Transformer Bobbins


From Xmultiple Engineering Dept.

Bobbins Overview

Xmultiple has a complete produced line of transformer products. In the case of an electrical transformer, inductor or relay, the bobbin is a permanent container for the wire, acting to form the shape of the coil (and ease assembly of the windings into or onto the magnetic core). The bobbin may be made of thermoplastic or thermosetting (for example, phenolic) materials. This plastic often has to have a UL or other regulatory agency flammability rating for safety reasons. Bobbins provide the alignment for the cores and therefore the bobbin is the critical element of any transformer. The bobbin or coil former are at the heart of the transformer and serves as its winding and termination platform. The bobbin or coil former supports the winding, channels the winding and provides a termination and connection method. Each bobbin is designed for use with a specific core shape, whether that core is ferrite, stacked laminations, or tape wound. There are many ways to design a transformer, so it is important to make the best bobbin and core combination selection. Product cost, availability, material limitations, safety agency requirements and, ease of production are all-important considerations.

Engineers designing products may ask which bobbin and core combination meet the form, fit and function of the design. However, cost considerations will not be far behind. The lowest cost core and bobbin combination with the widest availability are based on the square stack lamination sizes like E187, E24/25, E375, E21, E42/15. However, these square or rectangular center leg shapes may need to be shielded around the core with copper foil to reduce EMI, which will increase cost. Some Industry standards require 3750Vrms dielectric between the core and windings, and from winding to winding. To accomplish this effectively, a 3mm-isolation margin must exist on each side of the bobbin.

When designing and specifying a bobbin and core combination, attention is initially paid to the performance of the core. The designer must realize that the bobbin, and not the core, will have the greatest impact on the nature and ease of the winding operation, the winding lead termination, safety regulations and printed circuit board insertion. The bobbin will have a larger impact on the cost of manufacturing the assembly than the core.

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