In this paper, a voltage-bucking KY converter, which is KY converter's derivation is presented. KY converter is also called 1+D converter which is proposed by K.I.Hwu first in Taiwan. Compared with the traditional KY converter, the proposed KY converter's derivation can achieve energy flows from high voltage to low voltage. Combination with the proposed converter, KY converter can become bidirectional converter. For cascade, KY converter has 2+D and 1+2D gain, similarly, the proposed derivation has 1/(3-D) and 1/(3-2D). A detailed description and analysis of the derivation is presented along with the PLECS simulation.