Abstract: In experiments it is found that the wire wound spiral transformer would undergo breakdown during the charging period, sometimes even when the primary circuit just conducts. To explore the reason for breakdown and to improve insulation, a simplified multi-conductor transmission line method is used to deduce the inter-turn voltage distribution, under the assumption that the displacement current can be neglected. The results show that the voltage distribution varies with time, and it is influenced by circuit extrinsic parameters such as circuit mismatch, primary capacitor voltage, etc. With slight circuit mismatch, the transformer is likely to break down at the initial stage of charging. These characters can be utilized to design a better insulated transformer, by making the voltage distribut