Choices in switch mode power supply capacitors

Today's engineer is faced with several options when selecting capacitors for use in high reliability switch-mode power supply (SMPS) applications. Each type has advantages and disadvantages that are important for system performance and for long-term reliability. There are four options available: electrolytic, tantalum, film, and ceramic. This article addresses key attributes and characteristics of each, with special focus on the progress of the ceramic manufacturing industry.

 Electrolytic Capacitors (aluminum)

Typically, aluminum electrolytic capacitors, commonly known as electrolytic capacitors, are the first choice for SMPS applications. They offer an extremely high level of capacitance per unit volume at a cost that is much lower than alternative devices. Such high capacitance levels are achieved by using an extremely thin dielectric material in the form of an oxide layer. The oxide layer (Al₂ O₃ ) is deposited on an aluminum metal foil, which has been etched to increase overall surface area. This process is a very effective method of raising capacitance, however it also comes with several drawbacks that often reduce the electrolytic capacitor's suitability for some applications.

One limitation is the strict adherence to polarity that electrolytic capacitors require. Any reversal of applied voltage may result in a catastrophic failure. Electrolytic capacitors operated at higher ambient temperatures can also exhibit high levels of instability, a gradual loss in capacitance, and a significant loss in usable life time. With a typical maximum operating temperature of 105°C, these capacitors are not appropriate for military applications, which usually require 125°C. Operation at a very cold temperature may also reduce the effective capacitance and cause the device to exhibit higher dissipation factor and ESR.

Electrolytic capacitors are generally not a good fit for high frequency SMPS applications, as they exhibit high ESR at frequencies over 5 kHz, often forcing an engineer to place several capacitors in parallel in order to lower ESR. Shelf life of an electrolytic capacitor in a seemingly benign environment is also a concern, as the leakage current can increase with time.

 Tantalum capacitors

Tantalum electrolytic capacitors achieve very high capacitance values through the high porosity of their anode structure and consequentially large dielectric surface area. Although tantalums are generally thought to have more desirable performance characteristics than their aluminum counterparts, the long lead times and higher costs of tantalums may limit their usefulness to applications where size and mass are important and where electrolytics are not suitable.

Like electrolytics, tantalum capacitors require a strict adherence to polarity and are generally intolerant of high charge and discharge currents, especially if repetitive. Due to dielectric thickness constraints, they are usually restricted to a maximum voltage of 50 VDC and an upper operating temperature rating of 125°C. Although even the higher temperature ratings are achievable, performance does deteriorate and use at temperatures above 85°C generally requires a significant voltage de-rating.

ESR values for tantalum capacitors are also a concern and can even be higher than that of an equivalent aluminum electrolytic. Higher ESR is especially evident at frequencies over 100 kHz, where microstructural differences in internal resistance can result in a roll-off in capacitance by as much as 50%.

Tantalums also pose a number of safety risks. Inherent defects in the dielectric layer can result in catastrophic failure and the high oxygen content associated with manufacture of the cathode material can in extreme conditions, present a risk of fire. This potential problem is magnified in SMPS applications where banks of tantalum capacitors may be required to lower the effective ESR. Furthermore, like electrolytics, tantalum capacitors are generally not considered environmentally friendly due to their use of toxic materials.

Film capacitors

Film capacitors for SMPS applications mostly fall into one of two categories – metallized film or film/foil. Mmetallized film allows for a smaller size, lower mass and a lower cost per µF. Unlike other capacitor types, metallized film has the unique ability to 'self-heal' any flaws in its dielectric. Film/foil type capacitors are generally intended for higher continuous current applications such as a resonant circuit or a snubber circuit, where there is a high likelihood of transient exposure.

Although there is a wide range of dielectric materials that can be used for film capacitor design, the most common are polyester and polypropylene. Polyester designs feature a high dielectric constant and are readily available in a much thinner film gauge than polypropylene, allowing the capacitor to achieve a high volumetric efficiency at a lower cost. Polyester capacitors function at temperatures up to 125°C, but they exhibit a high dissipation factor, especially at higher frequencies, and lack adequate power dissipation which makes these designs more suitable for mostly low frequency, low current AC applications.

