This paper presents an analysis of the impedance in a medium-frequency resistance spot welding transformer. The medium frequency system consists of an input H-bridge inverter, a welding transformer and a full-wave center-tapped output rectifier that consists of two diodes. The welding transformer consists of a primary winding and two secondary disk windings. The resistive impedance determines the thermal conditions of the transformer and the reactance affects the medium frequency of the system’s power consumption. It is necessary to consider that due to the center-tapped rectifier, the current can flow in only one of the secondary windings at a time, which causes an unbalanced magnetic field strength distribution. Unbalanced distribution can cause high leakage fluxes which increase transformer impedance. The article presents the formulas for calculation of the impedance transformers. They are necessary in order to reduce copper losses and design a device with minimum power consumption. Some assumptions were made to reduce the problem of calculating the leakage field of the transformer to a one-dimensional field. The formulas are functions that depend on the relative values characterizing skin and proximity effects in the transformer windings. The graphs of these functions are given for convenience of use. An example of the calculation of the impedance is presented. It is shown that the typical design of the transformer is not effective due to the proximity and skin effects. The reactance exceeds the resistive impedance by several times when the frequency is more than 1000 Hz, despite the reduction of the leakage inductance. To reduce the impedance of the transformer it is necessary to provide the interleaving of primary and secondary windings.