When we first learn of capacitors at an academic level we learn the basics such as dielectric and farads. For most of an engineer’s undergrad career that is pretty much all you need to know about the energy storage devices. So, we enter the professional world as freshly minted engineers filled with knowledge of science, physics, and electronics. After a couple of years of working in the field, it becomes apparent that there is still much to be learned. For example, capacitor derating isn’t something taught in school. At some point some graybeard engineer comes to us and says, “don’t forget to derate that capacitor.” Huh? Derating? What is that? Capacitor manufacturing technology advances and changes like any other and as such the derating rules that the graybeards grew up with may also need some revision, especially those in the tantalum polymer family.
But Why Derate?
Derating is a term that is frequently used but not always in the right context or for the right reasons. We learned from the graybeards we must derate, but why? Derating is sound engineering practice. The reasons for doing it are diverse. Oftentimes one or more parameter of a device is derated to improve some type of performance. Said performance metric could be quiescent current or it could be life. In the case of tantalum capacitors voltage is derated in an effort to improve the life of the capacitor.
Derating the Right Amount
Once upon a time, the recommended amount of derating for tantalum capacitors was 50%. In other words, if you have a 5 V application, then, to extend the life of that capacitor you would need to buy a 10 V capacitor, not a 6.3 V capacitor. That rule is fine for the old style of tantalum capacitors that were constructed with an MnO2 cathode material. Although that rule held weight for about 30 years, the advent of tantalum polymer capacitors brought on a different derating paradigm. Thirty years of habit is hard to break, and many engineers were unnecessarily over derating their tantalum polymer capacitors.
When tantalum polymer capacitors were first introduced, 10% to 20% derating was recommended, as shown above. That is no longer strictly true. These derating rules remain true for some of the polymer capacitors in more mission-critical applications, such as space and automotive. For our commercial grade series of polymer capacitors, those of the T52x through T530 series the derating rules can be slightly modified to be consistent with their application. KEMET’s manufacturing techniques have improved and for our T52x-T530 tantalum polymer family the application voltage can be much closer to the rated voltage. We now recommend only 10% derating up to an operating temperature of 105°C. The image below is an excerpt from a datasheet that shows the new derating guidelines. With these new derating guidelines, you can create a more optimized design and not buy more voltage than you need.
Testing our Derating Guideline
It wasn’t guess-work to come to these derating guidelines we put our capacitors through a rigorous process that we call Surge Stress Step Testing (SSST). The methods of SSST are specified here. Without getting into the gory details, it is a rigorous test designed to exercise our parts at extreme conditions and find where their limits truly lie. From the results shown in the table above, it is evident that for those polymer KO capacitors of a rated voltage greater than ten, the failure rate is only 2 ppm as opposed to 1,700 for their older manganese dioxide counterparts. The moral of the story here is that KO caps are actually much less prone to failure and therefore don’t need as much derating as the MnO2 variety. Another thing to be noted here is that the failure rate for our AO-CAPs, which are our aluminum polymer capacitors have a failure rate of 0 ppm, which means they are extremely robust. We will save the topic of AO-CAPs for another article.
KEMET Leads the Way
KEMET is the main industry leader in tantalum polymer capacitors. We have taken our polymer technology from the commercial segments, to industrial, to automotive, and to space. You can browse our KO-CAP offerings using our powerful parametric search engine: Component Edge.