Planar transformers http://www.paytongroup.com/What_is_Planar_Technologyare devices used to exact standards with precise electrical characteristics such as capacitance, output, and aspect ratio. They are used to carry out military and aerospace projects.
Planar transformers are high frequency transformers used in isolated switchmode power suplies operating at high frequency. As opposed to convertional "wire-wound-on-a-bobbin" transformers, planar transformers usually contain winding turns made of thin copper sheets riveted together at the ends of turns in case of high current windings, or windings etched on a PCB in a spiral form. As the current conductors are thin sheets of copper, the operating freqiency is not limited by the skin effect. As such, high power converters built with planar transformers can be designed to operate at relatively high switching frequencies, often 100kHz or above. This reduces the size of required magnetic components and capacitors, thereby increasing power density.