January 7, 2010
News - Why You Should De-Rate Ca…
about a year ago
I couldn’t disagree more with most everything in this article.
The general rule of thumb is to derate by 30% (meaning use 70% of the rated voltage) unless you have a lot of ripple or transient currents. In circuits with high ripple or transients, you’ve got homework to do and shouldn’t be following a rule of thumb without a full understanding. In that case, 2-3x derating may save you. If you’re derating your bulk bypassing or filter caps by 2-3x it leaves you with an design that is unnecessarily expensive and large.
Despite the title, there is no discussion in the article of “why” to excessively derate capacitors. Electronic components are very precisely spec’d. Read and understand the datasheets. If the datasheet says “rated voltage X” it means that it will meet the specifications and it is ok to use at that voltage. Typically there will be a seperate “withstand voltage” that is significantly higher. Derating is done to ensure that there is sufficient margin in your design to ensure that you meet the component specs, not to ensure that the component performs.
Recommending avoiding tantalum capacitors is just silly. Tantalum capacitors have specific features that are sometimes very beneficial - although ceramics are getting to the point where they can fill in much of the time.
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