Reflections on Selecting Aluminum Electrolytic Capacitors

Aluminum electrolytic capacitors have become an important part of most electrical designs. As a result, the proper selection of these type of capacitors is crucial to the success of many designs. Engineers must understand how aluminum electrolytic capacitors work and how to select the most appropriate one for their design properly.

Overview

Aluminum electrolytic capacitors not only have a high capacitance per volume, but they can be used in nearly any electronic system, and are often used for filtering unwanted AC frequencies and energy storage among other applications. In addition, they are often used in DC-DC converters, inverters and in power supplies thanks to their high capacitance values and low impedance values.

Because of their efficiency, there has been much focus on improving aluminum electrolytic capacitor technology even further. New manufacturing processes and using the latest materials have resulted in significant improvements to their operating life, increased reliability and increased robustness. As a result, aluminum electrolytic capacitors have a long operating life – often operating for as many as 20 years,

All capacitors are made up of two electrically conductive material layers – or electrodes – that are divided by an insulator made up of a dielectric material. An electric field is generated by the placement of these layers, which allows for energy to be stored when a current is supplied to charge the capacitor. What makes aluminum electrolytic capacitor special is that the electrodes are created with aluminum foil. A conductive liquid is situated between the two aluminum electrodes. This is called an electrolyte. Through an electrochemical reaction, an oxide layer (Al2O3) is formed on one of the electrodes called the anode, which acts as the dielectric in an aluminum electrolytic capacitor.

Characteristics

A combination of the design and materials of give aluminum electrolytic capacitors their unique electrical characteristics. These characteristics offer significant advantages that make them uniquely ideal for many applications where other capacitors are less idyllic, and in many cases, make them the only solution that can be used for particular applications.

First, aluminum electrolytic capacitors have a large capacitance for voltage rating per volume, which results in the electrolytic having high volumetric efficiency. As a result, the capacitors offer a higher capacitance per volume than any other type of capacitor. This is the most important characteristic of aluminum electrolytic capacitors. When appropriately designed into an application, this characteristic can offer significant advantages over other competing technologies.

Second, most aluminum electrolytic capacitors have higher voltage ratings than other types of capacitors that do not. Aluminum electrolytic capacitors with high a DC voltage rating of 600V are easily sourced and commonly available for use in a wide variety of applications.

Third, these capacitors have extremely high energy storage capabilities because they have both a large capacitance and high voltage rating. The energy stored in a capacitor increases linearly with capacitance and exponentially with voltage.

Fully understanding these three characteristics is crucial background for selecting the right aluminum electrolytic capacitor for power applications. With that basic understanding, the following are four things that engineers should consider when evaluating aluminum electrolytic capacitors for their design.

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Figure 2: Construction of an aluminum electrolytic capacitor

Operating Lifespan

The most important consideration for aluminum electrolytic capacitors is the operating life required of the capacitor in the application. This is essential to know how long the application will function as designed before needing repair or replacement.

This may seem like a daunting task due to the many intricate factors involved, but due to reliability of the electronics, it is very predictable. Luckily, engineers do not need to consult with specialists or become experts themselves to determine operating lifespan. Rather, most reliable capacitor manufacturers provide web-based toolsto help engineers calculate the operating life of aluminum electrolytic capacitors within the specific design constraints of the application.

Voltage rating

Engineers should ensure that capacitors operate within the specified safe voltage operating range and ratings for aluminum electrolytic capacitors. Not only will this extend the operating life of the capacitor, but it will also keep them from unnecessary damage.

Often aluminum electrolytic capacitors are used in conjunction with power supplies, where they provide bulk capacitance. However, aluminum electrolytic capacitors can become polarized, therefore should only be used in DC voltage applications where DC rectification occurs.

When evaluating aluminum electrolytic capacitors, engineers should not to elect to use a capacitor with a voltage rating that is significantly higher than the application requires. When this is done, there is an equally strong increase in electrostatic resistance.

In addition, engineers should understand the application’s load requirements including ripple current, ambient temperatures, and possible voltage surges. By doing this, they will be able to extend the overall operating life of the application.

While many aluminum electrolytic capacitors can handle infrequent short-term surges and reverse voltages or voltages where the polarity is opposite, it is not recommended to have ongoing, regular surges or reverse voltages over the specified ratings. Overvoltages, reverse voltages and transients may damage the application. To combat reverse voltages, engineers should employ a diode that will prevent them. Capacitors purchased should always meet the specified application design requirements.

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Figure 3: Example of power supply circuit

Thermal properties

A capacitor heats up through the normal alternating electrical current that flows through it. This self-heat can cause power losses and is called ripple current. As a result, the ambient temperature of the application becomes a crucial part in the selection of aluminum electrolytic capacitors.

Cooling measures and the surface area of the aluminum electrolytic capacitor define what the thermal resistance or heat dissipation will be for the application. The heat dissipation is also restricted by the ripple current of the contact elements.

The ripple current rating of an aluminum electrolytic capacitor is typically specified at the upper category temperature. Not only is the ambient temperature of the application a significant factor, but so is the frequency of the AC signal, the thermal resistance and the equivalent series resistance.

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Figure 4: Aluminum electrolytic capacitor with heatsink (Source: TDK)

Equivalent Series Resistance

One of the largest concerns in power supply applications where an aluminum electrolytic capacitor is used is equivalent series resistance – the resistive component of an equivalent series circuit. The ripple current from the AC portion of the current causes power to dissipate through the equivalent series resistance in the capacitor. A higher frequency of the ripple current can cause the equivalent series resistance to increase, equivalent series resistance correspondingly. The higher the equivalent series resistance, the more power dissipated inside the capacitor, meaning that temperatures increase as more heat is generated.

However, engineers do not need to specify aluminum electrolytic capacitors with the lowest possible equivalent series resistance. Rather, they should specify a capacitor with a equivalent series resistance rating that works with the application’s ripple current.

Conclusion

It is important for design engineers to fully understand the capacitance, voltage ratings and energy storage capabilities of aluminum electrolytic capacitors when preparing to select them for any application. It is equally important for them to consider the operating lifespan, the voltage rating, the thermal properties and equivalent series resistance when selecting aluminum electrolytic capacitors for power applications.

By understanding these characteristics and considerations, engineers can select the best aluminum electrolytic capacitors for their applications with confidence.

TDK