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Capacitor Technologies Used in Filtering

Introduction

Capacitors are used to filter out system noise to obtain the best EMC performance of a product, usually in bypass or decoupling scenarios. Many common capacitor technologies are used in these filtering applications, each exhibiting unique behaviors. Although understanding each capacitor type and behavior is daunting and difficult to memorize, it is prudent that every aspiring engineer and technician involved in design for EMC at least have a rudimentary understanding of what capacitor technologies are available. This short article intends to provide a primer for you to obtain additional knowledge as required by your unique circumstance and filtering requirements.

Capacitor Technologies

Some of the most common capacitor technologies available for filtering applications today are:

  • Aluminum electrolytic
  • Aluminum polymer
  • Aluminum hybrid polymer
  • Ceramic
  • Plastic Film
  • Mica
  • MLCC
  • Tantalum

Only a small handful of the above capacitor technologies are addressed in this article.

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Characteristics of Capacitors

When considering the various capacitor technologies for filtering applications, categorizing capacitors by their differing characteristics is helpful. To keep things straight, it is helpful to construct a table that summarizes information according to the following:

Constructing this table and having it readily available helps one quickly determine if a certain capacitor technology is suitable or not for any specific filtering need.

Multilayer Ceramic Capacitors (MLCCs)

MLCCs are comprised of two classes, uniquely identified as Class 1 and Class 2.

Class 1

Class 1 NP0/C0G ceramic capacitors have relatively small values of permittivity; therefore, small values of capacitance are possible. This characteristic makes them suitable for high-frequency filtering applications. Additionally, their capacitance value is linear over temperature and they do not age much. Class 1 MLCCs have a very small DC-bias voltage dependency.

Class 2

In contrast to Class 1 MLCCs, the characteristics of Class 2 X7R, X5R, Y5V, etc., capacitors are at the other end of the extreme. They have relatively high values of permittivity (high values of capacitance are possible), a non-linear temperature dependency, do not age well, and have a high DC-bias voltage dependency.

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Film Capacitors

Film capacitors have a special structure for the electrodes, consisting of two different types: metal film and metallization on dielectric. The type of dielectric for each of these two different structures is paper and plastic film. The four types of dielectrics are Polyester (PETP), Polycarbonate (PC), Polypropylene (PP), and Polystyrene (PS).

Safety Ratings of Film Capacitors

One of the most important concepts to compliance engineering professionals regarding film capacitors is their safety ratings. In short, X-capacitors are used to filter out differential-mode noise in line-to-line situations. Y-capacitors are used to filter out common-mode noise line-to-ground. Due to leakage current limitations imposed by most product safety standards (the harshest of which are found in medical device regulations), the capacitance value of Y-caps is very low.

There are two different categories of X and Y-caps. X1 caps have an impulse rating of 4kV X2 caps are rated for 2.5 kV. Y1 caps have an impulse rating of 8 kV, whereas Y2 caps are rated 5 kV.

For more information covering the safety classes of capacitors, see IEC 60384-14 / UL 60384-14.

Pro Tip: Document each capacitor type’s sensitivity to humidity and temperature

When conducting your own research for each of the available capacitor technologies, take note of each one’s sensitivity to humidity and temperature. Based on your product’s Life Cycle Environmental Profile (LCEP), you will likely encounter certain types of unsuitable capacitors, and you can cross them off your list as potential candidates in your design.

Aluminum Electrolytic Capacitors

Aluminum electrolytic capacitors have been around forever and are made from well-proven technology. They support the highest voltages and have the largest range of capacitance values available due to their larger size. These capacitors’ expected lifetime is doubled for approximately every 10°C drop in temperature they see, which is below their maximum specified temperature rating.

Aluminum Polymer Capacitors

Aluminum polymer capacitors are a new technology type of capacitor. They look a lot like aluminum electrolytic capacitors, except the top has no vent. Other benefits of aluminum polymer capacitors over their electrolytic cousins are that they have a lower equivalent series resistance (ESR) value, they cannot dry out (they are made of solid polymers), and they have higher expected lifetimes. Their expected lifetime increases by a factor of 10 for approximately every 20°C drop in temperature they see, which is below their maximum specified temperature rating. The downsides of aluminum polymer capacitors are increased leakage current and susceptibility to vibrations.

Summary

This article briefly covered the latest capacitor technologies used in filtering. See the references and further reading suggestions below for a more in-depth review.

References and Further Reading

  1. Webinar: Introduction to the capacitor technologies and how to use them, Würth Elektronik Group.
  2. Hu, R., PCB Design and Fundamentals for EMC, RANDSpace Technology LLC, 2019.
  3. Keller, R.B., Design for Electromagnetic Compatibility – In a Nutshell, Springer, 2023.

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