits, a list of devices with sorting capabilities, and frequency response. Parameter calculations and automatic device selection are based on the poles of the Butterworth filter.

**REDEXPERT EMI Filter Design Tool Application Example**

The REDEXPERT EMI filter design tool is available online at www.we-online.com/filter-designer and is suitable for designing input and output filters for circuits such as AC/DC and DC/DC. The goal of this tool is to achieve noise suppression with as large an impedance mismatch as possible.

**Input filters for DC/DC circuits**

A good filter design requires as precise a specification input as possible. A large-capacity capacitor is usually connected in parallel at the input end of the DC/DC conversion circuit, so the equivalent series resistance (ESR) of the capacitor can be used as the input impedance. ESR values are typically in the range of 0.1Ω to 1Ω.

For example, the WCAP-AS5H series 865230557007 aluminum electrolytic capacitor of Würth Electronics has an ESR value of 100mΩ. To achieve 35dB attenuation at 350kHz, the tool will recommend a CL circuit (Figure 2) with component values C1 = 47.0pF, L1 = 240nH. Figure 3 shows the frequency response of attenuation, input impedance, and output impedance.

Figure 2: The start screen of the REDEXPERT EMI filter design tool, where filter parameters can be entered and the tool will recommend a suitable topology.

Figure 3: Filter component options, and frequency responses for loss, input and output impedance.

**DC/DC circuit output filter**

Today’s DC/DC circuits typically switch at higher frequencies, so an output filter is required to reduce high frequency noise and ripple. For example, suppose the output voltage has a ripple component of 50mV at 1MHz and needs to be reduced to 15mV. The required attenuation in dB (a) can be calculated from Equation 3.

(3)

After entering “Attenuation 11dB” and “Frequency 1MHz” at the input parameters, the tool suggests an LC topology of L1 = 470nH and C1 = 220nF.

refer to:

REDEXPERT EMI filter design tool: www.we-online.com/filter-designer

author:

Gerhard Stelzer studied electrical engineering and information technology at the Technical University of Munich, where he obtained a master’s degree in engineering. After that, he worked in the development of high-speed optical communication technology at Siemens AG. In 1995 he joined the magazine “Elektronik”, working in technical journalism. Gerhard will be Senior Technical Editor at Würth Electronics from 2021.

The Links: **SKT760/16E** **LQ150X1LG71** **ELEQUOTE**