November 18, 2021
Capacitor Derating
Just because you pick a capacitor that says 10 µF does not mean that it has 10 µF capacitance. A variety of factors can act to decrease capacitance. Those factors are time, temperature, and voltage. The capacitor choice for an IoT device designed to work in an air-conditioned office isn’t as critical as a near-Earth satellite with 10-year mission life.
Engineers have to derate components to ensure they perform as expected in their circuits. For example, an engineer that needs at least 6 µF for a decoupling capacitor might choose a 10 µF capacitor and derate it by factoring in the expected losses due to time and temperature. After several years of use, the capacitor might have an effective capacitance of 8 µF, which is still more than the 6 µF needed for the design.
Linear dielectric materials have no polarization “memory.” As soon as the electric field disappears, the atoms and molecules return to their original non-polarized state. Ferroelectric materials have a memory, and their capacitance at low reverse voltages is usually much less than specified due to the polarization hysteresis. For that reason, engineers tend to pick capacitors that are specified within 2x-3x their design voltage. The 2x-3x providers a small safety factor, and importantly, the operating voltage is still sufficient to reverse the polarization of the atoms and molecules. An X7R capacitor for a 5 V net might be rated no more than 10 V or 16V (common capacitor voltage ratings).
Temperature coefficient of linear dielectric materials
Most materials become increasingly disordered as their temperature increases, which means they aren’t as easily polarized. So at high temperatures, capacitance tends to decrease.
The relationship between change in capacitance and temperature for various dielectrics. (Data from Knowles Capacitor fundaments)
Temperature coefficient of ferroelectric dielectrics
Ferroelectric materials behave in a far different fashion. As the temperature of a ferroelectric material increases past particular threshold temperatures, the material can undergo a phase change. Phases are not limited to the common “solid, liquid, gas, plasma” you learn in secondary school but can include new atomic vibrational movement and perhaps crystalline rearrangement from cubic to tetragonal crystalline arrangement.
Time
Except for linear dielectrics, capacitors do not behave the same on day 500 as on day 1. The capacitance decreases over time. Capacitance can be recovered in a baking process (150° C) that heats the atoms enough that thermal motion is energetic enough to overcome polarization memory.
