Planar transformer
A planar transformer including first and second parts of a core with least one of the first and second parts having a center post that resides in an aperture of a PWB. The PWB includes primary windings and secondary windings that surround the center post, are magnetically coupled by the core, and are electrically independent. Separating an uppermost winding from the first part of the core is a first prepreg structure and separating a lowermost winding from the second part of the core is a second prepreg structure. There being no windings disposed between the first and second prepreg structures and the core. This planar transformer structure inhibits the windings from electrically shorting through the core when the planar transformer is subjected to shock or vibration. One variant includes the use of pre-form adhesives sheets located between the solder mask layers of the PWB and the core that further inhibit the windings from shorting through the core.
1 . A planar transformer comprising:
a core comprising a ferromagnetic material, the core including a first part and a second part, at least one of the first and second parts having a center post;
a printed wiring board having an aperture in which resides the center post, the printed wiring board comprising one or more laminates that each have a first side surface and an opposite second side surface, each of the first and second side surfaces including one of a primary winding and a secondary winding that are magnetically coupled by the core, the primary and secondary windings being electrically independent and surrounding the center post, the printed wiring board having an uppermost primary or secondary winding that is separated from the first part of the core by a first prepreg structure and a lowermost primary or secondary winding that is separated from the second part of the core by a second prepreg structure, each of the first and second prepreg structures comprising multiple prepreg layers, there being no primary or secondary windings disposed between the first prepreg structure and the first part of the core, there being no primary or secondary windings disposed between the second prepreg structure and the second part of the core, the printed wiring board including a first solder mask layer deposited on an uppermost surface of the first prepreg structure and a second solder mask layer deposited on a lowermost surface of the second prepreg structure, the first and second parts of the core being respectively bonded to the first and second solder mask layers;
the first and second parts of the core being respectively bonded to the first and second solder mask layers by a respective first and second pre-form adhesive sheet that each has a hardness in the Shore D hardness scale at a temperature of 70 degrees Fahrenheit;
each of the first and second prepreg structures and each of the first and second pre-form adhesive sheets being configured to inhibit the primary and secondary windings from electrically shorting through the core when the planar transformer is subjected to shock or vibration.