The high-frequency switching of transistors in power electronic (PE) converters is known to cause unintended common-mode (CM) current that flows through parasitically-coupled ground paths. One way to model these currents is to utilize time-domain simulations that capture switching dynamics and the corresponding parasitic response. Although potentially useful, the small time steps required can create a computational burden and limit the usefulness of the approach. In addition, access to internal hardware needed to characterize parasitic parameters is often limited.In this thesis, frequency-domain Thevenin equivalent circuits (TECs) are derived to model the CM behavior of PE converters. To do so, periodic linear time-varying (PLTV) analysis is used to develop Thevenin-like models that account for switching behavior of PE circuits. Subsequently, it is shown that in many applications these PLTV TECs can be reduced to traditional linear time-invariant (LTI) forms. Methods to experimentally characterize LTI TEC parameters and couple multiple TECs together for system-level analysis are then established. Finally, the TEC approach is extended to model converters in which common- and differential-mode (CM/DM) behavior are strongly coupled. Simulation and experimental results are used to validate the proposed TEC techniques.