A capacitor is a device that stores electricity in the form of an electric field. They have two conductors separated by a dielectric layer. The dielectric material is an insulator with the ability to polarize easily. When the two conductors have a voltage difference, the electric field creates an electric charge within the capacitor, creating stored electric energy. The amount of energy the capacitor can store is related to the geometry and size of the capacitors as well as the quality of the dielectric material.
Dielectrics enable the capacitor to have much greater capacitance, which is useful for storing charge for energy applications or tuning its frequency-response behavior in filtering applications. From a practical standpoint, dielectrics prevent capacitor failure via discharge or plate contact. The material in between plates can enable very small separation distances without the concern of the two conducting plates contacting. Furthermore, this document aims to highlight the advantages and disadvantages inherited by the different materials used and to provide an overview of their respective properties.
Quick Guide
Ceramic
Ceramic capacitors are made from ceramic materials that use conductive plates as electrodes. They are the most common type of capacitors due to their versatility in use, economically low cost, and smaller in comparison to others. Ceramic capacitors are known to maintain stability over a wide range of temperatures and can be used as general-purpose capacitors but are used in decoupling, bypass, filtering, RF, and timing circuits. Their size and low-cost lead them to be the primary choice for dielectric material until their limitations are reached.
Pros
- Inexpensive
- Non-Polarized
- Low source impedance
- Reliable at high altitudes
- Small size
- Good capacitor tolerance
- Stability over various temperature ranges (varies by Class)
Cons
- Max voltage limit
- Max transient current limit
- Capacitance is limited
Withing the ceramic family, there are different classes of material.
Class I ceramic capacitor materials include C0G and NP0. These materials offer a higher temperature range and more stable capacitance over the rated temperature range. Class II ceramic capacitors include X5R and X7R, which have a higher temperature coefficient. X5R capacitors have the lowest temperature rating, but offer the lowest cost.
For a more detailed dive into ceramic capacitor construction and types, read Kemet’s excellent article.
Electrolytic
Electrolytic capacitors are polarized capacitors that are typically aluminum and cylindrical. They go through an electrolytic process that forms an oxide layer that is used as the dielectric material. Keep in mind these are temperature and pressure-sensitive, meaning the value of capacitance can change more dramatically depending on temperature and pressure than other types of capacitors. These capacitors have a higher capacitance than other dielectrics and are usually found when substantial amounts of storage are necessary, like in power and audio circuits.
PROS
- Higher capacitance
- Available with higher working voltages
- Minimal PCB space for its capacitance
- Low cost for larger capacitors
CONS
- Shorter life span
- Temperature Sensitive
- Pressure Sensitive (not suitable for high altitude)
- Higher ESR
- High leakage current
- Polarized
- Prone to vibration issues due to size
Film
Film capacitors have a thin layer of polyester that is coated with a layer of metal on both sides, this is used as the capacitor’s electrode. Polyester film capacitors are the best type of capacitors when you need high stability, and/or low source impedance. They are usually relatively expensive in comparison to other dielectric materials. Also, they have a low dielectric constant meaning their capacitance is low for its size. Polyester film capacitors are frequently found in high-current power supplies, EMI filters, and other circuits with little tolerance for ESR/ESL and/or changes in capacitance due to pressure or temperature.
PROS
- Long shelf life
- Low ESR/ESL
- High working voltage limits
- High transient current limits
- Temperature Stability
- Low Leakage current
- Excellent Capitance Tolerance
CONS
- Very expensive
- Large in size
Mica
Mica capacitors go through a specialized manufacturing process that creates a thin layer of mica to be used as a dielectric material between plates. They have high working voltage limits, high stability accuracy and excellent insulating properties. They are commonly used in high-frequency products, high-voltage circuits, and audio equipment.
PROS
- High working voltage limits
- Low leakage current
- Low ESR/ESL
- Temperature stability
- Great capacitance tolerance
CONS
- More expensive
- Slightly larger
- Smaller capacitance
Tantalum
Tantalum capacitors are like electrolytic capacitors in that it has a metal plate as one of their electrodes, but instead of an oxide layer, the dielectric material is tantalum pentoxide. These capacitors are used where high capacitance and stability are important. Due to their high capacitance, tantalum capacitors can be found in power supplies and audio equipment.
PROS
- High capacitance
- Pressure difference tolerant
- ESR values are between ceramic and electrolytic values
- Stability over various temperature ranges
CONS
- Higher cost
- Sensitivity to overvoltage
- High leakage current
- Can fail short
- Polarized
- Average tolerance values
- More susceptible to market fluctuations due to material supply
Summary
Ceramic Capacitors are often the workhorse for a low voltage PCB design due to cost and size. Aluminum electrolytics are great when high capacitance is needed but there are enough draw backs that preclude them from higher reliability designs.
For their size, tantalum can pack in a decent amount of capacitance and often used on low voltage, medium capacitance, space constrained designs. Film and Mica capacitors both have strong benefits for very specific applications, but should be used selectively due to cost and size.
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