Choosing the Correct LC Filter Design for Your MLCC

A certain capacitance is required from the output capacitor of a switching power supply to keep the control circuit stable, and thus the output voltage as well. If the set output voltage reduces the capacitance value, this will affect the control circuit.

This will affect characteristics such as ripple, or behavior with sudden changes in load, and may lead to the specification not being met. With a filter placed at the input, e.g. for the switching frequency of a step-down converter (the input is always the critical side for step-down converters), the specified input voltage range of the converter and thus the applied voltage at the filter must be considered.

Otherwise, fluctuation of the filter's cutoff frequency will occur, impairing the operation of the filter and possibly leading to failure of the EMC test due to conducted interference.

The LC Filter

The LC filter is the filter type most widely and frequently used in electronics, and is constructed from an inductor and a capacitor.

Since this is a second order filter, it has a fall of -6dB at its cutoff point and steepness of 40dB/decade. A filter must generally have at least one frequency-dependent component.

Potential (excessively) high inrush currents are an important consideration when using a LC filter with an SMD ferrite, such as at the input of a switching regulator. Pulse-like inrush currents many times larger than the SMD ferrite's rated current can destroy the ferrite in the long term. The WE-MPSB series of components can be used to remedy this. These have a specified pulse tolerance.

Another point to consider when using SMD ferrites is the relationship between their impedance and the current flowing through them. The impedance is reduced by saturation of the ferrite material; depending on how high this current is, because a chip bead ferrite has no air gap. This also changes the filter properties. This relationship can also be reproduced in REDEXPERT. See Figure 7 in the ANP062 application note for an example of this characteristic, using component WE-CBF 742792113.

See section 3.2 in the ANP062 application note for an example which examines the voltage-dependency of capacitors in relation to their filter characteristics through the construction of two filter boards with various LC filter combinations and π filters. You can also take a closer look at the design (3.3), measurement setup (3.4) and properties of various MLCC components.

Simulation of Measured Results

The voltage-dependency of class 2 ceramic capacitors can also be simulated with LTspice. However, specific data are required for this.

The LTspice standard library includes a model of the real SMD ferrite. The LTspice library from Würth Elektronik includes models of the capacitors and can be downloaded from our website. This leaves the question of how to obtain capacitance values as a function of applied voltage. These can be extracted from REDEXPERT.

Visit Section 4 of the ANP062 application note for an in-depth look at the simulation.

Due to the demand for ever more compact electronic devices, filter components like the MLCC must be considered. We hope you learned more about the MLCC component!

For more information, check out our ANP062 application note. If you’re ready to try it yourself, contact us for your free samples.

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