Sunday, October 24, 2010

Derating Tantalum Capacitors…. Why?

A few weeks ago I was at Arrowfest, an annual electronics training event put on by Arrow, and the Kemet Sales Engineer started talking about derating Tantalum Capacitors……

The thought of “derating” always makes me wonder “Why don’t the Tantalum Capacitor manufactuer’s just mark the proper usable voltage on to the parts?”,  that way the embedded designers wouldn’t have to rely on rules-of-thumb or safety-margin  guidelines.  A history lesson is required to understand why……

In the 1950’s the need for new Lower ESR Capacitors spawned Bell Labs to invent the solid-electrolyte Tantalum Capacitor, and drive Richard Millard of Sprague Electric Company to further patent the improved “reform-step” manufacturing process in 1955. These Tantalums offered < 2 Ohm ESR and provided much improved power supply filtering for higher-speed Digital Circuits over their wet-electrolyte predecessors.
The voltage rating of Tantalum Capacitors is a function of the geometry of the Tantalum powder used to create the porous conductive pellet that provides all the surface area for the charge to be stored. The finer the powder the larger the overall surface area in a given volume, but the finer the powder the lower the voltage can be across the dielectric. (Engineering is all about trade-offs) 

The voltage that is marked on Tantalum Capacitors is the DC rated voltage at 85 Degrees C, but I can think of very few applications were you are concerned about a capacitors “DC” voltage only… it is the “AC” ripple voltage spec that is important. The AC voltage amplitude and frequency of the application will determine ultimately the power dissipation “Heat” the capacitor will have to withstand. This increased power dissipation with AC voltages & surge currents is what prematurely causes the cap's MTBF to decrease. The MTBF can be increased by using a higher DC rated voltage capacitor in a given application; hence this is where the derating came from.

Kemet recommends 50% voltage derating factor for the MnO2 cathode system tantalums; but for the polymer-based cathode systems they recommend a 20% voltage derating factor.

Now in my mind Kemet should just mark 5V on a 10V MnO2 cap and let me not even have to think about the derating, but I guess 60 years of history is hard to erase.

“Characterization of Tantalum Polymer Capacitors” By Erik K. Reed
“Voltage Derating Rules for Solid Tantalum and Niobium Capacitors” By T. Zednicek and J.Gill

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