On the other hand, polypropylene capacitors offer a much lower dissipation factor, making it the preferred choice for high voltage, high frequency AC, and high current DC applications. But, an upper operating temperature of 105°C limits the use of polypropylene film capacitors in military applications. For both polypropylene and polyester designs, the allowable temperature rise for film capacitors is generally only 15°C.

When using film capacitors in AC systems, strict adherence to maximum voltage ratings is essential to prevent corrosion of the insulation system, which can cause the dielectric to carbonize and the device to short circuit. Higher voltage ratings are achievable, but require a 50% linear de-rating when operating above 85ºC. When compared to the electrolytic family, film capacitors offer a significant improvement in ESR and equivalent series inductance (ESL) ratings. However, like those alternatives, film capacitors may also use toxic materials.

 Ceramic capacitors

Ceramic capacitors achieve a wide range of performance characteristics. One of their key attributes is the wide array of dielectric constants (K values) that they offer which allows ceramic capacitors to be tailored to specific applications. For instance, ceramics with low K values are typically distinguished by very stable performance characteristics making them ideal for those applications where precision is involved. On the other hand, a higher K dielectric may sacrifice stability but can achieve much higher capacitance values. Ceramic capacitors have low ESR, especially when compared to electrolytic and tantalum.  High dielectric constants, low ESR, and the ability to further increase capacitance through a multi-layer structure, allow ceramics to effectively compete with other technologies in SMPS applications.

Fig. 1:  Ceramic capacitor package styles

There are, of course, some drawbacks to using ceramic capacitors. One of the main concerns is their capability to withstand mechanical and thermal shock. Ceramic capacitors are manufactured in a ceramic kiln at very high temperatures. Once fired, the ceramic is characterized as an extremely strong material in compression, but limited in tension strength. In addition, variations in the coefficient of thermal expansion for capacitors materials and differences between the capacitor and the material which the capacitor is mounted can make these parts vulnerable to thermal shock.

Recent manufacturing advances have lessened the severity of these issues. In the mid-1990s, the U.S. military, NASA, and several established manufacturers collaborated to create minimum design/performance standards for ceramic capacitors. MIL-PRF-49470 established minimum standards for terminal strength, immersion, resistance to solder heat, high frequency vibration, life test, and more. It prompted major investments in material development and manufacturing equipment and facilities to ensure compliance.

Fig 2:  Table of capacitor characteristics

Using the right soldering process will have a positive impact on the performance and durability of a ceramic capacitor. Generally, infrared/convection oven reflow soldering is recognized as the preferred method; however, there are instances where hand soldering may be appropriate and additional precautions need to be taken. For more complete soldering recommendations, see API Technologies application note AN37-0012.

For SMPS applications, the most common ceramic dielectrics used are defined by EIA-198 as Class II stable materials, with X7R often being the first choice. Class II materials present several performance considerations, that tend to be overlooked and that the engineer should be aware of. One of those characteristics would be 'aging', which is defined as the gradual loss of capacitance after exposure to higher temperatures. Dielectric aging is predictable and is expressed as a percent per decade hour (for example, 1 to 10 hours, 10 to 100 hours, 100 to 1,000 hours, etc.) with typical values for Class II, X7R materials in the range of 2.5% or less. Understanding that this type of capacitance loss is less impactful over time, designers routinely incorporate a design margin that compensates for aging.

Typical dissipation factors for X7R capacitors are 1.5% to 2.5%. In some ac applications, this could result in high levels of self heating.

Ceramic materials are surprisingly sensitive to applied DC voltage, as they exhibit a decrease in dielectric constant and subsequent loss in capacitance. Capacitance reductions of as much as 40% at rated voltage occur for an X7R dielectric.

Ceramic capacitors offer a number of key advantages compared to alternative SMPS technologies that make them ideal for high reliability applications. As nonpolar devices, ceramic capacitors can be connected in any configuration. They also exhibit extremely low levels of high frequency ESR and ESL compared to electrolytic and tantalum capacitors, and this generally allows the design engineer to use a lower capacitance value.

X7R dielectric ceramics offer ±15% temperature stability over the entire -55° to +125°C operating range and their low ESR and ESL characteristics make them capable of handling high DI/DT and DV/DT levels. Ceramics are also maintenance-free, non-toxic and environmentally friendly.