In an isolated DC-DC converter utilized in a high-voltage DC power distribution system for data centers, a large surge voltage occurs across the secondary-side diodes owing to the transformer's leakage inductance, necessitating diodes with a large withstand voltage. However, these diodes cause large power losses owing to relatively large ON-resistances. First, this study analyzes surge occurrences using a high-frequency equivalent circuit. Second, the surge-suppression capability of a simple snubber capacitor is examined, and the surge-voltage evaluation through analysis using high-frequency equivalent circuits is described. This is followed by experimental confirmation with SiC diodes. Finally, the minimum capacitance of the surge-snubber capacitor is determined for miniaturization.