Surface Mount Transformer Theory

Surface mount electronic transformers refer to a common type of construction that permits attachment of transformers to a printed circuit board (PCB). Surface mount electronic transformers are sometimes wound on bobbins with integral SMT (gull wing,J-lead, etc.) pins.

Surface mount electronic transformers can use a variety of core materials: laminated or tapewound silicon steel alloys, nickel-iron alloys, cobalt alloys, powdered irons, ferrites, air cores, materials processed for square hysteresis loop properties and others.

Historically, transformers and other circuit devices have been mounted on PCBs using PTH (plated through hole) 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 via holes and then soldered to the copper circuitry. Increasingly, PCB manufacturers use solder pastes, adhesives and assembly processes that permit attaching transformer terminals to the PCBs without using via holes. Flat areas (known as solder pads) on the transformer terminals are soldered directly to copper circuitry surface, 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. It also increases PCB component density by allowing more components to be mounted on the opposite board face.

The toroidal coil, for example, 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: thermoplastics, phenolic, FR4, thermosets, Teflon™ and others. Most of these are molded. Others are machined. Some bobbins and headers are self­leading. In some scenarios the winding wire itself can be 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 including pot cores (round), RM, EP, E, El, EEM, EFD, U, UI, ER and many others, including even custom shapes. Surface mount transformers in these shapes are usually only available in the smaller sizes. However, designers are adapting more shapes and larger sizes to surface mount transformer applications. Designers may have mechanical and shear strength 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 and shock. Please consult with Gowanda regarding the optimum mounting configuration for your specific application and environment.