For the power systems in electric vehicle, a weight and efficiency are very important since it is directly related with driving range. Many previous researches proposed new systems to reduce a weight or to increase efficiency. However, although they improved the weight or efficiency, they usually degrade the other side. Therefore, this paper proposes a new power system which achieves high efficiency while don't degrade the weight of the power system by adopting frequency modulation control. The proposed idea is implemented for 48V-to-12V DC-DC converter of mild hybrid electric vehicle using buck converter.