An integrated transformer structure. In one embodiment, the primary transformer winding is formed using dielectrically isolated (DI) technology so as to isolate high voltage applied to the transformer primary from other components in the substrate. Alternatively, conventional junction isolated technology may be used, where physical separation between the integrated transformer and other components may be provided. In accordance with the present invention, the primary winding (18, 42) comprises a planar spiral formed with a low-resistivity material and incorporated with the substrate (20, 46). An insulating layer (26, 52) is then formed over the primary winding. A planar spiral configuration is also used to form the secondary winding (14, 44), where the secondary may be formed of a deposited metal and is formed on top of the insulating layer so as to be directly above the primary winding. The result is an effective aircore transformer structure capable of isolating thousands of volts.