Introduction to LC Filter Design with MLCCs: Whty the Applied Voltage Matters

Compact electronic circuits are in high demand, and this affects which active components – as well as which passive components – customers choose.

Thanks to ever-smaller casings, there are many opportunities for discrete design and smaller filter components. For example, LC filters used to be constructed with aluminum electrolytic capacitors because these offer a very wide range of capacitance values. This advantage, however, is becoming increasingly small.

Now, advances in ceramic capacitor technology have enabled the production of high-capacitance SMD ceramic capacitors (multilayer ceramic chip capacitors – MLCCs).

In terms of space, this is a huge advantage. However, there are some drawbacks.

In the next couple blog posts, we’ll take a closer look at the considerable influence of DC voltage on the capacitor, and hence the filter design. The focus is on an LC low-pass filter, as used as an input or output filter for switching regulators or a power supply filter for a module.

We hope you’ll follow along and learn a lot about MLCCs!

Types and Properties of MLCCs

MLCCs can essentially be divided into two types: those using class 1 ceramics, and those using class 2 ceramics.

Ceramics are very brittle materials, and their mechanical fragility increases with size. Therefore, the maximum size of MLCCs is limited, and care must be taken in the layout to reduce mechanical forces.

Class 1 and class 2 ceramics differ in various ways. Table 1 in the ANP062 application note shows the technical properties of ceramics currently used by Würth Elektronik.

The properties and tolerances of the different ceramic classes are defined by the IEC or EIA coding system. It should be mentioned that the IEC 60384-21 coding system is not normally used for class 1 ceramics, but there is one very well-known term: NP0. This has the same meaning as EIA code C0G. These are shown in figures 1 and 2 of our ANP062 application note.

NP0 has a very small tolerance over its temperature range: +/-30 ppm/°C. EIA coding is typically used with class 2 ceramics, including ceramics such as X7R or X5R. Depending on the application, the capacitor must have a particular capacitance to obtain the desired performance – e.g. for filtering. The relationship of this to temperature is shown in figure 4. X7R means that the capacitance may not vary by more than +/-15 % between -55 °C and +125 °C.

Thus, the capacitance value for a 10 μF class 2 ceramic may vary between 8.5 μF and 11.5 μF within the permitted temperature range. Any ceramic mixture that has this property is an X7R ceramic. In addition to this tolerance, there is also the manufacturer's delivery tolerance on the day of delivery. This is typically a further +/-10 %.

The ceramic class or code does not, however, define the composition of an X7R ceramic (powder particle size, material mix, etc.).

Moreover, any ceramic capable of holding its capacitance within the stated tolerance window across the temperature range can be described as X7R. This can vary between manufacturers. Therefore, the properties of the individual components must be closely compared to ensure the desired characteristics are obtained.

The so-called DC bias effect, i.e. the voltage dependence of the capacitance, has a very large influence on the capacitance. With class 2 ceramics, applied voltage causes a drop in capacitance. This is due to the internal structure of the barium titanate used as the base material. Using barium titanate does produce highly permeable ceramics, but these also have internal structures that respond to and become polarised by external electric fields. This results in a certain saturation of the material, and in turn leads to a drop in capacitance.

This characteristic is comparable to the saturation of ferromagnetic materials (e.g. ferrite material). Therefore, this material is also said to have ferroelectric properties. This relationship is illustrated in figure 5. It’s taken from the online-platform REDEXPERT by Würth Elektronik and shows the percentage decrease in capacitance with applied voltage, in this case using part no. WCAP-CSGP 885 012 206 026 (1 µF, 0603, 10 V, X7R) as an example.

Another effect can be used to prove that these are real measured data. At low voltage, this capacitor demonstrates a certain self-healing effect of the ceramic material. This could also be thought of as the ceramic needing to be 'woken up' first.

When voltage is applied, the healing and polarization process starts. Above a certain applied voltage (about 2.1 V in the example), the material becomes saturated and the available capacitance will be reduced. This characteristic must be recorded and examined for each individual component. With approximately 800 current catalogue items in the MLCCs category, this is a very laborious process. Würth Elektronik has recorded these data for every MLCC in its portfolio and integrated them into the REDEXPERT online-platform.

Up Next: Filter Design

The effect of voltage-dependent capacitance must be individually considered when selecting a capacitor for the application in question. In our upcoming posts, we’ll look at the various LC filter designs with MLCCs.

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