The interwinding capacitance of the transformer and the primary-side circuit nodes to ground capacitance are critical coupling path for common-mode (CM) noise in isolated power converters. In order to analyze the CM noise, the transformer interwinding capacitance is usually equivalent to lumped capacitance. In this paper, a generalized lumped capacitance model for constructing the CM noise equivalent circuit that is valid at low frequencies is derived. By simplifying the CM noise equivalent circuit based on Thevenin's theorem, the equivalent noise source (ENS) for CM noise is proposed. The ENS is expressed by a linear combination of voltages in the converter, where the coefficients of these voltages are determined by transformer winding structure and the circuit configuration. With the help of ENS, the CM noise of the converter under different circuit configuration can be systematically analyzed and conveniently compared, which develops an effective reference for selecting a proper circuit configuration. Finally, the lumped capacitance model and the ENS for CM noise are verified by the experimental results.