Electromagnetic field analysis is often used in the design of modern electrical equipment. However, electromagnetic field analysis is not often used in the design of power electronic devices, especially passive devices such as inductors and transformers. One of the major reasons for this is the improvement of switching devices. Switching devices used in power electronics have shifted from Silicon-based to Gallium Nitride (GaN) and Silicon Carbide (SiC) transistors. This has made it possible to reduce switching losses and increase the switching frequency. These features create high frequencies due to switching, causing parasitic inductance and parasitic capacitance effects to appear. This causes what was designed as an RC (resistor-capacitor) or RL (resistor-inductor) circuit element to operate as an RLC (resistor-inductor-capacitor) circuit due to parasitic effects. This is the reason why new electromagnetic field analyses that can take parasitic capacitance into account have been attracting attention in recent years. In this chapter, we explain the ringing phenomena caused by switching in GaN and SiC transistors and the reasons why it is difficult to calculate them by conventional electromagnetic field analyses and introduce the Darwin model of electromagnetic-quasistatic field analysis, which has attracted much attention in recent years, with examples.