EMC filter design at the push of a button

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EMC filter design at the push of a button



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EMC compliance of a product is annoying but unavoidable. At the same time, the design of EMI filters is by no means trivial. The Redexpert EMI Filter Designer tool simplifies filter design and helps save time and money.

The Redexpert EMI Filter Designer tool simplifies the design of EMI filters and helps save time and money.

(Image: Würth Elektronik eiSos)

Compliance with EMC (electromagnetic compatibility) compliance is often the last hurdle an electronic device must overcome in the development process. It is not uncommon for circuit and layout specific changes to be required. These then lengthen the product development cycle and increase costs. Therefore, Würth Elektronik now offers Redexpert EMI Filter Designer as part of its Redexpert tool family, with which EMI filters for conducted differential mode interference signals can be designed. A typical application is an input filter for DC/DC converters, especially when working with switching regulators that generate a lot of interference. Similarly, broadband interference suppression filters can also be calculated for other applications.

A filter circuit should implement a previously defined insertion loss in the selected frequency range. This can be achieved with a high impedance over the desired frequency range by maximizing the mismatch between load and source. The insertion loss a is defined as the ratio of the voltage without filter compared to the otherwise identical circuit with filter:

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0105537262 (Image: Würth Elektronik eiSos)

For a 2nd order LC filter, the cutoff frequency is fc following equation:

0105537268 (Image: Würth Elektronik eiSos)

However, this equation assumes ideal components. The winding resistance R is not taken into account for the inductanceDCthe capacitance between the windings and for the capacitor the equivalent series resistance ESR and the equivalent series inductance ESL etc.

The Redexpert EMI Filter Designer tool

Redexpert EMV Filter Designer takes these parameters into account in the frequency range up to 30 MHz, so that the simulation reflects the real electrical behavior of the components and the filter properties up to the 4th order much more precisely.

The EMI filter designer uses the input variables operating voltage, current, load/LISN (Line Impedance Stabilization Network) and interference source impedance, cutoff frequency and attenuation at a defined frequency to determine the most suitable topology as a suggestion.

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 response of the filter’s gain, input impedance, and output impedance. An overview again shows the input variables, the circuit, a bill of materials with an order function, and the simulated frequency curves. The calculation and subsequent automatic selection of the component values ​​is based on a Butterworth characteristic with the corresponding position of the bars.

Practical example with Redexpert EMI Filter Designer

Redexpert EMI Filter Designer is an online tool suitable for designing an input or output filter for AC/DC or DC/DC converters, etc. A typical input filter for a DC/DC converter is shown in Figure 1. The main purpose of the filter tool is to calculate pure signal attenuation for defined input parameters by achieving maximum impedance mismatch.

Unbalanced input filter for DC/DC converters

A good filter design requires circuit specifications that are as accurate as possible. In reality, DC/DC converters have a bulk capacitor on the input side in parallel with the input impedance, 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, assuming the WCAP-AS5H 865230557007 from Würth Elektronik as an electrolytic capacitor, the ESR value is 100 mΩ. Now, if you want to obtain a filter characteristic with an attenuation of 35 dB at 350 kHz, the tool recommends a CL circuit (Figure 2) and determines the values ​​of the components of C1 = 47.0 µF and L1 = 240 nH. Figure 3 also shows the frequency response for attenuation, input and output impedance.

Output filter for DC/DC converters

Because of the high switching frequencies of DC/DC converters common today, they require an output filter that reduces high-frequency interference and the residual ripple of the signal (ripple). For example, an output signal could have a 50 mV ripple component at 1 MHz that should be reduced to 15 mV. The required attenuation ai dB is given by the following equation:

0105544960 (Image: Würth Elektronik)

With the input parameters “Attenuation 11 dB” and “at frequency 1 MHz” and the selection of the output filter as an LC combination and Load Impedance = 50 Ω / Noise Source Impedance = 0.1 Ω, the tool now recommends an LC topology with L1 = 55nH and C1 = 4.7 µF. Regarding the topology, it should be noted that the interference source is always on the right side of the specified filter and the load on the left (Figure 1). This means that the filter must be integrated into the corresponding circuit in such a way that the inductance is placed directly on the DC/DC converter and the capacitor is in parallel with the load.

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