© 2020 The Institute of Electrical Engineers of Japan. Fast switching operation of Silicon carbide (SiC) and Gallium nitride (GaN) power semiconductor devices could be a severe electromagnetic interference (EMI) noise source of high-voltage power converter. This paper experimentally evaluates the dynamic characteristics of SiC and GaN power transistor in synchronous rectification DC-DC converter and characterized it as an EMI noise source. The results suggest that the characteristics of unipolar device of SiC MOSFET (metal-oxide-semiconductor field-effect transistor) and GaN HEMT (high-electron-mobility transistor) show large-amplitude and slow-damped ringing oscillation in the switching waveform. It will influence on the EMI level of power converter in several tens of megahertz range.