Why are electrolytic capacitors such a pain? Several reasons come to mind, but the A-#1, everybody’s-favorite is (or should be): reliability. In many product types, electrolytic caps exhibit field-failure rates that top the list of component-caused system failure. For example, a survey of system failure analyses presented by NIC Components concluded that capacitors accounted for 30% of failures—more than any other source.
And virtually all of it preventable.
Capacitors appear in designs as bi-parametric devices: They exhibit a capacitance—their raison d’être—and a maximum working-voltage rating. They are typically used where their high volumetric efficiency comes in handy, or where the amount of capacitance needed can’t be provided by other types, such as ceramic caps.
In reality, the characteristics of electrolytic caps include ripple-current limits, ESR (equivalent series resistance), tan δ (dissipation factor—an expression of ESR relative to capacitive reactance), ESL (equivalent series inductance), dielectric absorption, leakage current, and all of it parametric in temperature.
For electrolytic capacitors, temperature is indeed the enemy of reliability. A mere 10°C rise in internal operating temperature halves a capacitor’s expected lifetime. The flip side: big reliability wins for ensuring your caps keep their cool.
A capacitor’s ripple-current limit is, in effect, an expression of the device’s sensitivity to excessive temperature. Ripple current reflects in the device’s ESR as I2R heating at the heart of the device. A cool ambient can’t save an electrolytic capacitor from excessive current.
It’s common now for manufacturers to offer ripple current ratings that include a frequency-dependent coefficient. But be careful when comparing devices from different manufacturers: There is no standard reference frequency so comparing the base (non-parameterized) ripple-current rating between similar devices from different suppliers can be misleading unless you fold in each device’s frequency-dependent coefficient appropriate for your circuit.
The industry developed capacitor specifications back when power supplies were linear circuits operating from 50- or 60-Hz ac mains. Early supplies also powered circuits with low to modest dynamic currents compared to those of the modern day. Today, power converters are non-linear circuits, many of which operate with switching frequencies in excess of 1 MHz and support even faster load-current dynamics. Over this history, it took quite some time for capacitor manufacturers to specify ripple current even at 100 kHz. Now many, but certainly not all, provide ripple-current coefficients for frequencies out to 500 kHz or more.
Although excessive heat is the greatest hazard, it isn’t the only one. Excessive voltage stress is an obvious danger to capacitor reliability but so, too, are environmental factors such as shock and vibration and chemical contamination, particularly by halides.
You might start to think that electrolytic capacitors are simply junk components and should be avoided, but the facts are quite to the contrary: By careful specification of the components they use, one power-subsystem manufacturer has demonstrated capacitor lifetimes between 50 and 100 years, depending on the degree of conservatism for certain elements of the estimate. This range exceeds the expected useful lifetime of the products these capacitors inhabit, so this manufacturer has concluded that capacitors don’t reduce product reliability beyond the random failure statistics governing all components. So it can be done.
What can you do to improve capacitors’ reliability in your products? Here are a few suggestions:
• Recognize that capacitors only look simple. Read through the data sheet for the specific devices you’re considering and, for each parameter, consider how your circuit’s behavior compares to the device’s specified performance and operating range.
• Recognize that capacitors are not a good place to cheap out. Nothing alienates customers faster than premature product failures. Just ask Dell, for example, which one year had to spend nearly a half-billion dollars replacing motherboards populated by substandard capacitors. To the the company’s credit, it traced that particular mishap to a capacitor-manufacturing issue.
• The major capacitor manufacturers have multiple product lines that differ primarily by electrolyte chemistry and form, and by anode and cathode metallurgy and processing. Familiarize yourself with the various qualities each variation brings to your product and choose the device that meets your products’ and your customers’ needs.
• Choose capacitors with temperature ratings that don’t just meet but exceed your product’s expected environmental requirement—including unusual but foreseeable operating conditions. Remember, 10°C extra margin doubles the capacitor’s expected lifetime.
• Where possible, arrange your PCB layout to keep electrolytic capacitors away from heat sources. There are applications, such as power-supply bypass, where you must place capacitors as close as possible to their client device, say a hotter-than-the-surface-of-the-sun MCU, to minimize parasitic trace resistance and inductance. In such cases, try to orient the assembly so the capacitors gain the benefit of available convection cooling, and keep them out of the shadow of the heat source. Also, make power and ground traces as wide as possible to minimize their parasitics. Improvements can buy you a bit of additional distance between cap and client.
• If your product is to subject to shock and vibration, use capacitors in low-profile packages where possible. If large or leaded devices are necessary, provide additional support with an adhesive compound between the cap’s case and the PCB to relieve the electrical connections from excessive mechanical stress. Ensure that all coatings, molding or potting compounds, and adhesives are halide free to prevent chemical contamination.
• Check your circuit’s turn-on behavior to ensure that capacitors are not subject to voltage overstress. Even momentary over-voltage events can cause permanent damage and significantly shorten component life. If your circuit includes an inductive load, check the turn-off behavior as well.