Q Factor definition The Q factor of a capacitor, also known as the quality factor, or simply Q, represents the efficiency of a given capacitor in terms of energy losses. It is defined as: where QC is the quality factor, XC is the reactance of the capacitor, C the capacitance of the capacitor, RC is the equivalent series resistance (ESR) of the capacitor, and ω0 is the frequency in radians at which the measurement is taken. In an AC system, the Q factor represents the ratio of energy stored in the capacitor to the energy dissipated as thermal losses in the equivalent series resistance. For example, a capacitor that is capable of storing 2000 joules of energy while wasting only 1 joule has a Q factor of 2000. Since Q is the measure of efficiency, an ideal capacitor would have an infinite value of Q meaning that no energy is lost at all in the process of storing energy. This is derived from the fact that the ESR of an ideal capacitor equals zero. The Q factor is not a constant value. It changes significantly with frequency for two reasons. The first reason is the obvious ω0 term in the above equation. The second reason is that ESR is not a constant value with regard to frequency. The ESR varies with frequency due to the skin effect, as well as other effects related to the dielectric characteristics. A related term, called the dissipation factor(DF), is sometimes defined in capacitor datasheets instead of the Q-factor. In AC circuits the DF is simply the reciprocal value of Q. Why is the Q factor important? Most applications do not have to take the Q factor into serious consideration, and standard capacitors may be used in those applications. However, the Q factor is one [… read more]