Multiple output and small power modules are widely used in airborne devices. According to 270 V high-voltage DC input, a 7 output and 32W power module was developed to drive the motor. Combined with the requirements of the converter, the high efficiency, high power density, high voltage accuracy and low cross adjust rate are the main difficulties. A two stage converter composed of a half-bridge (D=50%) and buck was selected after analyzing four possible solutions. To optimize efficiency, the formula of converter's total loss was deduced, which varies with operating frequency and input voltage. The impact of frequency on the converter efficiency was also discussed with a conclusion that the optimal frequency is 100 kHz. The results also show that under the condition of same frequency and load, efficiency decreases with the increase of input voltage. Finally, a prototype was produced and experimental results were given: under the 100 kHz switching frequency condition, the efficiency decrease from 84.7% to 76.3% with the increase of input voltage and corresponding to the rated input voltage the efficiency is 80.2%, both of them are consistent with the theoretical analysis. In addition, the efficiency loss results also provide a basis for further optimization.