This paper focuses on leakage inductances which are bigger than any other parasitic inductances, and studies arelationship between leakage inductance and surge voltage on isolated DC-DC converter. As a result, we confirmed that the mechanism of the surge voltage in current mode DC-DC converter is completely different from that in voltage mode DC-DC converter. Moreover, in voltage mode DC-DC converter, the mechanism of the surge voltage in Forward topology is different from that in Full-bridge topology. Additionally, this paper proposes the equation to bring a peak value of surge voltage and the practical snubber design technique for current mode DC-DC converters. Finally, the validity of the proposed equation and design technique is confirmed by experiments and simulations.