Accurate modeling of power electronic converters, i.e., obtaining control to output transfer function, must include series resistance of capacitors and inductors. A simple method allows measurement of a capacitor’s Equivalent Series Resistance (ESR).

Capacitors are classified according to their dielectric’s type. Electrolytic capacitors are popular in power electronic circuits, due to their high volumetric efficiency and excellent price performance ratio. Unfortunately, their characteristics change with operating frequency, whereas an ideal capacitor’s impedance decreases with frequency. But, in the real world this is not seen in the laboratory. Increasing frequency up to a certain point leads to the expected impedance reduction but increasing frequency causes the impedance to increase, i.e., it acts like a resonance circuit. To model the behavior of an actual capacitor requires the addition of extra elements to the capacitor model. ESR is actually the resistance that a capacitor shows in the border between capacitor-like behavior and inductor-like behavior, i.e., resistance at the resonance frequency.

When modeling the dynamic behavior of power converters, ESR is important value because it predicts the output ripple of the converter as well as predicting capacitor lifetime. Power dissipated in ESR causes the capacitor’s temperature to increase and its capacitance and lifetime to decrease.

A simple and direct method for measuring ESR is proposed in [3], in which the ESR is determined directly by the ratio of the capacitor’s ripple voltage to ripple current. But the implementation is quite expensive and troublesome. To determine ESR using only voltage measurements, Chen et al. suggested that under some specific conditions, the inductor ripple current could be assumed to be constant and hence the output ripple voltage determines the ESR. However, the proposed method is limited and its precision is not high.

Here, we present a simple measurement technique for the determination of capacitor ESR.

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