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Image of KEMET Ceramic Capacitor Basics Pt 3 - Heating IR Loss 2

Looking at the equation on this slide, the current through a capacitor is determined by the AC voltage, ESR (equivalent series resistance), ESL (equivalent series inductance), the nominal capacitance, and frequency. Since ESL for MLCCs is usually very low, it has little impact on the current until it approaches the series resonant frequency. So, for this example, the designer will not consider the impact of ESL on the AC current. Given the equation, if the AC voltage is held constant, there are two conclusions: higher frequencies will result in higher AC currents, and higher capacitance results in higher AC currents. This means that at higher frequencies and higher capacitance values, even low AC voltages can generate high AC currents. For example, a 15nF capacitor operating at 100kHz and 10VRMS will result in an AC current of 0.094ARMS. A 150nF capacitor operating at 1MHz and 10VRMS will result in an AC current of 9.4ARMS. Note that all the AC current will go through the ESR of the MLCC generating I2R losses resulting in self heating. Higher AC currents will produce more heating and possible thermal runaway.

PTM Published on: 2022-07-06