DC/DC converters are widely used to produce various power supply voltages required for various electronic components on a board. Though DC/DC converter is more efficient than the linear regulator is, however, the switching noise becomes larger with the increase of the switching frequency. In particular, ringing noises at the sharp rising edge of the switching waveform generated by the effect of the parasitic inductance becomes a serious issue. This noise spreads out to the whole circuit board, and then generates electromagnetic interference (EMI), which often causes a malfunction of the electronic systems. In this paper, the methods to accurately reproduce the switching waveforms and to reduce ringing noises and resulting EMI from DC/DC converters were investigated. For this purpose, several evaluation boards were designed, and total electrical model was constructed by taking into account the parasitic inductances of traces on a board. First, frequency-domain analysis for the switching loop of the DC/DC converters was executed to find the optimum condition to suppress ringing noise effectively. Next, time-domain simulation was executed by considering the recovering time of MOS reverse current, which is not provided from device vendors. Finally, measured switching waveforms were well correlated with the simulated ones, and the optimal condition to suppress the ringing noise.