Radio Frequency (RF) & Wireless Design

Almost every industry uses Electrolytic Capacitors when the need or requirement to satisfy large amounts of capacitance (> 10 uF) need to be designed into a product. For most industries the defacto choice becomes the Electrolytic Capacitor. Only in cases where high reliability over a very long period of usage time (typically over 5 years) is where the Electrolytic Capacitors fall short and fall victim to failure due to aging and slow degradation.

So what causes this degradation and eventual failure ? Well its actually due to many things happening all at the same time, but most importantly are the heating effects both externally and internally.

The ambient temperature will reduce the life of an electrolytic capacitor by baking out the electrolyte and/or changing the composition of the chemical reaction between the foil and the electrolyte. In essence the density of the electrolyte as an insulator reduces its dielectric constant and therefore the capacitance is reduced.

Other effect such as voltage peaks or voltage surges may also perforate or create voids in the dielectric over time again reducing the total capacitance. Typical applications for electrolytics are as the front end 'bulk' filtering capacitance that does the heavy lifting for maintaining a constant voltage like a very short term (micro-second or milli-second) reservoir; kinda like a battery. 

However there are many low and high frequency high voltage responses that will be buffered by the electrolytic unless there are other transient protection devices to prevent voltage spikes or surges on the electrolytic capacitors. But one of the little understood effects is the self-heating that occurs from the Equivalent Series Resistance or ESR. 

An electrolytic capacitor has a usable lifespan that is set by most manufacturers by two operating conditions : temperature and time in hours. Most are rated for 85 C but others can be at 105 C, 125 C and up to 175 C. The minimum is typically 1000 hours and 2000 hours is probably the most common, but 20,000 hours is possible as well as a few at 100,000 hours !

The ESR will correlate to the number of hours by the power dissipated within the electrolytic when operating voltages (transients and voltage surges) occur. The power dissipated will be the ripple (AC) current times the internal ESR as P = I^2 * ESR. The power will be determined in watts and will raise the temperature beyond the ambient operating temperature for the duration of the transient or surge. In a very noisy environment and one without any front end protection, the lifespan of the capacitor may be reduced heavily.

In addition the transient response in terms of response time will also be affected by the ESR of the capacitor. Those will larger ESRs will not respond as quickly or efficiently as those will lower ESRs (see https://industrial.panasonic.com/ww/ss/technical/lc4 ). Therefore the frequency response of the capacitor is NOT dependent on just the capacitance but also the ESR as well !!

It's important to understand all electrical parameters when choosing an Electrolytic Capacitor, especially where device may operate for a very long period of time. For industries that depend upon high reliability, choosing the best electrolytic capacitor or choosing NOT to use one will definitely determine the lifespan of the product.

Its important to understand the 'typical' operating environment and 'typical' operating electrical conditions as well. For products that want to go beyond 'typical' and extend reliability to the end user, choosing working voltages that are 2X to 10x typical operating voltage, very low ESRs, and large operating hours (8,000 to 20,000 or more) at high temperatures 125C will give the product that edge over the competition.

Here's a lifespan calculator for electrolytic capacitors : https://www.illinoiscapacitor.com/tech-center/life-calculators.aspx