In this study, to develop the high power piezoelectric transformer for adaptor, PSN-PNN-PZT system ceramics were formulated as a function of $MnO_2$ addition and its dielectric and piezoelectric characteristics were investigated. Multi-layer piezoelectric transformer using an excellent composition was also fabricated and its electrical properties evaluated. The composition ceramics added to 0.5wt%$MnO_2$ showed the maximum value of $k_p=0.61$ and $Q_m=1.321$. As the output power of piezoelectric transformer is increased, its temperature rise increased. At the fixed 18W output power, the transformer was stably operated.