A modeling of thin rectangular bar of transverse-type Piezoelectric Transformer (PT) composed by three regions and made from PZT5A (lead zirconate titanate) ceramic material, is reported. The PT (partially metalized on the top and bottom sides) is dedicated to the galvanic isolation. Using the Legendre polynomial approach (PA) based on the polynomial development of the fields, taking into account the lateral dimension (2D study) and the gap (distance between the primary and secondary electrodes), results found are strongly matched through a comparison with those 3D finite element method (FEM). In addition, a good efficiency above 92 % is obtained for this converter. Furthermore, the PA permits to study the influence of the characteristic parameters and allows locating the gap length limit for best PT's performance.