Power Supply Design

Question: What can you tell me about aluminum electrolytic capacitor failure?

Original Question: We have been having frequent failures of power supplies. ... They were all purchased at the same time about 3 years ago. They have been working fine until the last 3 months. The power supply will 'pop', smoke and then take every component of the computer out (even the keyboard!). We have been going a few rounds with our PC vendor on this. K. H. 06/07/2001

Answer: It seems power supplies are often the weakest link. When I took a Photo-Shop class a couple of years ago at the local community college, I asked the person in charge of the computers about failures. The power supplies failed in about 20% of the computers each semester.

Because your computers are all failing after about three years, my first guess is that it is the aluminium electrolytic capacitors failing that start the sequence. Since these capacitors are not hermetically sealed, the electrolyte in these capacitor eventually evaporates causing increased ESR which causes increased heating, which causes the safety seal on the capacitor to pop, because if it does not pop, the capacitor explodes. Once these capacitors fail, they can cause all kinds of secondary failures.

A very high quality electrolyte used in military application may last about 20 years. Lower grade capacitors may last only a year or two, and failure of low grade (or over stressed) capacitors at three years does not seem unusual. Many industrial systems that use electrolytic capacitors have them replaced every three years as preventative maintenance. I just had my garage door opener repaired, and he replaced all the electrolytic capacitors in it as part of the service call. The manufacturer had designed the circuit board for easy pop-out replacement of these, since garages get hot and cold and shorten the life of electrolytic capacitors.

Electrolytic capacitors also fail for another reason. When placed in a design, they have a voltage safety margin, and the capacitors can take a voltage surge and not fail. However, over time, the capacitors reform to the voltage they are being used at, and any voltage surge may destroy them. For this reason, electrolytic capacitors are often periodical reformed as part of maintenance by applying the voltage they operate at and then slowly (over many hours) increase the voltage, while monitoring leakage current, until their surge rating is restored.

Failures can also be accelerated by cleaning with halogenated solvents, mounting method, and mounting orientation.

Replacing and reforming electrolytic capacitors was quite common before the current throw-away culture took hold. Now, when something breaks, it is usually cheaper to throw it away than it is to perform preventive maintenance or repair it.

Since there is tremendous price pressure on PC's, there is great pressure for a low-cost power supply, which often means several things. Low-cost components with limited life are often used, circuit components that might limit secondary failures such as you are experiencing are left out, and parts are often used at or beyond their ratings, causing poor reliability and short life.

For these reasons, for all my personal computers, I look at the length of the manufacturers' warranty first and only buy from those vendors who have the longest warranties. The last two computers I bought had three year warrantees when many competitors only offered one year. Since it is VERY expensive for manufacturers to repair computers under warranty, they usually pick parts and derating to make sure their product lasts at least as long as the warranty. Besides using higher quality parts, the warranty invites the manufacturer to design the circuits so a single failure will only damage one computer component and not wipe out the whole computer -- which the manufacture would have to replace under warranty.

In essence, you can only expect Oliver Wendell Holmes' 'one-hoss-shay' -- that destructs one hundred years to the day (The Deacon's Masterpiece). The only destruction date you have for your computer is the warranty. Since the manufacture's analysis tells him he will only have limited returns in the warrantee period, on the average, you can only expect a useful life of about twice the warranty period before you have to repair something. If your warrantee is one year, you can expect about two or three years of life. If it is a three year warranty, you can expect about six years or a little more of useful life.

The bottom line, do not buy from the lowest bidder, buy from the lowest bidder with the longest warranty. Also make sure the company is robust enough so they will still be in business during the warranty period.

This probably does not answer your question, but I hope it helps.