We considered of the buck and half bridge series DC-DC converter. It has good applications in areas with low voltage/high current, wide input voltage. The buck converter ratings and the half bridge converter ratings are $36~72V$ input and 22V/5A output, $19~24V$ input and 3.3V/30A output, respectively. Developed the buck and half Bridge series DC-DC converter ratings are of $36~72V$ input and 3.3V/30A output. The buck converter is operated with zero voltage switching process to reduce the switching losses. The $80.1\%~97.6\%$ of the efficiency is measured at $18.4{mu}H$ output filter inductance of buck converter. In the half bridge converter, the $86\%~96.4\%$ efficiency is measured at 150kHz switching frequency with PQI core. In the case of synchronized the buck and half bridge DC-DC converter, the measured efficiency is higher than that of the unsynchronized converter. In the synchronized converter, the maximum efficiency is measured up to $92.3\%$ with PQI core at 150kHz. 7A output.