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Consumer Capacitors Characteristics

2019/04/10

Shamelessly stolen from a blog here and edited for better reading. Saved it just in case I need it in the future. I'd suggest you to check out the post there, too.

Special thanks to Joe Zatarski for sharing his experience about various types of capacitors :)

Here's a quick run down of the types you’re likely to see in consumer electronics

Electrolytic capacitors

First is the electrolytic capacitor.

Traditionally, these have been used anywhere large amounts of bulk capacitance are needed, for filtering power supplies most often. They are constructed as a metal can filled with an electrolyte and two foils, usually aluminum, that make up the capacitor plates. One of the foils is generally anodized to produce an insulating oxide layer, though nonpolar types with both foils insulated exist as well.

These capacitors have the highest density of all the types, but they suffer from failure due to electrolyte evaporation, and sometimes oxide failures. They also do not have very accurate capacitance (a typical tolerance range for electrolytics is +80/-20%, though you can find +/-20%).

The lifetime of these caps is highly dependent on temperature, since higher temps can cause the electrolyte to evaporate faster.

Electrolytic capacitors used in linear power supplies (those operating in purely DC circuits, or in simple transformer+rectifier supplies) rarely fail in my experience, since they tend not to get very hot. There are exceptions to this, as poorly designed linear power supplies may place electrolytic capacitors near regulators that get hot, causing the capacitor to be at an elevated temperature, thus shortening it’s life.

On the other hand, electrolytic capacitors used in switch-mode power supplies (SMPS) operate at very high frequencies and take considerable amounts of ripple current. Every capacitor has an intrinsic loss, and this is often expressed as an ESR.

Power dissipated in a capacitor can be expressed as I^2*R, where I is the ripple current in Amps RMS, and R is the ESR. With higher ripple currents, the loss in the capacitor itself can become significant, and cause self heating of the capacitor.

For use in SMPS, special low-ESR capacitor types have become popular in the past few decades to reduce this loss and allow capacitors to take more ripple current without shortening the life of the capacitor. However, many of the cheaper brands of caps, most notably those coming from east asian manufacturers, have historically had problems in this regard.

There was a period in the late 90’s to early 2000’s known as the capacitor ‘plague’ where many manufacturers went with cheap low ESR capacitors from these manufacturers, only to find many power supplies were failing prematurely (within a year or two of manufacture).

Tantalum capacitors

Tantalum capacitors can be a variety of colors, but they are always a leaded part (as in, they have leads that extend out from the capacitor, unlike surface mount ceramics, for example). They will look kind of like a drop of material on the end of 2 leads, in thru-hole varieties, and can look like a rectangular cap, with two leads out of the ends in the surface mount variety. Tantalum capacitors, like most electrolytics, are polarized capacitors.

Tantalums are also generally used where bulk capacitance is needed for power supply filtering, but they don’t get nearly as large as electrolytics, and they aren’t quite as dense. They do have certain desirable characteristics, like lower ESR than electrolytics, which can be advantageous in certain applications.

Failure modes for tantalum capacitors is a bit more interesting, because they often fail as a short circuit, and they seem to fail for no apparent reason very often. Sometimes power supplies will protect themselves into a direct short, and nothing will happen other than the power supply failing to start. Other times, the power supply will kick on as normal, and deliver significant power into the shorted tantalum capacitor. In this case, the results are much more spectacular, with the tantalum capacitor often literally catching on fire and blowing up.

If you suspect there is a shorted tantalum cap on a board, and you’re not sure which one it is, a good way to narrow it down is to apply a controlled amount of current from a lab power supply in constant current mode, and find the hot component. Just watch out for excessively hot components, which may burn you (or in the case of the tantalum capacitors, start on fire).

Ceramic capacitors

Ceramic capacitors are generally a disk-type in thru-hole versions, and a small rectangular device in surface mount types.

Ceramic capacitors have better characteristics than most other capacitors, but they are not nearly as capacitively dense as electrolytics or tantalum capacitors. They can be made with fairly precise capacitance values, although there are a wide variety of ceramic capacitors with different dielectric types that have vastly differing characteristics.

There are generally two types, however. The first are the bulk capacitance dielectrics, which are dielectrics optimized for capacitance density. However, these dielectrics are highly nonlinear (they have heavy voltage coefficient, and temperature coefficient is not much better)

The other type, are used where the design calls for something more linear, or with higher accuracy, or with better temperature coefficient.

As far as failure of these capacitors, ceramics do not generally fail on their own. However, surface mount parts are prone to cracking in environments with high vibration or shock. To remedy this, they make special ‘soft termination’ capacitors that are less susceptible to this issue, but tend to cost more. When ceramic capacitors crack, they can short out. I have seen designs where two ceramic capacitors are placed in series, at right angles to eachother on a board, to reduce the likelihood of both ceramics cracking, and shorting out a power rail.

Film capacitors

These capacitors generally have the same advantages as ceramic capacitors. Again, there are a wide variety of dielectrics used in film capacitors, ranging from simple paper, to various polymers such as polyethylene, polypropylene, and even teflon/PTFE for certain special applications.

As far as failure modes, these capacitors are generally fairly stable. However, as expected, if the capacitor is mistreated, they can suffer drift in capacitance, or even short circuiting if exposed to bad enough transients.

A notable capacitor, are the Rifa safety capacitors made from a paper dielectric. These are common to find in X1/X2/Y cap applications, where the capacitors will be connected across line voltages. These capacitors are encased in an epoxy potting material, and the thermal expansion coefficients of the epoxy is not matched well, apparently, to the other capacitor components. Over the years, the capacitor will begin to crack, and eventually the capacitor will fail short, across the line, and release a large cloud of smoke. These capacitors, luckily, are not usually critical to the power supply, and can be removed without affecting function of the equipment. They are only there to provide some EMI isolation to prevent noise from the equipment from making it back into the power grid, or vice versa

TL;DR

The thing with capacitor failure, is that it’s highly dependent on the type of capacitor, as well as the application it’s used in.

Electrolytic capacitors

Tantalum capacitors

Ceramic capacitors

Film cap

Hopefully I have managed to teach a bit about the different common capacitor types, and give a new perspective on likely failures and some troubleshooting tips.