How to choose the right aluminum electrolytic capacitor?

Trying to find a suitable electrolytic capacitor for a particular application at the right price can be little hard. Also it needs a long operational life, but with the longer usage time and the problem becomes worse. So how to choose the right electrolytic capacitors?

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:

Rated Voltage

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.

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 theapplication, that is to say, 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

The capacitance measured depends 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 resultsfrom 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. Thiseffect 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 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 VoltagesCapacitors 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 withstandvoltage 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.

Cooling

The useful life values stated in our datasheets apply to aluminum electrolytic capacitors with natural cooling (i.e., the heat generated in the winding is dissipated through the case). It is possible to increase the permissible ripple current and/or prolong the useful life by means of additional cooling measures (e.g. heat sink or forced ventilation).As a large amount of heat is dissipated through the base of the case, a heat sink connected to the capacitor base provides the most efficient cooling. TDK offers specially designed versions of high voltage capacitors with screw or snap-

in terminals that can be mounted on a heat sink in order to ensure optimal heat transfer from the heat generation area via the base of the case of a heat sink.

Required Useful Life

The final key design consideration is the required useful life of the capacitor in the design. It is necessary to understand all of the factors already discussed, plus the life requirement, for an engineer to know that their design will last.There are a lot of factors and nuances involved, but engineers do not need to become in-depth expert specialists on aluminum electrolytic capacitors to be able to use them properly.

So all together there are 11steps to select a suitable capacitor:

Step 1: Determine the capacitance required by the design.

Step 2: Define the expected ambient operating temperature.

Step 3: Define the DC operating voltage to be applied to the capacitor.

Step 4: Bound the space available for the capacitor (if available space is a concern).

Step 5: Calculate the expected ripple current on the capacitor, per the design.

Step 6: Select some candidate capacitors. Select the minimum required for capacitance, temperature, and voltage ratings (steps 1, 2, & 3).

Step 7: Calculate the ripple current for the top candidates. Calculate using expected ripple current and the ESR of the candidate capacitors.

Step 8: Define required useful life of the capacitor per the design application.

Step 9: Calculate the useful life of the top candidates. Use TDK’s convenient online calculator that allows to input 15 load conditions that will provide accurate useful life information.