Surface Mount Transformers Theory

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Surface mount electronic transformers refer to a type of construction that permits attachment of surface mount transformers to a printed circuit board (PCB). Surface mount electronic transformers are usually wound on surface mount bobbins, but are also available as toroidal coils.

Surface mount electronic transformers can use a variety of core materials: laminated or taped wound silicon steel alloys, nickel-iron alloys, cobalt alloys; powdered irons and nickels; ferrite; air core; and/or core materials processed for square loop or round loop properties; and others.

Historically, transformers and other circuit devices have been mounted on PCBs using pin-thru technology. Transformer wires are terminated to pin type terminals. Holes are drilled in the PCB’s copper circuitry to accommodate the transformer pins. The transformer pins are inserted through these holes and then soldered to the copper circuitry. Engineers have developed solder pastes, adhesives, and assembly processes that permit attaching transformer terminals to PCBs without using holes. Flat areas (known as pads) on the transformer terminals are soldered directly to copper circuitry surfaces hence the term surface mount transformer. This process eliminates the need to drill holes for the pins, thereby reducing the cost to manufacture a PCB.

The toroidal coil is mounted on a header equipped with surface mount terminals. The bobbins (or headers), used with surface mount transformers, come in a variety of materials: plastics, phenolic, glass, Teflon and others. Most of these are molded. Some are fabricated. Some bobbins and headers are “self leading”. The winding wire is also used to form the surface mount terminal by looping the wire under a pre-formed flat edge thereby forming a reasonably flat terminal area.

Surface mount electronic transformers are available in a variety of shapes. Surface mount electronic transformers shapes include pot cores (round), “RM” (square pot cores), “EP”, “E”, “EI”, “EEM”, “EFD”, “U”, “UI”, “ER”, and some others including custom shapes. Surface mount transformers in these shapes are usually only available in the smaller sizes. Designers are adapting more shapes and larger sizes to surface mount transformer applications. Designers have mechanical concerns about the larger sizes. The weight of the larger sizes may exceed the weight that soldered surface mount pads can safely handle under vibration. Over time, designers hope to develop surface mount transformers (and inductors) in larger sizes.