How to Select the Right Aluminum Electrolytic Capacitor for your device ?

2025-03-15

Do you have a specific application in mind where you need to select an aluminum electrolytic capacitor? Sharing more details about your circuit, such as its operating voltage, frequency, and temperature conditions, can help me provide more tailored advice. Fisrt, let’s know more details about the SMD E-Cap:

The material of Aluminum electrolytic capacitor

Capacitors are composed of two electrically conductive material layers (electrodes) separated by a dielectric material (or insulator). Capacitors store energy in an electric field generated by this arrangement once a current is supplied to charge the capacitor. In an aluminum electrolytic capacitor, the electrodes are made out of aluminum foil. Between the two aluminum electrodes is a conductive liquid, called an electrolyte. Through an electrochemical reaction, an oxide layer is built upon one of the electrodes (the anode), which serves as the dielectric in an aluminum electrolytic capacitor.

The construction of an aluminum electrolytic capacitor.

Electrical Characteristics

The construction and materials of aluminum electrolytic capacitors give them some unique electrical characteristics, making them ideal for many applications.

Advantages of Aluminum Electrolytic Capacitors

The biggest advantage of aluminum electrolytic capacitors is that the electrolytics have high volumetric efficiency, i.e., a higher capacitance per volume than any commonly available capacitor. Aluminum electrolytics are often the only possible solution for certain applications. When selected and designed into the circuit properly, this advantage can be maximized.

Another advantage of aluminum electrolytic capacitors is the availability of high voltage ratings. Aluminum electrolytic capacitors with a DC voltage rating of 600V are readily available, meaning they can be used in a wide variety of applications.

Considering both the high capacitance and high voltage of aluminum electrolytics together produces another big advantage: energy storage.

The energy stored in a capacitor increases linearly with capacitance and exponentially with voltage.

Selecting a Capacitor for Power Applications

Understanding the fundamentals of aluminum electrolytics is the first step towards selecting the right one for a power electronics design. Here are the key design considerations:

Voltage Rating / Derating

Capacitor voltage ratings provide a safe operating range for a capacitor. Operating within these ratings prevents them from being damaged and extends their functional life. Aluminum electrolytic capacitors most commonly provide bulk capacitance to power supply voltage rails.

Example frequency converter circuit.

Because aluminum electrolytic capacitors are polarized, they are only used in DC voltage applications — after DC rectification in the example circuit. A capacitor should be selected taking into consideration the load condition of the application, i.e., operating voltage, surge, and transient voltages, ripple current, ambient temperature, cooling conditions, and expected useful life. It is not recommended to select a voltage rating much higher than required, as higher voltage ratings tend to coincide with higher ESR. In high ripple current applications like this one, higher ESR will cause significant problems.

Equivalent Series Resistance

Engineers learn about ideal capacitors early on in their education, but real-world capacitors are not ideal. Real capacitors can be modeled as an ideal capacitor with a few parasitic elements around it.

Equivalent circuit of a real capacitor.

In this image, CS is the ideal capacitive component of the equivalent series circuit. The capacitance measured will depend on both the temperature and the frequency of the signal used to make the measurement. ESR is the resistive component of the equivalent series circuit. ESR depends on both frequency and temperature, and is related to the dissipation factor by the following equation: ESR=tanδ/ω*CS, where tan δ is the dissipation factor and ω is the frequency applied. Finally, ESL is the inductive component of the equivalent circuit, and it results from the internal design of the capacitor and its terminal or lead configuration.

For the power supply application, equivalent series resistance (ESR) is of the most concern. The AC portion of the current seen by the capacitor, or the ripple current, causes power to be dissipated by the ESR in the capacitor. This effect varies with the frequency of the ripple current. The higher the ESR, the more power dissipated inside the capacitor, meaning increased heat generation and a shortened capacitor lifespan. It is not necessary to select the lowest-possible ESR available when specifying a capacitor for a power supply design, but it is recommended to select an ESR rating that works with the ripple current in the design.

Ripple Current

The term ripple current is used for the root mean square (RMS) value of the alternating current that flows through a device as a result of any pulsating or ripple voltage. Power losses resulting from this ripple current induce self-heating of the capacitor. The maximum permissible value of the ripple current depends on the ambient temperature, the ESR at the frequency of the AC signal, the thermal resistance, which is mainly determined by the surface area of the capacitor (i.e. heat dissipation area), and the applied cooling. Moreover, it is restricted by the ripple current capability of the contact elements.

The rated ripple current  (IAC,R) is usually specified at the upper category temperature and the reference frequency.

As thermal stress has a decisive effect on the capacitor’s life expectancy, the heat generated by the ripple current is an important factor affecting the useful life. These thermal considerations imply that it may be necessary under certain circumstances to select a capacitor with a higher voltage or capacitance rating than would normally be required by the respective application.

Surge, Transients, and Reverse Voltages

Capacitors are sensitive to transients, overvoltages, and reverse voltages. Typical aluminum electrolytic capacitors can withstand surge voltages 10 percent over their rating for short periods of time. Some capacitor types can withstand voltage pulses exceeding the surge voltage. As the requirements differ largely depending on the individual application, it is recommended to select the capacitor design to meet application specifications. It is always recommended that engineers understand the transients and overvoltages possible for capacitors in their designs.

Aluminum electrolytics are polarized capacitors that can suffer catastrophic damage from reverse voltages. Where necessary, voltages of opposite polarity should be prevented by connecting a diode. Reverse voltage of ≤1.5V are tolerable for a duration of less than one second, making diode protection viable. Aluminum electrolytics cannot withstand reverse voltages, even at levels ≤1.5V, continuously or repetitive operation. 

Welcome to contact with Hongda Capacitors for more details.

Email: judy@hongdacap.com.hk

Whatsapp&WeChat: +86 136 6989 7281