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Magnetic components play an important role in a power electronic converter system. They provide energy buffering, galvanic isolation and voltage ratio conversion and have significant impact on the overall performance of the system regarding power efficiency and dynamic behavior. On the one hand, the increasing demand for higher power density in the new generation of power converter systems brings new challenges to magnetic component design. On the other hand, the optimization of the whole power converter system including component selection and control design requires a good understanding of the magnetic components. Conventionally, magnetic design and system behavior evaluation rarely take into account the non-idealities of the magnetic components, so that the performance of the final hardware may differ substantially from the expectation. For this reason, improved models of magnetic components to be used in time-domain circuit simulation are desired. Magnetic circuits based on the permeance-capacitance analogy opens a new direction for modeling magnetic components, because magnetic non-idealities can be incorporated intuitively and the models are simple enough to be seamlessly integrated into system-level simulations. In this thesis, improved models of magnetic components are developed using the permeance-capacitance based magnetic circuits. In order to evaluate the core losses of a magnetic component, models of the frequency-independent hysteresis effect are proposed for commonly used core materials. Several variations of the Preisach model are used to accommodate different shapes of hysteresis loops. The models are valid over a wide range of field strength amplitude. They are valid for arbitrary excitations such as sinusoidal wave and PWM with DC bias. The magnetic circuit models are extended to cover frequency-dependent core losses. Two major loss mechanisms -- relaxation effect and eddy current effect -- are combined with the static hysteresis models, by introducing resistive components into the magnetic circuit. The relaxation effect model typically applies to ferrite materials under intermittent PWM excitation. The eddy current effect occurs in metal based cores, where the material conductivity contributes to the core loss with increasing excitation frequency. Both frequency-dependent and frequency-independent losses are physically present in the circuit model. Besides the main fluxes within the magnetic core, the leakage fluxes though the air have a considerable influence on the system behavior. The leakage flux coupling in multi-winding transformers leads to unbalances in the electrical circuit of complex power converter systems. The last part of the thesis presents a model for unbalanced leakage flux coupling in the permeance-capacitance based magnetic circuit, which intuitively reflects the impact of the winding placement on the unbalances in the electrical circuit. The models of magnetic components proposed in this work can be flexibly extended for a wide variety of core geometries and connected to arbitrary external electrical circuits consisting of switching devices and other passive components. The core losses and the unbalances due to leakage flux coupling can be evaluated under dynamic operation without any prior assumptions regarding the frequency spectrum and the magnitude of the excitation. The proposed models help to accurately simulate power converter systems under normal and unexpected operating condition