An automatic technique to obtain the equivalent circuit of aluminum electrolytic capacitors

This paper presents an economic and automatic experimental technique that allows the determination of the equivalent circuit of aluminum electrolytic capacitors at their operating conditions. The equivalent circuit of an electrolytic capacitor comprises an equivalent series inductance (ESL), an equivalent series resistance (ESR) and a capacitance (C). Both C and ESR values change with frequency, temperature and aging. The knowledge of the capacitor ESR value is essential to achieve the best design proposal for switch mode power supplies (SMPS) and in the selection of the best capacitors used in the DC link of adjustable speed drives (ASD). On the other hand, the accurate knowledge of the capacitor capacitance is also very important in the rectifiers design and in the choice of the best capacitor for the DC link of ASD. Moreover, the knowledge of both parameters allows the evaluation of the capacitor state condition. To implement the proposed technique, two distinct circuits should be considered. The first one accounts for the estimation of the ESR and reactance of the capacitor, while the second one allows the estimation of the capacitor capacitance. To implement the first circuit, it is necessary to put the capacitor under test in series with a resistor and connect them to a sinusoidal voltage. The relationship between the gain and phase of the capacitor voltage and current waveforms, obtained from sinusoidal fitting, gives enough information to compute both ESR and reactance of the capacitor. The second circuit is a simple charge/discharge circuit. From the relationship between the capacitor current and voltage, during discharge, it is possible to compute its capacitance using the least mean square algorithm.