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This paper presents two new methods for estimating the capacitance value of DC-link capacitors for single-phase AC/DC PWM converters. At no load condition, a controlled AC current (or a controlled AC voltage) with a lower frequency (30 Hz) than the line frequency is injected into the input side, which then causes AC voltage ripples at the DC output side. By extracting the AC voltage, current, and power components on the DC output side using digital filters, the capacitance value can then be calculated, for which the recursive least squares (RLS) method is used. The proposed methods can be simply implemented only with software and no additional hardware is required. The simulation results are illustrated to verify the proposed estimation algorithm.