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Smart power ICs are widely used in universal input power supplies. This paper presents a detailed analysis and scheme design for real-time small-signal modeling method for smart power IC. First, in this method, the switching duty cycle is converted into a voltage output signal based on the operational characteristics of the power IC. The power IC's frequency response is then obtained by measuring the duty cycle output signal and the power IC's control input signal. The modeling system is referred to as sampling the duty cycle of the power IC with a zero-order hold output. Using least-square identification, the smart power IC's dynamic model and transfer function are synthesized from the measurement data. The results of the modeling experiment are discussed, and practical guidelines and error amplifier (EA) design examples are presented for both continuous and discontinuous conduction modes (CCM and DCM). The effectiveness and practicality are verified through experiments.